Using geochemical data and modelling to enhance the understanding of groundwater flow in a regional deep aquifer, Aquitaine Basin, south-west of France

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Abstract

In deep aquifers the complex flow pattern originating from the geological structure often leads to difficult predictions of water origin, determination of the main flow paths, potential mixing of waters. All these uncertainties prevent an efficient management of the resource. In the context of the Aquitaine basin an original modelling approach suggests that geochemical data can be used to identify flow directions where geological and hydrogeological data are too scarce to provide sufficient information.

In the Eocene sands aquifer, the major patterns of groundwater geochemistry suggest the presence of two distinct areas within the aquifer. In the north and the east, waters exhibit sodium bicarbonate or sodium sulphate facies, and moderate total dissolved solids related to high sulphate concentrations. In the south, waters are characterised by calcium bicarbonate facies and low total dissolved solids.

Sulphur isotopic ratios provided key information on the origin of sulphur in solution (meteoric, gypsum dissolution, pyrite oxidation) and also on the intensity of the geochemical processes involved in the dissolution of minerals and the concentration evolution.

A geochemical model was developed to analyse the processes generating the chemical composition of each sampled water. At the aquifer scale, four main geochemical processes—dissolution, redox, acid–base reaction, exchange—of varying intensity could explain most of the observed spatial variability in groundwater composition.

Among several potential reaction schemes at each point, only one allowed to reproduce the independent variables (pH and 13C). The developed model was used to select the most probable water pathways at the aquifer scale. In this context, geochemistry clearly demonstrates the role played by subsurface structures on water flow velocities and residence time in their vicinity. In addition, the concentrations of several ions could only be justified by the aquitard–aquifer interactions.

Introduction

The chemical composition of groundwater is controlled by many factors that include composition of precipitation, geological structure and mineralogy of the watersheds and aquifers, and geochemical processes within the aquifer. The interaction of all factors leads to various water facies. Usually, major ions studies are used to define hydrochemical facies of waters and the spatial variability can provide insight into aquifer heterogeneity and connectivity (Murray, 1996, Rosen and Jones, 1998). With the development of geochemical modelling, trace, major and isotopic elements are used to infer the physical and chemical processes controlling the water chemistry and to delineate flow paths in aquifer (Eberts and George, 2000, Plummer and Sprinckle, 2001, Güler and Thyne, 2004, André, 2002).

The Eocene sands aquifer, located in the south Aquitaine Basin, constitutes an important water resource used for various purposes (thermal water, potable water…). This aquifer has been mostly studied since the end of the 50's along with the development of oil exploration in this part of France. The production wells and exploratory borings were used to draw structural maps and to define the hydrodynamic properties of the aquifer. More recently, the conflicts for water use have led to detailed studies of the hydrogeology of the aquifer (Labat, 1998). The hydrodynamic characterisation allowed identification of some main flow lines and the approximate behaviour of the aquifer near geologic structures.

The water chemistry has been first approached by a description of the spatial variation of some ions and by the calculation of the residence time (Blavoux et al., 1993). The approach presented here uses geochemical indicators and modelling, both linked to the structural or deep sedimentologic features in order to locate potential mass transfers. After a detailed analysis of the regional geology and the sampling locations used for observations, the physico-chemical, chemical and isotopic data are presented. Specific attention will be paid to isotopic data, particularly 34S.

Geochemical modelling is needed to deal with the interactions of several elements and dissolution processes. A pre-requisite step is devoted to a detailed inspection of the reactions that can occur, or not, in the studied aquifer. The modelling results are then validated using independent variables. Particular attention is paid to the points having an original behaviour or water composition that could not be predicted using the first group of assumptions.

The results of the geochemical modelling step are then interpreted in terms of potential flow at the aquifer scale. Similar regional approaches were used by several authors to interpret the geochemical evolution in regional aquifers (e.g. Hendry and Schwartz, 1990, Weaver and Bahr, 1991, Gerla, 1992, Sracek and Hirata, 2002). The quantitative approach of the reaction linked with the information on the aquifer solid and isotope data allowed to enhance the confidence in geochemical models. The modelling results were able to locate flow barriers at the regional scale and to elucidate the circulations in complex geological structure. The developed approach, involving the analysis of geochemical reactions, validation variables and aquitard–aquifer interactions, seems to be applicable to other basins.

Section snippets

Hydrogeologic and geologic settings

The Aquitaine Basin is limited in the east by the foothills of Montagne Noire, in the south by the North Pyrenean Piedmont, in the west by the Atlantic Ocean and in the north by the Poitou Plateau. We will be interested here in the southern sector of the basin located to the south of the Garonne river.

The Eocene sands aquifer, located in this part of the Aquitaine Basin, constitutes a major aquifer used for drinking water, agriculture, gas storage and as a thermal resource. This aquifer extends

Sampling and analyses

Waters were sampled during various field campaigns distributed over two years. This is due to the irregular use of some wells (irrigation, geothermal power heat). Owing to the residence time of the water in the aquifer, the time lags may not play a dominant role on the chemical composition of the waters and consecutive samplings at some wells showed a constant chemical composition.

Several parameters were measured in the field: temperature, pH, alkalinity and redox potential measurements were

Modelling strategy

A geochemical model is needed to quantify the relative importance of each chemical reaction at the sampled wells. During this first geochemical modelling step, only the system Na, Ca, Mg, Cl, S, CO3, H is considered. Si and K concentrations are not included due to the lack of information on the composition of the clay fraction of the aquifer, not accessible from cuttings samples. Moreover, the silicate equilibrium does not significantly modify the acid–base and redox equilibrium, and can thus

Modelling results

All groundwater samples were modelled using the approach described above (Fig. 8). It was possible to reproduce most of the water compositions (Table 4, Table 5, Table 6).

Among the processes, calcite equilibrium is the major factor influencing pH evolution, the pH value being also influenced by pyrite oxidation where it occurs. A good agreement is found between calculated and measured in situ pH, the maximum variations reaching only 0.3 pH unit except for two points having variation near 0.7 pH

Geochemical arguments for flow delineation

The developed model can also help to understand the connections between different sampled points. In fact a similar composition of two waters is not a proof of hydrodynamic connection. However, if an hydraulic connection is assumed, one must be able to explain the chemical processes that lead from one point to the other. This is the basis of how we used the geochemical model to delineate groundwater flow paths: some water paths are not possible and some preferential flow directions can be drawn

Conclusion

The results of this study show that detailed hydrochemical data coupled with geochemical modelling can help to elucidate the hydrologic and geologic factors controlling water chemistry on a regional basin.

Waters from the Eocene sands aquifer present mainly calcium bicarbonate facies with an evolution of the geochemical facies at several places. To understand these variations, detailed chemical and isotopic data were used; changes in water chemistry were interpreted and three distinct

Acknowledgements

The authors would like to thank TotalFinaElf—Stockage Gaz France for supporting this work.

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