Transport of contaminants in agricultural catchments during snowmelt: buffer strips vs. preferential flow paths

Knowledge of hydrological pathways is essential for an understanding of ability of riparian buffers to mitigate the impact of diffuse pollutants on freshwater ecosystems. We examined flow paths and source areas of contaminants in an agricultural catchment in NE Poland during snowmelts in 2009 and 2010. End-member mixing analysis showed that stream chemistry was controlled by the shallow runoff components. Their contribution to stream runoff varied significantly depending on soil frost, catchment wetness and the water input from snowmelt. In 2009 overland/rill flow was the main mechanism of runoff generation because of the low permeability of the frozen ground. Overland flow had the pronounced impact on stream chemistry during peak discharges, when it amounted up to 70% of discharge (Q). Riparian ground/soilwater runoff was important component of runoff throughout stream recession, when it contributed up to 50% of Q. High catchment wetness, lack of soil frost and large snowmelt input in 2010 resulted in enhanced infiltration and rapid and large groundwater response. Melt water and tile drain outflow were found to be the major peak runoff components. In 2009 overland flow together with discharge of shallow groundwater were responsible for the export of 88% of nitrate and 98% of orthophosphate. During snowmelt of 2010 70% of NO3− and 80% of PO43− moved via tile drain network. As migration of chemicals was controlled by preferential flow structures, it is very likely that vast majority of contaminants fluxes bypassed buffers and structures, which could constrain their impact on stream quality.