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Testing Groundwater and Surface Water

The National Water-Quality Assessment (NAWQA) Program, a major undertaking by the U.S. Geological Survey, is designed to examine the quality of both groundwater and surface water and is beginning to provide useful comparative data on water resources across the country -- including information on the occurrence of pesticides and nitrates. Under the program, 59 "study units," covering nearly 40 percent of the contiguous U.S. and encompassing almost 70 percent of water use, are being examined on a staggered research schedule reaching beyond the year 2000. Detailed study plans have been developed, including analyses of land uses in the watersheds that make up each unit and selection of numerous monitoring points.¹

Data collected thus far shows widespread occurrence of pesticides in streams and groundwater, with detection patterns generally correlated with land use and pesticide use patterns. This correlation is strong for streams and major rivers, but less so for groundwater, where the influence of unique local hydrogeologic conditions can be seen. Seasonal patterns can be discerned in many instances, with pesticide levels peaking after times of heavy usage and declining in the months following.

Exceptions to the common seasonal patterns appear to be for those surface waters where pesticide-contaminated "base flows" from groundwater contribute to the stream volumes in times of low rainfall. The study investigators also note that higher levels tend to last longer in reservoirs than in free-flowing streams. For urban streams, which showed the highest insecticide concentrations, these "seasonal pulses" likewise tended to last longer.²

As USGS staff explain the "most striking findings":

…[O]ne or more pesticides were found in almost every stream sample collected. More than 95 percent of the samples collected from streams and almost 50 percent of samples collected from wells contained at least one pesticide. … Major rivers, as well as agricultural and urban streams, had relatively high frequencies of detection.

Almost every sample of water and fish from streams and major rivers in all land use settings contained at least one of the pesticides that we measured. This means that, throughout the nation, almost every time and place that you observe a stream or river in a populated area you are looking at water that contains pesticides, inhabited by fish that contain pesticides. ³

USGS points out that the "good news" of these investigations has been that where federal drinking water standards or health advisory levels have been set (for a minority of the pesticides detected), these levels are exceeded infrequently on a year-round basis. Seasonal exceedances, lasting weeks or months, however, are not uncommon. What's more, the data indicate that "…most pesticides found in the environment usually occur as mixtures," with an average of seven pesticides found in each stream sample.

As Robert J. Gilliom explained in a March 1999 briefing on Capitol Hill, "Most stream samples with detections had 5 or more compounds and most ground-water samples with detections contained 2 or more pesticides." ⁴  Current EPA drinking water standards, however, address the health effects of single substances only and do not account for any possible additive or synergistic effects of chemical mixtures.  In fact, the existing standards also fail to consider the metabolites or breakdown products of the "parent" pesticide, despite the fact that, in many instances, detection of these metabolites is actually more common than detection of the pesticide itself.

In addition, the NAWQA data also show that there is "a high potential for pesticide impacts on aquatic life in some streams." ³ Since EPA water quality criteria to protect aquatic organisms have been established for relatively few compounds, the survey staff compiled aquatic life water-quality guidelines developed by EPA and Canada. They compared monitoring data to those standards and noted that guidelines "were exceeded for variable lengths of time in more than half of the urban and agricultural streams sampled." ⁴ More than 70 percent of urban areas studied had concentrations of one or more insecticides that exceeded a guideline.⁴ Notes USGS staff,

Long-term exposure to low-level mixtures of pesticide compounds, punctuated with seasonal pulses of high concentrations, is an exposure pattern that may not be adequately accounted for in present criteria. ³

As this ambitious NAWQA program moves forward, more areas will be studied and the number of data points from which to draw conclusions will increase substantially. Hopefully, the end result will be a better understanding of current patterns and indications of needed policy changes to decrease the impact of pesticides on water resources.

1.  Leahy, P.P. and T.H. Thompson, U.S. Geological Survey, The National Water-Quality Assessment Program, Open-File Report 94-70, available at <http://water.usgs.gov/nawqa/NAWQA.OFR94-70.html>.

2.  US Geological Survey staff have released a number of papers and made numerous presentations on the findings of the NAWQA program.  Information on individual study areas (or "study units") is available through the USGS NAWQA website at <http://water.usgs.gov/nawqa/nawqamap.html>.  In addition, the Survey periodically updates new information from its Pesticide National Synthesis Project.  Much of this information, including data summaries and maps of pesticide usage, can be found at <http://water.wr.usgs.gov/pnsp/>.  Information for this section of the report relies heavily on these sources, particularly "Pesticides in the Nation's Water Resources," a briefing by Robert J. Gilliom on March 19,1999 for the Water Environment Federation; "Pesticides in Surface and Ground Water of the United States:Summary of Results of the National Water Quality Assessment Program (NAWQA);"  Barbash, Jack E., et al, "Distribution of Major Herbicides in Ground Water of the United States," Water-Resources Investigations Report 98-4245, 1999; and Gilliom, Robert J., et al, "Testing Water Quality for Pesticide Pollution, Environmental Science & Technology, vol 33, issue 7, 1999.

3. Gilliom, Robert J., et al, "Testing Water Quality for Pesticide Pollution," Environmental Science & Technology, vol 33, issue 7, 1999 available at <http://pubs.acs.org/hotartcl/est/99/apr/test.html>.

4.  "Pesticides in the Nation's Water Resources," a briefing by Robert J. Gilliom on March 19,1999 for the Water Environment Federation at <http://water.wr.usgs.gov/pnsp/present/water/>.

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