Intrusion of Seawater into a Porous Fracture zone a Coupled Geohydrochemical Modelling Exercise

Abstract

ABSTRACTA coupled geochemical transport program PHREEQM2D has been tested in order to examine intrusion of seawater into fracture zones which can be connected to tunnels, such as nuclear-waste repositories on the sea shores. The code consists of a flow and transport program HST2D and a chemical equilibrium program PHREEQE. It uses a mixing cell approach, where chemistry and transport are solved separately. Ion exchange is calculated by means of a simple surface complexation model.In the present artificial exercise, replacement of groundwater by seawater in a connecting porous fracture zone was modelled without comparing the results to observed data. Dilution, mixing, ion exchange, and dissolution/precipitation reactions were examined.The results of modeling showed that the sulphate front of seawater moved some distance towards the tunnel during the simulations. The characteristic solutes of dilute groundwater were assumed to enter the tunnel in a short period of time. Amounts of sulphates decreased due to dilution and mixing. The chemical contents of waters were also affected by dissolution, precipitation, and ion exchange. Calcite dissolved and oxyhydroxides precipitated in oxidizing water. The strongest exchangeable ions in water, Ca and Na, were retarded due to ion exchange in the mixing zones. This caused an increase in dissolution of calcite in the mixing zones. Calcite would, however, start to precipitate in the mixing zone when the ion exchange capacity of rock would be consumed.Some important factors, such as retardation on oxide surfaces (adsorption models) and microbial activity (organic complexation models) have not been tested in the present exercise. Otherwise, the PHREEQM2D showed its applicability to modelling of chemical equilibrium, dilution, mixing, ion exchange, and elution/precipitation in discrete porous zones in transient conditions. It could be used to model temperature-enchanced, coupled geohyd-Γochemical, processes in porous zones consisting of densely fractured or crushed crystalline rock.