Long-term Denitrification Processes and Kinetics in a Crystalline Aquifersubject to Pumping from 1991

International audienceThe kinetic of denitrification associated to long-term mixing processes in heterogeneous aquifers is particularlychallenging to constrain. Specifically, chemical evolutions related to groundwater exploitation are cases that arepoorly known. It remains particularly unclear if long-term pumping whether enhances or slows-down the nitratereducing processes and what is the source of electron donor sustaining the reaction. The aim of this study isto investigate the dynamic of denitrification processes induced by long-term pumping in the Ploemeur aquifer(Britany, France) which has been operated for water supply since 1991. Several batch experiments have beencarried out in order to fully characterize the kinetics of the denitrification reaction involved. Batches consistedin crushed rock: more or less weathered granite and schists, and water sampled from the site. Denitrificationalways developed except in sterilized batchs. Denitrification rate was independent on the rock type but more onthe state of the bacterial community. Inorganic dissolved carbon only showed moderate variations while organiccarbon remained at low concentrations. Both observations make heterotrophic denitrification unlikely. A silicatedissolution was observed and detailed analysis of the cations quantified a main biotite contribution. The ironproduced by biotite dissolution accounts for the denitrification processes observed.Long term time-series analysis of the conservative elements recorded at the pumped well were used to determinemixing fractions from different compartments of the aquifer based on a Principal Component Analysis approachcoupled with an end-member mixing analysis. Discharge fractions were then used to quantify the denitrificationkinetic linked to pumping. With increasing concentration of Nitrate entering in the groundwater system since thebeginning of the operations, computations confirm that i) autotrophic denitrification processes are dominant andii) biotite plays a critical role in sustaining the Nitrate reduction processes. Both nitrate reduction and Sulfateproduction as well as Fluor release ratios support the hypothesis that biotite play a major role of electron donor inthis context.Field-scale pumping experiments in two other crystalline aquifers from Britany also showed similar denitrificationreactions without sulfate production in a stoichiometric ratio to nitrate consumption. Metagenomic analysisof these three aquifers showed a highly structured system. Microbial communities are dominated by potentialdenitrifiers in the upper weathered compartment while the deeper fractured system shows communities dominatedby Gallionellaceae group, both groups being potentially involved in the denitrification processes