Groundwater - Pollutants

Types of Major Contaminants in Groundwater

In the EPA's National Water Quality Inventory—1998 Report to Congress (the latest data available for information on specific contaminants in groundwater) thirty-one of the thirty-seven reporting states identified the types of contaminants they found in groundwater. The states said that nitrates, metals, volatile and semivolatile organic compounds, and pesticides were the pollutants found most often.

ARSENIC.

Arsenic is a naturally occurring element in rocks and soils, and is the twentieth most common element in the earth's crust. In groundwater, the presence of arsenic is largely the result of minerals dissolving from naturally weathered rocks and soils over time. The USGS reported that the nation's groundwater typically contains less than one or two parts per billion (ppb) arsenic. A ppb is equal to about one drop in an Olympic-size swimming pool. Moderate to high arsenic levels do occur in some areas throughout the nation in a pattern related to geology, geochemistry, and climate. Elevated arsenic concentrations in groundwater are commonly found in the West and in parts of the Midwest and the Northeast.

Arsenic research has shown that humans need arsenic as a trace element in their diet to survive. Too much arsenic, however, can be harmful. Arsenic can contribute to skin, bladder, and other cancers after prolonged exposure. In January 2001 the EPA proposed lowering the current maximum contaminant level (MCL) for arsenic in drinking water from fifty ppb to ten ppb. The effective date of the new arsenic rule was February 22, 2002. Public water systems must comply with the ten ppb level by January 23, 2006.

NITRATES.

Many scientists and geologists consider nitrates to be the most widespread groundwater contaminant. Many states use their presence as an "indicator" of the human impact on groundwater quality. Generally, a level of three ppb or more is considered indicative of human impact.

Nitrate contamination occurs most frequently in shallow groundwater (less than 100 feet below the surface) and in aquifers that allow the rapid movement of water. Regional differences in nitrate levels are related to soil drainage properties, other geologic characteristics, and agricultural practices. Nitrate in groundwater is generally highest in areas with well-drained soils and intensive cultivation of row crops, particularly corn, cotton, and vegetables. Low nitrate concentrations are found in areas of poorly drained soil and where pasture and woodland are intermixed with cropland. Crop fertilization is the most important agricultural practice for introducing nitrogen into groundwater. The primary source of nitrates is fertilizers used in agriculture and, in some areas, feedlot operations.

Nitrates are important because they affect both human and ecological health. They can cause a public health risk to infants and young livestock. In some areas of the country, substantial amounts of nitrates in surface water are contributed by groundwater sources. For example, the USGS reported in 1997 that groundwater discharge was a significant source of nitrate loading to tidal creeks, coastal estuaries, and the Chesapeake Bay.

PESTICIDES.

Pesticide concentrations in groundwater are generally low and rarely exceed EPA drinking water standards. In 1999 (the latest data available), the USGS reported that pesticide concentrations exceeded standards of guidelines in less than 1% of the wells sampled in the National Water Quality Assessment Program. This assessment, however, may be incomplete with respect to the overall health and environmental risks associated with pesticide presence in shallow groundwater. According to a 1999 report from the USGS (The Quality of Our Nation's Waters: Nutrients and Pesticides. Circular 1225), drinking water standards and guidelines exist for only forty-six of the eighty-three pesticide compounds identified. Of the sites where pesticides were detected, 73% had two or more compounds present. Continued research is needed to help reduce the current uncertainty in estimating risks to human health and to the environment from commonly occurring mixtures of pesticides.

METHYL TERTIARY BUTYL ETHER.

Methyl tertiary butyl ether (MTBE) is a volatile organic chemical that is added to gasoline to increase octane levels and to reduce carbon monoxide and ozone levels in the air. It is more soluble in water, and less likely to be degraded, than other common petroleum constituents. According to an October 2001 USGS report (MTBE and Other Volatile Organic Compounds—New Findings and Implications on the Quality of Source Waters Used for Drinking-Water Supplies), the EPA tentatively has classified MTBE as a potential human carcinogen. However, because of insufficient toxicity studies, the EPA has not yet instituted a drinking-water health advisory or standard.

In its National Water Quality Assessment Program (NAWQA), the USGS detected MTBE in about 5% of ambient groundwater samples collected across the nation. The concentrations typically were lower than the EPA's drinking-water consumer advisory concentration of twenty to forty micrograms per liter. Less than 1% of the groundwater samples exceeded the EPA consumer advisory concentration of twenty micrograms per liter. MTBE was most often detected in groundwater underlying urban areas (14% of wells tested in urban areas had MTBE). The USGS also studied MTBE concentrations associated with drinking-water supplies from selected communities in twelve states in the Northeast and Mid-Atlantic regions. MTBE was detected in 9% of community water systems. These concentrations were low; less than 1% exceeded the EPA consumer advisory concentration.

Studies of the distribution of MTBE showed that the frequency of MTBE detection increases with its use, typically associated with urban and populated areas. MTBE has been detected in about one out of five wells in MTBE high-use areas. The findings also showed that some areas are more vulnerable to MTBE contamination than others. The frequency of MTBE detection increases with larger community water systems, specifically those serving more than 50,000 people. The difference in vulnerability reflects, in part, natural features, land use, and human activities such as pumping and watercraft use.

What began as an effort to reduce air pollution has become a water-quality concern that has necessitated dozens of costly studies and created a public health risk. In 1999 the EPA appointed the MTBE Blue Ribbon Panel to provide independent advice and counsel on the policy issues associated with the use of MTBE and other oxygenates in gasoline. In July 1999 the panel recommended a package of actions designed to be implemented simultaneously to maintain air quality benefits while enhancing water-quality protection and ensuring a stable fuel supply at reasonable cost. The EPA initiated plans to phase out MTBE, but the Bush Administration cancelled those plans in 2001. As of 2004, more than half the states still allowed MTBE, though California and New York, which together accounted for about 40% of MTBE use, have banned its use since January 1, 2004.

Sources of Pollution

In 2000 the EPA requested that each state identify the major sources that potentially threaten groundwater in their state. Figure 4.9 shows the results of that survey. Thirty-nine states rated underground storage tanks as the most serious threat to their groundwater quality. Septic systems, landfills, industrial facilities, agriculture, and pesticides were also important contamination sources.

UNDERGROUND STORAGE TANKS.

Leaking underground storage tanks (USTs) have been identified by the EPA as the leading source of groundwater contamination since the mid-1990s, and were cited as such in the 2000 National Water Quality Inventory. In general, most USTs are found at commercial and industrial facilities in the more heavily developed urban and suburban areas. USTs are used to store gasoline, hazardous and toxic chemicals, and diluted wastes. Gasoline leaking from UST systems at service stations is one of the most common causes of groundwater contamination. The primary causes of leakage in USTs are faulty installation and corrosion of tanks and pipelines.

At one time, USTs were made of steel, which eventually rusted and disintegrated, releasing their contents into the soil. This led to the discovery that a contaminant in the ground is likely to become a contaminant of groundwater. One gallon of gasoline can contaminate one million gallons of water, or the amount of water needed for a community of 50,000 people. The fuel additive MTBE is particularly troublesome because it migrates quickly through soils into groundwater, and very small amounts can render groundwater undrinkable. Figure 4.10 shows how groundwater can be contaminated by leaking underground storage tanks.

In 1988 the EPA issued "comprehensive and stringent" rules that required devices to detect leaks, modification of tanks to prevent corrosion, regular monitoring, and immediate cleanup of leaks and spills. By December 1998 existing tanks had to be upgraded to meet those standards, replaced with new tanks, or closed. Existing tanks were to be replaced with expensive tanks made of durable, noncorrosive materials.

A March 5, 2003, report from the Government Accountability Office (GAO) (Environmental Protection: Recommendations for Improving the Underground Storage Tank Program) stated that as of December 2002 at least 19 to 26% of states still had problems with leaking underground storage tanks. Although 89% of the 693,107 tanks subject to UST rules had leak prevention and detection equipment installed, more than 200,000 tanks were not being operated or maintained properly. The states reported that because of inadequate operation and maintenance of the leak detection equipment, even those tanks with the new equipment continued to leak. In order to address the problems, the GAO recommended that Congress provide states more funds from the UST trust fund to ensure improved training, inspections, and enforcement efforts. In addition, the GAO suggested that Congress require the states to inspect tanks at least every FIGURE 4.9
Major sources of groundwater contamination, 2000
SOURCE: "Major Sources of Ground Water Contamination," in 2000 National Water Quality Inventory, U.S. Environmental Protection Agency, August 2002 three years and provide the EPA and the states with additional enforcement authorities.

Underground storage tank owners and operators must also meet financial responsibility requirements that ensure they will have the resources to pay for costs associated with cleaning up releases and compensating third parties. Many states have provided financial assurance funds to help UST owners meet the financial requirements. In about 95% of the cases, the EPA or the states have succeeded in getting responsible parties to perform the cleanups. Figure 4.11 and Figure 4.12 contain information about inspection frequency and compliance with underground storage tank regulations.

As of the end of 2004, about 447,000 releases of contaminants from corroded underground storage tanks had been confirmed. The EPA estimated that about half of those releases reached groundwater. According to a February 2005 EPA fact sheet, "Cleaning Up Leaks from Underground Storage Tanks," 92% (more than 412,000) of cleanups had been initiated, and 71% (more than 317,000) had been completed. As of the end of 2004, about 130,000 remained in the national backlog of cleanups required.

LANDFILLS AND SURFACE IMPOUNDMENTS.

Septic systems and landfills were the second- and third-largest sources of groundwater contamination, respectively, in 2000. (See Figure 4.9.) Landfills are areas set aside for disposal of garbage, trash, and other municipal wastes. Early environmental regulation aimed at reducing air and surface water pollution called for disposing of solid wastes—including industrial wastes—underground and gave little consideration to the potential for groundwater contamination. Landfills were generally situated on land considered to have no other use. Many of the disposal sites were nothing more than large holes in the ground, abandoned gravel pits, old strip mines, marshlands, and sinkholes.

The leachate (the liquid that percolates through the waste materials) from landfills contains contaminants that can easily pollute groundwater when disposal areas FIGURE 4.10
Groundwater contamination as a result of leaking underground storage tanks
SOURCE: "Figure 6-3. Ground Water Contamination as a Result of Leaking Underground Storage Tanks," in 2000 National Water Quality Inventory, U.S. Environmental Protection Agency, August 2002 are not properly lined. Landfills built and operated prior to the passage of the Resource Conservation and Recovery Act (RCRA) in 1976 are believed to represent the greatest risk. RCRA was enacted to protect human health and the environment by establishing a regulatory framework to investigate and address past, present, and future environmental contamination of groundwater and other media. The adoption of these new standards in 1976 forced many old landfills to close, as they could not meet the RCRA's safety standards, but in many cases the garbage dumped in them while in operation remains in place and is a threat to groundwater.

Surface impoundments are the industrial equivalent of landfills for liquids and are usually composed of man-made pits, lagoons, and ponds that receive treated or untreated wastes directly from the discharge point. They may also be used to store chemicals for later use, to wash or treat ores, or to treat water for further use. Most are small, less than one acre, but some industrial and mining impoundments may be as large as 1,000 acres.

Prior to RCRA, most impoundments were not lined with a synthetic or impermeable natural material, such as clay, to prevent liquids from leaching into the ground. This is particularly important since about 87% of impoundments were located over aquifers that were currently used as sources of drinking water. Aquifers located under nonlined impoundments are vulnerable to contamination. Less than 2% of the surface impoundments were located in areas where there is no groundwater or it is too salty for use. About 70% of the sites were located over thick and very permeable aquifers that would allow any contaminant entering its waters to spread rapidly. Groundwater protection was rarely, if ever, considered during site selection.

Since the passage of RCRA, landfills and surface impoundments have been required to adhere to increasingly stringent regulations for site selection, construction, operation, and groundwater monitoring to avoid contaminating groundwater. Prevention of groundwater contamination is largely the responsibility of state and local government. Examples of the more stringent requirements are landfill liners and groundwater monitoring. (See Figure 4.13.)

HAZARDOUS WASTE SITES.

Hazardous waste is an unavoidable byproduct of an industrial society. Many chemicals are used to manufacture goods. Hazardous waste generators can be large industries, such as automobile manufacturers, or small neighborhood businesses, like the local photo shop. Although the quantity of hazardous waste can be reduced through innovation and good management, it is impossible to eliminate all hazardous residue because of the demand for goods. The EPA estimates that between 2% and 4% of all waste is hazardous.

Contamination of groundwater with hazardous waste is frequently the result of historic indiscriminate waste disposal in landfills, impoundments, and dumps. Today, sites that handle hazardous waste or a mix of hazardous and nonhazardous waste are subject to very strict controls.

When a waste site is found to be so badly contaminated with hazardous waste that it represents a serious threat to human health (for example, contamination of groundwater used for drinking with known carcinogens), it is placed on the National Priorities List (NPL) established by the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA, commonly known as the Superfund). Sites placed on the Superfund list are eligible for federal intervention and cleanup assistance. As of February 11, 2005, 1,238 sites were listed on the NPL. Most were general sites such as industrial and municipal landfills and military bases. About one-fifth of the listed Superfund sites were municipal landfills. A list of contaminants, their sources, and their health effects is shown in Table 4.1.

INJECTION WELLS.

An injection well is any bored, drilled, driven shaft, or dug hole that is deeper than it is wide that is used for the disposal of waste underground. The EPA's underground injection control (UIC) program identifies five classes of injection wells:

FIGURE 4.11
Underground storage tanks in compliance with federal equipment requirements, by state, 2002
SOURCE: "Figure 1. Compliance with Federal Equipment Requirements Varies among States (Total Active Tanks per State)," in Recommendations for Improving the Underground Storage Tank Program, Testimony Before the Subcommittee on Environment and Hazardous Materials, Committee on Energy and Commerce, House of Representatives, by John Stephenson, Director, Natural Resources and Environment, U.S. General Accounting Office, March 2003, http://www.gao.gov/new.items/d03529t.pdf (accessed April 11, 2005)

Class I wells are used to inject hazardous and nonhazardous waste beneath the lowest formation containing an underground source of drinking water (USDW) within a quarter mile of the well bore.
Class II wells are used to inject fluids associated with oil and natural gas recovery and storage of liquid hydrocarbons.
Class III wells are used in connection with the solution mining of minerals that are not conventionally mined.
Class IV wells are used to inject hazardous or radioactive waste into or above a formation that is within a quarter mile of a USDW.
Class V wells are injection wells not included in Classes I through IV.

Each well class has the potential to contaminate groundwater. Classes I through IV have specific regulations and are closely monitored. Class V wells are typically shallow wells used to place a variety of fluids underground.

According to the EPA, there were more than 650,000 Class V injection wells in the United States in 2001 (http://www.epa.gov/safewater/uic/classvdetermination.html, April 2001). These wells are found in every state, especially in unsewered areas. There are many types of Class V wells, including large-capacity cesspools, motor vehicle waste disposal systems, storm water drainage wells, large-capacity septic systems, aquifer remediation wells, and many others. The waste entering these wells is not treated. Certain types of these wells have great potential to have high concentrations of contaminants that might endanger groundwater.

FIGURE 4.12
Frequency of inspection of underground storage tanks, by state, 2002
SOURCE: "Figure 2. Frequency of Inspections Varies among States (Total Active Tanks per State)," in Recommendations for Improving the Underground Storage Tank Program, Testimony Before the Subcommittee on Environment and Hazardous Materials, Committee on Energy and Commerce, House of Representatives, by John Stephenson, Director, Natural Resources and Environment, U.S. General Accounting Office, March 2003, http://www.gao.gov/new.items/d03529t.pdf (accessed April 11, 2005)

Class V injection wells are regulated by the UIC program under the authority of the Safe Drinking Water Act (SDWA). Class V wells are "authorized by rule," which means that they do not require a permit if they comply with UIC program requirements and do not endanger underground sources of drinking water. In December 1999 the EPA adopted regulations addressing Class V wells that were large-capacity cesspools and motor vehicle waste disposal wells. Under these regulations:

New cesspools were prohibited as of April 2000.
Existing cesspools had to be phased out by April 2005.
New motor vehicle waste disposal wells were prohibited.
Existing wells in regulated areas were to be phased out in groundwater protection areas identified in state source water assessment programs. These requirements were scheduled to be phased in over seven years.

AGRICULTURE.

As in surface water contamination, agricultural practices play a major role in groundwater contamination. Agricultural practices that have the potential to contaminate groundwater include fertilizer and pesticide application, animal feedlots, irrigation practices, agricultural chemical facilities, and drainage wells. The contamination can result from routine applications, spillage or misuse of pesticides and fertilizers during handling and storage, manure storage and spreading, improper storage of chemicals, irrigation practices, and irrigation return drains serving as direct conduits to groundwater. Fields with overapplied or misapplied fertilizer and pesticides can introduce nitrogen, pesticides, and other contaminants into groundwater. Animal feedlots often have impoundments from which wastes (bacteria, nitrates, and total and dissolved solids) may infiltrate groundwater.

Human-induced salinity in groundwater also occurs in agricultural regions where irrigation is used extensively. FIGURE 4.13
Cross section of a minimal groundwater monitoring system
SOURCE: Hazardous Waste: Compliance with Groundwatering Requirements at Land Disposal Facilities, U.S. General Accounting Office, 1995 Irrigation water continually flushes nitrate-related compounds from fertilizers into the shallow aquifers along with high levels of chloride, sodium, and other metals. This increases the salinity (dissolved solids) of the underlying aquifers. Overpumping can diminish the water in aquifers to the point where salt water from nearby coastal areas will intrude into the aquifer. Salinas Valley, California, is an example of the occurrence of saltwater intrusion. Eleven states identified saltwater intrusion as a major source of groundwater contamination in their 2000 305(b) reports to the EPA. (See Figure 4.9.)

SEPTIC SYSTEMS.

Septic systems were cited as the second most common source of groundwater contamination by thirty-one reporting states. Septic systems are on-site waste disposal systems that are used where public sewerage is not available. Septic tanks are used to detain domestic wastes to allow the settling and digestion of solids prior to the distribution of liquid wastes into permeable leach beds for absorption into soil. Wastewater is attacked in the leach beds by biological organisms in the soil and broken down over time. According to information collected by the National Small Flows Clearinghouse (1999), approximately twenty-three million Americans living mostly in rural areas use individual sewage disposal systems. Millions of commercial and industrial facilities also use these systems. American households dispose of 3.5 billion gallons of liquid waste into septic systems each day, and between 820 and 1,450 billion gallons of waste into the ground each year.

Improperly constructed and poorly maintained septic systems may cause substantial and widespread nutrient and microbial contamination to groundwater. This belief is based on some studies done in different parts of the country, which have shown nitrogen levels elevated above background nitrogen levels in areas with concentrated septic systems. There has been no systematic study done nationwide. Septic systems have also been blamed for some outbreaks of bacterial and viral disease associated with drinking groundwater.

TABLE 4.1
Drinking water contaminants, their sources, and potential health effects, 2003

Type	Contaminant	MCL or TT (mg/L)^a	Potential health effects from exposure above the MCL	Common sources of contaminant in drinking water	Public health goal
OC	Acrylamide	TT^h	Nervous system or blood problems;	Added to water during sewage/wastewater increased risk of cancer treatment	zero
OC	Alachlor	0.002	Eye, liver, kidney or spleen problems; anemia; increased risk of cancer	Runoff from herbicide used on row crops	zero
R	Alpha particles	15 picocuries per liter (pCi/L)	Increased risk of cancer	Erosion of natural deposits of certain minerals that are radioactive and may emit a form of radiation known as alpha radiation	zero
IOC	Antimony	0.006	Increase in blood cholesterol; decrease in blood sugar	Discharge from petroleum refineries; fire retardants; ceramics; electronics; solder	0.006
IOC	Arsenic	0.010 as of 1/23/06	Skin damage or problems with circulatory systems, and may have increased risk of getting cancer	Erosion of natural deposits; runoff from orchards, runoff from glass & electronics production wastes	C
IOC	Asbestos (fibers >10 micrometers)	7 million fibers per liter (MFL)	Increased risk of developing benign intestinal polyps	Decay of asbestos cement in water mains; erosion of natural deposits	7 MFL
OC	Atrazine	0.003	Cardiovascular system or reproductive problems	Runoff from herbicide used on row crops	0.003
IOC	Barium	2	Increase in blood pressure	Discharge of drilling wastes; discharge from metal refineries; erosion of natural deposits	2
OC	Benzene	0.005	Anemia; decrease in blood platelets; increased risk of cancer	Discharge from factories; leaching from gas storage tanks and landfills	zero
OC	Benzo(a)pyrene (PAHs)	0.0002	Reproductive difficulties; increased risk of cancer	Leaching from linings of water storage tanks and distribution lines	zero
IOC	Beryllium	0.004	Intestinal lesions	Discharge from metal refineries and coal-burning factories; discharge from electrical, aerospace, and defense industries	0.004
R	Beta particles and photon emitters	4 millirems per year	Increased risk of cancer	Decay of natural and man-made deposits of certain minerals that are radioactive and may emit forms of radiation known as photons and beta radiation	zero
DBP	Bromate	0.010	Increased risk of cancer	Byproduct of drinking water disinfection	zero
IOC	Cadmium	0.005	Kidney damage	Corrosion of galvanized pipes; erosion of natural deposits; discharge from metal refineries; runoff from waste batteries and paints	0.005
OC	Carbofuran	0.04	Problems with blood, nervous system, or reproductive system	Leaching of soil fumigant used on rice and alfalfa	0.04
OC	Carbon tetrachloride	0.005	Liver problems; increased risk of cancer	Discharge from chemical plants and other industrial activities	zero
D	Chloramines (as Cl₂)	MRDL = 4.0^a	Eye/nose irritation; stomach discomfort, anemia	Water additive used to control microbes	MRDLG = 4^a
OC	Chlordane	0.002	Liver or nervous system problems; increased risk of cancer	Residue of banned termiticide	zero
D	Chlorine (as CI₂)	MRDL = 4.0^a	Eye/nose irritation; stomach discomfort	Water additive used to control microbes	MRDLG = 4^a
D	Chlorine dioxide (as CIO₂)	MRDL = 0.8^a	Anemia; infants & young children: nervous system effects	Water additive used to control microbes	MRDLG = 0.8^a
DBP	Chlorite	1.0	Anemia; infants & young children: nervous system effects	Byproduct of drinking water disinfection	0.8
OC	Chlorobenzene	0.1	Liver or kidney problems	Discharge from chemical and agricultural chemical factories	0.1
IOC	Chromium (total)	0.1	Allergic dermatitis	Discharge from steel and pulp mills; erosion of natural deposits	0.1
IOC	Copper	TT^g; action level= 1.3	Short term exposure: gastrointestinal distress. Long term exposure: liver or kidney damage. People with Wilson's Disease should consult their personal doctor if the amount of copper in their water exceeds the action level	Corrosion of household plumbing systems; erosion of natural deposits	1.3
M	Cryptosporidium	TT^c	Gastrointestinal illness (e.g., diarrhea, vomiting, cramps)	Human and animal fecal waste	zero
IOC	Cyanide (as free cyanide)	0.2	Nerve damage or thyroid problems	Discharge from steel/metal factories; discharge from plastic and fertilizer factories
OC	2,4-D	0.07	Kidney, liver, or adrenal gland problems	Runoff from herbicide used on row crops
OC	Dalapon	0.2	Minor kidney changes	Runoff from herbicide used on rights of way

TABLE 4.1
Drinking water contaminants, their sources, and potential health effects, 2003 [CONTINUED]

Type	Contaminant	MCL or TT^a (mg/L)^b	Potential health effects from exposure above the MCL	Common sources of contaminant in drinking water	Public health goal
OC	1,2-Dibromo-3-chloropropane (DBCP)	0.0002	Reproductive difficulties; increased risk of cancer	Runoff/leaching from soil fumigant used on soybeans, cotton, pineapples, and orchards	zero
OC	o-Dichlorobenzene	0.6	Liver, kidney, or circulatory system problems	Discharge from industrial chemical factories	0.6
OC	p-Dichlorobenzene	0.075	Anemia; liver, kidney or spleen damage; changes in blood	Discharge from industrial chemical factories	0.075
OC	1,2-Dichloroethane	0.005	Increased risk of cancer	Discharge from industrial chemical factories	zero
OC	1,1-Dichloroethylene	0.007	Liver problems	Discharge from industrial chemical factories	0.007
OC	cis-1,2-Dichloroethylene	0.07	Liver problems	Discharge from industrial chemical factories	0.07
OC	trans-1,2-Dichloroethylene	0.1	Liver problems	Discharge from industrial chemical factories	0.1
OC	Dichloromethane	0.005	Liver problems; increased risk of cancer	Discharge from drug and chemical factories	zero
OC	1,2-Dichloropropane	0.005	Increased risk of cancer	Discharge from industrial chemical factories	zero
OC	Di(2-ethylhexyl) adipate	0.4	Weight loss, liver problems, or possible reproductive difficulties	Discharge from chemical factories	0.4
OC	Di(2-ethylhexyl) phthalate	0.006	Reproductive difficulties; liver problems; increased risk of cancer	Discharge from rubber and chemical factories	zero
OC	Dinoseb	0.007	Reproductive difficulties	Runoff from herbicide used on soybeans and vegetables	0.007
OC	Dioxin (2,3,7,8-TCDD)	0.00000003	Reproductive difficulties; increased risk of cancer	Emissions from waste incineration and other combustion; discharge from chemical factories	zero
OC	Diquat	0.02	Cataracts	Runoff from herbicide use	0.02
OC	Endothall	0.1	Stomach and intestinal problems	Runoff from herbicide use	0.1
OC	Endrin	0.002	Liver problems	Residue of banned insecticide	0.002
OC	Epichlorohydrin	TT^h	Increased cancer risk, and over a long period of time, stomach problems	Discharge from industrial chemical factories; an impurity of some water treatment chemicals	zero
OC	Ethylbenzene	0.7	Liver or kidneys problems	Discharge from petroleum refineries	0.7
OC	Ethylene dibromide	0.00005	Problems with liver, stomach, reproductive system, or kidneys; increased risk of cancer	Discharge from petroleum refineries	zero
IOC	Fluoride	4.0	Bone disease (pain and tenderness of the bones); children may get mottled teeth	Water additive which promotes strong teeth; erosion of natural deposits; discharge from fertilizer and aluminum factories	4.0
M	Giardia lamblia	TT^c	Gastrointestinal illness (e.g., diarrhea, vomiting, cramps)	Human and animal fecal waste	zero
OC	Glyphosate	0.7	Kidney problems; reproductive difficulties	Runoff from herbicide use	0.7
DBP	Haloacetic acids (HAA5)	0.060	Increased risk of cancer	Byproduct of drinking water disinfection	n/a^f
OC	Heptachlor	0.0004	Liver damage; increased risk of cancer	Residue of banned termiticide	zero
OC	Heptachlor epoxide	0.0002	Liver damage; increased risk of cancer	Breakdown of heptachlor	zero
M	Heterotrophic plate count (HPC)	TT^c	HPC has no health effects; it is an analytic method used to measure the variety of bacteria that are common in water. The lower the concentration of bacteria in drinking water, the better maintained the water system is.	HPC measures a range of bacteria that are naturally present in the environment	n/a
OC	Hexachlorobenzene	0.001	Liver or kidney problems; reproductive difficulties; increased risk of cancer	Discharge from metal refineries and agricultural chemical factories	zero
OC	Hexachlorocyclopentadien^e	0.05	Kidney or stomach problems	Discharge from chemical factories	0.05
IOC	Lead	TT^g; action level=0.015	Infants and children: delays in physical or mental development; children could show slight deficits in attention span and learning abilities; adults: kidney problems; high blood pressure	Corrosion of household plumbing systems; erosion of natural deposits	zero
M	Legionella	TT^c	Legionnaire's Disease, a type of pneumonia	Found naturally in water; multiplies in heating systems	zero
OC	Lindane	0.0002	Liver or kidney problems	Runoff/leaching from insecticide used on cattle, lumber, gardens	0.0002
IOC	Mercury (inorganic)	0.002	Kidney damage	Erosion of natural deposits; discharge from refineries and factories; runoff from landfills and croplands	0.002
OC	Methoxychlor	0.04	Reproductive difficulties	Runoff/leaching from insecticide used on fruits, vegetables, alfalfa, livestock	0.04