Multiple-indicator study of groundwater flow and chemistry and the impacts of river and paddy water on groundwater in the alluvial fan of the Tedori River, Japan

To investigate the source, flow paths, and chemistry of rich resources of high-quality, shallow groundwater in the alluvial fan between the Tedori and Sai rivers in central Japan, we analysed stable isotope ratios of H, O, and Sr and concentrations of major dissolved ions and trace elements in groundwater, river water, and paddy water. The Sr-87/Sr-86 ratios of the groundwater are related to near-surface geology: groundwater in sediment from the Tedori River has high Sr-87/Sr-86 ratios (>0.711), whereas that from the Sai River in the north of the fan has low Sr-87/Sr-86 ratios (<0.711). H-2 and O-18 values and Sr-87/Sr-86 ratios indicate that groundwater in the central and southern fans is recharged by the Tedori River, whereas recharge in the north is from the Sai River. Mg2+, Ca2+, Sr2+, HCO3-, and SO42- concentrations and H-2 and O-18 values in the groundwater are high in the central fan and, except for the northern area, tend to increase with distance from the Tedori River. There are linear relationships between Sr-87/Sr-86 ratio and the reciprocal concentrations of Sr2+, Mg2+, and Ca2+. These geochemical characteristics suggest that as groundwater recharged from the Tedori River flows towards the central fan, it mixes with waters derived from precipitation and paddy water that have become enriched in these components during downward infiltration. These results are consistent with our hydrological analysis and numerical simulation of groundwater flow, thus verifying the validity of the model we used in our simulation of groundwater flow. Copyright (c) 2016 John Wiley & Sons, Ltd.