Radon tracing of groundwater discharge into an Australian estuary surrounded by coastal acid sulphate soils

Widespread sulphidic deposits have accumulated in tropical coastal floodplains throughout the world. Sulphidic soils oxidize when floodplains are drained for urban and agricultural development. As a result, large amounts of sulphuric acid may be released to nearby waterways. Macropores may create excellent conditions for groundwater flow in coastal acid sulphate soils (CASS). An automated radon (²²²Rn) measurement system was used to quantify groundwater inputs into a tidally-dominated estuary that is known to be influenced by acid discharges from CASS (Richmond River Estuary, Australia). A high resolution radon survey along a 120-km long segment of the tidal river identified two areas of preferential groundwater inputs. Intensive time series measurements in one of those areas (the Tuckean Broadwater) demonstrated that groundwater inputs are highly variable over hourly and seasonal time scales and inversely related to surface water pH. Elevated radon concentrations (up to 12 dpm/L) and low pH (as low as 3.3) were observed in surface waters at low tide a few weeks after a large rain event. These results demonstrate that acidic waters are entering the estuary via tidally-modulated groundwater flow pathways. Groundwater discharge rates into drains in the Tuckean Swamp were estimated from a dual-assumption radon mass balance to be 0.09–0.16 and 0.56–0.89 m³ s⁻¹ during the dry and wet season, respectively (or 6–10 and 37–59 cm/day if the area is taken into account). While surface runoff increased only 2-fold in the wet season relative to the dry season, groundwater discharge rates increased ∼6-fold. Since groundwater can be a major driver of surface water quality, radon can be useful in CASS monitoring and management efforts.