Linking groundwater discharge to severe estuarine acidification during a flood in a modified wetland

Periodic acidification of waterways adjacent to coastal acid sulfate soils (CASS) is a significant land and water management issue in the subtropics. In this study, we use 5-months of continuous radon (²²²Rn, a natural groundwater tracer) observations to link estuarine acidification to groundwater discharge in an Australian CASS catchment (Tuckean Swamp). The radon time series began in the dry season, when radon activities were low (2−3 dpm L⁻¹), and the pH of surface water was 6.4. We captured a major rain event (213 mm on 2 March 2010) that flooded the catchment. An immediate drop in pH during the flood may be attributed to surface water interactions with soil products. During the post-flood stage, increased radon activities (up to 19.3 dpm L⁻¹) and floodplain groundwater discharge rates (up to 2.01 m³ s⁻¹, equivalent to 19% of total runoff) coincided with low pH (3.77). Another spike in radon activities (13.2 dpm L⁻¹) coincided with the lowest recorded surface water pH (3.62) after 72 mm of rain between 17 and 20 April 2010. About 80% of catchment acid exports occurred when the estuary was dominated by groundwater discharging from highly permeable CASS during the flood recession.