ABSTRACT
Pesticide data for ground water sampled across the United States between 1993 and 2003 by the U.S. Geological Survey National Water-Quality Assessment Program were evaluated for trends in detection frequency and concentration. The data analysis evaluated samples collected from a total of 362 wells located in 12 networks characterizing shallow ground water in agricultural areas and 6 networks characterizing the drinking-water resource in areas of variable land use. Each well network was sampled once during 1993-95 and once during 2001-03. The networks provide a broad overview of conditions across a wide range of hydrogeologic settings and in major agricultural areas that vary in dominant crop type and pesticide use. Of more than 100 pesticide compounds analyzed only 6 compounds were detected in ground water from at least 10 wells during both sampling events. These compounds were the triazine herbicides atrazine, simazine, and prometon; the acetanilide herbicide metolachlor; the urea herbicide tebuthiuron; and an atrazine degradate, deethylatrazine (DEA). Most detections of these compounds were at concentrations less than 0.12 micrograms per liter. Changes in concentrations between sampling events typically were less than 0.02 micrograms per liter. Data analysis incorporating information on changes in laboratory performance (as assessed through laboratory spikes) indicated that concentrations of atrazine, DEA, and prometon decreased significantly between sampling events. Trends in atrazine concentrations were investigated for correlations with potential explanatory factors. The best correlations were with factors indicating presence of recently infiltrated water.

LINKS

• USGS Report: Pesticides in the Nation's Streams and Ground Water, 1992-2001
• USGS National Water-Quality Assessment (NAWQA) Program