HYDROGEOCHEMICAL AND ISOTOPIC CONSTRAINS ON GROUNDWATER FLOW EVOLUTION IN THE MANAS RIVER BASIN IN ARID NORTHWESTERN CHINA

Environmental tracers (δ²H, δ¹⁸O, ⁸⁷Sr/⁸⁶Sr, CFCs) and major ion contents of various waters (river, reservoirs, springs and wells) have been used to investigate groundwater recharge, geochemical processes and groundwater residence times in the Manas River Basin, an arid region of northwestern China. Stable isotopes show that most groundwater and spring waters plot above the Global Meteoric Water Line (GMWL) but close to the Local Meteoric Water Line (LMWL) composed by the Global Network for Isotopes in Precipitation (GNIP) station, reflecting the meteoric origin and a lack of significant evaporation or isotope exchanges between water and the rock matrix. Groundwater δ²H and δ¹⁸O values show more homogenized values along the groundwater flow paths and with well depth, indicating inter-aquifer mixing progresses. The relationship between δ¹⁸O and Cl contents constrains two obvious different groundwater evolution progresses: (1) lateral recharge and mixing indicating a local groundwater flow system in the piedmont alluvial-oasis plain; and (2) leaching and vertical mixing corresponding to the longer flow paths and residence times with intermediate and regional groundwater flow systems from the northern oasis plain to the desert. Groundwater chemistry evolution is mainly controlled by rock dominance, like carbonate dissolution, and cation-exchange and mixing processes. End-member mixing analysis using ⁸⁷Sr/⁸⁶Sr ratios and dissolved species ratios reveals that carbonate dissolution from the southern mountain is mostly controlling Sr isotope geochemistry processes. Moreover, the remarkably small change of ⁸⁷Sr/⁸⁶Sr ratios indicates a well-mixed relatively homogeneous source of Sr along the groundwater flow paths. Three regimes are identified in the local groundwater flow system based on the CFC concentrations: (1) recharge zone, from the south mountain to the SHZ west, with a CFC-12 and CFC-113 binary mixing model fraction of 0.70-0.83 young water (groundwater apparent age of 29-35 a); (2) discharge zone characterized by a mixing fraction of 0.37-0.64 young water (37.5-42.5 a) in the SHZ north to the piedmont oasis plain, accompanied by upward discharge, like artesian springs; and (3) stagnant zone with mixing fraction of 0.09-0.50 young water (40-55 a) in the SHZ and Manas River east.