Geochemical and isotopic investigation of groundwater in the Yuncheng Basin, China: implications for groundwater quality and quantity in semi-arid agricultural regions

This thesis examines chemical and isotopic characteristics of groundwater from the Yuncheng Basin in north-central China, in order to understand the timing and mechanisms of recharge, controls on groundwater quality and the influence of climate and anthropogenic processes on groundwater quality and quantity. Groundwater radiocarbon activities range from 5.93 to 88.2 pmC, decreasing with depth in the Quaternary aquifer. Estimated groundwater residence times range from modern in the shallow unconfined aquifer unit (Q3 and Q4), to >20 k.a. in the semi-confined deep unit (Q1 and Q2). Residence times in deep groundwater increase from west to east, following the historic regional groundwater flow direction; this direction has been altered by pumping and groundwater now flows towards a cone of depression near Yuncheng City. The vertical recharge rate, calculated using age vs. depth relationships is ~1-10 mm/yr; this is lower than previous estimates using tritium in the soil zone nearby, indicating that vertical infiltration may have increased in modern times compared to historic times. δ18O and δ2H values in shallow modern groundwater are similar to rainfall during the summer monsoon, indicating recharge via direct infiltration. The δ18O and δ2H values in deep groundwater are significantly lower than modern rainfall, indicating recharge under a cooler climate than the present, during the late Pleistocene and early Holocene. The δ18O values increase from old to young groundwater, reflecting a broad temperature increase through the period of deep groundwater recharge. I/Cl and I/Br ratios correlate positively with δ18O values in the deep palaeowaters (r2 = 0.48 and 0.55), indicating greater delivery of I to the basin in rainfall during warm periods. This may be due to increased biological I production in warmer oceans. Shallow groundwater contains high nitrate concentrations (up to 630 mg/L); δ15N and δ18O values of nitrate are both generally between 0‰ and 5‰, indicating that synthetic fertilizers are the major source. Elevated nitrate concentrations (>20 mg/L) locally occur in deep groundwater, particularly near the Linyi fault, due to downwards vertical leakage. High TDS (up to 8450 mg/L), Br and Cl concentrations in shallow groundwater relative to rainfall indicate high levels of evapotranspiration in this water, due to flood irrigation and shallow water tables. Groundwater 87Sr/86Sr values are similar to those in local rainfall and carbonate minerals (0.7110 to 0.7120); trends in HCO3, pH and δ13C values indicate that carbonate weathering is a substantial source of groundwater DIC. However, groundwater is generally Na-rich and Ca-poor, and Na/Ca ratios increase along horizontal flow paths due to cation exchange, probably in clay lenses. Groundwater with high Na/Ca ratios also has high concentrations of F and As (up to 6.6 mg/L and 27 ug/L, respectively), that are a health risk. The F and As are likely enriched due to desorption of F- and HAsO4 2- from hydrous metal oxides in the aquifer sediments. Experiments conducted with sediments and synthetic water solutions indicate that greater mobilization of F and As occurs in Na-rich, Ca-poor water, hence, cation composition is an important control on F and As mobilization.