Problem: Widespread Contamination

Low levels of pesticides have been found in surface water in every region of the United States. Forty-three states have reported detecting traces of at least one of 143 pesticides and 21 by-products in groundwater (USGS, 1995). Contamination is seasonal, depending on the timing of pesticide applications and the level of runoff based on precipitation levels. In many streams, the annual average concentrations of pesticides are within regulatory standards for drinking water but may exceed water quality standards during peak usage times. However, quantifying these contamination levels is difficult. The difficulties in quantifying the threat of pesticides to water resources is explained by the USGS:

Our ability to assess the significance of pesticides in surface waters is limited by several factors.

• First, water-quality criteria have not been established for most pesticides and pesticide transformation products, and existing criteria may be revised as more is learned about the toxicity of these compounds.
• Second, criteria are based on tests with individual pesticides and do not account for possible cumulative effects if several different pesticides are present.
• Finally, many pesticides and most transformation products have not been widely monitored in surface waters.

These factors, and the lack of data on long-term trends, show significant gaps in our understanding of the extent and significance of pesticide contamination on surface waters. The results of this analysis indicate a need for long-term monitoring studies in which a consistent study design is used and more of the currently used pesticides and their transformation products are targeted (USGS, 1995).