Bulletin of the National Speleological Society - ISSN 0146-9517
Volume 33 Number 2: 73-85 - April 1971

A publication of the National Speleological Society

Preliminary Results on the Ground-Water Geochemistry of the Sierra de El Abra Region, North-Central Mexico
Russell S. Harmon

Abstract

Chemical analyses of 35 water samples collected from July 1969 to May 1970 were performed using standard wet-chemical and atomic absorption techniques for calcium, magnesium, sulfate, sodium, potassium, nitrate, and bicarbonate concentrations. In addition, most samples were analysed in the field at the time of collection for pH, calcium, bicarbonate, and dissolved oxygen concentrations.

Using the interpretive techniques of Carrels and Christ (1965) and Langmuir (1968), we found that most waters were saturated with respect to calcite and undersaturated with respect to dolomite The extremely high Ca2+/Mg2+ ratios of the cave waters indicate derivation from a high purity limestone (the El Abra Limestone), while the low ratios of the surface waters indicate a source outside the El Abra region. Carbon dioxide pressures, and thus calcium concentrations at a given saturation level, were found to be at least an order of magnitude larger than those commonly reported for karst waters of the eastern and southern United States. Calculations based on data from this study indicate a rate of carbonate solution of about 25 m3/year for an area of approximately 1,500 square kilometers.

The large volume of these Mexican caves compared to those of the more northern latitudes appears to be directly proportional to the greater amounts of carbon dioxide and water available for the solution process.

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