Abstract
This paper focuses on the chemical evolution of water during the exploitation of a fractured aquifer in a NO 3-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 SO 4 2 - and NO 3 - were observed after pumping was initiated. Nitrate concentration decreased during the first 200 days and then stabilized at ∼5 ± 1 mg/L, while SO 4 2 - 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 NO 3 - concentrations and a short residence time. The second area is directly related to the main pumped well, and characterized by reduced NO 3 - levels combined with an increased SO 4 2 - production, resulting from the denitrification processes in the pyrite-bearing fractures. Over the last few years, a SO 4 2 - increase unrelated to denitrification has been recorded in some wells. Based on the NO 3 - , SO 4 2 - and Fe concentrations, this is attributed to oxidation of S minerals, coupled to Fe III 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) O 2, (2) NO 3 - , and (3) Fe reduction.
Highlights
Over the past few decades, discharges of NOÀ3 from drains or shallow wells have been increasing due to anthropogenic inputs (Jordan et al, 1997).Correlations between anthropogenic inputs, fertilizer application, cultivation of N-fixing crops, and groundwater and river water pollution have been studied by numerous authors (e.g. Hill, 1978; Osborne and Wiley, 1988; Mason et al, 1990; Jordan and Weller, 1996)
The data set allows the distinction of two types of changes: (1) long-term variation of chemical composition; (2) short-term variations affecting SO24À and NOÀ3 concentrations and alkalinity, which are linked to rapid changes in the production rate
In the aquifer, pumping at a depth of 100 m since 1991 creates a fluid flow variation as indicated by the in situ hydraulic measurements which indicate that the maximum drawdown depression is located in the pumped well zone and that its influence radius increases with time
Summary
Over the past few decades, discharges of NOÀ3 from drains or shallow wells have been increasing due to anthropogenic inputs (Jordan et al, 1997).Correlations between anthropogenic inputs, fertilizer application, cultivation of N-fixing crops, and groundwater and river water pollution have been studied by numerous authors (e.g. Hill, 1978; Osborne and Wiley, 1988; Mason et al, 1990; Jordan and Weller, 1996). Over the past few decades, discharges of NOÀ3 from drains or shallow wells have been increasing due to anthropogenic inputs (Jordan et al, 1997). The spatial distribution of groundwater quality is related to groundwater contamination aspects It depends on water pathways, residence time, and on the soil and aquifer hydraulic properties (Grambell et al, 1975; Spalding and Exner, 1993; Brenner and Mondok, 1995; Martin et al, 2004). The silicate bedrock contains small aquifers that are usually located in the fracture system of the basement. Both large porous fractures and a high connectivity are required for the development of a large and sustainable water resource. Understanding the physics and the chemistry of fractured aquifers illustrates several problems that are mainly related to fluid flow heterogeneity and their relation with biochemical processes
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