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.