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. Investigating groundwater residence time and recharge sources is crucial forwater resource management in the alluvium aquifers of arid basins.Environmental tracers (chlorofluorocarbons, 3H , 14C , δ2H , δ18O ) and groundwater hydrochemical components are used forassessing groundwater mean residence times (MRTs) and aquifer recharge infault-influenced hydraulic drop alluvium aquifers in the Manas River basin(China). Aquifers under the Manas River upstream (south of the fault)contains very high 3H activity (41.1–60 TU), implying waterrecharge affected by the nuclear bomb tests of the 1960s. Carbon-14groundwater age correlates positively with distance from mountain area(3000–5000 years in the midstream to > 7000 years in thedownstream) and groundwater depth, but correlates negatively to a decrease in 3H activity (1.1 TU) and more negative δ18O values.This phenomenon reveals that the source of the deeper groundwater in thesemi-confined aquifer is paleo-meteoric recharge. Special attention has beenpaid to the estimation of MRTs using CFCs and 3H by an exponentialpiston flow model. The results show that MRTs vary from 19 to 101 years byCFCs and from 19 to 158 years by 3H . MRTs estimated from 3H are much longer than those from CFCs, probably due to thedifferent time lag of liquid ( 3H ) and gas-phase CFCs through theunsaturated zone. The MRTs estimated by CFCs show good correlations with pHand the concentrations of SiO2 and SO 4 2 - , which canprovide a possible approach to estimate first-order proxies of MRTs forgroundwater age. The young water fractions are investigated by the CFC binarymixing method in the south and north of the fault. Relatively modern rechargeis found in the south of the fault with young (post-1940) water fractions of87 %–100 %, whereas in the north of the fault the young waterfractions vary from 12 % to 91 %. This study shows that thecombination of CFCs and 3H residence time tracers can help inanalysing the groundwater MRTs and the recharge sources for the differentmixing end-members.

hydrology and earth system sciences

Application of environmental tracers for investigation of groundwater mean residence time and aquifer recharge in fault-influenced hydraulic drop alluvium aquifers