After rainfall events or irrigation, some water will evaporate or be absorbed by plants, while the rest will leave the fields either by surface runoff or by infiltration into aquifers. This runoff and infiltration is often contaminated with fertilizers and pesticides that are used in agricultural production. Contaminated water pollutes surface water bodies as well as groundwater aquifers, reducing the quality of water available for future uses (USGS, 2001b).
Agricultural pollution falls into the general category of non-pointsource pollution. As opposed to spatially concentrated pointsource pollution such as that generated by paper mills or sewage treatment facilities, non-pointsource pollution is geographically disperse. Because the pollution discharge occurs over a large area, non-pointsource pollution is generally more difficult to manage and treat than its pointsource counterpart. In addition, the effects of this pollution may take many years to notice, since the environmental residency and transport time for some toxins can be on the order of decades (USGS, 2001b).
The U.S. Environmental Protection Agency has declared non-pointsource pollution to be the leading cause of water pollution in the United States, with agriculture as the leading contributor of non-pointsource pollutants into rivers and lakes (EPA, 2001b). Agriculture is also one of the least regulated dischargers of effluents, primarily due to the difficult nature of quantifying non-pointsource pollution.
The problem of agricultural pollution pertains both to ground and surface water. According to the Environmental Protection Agency's Fact Sheet for 1994, ``... agriculture is the leading source of impairment in the nation's rivers, affecting 72% of the impaired river miles ... States attribute 56% of problems in lakes to agriculture'' (EPA, 1994, 2). The Office of Technology Assessment agrees, as stated in its document Environmental Tools for Policy Makers: ``[A]griculture is thought to be the single largest source of remaining river and lake water quality problems'' (OTA, 1995, 19).
With regards to groundwater, according to the 1994 EPA report, the most frequently cited pollutants are (EPA, 1994, 4):
Of these, the first and the last pertain directly to agriculture. This report analyzes analyze these two pollutants separately, with an emphasis on the sources, effects, and the potential for pollution removal or prevention.