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

Isotopes (delta O-18 and delta D, delta S-34(SO4) and delta O-18(SO)4, tritium and C-14) 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 delta D = 7.8 delta O-18 + 18.1. Precipitation from westerly circulation has a characteristic annual cycle of delta O-18 and delta D, high (delta O-18 > -5 parts per thousand) in summer and low (delta O-18 < -10 parts per thousand) at other times. This pattern was interrupted by an incursion of the Indian summer monsoon in August 2018, resulting in abnormally low delta O-18 and delta D values. Surface water in the upper SRB yields an evaporation trend of slope near 5, with an origin near delta O-1(8) = -10 parts per thousand( )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 delta O-1(8) and delta D 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 delta O-18 values between -14 and -20 parts per thousand on the LMWL, and may represent incursion of monsoonal circulation prior to 12 ka. Tritium and C-14 data identify post-bomb recharge, but C-14 is of limited use in dating older groundwater mixtures. Sulfate isotopes (delta S-34(SO)4 and delta O-1(8)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.

Automated summary

Isotopes were used to study the moisture sources in precipitation, as well as the sources of surface water and groundwater in the Shule River Basin. The results showed that groundwater originates from high-altitude precipitation and meltwater from ice archives. The study also identified different types of water in mixing trends and revealed the potential impact of climate change on water shortage.