Dynamics of groundwater-derived nitrate and nitrous oxide in a tidal estuary from radon mass balance modeling

We monitored submarine groundwater discharge (SGD) into the Werribee Estuary, Australia, using both chemical and physical methods. SGD occurred at hotspots where Rn-222 persisted through a 12 month survey period. A significant correlation between Rn-222 and NO3- (r(2) = 0.8, p < 0.01), as well as between Rn-222 and N2O (r(2) = 0.6, p < 0.01) at a Rn-222 hotspot, and much higher NO3- and N2O concentrations in groundwater relative to surface water suggest that elevated NO3- and N2O concentrations in the estuary were derived from SGD. Two sampling campaigns over 24 h revealed that variations of Rn-222, NO3-, and N2O were controlled by tide-induced hydraulic-head gradient fluctuations and, possibly to a much lesser extent, by tidal pumping and density-driven convection. A two-box Rn-222 mass-balance model was used to calculate the rate of SGD into two different layers across the pycnocline of the estuary. A higher total groundwater discharge rate of 0.12 +/- 0.09 m d(-1) was observed in the surface layer during ebb tide compared with 0.10 +/- 0.08 m d(-1) during flood tide. Fluxes of groundwater-derived NO3- and N2O were higher than the riverine flux at baseflow by more than 30 fold and 20 fold, respectively. SGD derived NO3- fluxes exceeded the mean annual riverine NO3- and TN fluxes by a factor of similar to 5 and similar to 3 respectively. SGD-derived N2O fluxes were 170 mu mol m(-2) d(-1), which are among the highest N2O fluxes observed in estuaries.