Studies of the Chemical Evolution of Natural Waters in the Hreppar-Land Geothermal Field, Iceland: An Aid to Geothermometry Interpretation

Abstract

The chemistry of natural waters in the Hreppar-Land low-temperature area, southern Iceland, is described. The area is located in Quaternary basaltic formations. Maximum subsurface temperatures just exceed The chemical composition of non-thermal waters in the area is controlled by the salt content of the precipitation, rock dissolution and supply of from the atmosphere and soil. The dissolved solids content of the geothermal waters is higher than that of the cold waters and it increases with temperature. The cause is increased rock dissolution. The of the cold waters is It rises sharply to as much as in the warm waters but falls again in hotter waters. At temperatures as low as rock dissolution and increase has progressed sufficiently to cause the water to precipitate some secondary minerals. These minerals act as sinks for the various elements they incorporate. Aqueous cation-proton activity ratios and aqueous acid concentrations indicate that equilibrium with secondary minerals is rather closely approached for all major elements (Si, Na, K, Ca, Mg, AI, Fe, C, sulphide-S and F) to temperatures as low as except for sulphate-S and, of course, Cold waters yield inconsistent results for the chalcedony, NaK-, NaKCa- and KMg-geothermometers due to disequilibrium conditions. At temperatures of about and higher the geothermometry temperatures have all converged to approximately the same value.