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Technical Memorandum on Surface Geophysical Surveys, Well 16 Source Characterization
Section 8 - SWMU B-28
8.1 - Site History and Description
SWMU B-28 is located approximately 100 feet west of B-19 and 1,500 feet south of Well 16 (Figure 1.1). Molten metal, small arms ammunition, and metal ammunition parts were disposed of at B-28. This was verified during the field investigation when two shallow trenches were encountered with fragments of molten metal exposed at the surface. Metal debris was also found scattered throughout the site. Both trenches trend northwest-southeast and are approximately 5 feet wide at the surface. The northern trench is a linear feature approximately 300 feet in length that intersects a drainage ditch to the northwest. The southern trench is slightly curved and extends approximately 100 feet. The topography of the site is flat and it is located in an open, grassy area with an occasional isolated tree (Figure 8.1).
8.2 - Geophysical Survey Results
Figure 8.2 and Figure 8.3 depict contour maps of apparent ground conductivity and in-phase EM data, respectively, for the north-south EM transects at SWMU B-28. EM readings were taken at a 50-foot station spacing with north-south and east-west orientations of the instrument. The in-phase EM data reveal two strong anomalies oriented in the northwest-southeast direction where readings reached a maximum of 95 ppt (Figure 8.3 and Figure 8.5). These anomalies are associated with the molten metal debris which is present on the surface at B-28. These results indicate that there are two northwest-southeast oriented trenches containing buried metal. The northern trench is approximately 300 feet long and 15 feet wide. The southern trench is approximately 100 feet long and 15 feet wide. The trenches are apparently shallow (<5 feet deep) according to field observations where the northern trench intersects the drainage ditch (Figure 8.1).
A GPR survey was also conducted at B-28 using the GSSI SIR-3 system. The survey was conducted in the east-west orientation across the northern trench at 25-foot intervals. As expected, a strong GPR response was recorded in the trench area where surface metal was exposed. The response was sharp and the GPR signals often had an "echo" response, a common reaction to the presence of surface metal. Figure 8.6 shows the results of GPR transect at B-28.
Two anomalies A and B, were identified at B-28 using EM and GPR methods (Figure 8.7). Anomalies A and B correspond to the northern and southern trenches, respectively, shown on Figure 8.1. The presence of two shallow trenches containing surface and subsurface metal debris resulted in high in-phase EM readings. Furthermore, a strong GPR response was recorded at the northern trench.