Groundwater characterisation and modelling of water–rock interaction in an argillaceous formation (Tournemire, France)

• Published in: Applied geochemistry Vol. 23; no. 8; pp. 2182 - 2197
• Main Authors: Beaucaire, C., Michelot, J.-L., Savoye, S., Cabrera, J.
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01-08-2008; Elsevier
• Subjects: Earth Sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Exact sciences and technology; Geochemistry; Pollution, environment geology; Sciences of the Universe; Alps; Dolomites; France; Europe; Eastern Alps; Western Europe; kaolinite; ion exchange; ground water; calcite; air; aluminum; boreholes; aquifers; sheet silicates; silicates; framework silicates; flow; fractures; models; sampling; oxidation; quartz; concentration; drilling; transport; carbonates; contamination; transmissivity; water-rock interaction; dolomite; migration; clay minerals; equilibrium; reduction; chemical composition
• ISSN: 0883-2927; 1872-9134
• DOI: 10.1016/j.apgeochem.2008.03.003

Chemical (Na, K, Mg, Ca, Cl, SO
4, HCO
3, Si, Al, Fe, Mn, TOC) and isotopic (
18O[H
2O],
2H[H
2O],
3H[H
2O],
14C[DIC],
13C[DIC],
34S[SO
4],
18O[SO
4]) data were obtained on groundwater collected from fractures inside the consolidated argillaceous formation of Tournemire (Aveyron) and from the aquifers surrounding this formation. Because of the very low transmissivity of such fractures (10
−10
m
2
s
−1), specific devices were developed, for limiting out-gassing and air-contamination of groundwater during the sampling. Two modelling approaches are proposed to account for the chemical evolution of fracture groundwater from Na–Cl–HCO
3 to Na–SO
4 type. The first one is based on mineral equilibria only and the second one also takes into account the ion-exchange reactions. The two modelling approaches are able to reproduce K
+ concentrations and they led to an almost constant
pCO
2 (10
−2.6
atm), which only depends on equilibrium with the calcite–dolomite–kaolinite–quartz assemblage. The modelling of Na
+, Ca
2+ and
HCO
3
+
behaviours is improved by taking the ion-exchange reactions into account.
In some boreholes, the time evolution of the chemical composition of fluids was greatly influenced by SO
4 reduction combined with oxidation of dissolved organic C. The isotopic composition of aqueous SO
4 in fracture groundwater confirms that SO
4 reduction processes have occurred since the drilling of these boreholes, but also previously, during the natural evolution of groundwater. These redox reactions constitute a source of aqueous inorganic C that has to be considered in the computation of groundwater “ages” with
14C. For the studied fracture groundwaters, the maximum age estimates range from 17 to 29
ka. This shows that fractures may induce relatively fast groundwater flow, compared to the very long migration times that are calculated by only considering diffusive transport in the rock microporosity.