Using stable isotopes and tritium to delineate groundwater flow systems and their relationship to streams in the Geum River basin, Korea

This study aims to clarify groundwater and stream water flow systems in the Geum River basin, Korea, which is an area with a crystalline basement using delta O-18, delta H-2, and H-3. Stable isotopes of precipitation showed clear differences between dry and rainy seasons due to the Asian monsoon. Groundwater and stream water samples were collected in the dry and rainy seasons in three hydrographic zones of the upper, middle, and lower zones. The contribution of rainy season precipitation to groundwater recharge based on deuterium excess was 71-89%, far exceeding the amount fraction of the season in total precipitation. Stable isotopic compositions of groundwater and stream water showed little seasonal variation, and indicated the dominance of high-altitude recharge in the upper zone. Groundwater with lower H-3 was mainly observed in that area, indicating a contribution of intermediate flow with a longer residence time. In contrast, stream water and some groundwater samples in the lower zone showed seasonal variability and signatures of 10-20% evaporation in their stable isotopic composition. The H-3 content of groundwater in the lower zone was high, indicating that the local flow system is dominant. These regional features also influenced the relative contributions of rainy season precipitation, dry season precipitation, and local recharge with evaporation signature, as estimated from delta O-18 and deuterium excess values, and H-3 in groundwater. Stream water from lower-order streams of the river had stable isotopic compositions similar to those in the upper zone, indicating a considerable contribution of groundwater discharge in the mountainous areas to the lower reach of the river. These results have implications for sustainable management of water resources in the basin during predicted changes in water usage and precipitation patterns due to climate change, as well as in regions with similar climatic and hydrologic conditions.