Stable carbon isotope ratios and the existence of a gas phase in the evolution of carbonate ground waters

The chemical and stable isotope compositions of unpolluted ground waters in carbonate terranes are a function of the pH, PCO2, 13C content of the ground water recharge, the 13C content of the carbonate rock, and the manner in which the rock is dissolved or precipitated. Physico-chemical models show that significantly different relationships exist between Ca2+ plus Mg2+, HCO3−, pH and 13C content of unpolluted ground waters when carbonate solution occurs in the presence or absence of a gas phase. A study was made of these relationships in waters from 21 springs and 13 wells in Nittany Valley, Pennsylvania. Assuming that CO2 in the recharge zone has a 13C contents below −21%. vs PDB, the data indicate that ground water flow to wells and springs, and carbonate rock solution probably occurs chiefly in the absence of a gas phase. This is in spite of the fact that most of such flow is under water table conditions. 13C contents averaged −12·3%. for the spring waters and −13·3%. for the well waters. Five well waters polluted by septic tank or sewage effluents had carbon isotopic compositions from −13·5 to −16·4%.,vs −11·3 to−12·7 %. for the eight well waters relatively free of organic wastes.