Processes controlling Ca and Mg mobility of natural waters in Skagafjordur, N-Iceland

In the Valley of Skagafjordur, N-Iceland, geothermal activity is widespread. Natural waters from the area and the highlands south of the valley have been divided into four groups on the basis of their composition and geology as 1) surface waters 2) soil waters 3) groundwaters in the lowlands and 4) groundwaters in the highlands. Soil waters show the highest CO2 partial pressure and Ca and Mg contents but the lowest pH, around 7. The pH of groundwaters is around 10 and their CO2 partial pressure and Ca and Mg contents are low. The Ca content of groundwaters is controlled by the solubility of calcite and that of Mg by the solubility of amorphous magnesium-silicate of the composition Mg4SiO3O10-X(OH)(2X). All the other water types are undersaturated with these minerals. Results indicate that vegetative soil cover enhanches the release of Ca and Mg into rivers and streams from rock derived material.