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

Investigating groundwater residence time and recharge sources is crucial for water resource management in the alluvium aquifers of arid basins. Environmental tracers (chlorofluorocarbons, H-3, C-14, delta H-2, delta O-18) and groundwater hydrochemical components are used for assessing groundwater mean residence times (MRTs) and aquifer recharge in fault-influenced hydraulic drop alluvium aquifers in the Manas River basin (China). Aquifers under the Manas River upstream (south of the fault) contains very high H-3 activity (41.1-60 TU), implying water recharge affected by the nuclear bomb tests of the 1960s. Carbon-14 groundwater age correlates positively with distance from mountain area (3000-5000 years in the midstream to > 7000 years in the downstream) and groundwater depth, but correlates negatively to a decrease in H-3 activity (1.1 TU) and more negative delta O-18 values. This phenomenon reveals that the source of the deeper groundwater in the semi-confined aquifer is paleo-meteoric recharge. Special attention has been paid to the estimation of MRTs using CFCs and H-3 by an exponential piston flow model. The results show that MRTs vary from 19 to 101 years by CFCs and from 19 to 158 years by H-3. MRTs estimated from H-3 are much longer than those from CFCs, probably due to the different time lag of liquid (H-3) and gas-phase CFCs through the unsaturated zone. The MRTs estimated by CFCs show good correlations with pH and the concentrations of SiO2 and SO42-, which can provide a possible approach to estimate first-order proxies of MRTs for groundwater age. The young water fractions are investigated by the CFC binary mixing method in the south and north of the fault. Relatively modern recharge is found in the south of the fault with young (post-1940) water fractions of 87%-100 %, whereas in the north of the fault the young water fractions vary from 12% to 91 %. This study shows that the combination of CFCs and H-3 residence time tracers can help in analysing the groundwater MRTs and the recharge sources for the different mixing end-members.