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Recharge processes and vertical transfer investigated through long-term monitoring of dissolved gases in shallow groundwater.

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Title: Recharge processes and vertical transfer investigated through long-term monitoring of dissolved gases in shallow groundwater.
Authors: de Montety, V.1 veronique.de-montety@univ-montp2.fr, Aquilina, L.2, Labasque, T.2, Chatton, E.2, Fovet, O.3, Ruiz, L.3, Fourré, E.4, de Dreuzy, J.R.2
Source: Journal of Hydrology. May2018, Vol. 560, p275-288. 14p.
Subjects: Groundwater pollution, Noble gases, Seasonal temperature variations, Groundwater recharge, Aquifers, Watersheds, Hydrologic cycle
Abstract: We investigated temporal variations and vertical evolution of dissolved gaseous tracers (CFC-11, CFC-12, SF 6 , and noble gases), as well as 3 H/ 3 He ratio to determine groundwater recharge processes of a shallow unconfined, hard-rock aquifer in an agricultural catchment. We sampled dissolved gas concentration at 4 locations along the hillslope of a small experimental watershed, over 6 hydrological years, between 2 and 6 times per years, for a total of 20 field campaigns. We collected groundwater samples in the fluctuation zone and the permanently saturated zone using piezometers from 5 to 20 m deep. The purpose of this work is i ) to assess the benefits of using gaseous tracers like CFCs and SF6 to study very young groundwater with flows suspected to be heterogeneous and variable in time, ii ) to characterize the processes that control dissolved gas concentrations in groundwater during the recharge of the aquifer, and iii ) to understand the evolution of recharge flow processes by repeated measurement campaigns, taking advantage of a long monitoring in a site devoted to recharge processes investigation. Gas tracer profiles are compared at different location of the catchment and for different hydrologic conditions. In addition, we compare results from CFCs and 3 H/ 3 He analysis to define the flow model that best explains tracer concentrations. Then we discuss the influence of recharge events on tracer concentrations and residence time and propose a temporal evolution of residence times for the unsaturated zone and the permanently saturated zone. These results are used to gain a better understanding of the conceptual model of the catchment and flow processes especially during recharge events. [ABSTRACT FROM AUTHOR]
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Database: Engineering Source
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Abstract:We investigated temporal variations and vertical evolution of dissolved gaseous tracers (CFC-11, CFC-12, SF 6 , and noble gases), as well as 3 H/ 3 He ratio to determine groundwater recharge processes of a shallow unconfined, hard-rock aquifer in an agricultural catchment. We sampled dissolved gas concentration at 4 locations along the hillslope of a small experimental watershed, over 6 hydrological years, between 2 and 6 times per years, for a total of 20 field campaigns. We collected groundwater samples in the fluctuation zone and the permanently saturated zone using piezometers from 5 to 20 m deep. The purpose of this work is i ) to assess the benefits of using gaseous tracers like CFCs and SF6 to study very young groundwater with flows suspected to be heterogeneous and variable in time, ii ) to characterize the processes that control dissolved gas concentrations in groundwater during the recharge of the aquifer, and iii ) to understand the evolution of recharge flow processes by repeated measurement campaigns, taking advantage of a long monitoring in a site devoted to recharge processes investigation. Gas tracer profiles are compared at different location of the catchment and for different hydrologic conditions. In addition, we compare results from CFCs and 3 H/ 3 He analysis to define the flow model that best explains tracer concentrations. Then we discuss the influence of recharge events on tracer concentrations and residence time and propose a temporal evolution of residence times for the unsaturated zone and the permanently saturated zone. These results are used to gain a better understanding of the conceptual model of the catchment and flow processes especially during recharge events. [ABSTRACT FROM AUTHOR]
ISSN:00221694
DOI:10.1016/j.jhydrol.2018.02.077