ESTIMATED DISSOLVED AGRICULTURAL MASS DISCHARGES USING ENVIRONMENTAL TRACERS IN A SEMI-ARID DRYLAND AGRICULTURAL WATERSHED

The goal of this research is to use environmental tracers to quantify the contributions of subsurface and surface runoff to estimate the loading of non-point source pollutants to rivers at multiple scales of study (field to catchment). Chemical hydrograph separation (HS) uses two conservative environmental tracers to quantify water flow into three components with time-dependent signatures: overland flow, soil-water, and groundwater flowpaths.

The study area is the Missouri Flat Creek 7,000 ha watershed located in a semi-arid dryland agricultural area near Pullman, WA. Ground and surface water samples have been collected bi-weekly, and have been analyzed for electrical conductivity, silica, turbidity, nitrate, silica, oxygen-18, and the two pesticides, lindane and triallate.

Ephemeral stream (edge-of-field) pesticide concentrations exhibit a first-order decline over two years following application for each of the two moderately hydrophobic pesticides studied. Nitrate concentrations are also greatest during the wet winter months following fall application associated with planting of winter crops. Using the HS results, and interpolated edge-of-field chemical concentrations for each flow component weighted by application information, we estimate pesticide and nitrate mass discharges on a continuous basis for various catchment areas. Pesticide and nitrate mass discharges so determined provide reasonable estimates of seasonal trend, magnitude and variability. Continuous pesticide mass discharge estimates overpredict observed lindane and triallate mass discharges by a factor ranging from approximately 1.4 to 2.5 and 2.0 and 3.8, respectively for the catchment areas modeled. Loss processes (including volatilization, sorption, and transformation) and/or non-distinct flowpath chemistry used in the HS may account for model overprediction. Additional work is being done to quantify these and other limitations of the HS method.

Chemical mass discharges estimated by the environmental tracer approach will be compared to those obtained from more traditional methods (e.g. interpolation through regression against water discharge). A combination of these two methods has the potential to provide important information on the location of in-stream processes affecting pollutant discharge.