Chloride/Magnesium Ratio of Shallow Groundwaters as a Regional Geothermal Indicator in Hawaii

Abstract

Because of the complex geological and hydrological conditions and the virtual lack of thermal springs, regional geothermal investigations in Hawaii require the use of techniques substantially different from those conventionally applied in other geothermal environments. The large number of hydrological wells in the state provides an appreciable source of groundwater chemical data. However, largely because of the island environment, interpretation of much of these data as geothermal indicators becomes ambiguous. Initially, SiO2 and temperature of groundwaters were used to identify thermally anomalous zones, but on a regional basis it has been found that these criteria are not always successful. As a further criterion for assessment, the Cl/Mg ratio of the groundwater has been used. On a state-wide basis, this ratio has been successful in further "screening" the SiO2-temperature selected sites, and in defining more specific areas which warrant further investigation. Temperature, SiO2 and Cl/Mg values for nearly 400 groundwater samples are included.