Oxido-reduction sequence related to flux variations of groundwater from a fractured basement aquifer (Ploemeur area, France)

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.