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Discussion papers | Copyright
© Author(s) 2018. This work is distributed under
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Research article 10 Apr 2018

Research article | 10 Apr 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Hydrology and Earth System Sciences (HESS).

Groundwater mean transit times, mixing and recharge in faulted-hydraulic drop alluvium aquifers using chlorofluorocarbons (CFCs) and tritium isotope (3H)

Bin Ma1,2, Menggui Jin1,2,3, Xing Liang1,4, and Jing Li1 Bin Ma et al.
  • 1School of Environmental Studies, China University of Geosciences, Wuhan, 430074, China
  • 2State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan, 430074, China
  • 3Laboratory of Basin Hydrology and Wetland Eco-restoration, China University of Geosciences, Wuhan, 430074, China
  • 4Hubei Key Laboratory of Wetland Evolution & Ecological Restoration, China University of Geosciences, Wuhan, 430074, China

Abstract. Documenting the transit times and recharge sources of mixed groundwater is crucial for water resource management in the alluvium aquifers of arid basin. Environmental tracers (CFCs, 3H, 14C, δ2H, δ18O) and hydrochemistry of mixed groundwater were used to assist our understanding of groundwater mean transit times (MTTs), mixing and aquifer recharge in faulted–hydraulic drop alluvium aquifers in the Manas River Basin (China). 3H activities of groundwater decrease with distance to the mountain from 41.1–60TU in the Manas River upstream (south of the fault), indicating the rainfall recharge since the 1960s, to as low as 1.1TU in the downstream (north of the fault). Carbon-14 groundwater ages increase with distance (3000–5000yrs in the midstream to >7000yrs in the downstream) and depth, as well as with more depleted δ18O values, confirming that the deeper groundwater is derived from paleometeoric recharge in the semi-confined groundwater system. MTTs estimated using an exponential-piston flow model vary from 19 to 101yrs for CFCs and from 19 to 158yrs for 3H, which show much longer MTTs for 3H than CFCs may be due to the time lag through the thick unsaturated zone. The thrust faults were found to play a paramount role on groundwater flow paths and MTTs due to their block water features, where the relatively long MTTs were found near the Manas City with shorter distance and smaller hydraulic gradients. The remarkable correlations between CFCs rather than 3H MTTs and pH, SiO2 and SO42− concentrations allow first-order proxies of MTTs for groundwater at different times to be made. The quite modern recharge in the south of the fault with young (post-1940) water fractions of 87–100% is obtained, while in the north of the fault in the midstream area the young water fractions vary from 12 to 91% based on the CFC binary mixing model. This study shows that the combination of CFCs and 3H transit time tracers have potential to study groundwater MTTs and identify the recharge sources for the different mixing end-members.

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Short summary
Groundwater supplies the largest and even only freshwater to industrial and agricultural production and living in the arid Northwest China. This research uses environmental tracers to enhance one's understanding of groundwater including the groundwater recharge sources, mean transit times and mixing characteristics. The results provide valuable implications for groundwater resources regulation and sustainable development and have its practical significance for other arid areas.
Groundwater supplies the largest and even only freshwater to industrial and agricultural...