Pesticides in Ground Waters of the United States: An Overview of Current Understanding.

J.E. Barbash, U.S. Geological Survey, 345 Middlefield Rd., Mail Stop 470, Menlo Park, CA 94025.

A comprehensive review of published information products in the subsurface (ground water and vadose zone), conducted as part of the USGS' National Water Quality Assessment (NAWQA), indicates that pesticides from every chemical class of these compounds have been detected in ground waters of the United States. Many of these compounds are commonly present at low concentrations in ground water beneath agricultural land. Little information is available on their occurrence beneath non-agricultural land, despite use in such areas (on lawns, golf courses, rights of way, etc.) that is often comparable to, or greater than, agricultural use. There is currently insufficient information to provide either a statistically representative view of pesticide occurrence in ground water across the United States, or an indication of any consistent changes in the severity or extent of the contamination over the past three decades. This is due largely to differences in analytical detection limits, well selection procedures, and other design features among studies conducted in different areas or at different times. Current approaches for distinguishing between pesticide contamination from "point sources" and from "nonpoint sources" appear to be inadequate for the task. Among the variety of natural and anthropogenic factors examined, those that appear to be most strongly associated with pesticide contamination of ground water are the depth, construction, and age of the sampled wells, the amount of recharge, and the depth of tillage. The approaches most commonly employed for predicting pesticide distributions in the subsurface, using either computer simulations, indicator solutes (e.g., nitrate or tritium), or ground-water vulnerability assessments, appear to provide unreliable predictions of pesticide occurrence in ground water. Such difficulties may arise in part from a general failure to account for the preferential transport of pesticides in the subsurface.