The National Water-Quality Assessment Project of the National Water-Quality Program of the U.S. Geological Survey (USGS) monitors rivers and streams in the U.S. for pesticides, including many pesticides commonly used in agriculture. Pesticide trend analysis in surface water differs from other common water-quality trend analyses, such as trends in major ions and nutrients, in that the seasonal hydrologic cycle has little effect on most pesticide concentrations. Pesticides have a strong seasonal response to use patterns and there may be one or more general periods of application depending on the region, crop, and target pests.  In addition, pesticide concentrations tend to have high numbers of concentrations below laboratory reporting levels and complex relations between streamflow and concentration as compared to other more naturally occurring water-quality constituents. These differences created the need for a specialized trend model, SEAWAVE-Q, which models a seasonal wave (SEAWAVE) and adjusts for streamflow (Q). The seasonal wave represents intra-annual variability in concentration and depends on the timing of application, the timing of maximum concentrations in surface-water samples, and the rate of decline in sample concentrations (a model half-life). The trend model assumes a linear trend across the period analyzed. For short trend periods, this assumption of a linear trend is adequate. However, as the period of record for analysis becomes longer (the USGS began sampling for pesticides in 1992), the assumption of linearity is problematic because of changes in pesticide regulation and use, some of which can be abrupt. Recently, the SEAWAVE-Q model has had an option added for long-trend periods that uses restricted cubic splines. This option allows for more flexibility in the time trend. Results with the new method will be compared to the old method for selected agricultural pesticides and sites across the U.S.

Modeling agricultural pesticide concentration and load trends in U.S. streams and rivers

256th National Meeting (2018)

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