Evaluating the impacts of intense seasonal groundwater pumping on stream–aquifer interactions in agricultural riparian zones using a multi-parameter approach

Abstract The flow and chemistry of groundwater in agricultural riparian zones are highly heterogeneous due to the human impacts as well as convergent flow conditions and variable stream–aquifer interactions. To evaluate impact of seasonal heavy pumping on the aquifers in agricultural riparian zones, we investigated groundwater and stream water using a multi-parameter approach integrating continuous monitoring of water level and temperature, and periodic measurements of hydrogeochemical parameters, water-stable isotopes, and microbial communities in agricultural riparian zones with contrasting patterns of land use and water use across the third order stream for two years in the mid-western South Korea. Intense groundwater use for warming of greenhouses in dry farmland during winter induced groundwater drawdown of up to 5 m, resulting in losing-stream condition and decreased groundwater temperature with considerable spatial variability. Groundwater contamination was clearly identified based on elevated concentrations of agriculture-derived solutes such as Ca2+ and SO42-, despite the negligible NO3- content under reducing conditions. Three-component mixing of groundwater based on δ18O and dissolved silica revealed major water sources of regional recharge, paddy soil water, and winter stream water, with considerable differences in their contributions between the riparian zones. The fraction of winter stream water in groundwater showed different temporal variability depending on aquifer connectivity with the stream as well as groundwater use. Principal component analysis of hydrogeochemical and isotopic parameters revealed the major processes of agricultural contamination, water–rock interactions, and the contribution of paddy-impounded water, which varied significantly with the extent of stream water infiltration. These results suggest that seasonal pumping substantially affect groundwater flow and its relationship to stream with considerable horizontal and vertical variability in riparian aquifers. This study highlights spatial and temporal variations of groundwater flow dynamics in riparian areas are effectively delineated by a multi-parameter approach, which can contribute to better understanding of stream-aquifer interactions and implementation of integrated water management in the areas with agricultural land uses and water uses.