CHARACTERIZING BEDROCK FRACTURE ORIENTATION AND DENSITY USING FRACTURE TRACE ANALYSIS, OUTCROP FIELD MAPPING, AND GEOPHYSICAL BOREHOLE METHODS IN FOSTER, RHODE ISLAND

The NIKE Site PR-79, in Foster, Rhode Island was part of a guided missile network built during the Cold War to provide air defense for the Providence metropolitan area. The site was designated a Formerly Used Defense Site (FUDS) property, administered by the U.S. Army Corps of Engineers, after trichloroethylene (TCE) concentrations exceeding the USEPA maximum contaminant level (MCL) of 5 µg/L were detected in two onsite groundwater supply wells and three neighboring residential bedrock wells. The purpose of this investigation was to characterize bedrock fractures, specifically their orientation and density, in the vicinity of the NIKE PR-79 Site, using integrated remote sensing based fracture trace analysis, outcrop field mapping and fracture trace verification, and geophysical borehole methods. Fracture orientation and fracture density are critical parameters to understanding the three-dimensional distribution of fractures in the subsurface, and ultimately, to determining the movement of groundwater in the area. Results of the three methods determined that fractures trend from northwest (NW) to northeast (NE) and are steeply dipping (>60°) towards the east. Average fracture density (regardless of orientation) was similar between the outcrop study (0.3 ft^-1) and the borehole study (0.8 ft^-1). Likewise, fracture spacing was similar in the outcrop study (3.3 ft/fracture) as in the borehole study (1.3 ft/fracture). The difference in fracture density and spacing was attributed to lithological differences in the vicinity of NIKE PR-79 Site. Results of the remotely sensed fracture trace study indicated that the area mapped as fine-grained quartz diorite was more densely fractured (45.4 fracture traces/mi²) than the areas mapped as Ponaganset Gneiss and South Foster Migmatite, which were similarly fractured (34.8 and 31.0 fracture traces/mi², respectively). Further investigation of bedrock wells including re-interpretation of lithology from Optical Televiewer and supporting logs is necessary to determine the fracture density on a lithological basis and estimate transmissive fracture trends below grade. Results from this study improve the overall conceptual site model of the NIKE PR-79 Site and will be used to construct a representative groundwater and surface water monitoring network.