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Appendix C Ground Penetrating Radar Investigation Phase 11 ESA Multi-Modal Station"Six-Block Pruiect" January 6,2603 Page 2 of 4 • Attenuation of electromagnetic waves transmitted through a medium is directly related to the electrical conductivity of the medium A highly electrically-conductive medium will attenuate electromagnetic waves rapidly, and the extent to which the electromagnetic waves penetrate the medium will be reduced Many of the minerals that comprise a typical clay soil (so-called "clay minerals") are highly conductive due to their strong ionic character In addition, clay minerals are hydrated minerals, and conductivity increases further with increasing salt content of the incorporated water Wet clays are even more difficult to penetrate with electromagnetic waves, as the additional water with its salt content only enhances the attenuation effects In the worst conditions, the depth of investigation of GPR may be limited to as little as one foot below ground surface (bgs) Previous work performed by Condor in the vicinity of the site and subsequently confirmed by drilling at the site indicates that the soils of downtown Stockton are predominantly clays, sandy clays, and silty clays, especially near the surface The presence of these soil types at the site limited the effective depth of this GPR investigation to an estimated six to eight feet bgs 1.2 INVESTIGATION METHODOLOGY AND INSTRUMENTATION The GPR investigation of selected areas of the site was performed to meet two objectives The primary objective of the investigation was to garner additional information pertaining to subsurface conditions at the proposed boring locations on selected private properties within the site The secondary objective was to investigate certain properties or portions thereof for evidence of suspect environmental conditions, such as the potential presence of USTs and other buried objects Prior to commencement of drilling activities on private properties, Condor representatives inspected individual properties to attempt to identify boring locations that were easily accessible, generally consistent with the proposed boring locations for the site, and strategically positioned with respect to . features typically associated with potential subsurface contamination To accomplish this last task, each property selected for drilling was visually inspected for features such as drains, paint rooms, asphalt and concrete damage and patches, hazardous materials containers and storage areas, indicators of potential USTs, etc Boring locations were selected to provide a higher likelihood of finding potential contamination After the boring locations for a property were selected, each boring location was investigated by collecting a series of GPR data The GPR data for most boring locations were collected by traversing six intersecting lines, three from west to east and three from south to north The lines were spaced approximately 15 feet apart and were approximately three to four feet in length, forming a three-foot by three-foot grid centered on each boring location Slight variations to this scheme were necessary for some boring locations based on site conditions The purpose of this methodology was to investigate each preliminarily identified boring location for underground piping, conduits, or other burred objects that would necessitate relocation of the boring After reviewing the collected data, the actual boring locations were marked in the portions of the grids with the least likelihood of being above a buried object If no portions of the grid appeared to be clear for drilling, entirely new locations were selected and additional data was collected and evaluated Whenever borings are advanced, drilling into an active utility line is a potentially expensive, time- consuming, and dangerous possibility, especially in older, developed urban areas To alleviate these concerns, the GPR investigation was conducted to supplement other safety precautions, including the notification of USA at least 72 hours before commencement of drilling and the advancement of the first five feet of each boring by hand augering The GPR investigation provided additional assurance of the feasibility of drilling at a particular location for two reasons notification of USA does not generally result 40 in identification of utility locations on private properties, and some utilities may be burred at depths