Marie Pettenati

Managed Aquifer Recharge: An Overview of Issues and Options

As covered in Chap. 2, many of the world’s aquifers are rapidly being depleted. Nearly one quarter of the world’s population – 1.7 billion people – live in regions where more water is being consumed than nature can renew (Gleeson et al. 2012). Over-exploitation occurs when groundwater abstraction is too intensive, for example for irrigation or for direct industrial water-supply like extracting fossil fuels (Pettenati et al. 2013; Foster et al. 2013). When groundwater is continuously over-pumped, year after year, the volume withdrawn from the aquifer cannot be replaced by recharge. Eventually, the groundwater level is much lower than its initial level and even when pumping stops, the aquifer has trouble rising once again to its original level. In continental zones, over-exploitation can lead to groundwater drawdown and, ultimately, to subsidence through development of sinkholes when underground caverns or channels collapse. In coastal areas, the decrease in groundwater recharge results in saltwater intrusion into the aquifer formation (Petalas and Lambrakis 2006; De Montety et al. 2008). Preserving local groundwater resources is an environmental and economic issue in coastal zones and is vital in an island context. The increasing demand for water caused by a growing population can lead to the salinization of groundwater resources if these are systematically over-exploited. Limiting the salinization of coastal aquifers is consistent with the groundwater objective of the European Union Water Framework Directive, which is to achieve a good qualitative and quantitative status by 2015. The economic advantage of preserving these threatened water resources is that, when there is a growing demand, a local water resource is sustained and there is no need to import water. Transporting water can cost 2–10 times more than limiting the intrusion of saltwater into a coastal aquifer.

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

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.

Autotrophic denitrification supported by biotite dissolution in crystalline aquifers (1): New insights from short-term batch experiments

We investigate denitrification mechanisms through batch experiments using crushed rock and groundwater from a granitic aquifer subject to long term pumping (Ploemeur, France). Except for sterilized experiments, extensive denitrification reaction induces NO3 decreases ranging from 0.3 to 0.6mmol/L. Carbon concentrations, either organic or inorganic, remain relatively stable and do not document potential heterotrophic denitrification. Batch experiments show a clear effect of mineral dissolution which is documented through cation (K, Na, Ca) and Fluoride production. These productions are tightly related to denitrification progress during the experiment. Conversely, limited amounts of SO4, systematically lower than autotrophic denitrification coupled to sulfur oxidation stoichiometry, are produced during the experiments which indicates that sulfur oxidation is not likely even when pyrite is added to the experiments. Analysis of cation ratios, both in isolated minerals of the granite and within water of the batch, allow the mineral dissolution during the experiments to be quantified. Using cation ratios, we show that batch experiments are characterized mainly by biotite dissolution. As biotite contains 21 to 30% of Fe and 0.3 to 1.7% of F, it constitutes a potential source for these two elements. Denitrification could be attributed to the oxidation of Fe(II) contained in biotite. We computed the amount of K and F produced through biotite dissolution when entirely attributing denitrification to biotite dissolution. Computed amounts show that this process may account for the observed K and F produced. We interpret these results as the development of microbial activity which induces mineral dissolution in order to uptake Fe(II) which is used for denitrification. Although pyrite is probably available, SO4 and cation measurements favor a large biotite dissolution reaction which could account for all the observed Fe production. Chemical composition of groundwater produced from the Ploemeur site indicates similar denitrification processes although original composition shows mainly plagioclase dissolution.