Isotope (δD, δ18O, 3H, δ13C, 14C) and chemical (B, Cl) Constrains on water origin, mixing, water-rock interaction and age of low-temperature geothermal water

The isotope (δD, δ18O, 3H, δ13C, 14C) and chemical (B, Cl) compositions of cold (streams, rivers, soil- and groundwater) and low-temperature geothermal water from northern Iceland were investigated in order to delineate the origin, mixing, water-rock interaction and the age of the waters. The waters showed a large range in temperature (0–104 °C) and chemical composition (TDS 7–954 ppm). Based on the δD (−54.9 to −125‰), δ18O (−7.8 to −16.7‰), B (0.001–1.57 ppm), Cl (0.3–500 ppm) content the waters were of meteoric origin that have been modified by primary rock leaching corresponding to rock to water ratio (ξ) of ∼0.001–0.1 kg of basalt dissolved per kg of water. At temperatures above ∼50 °C close to saturation was observed between the waters and common secondary minerals like clays, zeolites and oxides indicating these reactions control the concentrations of reactive elements. At lower temperatures deviation from equilibrium saturation was evident. The water age was constrained by 3H and 14C, the latter corrected for rock carbon using B, Cl and δ13CDIC relations. Most waters displayed modern age. Exceptions were 1) some waters at high ground having lower tritium content and older radiocarbon age; the trend considered to result from mixing between old glacial melt water and modern meteoric water, and 2) some low-temperature geothermal waters in lowland areas considered to be a mixture of old ʽice-ageʼ water, originating from last glacial times >12,000 years ago and modern and/or few decades old meteoric water. Thus, the chemical and isotope composition of cold and low-temperature geothermal waters in the region display a complex story, with waters originating from different conditions and of different age and later mixed and affected by water-rock interaction. These processes need to be considered and understood in order to develop a hydrogeological model of a groundwater system. •The chemical composition of geothermal waters are modified by secondary processes like water-rock interaction and water mixing.•Water origin and rock-to-water ratio of geothermal waters can be determined using δD, δ18O, B and Cl systematics.•Water age and mixing can be quantified using B, Cl, 3H, δ13C and 14C systematics.