Oceanography in the 21st Century - An Online Textbook
navigation bar for the online textbook


Groundwater Contamination

Residents in cities and many rural homes use groundwater for drinking and other household purposes. At the same time, groundwater is contaminated by many sources. How dangerous are pollutants in drinking water, where do they come from, and will they increase in the future?


Groundwater Contamination - The detrimental alteration of the naturally occurring physical, thermal, chemical, or biological
quality of groundwater. Further, groundwater contamination, for purposes of inclusion of cases in the public files and the joint
groundwater monitoring and contamination report, shall be limited to contamination reasonably suspected of having been
caused by activities of entities under the jurisdiction of the agencies identified in the Texas Water Code §26.406, TGPC rules, and subsequent legislative amendments. Groundwater contamination may result from many sources, including current and past oil and gas production and related practices, agricultural activities, industrial and manufacturing processes, commercial and business endeavors, domestic activities, and natural sources that may be influenced by, or may result from, human activities.

From Texas Groundwater Protection Committee (2005)

Sources of Contamination

Groundwater is contaminated by many activities such as those shown here.

sources of groundwater contamination
Sources of groundwater contamination. Click on the image for a zoom.
From US Environmental Protection Agency, Safe Drinking Water - Protecting America's Public Health Poster.

Landfills and Hazardous Waste Facilities
Texans dispose of approximately 30 million tons of municipal solid waste (durable and non durable goods, containers, food scraps, yard waste, inorganic waste, sludge from water and wastewater treatment facilities, septic tanks, construction and demolition debris) per year. 63% is residential. 37% is commercial and institutional. In 2005, there were 186 active landfills in Texas. They received 29.67 million tons of waste. Excluding construction waste, the per capita disposal rate in Texas 5.5 pounds per person per day. In 2005, U.S. residents, businesses, and institutions produced more than 245 million tons, which is approximately 4.5 pounds of waste per person per day. Almost all goes into landfills. (Texas Environmental Almanac and EPA Municipal Solid Waste).

Landfills contaminate groundwater when rain water leaks into aquifers below the landfill. Many early landfills did not have liners to trap rainwater that percolates through the landfill, and some newer landfills have liners that leak. The percolating water leaches toxic chemicals from batteries, broken fluorescent bulbs, electronic equipment, discarded household chemicals, and paints and solvents. Although landfills now prohibit toxic waste, and they are carefully regulated to prevent leakage to groundwater, many older sites are unlined and leak.

landfill modern landfill design
Left: A municipal landfill. Right: Design of a modern landfill.
Left from Aircraft Owners and Pilots Association. Right from Energy Information Administration A Modern Landfill.


Extensive herbicide use in agricultural areas (accounting for about 70 percent of total national use of pesticides) has resulted in widespread occurrence of herbicides in agricultural streams and shallow ground-water. The highest rates of detection for the most heavily used herbicides—atrazine, metolachlor, alachlor, and cyanazine—were found in streams and shallow ground water in agricultural areas. Insecticides were frequently detected in some streams draining watersheds with high insecticide use but were less frequently detected in shallow ground water because most insecticides are applied at lower levels than herbicides and tend to sorb onto soil or degrade quickly after application.
From USGS Water-Quality Patterns In Agricultural Areas

crop dusting ground spraying
Application of agricultural pesticides. Left: By a crop dusted. Right: From the ground.
From Colorado State University Environmental Health Advanced Systems Laboratory

Mining wastes include waste generated during the extraction, beneficiation, and processing of minerals. Extraction is the first phase of hard rock mining which consists of the initial removal of ore from the earth. Beneficiation is the initial attempt at liberating and concentrating the valuable mineral from the extracted ore. This is typically performed by employing various crushing, grinding and froth flotation techniques. Mineral processing operations generally follow beneficiation and include techniques that often change the chemical composition of the ore or mineral, such as smelting (iron and steel), electrolytic refining (aluminum) and acid attack or digestion. Copper and gold mines comprise 80% of the non-fuel facilities in the United States. They discard 90% to 99.99% of the mined rock, generating 1.3 gigatons of waste.
From Environmental Protection Agency web pages on mining.

Coal mines are another major source of contaminants. When pyrite rocks associated with coal mining are exposed to oxygen they are oxidized to generate acid mine drainage. The waste then flows into streams and infiltrates into aquifers.

A complex series of chemical weathering reactions are spontaneously initiated when surface mining activities expose spoil materials to an oxidizing environment. The reactions are analogous to "geologic weathering" which takes place over extended periods of time (i.e., hundreds to thousands of years) but the rates of reaction are orders of magnitude greater than in "natural" weathering systems. The accelerated reaction rates can release damaging quantities of acidity, metals, and other soluble components into the environment. For example, the overall pyrite reaction series [which occurs in high-sulfur coal mines] is among the most acid-producing of all weathering processes in nature.
From US Department of the Interior Factors controlling acid mine drainage formation.

acid mine drainage adams mine
Left: Pollution due to acid mine drainage in the Blackwater River of West Virginia. Right: Water collecting in the open-pit Adams Mine, an open-pit iron mine in Ontario, Canada.
Left: From US Department of the Interior Office of Surface Mining. Right: From Adams Mine Landfill Proposal.

Above Ground and Underground Storage Tanks
Gasoline stations, dry cleaners, and other industrial establishments store large quantities of liquids in tanks. Some are above ground, some are below ground. Homes is cold areas store heating oil in underground tanks or in basement tanks. Underground tend to cause groundwater contamination because small leaks often go undetected.

Nearly one out of every four underground storage tanks in the United States may now be leaking, according to the U.S. Environmental Protection Agency. If an underground petroleum tank is more than 20 years old, especially if it's not protected against corrosion, the potential for leaking increases dramatically. Newer tanks and piping can leak, too, especially if they weren't installed properly. Even a small gasoline leak of one drop per second can result in the release of about 400 gallons of gasoline into the groundwater in one year. Even a few quarts of gasoline in the groundwater may be enough to severely pollute a farmstead's drinking water. At low levels of contamination, fuel contaminants in water cannot be detected by smell or taste, yet the seemingly pure water may be contaminated to the point of affecting human health. Petroleum fuels contain a number of potentially toxic compounds, including common solvents such as benzene, toluene and xylene, and additives such as ethylene dibromide (EDB) and carbon-based lead compounds. EDB is a carcinogen (cancer-causing) in laboratory animals, and benzene is considered a human carcinogen.
From University of Missouri web page on Assessing the Risk of Groundwater Contamination From Petroleum Product Storage.

The EPA identified over 460,000 leaking underground storage tanks up to September 30, 2006. Steady cleanup work has progressed for over a decade and more than 350,000 contaminated sites have been cleaned up. The main concern now is contamination by methyl tertiary-butyl ether MTBE. The additive, or other additives with similar ability to oxygenate fuels, is required by the EPA to help reduce carbon monoxide emissions from cars in cold weather.

gasoline storage tank being removed from site
Gasoline storage tank being removed from site.
From Virginia Tech Groundwater Pollution Primer.

Septic Systems
Homes not connected to municipal sewer systems usually use septic systems to dispose of wastewater from toilets and drains. Waste water drains first into a septic tank where solids are separated from the liquid. Light solids such as fats rise to the surface, heavy solids sink to the bottom. The light solids remain until the tank is cleaned. Some of the heavy solids are decomposed by bacteria, some remain until the tank is cleaned. Relatively clear water from the tank drains into a field of pipes, the drain field or leach field, which slowly leak water into the ground. Most percolates downward and enters the water table, some is taken up by plants, some evaporates. The water is cleaned by natural remediation processes.

Water discharged into the ground includes nitrates and phosphorus which can contaminate aquifers or nearby streams.

septic system drain field
From Thurston County (Washington State) Public Health & Social Services Department web page on Inspecting Your Septic Tank.

Oil, Gas, and Industrial Injection Wells

Chapter 27 of the Texas Water Code (the Injection Well Act) defines an “injection well” as “an artificial excavation or opening in the ground made by digging, boring, drilling, jetting, driving, or some other method, and used to inject, transmit, or dispose of industrial and municipal waste or oil and gas waste into a subsurface stratum; or a well initially drilled to produce oil and gas which is used to transmit, inject, or dispose of industrial and municipal waste or oil and gas waste into a subsurface stratum; or a well used for the injection of any other fluid; but the term does not include any surface pit, surface excavation, or natural depression used to dispose of industrial and municipal waste or oil and gas waste.” All injection wells are regulated by either TCEQ (the commission in the Act) or the Railroad Commission of Texas TCEQ (RRC).
From Texas Commission on Environmental Quality web page on Injection Wells: Am I Regulated?

diagram of an injection well
Diagram of an injection well. From Pollution Issues

The US Environmental Protection Agency defines five classes of injection wells, all of which have important uses.

  1. Deep Wells Used to Inject Hazardous and Non-hazardous Waste Deep Below the Surface, EPA Class I

    Injection of hazardous waste into deep wells began in the United States in the 1960s. At that time, the chemical industry was looking for a safe, relatively inexpensive method for disposing of high volumes of waste that could be considered toxic. Technology was borrowed from the oil and gas industry to develop this new form of disposal ... There are 163 Class I hazardous waste injection wells located at 51 facilities. These are the only facilities that can accept hazardous waste generated offsite for injection.

    From EPA web page on Deep Wells (Class I)

    Class I wells are also used to dispose of non-hazardous industrial, low-radiation and municipal wastes. Because 89 % of the hazardous waste that is disposed of on land is disposed through Class I wells, they are the most strictly regulated type of well.

  2. Oil and Gas Injection Wells, EPA Class II
    This is the most common type of injection well.

    The oil and gas production industry accounts for a large proportion of the fluids injected into the subsurface. Typically, when oil and gas are extracted, large amounts of salt water (brine) are also brought to the surface. This salt water can be very damaging if it is discharged into surface water. Instead, all states require that this brine be injected into formations similar to those from which it was extracted. Over 2 billion gallons of brine are injected daily into injection wells in the US.

    The largest proportion of these brines are injected into formations that contain trace portions of extractable oil and gas. Injection of the brine can have the effect of enhancing production of oil and gas from the formations, thus secondary recovery of oil and gas depends heavily on injection. Furthermore, when States started to implement rules that prevented the disposal of brine to surface water bodies and soils, injection of this waste fluid became the prevalent form of disposal.

    Class II wells exist wherever there is production of oil and gas. There are approximately 167,000 oil and gas injection wells in the US, most of which are used for the secondary recovery of oil. In this process water is pumped into the formation that contains some residual hydrocarbons. A portion of the hydrocarbons are recovered, along with the injected water, by extraction or production wells. In a common configuration, one injection well is surrounded by 4 or more extraction wells. The recovered fluid is treated to remove most of the hydrocarbons in a device called a separator. The other type of oil and gas injection well is a disposal well. In this type of well, excess fluids from production and some other activities directly related to the production process are injected solely for the purpose of disposal.

    From EPA web page on Oil and Gas Injection Wells (Class II)

  3. Mining Wells, EPA Class III
    Wells are used to mine salt, sulfur, and uranium.

    A number of minerals are mined by using injection wells. In general the technology involves the injection of a fluid that contacts an ore which contains minerals that dissolve in the fluid. When the fluid is nearly saturated with components of the ore it is pumped to the surface where the mineral is removed from the fluid. More than 50% of the salt used in the US is obtained this way.
    From EPA web site on Mining Wells (Class III)

  4. Shallow Hazardous and Radioactive Injection Wells, EPA Class IV

    These wells are prohibited unless the injection wells are used to inject contaminated ground water that has been treated and is being injected into the same formation from which it was drawn.
    From EPA web page on Shallow Hazardous and Radioactive Injection Wells (Class IV)

  5. Shallow Injection Wells

    These are are injection wells that are not included in Classes I through IV. Class V wells inject non hazardous fluids into or above an aquifer. They are typically shallow, on-site disposal systems, such as floor and sink drains that discharge into dry wells, septic systems, leach fields, and similar types of drainage wells.
    From EPA web page on Shallow Injection Wells (Class V)

    The two most numerous types of Class V wells are storm water drainage and large capacity septic systems. Large cesspools and shallow waste disposal systems that receive or have received fluids from vehicular repair or maintenance activities, such as auto body or automotive repair, car dealerships, or other vehicular repair work, are now prohibited.

    Some Class V wells inject surface water to recharge aquifers, to control land subsidence, and to limit salt-water intrusion provided the injected water does not endanger underground sources of drinking water.

Despite the widespread use of injection wells, the Texas Commission for Environmental Quality has found few wells contaminating ground water supplies, and these have been cleaned up.

Types of Contaminants

Methyl Tertiary-Butyl Ether MTBE

Methyl tertiary-butyl ether (MTBE) is produced in very large quantities (over 200,000 barrels per day in the U.S. in 1999) and is almost exclusively used as a fuel additive in motor gasoline. It is one of a group of chemicals commonly known as "oxygenates" because they raise the oxygen content of gasoline. At room temperature, MTBE is a volatile, flammable and colorless liquid that dissolves rather easily in water. MTBE has been used in U.S. gasoline at low levels since 1979 to replace lead as an octane improver (helps prevent the engine from "knocking"). Since 1992, MTBE has been used at higher concentrations in some gasoline to fulfill the oxygenate requirements set by Congress in the 1990 Clean Air Act Amendments. Oxygen helps gasoline burn more completely, reducing harmful tailpipe emissions from motor vehicles. In one respect, the oxygen dilutes or displaces gasoline components such as aromatics (e.g., benzene) and sulfur. In another, oxygen optimizes the oxidation during combustion. Most refiners have chosen to use MTBE over other oxygenates primarily for its blending characteristics and for economic reasons. The Clean Air Act Amendments of 1990 (CAA) require the use of oxygenated gasoline in areas with unhealthy levels of air pollution. The CAA does not specifically require MTBE. Refiners may choose to use other oxygenates, such as ethanol.
From EPA web page on MTBE in Fuels.

The health effects of MTBE are not well understood. When inhaled in high concentrations, it causes cancer in some research animals. There is little data on its effects when humans ingest the chemical. EPA's Office of Water has concluded that available data are not adequate to estimate potential health risks of MTBE at low exposure levels in drinking water but that the data support the conclusion that MTBE is a potential human carcinogen at high doses. The EPA reviewed the available information on health effects in a 1997 advisory and stated that there is little likelihood that MTBE concentrations between 20 and 40 ppb in drinking water would cause negative health effects. The EPA Drinking Water Advisory recommends, but does not require, that concentrations be below 20 ppb in drinking water.

Chlorinated Solvents

Chlorinated solvents are volatile organic (carbon-based) compounds (VOCs) that contain chlorine. In general, chlorinated solvents have low water solubility and high volatilities and densities relative to other VOCs. They are used in aerospace and electronics industries, dry cleaning, manufacture of foam, paint removal/stripping, manufacture of pharmaceuticals, metal cleaning and degreasing, and wood manufacturing. Solvents also can be found in a variety of household consumer products including drain, oven, and pipe cleaners, shoe polish, household degreasers, typewriter correction fluid, deodorizers, leather dyes, photographic supplies, tar remover, waxes, and pesticides.

According to the [EPA Toxic Release Inventory] TRI, during 1998–2001, total on- and off-site releases of methylene chloride, PCE [perchloroethene], TCE [trichloroethene], and TCA [1,1,1-trichloroethane] averaged about 33 million pounds, 4 million pounds, 11 million pounds, and 0.5 million pounds, respectively. PCE is still the solvent of choice for 85 to 90 percent of the approximately 30,000 dry cleaners and launderers in the United States.

Solvents have been associated with both acute and chronic human-health problems. Some are suspected human carcinogens, and USEPA has set Maximum Contaminant Levels (MCLs) for solvents in drinking water at very low concentrations. Many of the solvents have water solubility that are high relative to their MCLs. This means that even small spills of some solvents can result in substantial ground-water contamination problems with respect to human health.
From USGS Occurrence and Implications of Selected Chlorinated Solvents in Ground Water and Source Water in the United States and in Drinking Water in 12 Northeast and Mid-Atlantic States, 1993–2002.

They become a problem when they leak from tanks, pipelines, and land fills, when they are spilled, and when they are disposed of improperly. They are the most commonly found volatile carbon-based compounds found in groundwater. They are strongly correlated with urban areas with high population densities and with groundwater having high oxygen concentrations. Perchloroethene was found most often. It was detected in 10% of the samples at levels exceeding 0.02 microgram per liter, and in 4% of the samples at levels exceeding 0.2 micrograms per liter. (Moran, 2006).

occurence of chlorinated solvents in groundwater and drinking water

Percent of water samples with chlorinated solvent levels that exceeded EPA Maximum Contaminant Levels. From Moran (2006).

Pesticides are any substance or mixture intended to prevent, kill, or repel any pest, including insects, weeds, mice, fungi, or bacteria. Thus, household chemicals to disinfect surfaces or to remove mildew are legally classified as pesticides.

Total pesticide use in the United States has remained relatively constant at about 1 billion pounds per year [excluding chlorine/ hypochlorates (2.6 billion pounds per year), wood preservatives (1 billion pounds per year), and special biocides (0.3 billion pounds per year)] , after growing steadily through the mid-1970s because of increased use of herbicides. Agriculture now accounts for 70 to 80 percent of total pesticide use. Most agricultural pesticides are herbicides, which account for about 60 percent of the agricultural use. Insecticides generally are applied more selectively and at lower rates than herbicides. Major changes in insecticide use have occurred over the years in response to environmental concerns, which have resulted in various restrictions on the use of organochlorine insecticides, such as DDT. Specifically, as the use of these persistent pesticides declined, the use of other, less persistent insecticides increased.
From USGS Sources of nutrients and pesticides

Glyphosate is by far the most commonly used herbicide, but it is of little concern because glyphosate doesn’t readily leach into water systems. Instead, it latches tightly to soil particles and degrades within weeks into harmless byproducts. By contrast, herbicides such as atrazine have been widely implicated in contaminating groundwater. (Service, 2007).

The US Geological Survey conducted a national survey of Pesticides in the Nation's Streams and Ground Water, 1992–2001, and issued a summary in 2006.

Among the major findings are that pesticides are frequently present in streams and ground water, are seldom at concentrations likely to affect humans, but occur in many streams at concentrations that may have effects on aquatic life or fish-eating wildlife. Human-health benchmarks were seldom exceeded in ground water. One or more pesticides exceeded a benchmark in about 1 percent of the 2,356 domestic and 364 public-supply wells that were sampled. The greatest proportion of wells with a pesticide concentration greater than a benchmark was for those tapping shallow ground water beneath urban areas (4.8 percent).
(Pesticides in the Nation's Streams and Ground Water, 1992–2001—A Summary).

pesticide frequency in US in water, groundwater, sediments, and fish
groundwater contaminants exceeding human health benchmarks
From US Geological Survey Pesticides in the Nation's Streams and Ground Water, 1992–2001—A Summary.

Atrazine is the pesticide most commonly found in rural water, being found in 20% of shallow groundwater sites surveyed by the US Geological Service as reported in Barbash (1999). Prometon is the most common pesticide in urban water, being found in 5% of shallow groundwater sites surveyed.

atrizine usage in US in 1997
Atrizine use in US in 1997. The map may be somewhat misleading because farmers now use mostly glyphosate to control weeds in corn.
From US Geological Service Pesticide Use Maps.

Metals (Natural and Anthropogenic)
Metals leach from landfills, old mines, and industrial sites. Batteries, electronic equipment, metal plating operations, metal smelters, all contribute. The US Geological Survey web page on Groundwater Quality lists the major natural and human-produced sources.

Small amount of metals are essential to life. Higher concentrations can be toxic. In addition, the toxicity of the metal also depends on its chemical compound. For example, metallic mercury is not toxic. But methyl mercury (CH3)2Hg is a highly toxic neurotoxin. It is produced by sulfate-reducing bacteria living in environments with low oxygen concentration. Chromium III is essential for humans, chromium VI is very toxic. Chromium VI compounds are readily soluble in water.

Nutrients are chemical elements that are essential to plant and animal nutrition. Nutrients include ammonia, urea, ammonium nitrate, and ammonium sulfate, potassium chloride, and diammonium phosphate. They enter aquifers from rain and irrigation water leaching the compounds from the soil after fertilizer was applied by households and farmers.

nitrogen from fertilizer mapAverage annual commercial fertilizer application rates for nitrogen in US of USA
Average annual commercial fertilizer application rates for nitrogen in US.
From Natural Resources Conservation Service, Model Simulation of Soil Loss, Nutrient Loss and Soil Organic Carbon Associated with Crop Production Appendix.

About 11.5 million metric tons per year (Mt/yr) of nitrogen in all forms is used in fertilizers in the United States. Ammonia represents about 32 percent of the total fertilizer nitrogen used; urea and urea-ammonium nitrate solutions together represent 37 percent; ammonium nitrate, 5 percent; and ammonium sulfate, 2 percent.

Phosphate rock, when used in an untreated form, is not very soluble and provides little available phosphorus to plants, except in some moist acidic soils. Treating phosphate rock with sulfuric acid makes phosphoric acid, the basic material for producing most phosphatic fertilizers. Phosphatic fertilizers include diammonium phosphate (DAP) and monoammonium phosphate (MAP), which are produced by reacting phosphoric acid with ammonia, and triple superphosphate, produced by treating phosphate rock with phosphoric acid. More than 90 percent of the phosphate rock mined in the United States is used to produce about 12 Mt/yr of phosphoric acid. Domestic consumption of phosphate in fertilizers has averaged 4.5 Mt/yr since 1994.

Potassium is found in potash, a term that includes various mined and manufactured salts; all contain potassium in a water-soluble form. Potash is produced at underground mines, from solution-mining operations, and through the evaporation of lake and subsurface brines. Minerals mined for potash include potassium chloride [KCl or muriate of potash (MOP)], potassium-magnesium sulfate [K2SO4·MgSO4 or sulfate of potash magnesia (SOPM)], or mixed sodium-potassium nitrate (NaNO3+KNO3 or Chilean saltpeter). Manufactured compounds are potassium sulfate [K2SO4 or sulfate of potash (SOP)] and potassium nitrate (KNO3 or saltpeter). The United States consumes about 11 Mt/yr tons of potash of all types and grades.
From U.S. Geological Survey Fact Sheet 155-99 on Fertilizers -- Sustaining Global Food Supplies.

Nitrate concentrations in groundwater is highest in areas of well-drained soils and intensive cultivation of row crops, such as corn, cotton, or vegetables. Low concentrations are found in areas of poorly drained soils and where pasture or woodland is intermixed with cropland in agricultural areas.
From Mueller and Helsel (1996).


Alley, R. B., J. Marotzke, et al. (2003). Abrupt climate change. Science 299 (5615): 2005--2010.

Moran, M. J. (2006). Occurrence and Implications of Selected Chlorinated Solvents in Ground Water and Source Water in the United States and in Drinking Water in 12 Northeast and Mid-Atlantic States, 1993–2002. U.S. Geological Survey. Scientific Investigations Report 2005–5268.

Mueller, D.K. and D.R. Helsel. (1996). Nutrients in the Nations's Waters—Too Much of a Good Thing? U.S. Geological Survey Circular 1136. 24pp.

Groundwater Pollution Primer, Virginia Tech.

Revised on: 4 August, 2009

navigation bar for the online textbook
Copyright and contact information for Our Ocean Planet
click here to get back to the table of contents click here to get back to the send an email to the person who designed this web page click here to send an email to the author click here to get back to the course schedule page click here to the table of contents click here to go back to the home page of oceanworld