Subsurface Pathways Dominate Streamflow Generation and Agrochemical Contamination in a Semi-Arid Agricultural Watershed: A Long-Term Environmental-Tracer Study

Understanding flow pathways and mechanisms that generate streamflow is important to understanding agrochemical contamination in surface waters in agricultural watersheds. This study tested the conceptual hydrologic framework developed in previous research in the semi-arid agricultural Missouri Flat Creek watershed, near Pullman Washington USA, using environmental-tracer methods. Oxygen-18 and electrical conductivity (EC) were monitored in tile drainage (12 ha catchment) and stream water (660 - 5700 ha catchment) from 2000 to 2008. Stream and tile discharges were greatest in winter with peak discharges occurring from January to March during each year. Tile drainage exhibited steady d¹⁸O values close to -14.7 per mil year round, changing briefly only during isolated events. Stream waters (derived predominantly from tile drainage and soil water seepage) were highly seasonal with mean winter d¹⁸O values of -14.5 per mil and mean summer values of -11 per mil. Seasonal variability in streamflow d¹⁸O was the result of in-stream evaporation rather than shifting streamflow sources. Winter precipitation accounted for 67% of total annual precipitation and was found to be the primary contribution to streamflow and tile drainage, while summer precipitation did not contribute appreciably. “Old” and “new” water partitioning in streamflow was not identifiable using d¹⁸O, but seasonal shifts of nitrate-corrected EC suggest that shallow soil pathways did contribute significantly to streamflow generation during winter (mean EC 200 mS/cm), while deeper soil pathways primarily generated summer streamflow (mean EC 250 mS/cm). Nitrate concentrations in streamflow and tile drainage peaked during high flow in winter with [N-NO₃^-] reaching 45mg/L, while summer [N-NO₃^-] was consistently 4 mg/L. Temporal patterns of EC and nitrate concentrations indicate that rapid shallow lateral soil water flow during winter was the most important pathway for nitrate loss to stream waters in the watershed.