Geophysical Survey Rock Hill Bleachery TBA Site Rock Hill, South Carolina EP-W-05-054 EPA, START 3, Region 4 Prepared for: Tetra Tech EM, Inc. October 12, 2012
Geophysical Survey Rock Hill Bleachery TBA Site Rock Hill, South Carolina EP-W-05-054 EPA, START 3, Region 4 TABLE OF CONTENTS Section Page Signature Page... ii Executive Summary... iii 1.0 Introduction... 1 1.1 Background... 1 2.0 Equipment and Methodology... 1 2.1 CMD-4... 1 2.2 EM61-MK2... 2 2.3 Radio Frequency EM Utility Locating Equipment... 3 2.4 X3M GPR... 4 3.0 Field Procedures... 5 4.0 Data Interpretation and Results... 6 Figures Figure 1 CMD-4 Conductivity data Figure 2 CMD-4 In-phase data Figure 3 EM61-MK2 data Figure 4 Site map and data interpretation (Whole site, 1 = 200 Scale) Figure 5 Site map and data interpretation (Northern part of the site, 1 = 100 Scale) Figure 6 Site map and data interpretation (Central part of the site, 1 = 100 Scale) Figure 7 Site map and data interpretation (Southern part of the site, 1 = 100 Scale)
Signature Page This report, entitled Geophysical Survey, Rock Hill Bleachery TBA Site, Rock Hill, South Carolina has been prepared for Tetra Tech EM, Inc. located in Duluth, Georgia. It has been prepared under the supervision of Mr. Jorgen Bergstrom at the request of and for the exclusive use of Tetra Tech EM, Inc. This report has been prepared in accordance with accepted quality control practices and has been reviewed by the undersigned. GEL Geophysics, LLC A Member of the GEL Group, Inc. Jorgen Bergstrom Senior Geophysicist John Reynolds Geophysics Specialist Scott D. Carney, P.E. Director October 12, 2012 Date ii
Geophysical Survey Rock Hill Bleachery TBA Site Rock Hill, South Carolina EP-W-05-054 EPA, START 3, Region 4 EXECUTIVE SUMMARY GEL Geophysics performed a geophysical survey at the Rock Hill Bleachery TBA Site in Rock Hill, South Carolina on October 1-5, 2012. This investigation was performed for Tetra Tech EM Inc. (Tetra Tech) to aid in characterizing the subsurface at the site. The investigation entailed the collection, processing, presentation, and interpretation of Electromagnetic (EM) and Ground Penetrating Radar (GPR) data. The objectives of the geophysical investigation were to determine the location of buried objects, cables, pipes and the depth and dimension of underground storage tanks, vaults and other buried structures. For this investigation GEL Geophysics used a GF Instruments CMD-4 EM ground conductivity and magnetic susceptibility instrument, a Geonics EM61-MK2 time domain EM system, MALA GeoScience X3M GPR system equipped with 250 MHz antennas, and RadioDetection RD 8000 and Pipehorn 800 radio frequency electromagnetic (RFEM) pipe and cable locators. Positioning data was provided by a Trimble 5800 Real Time Kinematic Global Positioning System (RTK/GPS) system and a TopCon GMS-2 GPS+GLONASS L1 system with a PG-A5 external GPS antenna. Due to site conditions not conducive for GPR signal propagation, GEL Geophysics relied heavily on EM methods for detecting subsurface features at the site. CMD 4 data was collected across the whole site with a 20-foot profile separation. EM61-MK2 data was then collected with a 3-foot profile separation in areas with high concentration of subsurface anomalies. GPR was used on concrete surfaces since the concrete was found to contain reinforcement bars or wire mesh and EM methods cannot detect features below reinforced concrete iii
The results of the geophysical survey produced a large number of anomalies. A large number of unknown underground utilities were detected at the site; however, it could not be determined if these utilities were abandoned or active. Thirty areas were detected which exhibit geophysical anomaly responses consistent with underground storage tanks, concrete vaults or concrete foundations. Approximately 300 objects up to 4 by 4 feet in size and less than 3 feet deep were detected across the site. These objects are likely larger chunks of building debris from the razed buildings. An estimated 2,000 smaller and/or deeper objects were also detected in the data sets but were not marked on any attached figure. There is a possibility that other subsurface features may exist at the project site that are not detectable by the geophysical systems due to either subsurface soil conditions or the occurrence of features below the signal penetration depth. To confirm the existence, exact location, and dimensions of features, GEL Geophysics suggests they be probed for verification. iv
Geophysical Survey Rock Hill Bleachery TBA Site Rock Hill, South Carolina EP-W-05-054 EPA, START 3, Region 4 1.0 INTRODUCTION GEL Geophysics performed a geophysical survey at the Rock Hill Bleachery TBA Site in Rock Hill, South Carolina on October 1-5, 2012. This investigation was performed for Tetra Tech EM Inc. (Tetra Tech) to aid in characterizing the subsurface at the site in advance of soil testing and remediation efforts. 1.1 Background Tetra Tech is currently involved in environmental site investigations at the Rock Hill Bleachery TBA Site in Rock Hill, South Carolina. These investigations involve assessing soil contamination at the approximately 24-acre site. To support this investigation, Tetra Tech invited GEL Geophysics to conduct a geophysical investigation of the site using Electromagnetic (EM) and Ground Penetrating Radar (GPR) and techniques. The objectives of the geophysical investigation were to determine the location of buried objects, cables, pipes and the depth and dimension of underground storage tanks, vaults and other buried structures. This investigation entailed the collection, processing, presentation, and interpretation of geophysical data. 2.0 EQUIPMENT AND METHODOLOGY The following is a brief introduction of the various geophysical equipment used at the site. 2.1 CMD-4 CMD 4 measures variations in electrical conductivity and magnetic susceptibility of subsurface materials. The conductivity is determined by inducing a primary
Geophysical Survey Rock Hill Bleachery TBA Site October 12, 2012 Rock Hill, South Carolina (tetr00312) Page 2 electromagnetic field and measuring the amplitude and phase shift of an induced secondary magnetic field. The secondary magnetic field is created by subsurface conductive materials behaving as an inductor as the primary field is passed through them. Terrain conductivity systems such as the proposed systems are commonly used to delineate variations in ground conductivity. There are two components of the induced electromagnetic field measured by the systems. The first is the quadrature-phase (out-ofphase) component that measures the bulk conductivity of soil and groundwater. The second is the in-phase component that measures the magnetic susceptibility and is therefore more sensitive to isolated metallic objects such as pipes, drums, underground storage tanks, and other metallic debris. By observing the response of the in-phase and quadrature-phase components, it is possible to differentiate whether a change in bulk conductivity is due to the presence of buried metallic objects or due to changes in subsurface soil conditions or pore fluid conductivity. The presence of metal buildings, fences, and other metallic surface objects cause interference and makes data interpretation for subsurface features near these objects difficult. GEL Geophysics therefore surveyed any metallic surface objects to facilitate the data interpretation. The CMD-4 system has an effective depth of exploration of up to approximately 20 feet below ground surface. However, the ability to detect small features decreases with depth. 2.2 EM61-MK2 Time Domain Electromagnetic (TDEM) systems use EM eddy currents to discriminating between moderately conductive earth materials and very conductive metallic targets. The EM61- MK2 consists of a portable coincident loop time domain transmitter and receiver with a 1.0-meter x 0.5-meter coil system. The EM61-MK2 generates 150 pulses per second and measures the response from the ground after transmission or between pulses. The secondary EM responses from metallic targets are of longer duration than those created by conductive earth materials. By recording the later time EM arrivals, only the response from metallic targets is measured, rather than the field generated by the earth material.
Geophysical Survey Rock Hill Bleachery TBA Site October 12, 2012 Rock Hill, South Carolina (tetr00312) Page 3 A GPS port on the logger allows simultaneous collection of EM61-MK2 and GPS data. After completion of the field data acquisition, the data is transferred to a PC for further processing and analysis. The system is mounted on a wheel assembly with the bottom coil approximately 1.5 feet above the ground, and the top coil approximately 2.8 feet above the ground. The EM61-MK2 detects a single 55-gallon drum at a depth of over 10 feet beneath the instrument, yet it is relatively insensitive to interference from nearby surface metal such as fences, buildings, cars, etc. 2.3 Radio Frequency EM Utility Locating Equipment Radio Frequency Electromagnetic (RFEM) utility locating equipment consists of a transmitter and a dual-function receiver. The receiver can be operated in a passive mode or in an active mode. The two modes of operation provide various levels of detection capabilities depending on the specific target or application. The EM system is operated in the active mode by either inducting or conducting a signal into the underground utility to be traced. A transmitter is placed over and in line with a suspected buried utility. The transmitter induces a signal, which propagates along the buried utility. As the receiver is moved back and forth across the suspected path of the utility, the trace signal induces a signal into the receivers coil sensor. A visual and audio response indicates when the receiver is directly over the buried utility. Another means of detecting in the active mode utilizes a method to conduct a signal within the buried utility. To accomplish this, a cable from the transmitter is clamped onto an exposed section of the buried utility and a signal propagates along the buried line. This technique minimizes any interference caused by parasitic emissions from adjacent cables in congested areas. When the system is utilized in the passive mode, the receiver is responding to a 60 Hertz cycle current energized by underground utilities. Interference can and may occur when buried utilities intersect or are adjacent to each other. This effect referred to as bleed-off may provide a false response to the identification of the tracked utility. Bleed-off is caused by utilities that may be energized in the active or passive mode.
Geophysical Survey Rock Hill Bleachery TBA Site October 12, 2012 Rock Hill, South Carolina (tetr00312) Page 4 2.4 X3M GPR GPR is an electromagnetic geophysical method that detects interfaces between subsurface materials with differing dielectric constants. The GPR system consists of an antenna which houses the transmitter and receiver, a digital control unit which both generates and digitally records the GPR data, and a color video monitor to view data as it is collected in the field. The transmitter radiates repetitive short-duration electromagnetic waves (at radar frequencies) into the earth from an antenna moving across the ground surface. These radar waves are reflected back to the receiver from the interface of materials with different dielectric constants. The intensity of the reflected signal is a function of the contrast in the dielectric constant between the materials, the conductivity of the material through which the wave is traveling, and the frequency of the signal. Subsurface features that commonly cause such reflections are: 1) natural geologic conditions, such as changes in sediment composition, bedding, and cementation horizons and voids; or 2) unnatural changes to the subsurface such as disturbed soils, soil backfill, buried debris, tanks, pipelines, and utilities. The digital control unit processes the signal from the receiver and produces a continuous cross-section of the subsurface interface reflection events. GPR data profiles are collected along transects, which are measured paths along which the GPR antenna is moved. During a survey, the distance of the GPR transects is measured in real-time with a calibrated wheel encoder. This allows for a correlation between the GPR data and the position of the GPR antenna on the ground. Depth of investigation of the GPR signal is highly site-specific and is limited by signal attenuation (absorption) in the subsurface materials. Signal attenuation is dependent upon the electrical conductivity of the subsurface materials. Signal attenuation is greatest in materials with relatively high electrical conductivities such as clays, brackish groundwater, or groundwater with a high dissolved solid content from natural or manmade sources. Signal attenuation is lowest in relatively low-conductivity materials such as dry sand or rock. Depth of investigation is also dependent on the antenna's transmitting frequency. Depth of investigation generally increases as transmitting
Geophysical Survey Rock Hill Bleachery TBA Site October 12, 2012 Rock Hill, South Carolina (tetr00312) Page 5 frequency decreases; however, the ability to resolve smaller subsurface features is diminished as frequency is decreased. The GPR antenna used at this site were internally shielded from aboveground interference sources. Accordingly, the GPR response typically not affected by overhead power lines, metallic buildings, or nearby objects. 3.0 FIELD PROCEDURES GEL Geophysics performed a geophysical survey at the Rock Hill Bleachery TBA Site in Rock Hill, South Carolina on October 1-5, 2012. All of GEL Geophysics field activities were supervised by a Senior Geophysicist and observed by Tetra Tech technical personnel. Buried pipe and cables were designated using a combination of GPR, EM, and RFEM pipe and cable locating equipment. The RFEM equipment was utilized in active mode by connecting the transmitter to an above ground utility feature and inducing a signal from the ground surface into the buried utility. Subsurface utilities were also detected in passive mode by scanning the site for radio and 60Hz signals from subsurface utilities. Identified buried pipes and cables were marked on the ground surface using marking paint and/or flags and the locations of these features were recorded using GPS. Approximate depth of underground utilities was also noted where available. CMD-4 data was collected across the site with a profile separation of approximately 20 feet except for the concrete covered areas in the southern part of the site. These areas where excluded since EM methods cannot be used to detected features below reinforced concrete. To facilitate the interpretation of the CMD-4 data, GEL Geophysics noted and measured the position of buildings, fences, utility surface features, surface metal, monitoring wells, and other features using GPS in order to determine if these objects were causing anomalies in the geophysical data and for positioning control. The CMD-4 data was processed and analyzed immediately following data collection and the results were used to guide where to collect more detailed data. Areas with high density of subsurface targets and with no concrete surface were scanned using the EM61-MK2 system. The reason for selecting EM61-MK2 in lieu of collecting CMD-4 data on a tighter grid was that the density of subsurface targets in these
Geophysical Survey Rock Hill Bleachery TBA Site October 12, 2012 Rock Hill, South Carolina (tetr00312) Page 6 areas was so high it would likely not have been possible determining size and location of individual targets using CMD-4. The CMD-4 has a larger signal footprint than the EM61-MK2 and is more sensitive to signal saturation from multiple targets in the vicinity of the instrument. In order to derive the most detailed image possible of subsurface features, EM61-MK2 data was collected in the areas mentioned above using a profile spacing of 3 feet. Targets identified in CMD data outside of the EM61-MK2 grids were located in the field and inspected using GPR and EM61-MK2. GPR signal penetration was found to be very poor at areas outside of the concrete covered areas due to highly conductive and wet soils. On average the signal penetration in those areas was no more than 2 feet. GEL Geophysics and Tetra Tech therefore decided not to collect GPR data on a grid across the site and focus the efforts on EM surveys instead. GPR signal penetration on concrete covered areas was approximately 3-4 feet. GPR data was collected in bi-directional grids across the concrete surfaces using a nominal profile spacing of 5 feet. The GPR data was analyzed through a combination of real-time data analysis by the operator and grid analysis in the office. All detected subsurface features across the site, grid corners and other features of interest were spatially positioned using GPS. Note that surface features were recorded for the sole purpose of facilitating the geophysical investigation and should not be considered geodetically surveyed. 4.0 DATA INTERPRETATION AND RESULTS The processed CMD-4 data is shown on Figure 1 (conductivity data) and Figure 2 (in-phase data). The CMD-4 data was compared with the locations of surface metal and anomalies caused by surface metal were disregarded. Based on this data set, areas were selected for more detailed investigation. EM61-MK2 data was collected across areas with a high anomaly density (Figure 3). GPR data was collected over concrete surfaces since EM methods cannot detect features below reinforced concrete. The data sets were
Geophysical Survey Rock Hill Bleachery TBA Site October 12, 2012 Rock Hill, South Carolina (tetr00312) Page 7 analyzed for subsurface pipes and cables, potential USTs, vaults and other larger obstacles, as well as for smaller objects containing metal. Using the geophysical tools described, GEL Geophysics identified a large number of potential underground utilities, 30 unknown objects that could potentially be USTs, concrete vaults, or larger concrete slabs, and approximately 300 smaller objects containing metal. The smaller objects are likely larger chunks of debris from the razed buildings or chunks of reinforced concrete. The smaller objects vary in size but are believed to not be larger than approximately 4x4 feet and not more than 3 feet deep. In some areas, the density of subsurface metal was too high to identify individual targets. These areas and all features mentioned above are shown on Figures 4-7. An estimated 2,000 additional smaller and possibly deeper metallic objects also exists throughout the site (not marked on Figures 4-7). The data on the figures is presented in South Carolina State Plane, US Survey feet, NAD 83 coordinate system. There is a possibility that other subsurface features may exist at the project site that are not detectable by the geophysical systems due to either subsurface soil conditions or the occurrence of features below the signal penetration depth. To confirm the existence, exact location and dimensions of features, GEL Geophysics suggests they be probed for verification.