Chemical and isotopic signatures of interstitial water in the French Chalk aquifer and water-rock interactions

A 700-m core has been drilled in the Chalk of the Paris basin (France) with the aim to know the evolution of the interaction between interstitial water and the matrix and to recognize the chemical stratification of the aquifer. The variation of the O-18 (from -4 to 0 parts per thousand vs SMOW) and H-2 (from -50 to 0 parts per thousand vs SMOW) contents point out a mixing between meteoric water and a marine solution. The total salinity also increases with depth from 5 to up to 30 mS.cm(-1) at the bottom of the Chalk series. The chemistry of the deep salty water is Ca2+/SO42- and Ca2+/HCO3- dominant and could be derived from connate water. The solid phase, through the isotope variations (C-13, O-18), show an increase of recrystallization of the grain matrix with depth.