Role of the riparian zone in controlling the distribution and fate of agricultural nitrogen near a small stream in southern Ontario

Uncultivated riparian areas can play an important role in reducing nutrient loading to streams in agricultural watersheds. Groundwater flow and geochemistry were monitored in the riparian zone of a small agricultural watershed in southern Ontario. Hydraulic and geochemical measurements were taken along a transect of monitoring wells extending across the riparian area into an agricultural field. Chloride and nitrate concentrations in groundwater samples collected from the agricultural field were much higher than in samples from the riparian area. A sharp decline in both nitrate and chloride concentrations was observed near the field–riparian zone boundary. It appears that increased recharge within the riparian zone, as compared to the artificially drained field, caused nitrate-rich groundwater from the field to be diverted downward beneath the riparian zone, thus limiting the input of agrochemicals to the riparian area and consequently protecting the stream from potential contamination. Geochemical data also indicated that nitrate was attenuated in the downward moving groundwater. Patterns of dissolved oxygen concentrations and redox potential in the subsurface coincided with the pattern defined by groundwater nitrate. These patterns indicated that conditions within the riparian zone and at depth near the field–riparian zone boundary were conducive to denitrification. A linear relation between the δ15N and δ18O values of nitrate from the monitored transect also supported denitrification as the primary nitrate removal mechanism. This study provides a new conceptual model of how riparian zones may prevent nitrate contamination of streams, and highlights the need for a complete understanding of both groundwater flow and geochemistry in riparian environments.