Flow dynamics and potential for biodegradation of organic contaminants in fractured rock vadose zones

We present an experimental approach for investigating the potential for bioremediation of Ž . volatile organic compounds VOCs in fractured rock vadose zones. The experimental work was performed with rock samples and indigenous microorganisms from the site of the United States Ž . Department of Energy’s Idaho National Engineering and Environmental Laboratory INEEL , located in a basalt flow basin where VOC contamination threatens the Snake River Aquifer. Our Ž . approach has four components: 1 establishing a conceptual model for fluid and contaminant Ž . distribution in the geologic matrix of interest; 2 identification of important features of liquid Ž . distribution by means of seepage experiments in the fracture plane; 3 identification of the presence and activity of microorganisms by non-destructive monitoring of biotransformations on Ž . rock surfaces at the micron-scale; and 4 integration of flow and biological activity in natural rock ‘‘geocosms’’. Geocosms are core-scale flow cells that incorporate some aspects of natural conditions, such as liquid seepage in the fracture plane and moisture content. Fluid flow and distribution within fracture networks may be a significant factor in the ability of microorganisms to degrade VOCs, as they affect the availability of substrate, moisture and nutrients. Flow visualization and tracer breakthrough curves in transparent fracture replicas for unsaturated inlet conditions exhibited the channelized and intermittent nature of liquid seepage. The seepage of Ž . water and non-aqueous phase liquids NAPLs of varying physical and chemical properties into an ) Corresponding author. Fax: q1-510-486-7079. Ž . E-mail address: [email protected] J.T. Geller . 0169-7722r00r$ see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S0169-7722 99 00095-9 ( ) J.T. Geller et al.rJournal of Contaminant Hydrology 43 2000 63–90 64 initially dry replica showed only subtle differences in liquid distribution. In contrast, the seepage of a NAPL into the fracture replica containing residual water resulted in complex trapping of NAPL along the solidrwaterrair contact lines and diversion of NAPL to previously dry parts of the fracture. We found that a mixed culture of viable bacteria exists on the natural rock surfaces. Ž . Microbial activity measurably changed in response to changing relative humidity RH . Biological activity in the geocosm produced changes in liquid surface tension and seepage patterns over time. q 2000 Elsevier Science B.V. All rights reserved.