NUTRIENT EXCHANGE BETWEEN THE RIPARIAN AND HYPORHEIC ZONES FOR A STREAM IN A LARGE AGRICULTURAL WATERSHED IN THE NORTH CAROLINA COASTAL PLAIN

Samples of surface water, riparian groundwater, and porewater were analyzed for major ions (Cl-, SO4-2, K+, Na+, Ca+2, Mg+2), total dissolved nitrogen (TDN), nitrate (NO3-), dissolved organic nitrogen (DON), and ammonium (NH4+) to evaluate the effect of nutrient exchange (e.g., denitrification and/or assimilation) and hyporheic-zone mixing for a 263-m section of a stream in a large Coastal Plain watershed with significant agricultural activity, West Bear Creek watershed in North Carolina. Surface water samples and riparian groundwater samples were collected bimonthly from eight wells from April 2005 to August 2006. Porewater samples were collected at intervals of 20, 30, and 60 cm below the top of the streambed in April and June 2006. Together, these data sets were used to quantify nutrient exchange (including nitrogen speciation) between the riparian zone, hyporheic zone and surface water.

Seepage meter measurements and measurements of hydraulic head gradient between surface and groundwater indicate that West Bear Creek generally receives groundwater from the underlying surficial aquifer. Concentrations of Cl- in surface water and porewater show no evidence of hyporheic-zone mixing to a depth of 60 cm (assumed to represent the bottom of the hyporheic zone). The majority (~85%) of nitrogen in both riparian groundwater and porewater was in the form of NO3-. The only consistent and significant change in nitrogen concentration between the riparian groundwater and porewater was a decrease in NO3-. There is a 46% average decrease in NO3- concentration from the riparian zone to the hyporheic zone; mean and maximum decrease in NO3- concentration was 7.24 mg/L and 17.9 mg/L, respectively. Since Cl- measurements preclude hyporheic-zone mixing at this depth, the observed decrease in NO3- suggests the occurrence of other processes such as denitrification and/or assimilation between the riparian zone and hyporheic zone. Streambed cores show suitable organic substrate to support denitrification. Future work will consist of N2 and Ar measurements to constrain the extent of denitrification between the riparian zone and hyporheic zone.