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Technical Memorandum on Surface Geophysical Surveys, Well 16 Source Characterization
Section 1 - Introduction
Surface geophysical surveys were performed at seven Solid Waste Management Units (SWMUs) at Camp Stanley Storage Activity (CSSA) as part of a contaminant source characterization study. In August 1991, chlorinated hydrocarbons were found in the groundwater at Well 16 at concentrations above the drinking water limit. CSSA requested that Parsons Engineering Science (Parsons ES) investigate the groundwater contamination, a project which began in September 1992 and is continuing to date. As the source of groundwater contamination is not known, part of the investigation is characterization of potential source sites (task 5). Geophysical surveys were performed prior to drilling and subsurface sampling to locate areas potentially related to past waste disposal activities. The geophysical surveys are the subject of this technical memorandum and are an integrated part of the Well 16 groundwater source characterization. A CSSA site map is shown on Figure 1.1.
During the investigation, two sites were added by CSSA (Well 16 and Southeast Well 16) which were not listed as SWMUs. After the investigation, a decision was made jointly to survey all the "open" areas within a 2,000-foot radius of Well 16. These open areas, not listed as SWMUs, are defined as those areas relatively free of vegetation cover, such as trees and dense shrubs. The open areas were broken up into the following six sites: North Pasture, South Pasture, Well 16 West, Well 16 East, Gate 6, and Salado Creek.
Geophysical surveys were performed at a total of fifteen sites. The approximate boundaries of the sites are shown on Figure 1.2. Seven of these sites are SWMUs. The remaining eight sites are not SWMUs but were added to the project at the request of CSSA. This technical memorandum contains a summary of the methods used during the geophysical surveys, data analysis, and a discussion of the results. A brief historical summary and description of each site is included in separate sections.
1.2 - Geophysical Survey Methods
Prior to collecting surface geophysical data, a grid system was established at each site which encompassed the areas of suspected ground disturbance. These grids consisted of staked locations separated by intervals ranging from 25 to 100 feet, depending on the size of the area and the amount of obstructions, if any. The grid systems and spacing used are shown on individual site base maps.
electromagnetic (EM) data were collected at 2-foot intervals along transects that were separated by 20 to 50 feet using the established geophysical survey grid. All EM measurements were recorded using a Geonics EM31-DL ground conductivity meter, and data for a majority of the sites were recorded with a Polycorder data logger. The conductivity meter consists of transmitter and receiver coils that are separated by 12 feet. The instrument has a nominal depth of penetration of approximately 16 feet when operated in the vertical-dipole mode. The instrument measures both quadrature- and in-phase components of an induced magnetic field. The quadrature-phase component is a measure of apparent ground conductivity while the in-phase component is more sensitive to the presence of ferromagnetic metal. A lateral variation in apparent ground conductivity indicates a lateral change in subsurface physical properties (i.e., related to degree of disturbance). Apparent ground conductivity is measured with a precision of approximately +/- 2 percent of the full-scale meter readings which corresponds to approximately 2 milliSiemens per meter (mS/m). The in-phase component of the EM-31 is the response of the secondary to primary magnetic field measured in units of parts per thousand (ppt). The primary magnetic field is due to the current source from the EM-31. The secondary magnetic field is due to induced currents within conductive material in the subsurface.
Data were collected by setting the instrument to record in an automatic vertical dipole mode. Readings were taken at 0.6 to 0.8 second intervals which corresponded to a reading every 2 feet along a given transect. Both apparent ground conductivity (i.e., quadrature-phase) and in-phase data were recorded. The operator aligned himself along a transect and paced, with the instrument parallel to the transect, between marked/staked stations that were separated by 20 to 50 feet. The variation in transect footage was related to the size of the site and the amount of obstructions, if any.
During the course of each field day, data were transferred from the data logger to computer diskettes. The data were processed using DAT31 software (Geonics, LTD) and contoured using Surfer software. For EM data that was not collected using the data logger, values were recorded on a log sheet, manually entered into a computer file, and contoured using Surfer software. Contour maps of apparent conductivity and in-phase data for each site were created. Cool colors were used for contours of lower conductivity and in-phase values, and warm colors were used for contours of higher conductivity and in-phase values.
1.2.2 Ground Penetrating Radar Survey
The ground penetrating radar (GPR) survey was performed using a Geophysical Survey Systems, Inc. (GSSI) SIR-3 system equipped with a Model SR-8304 graphic recorder, and 300 and 500 megahertz (MHz) antennas. In addition, a Sensors and Software, Inc. Pulse EKKO IV system with 100 MHz antennas was used. Acquisition parameters fot the Pulse EKKO IV consisted of 4-foot separation between transmitter and receiver electrode, station interval of 1 foot, sampling rate of 0.8 nanoseconds (nS), and a 250-fold vertical stack.
GPR is a surface geophysical technique which uses high-frequency electromagnetic energy. Pulses of short-duration electromagnetic energy are transmitted into the subsurface from the radar antenna that is moved across the ground surface at a slow and uniform pace. The radiated energy encounters inhomogeneities in electrical properties of the subsurface which causes some energy to be reflected back to the receiving antenna and some to be transmitted downward to deeper material. The amplitude r strength of the electromagnetic energy reflected from subsurface materials depends on contrasts in the electrical properties (conductivity and dielectric constants) of those materials. The reflected signal is amplified, transformed to the audio-frequency ranges, recorded, processed, and displayed. The recorded data displays the two-way travel time for a signal to pass through the subsurface, reflect, and return to the surface. Optimal subsurface conditions for use of GPR are dry sandy soils. The presence of even minor amounts of clay may effectively limit depth of investigation to less than a few feet due to absorption and reflection of the electromagnetic energy.
The observed time for the reflected signal to return to the antenna from a subsurface feature is an indication of the depth to the reflector. The two-way reflection time can be converted to depth if the electromagnetic wave velocity of the subsurface material is known. In the absence of such information, an approximate time to depth conversion can be estimated by using published values of material velocity for different soil types.
The interpretation of subsurface conditions is based on analysis of the recorded sections. Buried objects such as pipes and tanks are usually evident as prominent hyperbolic reflections on the GPR records. Subsurface soil changes can be difficult to interpret, but often can be discerned as a lateral change in the texture or reflection character of the GPR signal. Stratigraphic changes are often very prominent and may effect the GPR readings. The use of GPR to determine landfill boundaries and buried waste disposal trenches can be at times very successful due to contrasts in reflection character between the natural stratigraphy outside the trench boundaries and the disturbed soils within the disposal areas.
The following sections are brief historical summaries and descriptions of each site along with in-depth discussions of the geophysical survey results. Summaries and discussions are organized on a site by site basis. Figures for each site are included in that site section. The final section summarizes the findings of the geophysical surveys.