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Technical Memorandum on Surface Geophysical Surveys, Well 16 Source Characterization
Section 6 - SWMU O-1 (Oxidation Pond)
6.1 - Site History and Description
SWMU O-1 (oxidation pond) is located immediately northwest of B-4 (Figure 1.1). The oxidation pond was an evaporation pond constructed in 1975 and lined with vinyl plastic. Waste liquids and sludges from the bluing operation were collected and disposed of in the pond. The pond was subsequently closed in the fall of 1985 by filling it in with dirt. The plastic liner was deliberately damaged by a bulldozer during the filling operation as evidenced by small scattered pieces of the liner lying on the ground. The oxidation pond is a relatively small site with weathered limestone and white caliche at the surface and no vegetation. Limestone bedrock is apparently close to the surface in this area which corresponds with records indicating a depth of the pond at 2 to 5 feet. The site is flat, unobstructed, and surrounded by trees to the north, west, and south (Figure 6.1). There is evidence of scattered metal on the surface, some of which appears to have been subjected to extreme heat.
6.2 - Geophysical Survey Results
Figure 6.2 and Figure 6.3 depict contour maps of apparent ground conductivity and in-phase EM data, respectively, for the north-south oriented EM data at the oxidation pond. EM readings were taken at a 25-foot station spacing with north-south and east-west orientations of the instrument. The apparent conductivity contour map reveals a general outline of the pond boundaries. Conductivity values increase from 20 mS/m on the perimeter to 60 mS/m in the middle of the filled pond area. This relatively subtle increase in conductivity values is an indication that the ground has been disturbed. The oxidation pond lies in an open, well-delineated area (surrounded by trees on three sides) where limestone bedrock appears to be close to ground surface. The site was therefore a very shallow evaporation pond prior to closure. Furthermore, the constant in-phase EM readings infer the absence of buried metal objects.
Both the GSSI SIR-3 and Pulse EKKO IV data were collected at the oxidation pond. The GSSI SIR-3 data were collected along east-west and north-south oriented transects separated by 25 feet. By contrast to other sites surveyed using the SIR-3 system, excellent results were generated from this survey. Figure 6.4 is a representative line from this GSSI SIR-3 GPR survey which clearly delineates the location of the oxidation pond. The location of the GPR line is shown on Figure 6.1. GPR signals were recorded outside the boundaries of the pond where limestone rock is at, or close to, ground surface. However, inside the boundaries of the pond, there was little or no GPR response due to the presence of clay fill. The Pulse EKKO IV data did not clearly delineate the boundaries of the oxidation pond (Figure 6.5). The location of this Pulse EKKO IV GPR survey line is shown on Figure 6.1.
In summary, one relatively subtle anomaly was detected on the EM conductivity contour map (Figure 6.2) which generally outlines the boundaries of the oxidation pond. This EM anomaly was correlated with the results from the GSSI SIR-3 GPR data which also delineated the boundaries of the pond (Figure 6.6). Figure 6.6 shows that a close correlation was drawn between the two geophysical methods described above. Based on these results, the pond is interpreted to be roughly circular in shape with a diameter of approximately 75 feet and a depth of less than 5 feet.