Geochemical and isotopic studies of natural waters from the Tröllaskagi area, N-Iceland

Abstract

The objective of this study is to compile all existing geochemical and isotopic data of the waters in the Tröllaskagi area, N-Iceland. The waters were classified, reservoir temperatures evaluated, mixing processes studied and the origin and movement of the waters determined. From the ternary plots, the waters can be classified as cold groundwaters, volcanic waters and a combination of the two, with the exception of Hrísey which plots close to mature waters. The plots also suggest that all the waters originate from old systems. Most of the geothermal waters seem to have equilibrated with respect to specific minerals. Subsurface temperature prediction, using the chalcedony geothermometer with correction for dissociated silicic acid, is in the range 42-103°C. The chalcedony mixing model shows that the geothermal waters are not mixed and their subsurface temperatures range from 50 to 143°C. Both the binary plots and the Schoeller diagram suggest the absence of mixing except for the Hrísey waters, which seem to have mixed with sea water. Stable isotopes of δ18O and δD have been used in the study area to trace and determine the origin and movement of groundwater. The thermal waters in the Tröllaskagi area are more depleted than the local precipitation and in some cases more depleted than any precipitation on Iceland today. The isotopic values of the thermal waters found within the Tröllaskagi peninsula suggest a different origin for the different sites. It is possible to trace the origins of the thermal waters in Siglufjördur and Ólafsfjördur to the mountainous areas south of the thermal sites using isotopes, whereas the thermal waters in Dalvík and points further to the south are more depleted than any precipitation on the Tröllaskagi peninsula today.