In: Seminar publication; protection of public water supplies
from ground-water contamination
Environmental Protection Agency
For millennia, man has disposed of his waste products in a variety of ways. The disposal method might reflect convenience, expedience, expense, or best available technology, but in many instances, leachate from these wastes have come back to haunt later generations. This is largely because we have not thought out the consequences of our actions. Ground-water pollution may lead to problems of inconvenience, such as taste, odor, color, hardness, or foaming; but the pollution problems are far more serious when pathogenic organisms, flammable or explosive substances, or toxic chemicals or their by-products are present, particularly when long-term health effects are unknown.
Individual polluted ground-water sites generally are not large, but once polluted, ground water may remain in an unusable or even hazardous condition for decades or even centuries. The typically low velocity of ground water prevents a great deal of mixing and dilution; consequently, a contaminant plume may maintain a high concentration as it slowly moves from points of recharge to zones of discharge.
An oil-field brine holding pond was constructed adjacent to a producing well in central Ohio in 1968. Two years later when the well was plugged, the holding pond was filled, graded, and seeded. The chloride concentration in the ground water in the vicinity of the former pond still exceeded 36,000 mg/1 some 10 years after the operation began and 8 years after reclamation.
Scores of brine holding ponds were constructed in central Ohio during an oil boom in 1964; many are still in use. In 1978 a number of test holes were constructed within 200 feet of one such pond. Within this area shallow ground water contained as much as 50,000 mg/1 of chloride. Moreover, brine-contaminated ground water provides part of the flow of many streams and this has caused degradation of surface-water quality.29, 30, 31
Documentation of the migration of leachate plumes originating at garbage dumps and landfills is becoming increasingly abundant. Data show that under certain hydrologic conditions leachate plumes can move considerable distances and degrade ground water throughout wide areas. Furthermore, the problem is worldwide. Exler3z described a situation in southern Bavaria, Germany, where a landfill has been in operation since 1954 The wastes are dumped into a dry gravel pit. As Figure 116 illustrates, data collected from 1967 to 1970 showed the narrow lense-shaped plume had migrated nearly 2 miles.
Figure 116. Leachate from a Landfill
in Bavaria has migrated more than 2
Miles and the Ground Water has been degraded for narly 25 years
Figure 117. Thirty-Three Years after
disposal began the leachate from Aluminum
ore and mill tailings is still a problem in Keiser, Oregon
As Figure 117 illustrates, incompletely processed aluminum ore was dumped into a borrow pit in Keizer, Oregon from August 1945 to July 1946.33 The ore and mill tailings had been treated with sulfuric acid and ammonium hydroxide. When first recognized by local residents in 1946, the ground water was contaminated by more than 1,000 mg/1 of sulfate; many shallow domestic wells tapping the Recent alluvium were contaminated. In the Spring of 1948 the waste was removed from the borrow pit. Two wells, reportedly capable of producing more than 700 gpm (gallons per minute) were installed near the pit and the contaminated groundwater was pumped to waste for several months. By 1964 the contaminants had migrated more than a mile. No doubt some of the contaminants are still in the ground water at Keizer.
A well-documented study by Perlmutter and others 34 showed that disposal of chromium and cadmium-rich plating wastes from an aircraft plant on Long Island during a 20-year period contaminated a shallow aquifer. Figure 118 illustrates this study. The contamination was first discovered in 1942, and by 1972 the degraded ground water zone was about 4,200 feet long and 1,000 feet wide. The 1972 study demonstrated that the chromium-cadmium enriched cigar-shaped plume "had not only reached Massapequa Creek but was present in the stream as well as in the beds beneath it."35
Figure 118. More Than 36 Years After Disposal of Plating Wastes
the Ground Water Remains Polluted in South Farmingdale.
Figure 119. The Picric Acid, Which Has Been Found in the Ground
Water Near London for Decades, Originated at a World War I Munitions Plant.
During the middle and late 1930's grasshopper infestations were stripping the vegetation throughout wide areas in the Northern Great Plains. In western Minnesota partial control was obtained by a grasshopper bait consisting of arsenic, bran, and sawdust. Eventually the leftover bait was buried. In May 1972, a contractor drilled a well near his office and warehouse on the outskirts of a small town. During the next two and a half months 11 of the 13 individuals employed at the site became ill; two were hospitalized. They were suffering from arsenic poisoning. One sample of water from the well contained 21 mg/1 of arsenic. Analyses of soil from the site revealed arsenic concentrations ranging from 3,000 to 12,000 mg/l. Apparently the well was drilled in the vicinity of the grasshopper bait disposal site, which had long been forgotten by the local residents.
Wastes from munitions works include picric acid, a toxic, intensely bitter, pale yellow substance. Picric acid is not readily removed by traditional water treatment methods and its migration through either the unsaturated zone or the saturated zone does not appear to neutralize it.
During the World War I years of 1914-1918, wastes from the manufacture of explosives at a plant near the Thames River just northeast of London, England, were placed in abandoned chalk pits. Figure 119 illustrates the migration of these wastes. In the early 1920's water from a nearby well was first reported to have a yellow tint.37 Additional water samples collected between 1939 liq and 1955 also contained a characteristic yellow picric acid tint. Sampling ceased in 1955 when the pump was removed. By 1942 the pollutants had migrated at least a mile as indicated by another contaminated well. There is no reason to believe that the picric acid has been flushed from the aquifer. The ground water has certainly been polluted for 40 years, quite probably for more than 70 years, and very likely will be polluted for many more years to come.
Because of high evaporation and low recharge, waste disposal in arid regions can lead to long-lived groundwater quality problems. In the first place, salts are concentrated by evaporation to form highly mineralized fluids. Secondly, water supplies may not be readily available and, therefore, every effort must be made to protect existing sources.
Ground-water contamination in the desert environment near Barstow, California, was described by Hughes.38 Beginning around 1910, waste fuel oil and solvents from a railroad system were discharged to the dry floor of the Mojave River near Barstow. The first municipal sewage treatment plant was constructed in 1938; the effluent was discharged to the riverbed. Sewage treatment facilities were enlarged in 1953 and 1968. Effluent disposal was dependent on evaporation and direct percolation into the alluvial deposits.
At the U.S. Marine Corps base near Barstow, industrial and domestic waste treatment facilities first became operational in 1942; effluent disposal relied on direct percolation and evaporation. Some of the effluent was used to irrigate a golf course. Other sources of groundwater contamination were two nearby mining and milling operations.
As Figure 120 shows, analysis of well waters collected during the Spring of 1972 indicated the existence of two zones of contaminated ground water in the alluvial deposits of the Mojave River. The deeper zone, originating from the 1910 disposal area, exceeded 1,800 feet in width and extended nearly 4'/z miles in a downgradient direction. Its upper surface lies 60 or more feet below land surface. The second or shallow zone originates at the sewage treatment lagoon installed in 1938 and at the Marine Corps golf course. This zone consists of two apparently separate plumes. The upgradient plume extends nearly 2 miles downstream, while the plume originating at the golf course is nearly a mile long; the plumes are about 700 feet wide. Hughes estimated that the pollution fronts are moving at a rate of I to 1.5 feet per day. The Marine Corps well field lies in the path of these plumes; several domestic wells have already been contaminated. In this instance poor waste disposal practices, beginning nearly 75 years ago may cause water-supply problems at the Marine Corps base unless expensive corrective measures are undertaken.
Figure 120. Waste Disposal Beginning Nearly 70 Years Ago at Barstow, California is Now Threatening an Important Well Field at the Nearby Marine Base.
Figure 121. Ground-Water Pollution by Wastes from a Gasworks
Plant Near London Has Even Created a Fire Hazard.
From 1905 to 1967 wastes from a gasworks plant were deposited in abandoned gravel pits along the Lee River near Waltham Cross, a few miles northwest of London, England.39 Figure 121 shows that the tar acids, oils, and sulfate sludge infiltrated to contaminate the ground water over a wide area. Apparently the pollution was first detected in 1935, some 30 years after disposal began. At that time oil, floating on the ground water, emerged at land surface. Continual but slow accumulation of oil on and near the land surface led to hazardous conditions and, in 1943, the oil was ignited. Contaminated ground water was also encountered in new excavations where it appeared as high concentrations of sulfate in 1958 and as oily waters im 1961. In 1965, oily liquids also seeped into Pymmes Brook and the Lee River Navigation Channel following a substantial rise in the water table after heavy rains. Additional surface-water degradation occured in 1966 because of the discharge of oil from streamside seepage zones.
Ground water in the surficial sand and gravel deposit was contaminated over a wide area. Fortunately, most water supplies in this region are pumped from an underlying chalk, which generally is separated from the gravel by the London Clay. It is evident from this example that waste disposal, which began 80 years ago, continues to be troublesome and that ground-water contamination can indeed become a fire hazard.
All ground-water pollution is not necessarily bad. Inhabitants of Crosby, a small village in northwestern North Dakota, believed they produced the best coffee in the State because the water from which it was made contained "body". The rather highly mineralized water (dissolved solids = 2,176 mg/1, sulfate = 846 mg/1, chloride = 164 mg/l, and nitrate = 150 mg/1) used for brewing the coffee was obtained exclusively from an old dug well. The well, however, was constructed, probably near the turn of the century, at the site of the local river livery stable. Livestock wastes provided the peculiar flavor so characteristic of the coffee made in Crosby.
The manufacture of soda ash, caustic soda, chlorine, and allied chemicals began at Barberton, Ohio, shortly before the turn of the century. The plant discharged a mixture of calcium and sodium chlorides directly to the Tuscarawas River and to retention ponds. The discharge of chloride in 1966 averaged 1,500 tons per day.4t These wastes have led to serious ground-water pollution problems in eastern Ohio and have necessitated abandonment of streamside well fields at Barberton in 1926 and at Massillon and Coshocton in 1953.
Municipal wells at Zanesville, more than 135 river miles downstream from Barberton, have also been adversely affected by the chloride induced into the watercourse aquifer from the contaminated Muskingum River. Due to high treatment costs Zanesville officials considered abandoning their welt field in 1963. At the confluence of the Muskingum and Ohio Rivers, about 220 river miles below Barberton, is the city of Marietta. Almost 30 years ago, Marietta officials were concerned over the marked increase in chloride in municipal wells during the preceding 10 years.42 The cause, of course, was induced infiltration of the chloride-rich Muskingum River water.
It is evident that decades of poor waste-disposal practices at Barberton seriously impaired streamside aquifers and well fields for a distance of over 200 river miles. The soda ash plant at Barberton was closed in 1973 and waste discharges substantially reduced. Presumably, these water-quality problems will decrease in severity over the next several years, after a history of 90 years or more.
According to Mink and others43 mining operations in the Coeur d'Alene district of northern Idaho have been continuous for more than 90 years. Unfortunately, leaching of the ancient mining and milling wastes is now affecting the chemical quality of ground water in several areas, including Canyon Creek basin near Wallace. Here high concentrations of zinc, lead, copper, and cadmium occur in both ground water and soil samples.
In 1884 striking miners set fire to several deep coal mines in the vicinity of New Straitsville, Ohio. Still burning uncontrollably, the fires were started by disgruntled workers who rolled burning wood-filled coal cars into the shafts that honeycomb the ground under the town. In the years since, many wells have become contaminated, dried up, or produce water hot enough to make instant coffee.
Disposal of domestic, industrial, and municipal wastes, which probably began around 1872 through wells and sinkholes tapping a permeable limestone aquifer, was the birth of a contaminated area that now encloses some 75 square miles. By 1919 the practice of disposing of sewage at the northern Ohio town of Bellevue was well established and many wells had been contaminated. In the early 1960's some wells were reported to yield easily recognizable raw sewage.This problem began more than a hundred years ago and remains to this day.
A gasworks plant was built at Norwich, England, in 1815 and abandoned in 1830. Phenolic compounds, originating from whale oil, infiltrated and remained in the underlying chalk for at least 135 years when it contaminated a newly drilled well in 1950.These organic compounds, no doubt, are still there 170 or so years later.
Sources of Ground-Water Contamination
As water moves through the hydrologic cycle, its quality changes in response to differences in the physical, chemical, and biological environments through which it passes. The changes may be either natural or man-influenced; in some cases they can be controlled, in other cases they cannot, but in most cases they can be managed in order to limit adverse water-quality changes.
The physical, chemical, and biological quality of water may range within wide limits even though there are no man-made influences. In fact, it is often impossible or at least difficult to distinguish the origin (manmade or natural) of many water-quality problems. The natural quality reflects the types and amounts of soluble and insoluble substances with which the water comes in contact. Surface water generally contains less dissolved solids than ground water, although at certain times (generally during low flow rates) in areas where groundwater runoff is the major source of streamflow, the quality of both surface water and ground water is similar. During periods of surface runoff, streams may contain large quantities of suspended materials and, under some circumstances, a large amount of dissolved solids. Most commonly, however, during high rates of flow the water has a lower dissolved-mineral concentration.
Although the chemical quality of water in surficial or shallow aquifers may range within fairly wide limits from one time to the next, deeper ground water is characterized by nearly constant chemical and physical properties, at least on a local scale where the aquifer is unstressed by pumping. As a general rule, the dissolvedsolids content increases with depth and with the time and distance the water has traveled in the ground. A few uncommon water-quality situations exist throughout the country, reflecting unusual geologic and hydrologic conditions. These include, among others, thermal areas and regions characterized by high concentrations of certain elements, some of which may be health hazards.
For centuries man has been disposing of his waste products by burning, placing them in streams, storing them on the ground, or putting them in the ground using various methods. Man-made influences on streamwater quality reflect not only waste discharge directly into the stream, but also include highly mineralized or polluted surface runoff, which can carry a wide variety of substances. Another major influence on surface-water quality is related to the discharge of ground water into the stream. If the adjacent ground water is polluted, stream quality tends to deteriorate. Fortunately in the latter case, the effect in the stream generally will not be as severe as it is in the ground, due to dilution of the pollutant. See Reference 31 for example.
The quality of ground water is most commonly affected by waste disposal. One major source of pollution is the storage of waste materials in excavations, such as pits or mines. Water-soluble substances that are dumped, spilled, spread, or stored on the land surface or in excavations may eventually infiltrate to pollute ground-water resources. Ground water is also polluted by the disposal of fluids through wells and, in limestone terrains, through sinkholes directly into aquifers. Likewise, inflitration of highly mineralized surface water has been a major cause of underground pollution in several places. Irrigation tends to increase the mineral content of both surface and ground water. The degree of severity of pollution in cases such as these is related to the hydrologic properties of the aquifers, the type and amount of waste, disposal techiniques, and climate.
A major and widespread cause of ground-water quality deterioration is pumping, which may cause the migration of more highly mineralized water from surrounding strata to the well. The migration is directly related to differences in hydrostatic head between adjacent water-bearing zones and to the hydraulic conductivity of the strata. In coastal areas pumping may cause sea water to invade a fresh water aquifer. In parts of coastal west Florida, wild-flowing, abandoned artesian wells have salted, and consequently ruined, large areas of formerly fresh or slightly brackish aquifers.
The list in Table 10 shows that man-influenced groundwater quality problems are most commonly related to: (1) water-soluble products that are placed on the land surface and in streams; (2) substances that are deposited or stored in the ground above the water table; and (3) material that is stored, disposed of, or extracted from below the water table. Many of the pollution problems related to these situations are highly complex, and some are not well understood.
Table 10. Sources of Ground-Water Quality Degradation
Ground-Water Quality Problems that Originate on the Land Surface
1. Infiltration of polluted surface water
2. Land disposal of either solid or liquid wastes
5. Disposal of sewage and water-treatment plant sludge
6. De-icing salt usage and storage
7. Animal feedlots
8. Fertilizers and pesticides
9. Accidental spills
10. Particulate matter from airborne sources
Ground-Water Quality Problems that Originate in the Ground Above the Water Table
1. Septic tanks, cesspools, and privies
2. Holding ponds and lagoons
3. Sanitary landfills
4. Waste disposal in excavations
5. Leakage from underground storage tanks
6. Leakage from underground pipelines
7. Artificial recharge
8. Sumps and dry wells
Ground-Water Quality Problems that Originate in the Ground Below the Water Table
1. Waste disposal in well excavations
2. Drainage wells and canals
3. Well disposal of wastes
4. Underground storage
5. Secondary recovery
7. Exploratory wells
8. Abandoned wells
9. Water-supply wells
10. Ground-water development
Ground-Water Quality Problems that Originate on the Land Surface
Infiltration of Polluted Surface Water. The yield of many wells tapping streamside aquifers is sustained by infiltration of surface water. In fact, Figure 122 shows that more than half of the well yield may be derived directly from induced recharge from a nearby stream, which may be polluted. As the induced water migrates through the ground, a few substances are diluted or removed by filtration and sorption. This is especially true where the water flows through filtering materials, such as sand and gravel, particularly if these materials contain some soil organic matter. Filtration is less likely to occur if the water flows through large openings, such as those that occur in carbonate aquifers. Many pollutants, for example chloride, nitrate, and many organic compounds, are highly mobile, move freely with the water, and are not removed by filtration.
Examples of ground-water supplies being degraded by induced recharge of polluted surface water are both numerous and widespread. In the greatest number of cases, the pollution originated from the disposal of municipal or industrial waste directly into the stream, which was induced by pumping into adjacent aquifers. In hydrologic situations such as these, months or perhaps years may be required for the polluant to advance from the stream into the well. Once at the well, however, the aquifer between the well and the stream may be completely polluted, requiring years to recover once the source has been eliminated.
Figure 122. Induced infiltration of
water may lead to ground-water pollution
Land Disposal of Either Solid or Liquid Waste Materials. One of the major causes of ground-water pollution is the disposal of waste materials directly onto the land surface. Examples include manure, sludges, garbage, and industrial wastes. The waste may occur as individual mounds or it may be spread out over the land. If the waste material contains soluble products, they will infiltrate the land and may lead to ground-water pollution. Similar problems occur in the vicinity of various types of stockpiles.
Stockpiles. Perhaps the prime example of ground-water pollution caused by stockpiles is storage of de-icing salt (sodium and calcium chloride) used for highway snow and ice control. Not uncommonly, tons of salt are simply piled on the land surface
Figure 123. Leaching of Solids at the Land Surface. The
Ground-Water Pollution Under These Conditions is Rarely Anticipated.
awaiting use. As Figure 123 shows, the highly soluble material rapidly dissolves and either infiltrates or runs off into streams. In recent years, many highway officials have provided some protection for salt stockpiles by covering them with plastic sheets or storing the salt in sheds. This is not necessarily done to protect adjacent water resources, but merely to preserve the salt.
Dumps. In the past few years, investigators have begun to take a serious look at the environmental effects of dumps. As rainwater infiltrates through trash in a dump, it accumulates a wide variety of chemical and biological substances. The resulting fluid, or leachate, may be highly mineralized. As the leachate infiltrates, some of the substances it contains are removed or degraded. As Figure 124 shows, eventually the leachate may reach the water table where it flows in the direction of the regional ground-water gradient or toward a well. (In some places "sanitary fills" or dumps lie below the water table.)
Figure 124. Ground-Water Contamination Caused
By Leachate Infiltration from a Dump.
Disposal of Sewage and Water Treatment Plant Sludge. The sludge from treatment plants presents not only a significant waste disposal problem but one that is growing, significantly. The wastes include lime-rich sludge from water treatment plants as well as sewage sludge from waste water treatment plants. In recent years, municipal officials have attempted to solve the sewage-sludge problem by spreading the sludge on the land surface or filling abandoned strip mine pits with it. At first glance this may seem to be an effective means of disposal, but many exotic chemicals, derived from domestic, agricultural, municipal, and industrial wastes may exist in the sludge as soluble or relatively insoluble substances. When the sludges are used as fertilizers, the soluble compounds may infiltrate while the more insoluble compounds, many of which may consist of toxic metals, are removed and concentrated by plants. Much needs to be learned about the chemical and biological migration of numerous elements and compounds present in sludges.
Salt Spreading on Roads. In recent years, particularly since the construction of the interstate highway system, water pollution due to wintertime road salting has become an increasing problem. The effect is becoming even more severe as salt usage increases with a concomitant decrease in the use of sand. From a water quality vii point, the salting brings about deterioration of strea~ quality due to highly mineralized surface runoff, anc the infiltration of briney water causes ground-water pollution.
Accidental Spills of Hazardous Materials. A large volumt toxic materials are transported throughout the count by truck, rail, and aircraft and are stored in aboveground tanks; accidential spills of these hazardous materials are not uncommon. There are virtually no methods that can be used to quickly and adquately clean up an accidental spill or spills caused by explo sions or fires. Furthermore, immediately following a accident, the usual procedure is to spray the spill arf with water. The resulting fluid may either flow into stream or infiltrate the ground. In a few cases, the fluids have been impounded by dikes, which lead to even greater infiltration. In any case, water resource; may be easily and irreparably polluted from accident spills of hazardous materials.
Fertilizers and Pesticides. Increasing amounts of both fertilizers and pesticides are being used in the Unitec States each year. Many of these substances are highl toxic and, in many cases, quite mobile in the subsur face. Many compounds, however, become quickly a tached to fine-grained sediment, such as organic mat and clay and silt particles; a part of this attached mz rial is removed by erosion and surface runoff. In ma heavily fertilized areas, the infiltration of nitrate, a decomposition product of ammonia fertilizer, has gr ly polluted ground water. The consumption of nitrat rich water leads to a serious disease in infants commonly known as "blue babies" (methemoglobinemi,
In many irrigated regions, automatic fertilizer feed attached to irrigation sprinkler systems are becoming increasingly popular. When the irrigation-well pump shut off, water flows back through the pipe system i the well. This creates a partial vacuum in the lines th may cause fertilizer to flow from the feeder into the It is possible that some individuals are dumping fertil izers (and perhaps even pesticides) directly into the w to be picked up by the pump and distributed to the sprinkler system. In this case, it directly contributes to ground-water pollution.
Animal Feedlots. Animal feedlots cover relatively small areas but provide a huge volume of animal wastes. These wastes have polluted both surface and ground water with large concentrations of nitrate. Even small feedlots and liveries have created local but significant problems. In a few areas the liquid runoff from feed is collected in lined basins and pumped onto adjacen grounds as irrigation waters, providing a luxuriant growth.
Particulate Matter from Airborne Sources. A relatively mi source of ground-water pollution is caused by the fallout of particulate matter originating from smoke, flue dust, or aerosols. Some of the particulate matte water-soluble and toxic. An example of this type of pollution is airborne chromium-rich dust that discharged through the roof ventilators of a factory in Michigan and accumulated on the downwind side of the plant. As Figure 125 shows, the highly soluble chromium compounds rapidly infiltrated and polluted a local municipal drinking water supply. Along the Ohio River at Ormet, Ohio, the airborne discharge of fluoride from an aluminum processing plant has seriously affected dairy operations and fluoride concentrations in ground water at the plant exceeded 1,000 mg/1 in the mid 1970's.
Ground-Water Quality Problems that Originate in the Ground Above the Water Table
Many different types of materials are stored, extracted, or disposed of in the ground above the water table. Table 10 shows that water pollution can originate from many of these operations.
Septic Tanks, Cesspools, and Privies. Probably the major cause of ground water pollution in the United States is effluent from septic tanks, cesspools, and privies, although each site is small, as shown in Figure 126. Individually of little significance, these devices are important in the aggregate because they are so abundant and occur in every area not served by municipal or privately owned sewage treatment systems. The area that each point source affects is generally small, since the quantity of effluent is small, but in some limestone areas effluent may travel long distances in subterranean cavern systems.
Holding Ponds and Lagoons. The second major source of ground-water pollution is holding ponds and lagoons. As Figure 127 shows, these ponds and lagoons commonly consist of relatively shallow excavations that range in surface area from a few square feet to many acres. In some places they are euphemistically called "evaporation" ponds. Such ponds were commonly used to hold oil-field brines, and when the pond floors became sealed, the operators would disc them to increase infiltration. Holding ponds are also used to store municipal sewage as well as large quantities of wastes including a host of industrial chemicals. The latter are generally characterized by highly concentrated solutions that may contain toxic compounds.
Special problems develop with holding ponds and lagoons in limestone terrain where extensive near-surface solution openings have developed. In Florida, Alabama, Missouri, and elsewhere, municipal sewage lagoons have collapsed into sinkholes draining raw effluent into wide-spread underground openings. In some cases the sewage has reappeared in springs and streams several miles away. Wells producing from the caverns could easily become polluted and lead to epidemics of water-bornediseases.
Holding ponds are commonly considered to be liquidtight but the vast majority leak. Although rarely reported, ground-water pollution caused by leaking holding ponds at a large number of industrial sites has been so extensive that all of the water supplies on the plant property are unusuable for many purposes without treatment. As a result, expensive treatment plants have been required. Moreover, the ground water may be so cannot be pumped into adjacent streams.
Figure 125. Air Pollution Can Lead to ground-water Pollution
Figure 126 Percolation throug zone of aeration. Most of the
removal or degradatin processes funcition under these conditions
Figure 127. Schematic diagram showing percolation fon
through the zone of aeration and in an isotropic aquifer
Oil-field brines, a highly mineralized salt solution, are particularly noxious and without doubt they have locally polluted both surface and ground water in every state that produces oil. The brine, an unwanted byproduct, is produced with the oil. In many states it is disposed of by placing it in holding ponds from which it infiltrates into the ground. Commonly the oil well has been long abandoned before it becomes apparent that the adjacent ground water is polluted. This, in turn, may leave no possibility for recovery of damages by the landowner.
Sanitary Landfills. Sanitary landfills generally are constructed by placing wastes in excavations and covering the material daily with soil-thus the term "sanitary" to indicate that garbage and other materials are not left exposed to produce odors or smoke or attract vermin and insects. Even though a landfill is covered, however, leachate may be generated by the infiltration of precipitation and surface runoff. Fortunately many substances are removed from the leachate as it filters through the unsaturated zone, but leachate may pollute ground water and even streams if it discharges at the surface as springs and seeps.
At one site, rejected transformers and capacitors containing polychlorinated biphenyls from an industrial plant were disposed of in a municipal landfill. A number of stillbirths and birth defects soon occurred in cattle that drank water from a nearby stream. Analyses of the water showed large concentrations of PCB, the origin of which was, without question, the landfill.
Waste Disposal in Excavations. Following the removal of clay, limestone, sand, and gravel, or other material, the remaining excavations are traditionally left unattended and often are used as unregulated dumps. The quantity and variety of materials placed in dumps and excavations are almost limitless. Excavations also have been used for the disposal of liquid wastes, such as oil-field brines and spent acids from steel mill operations. Many other excavations serve as disposal sites for snow removed from surrounding streets and roads-snow that commonly contains a large amount of salt. Disposal of these and other wastes in excavations may lead to ground-water pollution.
Leakage from Underground Storage Tanks. A growing problem of substantial potential consequence is leakage from storage tanks and from pipelines leading to such tanks. Gasoline leakage has caused severe hazardous pollution problems throughout the nation. Gasoline floats on the ground-water surface and leaks into basements, sewers, wells, and springs, causing noxious odors, explosions, and fires. A single-wall steel tank has a life expectancy of 18 years and costs about $1 per gallon to replace. A cleanup operation will generally exceed $70,000.
Leakage from Underground Pipelines. Literally thousands of miles of buried pipelines crisscross the United States. Leaks, of course, do occur, but it may be exceedingly difficult to detect and locate them. Leaks are most likely to develop in lines carrying corrosive fluids. An example occurred in central Ohio where a buried pipeline carried oil-field brine from a producing well to a disposal well. The corrosive brine soon weakened the metal pipe, which then began to leak over a length of several tens of yards. The brine infiltrated, polluting the adjacent ground water, then flowed down the hydraulic gradient and discharged into a stream. During the ensuring months, nearly all of the vegetation between the leaking pipeline and the stream was killed. The leaking area of the pipe was detected only because of the dead vegetation and salty springs.
A vexing problem of chromium compounds that polluted several shallow wells in Michigan was traced to a leaky sewer transporting metal finishing wastes. Radioactive materials have also leaked from pipelines. The several leaks reported at the Hanford A.E.C. Works came about as a result of loaded, underground tanks settling differentially into the subjacent earth materials, causing the pipelines carrying radioactive waste to break at joints.
Artificial Recharge. Artificial recharge includes a variety of techniques used to increase the amount of water infiltrating an aquifer. It consists of spreading the water . over the land or placing it in pits or ponds, from which the water will seep into the ground, or pumping water through wells directly into the aquifer. As water demands continue to increase, there is no doubt that artificial recharge will become more popular as a ground-water management tool.
Waters used for artificial recharge consist of storm runoff, excess irrigation water, stream flow, cooling water, and treated sewage effluent, among others. Obviously the quality of water artificially recharged can have a deleterious effect on the water in the ground under certain conditions.
Sumps and Dry Wells. Sumps and dry wells may locally cause some ground-water pollution and, in places where these structures are adjacent to a stream, bay, lake, or estuary, may pollute such surface-water bodies and perhaps lead to a proliferation of algae and other water weeds. These structures are commonly used to collect runoff or spilled liquids, which will infiltrate through the sump. Sumps and dry wells are typically installed to solve surface drainage problems, so they may transmit whatever pollutants are flushed into the well to ground water.
Graveyards. Leachate from graveyards may cause ground-water pollution, although cases are not well documented. In some of the lightly populated glaciated regions in the north central part of the United States, graveyards are commonly found on deposits of sand and gravel, because these materials are easier to excavate than the adjacent glacial till and, moreover, are better drained so that burials are not below the water table. Unfortunately, these same sand and gravel deposits may also serve as a major source of water supply. Graveyards are also possible sources of pollution in many hard rock terrains where there are sinkholes or a thin soil cover.
Ground-Water Quality Problems that Originate in the Ground Below the Water Table
Table 10 lists a number of major causes of groundwater pollution produced by the use and misuse of space in the ground below the water table.
Waste Disposal in Wet Excavations. Following the cessation of various mining activities, the excavations are commonly abandoned; eventually they may fill with water. These wet excavations have been used as dumps for both solid and liquid wastes. The wastes, being directly connected to an aquifer, may cause extensive pollution. Furthermore, highly concentrated leachates may be generated from the wastes due to seasonal fluctuations of the water table. In the late 1960's at a lead-zinc mine in northwestern Illinois, processing wastes were discharged into an abandoned mine working. The wastes, moving slowly in the ground water, polluted several farm wells. Analyses of water from several of the polluted wells showed high concentrations of dissolved solids, iron, sulfate, and, more importantly, heavy metals and cyanide.
Drainage Wells and Canals. Where surficial materials consist of heavy clay, flat-lying land may be poorly drained and contain an abundance of marshes and ponds. Drainage of this type of land is generally accomplished with field tiles and drainage wells. As Figure 128 shows, a drainage well is merely a vertical, cased hole in the ground or in the bottom of a pond that allows the water to drain into deeper, more permeable materials. The pond water may be polluted which, in turn, leads to deterioration of water quality in the receiving aquifer.
Deepening of stream channels may lower the water table. Where the fresh-saltwater interface lies at shallow depths, lowering of the water table (whether by channelization, pumping, or other causes) may induce upward migration of the saline water; it may even flow into the deepened channel and pollute the surface water, as Figure 129 shows. Under these circumstances, reduction of the depth to fresh water can result in a rise in the level of saline water several times greater than the distance the fresh water level is lowered.
In some coastal areas, particularly in Florida, the construction of extensive channel networks has permitted tidal waters to flow considerable distances inland. The salty tidal waters infiltrate, increasing the salt content of the ground water in the vicinity of the canal.
Well Disposal of Wastes. For decades, man has disposed of liquid wastes by pumping them into wells. Since World War II, a considerable number of deep well injection projects have come into existence, usually at industrial sites. Industrial disposal wells range in depth from a few tens of feet to several thousand feet. The injection of highly toxic wastes into come of these wells has led to several water-pollution problems. The problems are caused by the pollution of fresh water due to direct injection into the aquifer as well as leakage of pollutants from the well head, through the casing, or via fractures in confining beds. Injection of liquid wastes near Denver by means of deep well disposal apparently caused an increase in the frequency of local earthquakes. Deep well injection in the vicinty of Sarnia, Ontario, caused several long-abandoned brine wells in Michigan to flow because of the greatly increased aquifer pressure.
Exclusive of oil-field brine, most deep well injection operations are tied to the chemical industry. Well depths range from 1,000 to 9,000 feet and average 4,000 feet. The deepest wells are found in Texas and Mississippi.
Figure 128. Diagram showing drainage of a pond into a aquifer through a drain well
Figure 129. Diagram showing
migration of saline water caused by lowering of water
caused by lowering of water levels in an effluent stream and streamside aquifer
hydraulically connected to an underlying saline water aquifer
As of October 1983, EPA reported the existence of at least 188 active hazardous waste injection wells in the United States. There was an additional 24,000 wells used to inject oil-field brine.
Properly managed and designed underground injection systems can be effectively used for storage of wastes deep underground and may permit recovery of the wastes in the future. Before deep well disposal of wastes is permitted by EPA, however, there must be an extensive evaluation of the well system design and installation, the waste fluids, and the rocks in the vicinity of the disposal well.
Underground Storage. The storage of material underground is attractive from both economic and technical viewpoints. Natural gas is one of the most common substances stored in underground reservoirs. However, the hydrology and geology of underground storage areas must be well understood in order to insure that the materials do not leak from the reservoir and degrade adjacent water supplies.
Secondary Recovery. With increased demands for energy resources, secondary recovery, particularly of petroleum products, is becoming even more important. Methods of secondary recovery of petroleum products commonly consist of injection of steam or water into the producing zone, which either lowers the viscosity of the hydrocarbon or flushes it from the rocks, enabling increased production. Unless the injection well is carefully monitored and constructed, fluids can migrate from a leaky casing or through fractures in confining units.
Mines. Mining has caused a variety of water pollution problems. These problems are caused by pumping of mine waters to the surface, by leaching of the spoil material, by waters naturally discharging through the mine, and by milling wastes, among others. Literally thousands of miles of stream and hundreds of acres of aquifers have been polluted by highly corrosive mineralized waters originating in coal mines and dumps in Appalachia. In many western states, mill wastes and leachates from metal sulfide operations have seriously affected both surface water and ground water.
Figure 130. Diagram showing
migration of saline water caused
by dewatering in a fresh-water aquifer overlying a saline-water aquifer
Figure 131. Upward leakage and flow
through open holes. Some important
aquifers have been ruined by improper drilling practices
Many mines are deeper than the water table, and in order to keep them dry, large quantities of water are pumped to waste. If salt water lies at relatively shallow depths, Figure 130 shows that the pumping of freshwater for dewatering purposes may cause an upward migration of the salt water, which may be intercepted by the well. The mineralized water most commonly is discharged into a surface stream.
Exploratory Wells and Test Holes. Literally hundreds of thousands of abandoned exploratory wells dot the countryside. Many of these holes were drilled to determine the presence of underground mineral resources (seismic shot holes, coal, salt, oil, gas, etc.). The open holes permit water to migrate freely from one aquifer to another. As Figures 131 and 132 show, a freshwater aquifer could thus be joined with a polluted aquifer or a deeper saline aquifer, or polluted surface water could drain into freshwater zones.
Figure 132. Downward leakage.
Contamination of One Aquifer
can affect others in a Multi-Aquifer system
Abandoned Wells. Another major cause of ground-water pollution is the migration of mineralized fluids through abandoned wells. In many cases when a well is abandoned the casing is pulled (if there is one) or the casing may become so corroded that holes develop. This permits ready access for fluids under higher pressure to migrate either upward or downward through the abandoned well and pollute adjacent aquifers. In other cases, improperly cased wells allow high-pressure artesian saline water to spread from an uncased or partly cased hole into shallower, lower-pressure aquifers or aquifer zones, resulting in widespread salt intrusion.
Figure 133. Diagram showing flood
a well through an improperly sealed gravel pack
Water-Supply Wells. Improperly constructed water-supply wells may either pollute an aquifer or produce polluted water. Dug wells, generally of large diameter and shallow depth, and poorly protected, are commonly polluted by surface runoff flowing into the well. As Figure 133 illustrates, other pollution has been caused by infiltration of water through polluted fill around a well or through the gravel pack. Still other pollution has been caused by barnyard, feedlot, septic tank, or cesspool effluent draining directly into the well. Many pollution and health problems can arise because of poor well construction.
Ground-Water Development. In certain situations pumping of ground water can induce significant water-quality problems. The principal causes include interaquifer leakage, induced infiltration, and landward migration of sea water in coastal areas. In these situations the lowering of the hydrostatic head in a freshwater zone leads to migration of more highly mineralized water toward the well site. Undeveloped coastal aquifers are commonly full, the hydraulic gradient slopes towards the sea, and freshwater discharges from them through springs and seeps into the ocean, as shown in Figure 134. Extensive pumping lowers the fresh-water potentiometric surface permitting sea water to migrate toward the pumping center. Figure 135 shows a similar predicament which occurs in inland areas where saline water is induced to flow upward, downward, or laterally into a fresh water aquifer due to the decreased head in the vicinity of a pumping well. Wells drilled adjacent to streams induce water to flow from the streams to the wells. If the stream is polluted, induced infiltration will lead to deterioration of the water quality in the aquifer.
Figure 134. Sea-Water intrusion is
overpumping of coastal aquifers
Figure 135. Diagram showing how a Pumping well can
induce highly mineralized water to flow from a saline
aquifer into a fresh-water aquifer
Natural Controls on Ground-Water Contamination
As Deutsch47 clearly pointed out, there are four major natural controls involved in shallow ground-water contamination. The first control includes the physical and chemical characteristics of the earth materials through which the liquid wastes flow. A major attenuating effect for many compounds is the unsaturated zone, which has been caved the "living filter". Many chemical and biological reactions in the unsaturated zone lead to contaminant degradation, precipitation, sorption, and oxidation. The greater the thickness of the unsaturated zone, the more attenuation is likely to take place. Below the water table, the mineral content of the medium probably becomes more important because various clays, hydroxides, and organic matter take up some of the contaminants by exchange or sorption. Many of the other minerals may have no effect on the contaminants with which they come into contact.
The second major control includes the natural processes that tend to remove or degrade a contaminant as it flows through the subsurface from areas or points of recharge to zones or points of discharge. These processes include filtration, sorption, ion-exchange, dispersion, oxidation, and microbial degradation, as well as dilution.
The third natural control relates to the hydraulics of the flow system through which the waste migrates, beginning with infiltration and ending with discharge. The contaminant may enter an aquifer directly, by flowing through the unsaturated zone, by interaquifer leakage, by migration in the zone of saturation, or by flow through open holes.
The final control is the nature of the contaminant. This includes its physical, chemical, and biological characteristics and, particularly, its stability under varying conditions. The stability of the more common constituents and the heavy metals are fairly well known although more complex than commonly realized. On the other hand, the stability of organic compounds, particularly synthetic organic compounds, has only recently come under close inspection and actually little is known of their degradation and mobility in the subsurface. This fact has been brought clearly to the attention of the general public by the abundance of reported incidences of ground-water contamination by EDB, TCE, and DBCP.
To a large extent, it is the aquifer framework that controls the movement of ground water and contaminants. Of prime importance, of course, is the hydraulic conductivity, both primary and secondary. In the case of consolidated sedimentary rocks, primary permeability, or that which came into being with the formation of the rock, in many respects is more predictable than secondary permeability, which came later and includes fractures and solution openings, among others. In sedimentary rocks similar units of permeability tend to follow bedding planes or formational boundaries, even if the strata are inclined. Permeable zones are most often separated by layers of fine-grained material, such as clay, shale, or silt, which serve as confining beds. Although leakage through confining beds is the rule rather than the exception, both water and contaminants are more likely to remain in a permeable zone than to migrate through a thickness of several units of different permeability. The movement of ground water and contaminants through larger openings, such as fractures, complicates the assumed picture. Figure 136 illustrates this movement. Not only can the velocity change dramatically, but in fracture flow, much of the attenuation capacity is lost, and it is difficult to predict local directions of flow.
The geologic framework, in conjunction with surface topography, also exerts a major control on the configuration of the water table and the thickness of the unsaturated zone. Generally speaking, a deposit of permeable surficial sand and gravel would be characterized by a water table that is relatively flat. In contrast, a covering of glacial till, which is typically fine-grained, would be characterized by a water-table surface that more closely conforms to the elevation of the land surface. The position of the water table is important not only because it is the boundary between the saturated and unsaturated zones, but also because it marks the bottom and, therefore, the thickness of the unsaturated material.
In many, if not most, contaminated areas, the water table has been or is intermittently
affected by pumping. The resulting cone of depression on the water table changes both the
hydraulic gradient and ground-water velocity resulting in flow to a discharging well. A
change in gradient and velocity also occurs in the vicinity of recharge basins (lagoons,
pits, shafts. etc.), because the infiltrating water forms a mound in the water table. As
Figure 137 shows, the mound causes radial flow and, therefore, contaminants will move in
directions that are differente than the regional hydraulic gradient, at least until the
mounding effects are over come and the regional gradient is reestablished.
Ground-water or interstitial velocity is controlled by the hydraulic conductivity, gradient, and effective porosity. Water movement throunh a permeable gravel with a gradient of 10 feet per mile averages about 60 feet per day, buy in a clay with the same gradient and no secondary permeability the water movement would be only about 1 foot in 30,000 years, In most aquifers, ground-water velocity ranges from a few feet per day to a few feet per year.
Figure 136. Movement of ground-water
through large openings
Carlston and others determined that the mean residence time of ground water in a basin in Wisconsin was about 45 days and in New Yersey about 30 days. This study shows that ground water may discharge into closely spaced streams in humid areas within a few days to a few months. On the other hand, in less permeable terrains ground water and contaminants may remain in the ground for years or even decades.
The causes of ground-water pollution are many, but it is the source that needs special consideration. For example, an accidental spill from a ruptured tank may provide a considerable volume of liquid with an extremely high concentration that is present only during a very short time span, but leachate generated from a landfill may consist of a large volume with a low concentration that spans a period of many years. Once it reaches the water table, the accidental spill might move as a conservative contaminant because of its high concentration despite the fact that it might be degradable in smaller concentrations. The leachate is more likely to be attenuated by microbial degradation, sorption, dilution, and dispersion.
In the case of landfills, leachate is a liquid that has formed as infiltrating water migrates through the waste material extracting water-soluble compounds and particulate matter. The mass of leachate is directly related to precipitation, assuming the waste lies above the water table. Much of the annual precipitation, including snowmelt, is removed by surface runoff and evapotranspiration; it is only the remainder that is available to form leachate. Since the landfill cover, to a large extent, controls leachate generation, it is exceedingly important that the cover be properly designed, maintained, and monitored in order to minimize leachate production.
Schuller and otherss0 described the effect of regrading, installation of a PVC topseal and revegetation of a landfill in Windham, Connecticut. As Figures 138 and 139 illustrate, field data clearly indicate chat the cover reduced infiltration and leachate generation, which caused a reduction in the size and concentration of the leachate plume.
According to an EPA estimatest, a disposal site consisting of 17 acres with 10 inches/year of infiltration could produce 4.6 million gallons of leachate each year for 50 to 100 years. This estimate, of course, is site dependent.
Feen and others52 described a landfill in Cincinnati, Ohio, and estimated leachate generation using a water balance technique. The data in Table 11 show that percolation through the landfill was calculated to occur only during January through April and in December. In this 50 acre landfill, the leachate generation averaged about 11,394,000 gallons/year, 949,475 gallons/month, or 31,216 gallons/day. Considering only the calculated values for the months when precolation was assumed to occur, leachate generation averaged about 113,900 gallons/day in January, 120,740 in February, 96,400 in March, 31,700 in April, and 17,500 gallons/day in December, in contrast to the annual daily average of 31,216 gallons.
The Cincinnati calculations appear unusual because the spring runotf in Ohio normally occurs during March and April and, since there is a soil-moisture excess at that time, most ground-water recharge takes place during this interval. Moreover, ground-water recharge may occur any time there is rain. Therefore, one must use caution when applying water balance techniques to estimate leachate generation. The method may provide a good estimate or long-term average, but it likely produces an estimate of the total volume of recharge that is too low.
Figure 138. Distribution of specific conductance, May 19, 1981
Figure 139. Distribution of specific conductance, November 12, 1981
The physical, chemical, and biological characteristics of leachate are influenced by: (1) the composition of the waste, (2) the stage of decomposition, (3) the microbial activity, (4) the chemical and physical characteristics of the soil cover and of the landfill, and (5) the time rate of release (recharge). Since all of the above can range within extremely wide limits, it is possible to provide only a general range in concentration of leachate constituents, as Table 12 shows.
It is also important to account for the fact that materials placed in landfills may range widely depending on the season. For example, many municipal landfills are used to dispose of snow and ice, which may contain large concentrations of calcium, sodium, and chloride from de-icing salts. This could lead to the generation of leachate that varies seasonally, particularly in regard to the chloride concentration. It is also important to remember that leachate collected from a seep at the base of a landfill should be more highly mineralized than the leachate present in the underlying ground water, which is diluted.
Table 11. Water balance data for Cincinnati, Ohio.
|Constituents||Operating landfill||Abandoned landfill|
|Hardness, mg/l as CaCO3||900||290|
|Total Iron, mg/l||40.4||2.2|
|Specific conductance, mmhos||3,000||2,500|
Table 12. Comparison of chemical characteristics of Leachate
operating landfill and a 20-year-old abandoned landfill in Southeastern Pennsylvania
Cyclic Changes in Ground-Water Quality
It is commonly assumed and often reported that natural ground-water quality is nearly constant at any particular site. Field data substantiate this assumption, and logic leads to the same conclusion if the aquifer is not submitted to a new stress. On the other hand, multiple samples from a single well are very likely to show slight changes in concentrations of specific constituents, due to differences in sample collection, storage, and analytical technique.
Conversely, investigators are finding with increasing frequency that ground-water quality, at least in shallow or surficial aquifers, can change significantly and rather rapidly, perhaps as much as an order of magnitude within a few hours or days, even though there is no source of man-made contamination.
Deeper or confined aquifers generally are characterized by a nearly constant chemical quality that, at any particular site, reflects the geochemical reactions that occurred as the water migrated through confining layers and aquifers from its recharge area to the point of collection or discharge.
The quality of deeper water can change, but generally not abruptly, in response to stresses on the aquifer system. Changes in hydrostatic head brought about by pumping, for example, may cause leakage of more highly mineralized water from adjacent units into the producing zone. This leakage may be due to fluid migration along the well casing or gravel pack or by leakage through confining beds or abandoned wells or exploration holes. Another cause of chemical change is waste disposal, particularly through well injection.
Surficial or shallow aquifers, however, are not well protected from chemical changes brought about by natural events occurring at the land surface or from man-induced pollution. Surficial aquifers are, in fact, the most susceptible to rapid and sometimes dramatic changes in quality. Some of the changes are related to man's activities; others are not.
The Concept of Cyclic Fluctuations
Several years ago Pettyjohn described cyclic fluctuations of ground-water quality. The mechanisms that lead to cyclic fluctuations will be discussed in greater detail here because both the cause and effect can have a significant impact on: (1) ground-water quality monitoring and determination of background quality, (2) transport and fate of organic and inorganic compounds, as well as bacteria and viruses, and (3) monitoring well design and installation.
Figure 140. Water-level surface and monitoring wells at
the contamined site.
The site that Pettyjohn used to develop the concept of cyclic fluctuation was a particularly well-instrumented area in central Ohio. The contamination area, shown in Figure 140, lies on the floodplain of the Olentangy River and is bounded on the east and south by small intermittent streams and on the west by the river. Underlain by shale, the alluvial deposits consist of sand, gravel, silt, and clay, that range from 15 to 35 feet in thickness. The water table lies from 1.5 to 5 feet below land surface and fluctuates a maximum of afoot or so throughout the year. Precipitation averages about 38 inches per year.
Oil production began at this site in mid-1964, but by July 1965, all wells had been plugged. Ground-water contamination occurred because of leakage of oil-field brine from three holding ponds. One pond (Skiles) was used from July 1, 1964 to June 30, 1965; about 126 barrels of brine were placed in it. Two other ponds (Slatzer), received 110,000 barrels of brine containing about 35,000 mg/1 of chloride over approximately the same time period.
When samples were first collected from 23 monitoring wells in July 1965, the aquifer locally contained more than 35,000 mg/1 of chloride. At this time the ponds were abandoned and the two Slatzer pits were filled with previously excavated materials that had formed surrounding embankments. The fact that the ground water contained higher concentrations of chloride than the original brine is most likely due to increased concentrations in the holding ponds brought about by evaporation.
As Figure 141 shows, an interesting relationship becomes apparent when examining the areal extent of ground-water contamination with time. Note that the area enclosed by the 1,000 mg/1 isochlor during 1965-66 changed monthly but the changes did not necessarily encompass smaller areas. The 1969 data show a similar phenomenon. This suggests that a linear flushing rate did not exist.
Of particular importance in the monitoring of this site are three adjacent wells, one screened at a depth of 9 feet and another at 23 feet, while a third is gravelpacked through much of its length (23 feet) and receives water from the entire aquifer. Figure 142 illustrates these three wells. The locations of these wells are shown in Figure 140 at the position marked A. It is assumed that the first two wells represent the quality that exists at depths of about 7 to 9 feet and 21 to 23 feet, respectively, and that the gravel-packed well provides a composite sample of the reservoir. It is also assumed that when the composite well had a higher concentration than both the deep and shallow wells, the most highly mineralized water was between 9 and 23 feet.
Figure 141. Areal extent of the contaminated area enclosed by
the 1,000 mg/l (1965, 1966) and the 500 mg/l isochlors during
selected months. Contours based on data from monitoring wells
Figure 143 shows the chloride fluctuations in the three wells during 1965, 1966, and 1969. Notice that at certain times the highest concentrations occur at the shallowest depths, at other times at the greatest depth, and at still other times the greatest concentration must lie somewhere in the middle of the aquifer. The only means for accounting for the variable distribution is intermittent reintroduction of the contaminant, which is puzzling in view of the fact that oil-field activities ceased in June 1965 before any of these samples were collected.
Figures 144 and 145 show another technique for illustrating the temporal-vertical distribution of chloride in the aquifer. These illustrations are based on monthly data obtained from the three adjacent wells. Concentrations at depths of 9 and 23 feet were measured; interpretation of the chloride distribution between these points was based on data from the fully penetrating well. In October and November 1965, the highest chloride concentrations were present at the shallowest depth but from December 1965 to April 1966 the highest concentrations were near the bottom of the aquifer. Furthermore, in November and December, the water in the middle of the aquifer was less mineralized than that above or below.
Figure 142. Completion details of three closely spaced monitoring wells
Figure 143. Fluctuation of chloride content in closely
spaced wells of different depths during 1965, and 1969
Although greatly reduced, in January 1969 the largest concentration of chloride was again at the shallowest depth, but the situation was reversed during April and May. During February and March the central part of the aquifer was less mineralized than adjacent parts. By August there was only a slight chloride increase with depth, but in September and October the greatest concentrations again appeared in the central part of the aquifer.
The chloride fluctuations that occurred during 1965-66 and 1969 are shown schematically in Figure 146. The October 1965 samples apparently were collected shortly after a recharge event, which leached salt from the ground and formed a highly concentrated mass. This slowly sinking mass (1) was subsequently replaced with less mineralized water. A month later, the first mass had reached and was migrating along the bottom of the aquifer when another recharge event occurred (2). By December, the second'mass had reached the bottom of the aquifer and was moving toward the river. Recharge events also occurred in January 1966 (3) and in February 1966 (4). The February event represented the spring runoff when evapotranspiration was minimal and the soil-moisture content exceeded field capacity over a wide area. This major period of recharge caused a large influx of salt and by March, the aquifer was contaminated throughout a wide area. This mass eventually discharged into the river.
In spite of the fact that brine disposal ceased by mid-1965, Figure 146 shows that the aquifer was recontaminated several times during 1969. Following an established pattern, small recharge events took place in January, February, and March 1969.
This study indicates that water soluble substances on the land surface or in the unsaturated zone may be intermittently introduced into a shallow aquifer for many years. The introduction of these contaminants is dependent upon the chemistry of the waste and the soil and upon the frequency of the recharge events. These recharge events are controlled by evapotranspiration, by the rate, duration, and intensity of precipitation, and by soil-moisture conditions.
Figure 144. Vertical distribution of chloride at the
contaminant site from october 1965 to april 1966
Figure 145. Vertical distribution of chloride at the
contaminant site during 1969
Figure 146. Schematic Diagram showing the cyclic movement of
masses of contaminated water through the aquifer during selected
months in 1965, 1966, and 1969. Stippled Areas for 1965-66 represent concentrations
in excess of 20,000 mg/l and for 1969 represent concentrations greater than 500 mg/l.
Throughout most of the year in humid and semiarid regions, the quantity of water that infiltrates and the amount of contaminants that are washed into an aquifer are relatively small. On the other hand, during the spring recharge period significant quantities of contaminants may be flushed into the ground over wide areas. Therefore, the major influx of contaminants occurs on an annual basis, although minor recharge events may occur at any time.
In order to account for cyclic fluctuations in groundwater quality it is assumed that: (1) the unsaturated zone may store a considerable volume of water-soluble substances for long periods of time, and (2) the main paths along which contaminants move through the unsaturated zone to the water table consist largely of fractures and macropores.
Large concentrations of many water-soluble substances are stored within the unsaturated zone. Nitrate storage has been extensively studied. For example, in parts of western Kansas, the top 3 to 4 feet of unplowed prairie soils have as much as 10,000 to 20,000 pounds of nitrogen per acre.55 A particularly interesting case in Texas was described by Kreitler.5ó In addition to natural substances, such as nitrate, many man-related contaminants are stored for years in the unsaturated zone and their presence may lead to continual recontamination of aquifers, as was the case adjacent to the Olentangy River in Ohio.
The long-term effects of oil-field brine holding ponds in central Ohio were investigated for several years by geology students at The Ohio State University. One pond adjacent to a producing well was excavated in 1968. Two years later, when the well was plugged, the holding pond was filled, graded, and seeded. Figure 147 shows that the chloride concentration in the ground a few inches below the water table in the vicinity of the former pond was 36,000 mg/1 some 10 years after operation began and 8 years after pond reclamation. This particular area is characterized by a thick and very dense glacial till of low primary permeability.
Figure 147. Chloride concentrations
in Shallow ground
water 8 years after reclamation of an Oil-field brine holding pond
A few miles to the north, three ponds were constructed. Apparently they had not been used for many months when the area was investigated since the chloride concentration in them ranged from 100 to 3,200 mg/I. In the vicinity of these ponds, 18 holes were hand augered to the water table where chloride concentrations ranged from 100 to 16,900 mg/1, as shown in Figure 148. Most of the wells contained less than 2,000 mg/1 chloride. In contrast to the area which contained the one pond mentioned above, this three-pond site consists of glacial till which contains several thin layers of sand and gravel. The higher permeability presumably accounts for the overall lower chloride concentration.
The preceding examples illustrate that substantial volumes of both naturally occurring and man-related chemical substances are stored in the unsaturated zone. The quantity of water-soluble substances in storage probably increases with decreasing grain size.
Figure 148. Chloride concentration in shallow ground water
in the vicinity of three inactive Oil-field brine holding ponds
It is commonly assumed that ground-water recharge cannot occur until there is a soil-moisture excess, that is until field capacity is exceeded. Increasing evidence clearly shows, however, that the concept of a distinct wetting front or pistonlike displacement flow through the main matrix of the soil is unlikely in a great many cases. In fact, in many situations, much of the flow occurs rapidly through macropores and fractures. In a study of the Missouri Ozarks, Aley showed that water entering macropores contributed five times as much recharge as did diffuse flow. Aubertin found that in sloping forested lands, macropores conducted large quantities of water to depths of 30 feet or more. In the majority of cases, at least during parts of each year, ground-water recharge probably is a function both of flow through macropores and fractures as well as displacement flow.
Some macropores and fractures can be exceedingly permeable. While drilling a test hole in north-central North Dakota with a rotary drilling rig, circulation was completely lost at a depth of about 20 feet. Even after mixing two bags of bentonite and a bag of bran in the drilling fluid, it was still not possible to regain circulation in a fine sand. At this time a curious circumstance was noticed-about 30 feet from the rig a fountain of muddy water was flowing from the trunk of an oak tree that had been cut down years before. Apparently, the drill bit had cut through the remains of a large decayed root of the dead tree.
In another case in the same general area, circulation was lost at a depth of about 60 feet while drilling through a thick section of glacial till. After mixing three bags of bentonite with the drilling fluid, circulation was finally regained but not before a spring had formed about 20 feet from the rig. In this case, the drill bit encountered a fracture at some unknown depth that was more permeable than the cutting- and mud-filled annulus of the hole.
The summer of 1980 in north-eastern Oklahoma was exceedingly hot and dry. Large, extensive desiccation crack systems were common throughout the finer textured soil areas within the state, including the author's backyard. Figure 149 shows one of several such fracture systems which was measured on August 7, 1980. This fracture system was about 72 feet long, was as much as 2 inches wide, and ranged from 12 to 14 inches deep. A metal probe could be pushed into the crack to a depth of 34 inches.
Two garden hoses, which provided a combined flow of 6 gpm, were placed in two separate locations in the fracture system. Water flowed into the fracture for 132 minutes (792 gallons). At no time did water overflow the fracture. When the water supply was shut off, the water level dropped 2 inches in 15 seconds. This primitive test clearly illustrates that fracture systems can collect, store and transmit large volumes of water. Furthermore, where extensive desiccation cracks exist, it would be unlikely that much, if any, overland flow could occur except, perhaps, in response to a heavy precipitation event from a convective storm.
Neither macropores nor fractures are commonly as large as those described above. Most may be barely detectable without a close examination. Ritchie and others suggested that the interfaces between adjacent soil peds also serve as macropores. Moreover, these openings need not extend to the land surface in order for flow to occur in them. Nonetheless, water can flow below the root zone in a matter of minutes.
Figure 149. Sketch map of a desiccation fracture in North-eastern Oklahoma
Thomas and Phillips suggested that this type of flow does not appear to last more than a few minutes or perhaps, in unusual cases, more than a few hours after "cessation of irrigation or rain additions".
The preceding discussion demonstrates that the unsaturated zone can store a large volume of water-soluble substances and that macropores and fractures can serve as highly permeable connecting routes between the land surface and the water table. The next consideration is the movement of salts within and from the unsaturated zone to the water table.
When a soil dries, desiccation cracks may form. Moisture on fracture walls may quickly evaporate and a strong capillary potential is developed that draws water from the adjacent soil matrix to the fracture walls. The soil water may be rather highly mineralized, either with natural substances, such as nitrate, or with contaminants that may have been spilled or disposed of on the land surface. Once the soil water reaches fracture or macropore walls and then evaporates, the salts remain as a water soluble lining.
During a period of rain or irrigation, Figure 150 shows that water may flow into these openings, dissolve the water-soluble lining, and rapidly flow downward. This may result in a highly concentrated solution that quickly reaches the water table and substantially degrades ground-water quality. Of course, some of the fluids and salts migrate back into the soil matrix because of head differences and the now reversed capillary potential.
Figure 150. Fractures and macropores
linings as a result of evaporation (Upper). These linings are dissolved
and flushed to the water table during recharge events
Even though there may be a considerable influx of contaminants through macropores and fractures to the water table following a rain, the concentration of solutes in the main soil matrix may change little, if at all. This is clearly indicated in studies by Shuford and othersó7 and again shows the major role of large openings. On the other hand, in the spring, when the soilmoisture content exceeds field capacity, some of the relatively immobile or stagnant soil water may percolate to the water table transporting salts with it. A similar widespread recharge period may occur in some places during the fall as a result of decreasing temperature and evapotranspiration and of wet periods that might raise the moisture content above field capacity.
In summary, the preceding suggests the following. During the spring, a large quantity of water-soluble substances may be leached from storage in the unsaturated zone over a wide area. The substances eventually reach the water table and cause significant change in ground-water quality. Although the quantity of leached substances is larger than at any other time during the year, the change may occur more slowly and the resulting concentration in ground water may not be at a maximum because of the diluting effect brought about by the major influx of water over a wide area. A similar but less dramatic change in quality may occur in the fall.
During summer months, ground-water quality changes would be expected to occur more rapidly, perhaps in a matter of hours, because of the large size and abundance of the macropores and fractures. These changes, however, may occur only over a relatively small area because of the local nature of convective storms.
Ecologic conditions in fractures and macropores should be quite different from those in the main soil matrix largely because of the greater abundance of oxygen. Resulting, one might well expect different microbial populations (types and numbers) and chemical conditions in macropores and fractures than in the soil matrix. Coupled with their far greater fracture permeability, this may help to explain why some biodegradable organic compounds or those that should be strongly sorbed actually may reach the water table and move with the ground water. This environment cannot be adequately examined by means of column studies.
Some public health investigators have reported70 that waterborne diseases seem to increase in the spring and fall. This might conform to and be the result of groundwater recharge through macropores and fractures.
In order to detect and evaluate cyclic fluctuations in ground-water quality and determine background concentrations as well, it will be necessary to install monitoring wells that can be used to measure vertical head differences and collect water samples from discreet sections of the aquifer. Moreover, it will be necessary to collect data frequently, perhaps weekly or even daily, until a pattern can be established.
Prediction of Contaminant Migration
In any ground-water pollution study it is essential to obtain the background concentration of a wide variety of chemical constituents, particularly those that might be common both to the local ground water and a contaminant. As mentioned previously, the water in shallow or surficial aquifers can undergo substantial fluctuations in chemical quality. Therefore, it is riot always a simple task to determine background concentrations, particularly of the more conservative constituents, such as chloride or nittate. In general, samples should be collected during dry periods and not during or within a week following a period of rain. Throughout much of North America the major period of ground-water recharge occurs in wetter periods of the year (generally in the spring), while ntinor recharge events occur during or immediately after a rain. These recharge events may flush water-soluble compounds from the unsaturated zone to the water table and may substantially change the chemical quality of the ground water. Since the quality of shallow ground water may fluctuate within fairly wide limits during short intervals, it is essential to determine background concentrations statistically by collecting several samples at different times and from different depths.
The severity of ground-water pollution is partly dependent on the characteristics of the waste or leachate, that is, its volume; composition concentration of the various constituents, time rate of release of the contaminant, the size of the area from which the contaminants are derived, and the density of the leachate, among others. Data describing these parameters are difficult to obtain and are lumped together into the term "mass flow rate", which is the product of the contaminant concentration and its volume and recharge rate, or leakage rate.
Once a leachate is formed it begins to migrate slowly downward through the unsaturated zone where several physical, chemical, and biological forces act upon it. Eventually, however, the leachate may reach saturated strata where it will then flow primarily in a horizontal direction as defined by the hydraulic gradient. From this point on, the leachate will become diluted due to a number of phenomena, including filtration, sorption, chemical processes, microbial degradation, dispersion, time, and distance of travel. Figure 151 illustrates some of these phenomena.
Filtration removes suspended particles from the water mass, including particles of iron and manganese or other precipitates that may have been formed by chemical reaction. Dilution by sorption of chemical compounds is caused largely by clays, metal oxides and hydroxides, and organic matter, all of which function as sorptive material. The amount of sorption depends on the type of pollutant and the physical and chemical properties of the solution and the subsurface material.
Figure 151. Phenomena which can dilute a leachate
Chemical processes are important when precipitation occurs as a result of excess quantities of ions in solution. Chemical processes also include volatization as well as radioactive decay. In many situations, particularly in the case of organic compounds, microbiological degradation effects are not well known. It does appear, however, that a great deal of degradation can occur if the system is not overloaded and appropriate nutrients are available.
Figure 152. Effect of differences in
on shapes of contamination plumes
Dispersion of a leachate in an aquifer causes the concentration of the contaminants to decrease with increasing length of flow. It is caused by a combination of molecular diffusion, which is important only at very low velocities, and dispersion or hydrodynamic mixing, which occurs at higher velocities in laminar flow through porous media. In porous media, different macroscopic velocities and flow paths that have various lengths are to be expected. Leachate moving along a shorter flow path or at a higher velocity would arrive at an end point sooner than that part following a longer path or a lower velocity, which results in hydrodynamic dispersion. Dispersion can be both longitudinal and transverse and the net result is a conic form downstream from a continuous' pollution source. As Figure 153 shows, the concentration of the leachate is less at the margins of the cone and increases toward the source. Because dispersion is directly related to ground-water velocity, a plume or slug will tend to increase in size with more rapid flow within the same period of time.
Since dispersion is affected by velocity and the configuration of the aquifer's pore spaces, coefficients must be determined experimentally or empirically for a given aquifer. There is considerable confusion regarding the quantification of the dispersion coefficient and many of the published values are fitted values that cannot be transferred.
Selection of dispersion coefficients that adequately reflect conditions that exist in an aquifer is a problem that can not be readily solved and herein lies one of the major stumbling blocks of chemical transport models.
Often confused with the term dispersion (DX = longitudinal dispersion and Dy = transverse dispersion) is dispersivity (ax, ay). Dispersion includes velocity: to transform from one to another requires either division or multiplication by velocity.
The rate of advance of a contaminant plume can be retarded if there is a reaction between its components and ground-water constitutents or if sorption occurs. This is called retardation (Rd). The plume in which sorption and chemical reactions occur generally will expand more slowly and the concentration will be lower than the plume of an equivalent nonreactive leachate.
Hydrodynamic dispersion affects all solutes equally while sorption and chemical reactions can affect various constituents at different rates. As Figure 154 shows, a leachate source that contains a number of different solutes can have several solutes moving at different rates due to the attenuation processes.
The areal extent of plumes may range within rather wide extremes depending on the local geologic conditions, influences on the hydraulic gradient, such as pumping, ground-water velocity, and changes in the time rate of release of contaminants.
The many complex factors that control the movement of leachate and the overall behavior of contaminant plumes are difficult to assess because the final effect represents several factors integrated collectively. Likewise, concentrations for each constituent in a complex waste are difficult to obtain. Therefore, predictions of concentration and plume geometry, at best, can only be used as estimates, principally to identify whether or not a plume might develop at a site and, if so, to what extent. Models can also be used as an aid in determining potential locations for monitoring wells and to test various renovation or restoration schemes.
A graphical solution (nomograph) was developed to provide a simple computational tool for the prediction of leachate plume movement and corresponding concentration. It is often necessary to estimate the potential distance of travel or length of time required for a plume to migrate some distance in the saturated zone from a point directly below a contaminant source. The concentration of conservative elements in the plume, such as chloride, can be estimated for some selected point in space and time along a flow path that extends directly downgradient from the source.
The nomograph is intended as a rapid means for obtaining an approximate solution. It also aids in understanding the model. It is one-dimensional (restricted to a line).
Figure 153. Ground water velocity exerts a Major control on
plume shape. Upper plume V = 1.5 ft/day and lower plume V = .5 ft/day
Figure 154. Plume of a Leachate containing several different solutes
The Wilson-Miller equation was reformulated to introduce scale factors and to provide the basis for the nomograph as follows:
Application allows mapping the center-line concentrations of the plume, with respect to time, in one direction (x distance) that is directly downgradient from the source. Dilution-dispersive mixing and retardation parameters are included in the solution. The equation and the nomograph apply to only one chemical constituent, such as chloride or dissolved solids, at a time.
Three scale factors are used in the nomograph as ratios with the primary variables x (distance), t (time), and QC,, (mass flow rate from source) in the forms of x/XD, t/TD, and QCo/QD. The y distance is set to zero. The scale factors are:
Figure 155. Nomograph for solutions
of time, distance, and
Concentration for any point along the principal direction of
XD = Dx
TD = Rd Dx
Two of the three ratios are computed directly and the third is then found using the nomograph. The factors provide two conveniences. First, the ratios are dimensionless except for concentration. Second, the scale factors combine the constant parameters, which makes it easier to repeat computations of concentration (C) for various positions along the x axis (x) or for different times (t).
The nomograph in Figure 155 is designed to provide a simple technique to estimate one of the following:
Application 1: The concentration (C) at a selected distance (x) and time (t).
Application 2: The distance (x) where a selected concentration (C) will occur at a given time (t).
Application 3: The time (t) when the concentration will reach a selected concentration (C) at a predetermined location.
As time passes, the concentration in a given area approaches a constant pattern known as steady state. Results for steady-state conditions can be determined for the first two applications as follows:
Application 1b: The maximum concentration (C). that would occur at a selected distance after a long time period.
Application 2b: The maximum distance (x) at which a selected concentration (C) will occur after a long time period.
The advantage of applications 1b and 2b is that it is possible to predict, for example, the maximum distance of plume migration for a given concentration threshold or limit. Such concentrations might be those that have been established as standards for the safe drinking water by EPA. Alternatively, it is also possible to predict the maximum leachate concentration that could be reached at a specified distance from the landfill or lagoon.
The estimate of distance (x), time (t), and concentration (C) may require an adjustment of the concentration value to correct for significant background concentrations. In applications 1 and lb the estimated concentration (C) must be added to the background concentration. In applications 2, 2b, and 3 the concentration value used must be the remainder after subtracting the natural background concentration.
The nomograph provides a visual representation of the plume concentration. The solution can be found easily for various locations, times, and concentrations. This leads to gaining a "feel" for the nature of the plume. As time passes, the concentration at a given location reaches steady state. The steady state value for concentration can be useful, for example, as a "worst case" scenario (maximum concentration reached in the infinite time). The upper line on the nomograph represents the time and distance at which steady state is reached. It is easy to see that before steady state, small changes in location or time correspond to large changes in concentration. In a sense, the steepness of the nonsteady state time lines indicates that the "leading edge" of the plume is relatively narrow and, therefore, passes a given location in a relatively short period of time. Behind the leading edge, the concentration remains constant at the steady-state value represented by the steadystate line on the nomograph.
The ground-water contamination case that is used in this example occurred in South Farmingdale, Nassau County, New York, and was described by Perlmutter and Lieber.73 Most of the data required for the solution described below were obtained directly from their report.
Contamination was caused by cadmium- and hexavalent chromium-enriched electroplating wastes that infiltrated from disposal basins into a shallow glacial aquifer. Apparently disposal began in 1941 and continued intermittently for several years. By the early 1960's a leachate plume originating at the disposal ponds extended downgradient about 4,300 feet, and was as much as 1,000 feet wide and as much as 70 feet thick. Figure 156 shows that the plume extends to the headwaters of Massapequa Creek, a small stream that serves as a natural drain for part of the contaminanted water.
Figure 156. Leachate Plume at South Farmingdale, New York
The surficial or upper glacial deposits, which are Pleistocene in age, extend from the land surface to a depth of 80 to 140 feet and lie on the Magothy Formation, a unit of stream deposits of Late Cretaceous age. The water table in the surficial deposits lies from 0 to about 15 feet below land surface.
The aquifer is in dynamic equilibrium and receives about 22 inches or about 1 mgd (million gallons per day) per square mile of recharge from precipitation. The water-table gradient averages about 1 foot in 500 feet and the water table undergoes an annual fluctuation of 2 to 3 feet. The direction of ground-water flow is southward from the disposal ponds toward Massapequa Creek. Estimates of ground-water velocity for the area range from 0.5 to 1.5 feet per day. Reportedly, the average velocity for the area is about 1 foot per day.
Chemical analyses of ground water in the South Farmingdale area indicate that the background concentration of hexavalent chromium is less than 0.01 mg/1. Likewise, the concentration is also less than 0.01 mg/1 in Massapequa Creek upstream from the area of the leachate plume. Along that stretch where the plume discharges into the stream, the concentration of chromium is substantially greater.
As much as 200,000 to 300,000 gallons per day of effluent (equivalent to 52 pounds per day of chromium) were discharged during the early 1940's to three disposal pits, which have a combined area of about 15,470 square feet. Since 1945 the volume of the waste stream has been reduced substantially and eventually a treatment plant was constructed. On two occasions the chromium concentration in the raw effluent was 28 and 29 mg/l.
The relatively clean nature (free of clay or organic matter) of the materials forming the surficial aquifer precluded any significant reduction in the chromium load in the plume. That is, ion-exchange during movement was negligible. Maximum chromium concentrations in the plume ranged from about 40 mg/I in 1949 to about 10 mg/1 in 1962.
The plume is about 200 feet wide at its origin at the disposal ponds.' It reaches a maximum length of about 4,300 feet and increases in width to about 1,000 feet. Assuming a velocity of 1.5 feet per day, this plume has an average longitudinal dispersion (Dx) of 105 ft2/day, a transverse dispersion (D ) of 21 ft2/day, and dispersivities of 70 ft (aX) and 14 ft. ~a~), respectively. Table 13 shows a summary of the required data. Application of the three methods to this example follows.
Figure 157. Applications 1a and 1b:
Using the Nomograph to Estimate
the concentration given values of distance and time
Table 13. Summary of Data for Example 1
m = 110 feet
n = 0.35
V = 1.5 ft2/day
Dx = 105 ft2/day
Dy = 21 ft2/day
Rd = 1
volume flow rate:
Q = 26,800 ft3/day
Co = 31 mg/l
mass flow rate:
QCo = 26,800 ft3/day x 31 mg/l
QCo = 52 lb/day
Application 1, illustrated in Figure 157:
To find concentration (C) for a distance (x) of 4,200 feet from the soure and time (t) of 2,300 days, calculate:
x = 4,200 ft = 60 (Locate at A)
XD 70 ft
References Pettyjohn, W.A. 1971. "Water Pollution by Oil-Field Brines and Related Industrial Wastes in Ohio." Ohio Jour. Sci., v. 71, no. 5, pp 257-269.
t = 2,300 days = 50 (Locate curve E)
Td 46.7 days
QCo = (26,800 ft3/day) (31 mg/l)
QD 1,800 ft3/day
= 460 mg/l (Locate at D)
QCD = 52 lb/day = .029 lb/ft3 (Locate at D)
QD 1,800 ft3/day
Using Figure 157 draw a line vertically from A to the intersection with the t/TD curve B, then horizontally from B to C and from C through the scale D to E, giving a concentration (C) of 2.6 mg/l
Application lb, illustrated in Figure 157:
To find the maximum concentration for a given distance for large time, use the steady-state line instead of curve (t/TD). Proceeding as above, a concentration (C) of 20 mg/l is read at F.
Application 2, illustrated in Figure 158:
To find distance (x) where a concentration (C) of 2.6 mg/l will occur at a time (t) of 2,300 days, calculate:
t = 2,300 days = 50 (Locate at D)
TD 46.7 days
QCo = (26,800 ft3/day) (31mg/l
QD 1,800 ft3/day
Using Figure 158, locate the selected concentration at A. Draw a line from A through B to C, then horizontally from C to curve D, and vertically to E, giving:
x = 60
Multiply by XD to determine distance (x):
x = ( x ) (XD) = (60) (70 ft) = 4,200 feet
Application 2b, illustrated in Figure 158:
To find the maximum distance at which a selected concentration will reach a given value for large time, use the steady-state line instead of curve (t/TD) in figure 158.
Figure 158. Application 2a: Using the Nomograph to Estimate
for Given Values of Concentration and time
Application 3, illustrated in Figue 159:
To find time (t) when the concentration (C) will reach 2.6 mg/l at location (x) of 4,200 feet, calculate:
x = 4,200 ft = 60 (Locate at D)
XD 70 ft
QCo = (26,800 ft3/day) (31 mg/l)
QD 1,800 ft3/day
= 460 mg/l (Locate at B)
Using Figure 159, locate the selected concentration at A, draw a line from A through B to C, then horizontally from C to an intersection with a vertical line from D, giving at E:
t = 50
Multiply by TD, to determine time (t):
t = ( t ) (TD) = (50) (46.7 days) = 2,300 days
If the lines intersect above the steady-state line, the concentration will not reach the given value at that location.
Figure 159. Application 3: Using the Nomograph to estimate
time given values of concentration and distance
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