Integrated approach for predicting geogenic contaminants in groundwater

Aquifers may naturally contain undesirable and toxic trace elements, known as geogenic contaminants. The presence of these micropollutants poses a major challenge for groundwater management, and has health, economic, and environmental consequences.

Areas with high concentrations, either elevated or exceed the guideline value of water standards, are generally identified through the analysis of groundwater data. However, it should be possible to predict these occurrences based on their presence in the solid matrices of aquifers and their mobilization controlled by the physicochemical conditions of the water.

To better predict the occurrence of inorganic natural pollutants, we have developed a methodology, based on an integrated approach to better understand the distribution of these elements in aquifers, the conditions controlling their presence, and their evolution in groundwaters.

The methodology combines two complementary approaches: i) predictions of geogenic elements content in rocks, including their speciation, using a source to sink methodology are cross-correlating with ii) hydrogeochemistry to identify water-rock interaction processes and the potential mobility of the elements according to physicochemical conditions (e.g., redox conditions). This makes it possible to predict the spatial distribution of geogenic elements (e.g. As, Se, F, etc.) as well as the processes of mobilization in groundwater.

Using a geographic information system (GIS), this study compares predicted occurrences using a source-to-sink (S2S) approach with available data including a large dataset on groundwater quality data (ADES data base, a national database publicly available). By interpretating the chemical composition of water, geochemical modelling via PHREEQC and geological data, it is possible to confirm but also to contribute to the S2S modelling.

The study focuses on aquifers linked to the Massif Central (France). Geological source-to-sink paleomaps and drilling data are used to correlate the availability in sedimentary deposits of elements such as arsenic (As), selenium (Se), fluorine (F), with the main periods of erosion, transfer, deposit and remobilisation between the end of the Cretaceous and the Miocene. Arsenic, in particular, is studied in various geological layers with a focus on its speciation and mobilization in confined aquifers such as the Beauce calcareous confined aquifer (Southern Paris Basin).

This approach presents a real interest in terms of groundwater quality, as it helps to anticipate water quality degradation linked to groundwater exploitation in aquifer impacted by the natural presence of geogenic metals.

This work is part of the PEPR OneWater DEESAC project (France 2030), illustrating a transdisciplinary approach combining geology, geophysics, geochemistry, and hydrogeology.