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Groundwater Investigation and Associated Source Characterization
Section 5 - Characterization of Potential Source Areas
Based on information generated during the preliminary assessment, seven potential source areas of groundwater contamination in wells 16 and D were identified. These sites, and additional areas around well 16,were investigated in 1995 through surface geophysical surveys, drilling and sampling of shallow boreholes, and an extensive soil gas survey. The first two activities were reported in previous technical memorandums (Parsons ES, 1995b, c, and d), and the third activity, the soil gas survey, is detailed in a technical memorandum in Appendix F. All three activities and results are summarized in the following sections.
5.1 - Surface Geophysical Surveys
Surface geophysical surveys were performed at a total of fifteen sites at CSSA to determine the presence and location of potential waste disposal areas (Figure 5.1-1). Seven of these sites were SWMUs and the remaining eight were sites added by CSSA for completeness of a surface investigation of well 16 source characterization. Other SWMUs at CSSA were surveyed under AL/OEB order 126 (Parsons ES, 1995c). EM surveys were conducted at all fifteen sites, and GPR surveys were conducted at nine sites. Two GPR methods were employed, one using the GSSI SIR-3 system and one using the Pulse EKKO IV system.
Overall the EM surveys were successful in determining the presence or absence of disturbed areas potentially containing waste material (i.e., metal debris, trash) that might be related to the groundwater contamination at well 16. GPR using SIR-3 equipment had limited success in determining the existence of trenches and/or disturbed areas within selected areas. This was probably due to interference from the surficial clay present at CSSA. With regard to the identification of buried trenches, the Pulse EKKO GPR method generated better results than the GSSI SIR-3 GPR method. Pulse EKKO data reinforced and at times added to the results of the EM surveys. For example, three inferred trenches were identified at SWMU B-4 during the Pulse EKKO GPR survey. The presence of these trenches at B-4 were not delineated as clearly from the EM survey results.
Both EM and GPR anomalies were detected at the following SWMUs: B-2, B-3, B-4, O-1, and B-28. No EM or GPR anomalies were detected at SWMUs B-1 and B-19. Some anomalies detected at areas designated Well 16 and Southeast Well 16 during the EM and GPR surveys were due to cultural disturbances (i.e., fences, underground piping, telephone lines, etc.) and thus were not included in the list of geophysical anomalies due to suspected past waste management activities. EM anomalies were observed at most of the "open" areas including the South Pasture, Well 16 West, Well 16, East, Gate 6, and Salado Creek. No EM anomalies were observed in the North Pasture. Table 5.1-1 lists the total number of anomalies detected at each of the fifteen sites using EM and GPR methods. Appendix I contains the surface geophysical maps for each site.
5.2 - Drilling and Sampling of Soil Borings
Thirty-four soil borings were drilled and sampled at seven SWMUs as part of the potential source characterization to determine the source of the chlorinated hydrocarbon contamination in groundwater (Parsons ES, 1995d). Soil boring locations were based on the results of the surface geophysical surveys. Table 5.2-1 summarizes the field activities.
Seven soil borings were redrilled at B-1, B-2, B-19, and B-28 and resampled. Because the laboratory exceeded holding times and had problems with base surrogate recoveries, fifteen soil/rock samples were recollected for volatile organic analysis, and two samples were analyzed for SVOCs. One boring was redrilled to collect a groundwater sample, but the boring did not produce water. However, borings at B-19 and B-28 produced water during the redrill effort, but not during the initial drilling. The redrill effort and results are discussed in the addendum to the soil boring technical memorandum (Parsons ES, 1995e).
Analytical methods were selected based on known or suspected waste materials at the seven sites, including solvents and metal debris. The metals were tested for potential closure actions. Soil and groundwater samples were analyzed for VOCs, SVOCs, and metals by methods SW8240, SW8270, and SW6010, respectively.
Two sets of comparison criteria were used. Texas Administrative Code (30 TAC 335 Subchapter S) medium-specific concentrations (MSCs) for soil-air ingestion/inhalation under Standard 2 health-based SWMU closures were used as soil comparison criteria. Groundwater analytical results were compared to the SDWA maximum contaminant levels. MSCs and MCLs are only for comparison and are not intended as recommendations for closure actions. Contaminants of concern included the chlorinated hydrocarbons detected in well 16 (cis- and trans-1,2-DCE, TCE, and PCE) and the metals cadmium, chromium, nickel and lead. Though Subchapter S MSCs do not include SVOCs, those concentrations detected at each site were reported in the technical memorandum in the analytical table accompanying each SWMU report section (Parsons ES, 1995d). Summary maps for each site are shown in Appendix J.
5.2.2.1 Volatile Organic Compounds
Volatile organics were detected in the soil/rock at B-2, B-3 and O-1, but were not detected at B-1, B-4, B-19, or B-28. PCE, TCE, and cis-DCE were detected above comparison levels in soil samples. Table 5.2-2 summarizes all analytes detected above comparison levels. Concentrations in bold font are those above comparison levels, and concentrations in regular font are below comparison criteria.
Toluene was detected below MSCs at equivalent depths in three borings located near the B-2 northern trench.
PCE, TCE, cis- and trans-1,2-DCE, and xylenes were detected in soil at B-3. All concentrations detected were below MSCs except for TCE, which was detected above MSCs in three borings. Locations of the samples whose concentrations are below MSCs are discussed in the technical memorandum.
PCE, toluene, and xylenes were detected at the oxidation pond (O-1). Only PCE was detected above MSCs in one sample and its duplicate. The locations and concentrations of the VOCs detected are further discussed in the technical memorandum.
Because the EPA indicated at the October 1994 meeting that monitoring wells were not appropriate until the source of groundwater contamination was defined, no wells were installed when drilling encountered water. However, grab samples of the water were collected form the open boreholes. Twelve groundwater samples were collected as part of the investigation. VOCs were only detected in two samples from sites B-3 and B-19, samples B3-SB1 (GW) and B19-SB2R (GW). PCE, TCE, cis-1,2-DCE, and vinyl chloride were all detected above their respective MCLs in the B-3 sample, and PCE was detected above MCLs in the B-19 sample (Table 5.2-2).
Cadmium, chromium, nickel, and/or lead were detected at concentrations above comparison criteria at the investigated sites except B-1 and B-19 (Table 5.2-2). Comparison values were based on whether the soil could easily be ingested (0 to 2 feet bgl) or whether it presents a potential for groundwater contamination (greater than 2 feet bgl). Cadmium and chromium were the only metals detected at depths less than two feet that exceeded MSCs. One sample was collected from B-2 and the other from O-1.
All four metals were detected above MSCs at concentrations greater than 2 feet bgl. Nickel and lead were detected in the same three samples, two from B-4 and one from B-3. Chromium was detected in two borings at B-3 and cadmium was detected in samples from two borings at B-28 and one at B-4.
With regard to characterization of potential source areas, samples from only two sites, B-3 and O-1, contained chlorinated hydrocarbons in soil above Subchapter S MSCs. One groundwater sample from B-3 contained the degradation chain of PCE, TCE, DCE, and vinyl chloride. A sample from the B-19 site redrill contained PCE at 0.002 ug/L above its MCL, however, as no B-19 soil samples contained PCE, this result was considered anomalous for the site. Therefore, the two SWMUs of primary consideration for additional source characterization were B-3 and O-1.
Metals detected above MCLs were not investigated further as part of the groundwater investigation, but CSSA has a separate project currently underway for closure of SWMUs in which these results were be evaluated and additional work performed if necessary.
The IDW generated from the potential source characterization of well 16 effort includes soils associated with drill cuttings from various SWMU at CSSA. The characterization of these soils was made by the use of the soil boring interval sampling analytical results. The results of analyses for these soils are presented in Table 5.2-2 and indicated that the soil were nonhazardous. Uniform hazardous waste manifests for the nonhazardous IDW generated during this investigation are included in appendix H.
Soil gas surveys were performed in two phases at thirteen sites on CSSA to determine possible source area(s) for the chlorinated hydrocarbon contamination present in groundwater. Selection of the sites was based on the geophysical anomalies due to suspected past waste management activities (Figure 5.1-1). Methodologies for the work are discussed in Section 3.9.2, Section 3.12.1, and Section 3.14.1. A detailed technical memorandum for the task, including figures and tables summarizing work on a site-specific basis, is presented in Appendix F. This section discussed the major points of the survey.
The first phase of work was conducted in a initial reconnaissance survey during June and July 1995. A total of 234 soil gas samples were collected from thirteen sites (Figure 5.3-1) and analyzed for PCE, TCE, DCE, and BTEX, to test for contaminants known in groundwater and also for possible fuel contamination. Evaluation of the test results showed that SWMUs O-1 and B-3 contained PCE, TCE, and DCE in significantly higher (three to four orders of magnitude) concentrations than other sites. Interestingly, most of the sites contained a very small amount of PCE in soil gas samples (0.1 to 1 ug/L). The initial soil gas survey results correlated with results from the drilling and sampling characterization effort that showed SWMUs O-1 and B-3 to contain chlorinated hydrocarbons in soil/rock samples and one B-3 groundwater sample.
Data from this work and other actions were discussed with EPA and TNRCC on October 19, 1995, in a technical presentation of work to date at CSSA. The soil gas initial results, combined with other data, strongly indicated that at least two sources of groundwater contamination were most likely SWMUs B-3 and O-1. At that meeting, among other decisions it was agreed that the soil gas survey would be expanded to determine the lateral extent of detectable soil gas concentrations to look for any other possible source areas and patterns of fractures. Also, the second survey would collect samples from a more detailed grid interval (5 to 12 feet) at O-1 and B-3 to better delineate the areas of soil gas "hot spots."
An additional 298 samples were collected in the detailed, follow-up investigation in November and December 1995. Because the geochemist conducting the reconnaissance survey had recognized peaks on the field GC as probable concentrations of carbon tetrachloride and chloroform in samples from the SWMU B-3, these compounds were added to the other target analytes tested. The detailed survey focused on sites B-3, B-4, the south pasture, the area between well 16 and B-3 (southeast well 16), O-1, Salado Creek area, and the area between B-3 and B-4. To extend the survey further east, additional samples were attempted in the eastern portion of SWMU B-3, but the effort was not very successful due to limestone very near or at ground surface.
The survey results confirmed the reconnaissance survey findings of much higher PCE soil gas concentration at SWMUs O-1 and B-3. Results in other areas were not indicative of source concentrations of chlorinated solvents, but low levels of PCE in soil gas were found to extend to the well 16 area, east of B-3 and B-4, west to well 4, and southwest to the corner of the CSSA H and I area, and southeast along Salado Creek.
Summaries of detected compounds and their maximum concentrations from both the reconnaissance and detailed surveys are shown in Table 5.3-1. PCE was detected at all thirteen sites, while BTEX were not detected in any soil gas samples from any site. Figure 5.3-2 (PDF Version) is a compilation of PCE data from all the sites, and summarizes locations of PCE greater than 0.1 ug/L up to concentrations greater than 100 ug/L.
The soil gas survey results indicated that both TCE and PCE were detected at concentrations indicative of source areas at SWMUs O-1 and B-3. The oxidation pond was determined to be a source of PCE contamination as indicated by PCE concentrations greater than 10,000 ug/L. In relation to PCE concentrations, only minor amounts of TCE were detected at the oxidation pond, which is suggestive of aerobic conditions that do not allow much degradation of PCE into breakdown compounds.
At B-3, TCE concentrations greater than 1,000 ug/L and PCE concentrations greater than 100 ug/L indicate a probable source area is likely located in the central portion of the site near the existing gravel road with an additional TCE source located along an abandoned road to the west of the main gravel road. DCE was also detected in association with TCE and PCE anomalies. The occurrence of DCE is probably the results of the anaerobic decay of PCE and TCE.
Geologic barriers can cause perturbations in the distribution of compounds in soil fgas. The area of low PCE concentrations on the west side of B-3 is probably the result of clay layer blocking the migration of PCE into the shallow soil gas at these locations (Figure 5.3-2) (PDF Version). The sudden PCE concentration decrease to the northwest of B-3 in the southeast well 16 area could be related to a geologic structure or a stratigraphy barrier such as a clay layer. The occurrence of PCE between SWMUs B-3 and B-4 could indicate that a preferred migration pathway exists between the two SWMUs. Similarly, the occurrence of PCE in the shape of a lobe extending from B-3 to at concentrations greater than 1 ug/L could indicate a preferred migration pathway from B-3 to the south pasture. The soil gas results suggest that well 4 is located near the western edge of the groundwater contamination. Similarly, evaluation of the PCE summary map suggests that three other preferred pathways may exist as suggested by linear trends in the PCE data and the frequent occurrence of TCE in these zones as well.
The distribution of PCE in soil gas probably reflects migration of this compound from source areas by diffusion and by transport in perched groundwater systems. These perched systems could have gradients different from that indicated by monitoring well data with movement dictated by solution features and other zones of secondary permeability. The initial movement of perched water zones from source areas may also be related to topography with flow from geographically higher areas such as O-1 and B-3 into topographically lower areas near Salado Creek.
Although PCE was detected at all the other sites and TCE was also detected at several SWMUs, no source areas capable of causing the groundwater contamination observed in Well 16 or the other CSSA wells were indicated at the other sites. No soil gas anomalies were present at Gate 6 and only one minor PCE anomaly, less than 1 ug/L, was present at B-2.
The occurrence of PCE and TCE at sites adjacent to or near B-3 and O-1 is probably the result of migration from source areas in B-3 and the oxidation pond as suggested by the general decrease in PCE concentrations from those source areas (Figure 5.3-2) (PDF Version). PCE was detected from source areas at B-3 and the oxidation pond to Wells 16, D, and 4. TCE and DCE were also detected to the west of B-3 in the area southeast of Well 16, and TCE was also detected in the south pasture. The occurrence of DCE and TCE at these sites probably represent migration from SWMU B-3. The distribution of PCE, TCE, and DCE in soil gas suggests that the source of the solvent contamination at well 16 is SWMU B-3, but the highest PCE concentrations in soil gas were detected at O-1. Given that both sites are considered downgradient of well 16, and the subsurface conditions of fracturing and faulting in limestone, both sites are likely source areas.