Groundwater-recharge connectivity between a hills-and-plains' area of western Taiwan using water isotopes and electrical conductivity

•Water isotope study is conducted on the hills-and-plains' region of western Taiwan.•Summer rainfall is a major source for groundwater and streamwater in the study hill.•Hill groundwater is a vital source for its adjacent plain groundwater.•Hill groundwater supplying the plain groundwater mostly relies on deep circulation.

SummaryWater isotopes (δD, δ18O) as well as electrical conductivity were employed to clarify hydrological relationships among precipitation, stream water, and groundwater in the Douliou Hills (DLH) of Taiwan. They are also used to evaluate the importance of groundwater recharge sources in the proximal fan of Choshuichi alluvial plain (CSAP), which is connected to the eastern extreme of the DLH. Results indicate that the most important source of groundwater in the DLH comes from summer precipitation. In most cases the summer precipitation recharges stream water via base flow. Based on δ18O and EC values, groundwater in the proximal fan of the CSAP can be divided into groundwater from 5 wells nearby the DLH (near-DHL group) and groundwater from 6 wells nearby the Choshui Stream (near-CS group). Semi-quantitative results calculated by ternary end-member mixing analysis (EMMA) for δ18O and electrical conductivity values indicate that for the near-DLH group, precipitation in the CSAP contributes 40-50% of water to the proximal-fan groundwater while DLH groundwater donates 20-55% and Choshui Stream water, 0-29%. Precipitation in the CSAP is the most important recharge source. DLH groundwater's contribution is more notable in deep groundwater and Choshui Stream water's contribution decreases with distance from the Choshui Stream. For the near-CS group, anthropogenic pollution adds ions to the groundwater restricting the effectiveness of the ternary EMMA approach. This study provides new insights into groundwater recharge in the CSAP. It should be applicable to future studies of hill-and-plains' water connectivity in other regions.