The role of biofilms in subsurface transport processes

Landfill and radioactive waste disposal risk assessments focus on contaminant transport and are principally concerned with understanding the movement of gas, water and solutes through engineered barriers and natural groundwater systems. However, microbiological activity can impact on transport processes changing the chemical and physical characteristics of the subsurface environment. Such effects are generally caused by biofilms attached to rock surfaces. This paper will present the results of an experimental study of the significance of biofilm growth on groundwater flow and the transport of contaminants in intergranular and fracture porosity flow systems. Risk assessments for landfills and geological repositories for radioactive waste are primarily based on the precepts of contaminant transport; and are concerned with understanding the movement of gas, water and solutes through engineered barriers and natural groundwater systems, within the concept of ‘Source’, ‘Pathway’ and ‘Receptor’. The emphasis on solute migration for landfill investigations is reflected in the theoretical development used during numerical simulation. However, microbes living in such environments can have an impact on transport processes (Chapelle 2000; Cunningham et al. 1997; Fredrickson et al. 1989; Keith-Loach & Livens 2002; West & Chilton 1997; West et al. 2006). Microbial activity in any environment is generally located on chemical or physical interfaces, usually within biofilms, and the impacts can be both physical (e.g. altering porosity) and/or chemical (e.g. changing pH, redox conditions) and may result in intracellular or extracellular mineral formation or degradation (Beveridge et al. 1997; Ehrlich 1999; Konhauser et al. 1998; Milodowski et al. 1990; Tuck et al. 2006). Where biofilm growth in a crystalline host rock promotes mineral precipitation it can reduce water inflow and this can be a positive effect for limiting the transport of contaminants. Recent experimental work has investigated some of these chemical and physical effects in more detail in the context of the geological containment of radioactive waste. This paper will examine some of these studies and will indicate the significance of biofilms in influencing both granular and fracture flow.