Nitrous oxide sources and sinks in coastal aquifers and coupled estuarine receiving waters

Sources and sinks of the atmospherically reactive gas nitrous oxide (N2O) were determined in the heavily nutrient loaded Childs River in Cape Cod, MA. Surface waters were supersaturated and bottom waters were depleted with N2O throughout the system. In apparent septic effluent plumes, N2O concentrations reached 3 orders of magnitude above atmospheric equilibrium. Because nitrate and N2O concentrations correlated in groundwater entering the estuary, septic tank effluent appeared responsible for the supersaturated concentrations of N2O in surface waters. A hyperbolic function fit nitrate and N2O concentrations in the water column of the estuary with a maximum supersaturation of approximately 60 nM. From surface water supersaturation we predicted a release of 480 nmol N2O m−2 h−1 to the atmosphere in the summer. Property plots of salinity vs. bottom-water N2O suggested a benthic sink of N2O. Consistent with this trend, sediments consumed rather than released N2O in most flux measurements. Nutrient loading did not directly alter benthic N2O flux, potentially because stratification limited exposure of sediments to nitrate-rich surface waters, but macroalgal cover increased benthic N2O consumption. Sediment N2O consumption averaged 111 nmol N2O m−2 h−1 and correlated with oxygen uptake. Losses from the system to the atmosphere and sediments exceeded inputs of N2O contaminated groundwater, which suggests missing N2O sources.