Agricultural Run-Off as a Source of Drinking Water Contamination

Agricultural run-off from crops is a a concern because it may contain fertilizer and pesticides. It enters our water sources by seeping through the soil to ground water or entering streams as surface run-off.

Populations in agricultural areas are at highest risk for exposure to these contaminants due to a number of factors:

Approximately 95% of rural populations obtain their drinking water from  private water supplies, usually wells.

Private supplies are not regulated under the Safe Drinking Water Act and do not have to meet the health based standards under the Act. Therefore, the water sources are not monitored for maximum contaminant levels as public water systems are.

Because agricultural run-off is a diffuse  source of pollution, it is  hard to control. It is  the number one catergory of  non-point source pollution in rivers and lakes (1). Non-point source is most simply defined in contrast to point source pollution,which comes from a specific place such as a pipe or smokestack. Contaminant concentrations arising from industrial or other point sources can easily be measured at the 'end of the pipe'. Non-point source pollution is difficult to  assess because the source is  spread over a large area, as in agricultural or mining regions.

Pesticides are a threat to health due to their inherent toxicity. There are many types of pesticides and exposure depends upon many factors. For an in depth discussion of factors surrounding pesticide exposure and human health ( provided by the Iowa State Extension at Purdue), click here.

Nitrates are the major constituent of fertilizer which poses a  health threat, particularly in infants under 6 months of age.  Nitrite, converted from nitrate in the infant's digestive tract, binds to hemoglobin in the blood, preventing oxygen transport..Excessive amounts of  Phosphate  can degrade water quality. Nitrates and phosphates cause eutrophication of surface waters such as lakes, streams and rivers. Eutrophication is a natural process of enrichment of a water body by the addition of plant nutrients. This causes excessive growth of algae and results in oxygen depletion. Many species cannot survive under these conditions and waters begin to smell putrid from large quantities of decaying matter undergoing anaerobic degradation

Agricultural land is a source of drinking water contamination largely due to current agricultural practices. These practices include:

Over fertilization. This occurs as a result of the common practice of combining the use of fertilizer and manure, and is due to underestimating the nutrient value of manure.

Crop specialization. This practice requires the use of larger quantities of pesticides in order to maintain high productivity of a single type of crop.

Lack of crop rotation and cover crops. This practice results in land erosion and use of more fertilizer and pesticides. Cover crops prevent land erosion because the roots of plants create a net which hold the soil in place during precipitation and irrigation. Some types of cover crops store nitrogen in their root systems. This can be tilled into the land and used as a source of nitrogen fertilizer for future crops. Crop rotation is the practice of alternating the type of crop planted. It enhances soil quality,  allowing the use of less fertilizer. It also helps limit pest infestation, which results in the use of less pesticides.

There are numerous alternative practices which can decrease the impacts the agricultural industry has on our drinking water resources. For example, catchment ponds can be used to treat pesticide run-off and vegetative buffer zones around canals can extract nitrogenous waste and phosphates. In order to change common practice in the industry new approaches must be shown to be economically sound and effective.

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References

1. EPA, Office of Water. Nonpoint Source Pollution; The Nation's Largest Water Quality Problem, Nonpoint Source Pointer No. 1; EPA 841-f-96-004A. URL: http//www.epa.gov/OWOW/NPS/facts/point1.htm

This page was prepared by T.L. Pedersen , UCD EXTOXNET FAQ Team.
June 1997