Soil gas geochemistry and diffuse CO2 flux from soil, together with gas concentrations in 7 production wells and 10 steam vents in the Reykjanes geothermal field (Iceland), were studied with the aim of: 1) studying the potential use of the CO2/He ratio in soil gas to constrain CO2 retention in calcite in the bedrock; and 2) quantifying the effect of 100 MWe power production from the Reykjanes geothermal reservoir on surface activity. The presence of biogenic CO2 in the soil gas prevented the use of the CO2/He ratio for the quantification of CO2 retention in calcite. The CO2/He ratio in about 20 soil gas samples was, however, low and consistent with CO2 retention. Our results suggest that detailed soil gas and soil temperature mapping may provide excellent information for defining drilling targets. The surface activity in the Reykjanes geothermal field increased noticeably after the commissioning of the 100 MWe Rey-Wanes Power Plant in 2006. When increased steam flow from steam vents is taken into account the total increase in surface activity in Reykjanes between 2004 and 2007 is of the order of 50%. (c) 2015 Elsevier Ltd. All rights reserved.