Compartmentalization of physical and chemical properties in hard-rock aquifers deduced from chemical and groundwater age analyses

Abstract

International audienceUsing groundwater age determination done through CFC analysis and geochemical data obtained from seven sites in Brittany (France), a hydrogeochemical model for hard-rock aquifers is presented. According to the geological structure, three zones can be defined: the weathered layer, about 30 m thick; the weathered-fissured layer (fractured rock with a high density of fissures induced by weathering), which represents a transition zone between the weathered zone and the lower fractured zone; and the unweathered part of the aquifer. (1) The weathered layer (alterites) is often considered as a porous medium and is the only part frequently used in hard-rock aquifers. Recent apparent ages (0­10 a) are observed in the groundwater fluctuation zone in a thin layer, which is from 1­2 m-thick in the lower parts and 10­15 m-thick in the upper parts of the catchments. Below this thin layer, the groundwater apparent age is high (between 10 and 25 a) and is unexpectedly homogeneous at the regional scale. This groundwater apparent age contrast, which also corresponds to a Cl- concentration contrast, is attributed to rapid lateral transfers in the fluctuation zone which limit water transfer to the underlying weathered zone. Groundwater chemistry is characterized by and Cl- concentrations related to land uses (high in agricultural areas, low in preserved ones). (2) At the interface between the weathered and the weathered-fissured layers a strong biogeochemical reactivity is observed. Autotrophic denitrification is enhanced by a higher availability of sulfides. (3) Under this interface, in the weathered-fissured layer and the underlying fractured deep part of the aquifer, groundwater apparent age is clearly correlated to depth. The vertical groundwater velocity is estimated to be 3 m/a, whatever be the site, which seems to indicate a regional topographic control on groundwater circulation in the deep part of the aquifer. In this deep part, groundwater chemistry is modified by water­rock interaction processes as indicated by Ca and Na concentrations, and a slight sea-water contribution (from 0.1% to 0.65%) in the sites close to the seacoast. One site inland shows a saline and old end-member. The global hydrogeochemical scheme is modified when the aquifer is pumped at a high rate in the fissured-weathered layer and/or the fractured layer. The increase in water velocity leads to a homogeneous groundwater apparent age, whatever be the depth in the weathered-fissured and fractured layers

Residence time, mineralization processes and groundwater origin within a carbonate coastal aquifer with a thick unsaturated zone

“…The analytical uncertainty is estimated to be 1% to 3% for CFCs and near 5% for SF 6. Thus, the global uncertainty on residence time, including sampling, analysis, selection of the recharge temperature in Henry's law, and dispersion-adsorption effects, is estimated to be from 2 to 11 a (Ayraud et al, 2008;Labasque et al, 2006;Labasque et al, 2014).…”

“…The calculations assume an average elevation for the samples of 100 m ASL. Excess air calculation is used to correct SF 6 data, which could be greatly influenced by this parameter in contrast to CFCs (Ayraud et al, 2008;Maiss and Brenninkmeijer, 1998;Plummer et al, 2001). …”

“…Noble gases are extracted by head-space extraction with a He phase (Ayraud et al, 2008;Labasque et al, 2006). Ar and Ne were subsequently measured using a micro-gas chromatograph (GC 3000, SRA instruments) at the OSUR Geosciences Laboratory of the University of Rennes 1, France.…”

Timescales of regional circulation of saline fluids in continental crystalline rock aquifers (Armorican Massif, western France)

“…2). Moreover, previous studies in the Armorican basement have identified a compartmentalization of these crystalline rock aquifers from hydrological, geochemical, geological and geophysical data (Ayraud et al, 2008;Roques et al, 2014), and from regional-scale numerical modeling (Goderniaux et al, 2013). Geochemical studies also showed specific chemical compositions of the water in the deep fractured aquifer, potentially including saline groundwaters Ayraud et al, 2008;Pauwels et al, 2013).…”

“…The study focuses on the Armorican basement (western France), where several authors have reported evidence of saline fluids (Ayraud et al, 2008;Pauwels et al, 2010). Three main transgressions were recorded during Mio-Pliocene to Pleistocene times, the latest being the highest.…”