A field analog of CO2−closed conditions in a karstified carbonate aquifer (Nerja Cave Experimental Site, South Spain)

Abstract

We present new data that illustrate the hydrochemical evolution of groundwater along a flow line in the Triassic marbles around the Nerja Cave, South Spain. Water dissolves calcite and dolomite, and then CaSO4. The environment is locally rich in CO2 (up to near 60,000 ppmv) and consequently the water increases significantly its content in Ca2+, Mg2+, HCO3− and SO42− along the flow, with EC values between 500 and 900 μS/cm. The pH values are typically in the 7–8 range, and the equilibrium PCO2 of the water varies between 10−1.5 and 10−2.5 atm. In the considered flow line there is a relatively deep borehole (S2: 380 m; 280 m saturated) that shows pH values around 10 and equilibrium PCO2 of 10−6 atm, with EC values generally in the 150–200 μS/cm range. Most of its solutes derive from rainwater concentration, together with the dissolution of carbonate minerals in a system closed to CO2. For this reason we consider S2 to be a field analog of such conditions. The nearly stagnant water of this well also shows evidence of sulphate reduction. Unlike its solute contents, isotopically (δ2H and δ18O) the water of S2 does not show any modification with respect to the other points along the flow line.