INVESTIGATION OF THE EFFECTS OF REACTION INDUCED GAS BUBBLE FORMATION ON GROUNDWATER HYDROLOGY AND GEOCHEMISTRY: REACTIVE TRANSPORT MODELLING

The production of gas bubbles as a result of biogeochemical reactions can have a significant effect on groundwater chemistry, solute transport and aquifer permeability. In contaminated aquifers these effects must be quantified in order to properly assess contaminant degradation rates and accurately predict contaminant removal potential. Our research applies an integrated approach, involving field, laboratory and modelling studies to investigate the effects of gas bubble production on the geochemistry and hydrology of a petroleum hydrocarbon contaminated aquifer undergoing methanogenesis.

In previous work at a crude oil spill site near Bemidji, MN we have demonstrated in a qualitative manner that the production of gas bubbles in the methanogenic portion of the aquifer has lead to the stripping of non-reactive gases, Ar and N2, from the groundwater, and that concentrations of these non-reactive gases can be used to quantify the degassing process and dissolved gas transport. However, quantifying these processes is complicated, for example, by inflow of uncontaminated water into the methanogenic zone, the presence of other gases such as CO2 and their pH dependent speciation, and changes in aquifer permeability due to the presence of gas bubbles. More sophisticated assessment tools are necessary.

A reactive transport code has been developed which describes reaction induced formation of a gas phase below the water table, equilibrium partitioning of all gases into the gas phase, and permeability changes due to variations in gas phase saturation. The model is based on the multi-component reactive transport code MIN3P, which is broadly applicable to geochemical problems involving kinetically controlled redox and mineral dissolution/precipitation reactions along with equilibrium hydrolysis, aqueous complexation, ion exchange and surface complexation reactions. The result is a versatile model applicable to a wide range of groundwater problems where gas bubbles affect the hydrogeology of an aquifer.

The enhanced version of MIN3P is used to investigate the spatial and temporal trends in dissolved gas concentrations observed at the Bemidji site.