Noble gas composition and ³H/³He groundwater ages in the Gardermoen Aquifer, Norway: Improved understanding of flow dynamics as a tool for water management

This study presents a novel data set of noble gas compositions and ³H/³He measurements in groundwater samples from the Gardermoen Aquifer in Norway. The motivation was to test the applicability of noble gases as tracers in constraining the conceptual model and improve the understanding of deeper parts of the aquifer. Flow models have been used as tools for water resource management at the Oslo international airport, which is located within the aquifer recharge area, and bordering towards protected nature reserves. Current models are well calibrated for upper parts of the aquifer, and in line with new noble gas data. However, in some areas large inverse pressure gradients are observed. Interpretation of noble gas data proved useful in understanding these pressure deviations as a result of water retention in low permeability layers of clay and silt. Noble gas composition, isotopic ratios and tritium water ages define different water "types". Mixing along flow path or as a consequence of pumping were found to be indicators of geological heterogeneity, and were used to evaluate degree of flow separation as well as vertical versus horizontal flux. ³H/³He water ages varied between 0 and 50 years, increasing with depth below the groundwater table and along flow paths. These data indicate lower vertical permeability and longer residence times in deeper parts of the aquifer than suggested by previous models. This study is of high relevance in the continued water resource management scheme at Gardermoen, for improving existing models and in sound, long-term monitoring of groundwater, ensuring sustained influx towards sensitive biotopes.