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

Abstract

The chemical and stable isotope compositions of unpolluted ground waters in carbonate terranes are a function of the pH, P CO 2 , 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 Ca 2+ plus Mg 2+, HCO 3 −, 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 CO 2 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.