Heated Biofilm Growth in a Planar Fracture for Reduction of Hydraulic Aperture

Abstract

The objective of this study was to examine the effect of heat on the growth of a biofilm in a parallel glass plate fracture table. Groundwater was collected from a limestone aquifer and amended with a nutrient mixture to stimulate the indigenous microbial population. The amended water was heated to approximately 30oC in an upgradient reservoir attached to the fracture table and recirculated through the 2-m long, 0.6-m wide, parallel glass plate fracture having an approximate fracture aperture of 2000 μm. The fracture was maintained at approximately 10oC to simulate natural in situ groundwater temperature and the upgradient reservoir maintained at 30oC. Geochemical parameters and bacterial counts were measured regularly throughout the biostimulation to monitor biofilm growth in the fracture. Hydraulic tests and tracer experiments completed before and after the biostimulation were used as the primary indicators of the successful bioclogging of the fracture. Geochemical parameters measured throughout the trial revealed an increasingly reducing environment capable of supporting the development of a diverse biofilm. Direct and indirect bacterial counts revealed the dominant bacteria within the system included common groundwater bacteria pseudonomads, enteric, and slime-forming bacteria. Heterotrophic bacteria were also present in significant concentrations. Visible clusters of biofilm were observed on Day 2 of the trial with a fully-connected biofilm observed by Day 7. The biofilm impacted the groundwater flow through the fracture resulting in an approximately 2.75-hour delay in the tracer’s breakthrough during the tracer experiment completed on Day 13 of the trial compared to an experiment conducted during the initial stages of biofilm development. Based on the results of the tracer experiment, the biofilm growth reduced the velocity of the groundwater by 9.8%, the fracture aperture by 37.8%, and increased the bulk dispersivity to 50mm. Recommendations for future work include the application of heated biostimulation at the field scale in a well-characterized, isolated fracture.