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CHARACTERIZATION OF FRACTURED BEDROCK FOR STEAM INJECTION
Citation:
Davis*, E L. AND K. Novakowski. CHARACTERIZATION OF FRACTURED BEDROCK FOR STEAM INJECTION. Presented at Third Conference on Remediation of Chlorinated and Recalcitrnt Compounds, Monterey, CA, 05/20-23/2002.
Description:
The most difficult setting in which to conduct groundwater remediation is that where chlorinated solvents have penetrated fractured bedrock. To demonstrate the potential viability of steam injection as a means of groundwater clean-up in this type of environment, steam will be injected into an area of known TCE contamination present in the upper 100' of an unconfined, fractured limestone underlying the Quarry Site at the Loring Air Force Base in Limestone, Maine. USEPA's SITE program will be evaluating steam injection remediation in this type of environment by monitoring the heating of the subsurface, the recovery of contaminants, and pre- and post-steaming contaminant concentrations and distributions. In order for this steam injection to be successful in recovering a significant portion of the contaminant mass, adequate amounts of steam must be injected along the right pathways and a pathway for the mobilized contaminants to reach a recovery well. Thus characterization efforts are aimed at characterizing both the fracture framework of the rock and the location and quantities of contaminants.
To conduct the characterization and provide an array of injection and extraction wells for the steam injection, a total of seventeen new boreholes were drilled in the most contaminated area of the site. Contaminant distributions were determined by measuring the contaminant concentrations in both the open fractures and in the unfractured matrix by extracting contaminants from rock chips obtained from core adjacent to fracture surfaces using methanol, and then analyzing concentration in the methanol by EPA Method 8260. Samples of groundwater from discrete fracture zones were obtained using a sampling system with which discrete zones of boreholes were isolated using a straddle-packer system.
The characterization of the fractures in which the contaminants are present is aimed at developing a three dimensional picture of the fracture network. Cores obtained during the drilling show the fracture frequency, type and location. Acoustic televiewer logs were used to help to determine fracture orientation in order to develop the three dimensional picture. Hydraulic conductivity tests were performed on 10 to 20 foot sections of boreholes to locate the hydraulically active fractures in the system. From these tests, transmissivity and fracture aperture were calculated to estimate the potential steam uptake in each fractures. The transmissivity data in conjunction with the three dimensional framework was also used to determine the potential interconnectivity of the fractures which are hydraulically active between the boreholes. The potential interconnectivity is critical for the efficient transport of steam and vaporized contamination. To verify the conceptual model for interconnectivity, pulse interference tests will be conducted between boreholes early in the next field season. Based on the results of the drilling, rock and groundwater sampling and on the hydraulic testing, a conceptual model for the fracture framework in the steam injection area has been developed.