Cl/Mg Ratio of Hawaiian Groundwaters as a Regional Geothermal Indicator

Abstract

During the regional assessment for potential geothermal areas in Hawaii, several quantitative and qualitative chemical geothermometers were applied to groundwaters with little apparent success. The problems most commonly encountered arose from variable amounts of seawater contamination of basal aquifers and differences in rock chemistry, groundwater residence time, and recharge rates. One qualitative geothermometer which seems to have overcome many of these difficulties is the Cl/Mg ion ratio. The utility of this ion ratio arises from (1) the relatively unreactive nature of the chloride ion in low to moderate temperature groundwater and (2) the temperature sensitivity of the reactions undergone by magnesium in groundwater. The application of this ion ratio to on file groundwater chemistry data has allowed us to differentiate between nonthermal and thermal groundwater chemical anomalies.