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

Abstract

• The Indian summer monsoon invaded into the upper Shule River Basin in August 2018, interrupting the usual pattern of westerly moisture flow. • Surface water and groundwater originate from both recent precipitation and ancient precipitation archived in ice. • Groundwater with pre-evaporation δ 18 O < −14‰ may represent incursions of monsoonal moisture prior to 12 ka in the Hexi Corridor. • δ 34 S SO4 and δ 18 O SO4 in both surface water and groundwater fail to distinguish agricultural sulfate pollution from natural sulfate sources. Isotopes (δ 18 O and δD, δ 34 S SO4 and δ 18 O SO4 , tritium and 14 C) 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 = 7.8δ 18 O + 18.1. Precipitation from westerly circulation has a characteristic annual cycle of δ 18 O and δD, high (δ 18 O > −5‰) in summer and low (δ 18 O < −10‰) at other times. This pattern was interrupted by an incursion of the Indian summer monsoon in August 2018, resulting in abnormally low δ 18 O and δD values. Surface water in the upper SRB yields an evaporation trend of slope near 5, with an origin near δ 18 O = −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 δ 18 O 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 δ 18 O values between −14 and −20‰ on the LMWL, and may represent incursion of monsoonal circulation prior to 12 ka. Tritium and 14 C data identify post-bomb recharge, but 14 C is of limited use in dating older groundwater mixtures. Sulfate isotopes (δ 34 S SO4 and δ 18 O SO4 ) 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.