Geochemical modelling of natural geo-/hydrochemical stratification dominated by pyrite ocidation and calcite dissolution in a glaciofluvial Quaternary deposit, Gardermoen, Norway.

Abstract

The PHREEQC geochemical model has been used to reproduce on a geological time scale an abrupt shift between an oxic and an anoxic zone, in the Gardermoen glaciofluvial delta deposit. Groundwater in the aquifer has a major shift in chemical signature three metres below the groundwater table indicating that calcite dissolution and pyrite oxidation are the dominating weathering processes. The results of the modelling have been compared with those of water samples collected from multi-level samplers with microscreens located to one metre apart from the groundwater table and five metres down. The sampling scheme was not detailed enough to resolve the exact location of the weathering horizons. The model suggests that the calcite dissolution front is located slightly below the pyrite oxidation front. This causes a significant drop in predicted pH in a zone where other buffering mechanisms than calcite dissolution may become active. Calcite and pyrite in the sediments are of such concentrations that their respective dissolution fronts move at approximately equal rates in the aquifer. The depletion of these minerals has caused a distinct chemical separation in the aquifer between an oxic\/low ionic strength and anoxic\/hihg ionic strength zone, that will determine the fate of a contaminant entering the saturated zone.