Pesticides in Drinking Water
The National Water-Quality Assessment Program (NAWQA) and the U.S. Environmental Protection Agency recently completed a two-year pilot monitoring program for assessment of pesticide concentrations in drinking water. The program was designed as a first step toward a long-term goal of characterizing human exposure to pesticide residues in drinking water derived from surface-water sources. Reservoirs were selected for sampling because they are important sources of drinking water and because the integrate pesticide loadings within their watersheds.
Twelve water-supply reservoirs and community water systems were sampled, ranging in size from 120 to 92,600 acre-foot normal capacity, with contributing watersheds ranging from about 3 to 785 square miles. The sites are located in California, Indiana, Louisiana, Missouri, New York, North Carolina, Ohio, Oklahoma, Pennsylvania, South Carolina, South Dakota and Texas. Samples were collected from the raw water intake and the finished water tap located at the entry point to the distribution system. Each site generally was sampled every quarter, with bi-weekly sample collection during a four-month period coincident with extensive pesticide applications.
Analysis included 186 pesticides and degradation products. A total of 116 compounds were detected in at least one sample of raw water or of finished water. Many of the compounds, however, were detected in fewer than 5 percent of the samples and at concentrations very near the reporting level, which ranged from 0.001 micrograms to about 0.1 micrograms per liter. Detection does not necessarily translate to risk. The NAWQA program measures chemicals at very low concentrations, often 10 to 1,000 times lower than EPA standards and health advisories. The low-level sampling is used to detect and evaluate emerging issues, as well as track contaminant levels over time.
Widely used herbicides were the most frequently detected compounds in raw and finished water (detected in 36% to 96% of raw water samples and in 19% to 96% of finished water samples). None of the herbicides was detected in finished water at a concentration greater than a national drinking water criterion.
Insecticides occurred less frequently than herbicides, but also were detected in both raw and finished water.