Fate of nitrate and sulfate in waters pumped from a fractured aquifer. In : RST, 20-25 septembre 2004, Strasbourg, France

Many groundwaters of French Brittany show nitrate concentration exceeding 50mg/L. / Supplying of Ploemeur town (10,000 inhab ) with drinking water owes to pumping / at 40-100 m depth from a fractured aquifer is located in a granite/schist shear zone, / Monitoring of chemical composition of pumped water for more than ten years / revealed that nitrate concentration never exceeds 10mg/L while in surrounding wells / it ranges from 30 to 120mg/L. / The SO4 and NO3 concentration evolved rapidly during the first two years after / pumping initiation, and show a anticorrelated evolution which is attributed to / autotrophic denitrification related to microbial activity. / 5FeS2 + 15NO3- + 10H+ ?> 15/2 N2 + 5Fe2+ +10 SO42- + 5H2O / In the upper part of the aquifer, SO4 and Fe concentrations became extremely high / during the last years, This is attributed to more reducing conditions, related to the / continuous high flow in the system, initiating sulfide oxidation coupled to Fe3+/Fe2+ / reduction : / FeS2 + 14 Fe3+ + 8 H2O &