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
This paper focuses on the chemical evolution of water during the exploitation of a fractured aquifer in a NO3-rich agricultural environment. During a ten year period, both production rate and chemical parameters were continuously measured in tap water obtained from a deep-water plant in Brittany, France. Changes in and were observed after pumping was initiated. Nitrate concentration decreased during the first 200 days and then stabilized at 5 ± 1 mg/L, while concentration increased rapidly over this period and then showed a steady state increase (0.01 mg/L/day). These changes are attributed to the development of equilibrium between the physical flow parameters and the chemical kinetics of autotrophic denitrification processes that occur in the pyrite-bearing fractures. The chemical characteristics of the groundwaters collected in 18 wells located around the site allow identification of two different areas. One is weakly influenced by pumping and is characterized by high concentrations and a short residence time. The second area is directly related to the main pumped well, and characterized by reduced levels combined with an increased production, resulting from the denitrification processes in the pyrite-bearing fractures. Over the last few years, a increase unrelated to denitrification has been recorded in some wells. Based on the , and Fe concentrations, this is attributed to oxidation of S minerals, coupled to FeIII reduction. Exploitation of the aquifer has led to a rapid transfer of the waters within the deep fractures. Their high velocities strongly control the chemical parameters and have led to a redox sequence that has promoted S oxidation, coupled with (1) O2, (2) , and (3) Fe reduction
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