11:45 AM - 12:00 PM
[HTT21-05] Application of Multiple-indicator to groundwater flow and chemistry study in the alluvial fan
Keywords:Strontium isotope, Paddy field, Groundwater-river interaction
The δ2H and δ18O values in the shallow groundwater are relatively low along the Tedori River and increased with the distance from the river; this trend would be caused by dilution effect by the river water. However, there is little contribution of the paddy water with high values reflecting the influences of the evaporation effect. Concentrations of Mg2+, Ca2+, Sr2+, HCO3-, and SO42- in the groundwater have a similar distribution trend: lower along the Tedori River and higher in the central area of the fan where paddies are relatively widespread. The 87Sr/86Sr ratios of the groundwater are related to near-surface geology. The groundwater in sediment from the Tedori River has relatively high 87Sr/86Sr ratios, whereas that from the Sai River in the north of the fan has low 87Sr/86Sr ratios. The river water of the Tedori River is also high ratio reflecting sediments in the upper river basin. The three stable isotopes indicate that the groundwater in the central and southern fans is recharged from the Tedori River, whereas recharge in the north is from the Sai River.
There are linear relationships between 87Sr/86Sr ratio and the reciprocal concentrations of Sr2+, Mg2+, and Ca2+. These geochemical characteristics suggest that groundwater recharged from the Tedori River flows towards the central fan and then runs off the sea. It mixes with waters from precipitation and paddy water that have become enriched in these components during downward infiltration. The results obtained by application of multiple-indictor are consistent with our hydrological observation results: groundwater contour maps and river water balances, and groundwater flows reproduced by the three dimensional numerical simulation.