Recharge history and controls on groundwater quality in the Yuncheng Basin, north China

Environmental isotopes and water quality indicators (e.g. TDS and NO3 contents) were used to characterize the age, recharge history and controls on the quality of groundwater resources in the Yuncheng Basin, north China, where extensive extraction occurs for agriculture and domestic supply. d18O and d2H values as low as -10.6% and -73%, respectively, together with low radiocarbon activities (<20 pmC) show that deep groundwater comprises palaeowaters largely recharged in the late Pleistocene (~10 to 22 ka B.P.) under a cooler climate than the present. Shallow groundwater has higher radiocarbon activities (>70 pmC), indicating a significant component of modern (post-1950s) recharge. The shallow groundwater has higher d18O and d2H values (up to -8.1% and -54%) that are similar to those in modern summer monsoon rainfall, indicating that current recharge is via direct infiltration and/or leakage of ephemeral streams during heavy rain events. Historic recharge rates estimated using radiocarbon ages are between 1 and 10 mm/year, corresponding to <2% of local rainfall, which is lower than estimates of modern recharge nearby based on tritium data. The relationship between groundwater d18O values and 14C ages is similar to that observed in groundwater from other basins in northern China, confirming that much deep groundwater in the region is palaeowater, and suggesting that a broad scale assessment of groundwater residence times may be made from d18O and d2H values. Most deep groundwater has low TDS and nitrate concentrations (median 1090 mg/L and 1.8 mg/L, respectively), while shallow groundwater has TDS contents of up to 8450 mg/L (median 2010 mg/L) and NO3 concentrations up to 630 mg/L (median 31 mg/L).