COUPLING OF BIOGEOCHEMICAL AND PHYSICAL PROCESSES IN CONTAMINATED AQUIFERS (Invited Presentation)

The transport and attenuation of contaminants in the subsurface is controlled by both bio/geochemical and physical processes. In many cases, the fate of contaminants is strongly affected by the coupling between geochemical and physical processes. This is particularly true where gas bubbles are present. Gas bubbles can result in transport of gaseous contaminants in the gas phase, can result in a reduction in hydraulic conductivity, water flow and contaminant transport rates in aquifers, and can result in changes in the redox environments affecting contaminant degradation. Examples are presented to demonstrate the effects of gas bubbles and coupled processes on contaminant fate. At a petroleum hydrocarbon site near Bemidji MN, biogenically produced gas bubbles are shown to impede contaminant transport from the source zone, and physically entrapped gas bubbles enhance aerobic degradation downgradient of the source zone. In granular iron treatments systems, hydrogen gas generation results in gas bubble formation and permeability reduction; however, the degree of gas bubble formation is controlled by reaction rates, water velocity, and solution chemistry. Reactive transport modelling is used to quantify coupling in these complex systems.