Determining groundwater inflow and Si behavior in a wetland using 222Rn mass balance and multidisciplinary approach

Abstract

Studying groundwater inflow and quantifying its flux into aquatic systems remain challenging because of geological and topographical heterogeneities. In this study, we measured the 222Rn activities in the surface water and porewater of a shallow wetland, the Baekseok Reservoir Wetland, to investigate the groundwater flow system and quantify the groundwater inflow rate. Three field campaigns were carried out in April 2018, September 2018, and February 2019 to determine the spatial and temporal variations in the 222Rn activity. The 222Rn activities were significantly higher along the western boundary of the wetland than the eastern boundary, suggesting a significant difference in the groundwater inflow rate, which reflects different hydrological characteristics on the two boundaries of the wetland. The results of hydrogeological investigations (electrical resistivity survey and monitoring of groundwater and surface water levels) and microbiological analyses were consistent with the 222Rn distribution results, supporting our hypothesis that the groundwater inflow qualitatively and quantitatively exhibits a spatial difference. Based on the mass balances of water and 222Rn under a steady-state condition, the groundwater inflow rates were estimated to be 3.2 × 102, 1.7 × 103, and 7.6 × 102 m3 day−1 in April, September, and February, respectively. The highest groundwater inflow rate in September may be due to the relatively high hydraulic gradient resulting from intense rainfall events. The water residence time of the wetland estimated from the 222Rn mass balance results was approximately one month regardless of the seasons. In addition, the groundwater-derived dissolved silica flux accounted for ~60% of the total dissolved silica input flux of the wetland, highlighting the importance of groundwater-derived dissolved silica. Our multidisciplinary approach to identify and quantify groundwater inflow has important implications in determining groundwater sources in aquatic systems and the behaviors of other substances, such as dissolved nutrients.