Background and Description of Methods

In September 2009, the U.S. Geological Survey (USGS) was requested to assist the Environmental Protection Agency (EPA) Region 4 Superfund Section in the development of a conceptual groundwater flow model in the area of the Mills Gap Road contaminant investigation near Asheville, North Carolina (Site ID A4P5) through an Interagency Grant and work authorization IAD DW number 14946085. The USGS approach included the application of established and state-of-the-science borehole geophysical tools and methods used to delineate and characterize fracture zones in the regolith-fractured bedrock groundwater system. Borehole geophysical logs were collected in eight wells in the Mills Gap Road project area from January through June 2010. These subsurface data were compared to local surface geologic mapping data collected by the North Carolina Geological Survey (NCGS) from January through May 2010.

Borehole geophysical logs and surface geologic mapping methods were used to characterize both subsurface and surface features in the fractured bedrock and overlying regolith. As in most areas of the Piedmont and Blue Ridge Physiographic Provinces in the southeastern United States, the groundwater system in the metamorphic and igneous rocks is complex and directly related to multiple periods of structural deformation, metamorphism, and igneous intrusion. The groundwater system in the Blue Ridge Physiographic Province consists of two components—a shallow regolith component that may include soil, saprolite, debris flow material, colluvium, and alluvium, and a deeper fractured-bedrock component (Chapman and others, 2005). Where present in the Blue Ridge Physiographic Province, the regolith is the primary storage reservoir and is the source of recharge to the bedrock fractures (Heath, 1980, 1983, 1984, 1994; Heath and Jennings, 1995). The bedrock has little primary porosity except where secondary openings are present in the form of fractures and other discontinuities. These secondary openings are the primary source of permeability. The bedrock is described by the NCGS as composed of the regional Ashe Metamorphic Suite-Tallulah Falls Formation, a metamorphosed and deformed package of sediments of Late Proterozoic to Middle Ordovician age that are interlayered with minor mafic intrusives and volcanics. Lithologies consist of metagraywacke, schistose metagraywacke, garnet mica schist, amphibolite, quartz and quartz-tourmaline veins, and lesser zones of fault breccia and gouge (Wooten and others, 2010). The observed regolith component of the groundwater system in the study area is described by Wooten and others (2010) as consisting of transported colluvial and alluvial deposits and residual soil formed from the in-place weathering of bedrock. MACTEC, Inc. (2009) reported following a Phase 1 Remedial Investigation of the Mills Gap Road site that the zone of overburden (regolith), determined from boreholes at the CTS Corporation of Asheville site, ranges in thickness from 28 to 81 feet (ft) below land surface (bls). This site is the location of a former electroplating facility, hereafter referred to in this report as the CTS site.

Dominant structural features and discontinuities described in the report by Wooten and others (2010) describe regional bedrock foliation and compositional layering as principally striking to the north-northeast and northeast and, to a lesser extent, to the north-northwest, except in the vicinity of the Mills Gap Fault Zone (MGFZ). Foliation, compositional layering and a younger mylonitic foliation are incrementally realigned near the MGFZ from a regional northeast-southwest strike trend to a west-northwest—east-southeast trend that is subparallel to the fault zone.

Three surface joint sets are described in the report by Wooten and others (2010). One joint set is associated with the MGFZ and is grouped into the following strike azimuths: 115 degrees (°) and 295° parallel to the fault and 85° and 265° conjugate to the fault trend. A second joint set includes strike azimuths of 25° and 205° that are subparallel to the predominant northeast-southwest bedrock layering and regional foliation, as well as north-northeast trending outcrop-scale faults, and north-northeast-trending secondary quartz and tourmaline veins. The third joint set includes strike azimuths of 145° and 325°, northwest-southeast striking that may also include some joints associated with the MGFZ, and 175° and 355°, regional north-south striking joints that may include some joints subparallel to the predominant north-south striking foliation (Wooten and others, 2010).

From January through June 2010, borehole geophysical logs were collected from a total of eight open-borehole bedrock wells in the Mills Gap Road investigation area in the vicinity of the CTS site (table 1; fig. 1). Six of the wells (AW-4, AW-5, AW-7, ERT-7, Oaks-2, and ERT-6) were located in the Oaks subdivision, approximately 0.5 mile (mi) northeast of the site; one well (CHR) was located on Chapel Hill Church Road, approximately 0.37 mi northeast of the site; and one well (Well 1) was located on Concord Road, approximately 0.25 mi southeast of the site. Well depths ranged from 152 to 705 ft bls (table 1). From the compilation of casing depths listed in table 1, the regolith thickness inferred ranges from 6.5 to 70 ft. Additionally, fracture orientations from acoustic televiewer (ATV) image logs were available for four wells on the CTS site (fig. 1; table 1). Logs collected from each of the eight wells included caliper, electrical resistivity, natural gamma, fluid temperature and resistivity, heat-pulse flowmeter (both ambient and stressed), and optical televiewer (OTV). ATV logs were run in two wells as part of the quality-assurance procedures for the OTV tool. Field notes from geophysical logging activities are included in Attachment 1. Fracture zones were delineated at depth in each well by using all of these borehole logs. The fracture delineations were then used to guide the selection of downhole straddle-packer sampling by the EPA Environmental Response Team (ERT) and their contractors. Fracture orientations were determined from OTV images. The fracture orientation data were compared and used along with surface geologic mapping data to build a conceptual model of groundwater flow in the study area. Results of the NCGS surface geologic mapping study are described in Wooten and others (2010).

First posted March 28, 2011