IDENTIFICATION OF DISCRETE HIGH TRANSMISSIVITY ZONES IN SPARSELY FRACTURED LIMESTONE–AN INTEGRATED APPROACH USING FRACTURE TRACE ANALYSIS AND SURFACE AND BOREHOLE GEOPHYSICAL METHODS - LORING, MAINE

A chlorinated-VOC-contaminated ground water plume emanates from an abandoned rock quarry at the former Loring Air Force Base in Limestone, Maine. Fractured Ordovician-Silurian limestone units underlie the site. Predominant fracture orientations strike NW-SE and NE-SW, although other orientations are locally important. A bulk hydraulic conductivity value of 0.08 feet/day was reported from a bedrock pump test in the quarry. However, discrete fracture zones of significantly higher conductivities are locally superimposed on the low-conductivity matrix. The spacing, continuity, and hydraulic significance of such zones remained poorly understood, as previous work included only limited efforts to target drilling locations to specific fracture zones. Our central focus in this investigation, therefore, was to locate downgradient monitoring points along primary contaminant migration pathways, with the goal of establishing a technically defensible compliance boundary for long-term monitoring (LTM). Limited fracture-trace and structural analyses were integrated with the existing hydrogeologic database to locate 2-D resistivity traverses in key downgradient locations. We subsequently obtained dipole-dipole resistivity cross sections along four traverses across suspected fracture trends. The project team located three boreholes on fracture zones inferred from low-resistivity anomalies. In addition, two boreholes were located along portions of the 2-D resistivity traverses which did not exhibit distinct anomalies. Following drilling, we logged the uncased boreholes with downhole geophysical tools including fluid temperature, fluid resistivity, SPR, SP, natural gamma, normal resistivity, caliper, acoustic televiewer, and heat-pulse flowmeter. In addition to collecting water quality samples from specific packered zones, we also conducted specific capacity tests on inferred conductive fractures. All of this information was used to select up to two zones per location for installing permanent monitoring well screens.

Wells drilled in geophysically-inferred fracture zones validated the utility of the overall approach, and served to contrast the hydraulic properties of fractures in anomaly-targeted wells versus non-targeted wells. LTM may now proceed with a greater level of confidence.