Published 09/07/2018
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Compartmentalization of physical and chemical properties in hard-rock aquifers deduced from chemical and groundwater age analyses

O.Bour , P.Davy , A.Pierson-Wickmann , L.Aquilina , E.Fourre , T.Labasque , H.Pauwels , P.Merot , V.Durand , J.Molenat , V.Ayraud , C.Tarits , P.Le

Groundwater from hard-rock aquifers constitutes a water resource that is important but vulnerable to pollution. There are various sources of groundwater contamination which may have important consequences on if anthropogenic activities could led to a high degree of pollution of shallow groundwaters, recent studies have demonstrated that these aquifers can represent a major water resource that has not been fully investigated yet. Due to a decreasing weathering intensity from surface to depth, hard rock aquifers are divided into 3 major zones.
Using groundwater age determination done through CFC analysis and geochemical data obtained from seven sites in Brittany , 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 , which represents a transition zone between the weathered zone and the lower fractured zone; and the unweathered part of the aquifer. The weathered layer is often considered as a porous medium and is the only part frequently used in hard-rock aquifers. Recent apparent ages 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 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. 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. 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 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.
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