Isotopic evidence for a link between agricultural irrigation and high arsenic concentrations in groundwater

An isotope-based survey was carried out in the Datong Basin, northern China to investigate the hydrogeology of groundwater with high arsenic concentrations. Oxygen isotope (δ18O), hydrogen isotope (δD) and radioactive hydrogen isotope (3H) measurements were conducted with the aim of characterizing the groundwater origins and flow dynamics in this arsenic-contaminated groundwater system. Groundwater dating results from 3H measurements show that groundwaters from 20m ~ 70m have a wide range of ages (10a~ 191a), indicating diverse groundwater sources. In contrast, deeper groundwaters (70m ~90m) display a narrower age range (35a ~ 47a). In addition, the shallow-aquifer (<70m), groundwaters exhibit wide variations in δ18O and δD, from -12.7% to -6.96% and -97.1% to -49.8%, respectively. Deep groundwaters (>70m) possess relatively narrower isotopic ranges and mostly lighter isotopic ratios, from -12.8% to -8.88% and -97.6% to -71.7%, respectively. Comparison with the local meteoric water line shows that groundwater δ18O and δD values plot with a shallower slope, consistent with the arid-semiarid climate of the Datong Basin, as well as a meteoric origin of the groundwater, and points to precipitation as the dominant source of recharge to the deeper aquifers in the study area. Groundwaters with high arsenic concentrations (100μg/L ~ 309μg/L) mainly occur in aquifers at depths between 20m and 70m, while shallower (<20m) and deeper (>70m) groundwaters carry relatively lower arsenic concentrations (<50μg/L). This result differs from previous studies[1] [2], which documented that groundwaters with high arsenic concentrations occur primarily in the upper aquifers (<50m). It is striking that the groundwaters with elevated arsenic concentrations are also those with the greatest diversity of tritium ages and dispersion of δ18O and δD values, suggesting that a single process may explain all three data sets. One explanation is that extensive irrigation with groundwaters from various depths (10m~200m) induces the age diversity of the shallow groundwaters. This interpretation is supported by the variations in the δ18O and δD data. Intensive agricultural activities in the Datong Basin, including extensive pumping of irrigation water from aquifers of various depths, may be changing the hydrology of the shallow groundwater system and directly affecting the arsenic distribution in the groundwater. [1] Shvartsev, S. L. and Wang, Y. X. (2006). "Geochemistry of sodic waters in the Datong intermountain basin, Shanxi Province, northwestern China." Geochemistry International 44(10): 1015-1026. [2] Xie, X. J., Ellis A., Wang, Y. X., et al. (2009). "Geochemistry of redox-sensitive elements and sulfur isotopes in the high arsenic groundwater system of Datong Basin, China." Science of the Total Environment 407(12): 3823-3835.