Salinity increase with depth in fractured aquifers
Résumé
Fractured aquifers constitute an interesting water resource. Due to anthropogenic activities, the upper part of these aquifers [0-30 m] is often highly polluted. (nitrate concentrations above 50 mgL-1 in French Brittany). However, the deeper part of these aquifers [30-150 m] may constitute an alternative for water exploitation and present unpolluted water due to long residence times. We investigated a site exploited for drinking water at a depth of 100 m. A clear reactivity has developed close to the pumping well, pumping inducing autotrophic denitrification reactions with pyrite. The site also presents higher salinities than surface waters with Cl concentrations ranging from 50 to 100 mgL-1. Salinity increases with depth and also increases with time in the abstraction well in relation to pumping. Temperature anomalies indicate that the more saline waters originate from at least 300 m depth. Chemical (major, traces, Br/Cl ratios) and isotopic analyses (O, D, Sr, B) indicate that the saline waters present a marine signature. Noble gases present an argon enrichment which indicates either an argon production or a recharge temperature of 5°C which would require a large regional recharge following the last glacial maximum. This example indicates that French Brittany may present saline waters at shallow depth and require a coupled investigation of biogeochemical and mixing processes.