MULTI-FACETED HYDRAULIC CHARACTERIZATION OF AN ISOLATED BEDROCK FRACTURE IN THE BLUE RIDGE PROVINCE

This study focuses on the determination of hydraulic properties of an isolated fault-zone fracture that intersects two wells at the Fractured Rock research Site (FRS) in Floyd County, Virginia, located in the Blue Ridge physiographic province. Previous research conducted has revealed important information concerning the structure and behavior of crystalline bedrock at the FRS site and its relation to regional hydrogeology of the province. However, little has been done to investigate hydrologic processes at FRS at the fracture scale.

Using a combination of optical televiewer and heat-pulse flow meter logs, fractures intersecting the borehole walls were identified both visually and hydraulically. Isolation of the fracture of interest with packers at each well during aquifer testing allowed us to calculate inter-borehole transmissivity and storativity. Transmissivity was found to vary by up to two orders of magnitude depending on whether packers were used to isolate individual fractures. A forced gradient tracer test was conducted to investigate the behavior of flow within one particular fracture. Using a conservative bromide tracer, fluid particle travel time, average fluid velocity, dispersivity, and average fracture porosity between the wells were calculated. In addition, one-dimensional flow was confirmed, and evidence of strong channelization within the fracture was observed using the shape of the breakthrough curve and tracer concentration levels.

Because of the structural complexity of fractured rock aquifers, a comprehensive description of their hydraulic characteristics can only be obtained through multiple lines of testing. Differences between the parameter values calculated in this study and those determined from previous investigations at FRS highlight the complex heterogeneity of this and other fractured rock aquifers.