Modelling of biochemical processes in rocks:oxygen depletion by pyrite oxidation-model development and exploratory simulations

摘要

The purpose of this work is to gain understanding about one of the potential oxygen consuming processes occurring in deep subsurface crystalline rocks.A model for the depletion of oxygen by microbially mediated oxidation of pyrite along a deep subsurface conducting fracture was developed.In the model,a previously undisturbed fracture is suddenly invaded by oxygenated water.This could happen after a glacial period when the melt water penetrates down in the deep rock.We assume that no degradable organic matter is present.The oxygen in the water will be scavenged by the pyrite minerals in the rock matrix and in secondary minerals in any fracture infill.In this study the intrusion depth of oxygenated water is simulated assuming consuming reaction.Bacterial growth was modelled using Monod's dual limitation equation with oxygen and dissolved pyrite as the two limiting substrates.The bacteria grow as a biofilm on the fracture surface and are also present in the flowing water.The oxygen is transported to the microbes in the biofilm by the flowing water in the fracture.The ferrous iron and the sulphur from the pyrite move by molecular diffusion in the rock matrix.The rock matrix,near the fracture,is gradually depleted of pyrite and a dissolution from propagates away from the fracture.