The effect of bank seepage on riparian hydrology and nitrogen biogeochemistry

The effect that bank seepage has on riparian hydrology and nitrogen biogeochemistry was studied along two lowland, headwater agricultural streams outside Toronto, Ontario from April to October 2003 & 2004. Hydrologic flow paths were combined with stream and subsurface water chemistry at both sites along a 100 m reach of stream. Bank seepage at the Vivian Creek site was seasonal and varied with temperature and precipitation patterns, leading to several subsurface flow reversals. The West Duffins Creek riparian zone was subject to bank seepage for the duration of the study. During the summer months the stream served as the principal source area of water for the riparian zones, confirmed through analysis of chloride gradients. Stream-origin nitrate was removed from, and ammonium concentration increased substantially in, the subsurface water within in a short distance inland at both sites, facilitated by consumption of dissolved oxygen to anaerobic levels. In situ co-injections of bromide and nitrate were performed at both sites under base flow in a 25-m 2 grid of piezometers adjacent to the stream and revealed that bank seepage occurs via preferential flowpaths and patterns of nitrate removal can be classified as either a rapid, lag, or saturation response scenario. This research indicates that the process of bank seepage may occur in temperate climates along lowland headwater streams more frequently than considered in the literature.