Deep groundwaters are normally characterized by long residence times in the aquifer and by being well evolved geochemically. At Gardermoen the estimated average groundwater residence time is in the range of 20-30 years. In this study regional and/or depth related chemical differences in groundwater chemistry have been discussed and related to natural geochemical processes and anthropogenic input, based on a set of groundwater samples.
Calcite dissolution is confirmed to be the dominating weathering process and the aquifer may be classified as a Ca2+ and HCO3- system. Most of the dissolution happens in the upper saturated zone, and approximately 48.5% of the bicarbonate may be assigned to calcite dissolution. Geochemical modelling results (by the use of PHREEQC) confirms weathering of silicates along the flowpath to be minor, with plagioclase contributing the most to the groundwater chemistry, followed by chlorite and K-mica (K-feldspar). Deeper groundwater at Gardermoen has reached the post oxic zone, and advanced to manganese or iron reducing conditions. Pyrite oxidation is the only natural source of sulphate within the aquifer. Sulphate reduction was not evident as is expected along with the observed low oxygen saturations, i.e. most of the pyrite oxidation occurs in the oxic upper saturated zone. The observed relationship between bicarbonate and sulphate indicates that protons released from pyrite oxidation do contribute to calcite dissolution, but to a small extent.
TLC-FID analyses yielded reliable quantitative estimations of the organic fraction. A tripartite configuration in the peak representing the polar fraction in the chromatograms was observed in all samples, and may be viewed as a characterisation of the distribution among organic acids in the groundwater at Gardermoen, with respect to polarity. Based on this, carboxylic acids are most abundant, followed by phenolic and hydroxylic, respectively.
3H-3He dating performed during this study has not yet been concluded and comprises too few samples in order to establish a regional overview of residence times. The current dataset displays, however, increasing ages relative to the depth below the groundwater table, corresponding to the general assumption on residence times.