Hydrochemical and isotopic characterisation of the Bathonian and Bajocian coastal aquifer of the Caen area (northern France)

This paper describes the geochemical evolution of groundwater in the Bathon ian and Bajocian aquifer along its flowpath. Since this aquifer represents one of the main sources of fresh water supply in the Caen area and has been subjected to a Holocene marine intrusion, its management requires a sound knowledge of (1) the primary conditions and (2) the potential influence of either natural or anthropogenic pressures. Groundwater vertical sampling va lidity is discussed with the contribution of high resolution temperature lo gging. The main processes of geochemical evolution along a groundwater flow line and the sea-water intrusion characteristics are discussed using ionic concentrations (Br-, F- and major elements) and isotopes (water delta(2)H and delta(18)O, TDIC delta(13)C and A(14)C, sulphate delta(18)O and delta(3 4)S). As the C-13 content of TDIC is used as a tracer of water-rock interac tion, it shows evidence of specific chemical and isotopic evolutions of gro undwater within the aquifer, both related to water-rock interaction and min eral equilibria in groundwater. All the: above-mentioned tracers evolve dow nflow: cation concentrations are modified by exchange with clay minerals al lowing a high F- concentration in groundwater, whereas Br- and SO42- concen trations; appear to be redox condition dependant. Superimposed on these geo chemical patterns, delta(18)O and delta(2)H compositions indicate that aqui fer recharge has varied significantly through time. The chemical evolution of groundwater is locally affected by a salty water intrusion that is chara cterised by mixing between Flandrian fresh water and sea-water which has in teracted with peat as evidenced by a high Br-/Cl- ratio and SO42- reduction . (C) 2000 Elsevier Science Ltd. All rights reserved.