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

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• 作者：C. Beaucaire ; J.-L. Michelot ; S. Savoye ; J. Cabrera
• 刊名：Applied Geochemistry
• 出版年：2008
• 出版时间：August 2008
• 年：2008
• 卷：23
• 期：8
• 页码：2182-2197
• 全文大小：653 K

文摘

Chemical (Na, K, Mg, Ca, Cl, SO₄, HCO₃, Si, Al, Fe, Mn, TOC) and isotopic (¹⁸O[H₂O], ²H[H₂O], ³H[H₂O], ¹⁴C[DIC], ¹³C[DIC], ³⁴S[SO₄], ¹⁸O[SO₄]) 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 m² 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–HCO₃ to Na–SO₄ 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 pCO₂ (10^−2.6 atm), which only depends on equilibrium with the calcite–dolomite–kaolinite–quartz assemblage. The modelling of Na⁺, Ca²⁺ and 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 SO₄ reduction combined with oxidation of dissolved organic C. The isotopic composition of aqueous SO₄ in fracture groundwater confirms that SO₄ 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.