National Study Design for Assessing Pesticides in Streams and Ground Water.

Robert J. Gilliom, U.S. Geological Survey, WRD, Placer Hall, 6000 J Street, Sacramento, California, 95819.

Concern about pesticides in the Nation's streams and ground water is one of the highest priority issues under study as part of the National Water Quality Assessment program. The national study design is based upon the use of a consistent study approach and sampling strategy in all 60 study units to assess the occurrence and levels of a broad range of pesticides in streams and shallow ground water associated with specific land-use and hydrologic settings. Land-use and hydrologic settings are chosen for study based upon a combination of local priorities in each study unit and national priorities for representing particular settings because of their regional extent or national importance. As a basis for national-level design, agricultural and urban land-use settings in all study units are classified in relation to national patterns in such factors as major crops and urban development densities. The land-use classifications, combined with hydrologic features, support a comparative assessment among occurrences and levels of pesticides in different settings. Major streams and aquifers affected by multiple settings are also assessed. Initial results from eight of the first 20 study units show contrasts and similarities in pesticides present in streams and ground water in different settings of the Nation and indicate how patterns will be interpreted in relation to present knowledge of pesticide use practices and the behavior of pesticides in particular hydrologic environments.