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Environmental Cleanup Plan for AOC 65
Section 3 - Geophysical Investigation
Parsons will perform surface geophysical surveys near AOC 65/Building 90 to identify the location, orientation, and other pertinent properties of subsurface features that may influence or control migration of contaminants in the area. Results of the surveys will be used to assist in selecting appropriate locations and depth for piezometers, the vapor extraction wells (VEWs), and vapor monitoring points (VMPs) to be installed as part of the soil vapor extraction treatability study and groundwater recharge study. Additionally, results of these surveys will provide information for evaluating the application of geophysical technologies to other areas at CSSA.
The geophysical surveys will be undertaken in a phased approach where the results from one phase will guide the work of successive phases. The initial phase consists of resistivity and limited microgravity around AOC65/Building 90. Successive phases of the geophysical investigation may include expanding resistivity and microgravity into other areas located to the south and west of Building 90, two-dimensional seismic profiling over a broad area (at least 100 acres) and three-dimensional electrical resistivity surveying over a small area (less than �-acre). Three-dimensional seismic data will be acquired and borehole geophysical methods will be applied in later phases.
Electrical resistivity imaging (ERI) will be performed to map the distribution of faults, fractures, and karst features present in the shallow subsurface. ERI is a main geophysical technique planned for use at the site because the distribution of true resistivities of the subsurface materials can be inverted yielding a 2-D cross-section of the subsurface. The ability of ERI to detect faults, fractures, and karst features is based on the fact that these geological targets frequently exhibit a discernible electrical resistivity contrast from the surrounding, undisturbed bedrock due to the increased amounts of air, water, or clay particles within them.
The study area for ERI will encompass the area in the vicinity of Building 90 as well as the off-post area near the southwest corner of CSSA. The exact locations of the ERI survey lines will depend on the orientation of geological features, underground utility locations, and space limitations. Following processing of the resistivity data, results will be included in a summary report for presentation to AFCEE and CSSA.
Three-dimensional resistivity may be performed on very localized areas to generate 3-D images of subsurface features. If performed, the 3-D ERI surveys will be conducted using the same equipment utilized for the 2-D surveys with the electrode cables arranged in a 2-D pattern on the ground surface over the area of interest. Due its limited areal coverage, the 3-D ERI method will be limited to surveys over areas less than 1/2 acre. Results of the 3-D ERI surveys will be included in the summary report submitted to AFCEE and CSSA.
Microgravity surveys will be performed to help identify and delineate karst features that may be present. Microgravity is a geophysical method that can detect extremely small variations in the earth�s gravitational field due to changes in the density of subsurface materials. The lateral density variations of interest in this investigation are those arising from either air or water-filled karst features, which are both less dense than the surrounding bedrock and therefore create measurable decreases in the gravitational field. Microgravity profiles will be acquired along the same transects as some of the ERI profiles to complement the ERI data.
Very low frequency electromagnetic (VLF) surveys will be performed to assist with the delineation of bedrock fractures. The VLF method utilizes electromagnetic signals from transmitters located throughout the world to identify conductive linear subsurface features such as fractures. In the U.S., the Department of Defense normally uses VLF transmitters for communication purposes. At distances greater than 500 km from a transmitter the EM wavefront can be approximated as a plane wave and utilized for subsurface characterization. If a fracture is present in the bedrock and the orientation of the fracture is aligned toward a VLF transmitter, the primary EM field will generate a secondary EM field in the fracture. The system measures the EM fields and if a secondary field is present, the location can be determined. The instrument also compares the phase shift between the primary field and the secondary field inducted in the ground. This amount of the phase shift is a function of the size and conductivity of the feature.
3.5 - Ground Penetrating Radar
Ground penetrating radar surveys (GPR) surveys will be performed within areas that have been identified for further investigation based on the results of ERI and microgravity surveys. The objective of the GPR survey will be to complement the ERI data by collecting GPR profiles along some of the same transects as the ERI profiles. The GPR method works by transmitting radar waves into the subsurface where they are partially reflected at boundaries with differing electrical properties, and measuring the total transit time for reflected waves to return to the surface. GPR data can be collected as a continuous profile that provides highly detailed information on the shallow stratigraphy.
2D and 3D seismic reflection data will be acquired at the site to assist with delineating geological targets of interest. The objectives of the seismic surveys are to accurately image the site stratigraphy so that fault and fracture zones can be delineated and image any karst features that may be present in the subsurface. A geophysical subcontractor will provide the 2D and 3D seismic surveys and Parsons personnel will provide oversight services and data management for this field effort.
Borehole geophysical methods will be applied to the borings advanced as part of the soil vapor extraction treatability study and groundwater recharge study. In addition to the standard borehole logging methods such as caliper and natural gamma logging, other borehole geophysical logging tools are being considered such as the optical televiewer and borehole radar. The optical televiewer will present a 360-degree digital image of the borehole that will allow direct viewing of the fracture depth and orientation, as well as lithologic information such as color, grain size, and texture. Single-hole, directional borehole radar can be used to identify the location and orientation of fracture zones. Borehole radar can provide information on the orientation and density of fractures some distance away from the borehole depending on the electrical resistivity of the bedrock.