Challenges of Artificial Recharge of Aquifers: Reactive Transport Through Soils, Fate of Pollutants and Possibility of the Water Quality Improvement

Abstract

The unsaturated zone acts indeed as a natural reactive filter and can reduce or remove microbial and organic/inorganic contaminants through biogeochemical processes enhancing mass transfer between phases (soil – water – gases). The performance of the soil to purify the infiltrated water is based on both chemical, geo-biochemical and hydrodynamic coupled processes in a porous medium. The geochemical reactivity of soil minerals and the biodegradation of organic matter involving microbial mediated redox-reactions are the key reactions characterizing the water cleaning capacity of a soil. The reactive transport mechanisms induced by aquifer recharge using secondary or tertiary treated wastewaters still containing metals, metalloids and organic matter as pollutants is studied through laboratory and pilot experiments. This technology targets the geochemical reactivity and dynamics of soil to improve water quality while maintaining environment quality and protecting other resources (aquifers, agricultural production, soil, etc.). Obviously, the dilemma to meet these both constraints becomes a real challenge. This study aimed to develop a general concept based on the control of the physical, chemical and microbial keys processes easy to integrate in the numerical predictive and quantitative tools. The reactive transport modeling is carried out in order to identify the relevant processes controlling the filtration capability of the soil. Some results of ongoing projects based on the understanding of reactive transport processes will be presented. The technologic challenges emerged from the environmental safety issue and from the artificial recharge study will be discussed. Artificial groundwater recharge of aquifers by percolation through the unsaturated zone (UZ) is a technique to enhance the water quality for drinking water supplies. The performance of the UZ to purify the infiltrated water is based on chemical, geobiochemical and hydrodynamic coupled processes in a porous medium. The geochemical reactivity of soil minerals and the biodegradation of organic matter involving microbial mediated redox-reactions are the key reactions characterizing the epuration capacity of a soil. In order to improve our understanding of the physical and chemical phenomena controlling the efficiency of such process, a series of projects in a coastal aquifer in south-eastern France are built between Veolia and BRGM. The projects are based on the integration of numerical simulations with calibrated parameters on laboratory, pilot experiments and field aquifer characterization. The site characterizations and numerical simulations tend to show the development of “filtrating zones” by combination of various physico-chemical and thermokinetic processes. On the other hand, the mixing between infiltrating recharge waters and seawater can have important impact on the dissolution of carbonate minerals and precipitations of sulphate minerals. The results will be extrapolated to the real (industrial) system to elaborate exploitation scenarios and sensitivity analysis.