Transverse and Longitudinal Streambed Flux and Nitrate Removal in an Agricultural Stream Reach

Abstract:

Late spring and summer streambed surface water - groundwater (SW-GW) flux and nitrate removal were estimated for Leary Weber Ditch, IN, along a reach near the confluence with Sugar Creek, IN, using data collected in 2004 and 2005. Stream stage, groundwater heads and temperatures were continuously monitored in streambed piezometers and stream bank wells for two transects across Leary Weber Ditch. Periodic manual measurements of stage, head and temperature were also made upstream and downstream of these transects. The data were used to develop two-dimensional (2D), cross-sectional (transverse) models of streambed water, heat, and nitrate transport perpendicular to the stream and a 2D longitudinal model along the axis of the stream. Model-estimated SW-GW exchange was influenced by physical heterogeneity in the streambed, stream channel topography, and local GW heads. Transverse modeling indicated that SW-GW exchange was generally greater near the banks and decreased toward the center of the channel. However, streambed heterogeneity modified this pattern with smaller flux at locations where fine silt was observed in cores, and greater and more temporally variable flux where the fine silt was absent. Transverse SW-GW exchange occurred mostly through lateral GW flow in shallow sediments, with different GW heads on the stream banks leading to transverse flow underneath the streambed at depth. The longitudinal modeling indicated that stream channel topography and streambed heterogeneity resulted in hyporheic flow paths within the streambed. Nitrate bearing stream water entered the streambed as a result of hyporheic flow, rapid stream stage rise during high flow events, and lowered GW levels leading to losing conditions. Field measurements of streambed nitrate concentration and simulation results suggest rapid denitrification in the anaerobic streambed, such that streambed nitrate removal was primarily determined by spatial and seasonal SW-GW exchange patterns.