| Why the Concern about Agricultural Contamination in Groundwater? | ||||||||||||||||||||||||||||||||||||||||
| Over 90% of rural Americans and half of all Americans rely on groundwater as their source of drinking water. In addition, more than one-third of all irrigation water in the United States comes from groundwater. Although groundwater has been valued as a source of pure water, many groundwater supplies have become contaminated. Contamination is found in both rural and urban areas; agriculture serves as a main source of rural groundwater contamination. | ||||||||||||||||||||||||||||||||||||||||
Nitrate and pesticides are the major agricultural groundwater contaminants. Low nitrate levels occur naturally in some groundwater. Additional nitrate forms in soil from the nitrogen (N) in fertilizers, animal manures, and N-fixing plants such as legumes. Nitrate is highly soluble in water, so it can leach readily through well-drained soils to the water table below. Too much N fertilizer or animal manure, improper timing of fertilizer applications, or excessive irrigation can lead to nitrate groundwater pollution. Septic systems are another source of N contamination. They may be the primary source of groundwater nitrate in densely developed suburban areas. Pesticides are much less common contaminants than nitrate, but they have been found in groundwater in nearly all 50 states. The properties of the pesticide, type of soil, management of the crop, and local climate all influence the risk of pesticides leaching to groundwater. Nitrate and pesticide pollution of groundwater may come from both normal field practices and from point sources such as leaks, spills, and direct well contamination. | ||||||||||||||||||||||||||||||||||||||||
Nitrates and pesticides are health risks even at low concentrations in drinking water. Infants who consume too much nitrate can develop a blood disorder called methemoglobinemia (also called blue-baby syndrome). Nitrate converts to nitrite in infant digestive systems. Then in the blood, nitrite changes the hemoglobin, preventing it from carrying oxygen. The infant suffers oxygen deprivation, and in severe cases, death. Methemoglobinemia is a problem only in the first 6 months of life, and often in infants who already have diarrhea. Nitrate levels as low as 10 parts per million (ppm) in drinking water have caused methemoglobinemia. Very few cases of the disease have been reported in the United States in recent years, but it is likely that most cases are not reported. To protect infants from drinking nitrate-polluted water, the U.S. Environmental Protection Agency (US-EPA) drinking water standard is 10 ppm for nitrate-N. Although humans more than 6 months old are not affected by nitrate in drinking water, ruminant animals such as cattle and sheep are susceptible. High pesticide concentrations can cause a variety of acute (immediate) toxic effects, depending on the type of pesticide. Pesticide concentrations in groundwater are seldom, if ever, high enough to cause acute toxicity. The main concern with groundwater is over the cumulative, chronic health effects of long-term, low-dose pesticide exposure. The US-EPA is setting drinking water standards for pesticides to reduce the risk of unsafe long-term exposures. These standards are called maximum contaminant levels (MCL). They are based on low-dose toxicity studies, but include economic factors as well. Drinking water is not the only concern. Groundwater also contributes to the recharge of lakes, streams, and wetlands. In some cases, these surface water ecosystems may be affected by the recharge from contaminated groundwater. Groundwater standards recently adopted by the Washington Department of Ecology (Ecology) are designed to protect all of these beneficial uses of groundwater, not just its use as drinking water. | ||||||||||||||||||||||||||||||||||||||||
| Some pesticides are probable carcinogens. Extended exposure to low doses of these pesticides in drinking water may increase the risk of cancer. Long-term use of water containing a carcinogenic pesticide at the US-EPA's MCL level may increase the cancer risk by as much as 1 in 10,000 for some pesticides. For many pesticides the risk is much less. Although there is some evidence linking nitrate in meats with increased cancer risk, no link has been found between groundwater nitrate and cancer. | ||||||||||||||||||||||||||||||||||||||||
In 1990, the US-EPA completed a nationwide survey of the frequency and concentration of nitrate and pesticides in drinking water wells. Over 100 pesticides, 25 pesticide breakdown products, and nitrogen (N) as nitrate and nitrite were evaluated in about 1,300 wells. Based on this study, the US-EPA estimates that over half of the drinking
water wells in the United States have elevated nitrate levels (Fig. 1).
Nitrate in most of the wells is far below the 10 ppm (parts per million)
drinking water standard, and many wells are at or near background levels
that occur naturally. The survey predicts that about 1,100 of the 95,000
community wells and 250,000 of the 10.5 million rural domestic wells in
the United States have nitrate levels above the MCL of 10 ppm (Fig. 1).
The US-EPA also found pesticides widespread in groundwater, although in far fewer wells (Fig. 2). A few pesticides caused most of the contamination. The US-EPA estimated that about 10% (10,000) of community wells and 4% (450,000) of domestic wells contained detectable pesticides. Although most of the pesticide levels are very low, the survey predicts that over 60,000 wells (about 0.5% of United States drinking water wells) may have pesticides above the US-EPA MCL.
Washington State has pesticide- and nitrate-contaminated groundwater. An Ecology study of 81 wells in Franklin, Whatcom, and Yakima counties found nitrate in most wells (Table 1). The most serious pesticide contamination was caused by soil fumigants (DCP, EDB, and DBCP) that are no longer registered for use. They had leached to groundwater and remained there for several years. The study focused on agricultural areas known to be vulnerable to groundwater contamination. While the results cannot be generalized throughout the state, they do indicate the potential for groundwater contamination in other vulnerable areas. | ||||||||||||||||||||||||||||||||||||||||
The risk of groundwater contamination is highest where nitrogen or pesticide use is high and where groundwater is vulnerable. Vulnerable areas are those where nitrogen or pesticides on or near the surface can move readily into the groundwater. Examples include sites with shallow, unprotected groundwater below very permeable and often sandy soils. Shallow, poorly constructed, or poorly maintained wells also increase the potential for contaminants to move into the groundwater.
Nitrate contamination tends to increase with heavy fertilizer or manure
applications and in areas with many septic systems. The risk of pesticide
contamination is high with leachable pesticides and heavy use, or where
spillage occurs during storage, mixing, and equipment clean up. | ||||||||||||||||||||||||||||||||||||||||
| Knowledge of pesticide and nitrate contamination of groundwater should be a call to action, but not a cause for panic. Most wells do not contain pesticides. Although most wells do have nitrate, the level will not affect human health in most cases. For the many wells that are contaminated, however, there is no easy solution. No ready technology exists to remove nitrate or pesticides from groundwater, and natural groundwater cleansing may take years or decades. Nitrate removal from a private water well can be complicated and expensive, although carbon filtration can remove some pesticides. Unfortunately, in-home water treatment requires regular attention by the residents. The best long-term solutions for a polluted well are to import water from elsewhere or to dig a deeper well. Even replacing the well does not guarantee cleaner water. Washington's best course is aggressive programs of prevention. | ||||||||||||||||||||||||||||||||||||||||
| The key to preventing nitrate pollution is matching nitrogen fertilizer
applications to crop need. For example, set realistic crop yield goals and
apply only enough nitrogen to meet those goals. Apply the nitrogen when
the crop needs it. Similarly, apply only enough manure to meet crop N needs.
Avoid using soil simply as a manure disposal system. For pesticides, the
keys are applying only those pesticides that are required, choosing lower-risk
pesticides when possible, and careful pesticide handling and application.
Integrated pest management can reduce the need to use pesticides. While
these approaches seem simple, they require high-quality management to carry
out. Ecology is developing an Agricultural Pesticide and Nutrient Management Strategy that will provide direction for future activities to protect groundwater. Other fact sheets in this series have more detail on managing nitrate and pesticides to prevent contamination. Additional research and education are still needed to meet the challenge of halting agricultural contamination of Washington's vital groundwater resource. | ||||||||||||||||||||||||||||||||||||||||
| Partial funding for publications in this series on Groundwater Protection was obtained through U.S. Environmental Protection Agency nonpoint source pollution grants administered by the Washington State Department of Ecology. | ||||||||||||||||||||||||||||||||||||||||
By Craig G. Cogger, Ph.D., Washington State University Extension Soil Scientist, WSU Puyallup Research and Extension Center; and Craig B. MacConnell, M.S., WSU Whatcom County Cooperative Extension Chair and Horticultural Agent. The authors acknowledge the contributions of Christopher F. Feise, Ph.D., Washington State University Extension Western Washington Water Quality Coordinator and Groundwater Fact Sheet Project Coordinator, WSU—Puyallup Research and Extension Center; John H. Pedersen, Ph.D., P.E., Consulting Technical Editor and retired manager of the Midwest Plan Service, Iowa State University, Ames; and Ronald E. Hermanson, Ph.D., P.E., WSU Extension Agricultural Engineer and Water Quality Project Leader, WSU—Pullman. Issued by Washington State University Cooperative Extension and the U.S. Department of Agriculture in furtherance of the Acts of May 8 and June 30, 1914. Cooperative Extension programs and policies are consistent with federal and state laws and regulations on nondiscrimination regarding race, color, gender, national origin, religion, gender, age, disability, and sexual orientation. Evidence of noncompliance may be reported through your local Cooperative Extension office. Trade names have been used to simplify information; no endorsement is intended. Published August 1991. Reprinted June 1994. Subject code 376. A. EB1632 | ||||||||||||||||||||||||||||||||||||||||
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