Characterization of subsurface bedrock fractures at GMH Electronics EPA Superfund Site near Roxboro, NC

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The US Environmental Protection designated a former electronics manufacturing facility as a Superfund cleanup site near Roxboro, North Carolina. Operations at a former gasoline station/store and electronic component production facility may have resulted in the presence of a volatile-organic compound (VOC) plume in groundwater in the area; referred to as the "GMH Electronics Site".

The major objective of this study was to characterize the subsurface bedrock fracture orientations and interconnectivity in selected open-borehole wells near and adjacent to the GMH Electronics Superfund site. These interpretations of subsurface fracture orientation will then be correlated with surficial geologic mapping being conducted in the study area.

 

Objective:

The major objective of this study was to characterize the subsurface bedrock fracture orientations and interconnectivity in selected open-borehole wells near and adjacent to the GMH Electronics Superfund site. These interpretations of subsurface fracture orientation will then be correlated with surficial geologic mapping being conducted in the study area.

Map of GMH Electronics Superfund Site location and surrounding geology
Map of GMH Electronics Superfund Site location and surrounding geology

This information will be used to develop a preliminary understanding of hydrogeologic controls on flow through the fractured bedrock part of the groundwater system, as well as potential sources areas of contamination and identification of areas where residential wells and surface waters in the area could be impacted by contaminant transport through the fractures. The improved understanding of flow in the bedrock fractures can be used by the USEPA and State regulatory agencies to guide future monitoring and remediation efforts.

Approach:

The approach to this study consisted of the following work tasks, in sequential order:

  1. Geophysical logging of selected open-borehole wells. Wells will be selected with regard to importance toward delineation of the areal and vertical contamination in the bedrock fractures.
  2. Delineation of primary contributing fracture characteristics including depth, aperture, orientation, weathering, geologic setting (rock type, proximity to lithologic contact, cross-cutting or parallel to foliation) for each well logged. Presentation of primary fracture orientations (strike direction) for each well in map format.
  3. Deployment of passive diffusion bag (PDB) samplers near selected fracture zones in the open borehole wells. Compilation of volatile organic compound analytical results from the PDBs.
  4. Compilation and presentation of surface geologic mapping data collected by the NCGS.
  5. Correlation and presentation of primary fracture orientations and subsurface geologic setting with surface geologic mapping data.
  6. Continuous and periodic water level monitoring to determine potential interconnectivity of bedrock fracture zones.

 

Current Investigations:

From November 2012 to May 2013, the USGS assisted EPA in monitoring water levels in wells during well construction activities. Continuous water levels were recorded at intervals of one and fifteen minutes, to monitor potential response to the nearby drilling of new bedrock wells. Borehole geophysical logs were run in four newly constructed bedrock wells.

The study characterized the subsurface bedrock fracture orientations in selected open-borehole wells near and adjacent to the GMH Electronics Superfund site. These interpretations of subsurface fracture orientation were correlated with surficial geologic mapping being conducted in the study area. This information will be used to develop a preliminary understanding of hydrogeologic controls on flow through the fractured bedrock part of the groundwater system, as well as potential sources areas of contamination and identification of areas where residential wells and surface waters in the area could be impacted by contaminant transport through the fractures.

The improved understanding of flow in the bedrock fractures can be used by the USEPA and State regulatory agencies to guide future monitoring and remediation efforts.

Past Investigations:

In 2011, the USGS collected borehole geophysical logs and images, and conducted water quality sampling in order to develop a conceptual groundwater model for the assessment of current contaminant distribution and future migration of contaminants. Data compilation efforts included geologic mapping of more than 250 features, including rock type and secondary joints, delineation of more than 1,300 subsurface features (primarily fracture orientations) in 15 open borehole wells, and the collection of passive diffusion-bag samples from 42 fracture zones at various depths in the 15 wells.

The data was published as Data Series 762, Geophysical Logging and Geologic Mapping Data in the Vicinity of the GMH Electronics Superfund Site near Roxboro, North Carolina