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SWMU B-20 Investigation Report Addendum
Section 4 - Conclusions
The overall objective of the second phase of the investigation at the B-20 site was to continue to prepare the site for eventual closure under either RRS1 or RRS2. The scope of the second phase was based on recommendations for additional work made in the RI report (Parsons ES, 1995a). These recommendations primarily address identifying possible remaining UXO at the site and around the site's perimeter, and better delineating the extent of contamination.
Some of the data gaps identified in the RI report were addressed in full, but some could not be completed due to budget and schedule constraints. All of the information gathered during the first and second phases of the RI will eventually be used in determining the most technically feasible and cost effective method for remediating the site.
This section describes the conclusions of the second phase of the investigation. These conclusions are used in Section 5 to recommend additional work at the site to remediate it to either RRS1 or RRS2 criteria.
Clearing vegetation and using a magnetometer during the surface sweep for UXO resulted in a significant increase in the number of UXO items identified. A total of 506 UXO items have been found on the surface of the B-20 site and the perimeter area. Three of the items were considered to be too dangerous to move. All of the UXO identified at the site has been destroyed.
The extent of the area cleared was increased during this investigation to address UXO identified outside of the original site boundary. The area cleared was extended at least 50 feet from the furthest UXO item identified. In some areas, this resulted in an area extending over 500 feet from the original site boundary. The surface of the area shown in Figure 3.5 has been carefully and systematically swept for UXO. In fact , the original site has been swept twice for surface UXO. The type of UXO most likely to still remain at the site is raw TNT. Several pieces of raw TNT were identified during the sweep; however, it is difficult to identify on the ground surface.
In addition to the site and the perimeter area, a large (approximately 32 acre) area north of the site was visually swept for UXO. This area was added to the original scope of the phase II RI when three UXO items were identified approximately 0.25-mile from the site. During the sweep of this 32 acre area, 14 UXO items were found. Vegetation, which was not cleared for the sweep in this area, may have obstructed visual identification of UXO.
Metals
Additional surface soil sampling was conducted during the phase II RI to better delineate the area in which metals concentrations in surface soils exceed background. This sampling was conducted in the south-central portion of the site, where barium, cadmium, lead, and mercury concentrations exceeded background in the first phase RI. Analytical results of these additional samples indicated that a greater amount of surface soil than estimated in the phase I RI needs to be removed or remediated to meet RRS1 criteria. Furthermore, the extent of contamination may not yet be fully delineated, particularly on the south side of the site. The current estimated volume of surface soil exceeding RRS1 criteria is 17,900 yd3. The estimation is based on an average soil depth of one foot. Surface soils in this large area do meet RRS2 criteria.
During the phase II RI, no additional sampling was done at the small areas where metals concentrations exceed both RRS1 and RRS2 criteria. Surface soils do not meet industrial facility MSCs at two small ammunition disposal areas in the northern portion of the site and the B-21 small ammunition disposal area on the east side of the site.
Explosives
Samples were collected near locations in which raw TNT was identified and analyzed for TNT and RDX using an immunoassay test kit. The results indicated that only very small amounts of TNT had contaminated soils directly underneath each individual piece of TNT. Results from samples collected a foot away showed no TNT contamination.
Although several very small (up to 2-inch) pieces of TNT were found during the sweep, samples were only collected under two of them. Contamination at the other locations is considered to be similar. TNT is quite insoluble and readily binds to soil. The controlled burn planned for the site may likely "remediate" these very low concentrations of explosives on the surface of the site.
The possibility oif buried UXO in craters and in the northern portion of the site was addressed during the second phase of the RI. Since the standard operating procedure for UXO disposal was to bury it in the craters for detonation, the first phase RI concluded that some UXO may remain there. The features of the northern 6 acres of the site and the lack of records for that area led to the conclusion in the RI report that there may be UXO buried in that portion of the site. The objective of the second phase of the investigation was to confirm or deny the presence of UXO in these areas, and to remove any UXO identified.
Magnetic anomalies had been identified at 10 craters during the first phase of the RI. These anomalies were re-identified, excavated, and removed at all but 2 of these craters during this investigation. The two craters (14 and 15) not addressed are located in a portion of the northern 6 acres (grids A2 and A3) which was not surveyed due to budget and schedule constraints. Most of the anomalies in the craters were caused by scrap metals fragments or clay lenses. Only one UXO item (a 20mm projectile) was identified.
The magnetometer survey and anomaly identification in the northern portion of the site was done on a grid-by-grid basis. Six of the eight grids delineated on the surface were investigated. A total of 300 buried UXO items were identified in this area. The items were scattered throughout, but the majority of the items were found on the north side of teh 6-acre area. The two grids which were not surveyed are located adjacent to the grids which contained a large number of UXO; therefore, these grids may also contain a significant amount of buried UXO.
Most of the anomalies detected in the north 6 acres were caused by metal-containing debris, such as metal cans and wire. therefore, it appears that this portion of the site was used for some sort of solid waste disposal. Many of the debris items were removed during the investigation, but in some cases the volume of waste material was too large to practically be removed.
Metals levels in three borings (SB2, SB9, and SB10) drilled during the first phase of the RI exceeded both RRS1 and RRS2 criteria. To determine if metals levels in these borings could leach, borings were drilled immediately adjacent to these locations and samples for leachate analysis were collected. Two samples were collected from each new boring and were analyzed using TCLP and SPLP. The metals in rock/soil exceeding RRS1 and RRS2 criteria were arsenic and mercury. Arsenic and mercury concentrations in the leachate samples did not exceed MCLs, and are therefore considered to meet RRS1 and RRS2 criteria.
However, the lead level in leachate from one sample did exceed the MCL. This sample was collected in soil from a boring near two small ammunition disposal areas, from which the lead may be migrating.
Surface water contamination was not addressed during this investigation. Due to the drought conditions during the phase II RI, most of the areas which contained surface water during previous investigations were completely dry. By March 1996, neither the small pond nor the craters contained any water. Even the large livestock pond located northeast of the site was almost dry.