Distribution of isotopic ages of groundwater and its evolution and sustainable utilization in the Yuncheng Basin

Through analyses of radiocarbon and other isotopes/geochemical indicators of groundwater in the Yuncheng Basin, it is found that groundwater in the middle and deep layers of the basin was recharged approximately 22 to 3 k.a. before present (radiocarbon activities between 6 and 38 percent modern carbon). The shallow groundwater (71 to 89 percent modern carbon) comprises modern water (e. g. recharged post-1950s), or has a significant mixing component of modern water. Stable isotope compositions of deep groundwater (δ18O, -10‰; δD, -70‰) indicate that the paleowaters in deep layers was recharged under a cooler climate than the present, while the values of shallow groundwater (δ18O , -8‰; δD, -51‰) are similar to the average stable isotope compositions of modern rainwater from the East Asian monsoon. The average nitrate concentration of the shallow groundwater (31 mg/L) is much higher than in the deep groundwater (1.8 mg/L); and stable isotope values (δ15N－δ18ONO3) indicate that nitrate is largely derived from synthetic fertilizers. In addition, the TDS of the shallow groundwater reaches much higher values (up to 8.5 g/L), compared to that of the deep groundwater (average TDS of 1.1 g/L), due to evapotranspiration and mineral dissolution. In the study area, the deep groundwater (which has a relatively good water quality) is exploited intensively. High levels of drawdown coupled with the influence of fractures (that provide preferential migration pathways) have resulted in leakage of the shallow, poor water quality groundwater into the intermediate and deep layers. This has begun to affect the quality of the deeper groundwater. If it occurs on a large scale, the water quality in the intermediate and deep layers is likely to deteriorate, with increasing concentrations of nitrate and total dissolved solids.