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

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.