Geochemical evolution of groundwater in the unsaturated zone of a karstic massif, using the PCO2–SIcPCO2–SIc relationship

SummaryIn karstic environments, groundwater is strongly influenced by CO2 partial pressure variations of air present in the infiltration zone of these aquifers. In order to characterize the geochemical changes in groundwater as it moves through the infiltration zone, we monitored various rising springs in the perched karstic aquifer of Cussac (Dordogne, France), and measured the CO2 partial pressure in air of a nearby cavity (the Cussac Cave) for 24 months. Our method is based on the relationship between the saturation index with respect to calcite (SIc) and the CO2 partial pressure at atmospheric equilibrium with water. We distinguished a value for this last parameter when water is at equilibrium with respect to calcite (SIc = 0) called saturation CO2 partial pressure. The use of this parameter can provide information on flow conditions and relationships between water, air, and rock. Cussac aquifer is a suitable area to apply these methods because of its small size, numerous springs, and a cave that provides data for CO2 partial pressure condition inside the massif. Results show that most of the calcium-carbonate mineralization is acquired in the epikarst followed by a precipitation phase in the upper part of the infiltration zone. Groundwater reaches the saturated zone with some degree of saturation depending on CO2 partial pressure variations in air inside the massif.

► We reconstitute the air inside the kart environment, using measurements on water.
► Measurements are made on waters from epikarst to saturated zone and inside a cave.
► We define water’s intrinsic parameters using calcium carbonate equilibrium equations.
► We describe the water conditions of flowing upstream from a system outlet.
► Degassing/precipitation stages occur in the different compartments of the aquifer.