Known (Updated): Several investigations have been conducted to assess groundwater contamination in Well 16, located north of SWMU O-1. Chlorinated hydrocarbons were detected in Well 16 in 1991 at concentrations above drinking water standards, prompting investigations at seven potential source areas. Soil samples collected from borings completed at SWMU O-1 indicated elevated levels of tetrachloroethene (PCE), chromium and cadmium above background comparison concentrations. Due to a PCE "hot spot" discovered during a 1995 soil-gas survey and the poor integrity of the liner observed in January 1996, SWMU O-1 is implicated as a potential source area for the contamination detected in Well 16.

Because SWMU O-1 is a potential source of groundwater contamination, the EPA requested in the 3008(H) Administrative Order that Interim/Stabilization Measures (IM) be completed at the site. The 3008(H) Order indicates that the O-1 IMs are, at a minimum, to include in-situ treatment or excavation and removal of contaminated soil or sludge. The goals of these efforts are to mitigate migration of contamination and to expedite O-1 closure under state requirements.

Previous Investigations (Updated): Source area characterization activities began with surface geophysical surveys performed during January through March 1995. A large anomalous area was detected at SWMU O-1 during EM and GPR surveys. Based on this geophysical data, four soil borings were drilled within SWMU O-1 to investigate portions of each area exhibiting apparent geophysical anomalies. Results of analytical data gathered from the investigation indicated levels of PCE, chromium and cadmium above comparison concentrations. These concentrations are listed in Table O1-1. Locations of these borings are shown in Figure O1-1. Only three VOCs were detected in the twenty samples analyzed by method SW-8260A: toluene, m,p-xylene and PCE. Most of the soil boring samples in which these analytes were detected were shallow surface soil samples. However, PCE, toluene, and xylene were detected at low concentrations in SB1 at a depth of 26.5 to 27.5 feet bgs.

A liner integrity investigation was conducted in the fall of 1995 and spring of 1996. A number of samples were collected at various times during this investigation. Results are summarized in Table O1-2. The highest VOC concentrations were measured in surface soil sample #4 (OX-1-96-SS4) which was located within the limits of the excavated area during the liner investigation.

The analytical results of the metals are also presented in Tables O1-1 and O1-2 with corresponding background concentration ranges at CSSA and criteria for TNRCC RRS2. Metals levels for the fill or Tarrant Association soil samples are within the normal range of background concentrations at CSSA in all samples collected during the previous investigations except cadmium and chromium. In soil boring #2 (OX-SB2, 0 to 2 feet bgs) the detected concentrations of chromium (730 mg/kg) and cadmium (4.8 mg/kg) were significantly greater than the calculated maximum background concentration for CSSA soils. Arsenic, barium, copper, lead, mercury, nickel and zinc were detected at concentrations less than those defined as background. The natural concentrations of metals at CSSA are greater than the levels provided in the TNRCC risk reduction rules, and therefore are used to determine whether sampled metal concentrations detected are indicative of contaminated backfill material or naturally occurring conditions at SWMU O-1.

A subsequent soil gas survey of SWMU O-1 during the summer and fall of 1995 identified PCE concentrations as high as 60,000 ppbv at depths of 1.0 to 3.5 feet bgs, as shown in Figure 01-2 (Groundwater Investigations and Associated Source Characterization, Parsons ES, 1996d). In November 1995, additional surface soil sampling was accomplished near soil boring OX-SB2, at which time a piece of liner was noted at 1.5 feet bgl. The observation indicated that the liner was not intact since being bulldozed in 1985. This observation prompted a liner integrity investigation in January 1996. The purpose of the investigation was to dig a test pit above any intact existing liner; however, it was observed that the liner was destroyed. Approximately 80 cubic yards of soil to a depth of 3.5 feet bgs was excavated in an attempt to collect data or provide evidence of any potential limestone fractures. During the spring and summer of 1996, the excavated soil was replaced into the excavated area and an additional soil gas sampling effort was completed within the SWMU O-1 area.

In order to address the inorganic metal soil contamination at SWMU O-1, electrokinetic remediation benchmark and field treatability studies were conducted. These studies assessed the applicability of electrokinetics to remediate the inorganic metal contamination in soil. The benchmark study indicated that electrokinetic remediation would transport VOCs through the O-1 soils to the cathode well. However, to get this process to be successful, a large amount of acid had to be added to the extremely alkaline soils to acidify the soils. In April 1997, three trenches were excavated in an attempt to locate soil areas with the highest concentrations of PCE, chromium and cadmium for the field study. The electrokinetic field study began on 7 July 1997 with the beginning of the electrokinetic well construction activities, and was completed on 23 December 1997, when the process unit for the field study system was turned off. The treatability study results for PCE indicate that the electrokinetic process will transport VOCs through electro-osmosis to the cathode well. The field study concluded that the amount of chromium removed from the study area was entirely dependent on the efficiency of soil acidification, and that this option was not feasible for SWMU O-1 due to the predominance of alkaline limestone at the site. Due to the low concentration of cadmium observed in the field study area, electrokinetic test results were inconclusive for cadmium within the study area. The infeasibility of using this treatment option at O-1 was based on the costs for the large amount of acid needed to acidify the alkaline soils, and the fact that the unheterogeneous nature of the soils at O-1 would make it difficult to locate the areas with the largest amount of contaminants for removal. A further discussion of this investigation may be found in the Electrokinetic Test Report for SWMU O-1 (Parsons ES, 1999), which is included in Volume 4.

Interim Measures Activities: Activities at SWMU O-1 planned in order to accomplish 3008(h) Order IM requirements will include:

Determine lateral extent of contamination in soil. Vertical extent of contamination in the soil strata is assumed to be from the surface down to bedrock. It is estimated that approximately 20 borings will be required to define the extent of contamination. Each boring will be completed down to bedrock, and are anticipated to be 1 to 4 feet in depth. One composite soil sample will be collected and analyzed from each soil boring for PCE, , toluene and m,p-xylene (Method SW8260A), cadmium (Method SW7131) and chromium (Method SW 6010A). The soil borings will be drilled in phases, with the number of phases to be determined by the analytical results of the soil borings. The initial phase of soil borings will be completed in a circular fashion surrounding the historic O-1 pond boundary. Two concentric circles will be completed around the O-1 pond so that the borings (and samples collected) are offset from each other. The next phase(s) of soil borings will be dependent upon analytical results of collected soil samples. If metal contaminants of concern (listed above) are found above background levels or VOCs are detected in a soil sample, then an additional boring will be installed and sampled further from the O-1 pond boundary until VOCs are non-detect and metals are below background levels.

Excavate contaminated soils. Soil boring analytical results from the soil strata will be used to define the extent of excavation and the amount of soil requiring excavation. The outer most samples (the furthest from the O-1 pond boundary) that meet the "clean closure" criteria listed above will be used to define the extent of excavation. The soil will be excavated down to bedrock to the lines of extent.

Collect waste characterization samples. Excavated soils from within the historic O-1 pond boundaries will be stored at the site in roll-off boxes in approximately 15 cubic yard batches. Approximately 14 batches are anticipated equaling approximately 200 cubic yards. Waste characterization samples will be collected at a rate of one per 15 cubic yards from the historic O-1 pond area and analyzed by TCLP for PCE, toluene, m,p-xylene, cadmium and chromium. Waste characterization of the soil in these small batches will facilitate solidification measures if necessary. The remaining excavated soil (estimated 1,400 cubic yards) removed from outside of the historic O-1 pond boundary will be sampled at a rate of one per fifty cubic yards of excavated soils and analyzed by TCLP for PCE, toluene, m,p-xylenes, cadmium and chromium. This soil will be transported immediately after excavation to an authorized landfill facility, or staged in an appropriate area for staging of the excavated soils. An estimated total of 1,600 cubic yards of contaminated soil will be excavated and will require treatment and/or disposal.

If perched water is observed in the excavation, collect one water sample and analyze for PCE, 1,2-DCE, TCE, vinyl chloride, toluene and m,p-xylene (Method SW8260A), cadmium (Method SW7131) and chromium (Method SW6010A).

Identify the soils requiring disposal. Using TCLP analysis results compared to hazardous waste concentrations listed in Table O1-3, it is anticipated that most of the soils will meet non-hazardous contaminant levels upon excavation and can be disposed of at an appropriate non-hazardous landfill. If the analytical results for TCLP analyses are either equal to or above the levels listed in Table O1-3, then the soils will be stabilized with Portland cement. A waste analysis plan for the treatment of soils by stabilization is included in Volume 1-4, behind the RL33 Addendum tab, of this encyclopedia. It is estimated that only 200 cubic yards of soil from within the historic boundary of O-1 will require stabilization prior to disposal.

Determine vertical extent of contamination in bedrock. Three to five soil borings are anticipated to be required to define the vertical extent of contamination in bedrock. These borings will be completed in the bedrock in the excavated area only, and will be completed before the excavation is backfilled. If necessary, future soil borings may be completed to define lateral extent of contamination in the bedrock. Two samples will be collected from each boring, one at the rock surface, and one at the bottom interval of each boring. Based on previous sampling results (PCE in 26-27 feet interval of O1-SB1) presented in Table O1-1, the borings are anticipated to begin from approximately 1 to 4 feet bgl to a total depth of 27 feet. Each soil sample will be analyzed for PCE, 1,2-DCE, TCE, vinyl chloride, toluene and m,p-xylene (Method SW8260A), cadmium (Method SW7131) and chromium (Method SW6010A).

Table O1 - 3 - Analytical Definition of Hazardous Waste

Survey sampling locations and boundary of the excavated area at SWMU O-1 using CSSA’s GPS unit.

Based on the findings of bedrock contamination evaluation, it is anticipated that a 2-foot clay liner (10E-07 permeability) or equivalent synthetic liner system will cap the excavated area. The excavation will be backfilled with basic clean fill and a liner system installed that will be sloped for rainfall drainage and include 6 inches of seeded topsoil. The liner system will aid in preventing the percolation of groundwater through underlying potentially contaminated limestone.

Closure Activities: Partial closure of SWMU O-1 will be completed under requirements set forth in Section 3 of the Work Plan Overview. IM data and previous investigation data and results will be used as a basis for partial closure. Once the soil is excavated during IM activities, closure of O-1 for soils will meet RRS1, by removal and/or decontaminating all waste and contaminated media to background levels unaffected by waste management activities. However, to obtain a full closure for O-1, groundwater analysis must also meet risk reduction standards. The presence of PCE in bedrock during the March 1995 drilling activities indicates contamination above background levels for PCE in bedrock. Future closure possibilities with regard to groundwater at O-1 will be evaluated in the future. The amendments to the project SAP and HASP are included in Volumes 1-4 and 1-5 of this Encyclopedia, respectively.