CCYK NAWQA Study - Publications

Pesticides in public supply wells of Washington State

By Sarah J. Ryker and Alex K. (Sandy) Williamson, U.S. Geological Survey, Tacoma, Washington


The Federal Safe Drinking Water Act requires quarterly pesticide monitoring of Washington's 5,400 Class A public water supply wells, beginning in 1995. Forty percent of Washington's Class A public water systems have only 5-100 connections. For these systems, the cost per household could reach $70 per year for each well in the water system.

In 1994, the U.S. Geological Survey (USGS) cooperated with the Washington State Department of Health (WDOH) in the first statewide assessment of Washington public supply well vulnerability to pesticides. WDOH contract laboratories sampled 1,326 public supply wells across Washington State. The USGS analyzed 220 duplicate samples in the Puget Sound Basin and the Central Columbia Plateau.

The sampling results were used to derive risk factors for pesticide detection. These risk factors were applied to public supply wells across Washington State. Low- and moderate-risk wells were granted full or partial waivers from quarterly monitoring.

Most pesticide detections were at very low concentrations, well below the U.S. Environmental Protection Agency (EPA) maximum contaminant level for drinking water.

Rates of pesticide detection in 4 sampling regions:

  • Puget Sound Basin
    WDOH: 5%
    USGS: 20%
  • Yakima Basin
    WDOH: 6%
  • Central Columbia Plateau
    WDOH: 16%
    USGS: 55%
  • Other Washington
    WDOH: 7%

CITATION: Ryker, S.J., and Williamson, A.K., 1996, Pesticides in public supply wells of Washington State, in Agriculture and Water Quality in the Pacific Northwest Conference Proceedings, Yakima, Washington, October 1996, p. 19.

Central Columbia Plateau - Yakima River Basin NAWQA Study
NAWQA Program Bibliography
Water Resources of Washington State
U.S. Geological Survey
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Last modified: Wed Mar 29 12:10:59 2000