Geochemical Modelling of Water-Rock Interaction in Deep Groundwater

Abstract

A deep borehole (1001 m deep) was drilled in a granitic intrusion in Lavia in southwestern Finland in 1984 within the framework of the site investigation programme for spent fuel disposal /1/. Groundwater samples were taken from five depths in the borehole in 1984-1985 /2/. This paper describes the attempts to evaluate the geochemical interaction between groundwater and bedrock minerals in the borehole. The modelling of the water-rock interaction in the borehole was performed by using the geochemical codes PHREEQE and EQ 3/6. The modelling was principally performed with PHREEQE. For comparison some cases were also calculated with EQ3/6. Based on the chemical analysis of the water including also pH, Eh and temperature, the distribution of species in solution is solved by the aqueous model using mass action and mass balance equations. From the calculated distribution of species equilibrium partial pressures of gases are calculated, as well as the saturation state of the solution with respect to appropriate mineral phases. On the other hand based on the identified minerals the model was also used to calculate the dissolution of the minerals found in the borehole into water in order to test which minerals are the main sources of the elements in groundwater. The major elements in groundwater were found to occur mainly in ionic form. The mineral saturation indexes (SI) are presented in Figure 1. Quartz is in equilibrium with water at every groundwater sampling level. The high saturation indexes found for kaolinite and muscovite are in agreement with the fracture minerals identified in the borehole. Calcite is dissolving at the depth of 94–99 m, which is in accordance with the distribution of calcite in factures. The behaviour of ores (pyrite, goethite and hematite) is analogous with mineral analyses of fractures. According to the fluid mineral stability diagram kaolinite is the stable mineral phase at the lower and upper sampling levels. At the depth 422–427 m the equilibrium is in the muscovite field. Both SI- and fluid mineral stability diagrams indicate more stagnant groundwater conditions at the 422-427 m level than at the lower and uppermost sampling levels of the borehole. According to the calculations based on the minerals found in the borehole the main sources for sodium and calcium are montmorillonite + albite and calcite + fluorite, respectively. The results obtained so far are considered only tentative as they depend on several factors such as the correct thermodynamic constants, the accurate analysis of all dissolved species, the physico-chemical parameters and the composition of mineral phases, which all together control the solute content.