An isotope study of the Shule River Basin, Northwest China: Sources and groundwater residence time, sulfate sources and climate change

Abstract

Isotopes (δ18O and δD, δ34SSO4 and δ18OSO4, tritium and 14C) were employed to reveal moisture sources in precipitation and sources of surface water and groundwater, as well as groundwater residence times and sulfate sources in the Shule River Basin (SRB). Groundwater originates in the Qilian Mountains as high-altitude precipitation and meltwater from ice archives. The local meteoric water line (LMWL) is δD = 7.8δ18O + 18.1. Precipitation from westerly circulation has a characteristic annual cycle of δ18O and δD, high (δ18O > −5‰) in summer and low (δ18O < −10‰) at other times. This pattern was interrupted by an incursion of the Indian summer monsoon in August 2018, resulting in abnormally low δ18O and δD values. Surface water in the upper SRB yields an evaporation trend of slope near 5, with an origin near δ18O = −10‰ on the LMWL. Other catchments of similar altitude in the Qilian Mountains have evaporation trends with different origin points, indicating different input fractions of meltwater from ancient ice for each catchment. Groundwater δ18O and δD data plot along mixing trends, different in each sub-basin, between three water types: (1) recent Shule River runoff; (2) water like that archived in the Dunde ice sheet, representing precipitation over the last 12 ka; and (3) evaporated water that cannot be explained as precipitation from the last 12 ka. Type (3) water originated as water with δ18O values between −14 and −20‰ on the LMWL, and may represent incursion of monsoonal circulation prior to 12 ka. Tritium and 14C data identify post-bomb recharge, but 14C is of limited use in dating older groundwater mixtures. Sulfate isotopes (δ34SSO4 and δ18OSO4) in dissolved sulfate from groundwater and surface water indicate mixing of sulfur derived from evaporite and sulfide, but do not identify sulfate pollution from fertilizer. Future climate change may lead to water shortage as ancient ice is consumed by melting.