Potential risks to humans and aquatic life implied by NAWQA pesticide findings can be only partially addressed by comparison to established standards and guidelines for several reasons. First, many pesticides and their breakdown products do not have standards or guidelines. Second, streams and ground water in basins with significant agricultural or urban development almost always contain complex mixtures of pesticides and nutrients. More than one-half of all stream samples in the study contained five or more pesticides, and nearly one-fourth of all groundwater samples contained two or more. Existing water quality standards and guidelines typically are based on tests of individual concentrations and do not account for commonly found mixtures or byproducts of chemical degradation. Third, standards and guidelines usually are based on long-term exposure to constant concentrations of contaminants, rather than lengthy periods of low concentrations punctuated by brief, seasonal pulses of high concentrations that emerged in almost every basin sampled.And finally, many possible impacts on aquatic organisms have not been tested. Potential effects on reproductive, nervous, and immune systems, as well as on chemically sensitive individuals, are not yet well understood. For example, many of the 20 most frequently detected pesticides in this study, although detected at relatively low concentrations, are suspected endocrine disrupters that have potential to affect reproduction or development of aquatic organisms or wildlife by interfering with natural hormones. A top priority for future research should be to enhance the understanding of risk to humans and aquatic life by collecting information that reflects these complexities in contaminant occurrence and the nature of exposure and potential effects.

Nutrients generally do not pose a health risk for residents whose drinking water comes from streams or from aquifers buried relatively deep beneath the land. Health risks from elevated nitrate increase in those aquifers located in geologic settings, such as in sand, gravel, or karst (weathered carbonate rock), that enable rapid movement of water. The most prevalent nitrate contamination found in this study was in shallow ground water (less than 100 feet below land surface) beneath agricultural and urban areas. Because of its proximity to the land surface, the shallow groundwater is younger and more vulnerable to contamination from human activities than deep groundwater. For example, about 15 percent of all shallow groundwater sampled beneath agricultural and urban areas exceeded the drinking-water standard for nitrate. These findings are of particular importance in rural areas where shallow water commonly is used for domestic supply. Homeowners may not be aware of possible contamination because domestic wells are not monitored regularly, as is required for large public-supply wells. In addition, many homeowners in newly residential areas that rely on domestic wells may not know that chemicals leached from previously farmed land can remain in shallow, slow-moving groundwater for decades.