Origin and residence time of groundwater based on stable and radioactive isotopes in the Heihe River Basin, northwestern China

Study region: The Heihe River Basin (HRB) is one of several arid basins in which runoff from the Qilian Mountain recharges basin aquifers. Study focus: A basin-wide dataset (δ18O, D, 3H and 14C) is used to determine the present and past relationships between precipitation, surface runoff and recharge, to constrain groundwater residence times, and to infer Holocene climate change. New hydrological insights for the region: Groundwater in the upper region (UR) of HRB has (δ18O, δD) clustered near (−8.0, −46‰), consistent with present-day Qilian Mountain precipitation. Tritium of groundwater >26 TU indicates post-bomb recharge. Mountain runoff provides recharge to alluvial-fluvial aquifers in the Middle Region (MR) and Lower Region (LR) along the main river of the HRB. Between 1986 and 2001, anthropogenic tritium releases affected north-central China, affecting HRB precipitation. Irrigation reflux strongly affects isotopes in basin groundwater, generating anomalous samples with low tritium and post-bomb 14C, or high tritium and pre-bomb 14C. Stable isotopes in Qilian Mountain runoff have evolved in response to climate change. A 1‰ shift in δ18O since 1960 coincides with drying of the Aral Sea, possibly affecting moisture advected from the west. A 6–8‰ shift before 12 ka may indicate the former extent of the South Asian monsoon. Keywords: Heihe River Basin (HRB), Groundwater origin, Residence time, Stable and radioactive isotopes, Northwestern Chin