BIOGEOCHEMICAL PROCESSES DURING THE INFILTRATION OF RIVER WATER INTO AN ALLUVIAL AQUIFER

Biogeochemical processes occurring during infiltration of surface water from the Lot River into an alluvial aquifer are described using chloride as a natural tracer of water mixing in a well field where a Cl- rich aquifer water is recharged with a Cl--poor river water. Near the river bank a slightly reduced zone (depleted in O2, DOC, NO3, Na, and K and enriched in Mn, Ca, Mg, bicarbonate, and silica) is observed. Sulfate behaves conservatively. Nearest to the infiltration zone some of the pH-regulating processes are not at equilibrium. These phenomena can all be explained by bacterial degradation of organic matter in the river bank sediments and weathering of minerals along the infiltration path. In some cases (degradation of DOC and dissolution of calcium and magnesium carbonates) a semiquantitative confirmation of the stoichiometry of the reactions is given. Zinc is efficiently filtered after the first 10-15 m of the bank sediment-alluvion system. Some chemical changes occurring in the reduced zones are reversible (depletion of dissolved oxygen, dissolution of Mn). Others are not.