Groundwater seepage as a driver of CO₂ evasion in a coastal lake (Lake Ainsworth, NSW, Australia)

In recent years, it has become apparent that carbon dioxide (CO₂) emissions from inland water bodies and lakes are an important component of the global carbon cycle. Large-scale lake heterotrophy is thought to be a major driver of CO₂ production and may mask other processes such as groundwater input. This study uses radon (²²²Rn, a natural groundwater tracer) to quantify groundwater discharge, and estimates CO₂ outgassing to determine the contribution of groundwater-derived CO₂ inputs into Lake Ainsworth (New South Wales, Australia). Lake Ainsworth was a source of CO₂ to the atmosphere throughout the study period with outgassing rates ranging from 10.6 to 152.3 mmol m² day⁻¹. Annual groundwater fluxes were determined using a radon mass balance equated to about 55 ± 50 % of the total volume of water input (via direct precipitation and groundwater) into the lake. In spite of large uncertainties, groundwater seepage was a source of CO₂ supersaturation in Lake Ainsworth equivalent to 13 ± 25 % of total CO₂ outgassing rates. Hence, groundwater discharge may need to be considered for carbon budgets of other lakes.