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

Abstract

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