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SWMU B-20 Investigation Report Addendum

Section 3 - Results of Current Investigation

As stated in Section 1, the scope of the second phase of the RI was based on conclusions of the first phase of the investigation. The first phase of the RI defined data gaps and areas in which UXO may have remained. Table 3.1 presents conclusions of the first phase of the RI and lists how each conclusion was addressed in the second phase of the RI. This section presents the results of this additional investigation.

3.1 - Surface Soil Sampling and Analytical Results

Surface soil samples from specific areas of the site were collected to better delineate the extent of contamination and to aid in determining the volume of material to be removed or remediated to comply with the RRS1 cleanup criteria. During the second phase of the RI, the majority of the samples were collected to define the extent of metals contaminations. However, a limited amount of sampling was done to address explosives contamination in certain areas. Phase II RI surface soil sampling locations are shown in Figure 3.1.

3.1.1   Explosives Analyses

According to the project plan amendment (Parsons ES, 1995b), twenty subjectively located surface soil samples in the vicinity of the rocket pipe stand, SS19, and SS20, were to be analyzed for explosive residues (RDX and TNT) to quantify the extent of explosive contamination in those areas and more accurately define the volume of soil that would need to be removed in order to meet RRS1 or RRS2 cleanup criteria. This sampling was planned to be conducted after a controlled burn of the site. However, due to drought conditions, a controlled burn could not be conducted at the site. A controlled burn will likely destroy a majority of the possible explosive residue remaining on the surface.

During this field effort, additional small pieces of raw TNT were found in the vicinity of the rocket pipe stand, SS19, and SS20. Preliminary field tests were performed on two samples (SS60 and SS61) adjacent to a raw TNT fragment located in the vicinity of SS20. The first sample which was collected directly underneath a 1-inch-diameter fragment showed levels of TNT greater than 5.0 ppm (upper detection limit), and RDX less than 0.5 ppm. The second test was conducted on the surface soils located 4 feet downgradient of the same fragment. No TNT or RDX were detected in this sample. Additional tests were conducted on samples collected only a foot from the location at which the TNT piece was found. No explosives residues were detected in the samples. As supported by field test results presented herein, TNT is not expected to migrate significantly in soils due to its low solubility (200 mg/L). TNT contamination of surface soils at B-20 is very localized and limited to areas directly under the raw TNT pieces.

Analytical results which exceed RRS2 criteria (MSCs for industrial facilities) are presented in Figure 3.2. This figure includes results from the phase I RI.

3.1.2   Metals Analyses

Sixteen samples (SS44 through SS59) were collected and tested for one or more of the following analytes: lead by EPA method SW7420, mercury by method SW7471, and barium and cadmium by method SW6010 (Parsons ES, 1995a). These four metals were identified in the RI report as metals exceeding RRS1 criteria. The sampling locations are shown in Figure 3.1. The analyses conducted at each location are listed in Table 3.2. The data validation summary report is included in Appendix D.

Background levels used for comparison were from the corresponding soil type, and were statistically calculated and reported in the RI report. The dominant soil types at B-20 for which elevated metal values were identified during phase I of the RI (Parsons ES, 1995a) are Brackett-Tarrant association soils (Bte) and Crawford and Bexar stony soils (Cb). The background metals concentrations for these two soil types are also listed in Table 3.3.

Table 3.3 demonstrates that the cleanup value (RRS1) for a given compound can vary significantly between soil types. For instance, the background value for lead in Cb soils is 133 mg/kg, but the background level in Bte soils is only 128 mg/kg. Cleanup to the highest background level for all three soil types (i.e. 133 mg/kg site wide, not just in the Cb soil area) is considered to be a more appropriate remediation level for the entire site. Standardizing background soil values for metals across the site offers a more consistent means for delineating contaminant extent. The high values for each metals are highlighted on Table 3.3.

Figure 3.3 shows the areas exceeding the standardized RRS1 cleanup criteria. The volume of soil that exceeds the RRS1 cleanup standard is 17,885 yd³. This volume was derived using an estimated average depth of one foot of soil across the site. This is a rather conservative estimation because, in many instances, bedrock is encountered at less than 0.2 feet below the surface. However, this estimate accounts for the occasional mounds of displaced soil near craters that can be greater than 5 feet deep.

3.2 - Subsurface Soil Analytical Results

Results from the RI showed that RRS1 cleanup criteria were exceeded at SB2 (mercury at 14.0 feet), SB9 (arsenic at 4.0 feet and 13.7 feet), and SB10 (arsenic at 4.0 feet and 14.6 feet) (Parsons ES, 1995a). As concluded in the RI, since most of these samples were collected within rock and since groundwater was not encountered in these borings during drilling, it is unlikely that these metals would have an adverse affect on human health or the environment. However, closure of a non-residential facility under RRS2 requires (30 TAC 335.559(g)(2)) a demonstration that contaminants in soil do not pose the potential for future release of leachate in excess of the groundwater concentration considered to be protective for non-residential worker exposure. To make this comparison possible, three soil borings (SB11, SB12, and SB13) were drilled adjacent to the three previously drilled locations (SB10, SB9, and SB2, respectively) at which metals were detected above RRS1 and RRS2 standards. Locations of the soil borings are shown in Figure 3.4. Leachate (SW-1311 and SW-1312) of samples collected from the borings was analyzed for arsenic, barium, cadmium, lead, and mercury as listed in Table 3.4.

Lithologies encountered during drilling were consistent with those previously observed during the phase I B-20 RI. Bedrock was encountered between 4 and 6 feet BGL in SB11 (adjacent to SB10) and SB12 (adjacent to SB9) and was present at the surface of SB13 (adjacent to SB2). Groundwater was not encountered during the drilling; however, a thin (less than 6 inches thick) moist limestone zone was found at 14.3 feet BGL in SB13. This damp zone was found at 10 feet BGL in the corresponding borehole, SB2. Each borehole was drilled to 16 feet and grouted. SB13 (adjacent to SB2) was left open for a period of two hours prior to grouting to see if groundwater would accumulate in the borehole. At the time of grouting, the borehole was still dry. During the RI, groundwater which accumulated in SB1, approximately 260 feet from SB2 and SB13, was sampled and no contaminants were detected. The drilling logs for borings SB11, SB12, and SB13 are included as Appendix E.

Two samples were collected from each boring at depths which corresponded to previous sampling intervals. An additional sample was collected from teh slightly moist zone at the bottom of the SB13 (SB2) core. The results of both the toxicity characteristic leaching procedure (TCLP) and the synthetic characteristic leaching procedure (SPLP) metals analyses are listed in Table 3.4, and the maximum contaminant levels (MCLs) for groundwater as listed in Appendix II of 30 TAC 335 are included in the table for comparison. Neither arsenic nor mercury was detected in the leachate recovered from these samples.

A high level of lead (0.054 mg/L) was detected in the leachate from B20-SB12 (2.0 to 4.0 feet). This concentration is above the groundwater standard of 0.015 mg/L for lead (Appendix II 30 TAC 335). During the RI, the corresponding subsurface soil sample from SB9 contained only 19 mg/kg of lead, well below both RRS1 and RRS2 cleanup criteria. However, high lead levels have also been detected in surface water samples collected in craters near SB12. Therefore, it appears that lead in this area of the site may be quite soluble.

3.3 - Unexploded Ordnance Screening

The RI Report recommended additional UXO identification activities at the B-20 site. Surface clearance was recommended for the entire site and for areas outside the site, as conflagration patterns dictated. Subsurface clearance was recommended at the craters and in the northern portion of the site (approximately 6 acres). During the second phase of the RI, much of this recommended work was completed. The second phase of work was conducted from December 11, 1995 through March 15, 1996. This section presents the findings of the UXO clearance activities.

UXO screening activities were continued during this investigation by qualified professionals from UXB International in Chantilly, Virginia. These UXO professionals, graduates of the U.S. Naval Explosive Ordnance Disposal (EOD) School in Indian Head, Maryland, are experienced in the identification and handling of UXO. The UXO professionals took a conservative approach in identifying UXO. When in doubt as to whether an item actually contained explosive residue, they always assumed that the item did contain explosives and deemed it UXO. Items too dangerous to move were left in place for later detonation. All other UXO items were transported to a central location on site for detonation, as described in Section 3.3.5.

A large amount of scrap metal was scattered across the surface of the site. During the second phase of the RI, as much of this material as possible was collected and placed in dumpsters. CSSA later disposed of this material through a metal recycling facility.

3.3.1   Surface Clearance of Site and Surrounding Area

As recommended in the RI Report, the site and the area around the perimeter of the site was swept for possible remaining UXO on the surface. The UXO sweep was conducted in a manner similar to that of the first phase investigation. UXO personnel surveyed the entire area shown in Figure 3.5 in 5-foot-wide traverses and visually identified potential UXO on the ground surface. A Schonstedt GA-52B Locator, a magnetometer which identifies ferrous items up to a depth of 10 feet, was used to aid in UXO identification.

The boundaries of the area outside the site were defined by conflagration patterns identified during the first phase of the RI, review of historical aerial photographs, and field identification of UXO during the second phase of the RI. Historical aerial photographs and first phase RI findings indicate a potential for UXO north and south of the site. Therefore, the focus during this investigation was on those areas. When UXO was discovered in those areas during the second phase investigation, the boundary was extended by 50 feet from the furthest locations, until no additional UXO was found.

The entire B-20 site and surrounding area was cleared of vegetation to further aid in the identification and removal of UXO that may not have been detected during the phase I RI UXO sweep. Vegetation clearing activities consisted primarily of trimming or clearing junipers less than 10 feet tall and removing brush. Junipers, which develop dense growth near the ground, posed the greatest obstacle to UXO identification. Vegetation was left undisturbed to the greatest extent possible without compromising the UXO clearing activities. For example, live oaks and brush oaks were either left undisturbed or trimmed if necessary. All clearing techniques were approved in advance by the USFWS (USFWS, 1995).

The equipment used to clear the vegetation included a bobcat skid loader with a tree-shear attachment, backhoe, IT24 loader, and chainsaws. Upon clearing the brush, the backhoe and IT24 loader were used to pile the debris for burning. Several piles of brush were burned on February 1, 1996. This burning activity was approved by the county fire marshal, and was conducted by a team of UXB International and CSSA personnel. During all burning activities, great care was taken to prevent the fire from burning out of control. Water trucks from CSSA and emergency technicians were on site at all times during the burning events. Due to drought conditions in February and March 1996, the controlled burn of the site was postponed and could not be scheduled during the remainder of the UXO clearing activities.

The number of each of the UXO types found at the B-20 site is listed in Table 3.5. Many of the items were broken pieces in which explosive residues were trapped, but did not detonate. Most of the items contained (or were suspected of containing) secondary explosives such as TNT, cyclotetramethylene tetranitramine (HMX), and RDX. Some small pieces of raw secondary explosives were also observed at the sites. However, several items, such as fuzes, were suspected of containing primary explosives, such as lead styphnate and lead azide.

Locations where UXO items were found are shown in Figure 3.5. Items found at the B-20 site consisted primarily of 20mm projectiles (325 of 812 items). Other common items included fuzes (84 of 812 items), 75mm projectiles (38 or 812), and 20-pound fragmentation bombs (18 of 812). Two additional BLU 26 antipersonnel bomblets were identified during the second phase of the RI. These items, along with the BLU 26 found during the first phase, were left in place for later destruction (as described in Section 3.3.5).

3.3.2   Surface Clearance in Area North of Original B-20 Boundary

During the UXO clearing activities, three 20-pound fragmentation bombs containing TNT were discovered approximately 1/4-mile north of the site. These UXO items were found in an area well outside of the area around the site being cleared of vegetation and swept for UXO.

To address this large area between the UXO and the site boundary (shown in Figure 3.6), the UXO professionals visually swept the surface, without clearing vegetation. The area swept in this manner covers approximately 32 acres and extends approximately 1/4-mile north of B-20. Boundaries of the area were surveyed using a Global Positioning System (GPS) unit. A total of 14 UXO items were identified in this area. UXO consisted of 20-pound fragmentation bombs, with the exception of one 75mm projectile. Locations of UXO found in this area are shown in Figure 3.6.

3.3.3   Subsurface Clearance in Northern Portion of Site

Historical aerial photographs indicate that the northern portion of the site (approximately 6 acres) was used prior to the 1980s. However, no records regarding use of this area are known to exist. Observations made during the first phase of the RI also indicate that the area was once used for disposal activities. These observations include the following:

Based on these results, the RI report (Parsons ES, 1995a) recommended that a magnetometer survey be conducted in the entire northern portion of the site, and that any subsurface anomalies be excavated and removed. This work was initiated during the second phase of the RI in the area shown as grids on Figure 3.7.

The surface features within this area consist of rolling mounds, pits, heavy brush, and trees. As described in Section 3.3.1, vegetation was cleared in order to adequately sweep the area for UXO. Upon completion of the vegetation clearing, the site was divided into eight grids labeled A1 through A4 and B1 through B4. Figure 3.7 depicts all eight grids. Grids A2 and A3 each measured approximately 200 feet by 200 feet, and the other grids were considerably smaller. Due to budget and schedule constraints, grids A2 and A3 were not swept during this investigation.

A Foerster Ferex (Mk-26) Ordnance Locator was used to detect subsurface ferrous metal. As described in the RI report, this magnetometer is the most recent military-approved locator, and is in use by U.S. military EOD teams. The Foerster Ferex locator is a hand-held unit which uses two fluxgate magnetometers aligned and mounted a fixed distance apart to detect changes in the earth's ambient magnetic field. These changes in the magnetic field can be due to ferrous metal or magnetic disturbances in soil. The Mk26 locator provides both an audio and metered signal and detect subsurface ferrous metal items to a depth of up to 19 feet.

Over 37,000 magnetic contacts were identified in the six grids surveyed. These contacts were caused by clay lenses, metal-containing debris such as cans and wire, and, to a small extent, UXO. A total of 300 subsurface UXO items were identified in the six grids. In addition, over 19,000 pounds of scrap metal were excavated and removed. Table 3.6 lists the number of magnetic contacts, pounds of scrap metals, and number of UXO items in each grid.

The subsurface UXO items identified during this investigation were generally scattered throughout the grid areas, with a higher concentration of UXO found on the north side of the 6-acre area. Three craters are located in two of the north grids (B3 and B4). No previously identified craters are located in grid B2. Other than a higher concentration of UXO on the north side, no patterns in the locations could be identified. Based on the scattered UXO locations, the amount of general debris found during the excavations, the hummocky topography, and the thick soil layer present in the north portion of the site, it appears that this area may have been used for landfill type disposal activities. The soil and waste material, including a small amount of UXO, appeared to have been reworked by earth-moving equipment.

The paragraphs below describe in greater detail the results of the magnetometer survey and anomaly excavation activities at each grid.

Grid A1

Grid A1 comprises approximately 0.90 acres in the southwest corner of the 6-acre area. A total of 3,126 magnetic contacts were identified and approximately 1,635 pounds of scrap metal were collected and placed in CSSA-supplied dumpsters. A large number of the contacts were caused by metal debris, including metal cans (gallon and 5-gallon sizes), wire, and miscellaneous trash. Two disposal areas were identified in grid A1: one in the northern end of the grid and one in the southern end. Each of these disposal areas covered approximately 1,000 ft². Subsurface material in almost the entire grid area showed evidence of having been burned.

Only 16 UXO items were identified in grid A1. The items are included in the list of UXO found at the B-20 site presented in Appendix F (numbers 274 through 289). The majority of the items (six of sixteen) were fuzes. The items were scattered throughout the entire portion of the grid, and no pattern could be ascertained.

Grids A2 and A3

Grids A2 and A3 each comprises approximately 1 acre in the south-central portion of the 6-acre area, as shown in Figure 3.7. These subsurface of these grids was not cleared for UXO during this investigation due to budget and schedule constraints.

Grid A4

Grid A4 comprises approximately 0.70 acres in the southeast corner of the 6-acre area, as shown in Figure 3.7. A total of 7,059 magnetic contacts were identified and approximately 1,980 pounds of scrap metal were excavated and placed in dumpsters. As at grid A1, the majority of the contacts were caused by metal debris, including metal cans, wire, and miscellaneous trash. A total of 17 subsurface UXO items were found at grid A4. The items are also included in the list of UXO found at the B-20 site presented in Appendix F (numbers 717 through 733). Seven of the 17 UXO items identified in grid A4 were M15 WP grenades, and three were mine actuators.

Grid B1

Grid B1 comprises approximately 0.40 acres in the northwest corner of the 6-acre area. A total of 4,794 magnetic contacts were identifed and approximately 1,175 pounds of scrap metal was excavated and placed in dumpsters. Most of the magnetic contacts were caused by metal-containing trash and wire. Only 3 subsurface UXO items (one nose section of a 20-pound fragmentation bomb and two 30 caliber blanks) were identified. The items are listed in Appendix F (numbers 298 through 300). These items were found on the north side of the grid (between 15 and 35 feet south of the gravel road). Two trenches containing wire and other metal waste were also identified within grid B1. The trenches, each approximately 10 feet by 30 feet, were located south of the location in which the three UXO items were found.

Grid B2

Grid B2 comprises approximately 0.90 acres in the north-central portion of the 6-acre area. Of all the grids surveyed, grid B2 had the highest number of magnetic contacts and the most subsurface UXO. A total of 13,459 contacts were identified and approximately 3,540 pounds of scrap metal were removed. A total of 60 UXO items were found in grid B2. The UXO items found in the grid are listed in Appendix F (numbers 734 though 794, except 741).

Grid B3

Grid B3 comprises approximately 0.70 acres in the north-central portion of the 6-acre area, as shown in Figure 3.7. A total of 7,916 magnetic contacts were identified and approximately 7,917 pounds of scrap metal were removed. As at other grids, most of the contacts were caused by metal cans, wire, and miscellaneous trash which had been disposed in the area. Of all the grids surveyed, the largest number of UXO items were identified in grid B3. A total of 144 subsurface UXO items, including 20mm projectiles, 75mm projectiles, 20-pound fragmentation bombs, fuzes, mine actuators, grenades, and many other types of items, were found. The items are listed in Appendix F (numbers 361 through 716).

Grid B4

Grid B4 comprises approximately 0.20 acres in the southeast corner of the 6-acre area. A total of 1,256 magnetic contacts were identified and approximately 3,435 pounds of scrap metal were removed. Metal cans, wire, and miscellaneous trash disposed of in the area caused many of the magnetic contacts. A total of 60 UXO items (listed as numbers 301 through 360 in Appendix F) were identified in grid B4. The majority of the items were 20mm projectiles (31 of 60) or fuzes (10 of 60).

3.3.4   Subsurface Clearance of Craters

Magnetic anomalies were identified in ten craters (1, 3, 4, 5, 6, 7, 8, 9, 14, and 15) during the first phase of the RI (Parsons ES, 1995a). At that time, these anomalies were identified solely for the purpose of providing clearance for drilling soil borings and they were not investigated further. During the second phase of the RI, the anomalies in these craters (with the exception of craters 14 and 15, which are located in grids A2 and A3) were re-identified and investigated through excavation. Crater excavation activities took place on December 18 and 19, 1995.

A Foerster Ferex (Mk-26) Ordnance Locator was used to relocate the anomalies. Once the ferrous anomalies were re-identified, a backhoe was used to excavate the soil overlying the suspected anomalies. Craters 1, 6, 7, 8, and 9 were cleared specifically for use for detonation of UXO identified during this investigation.

Many of the anomalies identified during the first phase of the RI were determined to be caused by fragmented metal pieces scattered at and near the surface. In addition, it was found that shallow metallic-rich clay lenses were also causing some anomalies. These lenses contained sufficient magnetic properties for the Mk-26 to read. Once excavated, the clay lenses caused only minor magnetic disturbances. The only live item in the craters investigated was one 20mm projectile in crater 9. The investigation and subsequent excavation of each of the craters is described below and illustrated in Figures 3.8 through 3.15. Crater locations are shown in Figure 3.4.

Crater 1

Three anomalies were identified within crater 1. After investigating each anomaly, no live or suspect ordnance was discovered. One anomaly was due to scattered scrap metal pieces and the other was due to metallic-rich clay lenses. Figure 3.8 shows the approximate location of each anomaly with a description of its contents. The anomaly identified in the center of the crater during the first phase of the RI was determined to be a result of scrap metal at the surface. 

Crater 3

The ground surface at crater 3 was reswept two times with the Mk-26 during the second phase of the RI. On both occasions, the original contacts were identified as resulting from small scattered scrap metal fragments at the surface, and therefore no excavations were made. Figure 3.9 shows the approximate location of each anomaly identified within crater 3 during the first phase of the RI.

Crater 4

Four anomalies were identified within crater 4 during this investigation. A backhoe was used to excavate to a depth of three feet at each anomaly. No live or suspect ordnance was discovered. Three anomalies were found to be caused by clay lenses. One anomaly was caused by scrap metal fragments at the surface. Figure 3.10 shows the approximate location of each anomaly with a description of its contents.

Crater 5

Two anomalies were identified within crater 5. After investigating each anomaly, no live or suspect ordnance was discovered. One anomaly was caused by metal conduit on the slope of the crater. At the location of the other anomaly, the excavation extended to bedrock and no UXO or metal scrap was encountered. Therefore, this anomaly was attributed to clay lenses. Anomalies in crater 5 are depicted in Figure 3.11.

Crater 6

Three anomalies were identified within crater 6 during the first phase of the RI, but could not be re-identified during this investigation. To ensure that there were no buried UXO, trenches were dug at each former anomaly location. No live or suspect ordnance was discovered in any of the trenches. The anomalies were due to scattered metal fragments and/or clay lenses. Crater 6 is shown in Figure 3.12.

Crater 7

One anomaly was identified within crater 7. After investigating the anomaly, no live or suspect ordnance was discovered and the anomaly was found to be caused by clay lenses. The trench excavated to determine the cause of the anomaly extended to bedrock. Figure 3.13 shows the approximate location of the anomaly in crater 7.

Crater 8

Two anomalies were identified within crater 8. The anomalies were excavated and were found to be caused by scrap metal fragments and not UXO. Bedrock was encountered at a depth of 2 feet at the bottom of the crater. Figure 3.14 depicts crater 8.

Crater 9

Two anomalies were identified within crater 9. As shown in Figure 3.15, the anomalies were found to be caused by one 20mm projectile, copper wiring, and scrap metal fragments. Figure 3.15 shows the approximate location of each anomaly with a description of its contents. All movable suspect or live ordnance was temporarily stored in crater 9 until they were blown-in-place as described in Section 3.3.5.

3.3.5   UXO Removal and Destruction

UXO discovered and exposed during visual and magnetometer sweeps was removed from its original location and placed in a designated location (crater 9) unless the professional judgment of UXB International experts disctated that the item could not be moved. All removal activities were conducted by UXB International. All buried UXO were removed by a qualified UXO professional with a backhoe or other appropriate heavy equipment. All ordnance deemed too dangerous to move was clearly marked with stakes, fluorescent paint, and caution tape to distinguish its location.

On January 31 and May 6, 1996, the Army 137th Explosive Ordnance Detachment (EOD) stationed at Fort Sam Houston Army Depot in San Antonio, Texas destroyed UXO which had been found at the site until those respective dates. Interim approval for the destruction of these items had been granted by TNRCC on January 29, 1996 (TNRCC, 1996). The disposal of unmovable ordnance was accomplished by a blow-in-place (BIP) method. As described in Section 3.3.1, a total of three UXO items identified during both phases of the investigation were blown-in-place. All movable items were destroyed in crater 9.

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