Title: The fluid geochemistry of Ölkelduháls and Hveragerði geothermal areas – SW Iceland

Abstract

The surface geothermal water and steam vent chemistry of the Hveragerði and Ölkelduháls
geothermal areas SW Iceland were studied. In total 43 samples were collected of cold springs and
rivers, hot springs, and steam vent discharges and their chemical composition analyzed.
The chemical composition of geothermal waters at surface was characterized by mildly acid to
alkaline pH of 6.06-8.69 and low Cl concentration of 3.32-7.34 ppm and with SiO2 and CO2 generally
being the most important dissolved elements with concentration of 38.5-217 and 6.43-486 ppm,
respectively. Surface geothermal fluids are considered to be sourced from three end-member waters
and mixture of: (1) boiled reservoir liquid, (2) condensed steam and (3) non-thermal water.
Relationship between Cl, CO2, SO4 and temperature show evident signatures that surface geothermal
fluids in the area are dominantly steam-heated waters with variable mixing ratios between condensed
steam and non-thermal waters. No boiled reservoir liquids were observed at surface, these
considered to represent boiled liquid fraction of reservoir geothermal fluids.
The chemical composition of steam vents was dominated by water (>99 mol%) followed by CO2
(499-6,587 µmol/mol), H2S (17.9-260.4 µmol/mol) and H2 (10.2-194.2 µmol/mol). The composition of
steam vents produced upon depressurization boiling of geothermal reservoir fluids differ within the
region with steam vent gases being enriched in CO2 and H2S at Ölkelduháls relative to the
Hveragerði region, those differences may be related to different heat source for both regions,
Hrómundartindur volcanic systems on Ölkelduháls and Grændalur extinct volcanic system in
Hveragerði. Gas geothermometry of steam vents estimate temperatures between 230-280°C in Hveragerði
and 280-300°C in Ölkelduháls.
The surface manifestations were influenced by seismic events, the 2008 earthquake could either open
and close fractures or faults rupture and affect the appearance of geothermal manifestations,
especially alkaline hot springs reported on previous studies on Hveragerði town (boiled hot spring)
and the vicinity of Varmá river that were not recognized during the 2020 survey; however, fumaroles
keep the same characteristic in terms of CO2, H2S and H2
concentration.