| Summary |
Surface water and groundwater interaction have been studied extensively. Samples of surface water, riparian groundwater, and hyporheic zone porewater were analyzed for major ions (Cl̄, K⁺, Na⁺, Ca⁺², Mg⁺²), total dissolved nitrogen (TON) and nitrate (NO₃̄) 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 small Coastal Plain watershed with significant agricultural activity (West Bear Creek (WBC) watershed in North Carolina). The entire watershed (WBC and its confluence Bear Creek) was also monitored at nine locations to evaluate this watershed's nutrient contribution to the Neuse River Basin. Surface water samples were collected from the center of the stream beginning in December 2005 and continuing bi-monthly until December 2006. Riparian groundwater samples were collected bi-monthly from seven wells during April 2005 to December 2006. Hyporheic zone porewater samples were collected at 35 cm below the top of the streambed on the same timetable as the surface water. However, intervals of 20, 35, and 60 cm below the top of the streambed were collected 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. Piezomanometer measurements and 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 hyporheic porewater show no evidence of hyporheic zone mixing to a depth below 20 cm except at site 437 m. The majority of nitrogen in both riparian groundwater and porewater was in the form of NO₃̄. The only consistent and significant change in nitrogen concentration between the riparian groundwater and hyporheic porewater was a decrease in NO₃̄. There is a 46% decrease in NO₃̄ concentration from the riparian zone to the hyporheic zone; mean and maximum decrease in NO₃̄ concentration was 7.24 mg N/L and 17.9 mg N/L, respectively. The use of sodium (Na⁺) to preclude dilution as a removal mechanism was employed within the study reach. Results suggest the occurrence of other processes such as denitrification and/or assimilation between the riparian zone and hyporheic zone. Analysis of the watershed samples indicates that overall nitrate concentration does not vary much downstream. Flux, however, increases downstream. Nitrate concentration and flux of the Neuse River above and below the confluence with Bear Creek increased throughout most of the study period, suggesting that the agriculturally-dominated Bear Creek watershed has a dramatic impact on the Neuse River's nutrient budget. |
