Site Description and History

SWMU B-25 consists of a shallow, north-south trench with a soil mounds on each end, as well as the area that surrounds the trench, as shown in Figure B25-2.  The north and south mounds are 4 and 2 ft high, respectively.

SWMU B-25 is a suspected site for waste management, although no CSSA documentation of the trench or its use is known. Currently, SWMU B-25 is not used, although the trench remains open.  Initial field investigations revealed an empty gas can and some abandoned wooden pallets at the site.

Summary of Previous Investigation Results

Geophysical Survey

An EM geophysical survey was performed in March 1996 at SWMU B-25.  One anomaly, located west of the trenches and mounds, was revealed by the survey as a subsurface disturbance potentially related to metallic waste disposal.

Soil Borings

In March 2000, soil samples were collected along the length of the trench and mounds at SWMU B-25.  Three borings were advanced, with SB01 in the north mound, SB02 in the central portion of the trench, and SB03 adjacent to the anomaly identified in the 1996 geophysical survey.  Surface and subsurface soil and rock samples were collected and analyzed for metals, VOCs, and SVOCs.  No evidence of waste was encountered in any of the borings.

In the area of the geophysical anomaly (at SB03), chromium and zinc concentrations slightly exceeded RRS1 closure standards for a subsurface sample (collected from 9 to 9.5 ft bgs), but none exceed the Texas specific background concentrations.

Boring SB02, located in the center of the trench, contained zinc in excess of RRS1 closure standards in the surface soil sample as well as a subsurface sample (3 to 3.5 ft bgs).  Reported chromium concentrations for this subsurface sample also exceeded RRS1 standards.  Again, none exceeded the Texas specific background concentrations.  Based on the drilling results and the lack of geophysical anomalies, it appears that the trench was not used for waste management activities.

Samples from SB01, located in the northern mound, indicated arsenic in the surface soil sample and chromium, copper, and zinc in the subsurface, all in concentrations higher than RRS1 closure standards.  Except for arsenic, the concentrations were only slightly above RRS1 standards.

All VOCs were below RRS1 closure standards, with the exception of three detections of bis(2-ethylhexyl)phthalate. However, because bis(2-ethylhexyl)phthalate was also detected in the equipment blank, it is concluded that its presence is related to sampling equipment.

Closure Plan and Sampling Recommendations

All waste encountered must be removed from CSSA for off-post disposal to pursue RRS1 closure with respect to the soil unit at SWMU B-25.  Geophysical survey results indicate that there is no buried metal within the trench.  However, test pits will be excavated within the trench area to confirm that no other waste has been disposed of in the trench.  In addition, the small anomaly located in the northwest portion of the site warrants excavation to determine if it is associated with past waste management activities.  Finally, soils in the vicinity of RW-B25-SS01/SB01 and RW-B25-SS02 will be excavated, characterized, and disposed off-post.  Stockpiled soil present at the site can be used to regrade the site if sample analysis confirms that it meets RRS1 criteria.

Groundwater and competent bedrock will be addressed as part of the groundwater operable unit.  The soil excavation will extend to the depth that waste or waste residue is encountered or to bedrock, whichever comes first.

The estimated volume of waste material and waste residue present at SWMU B-25 could be up to 500 CY, if the trench does contain waste.  Based on RFI analytical results, this waste meets non-hazardous Class 2 criteria.  Confirmation samples will be collected following removal/remediation to verify that no contamination remains.  Analysis should include arsenic (SW-7060A), chromium, copper, and zinc (SW-6010B).  Samples will be collected at a frequency of one per 100 linear ft of sidewall, with a minimum of one sample per sidewall.  If the bottom of the excavation is bedrock, bottom samples will be collected at a frequency of one per 20,000 ft² of bottom surface area, with a minimum of two samples per excavation area.  If the bottom of the excavation is soil, bottom samples will be collected at a frequency of one per 100 ft (100-ft grids).

Following excavation of waste and waste residue, any resulting trenches will be backfilled with clean fill and the site restored.

Summary Of Planned Work

1. Amend CSSA’s Waste Profile using B-25 RFI data.  RFI data indicates that the waste at this site meets nonhazardous Class 2 criteria.

2. Collect one sample from each soil mound to determine if concentrations meet RRS1 criteria.  Each sample should be analyzed for arsenic, barium, cadmium, chromium, copper, lead, mercury, nickel, and zinc.  If this material meets RRS1 criteria, it can be used as backfill for any site excavations.

3. Excavate test pits at the anomaly and within the trench to determine if there is any buried waste.  If practical, separate metal debris for recycling.  It is estimated that a total of up to 500 CY of material from SWMU B-25 could require excavation and disposal.

4. Excavate soils in the vicinity of SS01 and SS02, where above-background concentrations of arsenic and zinc were detected in 2000.  Also, dispose of any waste (wooden pallets, empty gas can) on the ground surface.  Dispose of all waste and soils containing metals at concentrations above CSSA background criteria at Covel Gardens Landfill under waste profile CG‑25591 C-4.

5. Collect samples of the excavation sidewalls and bottom.  Samples will be analyzed for arsenic, chromium, copper, and zinc, based on the results of samples collected at the site in 2000. 

6. Continue excavation until RRS1 soil criteria or competent bedrock, whichever comes first, is reached.

7. Re-grade the site with clean fill and reseed.

8. Validate data and prepare Closure Report for SWMU B-25.