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[1] Through a series of groundwater flow and 4 He transport simulations, this study illustrates the conceptual and practical gains achieved by expanding a two-dimensional (2-D) model to a true 3-D one through an application in the Carrizo aquifer and surrounding formations in southwestern Texas. The 3-D simulations allow for a more detailed and accurate definition of the heterogeneities of the system by specifically identifying and differentiating processes that directly impact the three-dimensional hydraulic conductivity field. It is shown that while hydraulic conductivity decreases exponentially along the regional groundwater flow direction, such decrease is better described as a function of depth rather than recharge distance. This relationship reflects the combined influences of differential compaction of the media as well as downdip lithological change. The intrinsic permeability derived from this relationship agrees with field information and with previous findings obtained for the continental crust for depths 2 km, suggesting that for large scales, decrease rate of permeability with depth is independent of the media. Results also suggest that the solution for groundwater flow simulations based on calibration of hydraulic heads depends on the ratio between hydraulic conductivities of different formations, indicating that an infinite number of solutions are available for calibration of 3-D groundwater flow models. Understanding how geological processes directly affect the 3-D hydraulic conductivity field at the regional scale is essential not only to hydrogeological applications but also at improving our understanding of the Earth's crust and mantle dynamics by allowing for a more accurate quantification of helium and heat fluxes.

Deep subsurface aquifers despite difficult access, represent important water resources and, at the same time, are key locations for subsurface engineering activities for the oil and gas industries, geothermal energy, and CO2 or energy storage. Formation water originating from a 760 m-deep geological gas storage aquifer was sampled and microcosms were set up to test the biodegradation potential of BTEX by indigenous microorganisms. The microbial community diversity was studied using molecular approaches based on 16S rRNA genes. After a long incubation period, with several subcultures, a sulfate-reducing consortium composed of only two Desulfotomaculum populations was observed able to degrade benzene, toluene, and ethylbenzene, extending the number of hydrocarbonoclastic–related species among the Desulfotomaculum genus. Furthermore, we were able to couple specific carbon and hydrogen isotopic fractionation during benzene removal and the results obtained by dual compound specific isotope analysis (𝜀C = -2.4‰ ± 0.3‰; 𝜀H = -57‰ ± 0.98‰; AKIEC: 1.0146 ± 0.0009, and AKIEH: 1.5184 ± 0.0283) were close to those obtained previously in sulfate-reducing conditions: this finding could confirm the existence of a common enzymatic reaction involving sulfate-reducers to activate benzene anaerobically. Although we cannot assign the role of each population of Desulfotomaculum in the mono-aromatic hydrocarbon degradation, this study suggests an important role of the genus Desulfotomaculum as potential biodegrader among indigenous populations in subsurface habitats. This community represents the simplest model of benzene-degrading anaerobes originating from the deepest subterranean settings ever described. As Desulfotomaculum species are often encountered in subsurface environments, this study provides some interesting results for assessing the natural response of these specific hydrologic systems in response to BTEX contamination during remediation projects.

[1] On the basis of chloride concentrations of pore water in the Tournemire massif (part 1), a conceptual model for mass transport in argillites by diffusion is proposed. From this conceptual model and current knowledge of the geological history of the massif, one-dimensional numerical simulations are formulated for chloride transport in Tournemire massif over the past 53 Ma. Good agreement between experimental data and calculated values for both diffusion coefficients and concentrations of chloride confirms that diffusion is the main process for mass transport in the massif. This model is also tested using deuterium contents of pore water, applying variable concentrations to meteoric water (circulating in system boundary layers) based on the thermal dependency of its isotopic composition. These simulations reveal the likely important role of lithologic heterogeneities, such as fractures, in the horizontal distribution of tracer concentrations.