Pesticides

An Initial Attempt to Evaluate Pesticide Levels in Sligo Creek

This is an attempt to evaluate data from water quality sampling of Sligo Creek by the U.S. Geological Survey on August 23, 1994. The sample was done as part of the study Water Quality in the Potomac River Basin, Maryland, Pennsylvania, Virginia, West Virginia, and the District of Columbia, 1992-96 (USGS Circular 1166), a report released in June 1998. The study found that “a wider variety of pesticides was detected more frequently in small streams than in ground-water samples,” and that “concentrations of pesticides in streams and ground water of the Potomac River Basin are usually not threatening to human health or most ecosystems, based on current standards and understanding.”

However, the lead author of a more recent study released in March 2006 aggregating the findings of the Potomac River basin study with studies of river basins around the country, Pesticides in the Nation’s Streams and Ground Water, 1992–2001 (USGS Circular 1291), which stated that “the potential effects of contaminant mixtures on people, aquatic life, and fish-eating wildlife are still poorly understood and most toxicity information, as well as the water-quality benchmarks used in this study, has been developed for individual chemicals. The common occurrence of pesticide mixtures, particularly in streams, means that the total combined toxicity of pesticides in water, sediment, and fish may be greater than that of any single pesticide compound that is present. Studies of the effects of mixtures are still in the early stages, and it may take years for researchers to attain major advances in understanding the actual potential for effects.”

In the table below, pesticide levels found in Sligo Creek are compared with those in Paint Branch, Cabin John Creek, Accotink Creek, and the standards for pesticides given by the Environmental Protection Agency or other sources when available. Chlorpyrifos is the pesticide which had a higher level in Sligo Creek than the other three reference sites used here, the one time in 1994 Sligo was sampled. The Atrazine level also was comparatively high. None of the pesticides tested for had levels above any known standard for drinking water or aquatic life, though as mentioned little is known about the effects of mixtures of pesticides. If you have any further information please contact [email protected].

Sources of Standards

¹National Primary Drinking Water Standard

²National Secondary Drinking Water Standard

³2004 Edition of the Drinking Water Standards and Health Advisories

⁴Summary of Compound Detections and Concentrations
(A chapter from USGS Circular 1166: Water Quality in the Potomac River Basin, Maryland, Pennsylvania, Virginia, West Virginia, and the District of Columbia, 1992-96)

⁵Maryland State Numerical Criteria for Toxic Substances in Surface Waters

⁶EPA: Pesticides – Specific Chemical Fact Sheets

⁷Water Quality Benchmarks
(Appendix 3 from USGS Circular 1291: Pesticides in the Nation’s Streams and Ground Water, 1992–2001 (published March 2006)

⁸EPA: Aquatic Life Water Quality Criteria

⁹EPA: Current National Recommended Water Quality Criteria

Other Water Quality Parameters Measured