Groundwater Pollution and Remediation

Any artificially induced change in the natural quality of groundwater can be considered groundwater pollution. Groundwater pollution remediation is concerned with preserving the resource’s beneficial use, which is frequently a potable water supply. Groundwater contaminants are usually removed with "pump-and-treat" technologies. These technologies involve initial groundwater extraction, followed by treatment at the surface prior to reinjection or consumption.

Groundwater Contamination

Groundwater contamination may occur as a result of either natural or human causes. When human activities cause degradation of natural water quality, the term pollution is used. There are numerous statutes related to groundwater quality in the United States. One of the most important is the Safe Drinking Water Act of 1974. As a result of this legislation, the US Environmental Protection Agency (EPA) issued primary drinking water regulations in 1975, which have since been modified as new information regarding environmental exposure became available. The environmental standards in the regulations list maximum permissible concentrations, based on health criteria, of many organic and inorganic chemicals. The regulations apply to all public water systems, but individual groundwater supplies would ideally be of the same quality. Although the concentration limits for chemical contaminants are very low, theoretically, synthetic organic chemicals should not be present at all. Other chemical components, such as chloride, copper, iron, zinc, and manganese, have recommended maximum levels based on aesthetic or taste criteria.

Much groundwater pollution is the result of microbiological contamination. The permissible limits for an untreated water supply are 100 colonies of total coliforms (bacteria) per 100 milliliters and 20 colonies of fecal coliforms per 100 milliliters. Disease-causing organisms such as E. coli bacteria typically come from animal wastes. Contaminated water may cause typhoid fever, cholera, bacillary dysentery, paratyphoid fever, and even polio and infectious hepatitis. Between 1971 and 1977, there were 192 outbreaks of water-borne diseases in the United States and vastly more in developing countries. More stringent regulatory controls on the use and conservation of groundwater resources in North America since that time have drastically reduced the number of water-borne diseases, although they have not eliminated the problems that can arise due to human error or negligence.

Point Sources

Groundwater pollution sources are often identified by the designations "point" and "nonpoint." Point sources of groundwater contamination can be identified as a single isolated location from which the polluting material emanates. Nonpoint sources are those from which a polluting material emanates over a broadly delineated area. There is some overlap in the practical application of these definitions in that a single source for a specified pollutant may be considered a point source even though it extends over a large area—for example when one field of 100 hectares in area drains excess nitrate into an adjacent waterway in runoff water. Potential point sources thus may include such conditions as confined animal operations, land application of wastewater and sewage, a solid waste landfill, a leaking underground fuel tank, and septic tanks. Industrial point sources can include injection wells, hazardous and radioactive waste sites, mining activity sites, oil and gas production fields, chemical spills, and leaking underground storage tanks.

Additionally, saltwater intrusion can cause groundwater contamination, as can urban storm sewage waters. Storm runoff from streets and parking areas contains petroleum products, synthetic chemicals, and various metals as residues from automobile traffic, which may contaminate the groundwater system if the storm drains have leaks or if the runoff is not collected. Similarly, sewer lines may also cause groundwater contamination. In many cold climate areas, technical-grade salts used for de-icing roads are common pollutants.

Confined animal operations include poultry operations, milking barns, and feedlots for beef production. With hundreds or even thousands of animals confined in an area, the soil is typically not able to assimilate the animal wastes. Storm runoff can easily pollute both ground and surface waters. Animal wastes inject nitrogen compounds, phosphates, chloride, metals, organic chemicals, and bacteria into the environment and potentially into the groundwater. Nitrate-nitrogen is perhaps the most significant contaminant that may reach the groundwater. Similar contamination is possible from the improper application of fertilizers, from sewage and wastewater, and from poorly functioning septic tanks. Nitrate salts in groundwater effectively promote the growth of algae and the overgrowth of aquatic plants, typically to the detriment of the biological oxygen demand of a water system. Usually, bacteria, viruses, and phosphates are removed by interaction with the soil.

The many millions of septic tanks and cesspool systems that are in use throughout North America result in billions of liters of partially treated sewage entering the groundwater system annually. This is a tremendous source of potential contaminants if the system does not operate properly. Septic system failures result from inadequate length of flow through the soil, degradation over time of the physical components of the septic system, or, more commonly, from simply overloading the system.

Hazardous Wastes

Hazardous injection wells and radioactive and hazardous waste sites may contaminate groundwater with various heavy metal salts and inorganic chemicals, such as arsenic, lead, chromium, and uranium, and with toxic organic compounds. The Love Canal incident has received wide publicity but is only one example of groundwater contamination by a hazardous waste site. Between 1947 and 1953, the abandoned Love Canal in New York was used as a hazardous waste dump. Thousands of drums of chemicals were disposed of in the water and on the banks of the canal. After the site was sold in 1953, the canal was filled with dirt, and schools and houses were built nearby. In 1970, chemicals such as chloroform, benzene, toluene, and many other dangerous and carcinogenic compounds were detected in the air in basements of the houses. Chemicals from leaking and disintegrating drums had contaminated the groundwater and seeped through basement walls. The result was an observed incidence of cancers well above the national average, elevated incidence of several other medical conditions, plummeting property values, and several extensive lawsuits.

Hazardous industrial wastes have sometimes been disposed of by means of deep injection wells that inject the fluids into salty water zones lying below freshwater aquifers. Although the reported cases of groundwater contamination from such sources are low, the potential for contamination exists if the injection well leaks back to the freshwater level. Accidental spills of inorganic or organic chemicals on the surface may soak through the soil into the groundwater. Chemical spills may occur at the site of production, at the site of use, or during transportation.

Leachate and Petroleum Products

There are more than 100,000 industrial and municipal landfills in North America, disposing of several hundred million tons of waste per year. A city of 1 million people typically generates enough refuse in a year to cover approximately 145 football fields 15 meters deep with garbage. In a landfill or other open repository, the waste materials slowly break down over time as they interact with each other and their environment, releasing their molecular components to water that seeps through and from the material, leaching thousands of different decomposition products from the mass. Leachate produced by the decomposition of the large volumes of buried municipal waste poses a great potential threat to groundwater quality.

Although the composition of leachate is variable, it is enriched in dissolved solids (especially chloride and metals) and will consume a large amount of oxygen (measured as chemical oxygen demand), primarily because of its high concentration of undecomposed organic material. Methods for groundwater protection at landfills include surface-water control, covering the waste with soil (clays) that does not transmit significant amounts of precipitation into the landfill, and liners (natural clay or synthetic) that keep the leachate from leaving the burial site. Many large sites include collection points from which leachate can be removed for processing, as well as vent systems for trapping and collecting methane gas produced during the decomposition process.

Underground storage tanks pose a similar risk of groundwater contamination. Petroleum-product storage tanks, the primary feature of the many thousands of service stations in North America, constitute the major type of underground storage tanks. Faulty tanks—resulting from defective construction, improper emplacement, or physical breakdown and rusting—leak these organic chemical materials directly into the soil, where they are free to migrate into the groundwater. Leaky oil wells and distribution pipelines may also contaminate the groundwater with petroleum and petroleum by-products. Further, millions of liters of gasoline directly enter the environment annually, a bit at a time, as the drops that fall from the ends of filler nozzles at each use. Larger spills of gasoline at gas stations and by tanker trucks ruptured in traffic accidents also have contaminated groundwater. Because oils and gasoline are immiscible (will not mix with) and are less dense than water, most of the pollution from these sources floats on top of the groundwater. Small amounts of petroleum products, however, can dissolve in the water and contaminate the entire aquifer.

Acidic Mine Waters and Saltwater

Acidic mine waters, rich in metals such as lead, zinc, and cadmium, are common in many ore and coal mining operations. Pyrite (commonly known as fool’s gold) is composed of iron and sulfide. When pyrite interacts with air and water under appropriate conditions, sulfuric acid is produced, which dissolves more metals from the rock. Protecting groundwater from contamination produced by mining operations is difficult, especially for deep mines.

Saltwater contamination of groundwater (intrusion) may result from oil and gas wells, as well as from freshwater wells. Most deep oil and gas wells encounter brines (salty water), which are returned to their sources with injection wells. If either the petroleum wells or the injection wells have leaks, saltwater may enter more shallow freshwater aquifers. In many areas of the country, especially in coastal areas, salty water underlies freshwater. If freshwater wells (for example, irrigation wells) pump too much water, the level of the freshwater can be lowered to the point that the underlying saltwater encroaches on the freshwater aquifer recharge zone, contaminating it.

Nonpoint Sources

Nonpoint (diffuse) sources of contamination are equally important. As much as 50 percent of the total water pollution problem has been attributed to nonpoint sources. Although these two sources of pollution are used for discussion purposes, there is often a gradation between point and nonpoint sources. For example, if only a few septic tanks in an area are causing pollution, each of the offending septic tanks would be considered a point source of pollution. The overall area in which those septic tanks are located may be contaminating the groundwater over a broad area; however, there are no distinct points from which the pollution emanates, and the contamination would be considered nonpoint source pollution.

The contamination of the groundwater in Long Island, New York, is an example of nonpoint source pollution of groundwater from septic tank discharges. The type of soil and the concentration of a large number of septic tanks in a small area have resulted in broad, general contamination of the groundwater by nitrate, ammonia, and detergents. Nitrate-nitrogen and ammonia-nitrogen concentrations in this area have exceeded 20 milligrams per liter, which is well above the acceptable concentration of nitrogen in drinking water. As a result, the water must be treated before use. Atmospheric deposition (both wet and dry) of chemicals downwind of smokestacks in urban areas can cause nonpoint pollution of groundwater over a large area.

Agricultural practices cause a significant amount of nonpoint source pollution. Contamination by pesticides and fertilizers that incorporate nitrates is most significant because they are spread over large areas and may migrate through the soil into the groundwater. Irrigation waters can also cause contamination in arid areas. The water in wet soil dissolves salts, allowing the salts to move to the surface. As the water evaporates, the dissolved salts and other chemicals are concentrated in the remaining water at the surface, sometimes rendering the soil unsuitable for certain crops. Prolonged periods of wetness due to extensive irrigation or precipitation can then carry the enhanced salt load deep into the soil, where it can then soak into the groundwater system. This water, enriched in dissolved chemicals, may then be pumped up from groundwater sources and used for irrigation again. This cycle may be repeated several times, depending on the flow rate of the groundwater in replenishing the wells, ponds, or streams from which it has been pumped, eventually producing groundwater with very high concentrations of chemicals.

Treatment Technologies

Selection of a groundwater treatment system for groundwater pollution remediation is normally a three-step process consisting of an initial groundwater investigation to determine the type and concentration of contaminant and extent of pollution, the establishment of cleanup goals, and the selection of treatment technology. The last step almost invariably comprises groundwater extraction, surface treatment, and disposal or reinjection, an overall process termed pump-and-treat remediation.

Treatment technologies may be categorized as biological, chemical, or physical. In biological treatment methods, the contaminants are metabolized by aerobic or anaerobic microorganisms. Chemical treatment methods are based on the use of a chemical reactant to immobilize or break down a contaminant and typically include adsorption and precipitation stages. Physical treatment technologies utilize a physical property, such as a contaminant’s molecular weight or solubility, as the basis for separating the contaminant from the polluted groundwater. These technologies include air stripping, reverse osmosis, and electrodialysis.

Air Stripping

The basic concept behind air stripping is mass transfer, whereby the contaminant in water is transferred to a solution in the air. Contaminated water is brought into contact with air, either by passing air through the water or by injecting the water into the air as a fine spray. Some of the volatile contaminants become vapors and partition between the air and the water. That is to say, a portion of the volatile components transfer to the air and are removed from the water as the vapors are carried off in the airstream. The transfer of a contaminant is related to its vapor pressure in air relative to its solubility in water. This ratio is formally expressed as Henry’s law: H = Cig/Cil, where Cig is the equilibrium concentration in the gas phase (grams per cubic meter), Cil is the equilibrium concentration in the liquid phase (grams per liter), and H is the Henry’s law constant. The Henry’s law constant can be used to predict a contaminant’s strippability; the higher the constant, the greater the strippability.

As mentioned, water is brought into contact with air either by putting air through water or by putting water through air. Two systems that use the air-through-water methodology are diffused air aeration and mechanical surface aeration. Diffused air aeration is a procedure in which compressed air is injected into a tank of water through a porous base plate or through perforated pipes. In mechanical surface aeration, an impeller creates a turbulent mixing of air and water.

In each of the air-through-water systems, the mass transfer effect occurs at the bubble surface. In water-through-air aeration systems, the mass transfer is facilitated by the creation of thin water films or small water droplets. Water-through-air system configurations include crossflow towers and tray aerators, in which the air flows crosscurrent or countercurrent, respectively, to water flowing downward over trays or slats; spray basins, in which water is sprayed from a network of nozzles within a basin into the air in fine droplets to be subsequently collected as they fall back into the basin; and packed towers. Packed towers utilize a countercurrent flow scheme. Air is blown up the tower while water trickles down over an inert (chemically inactive) packing, commonly polypropylene moldings.

Carbon Adsorption

In carbon adsorption, contaminants are removed from water by their attraction and binding to the surface of the activated carbon adsorbent. Intermolecular attractive forces between the carbon atoms and the molecules of various contaminants act rather like magnets, making the molecules adhere loosely to the carbon particle surfaces or adsorb. Adsorption performance is estimated with a liquid adsorption isotherm test. When contaminated water is mixed with activated carbon, the contaminant concentration decreases to an equilibrium concentration, at which point the number of molecules leaving the surface of the adsorbent is equal to the number of molecules being adsorbed. The relationship between adsorption capacity and equilibrium concentration, known as an adsorption isotherm, is described by the Freundlich equation: X/M = KCel/n, where X/M is the adsorption capacity, or milligram volatile organic carbon (VOC) adsorbed per gram of activated carbon; Ce is the contaminant concentration at equilibrium VOC milligram per liter and K and l/n are empirical constants. From the isotherm, the adsorption capacity of the carbon for a contaminant can be estimated using the X/M value that corresponds to the incoming water contaminant concentration.

Activated carbon is produced by high-temperature pyrolysis (the chemical change of a substance by heat) of an organic material to produce a carbon char, followed by partial oxidation at high temperatures in an oxygen-poor atmosphere. During partial oxidation, the oxidation occurs along planes within the carbon, creating macropores and micropores, thereby greatly increasing the surface area of the carbon. The resulting surface area can be up to 1,400 square meters per gram.

During activated carbon treatment, the contaminated water is placed into contact with the carbon for between fifteen and sixty minutes at a surface loading rate of 2 to 17 gallons per minute per square foot. In contact systems using granular carbon, the contaminated water flows either through a fixed bed of carbon or a moving bed, in which the carbon moves down the column under gravity countercurrent to the flow of water. The main advantage of the moving bed is reduced suspended solids removal, although this advantage is rarely a factor in groundwater treatment. The flow of water in fixed-bed systems may be either up or down through the bed, as it does not depend on gravity to induce water flow. System configurations may be a single column or multiple columns arranged in parallel or in series. In powdered carbon systems, the carbon is mixed with the contaminated water, typically in the clarifier of an activated sludge system, and allowed to settle before disposal. Upon exhaustion of granular carbon’s adsorptive capacity, the carbon is removed and regenerated in a furnace, where the high temperature destroys the adsorbed organic matter.

Reverse Osmosis and Ion-Exchange Treatment

The term "osmosis" describes the phenomenon in which certain types of membrane will permit the passage of a solvent but restrict the movement of solutes such that water molecules will pass through a semipermeable membrane from a weak solution to a strong solution, eventually equalizing the solute concentrations on both sides of the membrane. In reverse osmosis, pressure (typically 200 to 400 pounds per square inch) is applied to force the water from a contaminated groundwater source through a membrane. Contaminant movement is retarded by the membrane, and purified water is obtained as it passes through to the other side. Cellulose acetate or polyamide membranes in tubular, spiral-wound, or hollow-fin fiber configurations are used.

The electrodialysis process uses electric potential rather than pressure to remove ions from a solution. Two ion-selective membranes, one that is permeable to cations (positively charged ions) and one that is permeable to anions (negatively charged ions), partition the contaminated water from the brine solution and the electrodes. When an electric current is passed across the cell, the cations migrate through the cation-permeable membrane toward the cathode. The anions, however, are prevented from migrating by the membrane. At the anion-permeable membrane, the converse occurs and, consequently, both cations and anions are removed from the contaminated water. In operational electrodialysis systems, several hundred alternate anion and cation permeable membranes, spaced at 1-millimeter intervals, are placed between a single set of electrodes. The passage of water between the membranes usually takes 10 to 20 seconds, during which time 25 to 40 percent of the ions are removed. Depending upon treatment goals, the water may pass between 1 and 6 membrane stacks.

Precipitation is the process whereby soluble inorganic contaminants are converted, by the addition of reagents, to insoluble precipitates, which are then removed by flocculation and sedimentation. With respect to dissolved metals, precipitation is achieved by increasing the concentration of the anion of a slightly soluble metal-anion salt, usually carbonate, hydroxide, or sulfide.

On-site Treatment

In addition to contaminant pump-out, reverse osmosis, and ion-exchange treatment of inorganic contaminants, and air stripping-based treatment of organic contaminants, two on-site treatments have been under evaluation. On-site nitrate removal involves the use of autotrophic denitrifying bacteria, which can catalyze enzymatically the reduction of nitrate to nitrogen gas. Degradation of hydrocarbons by naturally occurring groundwater microorganisms is expected to treat gasoline-related contaminants. Fully implemented pump-and-treat systems, meanwhile, are estimated to yield a total of 70,700 acre-feet per year of remediated groundwater.

Studying Groundwater Pollution

Although it is possible to determine if a well is polluted, it is difficult to determine the extent of aquifer pollution. Many wells must be drilled and designed to collect water from various levels within the aquifer to evaluate the full extent of aquifer contamination. This approach is obviously expensive and is used only in areas strongly thought to be polluted. Monitoring wells are located near waste disposal sites to monitor the water quality in the vicinity. The locations and design of the monitoring wells are based on detailed studies of the geology (including soil) and hydrology of the site. Water samples are collected from these wells at regular intervals to monitor any degradation of the water. The parameters selected to monitor are those associated with wastes that are soluble in water. For example, chromium, lead, and copper might be used for monitoring a hazardous waste site that accepts waste from metal-processing plants. Early detection of pollution allows the source of contamination and a relatively small portion of the aquifer to be cleaned up before the entire aquifer is irreparably damaged.

Computer modeling of the transport and deposition of contaminants has become a major method in studying groundwater pollution. These types of studies require not only computer and mathematical expertise but also considerable knowledge of hydrology, geology, and chemistry. The direction and rate of groundwater flow are important. The type of rock and the presence of fractures and faults, which affect the movement of groundwater, are also important factors. Equally important is the solubility of chemicals and their reaction with the soil and aquifer material. These models are helpful in designing monitoring networks for sites. In addition, these models can be useful in designing a remediation program for polluted portions of aquifers.

Importance of Groundwater Quality Protection

High-density populations of organisms, including humans, usually encounter waste disposal problems. These problems are especially critical for industrialized societies that produce large quantities of municipal and hazardous wastes. Disposal of these large volumes of wastes has often resulted in major contamination of groundwater. In the United States and many other parts of the world, groundwater is a vital source of drinking water. It may be distributed through a community system or private wells. Groundwater is also an important source of irrigation water, especially in the western part of the United States. The quality of the groundwater is important for its agricultural use. If the water becomes too salty, plants cannot grow. If the water contains high concentrations of trace metals (for example, selenium), then plants may concentrate the metal and pose a health problem.

Groundwater pollution is more critical than surface water for two reasons. First, it is more difficult to gain access to groundwater; therefore, it is more difficult to determine groundwater pollution and clean up the contamination. Groundwater pollution cannot be seen and is detected only when a well or a spring becomes noticeably polluted. Second, groundwater movement through aquifers is usually very slow, so remediation of the groundwater will also be slow. Because it is difficult and expensive to clean up an aquifer once it has been polluted, considerable effort should be spent in the careful design of installations and monitoring systems and in land-use planning.

Land-use management is crucial to protecting groundwater quality. For example, if soils in an area are too thin or of the wrong type to allow natural treatment of polluted surface waters from feedlots, these operations should be banned in this area. In some cases, agricultural contamination of aquifers has occurred because too much pesticide or fertilizer was used for the soil and vegetation conditions in an area. Mapping of faults and fractures, which often are zones of increased groundwater movement, can be useful in land-use planning. Limestone areas are often very susceptible to groundwater contamination because the rocks are usually fractured, and these fractures may be enlarged as the groundwater dissolves the rock. Caves, sinkholes, and disappearing streams also are often associated with limestone aquifers. These features do not allow natural filtration of recharge water; therefore, the groundwater may be quickly polluted by surface waters.

Context

Approximately 38 percent of all US drinking water comes from groundwater, most of it from freshwater aquifers, and almost half of the global drinking water comes from groundwater. Groundwater is essential for drinking water, irrigation, and industrial use. Pollution poses a significant, ongoing threat to this vital resource.

Information compiled by the EPA in the early twenty-first century indicated that around 20 percent of all drinking water systems and around 30 percent of the systems in municipal areas using groundwater as their source show at least trace levels of volatile organic carbons. The true extent of groundwater contamination is, however, probably much greater than that indicated by the EPA data. Potential point sources of pollutants alone include tens of thousands of hazardous waste sites and landfills, as well as millions of septic systems. Given possible widespread groundwater contamination, increasing public awareness of the importance of this resource and the health implications of its contamination, and the increasing prominence of environmental issues on the political agenda, the comparatively new technology of groundwater pollution remediation will become an increasingly important component of environmental management.

In the first decades of the twenty-first century, understanding of chemical contamination of groundwater increased. Chemical contamination of groundwater is caused by human activity and negatively impacts human health in a long term but poorly understood manner. After the source of chemical contamination is identified, it can take decades to reverse the damages caused to the precious resource.

Principal Terms

air stripping: the process of passing contaminated water through an aeration chamber, causing the organic contaminants to volatilize into the gaseous waste stream

aquifer: a saturated underground rock or sediment formation from which water is typically withdrawn for use

carbon adsorption: the process of pumping contaminated water directly through carbon filters, to capture contaminants by adsorptive binding to the surface of the carbon particles

contaminant: any natural or unnatural component that is introduced into the environment in concentrations greater than those normally present

flocculation: a slow process by which suspended particles are gathered together to form larger particles that can then be removed by physical means

ion exchange: the reversible switching of ions between the water being treated and an ion exchange resin by which undesirable ions in the water are exchanged with acceptable ions on the resin

leachate: contaminated fluid produced by the passage of water through decaying garbage in a landfill

nonpoint source: a large, diffuse source of contamination

osmosis: the passage of a solvent, usually water, from a solution of relatively low solute concentration into a solution of higher solute concentration across a semipermeable membrane

point source: a single, defined source of contamination

precipitation: the chemical conversion of a dissolved soluble material into an insoluble one that subsequently settles out of the solution

Bibliography

Al-Hashimi, Osamah, et al. "A Comprehensive Review for Groundwater Contamination and Remediation: Occurrence, Migration and Adsorption Modelling." Molecules, vol. 26, no. 19, 2021. doi.org/10.3390/molecules26195913. Accessed 15 Apr. 2023.

Appelo, C. A. J., and Dieke Postma. Geochemistry, Groundwater, and Pollution. 2nd ed. CRC Press, 2005.

Brassington, Rick. Field Hydrogeology. 5th ed., John Wiley and Sons, 2024.

Batu, Vedat. Applied Flow and Solute Transport Modeling in Aquifers. CRC Press, 2006.

Brimblecombe, Peter et al., eds. Acid Rain: Deposition to Recovery. Springer, 2010.

Dillon, P. J., ed. Management of Aquifer Recharge for Sustainability. Taylor & Francis, 2002.

"Groundwater Contamination." Groundwater Foundation, www.groundwater.org/get-informed/groundwater/contamination.html. Accessed 30 July 2024.

"Groundwater Use in the United States." The USGS Water Science School, USGS, 18 June 2018, water.usgs.gov/edu/wugw.html. Accessed 30 July 2024.

Li, Peiyue, et al. “Sources and Consequences of Groundwater Contamination.” Archives of Environmental Contamination and Toxicology, vol. 80, no, 1, 2021, pp. 1-10, doi:10.1007/s00244-020-00805-z.

Moore, John E. Field Hydrogeology. 2nd ed. CRC Press, 2012.

Madhav, Sughosh, and Pardeep Singh. Groundwater Geochemistry Pollution and Remediation. Wiley-Blackwell, 2021.

Randolph, John. Environmental Land Use Planning and Management. Island Press, 2004.

Simon, Franz-Georg, et al. Advanced Groundwater Remediation: Active and Passive Technologies. Thomas Telford Publishing, 2002.

Van Beynen, Philip E. Karst Management. Springer, 2011.

Water Quality Instrumentation: Principles and Practice. Water Environment Federation, 2022.