Relationships among groundwater age, denitrification, and the coupled groundwater and nitrogen fluxes through a streambed

The relationships among coupled groundwater and nitrogen (N) fluxes, groundwater age, and denitrification were examined for a section of West Bear Creek, an agricultural stream in the coastal plain of North Carolina, United States. Simultaneous streambed measurements of hydraulic conductivity (K) and hydraulic head gradient (J) and the concentrations of NO3- ([NO3-]), dissolved gases, and chlorofluorocarbons in groundwater were interpolated, mapped, and (for water flux v = KJ and nitrate flux f(NO3) = v[NO3-]) integrated over the streambed area. Nitrate and dissolved organic N accounted for 92 and 8% of N flux through the streambed, respectively. Streambed maps show a band of greater groundwater age, and lower [NO3-] and f(NO3), running through the center of most of the study reach. Nitrate flux (f(NO3)) exhibits this "center-low'' pattern even though one of its controlling factors, groundwater flux (v), has on average the opposite "center-high'' pattern. An inverse relationship between [NO3-] and age is indicative of fertilizer as the primary source of groundwater NO3-. Denitrification reduced mean f(NO3) by similar to 50%, from 370 mmol m(-2) d(-1) (what it would have been in the absence of denitrification) to 173 mmol m(-2) d(-1) (what it actually was). Measurement of both groundwater age and v made possible a new method for estimating flow-weighted mean groundwater age (tau(FWM)), an important aquifer hydraulic characteristic related to groundwater storage and recharge rate. This method gives tau(FWM) = 30 years, which, along with the overall distribution of groundwater ages, suggests the possibility of a significant time lag between changes in N fertilizer application rates and NO3- flux from groundwater to West Bear Creek. Differences in streambed groundwater chemistry between the left and right sides of the streambed suggest differences in agricultural practices on opposite sides of the stream.