critical concern is, to what extent have pesticides contaminated the environment? One way to address the question is to examine the occurrence of pesticides in surface water, groundwater, and air. An analysis of such occurrences in the United States has recently been reported under the auspices of the National Water-Quality Assessment Program of the US Geological Survey (USGS) (Majewski and Capel 1995, Barbash and Resek 1996, Larson et al. 1997). The reports compile and synthesize the many occurrence studies that have been conducted in the United States. In addition, the results of a recent data-collection program for groundwater conducted by USGS have been reported (Kolpin et al. 1998). The results from the latter study are used below to illustrate the occurrence of pesticides in the environment.

To examine pesticide occurrence in groundwater, samples were collected from recently recharged groundwater (generally less than 10 years old) in 41 land-use areas scattered throughout the United States. As reported in Kolpin et al. (1998), pesticides were detected at 54.4% of the 1034 sampling sites. Many pesticides were found; 39 of the 46 that were looked for were detected. Pesticide concentrations were generally low; more than 95% of the detected concentrations were less than 1 µg/L. Maximal contaminant concentrations developed by EPA for drinking water, which have been established for 25 of the 46 pesticides, were exceeded for only one pesticide at a single location. Pesticides were detected in both agricultural (56.4%) and urban (46.6%) locations.

Those results indicate that pesticides are widespread in shallow groundwater. As noted in Larson et al. (1997), they are also widespread in surface waters. The potential impact on human health is uncertain, given the relatively low concentrations and the lack of understanding of the impact of low concentrations on human health. In addition, more than one pesticide is often found at a site. For example, two or more pesticides were detected in shallow groundwater at 73% of the sites where pesticides were detected (Kolpin et al. 1998). The impact of chemical mixtures on human health is unclear. The widespread detection of pesticides in urban areas illustrates the fact that pesticide use and its associated problems are not confined to agricultural applications. Clearly, strategies for minimizing the impact of pesticides on human health and the environment will be successful only if all uses of pesticides are considered.

Ecological Problems: Impacts on Nontarget Organisms

The basic intent in the design of pesticides is to produce substances that are highly toxic to pest species, but much less toxic to the nonpest species so that there can be a useful margin of safety. The toxicity to each species is related to the characteristics of the substance and to the dose

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