Quantifying groundwater carbon dioxide and methane fluxes to an urban freshwater lake using radon measurements

Freshwater lakes can play a significant role in greenhouse gas budgets as they can be sources or sinks of carbon to the atmosphere. However, there is limited information on groundwater discharge being a source of carbon to freshwater lakes. Here, we measure CO2 and CH4 in the largest urban freshwater lake in the metropolitan area of Sydney (Australia) and quantify groundwater discharge rates into the lake using radon (Rn-222, a natural groundwater tracer). We also assess the spatial variability of radon, CO2 and CH4 in the lake, in addition to surface water and groundwater nutrient and carbon concentrations. Results revealed that the lake system was a source of CO2 and CH4 to the atmosphere with fluxes of 113 +/- 81 and 0.3 +/- 0.1 mmol/m(2)/d, respectively. These calculated CO2 fluxes were larger than commonly observed lake fluxes and the global average flux from lakes. However, CH4 fluxes were lower than the average global value. Based on the radon mass balance model, groundwater discharge to the lake was 16 +/- 10 cm/d, which resulted in groundwater-derived CO2 and CH4 fluxes contributing 25 and 13% to the overall greenhouse gas emissions from the lake, respectively. Radon, CO2 and CH4 maps showed similar spatial distribution trends in the lake and a strong relationship between radon, NO3 and NH4 suggested groundwater flow was also a driver of nitrogen into the lake from the western side of the lake, following the general regional groundwater flow. This work provides insights into groundwater and greenhouse gas dynamics in Sydney's largest urban freshwater lake with two implications for carbon budgets: to incorporate urban lakes in global carbon budgets and to account for, the often ignored, groundwater discharge as a source of carbon to lakes. (C) 2021 Elsevier B.V. All rights reserved.