Greenhouse gases and submarine groundwater discharge in a Sydney Harbour embayment (Australia)

Abstract

We investigated whether submarine groundwater discharge (SGD) traced by radon (222Rn, a natural groundwater tracer) may drive carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) in surface waters in Chowder Bay, a marine embayment in Sydney Harbour, Australia. A radon mass balance revealed significant groundwater discharge rates into the bay (8.7 ± 5.8 cm d−1). The average CO2, CH4, and N2O concentrations in the subterranean estuary were 3.5, 7.2, and 2.8 times higher than the average surface water concentrations, indicating the possibility of coastal groundwater as a source of greenhouse gases to the bay. SGD-derived fluxes of greenhouse gases were 5.02 ± 2.28 mmol m−2 d−1, 5.63 ± 2.55 μmol m−2 d−1, and 1.72 ± 0.78 μmol m−2 d−1 for CO2, CH4 and N2O, respectively. The average CO2 evasion rate from surface water was 2.29 ± 0.46 mmol m−2 d−1 while CH4 and N2O evasion rates were 12.89 ± 3.05 and 1.23 ± 0.25 μmol m−2 d−1 respectively. Therefore, groundwater-derived greenhouse gas fluxes accounted for >100% CO2 and N2O and ∼43% of CH4 surface water evasion, indicating SGD is likely an important source of greenhouse gases to surface waters. However, this may be due to observations being performed near the SGD source, which may overestimate its contribution to the wider Sydney Harbour. Over a 20-year time frame, the combined emissions of CH4 and N2O from surface waters to the atmosphere accounted for 25% of the total CO2-equivalent emissions. Although this study gives preliminary insight into SGD and greenhouse gas dynamics in Sydney Harbour, more spatial and temporal resolution sampling is required to fully constrain these processes.