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Groundwater Investigation and Associated Source Characterization
Section 6 - Oxidation Pond Characterization
The oxidation pond, also referred to as the evapotranspiration pond and SWMU O-1, was reportedly constructed in 1975 (CSSA, 1992). The pond reportedly measured approximately 42 feet by 60 feet by 2.5 feet in depth and was lined with vinyl plastic. Wastes from Building 90-1 (spillage, change-out, etc.) were trucked to the oxidation pond from an exterior 1,000-gallon settling tank. The frequency of delivery to the pond varied upon the level of bluing activity. In 1982, an estimated 24,000 gallons were contained in the pond (CSSA, 1992).
The Texas Department of Health conducted sampling efforts of the oxidation pond material (liquid and sludge material) during a period of April 1984. The samples were delivered to Brooks AFB for analysis. The results of analyses from the April 1984 sampling effort indicated that the material was nonhazardous for metals concentrations. However, no data were available to assess the VOC (specifically PCE) concentrations.
In 1985, RRAD prepared a "recommended procedure" for closure of O-1 (RRAD, 1985). In the fall of 1985, the oxidation pond was bulldozed destroying the pond liner and filled with surrounding soils (CSSA, 1992). No records are available to indicate that disposal of the contained sludge or residue occurred before destruction of the liner.
This section includes an overview, a summary of the oxidation pond characterization activities to date (Section 6.2), and an identification of remedial alternatives (Section 6.3). A summary of conclusions for the oxidation pond follows (Section 6.4).
6.2 - Characterization Activities
Characterization activities included review of historical information, geophysical surveys, surface and subsurface soil sampling, and a liner integrity investigation. Chlorinated hydrocarbons were first detected in well 16 at concentrations above drinking watercriteria in 1991, prompting an investigation of the possible source areas that contribute to the contamination of groundwater. Source characterization began with surface geophysical tasks performed at seven potential source areas (Parsons ES, 1995b). Both EM and GPR surveys were conducted at O-1, and a plan view of the interpreted geophysical anomaly location is shown on Figure 6.2-1. Based on the CSSA personnel communications that the pond liner was destroyed during excavation, and geophysical data indicating an area of disturbance about 110 x 60 feet to a depth of less than 10 feet, four soil borings were drilled at the potential area to investigate portions of the area within the geophysical anomaly (Parsons ES, 1995d). The results are summarized in Table 6.2-1, and "hot spots" from these soil borings are shown on Figure 6.2-2. The oxidation pond was initially estimated at 8 to 10 feet deep; however, two of the three soil borings show fill material to a depth of only 2 feet, while a third indicates fill material to 4 to 5 feet bgl (Appendix A).
A reconnaissance and a detailed soil gas survey of SWMU O-1 accomplished in the summer and fall of 1995 identified PCE in the soil gas samples associated with the geophysical anomaly. PCE was detected in soil gas concentrations as high as 80,000 parts per billion volume (ppbv) (Appendix F). Distribution of PCE in soil gas at the oxidation pond is shown in Figure 6.2-3. The presence of PCE in soil gas concentrations much higher than other areas around well 16 has implicated O-1 as one of two likely source areas for groundwater contamination.
Parsons ES gave a technical presentation of groundwater investigation and source characterization actions to date at CSSA on October 19, 1995, for EPA, TNRCC, AL/OEB, and AFCEE representatives. Based on the characterization of potential source areas through geophysical surveys, drilling, sampling, and analysis, and soil gas surveys, the results strongly showed that the two most likely source areas of groundwater contamination are SWMUs O-1 and B-3. The regulatory agencies and CSSA agreed at the meeting that the next step in the groundwater investigation project would be towards source removal at those two sites.
During November 1995, additional surface soil sampling was conducted to provide data specifically on soils near soil boring OX-SB2 (Figure 6.2-2). In addition, a 5-gallon sample of the soil material was collected and provided to EET of Austin for a benchscale soil washing test. The observation of a piece of a liner at 1.5 feet bgl was noted during the November sampling event.
As a result of a meeting on December 7, 1995, with CSSA, AL/OEB, AFCEE, and Parsons ES personnel, a decision was made to further investigate the area and to include the following activities:
| Collect soil samples above the intact liner for chemical and physical analyses; | |
| Collect and analyze undisturbed soils for metal background comparison; | |
| Perform air influence testing, if liner is not competent, for soil vapor extraction treatability study; and | |
| Perform benchscale testing of VOC volatilization from soils and determination of liner competence which includes: | |
| Remove soils above existing liner. Spread soils on plastic, and berm with clean soils and plastic around the excavated soils. Collect one composite sample of excavated soils for VOC analysis. | |
| Return each 3 to 6 weeks and repeat sampling of excavated soils. Evaluate analytical data during the test period to assess the probable reduction of VOCs in soil. | |
| Use data to aid in determining final remedial option to best treat O-1 soils for removal of VOCs. |
6.2.1 Summary of Liner Integrity Investigation
A liner investigation was initiated in January 1996. The investigation was to use a backhoe to dig a test pit above the existing liner, without disturbing the liner. The excavated "hot" soils were to be placed on plastic within a bermed area. Upon initiating the liner integrity investigation it was apparent that the liner was indeed destroyed. A decision was made in the field to excavate to the limestone bedrock in order to find visual evidence of any potential limestone fractures.
During the investigation, liner material that was found was no larger than 3 feet in diameter. The liner removed was a nylon reinforced polyethylene material, and possibly a second liner material which resembled a form a dense rubber. Much of the liner existed in small sections, which were collected and placed into 55-gallon drums and characterized for disposal (Section 6.2.3). The excavation was completed to bedrock limestone. No rock fractures were observed.
Approximately 80 yd3 of soil material was excavated during the liner investigation, of which 19 yd3 of "hot" soils were placed into a plastic-lined bermed laydown area. The remaining 61 yd3 of soils were placed near the excavation area, the bermed "hot" soils laydown area and approximate location of other excavated soils. Soil samples taken during the liner investigation included a grab sample of the excavated soils and a composite sample of the soils placed into the bermed soil laydown area. Subsequent composite samples of the bermed soils laydown area were taken during late January and early March 1996 to measure the effect of volatilization.
In April 1996, the soils within the bermed laydown area were removed and placed with the original excavated soils. Soil grab samples were collected within the excavated area at the center bottom and along the north, east, and south walls. In addition, composite soil samples were collected from the surface underneath the former bermed laydown area and the excavated soil placement area. Results of these soil analyses are discussed in Section 6.2.2 below.
The samples collected during the liner integrity investigation include two samples (Ox-1-96-SS1 and Ox-1-96-SS2) which were taken near soil boring SB-2, two samples Ox-1-96-SS3 (grab sample) and Ox-1-96-SS4 (composite sample of hot soils). Subsequent composite samples of the hot soils were taken in February 1996 and identified as O-pond-SP and Ox-1-96-SS5.
Discrete samples taken within the excavation, April 1996, were identified as Ox-1-96-SSNW (north wall), Ox-1-96-SSSW (south wall), Ox-1-96-SSEW (east wall), and Ox-1-96-SSCB (center bottom). In addition, composite samples of the excavated soils and soils underlying the former berm were identified as Ox-1-96-SSES and Ox-1-96-SSBF, respectively.
The sample identified as Ox-1-96-OL represents the old liner material which was removed during the liner integrity investigation. Sample Ox-1-96-NL is the plastic material, "new liner, " which was used to create the bermed soil laydown area. The liner materials were removed, segregated, and placed into 55-gallon drums during the investigation period. Analytical results of the liner characterization are presented in Section 6.2.2.
6.2.2 Characterization and Analytical Results
Soils in the oxidation pond area of concern are predominantly fill, consisting of gravelly clay with marly limestone and caliche fragments, along with an identified sand layer, presumably the liner bedding sand (Parsons ES, 1995d). Figure 6.2-5 shows a cross-section of the site developed from soil boring logs (Appendix A) and visual observations of fill samples collected from the site. Based on excavation activities associated with the liner integrity investigation, depth to limestone is approximately 3.5 to 4 feet. Clayey gravel soil in the fill material is similar to logged surrounding soils. Soils in the O-1 area are classified as Tarrant association, gently undulating (Figure 2.4-3).
The surface geology in the O-1 area is mapped as the upper member of the Glen Rose Formation, with outcrops of the lower member mapped to the west and south (Figure 8.3-4; geologic mapping discussed in Section 8.3.1). The contact between the members, a key marker bed about 2 feet thick at CSSA, was not observed during logging of rock core at O-1. Interbedded marly limestone with occasional fossil fragments is found from 3.5 to about 13 to 14 feet bgl, underlain by a 2 to 3-foot thick hard limestone with vertical fractures and iron staining. The bedding plane appears to dip slightly to the southeast, which is consistent with regional trends (Section 8.3.1). The depth to the bottom of this limestone layer was not determined in the southeast boring, but using bed thickness from the middle boring, it would seem to be about total depth of 15.5 feet. The hard limestone is underlain by marly interbedded limestone, and some calcite infilling of solution cavities is observed at around 22 to 24 feet bgl. The marly interbeds are underlain by a hard, massive limestone at about 27 feet bgl in the two borings drilled to 30 feet bgl. The limestone is interpreted to extend across the area.
Analytical results of soil samples collected to date from the SWMU O-1 area are presented in Table 6.2-2. Table 6.2-2 also includes the contaminant concentration criteria for closure under 30 TAC 335 Subchapter S Risk Reduction Rule Standard 1 and Standard 2.
The investigations to date provide data which indicate contaminants of concern for the SWMU O-1 area are PCE, cadmium, and chromium. Highest concentrations of PCE, cadmium, and chromium contained within SWMU O-1 were reported as 1,390 mg/kg, 4.8 mg/kg, and 1,300 mg/kg, respectively. Analytical results to date have only indicated the concentrations of contaminants of concern. As indicated in Table 6.2-1, soil boring OX-SB1 collected at 26.5 to 27.5 feet bgl contained concentrations of PCE, toluene, and xylene above nondetect levels. These results may not be indicative of contamination within the limestone in the O-1 area, but determination of the extent of contamination would be required to properly evaluate the site through additional sampling of the limestone beneath the O-1 area.
To achieve closure of SWMU O-1 under Standard 1 requirements, contaminants must be remediated or removed to background levels. Background levels for the constituents of concern within O-1 soils are approximately 0.45 mg/kg for cadmium, 40 mg/kg for chromium, and <0.005 mg/kg for PCE. Characterization results indicate that PCE, cadmium, and chromium are at levels above closure criteria. Remedial technologies exist which may remove these contaminants to levels appropriate for closure of O-1. Section 6.3 presents an identification and prescreening of remedial technologies for the oxidation pond O-1.
The IDW generated from the liner investigation activities included liner material and PPE. The PPE material was placed into plastic trash bags and discarded along with general plant trash. Liner material, labeled as new and old liners, were placed into 55-gallon containers. The new and old liner materials were composited and analyzed for TCLP VOCs and TCLP chromium. Results of analyses for samples Ox-1-96-OL and Ox-1-96-NL are presented in Table 6.2-2. Comparison of analytical results to 40 CFR 261.24 Table 1 show that the liner material can be characterized as nonhazardous. The liner material was classified as a nonhazardous class 1 waste per 30 TAC 335 Subchapter 1. Results were provided to CSSA for determination of disposal options.
6.3 - Identification and Screening of Remedial Alternatives
The identification and prescreening of remedial technologies provides a quick and relatively inexpensive indication of whether identified technologies are potentially viable for soil remediation. The prescreening of remedial technologies can also provide preliminary estimates of the cost and performance data necessary to further investigate the remedial technologies and/or conduct a remedial design study. The prescreening and screening of remedial alternatives includes an evaluation of treatment technologies, and is based on effectiveness, implementability, and cost.
The remedial alternatives selected for screening are based on proven treatment technologies, however, innovative technologies may be suited for remedial activities at the oxidation pond because of its relatively small size and CSSA's intent to remediate on site, as necessary. Remedial alternatives are identified through a process in wh8ich fate and transport of contaminants are known.
Fate and transport of contaminants is dependent upon almost every characteristic and property of not only the contaminant but also the soils at the site of contamination. Adequate knowledge of the site soil characteristics as well as contaminant properties and how they affect the target contaminants and their transport is critical in the applicability of different technologies and in choosing the best remedial alternative. Contaminant properties such as volatility and water solubility significantly affect results of remedial technologies and may preclude their use. Similarly, some soil types are not amenable to certain types of treatment. For example, soils high in clay or natural organic content generally have high adsorption potential for contaminants. This can make contaminant removal more difficult for many technologies such as soil vapor extraction for volatile organic compounds. Other important soil properties that can affect remedial action include porosity and permeability. Both are important factors for removing contaminants from tight pore spaces. In addition, if contaminants have migrated below the water table, then certain treatments like soil vapor extraction and bioventing are rendered ineffective unless the water table can be dropped and the unsaturated zone increased and exposed. Thus, remedy screening is effective in reducing the technology options to those most viable.
The oxidation pond at CSSA contains chromium and PCE in soil media. Because these two contaminants have very different chemical and physical properties, selection of one alternative to attempt remediation of both is very difficult and usually not effective to attain cleanup levels.
The technologies potentially applicable to the remediation of contaminated soils are presented in this section. Response actions for soil remediation include source removal actions, and in situ soil treatment technologies. Generally, in situ treatment technologies are less expensive than ex situ treatment processes because no excavation is required. These techniques include chemical treatment, thermal treatment, and biological treatment. In situ techniques have a broad range of applicability. Critical factors when considering treatment technologies are areal extent, depth and volume of contamination, concentration, soil characteristics, and site hydrology, the last of which is not a treatment concern when soils are excavated.
The various options were evaluated and screened for use at the oxidation pond based on their effectiveness, implementability, and relative cost. A summary of the prescreening technologies are presented in Table 6.3-1. Those alternatives retained for additional consideration are discussed below.
Soil/Rock washing is a physical/chemical separation technology in which excavated soil is pretreated to remove large objects and soil clods and then washed with fluids to remove contaminants. To be effective, soil/rock washing must either transfer the contaminants to the wash fluids or concentrate the contaminants in a fraction of the original volume, using size separation techniques. In either case, soil washing must be used in conjunction with other treatment technologies. Either the washing fluid or the fraction of soil containing most of the contaminant, or both, must be treated.
Generally, the soil washing process is accomplished in three phases. The first phase is the soil preparation of the excavated soil. Soil preparation involves mechanical screening of the soil feedstock to remove debris and oversized materials. Preparation may also include size reduction by the use of shredder or hammer mills. The maximum size of soil/rock varies with equipment, but ranges from 3/8-inch to 2-inch diameter particles.
The next phase is the soil washing process. The soil is mixed with water to form a slurry and then several separation/classification technologies are utilized to remove the contaminants such as screening, hydrocycloning, gravity separation and froth flotation. Intensive contact between the soil grains and the wash fluid causes the soil contaminants to dissolve and disperse into the water. Cation exchange capacity measurements need to be evaluated before actual design of this remedial option can proceed.
The final phase treats the remaining fine soils (silt and clay) and contaminated water mixture. Soil washing is applicable to a wide variety of contaminants. Available chemical and physical data (including averages and ranges) and the volumes of the contaminated soil requiring treatment should be identified. Hot spots require separate characterizations so they can be properly addressed in the treatability tests. Soil washing may be applicable to some, but not to all parts of the site.
Phytoremediation uses plants to leach toxic heavy metals from soil. The technology,, while not new, is relatively unproven technology which varies greatly with soil conditions. The economics of phytoremediation are still being investigated. Depth of soil remediation will be limited to the uptake capabilities of selected plants, extensive time as well as sampling data to determine the effectiveness associated with the technology.
Electrokinetics uses direct electrical current in the soil to transport soil water and dissolved ions to one of the electrodes. This is an innovative technology that has yet to be proven for large scale operations. Electrokinetic remediation is a possible technique for in-situ removal of metal contaminants from both saturated and unsaturated soils.
In electrokinetic remediation, electrodes are implanted in the soil, and a direct current is imposed between the electrodes. The application of direct current leads to a number of effects: ionic species and charged particles in the soil water will migrate to the oppositely charged electrode, and along with this migration, a bulk flow of water is induced, usually toward the cathode (electroosmosis) (Hunter, 1981). The combination of these phenomena leads to a movement of contaminants towards the electrodes. Contaminants arriving at the electrodes may potentially be removed from the soil in several ways including electroplating or adsorption onto the electrode, precipitation or co- precipitation at the electrode, pumping water near the electrode, or complexing with ion-exchange ions (Mattson and Lindgren 1993).
Soil vapor extraction is generally an in situ based remediation technique. However, SVE techniques can be applied as an ex situ remediation technique. The basic theory of soil vapor extraction is to apply a negative pressure, or vacuum, to the subsurface to create a pressure gradient. This gradient produces advective air flow which will remove vapor-phase compounds and also promote continued volatilization of organic compounds adsorbed in soils. The vacuum is created by using blowers or vacuum pumps, and is applied to the subsurface soils through extraction wells. SVE technology is proven and generally accepted by the EPA, and is currently being used at SWMU B-3. Thus pilot scale testing could be minimal at the O-1 site.
6.4 - Summary and Conclusion of SWMU O-1 Characterization
Based on the results of the geophysical survey and liner investigation, the estimated area of O-1 that requires treatment for contamination is 2,000 square feet. The estimated average thickness of this potentially contaminated soil is 3.5 feet (based on observations made during liner integrity investigations), which totals 7,000 cubic feet (or 260 cubic yards). The average porosity of the fill material in the trench was assumed 30 percent, which converts into a bulk density of 1.85 g/cm3 (or 115 lb/ft3). Based on these assumptions, the total mass of fill material in the trench requiring treatment is approximately 805,000 pounds (365,910 kilograms) of solid material.
The three contaminants requiring treatment based on the comparison to TNRCC risk reduction rule criteria are PCE, cadmium, and chromium. The average concentrations of these compounds are 267.3 mg/kg, 1.62 mg/kg, and 391.4 mg/kg, respectively. Based on these estimates, the total quantities of PCE, cadmium, and chromium needed to be extracted from the O-1 area are 97.8 kilograms (215.7 pounds) of PCE, 0.6 kilograms (1.32 pounds) of cadmium, and 143.2 kilograms (315.1 pounds) of chromium. These estimates are based on the limited characterization data that was collected. Additional characterization data may be necessary to obtain more accurate estimates of the mass of contamination requiring treatment.
Results of the volatilization study associated with the liner investigation indicate that the 19 cubic yards of "hot" soils placed into the bermed laydown area initially contained 180 pounds of PCE. This volume represents approximately 83 percent of the total volume of PCE estimated to be contained in SWMU O-1. Figure 6.4-1 presents a graph of the PCE removed from the hot soils over the 3-month study period. As the graph indicates, a significant amount of the PCE was volatilized over a 1-month period. Heavy metals contained in the O-1 area have not been addressed in the investigation activities to date, other than identification of concentrations and an estimation of total mass contained within O-1.
Evaluation of data from the SWMU O-1 investigation indicates that the horizontal or vertical extent of contamination has yet to be determined. Possible fractures or faults in limestone underlying the soils of O-1 are potential pathways to groundwater contamination identified in well 16. Two faulted blocks have been interpreted just south of the area (Section 8.3.5), though interpretation of boring logs at O-1 did not reveal any faults through the site. Therefore, the full extent of contamination may be required in order to close the SWMU. Characterization results do not verify the level of PCE or chromium in the underlying limestone.
Additional investigations associated with the preliminary screening of remedial technologies associated with new or emerging technologies is also recommended. Review of new or emerging remediation technologies would allow CSSA to possibly provide innovative remedial actions. Treatability studies of remedial technologies, which pass the prescreening and detail evaluations, can also be effective for the O-1 closure schedule and costs.