The chemistry of trace elements in surface geothermal waters and steam, Iceland

Abstract

The geochemistry of trace elements in surface geothermal fluids in Iceland was studied. The sampled fluids included hot springs, mud pots, steam vents and soil solutions with temperatures ranging from 4 to 100°C, pH between 2.01 and 9.10 and total dissolved solids between 86 and 4375ppm. The surface geothermal waters may be categorized into three groups based on their chemical composition, namely NaCl waters, steam-heated acid-sulfate waters and mixed waters. NaCl waters with pH >8 are considered to represent aquifer geothermal fluids that have undergone boiling in the upflow. They contained only low concentrations of most metals, <0.1ppb of Cd and Co, <1ppb of Ni, Pb, Cr and Cu and <10ppb of Zr, V and Zn, whereas somewhat higher concentrations of Ba (0.06–15ppb), Sr (1.2–107ppb), Cs (0.08–19ppb), Rb (7.1–163ppb), Li (18–380ppb), As (<0.2–252ppb), Sb (0.05–40ppb), Mo (0.16–50ppb), W (2.4–88ppb), Mn (0.1–163ppb), Fe (1.8–157ppb) and Al (21–1510ppb) were found. Steam-heated acid-sulfate waters typically had a pH <4. They form by condensation and mixing of steam in shallow non-thermal water. Compared to NaCl waters, steam-heated acid-sulfate waters typically had high concentrations of Al (0.02–267ppm), Fe (0.66–360ppm), Mn (44–4231ppb), V (1.1–1120ppb), Cr (0.15–660ppb), Zn (3.1–633ppb), Ni (0.20–192ppb), Cu (0.09–121ppb), Co (0.02–90ppb), Ba (1.0–60ppb) and Sr (2.8–316ppb), whereas concentrations of Li (<0.03–57ppb), Cs (<0.01–0.77ppb), Rb (0.12–24ppb), As (<0.1–61ppb), Mo (<0.01–14ppb), Sb (<0.01–25ppb) and W (<0.01–6.9ppb) were lower. Mixed waters are mixtures between NaCl waters, steam-heated acid-sulfate waters and non-thermal water, and showed chemical characteristics of these end-member waters. The steam discharged by the steam vents was found to carry trace elements including B, As, Cu and Cd in the ppt to ppb concentration range. The geochemistry of Li, Cs, Rb, As, Mo, Sb and W in surface geothermal waters was dominated by rock leaching, together with mixing of condensed steam and non-thermal surface waters at a low pH and with incorporation into secondary minerals including aluminum silicates and sulfides under alkaline conditions. In contrast, aqueous Ba and Sr concentrations were largely influenced by the formation of secondary minerals including sulfates, carbonates and Ca-minerals in all water types. At a low pH, the behavior of Al, Fe, Mn, Co, Ni, Zn, Cd, Cu, Pb, Cr and V was generally dominated by rock leaching, though occasionally by mineral precipitation in the case of Fe, Al, Cu, Co and Zr. With increasing pH, these metals became immobile due to their incorporation into secondary sulfide (Fe, Co, Ni, Zn, Cd, Cu, Pb), (hydr)oxide (Al, Fe, Cr, V, Zr) and aluminum- silicate (Al) minerals limiting their aqueous concentrations.