Pesticide and nitrate transport in an agriculturally influenced stream in Indiana

Agrochemicals can be transported from agricultural fields into streams where they might have adverse effects on water quality and ecosystems. Three enrichment experiments were conducted in a central Indiana stream to quantify pesticide and nitrogen transport dynamics. In an enrichment experiment, a compound solution is added at a constant rate into a stream to increase compound background concentration. A conservative tracer (e.g., bromide) is added to determine discharge. Water and sediment samples are taken at several locations downstream to measure uptake metrics. We assessed transport of nitrate, atrazine, metolachlor, and carbaryl through direct measurement of uptake length (S _w ), uptake velocity (V _f ), and areal uptake (U). S _w measures the distance traveled by a nutrient along the stream reach. V _f measures the velocity a nutrient moves from the water column to immobilization sites. U represents the amount of nutrient immobilized in an area of streambed per unit of time. S _w varied less than one order of magnitude across pesticides. The highest S _w for atrazine suggests greater transport to downstream ecosystems. Across compounds, pesticide S _w was longest in August relative to October and July. V _f varied less than one order of magnitude across pesticides with the highest V _f for metolachlor. U varied three orders of magnitude across pesticides with the highest U associate with sediment-bound carbaryl. Increasing nitrate S _w suggests a lower nitrate demand of biota in this stream. Overall, pesticide transport was best predicted by compound solubility which can complement and improve models of pesticide abundance used by water quality programs and risk assessments.