The impact of biofilm on the process of back diffusion from a contaminated rock matrix

Groundwater remediation in fractured rock settings is complicated by the diffusion of contaminant mass into the rock matrix and the subsequent back diffusion into the fractures. The process of back diffusion, in particular, leads to extended periods of low-level contamination in the fracture network that persists long after the source area is hydraulically or otherwise removed.The objective of this work was to establish biofilm development on a rock surface, or potentially within the rock matrix micropores, that would cause a significant reduction in diffusive mass transfer through a rock matrix from one adjacent reservoir to another. The biofilm is expected to cause a decrease in effective porosity as well as effective diffusion coefficient within the system. The experiment was conducted using double reservoir diffusion cells to assess diffusion of two conservative tracers, Bromide (Br -) and 2,6-Difluorobenzoic acid (DFBA), across dolostone rock coupons (0.7-1.2cm in thickness, porosity of 3.5-7.3%). A semi-analytical model that accounted for removal of sample volumes was successfully developed and used to design and analyze the experimental results. The optimal design included an exit reservoir that was 5 times the volume of the source reservoir. Four diffusion cell pairs were constructed from adjacent rock samples. A biofilm was stimulated over a 24 day growth period on the rock of one cell of each pair. Substantial biofilm growth was confirmed on the rock coupons through use of the Biological Activity Reaction Test (BART(TM)) and the Dubois et al. (1956) colorimetric method of glucose determination.A comparison of matrix diffusion through biofilm stimulated and non-biofilm rock was done and analysis of the experimental and model fit results show that the biofilm exerts little effect on rock matrix diffusion. Reasons for this can be lack of formation of a densely packed, diffusion limiting, biofilm or lack of attachment to or penetration within the rock pore space. Recommendations include a more detailed analysis of the type and extent of biofilm formation, the use of a less diverse, low concentration nutrient media over a longer growth period, and selecting a nutrient media that stimulates high EPS producers. Future work could also include examining biofilms under flowing water conditions.Biofilms are layered bacterial structures that consist primarily of microbial cells, organic polymers which are attached to a solid substratum by a microbially secreted slime called extracellular polymeric substance (EPS). Biofilm formation on rock fracture surfaces can be induced through the stimulation of indigenous bacterial species via the addition of nutrients. The extensive EPS polymer network can potentially limit diffusion through the biofilm and thus across the rock surface they colonize.