Processes controlling the episodic steamwater transport of atrazine and other agrichemicals in an agricultural watershed

Episodic streamwater transport of atrazine (a common agricultural herbicide ) and nutrients has been observed throughout agricultural watersheds in the United States and poses a serious threat to the quality of its water resou rces. Catchment-scale atrazine and nutrient transport processes after agric ultural application are still poorly understood., and predicting episodic s treamwater composition remains an elusive goal. We instrumented a 1.2-km(2) agricultural catchment near Harrisonburg, Virginia, and examined streamwat er. overland flow, soil water, groundwater, and rainfall during the summer of 1998. Storm chemographs demonstrated different patterns for constituents derived primarily from weathering (silica and calcium), compared to consti tuents derived primarily from early spring land applications (nitrate, atra zine. DOC, potassium, chloride, and sulfate). During storms, the concentrat ions of silica and calcium decreased, the atrazine response was variable, a nd the concentrations of nitrate, DOC, potassium, chloride, and sulfate inc reased; the elevated nitrate signal lagged several hours behind the other e levated constituents. Graphical and statistical analyses indicated a relati vely stable spring-fed baseflow was modified by a mixture of overland flow and soil water. A rapid, short-duration overland-flow pulse dominated the s treamflow early in the event and contributed most of the potassium, DOC, ch loride, suspended sediment, and atrazine. A longer-duration soil-water puls e dominated the streamflow later in the event and contributed the nitrate a s well as additional potassium. DOC. sulfate, and atrazine. The contributio ns to the episodic streamflow were quantified using a flushing model in whi ch overland-flow and soil-water concentrations decreased exponentially with time during an episode. Flushing time constants for the overland-flow and soil-water reservoirs were calculated on a storm-by-storm basis using separ ate tracers for each time-variable reservoir. Initial component concentrati ons were estimated through regression analyses. Mass-balance calculations w ere used for flow separations and to predict the observed streamwater compo sition. Model forecasts indicated that reduced fertilizer and pesticide app lication (rather than elimination of overland-flow or soil-water contributi ons) was necessary to improve the episodic streamwater composition. This st udy provides important additional understanding of the catchment-scale proc esses by which land-applied pesticides and nutrients can move through agric ultural systems. (C) 2001 Elsevier Science B.V. All rights reserved.