Summary:
The chemical composition of geothermal fluids may be altered upon ascent from the reservoir to surface by processes including boiling, degassing, mixing, oxidation and water – rock interaction. In an attempt to quantify these processes, a three step model was developed that includes: (1) defining the composition of the end-member fluid types present in the system, (2) quantifying mixing between the end-members using non-reactive elemental concentrations and enthalpy and (3) quantifying the changes of reactive elements including degassing, oxidation and water – rock interaction. The model was applied to geothermal water at Vonarskard, Iceland, for demonstration having temperatures of 3–98 ° C, pH of 2.15–9.95 and TDS of 323–2250 ppm, and was thought to be produced from boiled reservoir water, condensed steam and non-thermal water. Most geothermal water represented mixture of non-thermal water and condensed steam whereas the boiled reservoir water was insignificantly mixed. CO2 and H2S degassing was found to be quantitative in steam-heated water, with oxidation of H2S to SO4 also occurred. In contrast, major rock forming elements are enriched in steam-heated water relative to their mixing ratios, suggesting water – rock interaction in the surface zone. Boiled reservoir water observed in alkaline hot springs have, however, undergone less geochemical changes upon ascent to surface and within the surface zone.