Richelle presented the 2003 Darcy lecture, which included the results of investigation and modelling of the migration of agricultural chemicals to groundwater. The pathways by which agricultural chemicals (nonpoint pollutants) move from fields to streams vary in time, exemplified by work from the semi-arid Palouse River watershed in eastern Washington. Precipitation is strongly seasonal, occurring primarily in Nov-June in this region, and the major land use in the region is dryland agriculture. We observed high in-stream nitrogen (predominantly nitrate) and pesticide concentrations associated with high runoff. Soil pore water samples within agricultural fields show that both the pesticides and nitrate can be transported to shallow groundwater over relatively short time frames (weeks) even though average annual recharge is very low (~5 cm/year). Time-series observations of soil pore water and stream chemistry have allowed us to begin to discern the flowpath and reservoir from which the agricultural chemicals are derived. We use environmental tracers to separate the stream hydrograph for source components and to model stream pollutant concentrations at multiple watershed scales (~6 to 5000 ha drainage areas). This work suggests that a majority of annual nitrate discharge is transported to streams via the groundwater pathway while the converse is true for the pesticides. Ultimately, this work will provide input useful to sustainable water-resource management.
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