SAMPLING AND ANALYTICAL METHOD FOR GEOCHEMICAL SURVEY OF DISSOLVED GASES IN BRACKISH THERMAL WATERS

Abstract

Dissolved gases are often utilized in geochemical studies for investigating hydrothermal and/or magmatic contributions to shallow aquifers. Between the several methods applied for analyzing dissolved gases in water, the head space equilibration technique is probably the most used because it does not require any strain before or during the water sampling in the field and can be adapted to various aims in the laboratory, with multispecies analysis by common gas chromatography. This type of technique has been developed and applied. We propose a revised method of gas–liquid partitioning at equilibrium that allows an host gas aliquot to be introduced directly into the gas chromatograph (GC) and accurate analysis of dissolved O2, N2, He, CO2, CH4, CO, H2, and Ar in water samples with salinities ranging from 0 to 40 PSU. Provided that the analyzed gases represent close to 100% of the originally dissolved gases, a mass balance relative to the host gas can be used to obtain an independent estimate of the overall quality of the major species determination. The method allows the rapid characterization of several sites and represents a useful tool for geochemical surveys. A comparison between replicate samples analyzed using different procedures demonstrates the efficiency of the method and indicates that the abundances of the main dissolved gases can be obtained, which is helpful for determining the underlying geochemical processes. A rigorous numerical approach allows the molar fraction in the headspace to be estimated when the natural abundances of dissolved gases are known within a given range, thus allowing the optimal choice of the volume ratio between the headspace and solution to conform to the detection range of the used GC.