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March 2002 Off-Post Quarterly Groundwater Monitoring Report
Section 2 - Previous Off-Post Groundwater Sampling Procedures
Both on- and off-post groundwater monitoring are being conducted at CSSA. The quarterly off-post groundwater monitoring program was initiated in September 2001 and on-post groundwater monitoring was initiated in 1991 in response to VOC contamination detected in CSSA drinking water supply well CS-16. Sampling of on-post wells was continued periodically until the CSSA quarterly groundwater monitoring program for on-post wells was initiated in December 1999. Results from on-post groundwater sampling are presented in the March 2002 On-Post Quarterly Groundwater Monitoring Report (Parsons, June 2002).
Off-post sampling was first performed in 1995 in response to VOC contamination detected in on-post well CS-16. At the time, four off-post wells (RFR-3, RFR-8, JW-30, and OFR-2) were sampled. Analytical results for the December 1995 sampling event can be found in the Environmental Encyclopedia, Volume 5, Groundwater Investigation and Associated Source Characterization Report (June 1996), Section 8, specifically Table 8.2-5 and Table 8.2-6, or in the Cumulative Off-post Analytical Tables located in Volume 5, Groundwater, Introduction to the Groundwater Monitoring Program, Table 8 and Table 9 of the Environmental Encyclopedia. Cumulative Off-post Well Sampling Locations are shown on Figure 1.1. No VOC detections were reported from the four wells sampled in 1995.
In September 1999, wells JW-30, RFR-3, and RFR-8 were resampled to comply with the Interim/Stabilization Measures requirements in the RCRA 3008(h) Administrative Order on Consent. Analytical results for the September 1999 sampling event can be found in the Environmental Encyclopedia, Volume 5-1, December 1999 Quarterly Groundwater Monitoring Report, specifically Table 9 or in the Cumulative Off-post Analytical Tables located in Volume 5, Groundwater, Introduction to the Groundwater Monitoring Program, Table 8 and Table 9 of the Environmental Encyclopedia. Methylene chloride was the only VOC detected in 1999, and the detection is attributed to laboratory method contamination.
An additional well located at the southwest corner of CSSA, LS-7, was sampled in December 1999 to determine whether VOCs may have migrated to the southwest. Quarterly sampling at this location continued after December 1999 because VOC levels were detected. Quarterly sampling of LS-7 was included with on-post sampling events until sampling of other off-post wells was initiated in September 2001.
The off-post groundwater monitoring program was expanded in August 2001 to determine whether other wells in the vicinity of LS-7 were impacted, to ensure that appropriate response actions were taken, and to assist with the assessment of off-post plume migration. This work included sampling of four additional private and public drinking water supply wells (LS-2, LS-3, LS-5, and LS-6) near the southwestern corner of the post. These wells are all located downgradient of a potential VOC source area identified at Building 90/AOC-65, and their locations are shown on Figure 1.1.
The off-post groundwater monitoring program was again expanded in September 2001 to determine whether other wells along the south and west perimeter of CSSA were impacted, to ensure that appropriate response actions were taken, and to assist with assessment of the off-post plume migration. This work included sampling 15 additional private and public drinking water supply wells (DOM-1, FO-22, FO-J1, I10-2, JW-6, JW-9, JW-12, JW-13, JW-14, LS-1, LS-4, RFR-6, RFR-7, RFR-9, and RFR-10). These wells are all located downgradient of the potential VOC source areas identified near Building 90/AOC-65 (Plume 2), and SWMUs B-3 and O-1 (Plume 1). These well locations are shown on Figure 1.1.
In October 2001, two new wells (OFR-3 and RFR-11) were added to the program in response to laboratory results collected from nearby well RFR-10 which indicated concentrations of PCE at levels exceeding the MCL.
The December 2001 event expanded the off-post groundwater monitoring program again, adding three additional wells (HS-2, HS-3, and I10-4) to the program. Wells HS-2 and HS-3 (located farther southeast of Building 90/AOC-65) and I10-4 (located farther southwest of the post) were sampled because the extent of PCE contamination had not been defined by data from wells sampled through October 2001. The TNRCC also collected samples from wells HS-2, HS-3, and I10-5 during the December 2001 event. The results from the laboratory contracted by the TNRCC do not confirm or deny the results of CSSA sampling because the method detection limits used by the TNRCC laboratory were higher. These well locations are shown on Figure 1.1.
Prior to the September 2001 sampling event, all off-post wells were sampled from existing taps located as close to the wellhead as possible. By the September 2001 monitoring event, all wells needing wellhead taps had them installed. This enabled the sample to be collected prior to groundwater entering the storage tank and minimized chances of VOC volatilization and aeration. Samples were collected after the wells were adequately purged as indicated by stabilization of the pH, conductivity, and temperature parameters. Generally, this required an average of 20 gallons to be purged prior to sample collection from most wells.
2.1 - Previous Off-post Groundwater Sampling Results
Investigation following the 1991 detection of VOCs in groundwater at well CS-16 identified SWMU B-3 and SWMU O-1 as potential source areas. The 1995 and 1999 off-post groundwater monitoring targeted wells near SWMU B-3 and SWMU O-1, located in the north-central portion of CSSA Plume 1. The December 1999 detection of VOCs in off-post well LS-7 prompted off-post groundwater monitoring, targeting wells near a newly identified potential source area at Building 90/AOC-65, at the southwest corner of CSSA Plume 2.
No VOC contamination was reported in the four off-post wells sampled in 1995 (RFR-3, JW-30, RFR-8 and OFR-2), located west of CSSA. With the exception of methylene chloride, a common laboratory contaminant, no VOC contamination was reported upon resampling of three of these wells in September 1999 (RFR-3, JW-30, and RFR-8). Low levels of methylene chloride (0.17 and 0.15 �g/L) were detected in RFR-3 and RFR-8 during the September 1999 sampling event. (Figure 1.1).
Methylene chloride has been reported on- and off-post since 1992. However, each time methylene chloride has been detected in a sample, it has also been present in the analysis method blank, indicating the likelihood that this analyte was introduced as a laboratory contaminant and is not present in the groundwater. There are no known historical uses of methylene chloride on-post. Additionally, methylene chloride has not been detected in the same wells consistently over time, enhancing the position that methylene chloride is a laboratory artifact. The earliest detection of methylene chloride was in November 1992, with sporadic detection in September 1999, December 2001, and again in March 2002. The March 2002 detections of methylene chloride are discussed in Section 4.
Well LS-7 was first sampled in December 1999. Analytical results from LS-7 indicated low levels of PCE (2.51 �g/L) and TCE (0.3 �g/L) in the groundwater. Since this detection in 1999, CSSA began sampling LS-7 quarterly in conjunction with its on-post groundwater monitoring program (September 2001 On-post Quarterly Groundwater Monitoring Report).
In March 2000, sampling of LS-7 reported PCE and TCE at concentrations of 2.87 mg/L and 0.5 mg/L, respectively. These detections were above the laboratory reporting limit (RL) but below the applicable MCL. By June 2001, the PCE concentration increased to 4.0 �g/L. The increase in concentrations of VOCs prompted CSSA to install a GAC treatment system on August 7, 2001. This decision was based on linear regression statistics from the March 2000 through July 2001 sampling events, that indicated an exceedance of the MCL could possibly occur by the end of calendar year 2001. Figure 2.1 demonstrates PCE and TCE concentration trends in LS-7 since December 1999.
In August 2001, analyses of water samples collected from four other private/public drinking water wells located near the southwest corner of CSSA (LS-2, LS-3, LS-5, and LS-6) indicated VOC contamination. Concentrations ranged from 0.28 to 6.5 mg/L for PCE and 0.26 to 0.90 mg/L for TCE. PCE levels in LS-6, which were measured at 6.5 mg/L, was the first measured exceedance of the MCL of all off-post samples tested. Based on these findings, CSSA immediately provided bottled water to residents using LS-6 and installed a GAC filtration system on August 15, 2001 (CSSA Off-Post Monitoring and Response Plan, [the Plan] 2001). Figure 2.1 demonstrates PCE and TCE concentration trends in LS-6 since August 2001.
In September 2001, the following off-post wells were sampled in conjunction with on-post quarterly groundwater sampling (DOM-2, FO-22, FO-J1, I10-2, JW-6, JW-9, JW-12, JW-13, JW-14, LS-1, LS-4, LS-6, LS-7, RFR-6, RFR-7, RFR-8, RFR-9, RFR-10). Results for off-post wells were segregated into a separate report to present this large amount of data in a more organized and concise fashion. This report is located in Volume 5-1, Groundwater, Off-post Quarterly Groundwater Monitoring, September 2001 Off-post Quarterly Groundwater Monitoring Report, (Parsons, 2002).
VOC contamination was detected in wells I10-2, JW-14, LS-1, LS-6, LS-7, and RFR-10. Concentrations ranged from 0.12 to 14.0 mg/L for PCE and 0.23 to 7.5 mg/L for TCE. PCE and TCE levels in RFR-10, which were measured at 14.0 and 7.5 mg/L, exceeded the MCLs of both compounds (5.0 mg/L). Based on these findings, CSSA immediately provided bottled water to residents using RFR-10 and installed a GAC filtration system on October 9, 2001. This also prompted CSSA to sample other wells in the vicinity of RFR-10 in early October 2001.
Prior to the December 2001 quarterly sampling event, samples were collected from selected wells of concern based on results from the September 2001 monitoring event. Seven wells were sampled various times from late September through November 2001 (LS-2, LS-3, LS-5, LS‑6, LS-7, OFR-3, RFR-10, and RFR-11). VOC contamination was detected in wells LS-2, LS-3, LS-2-LS-3/entry point (EP), LS-5, LS-6, LS-7, OFR-3, RFR-10, RFR-11. Concentrations ranged from 0.15 mg/L to 20.89 mg/L for PCE and 0.16 mg/L to 9.4 mg/L for TCE. In October 2001, wells RFR-11 and OFR-3 were sampled for the first time in response to the concentration detected in well RFR-10. RFR-11 had concentrations of PCE at 16.0 mg/L and TCE at 0.35 mg/L, PCE exceeding the MCL of 5.0 mg/L. Based on these findings, CSSA immediately provided bottled water to residents using RFR-11 and installed a GAC filtration system on October 16, 2001. Figure 2.2 shows PCE and TCE concentration trends in wells RFR-10 and RFR-11 since September 2001.
In the December 2001 quarterly monitoring event, VOC contamination was detected in wells LS-7, OFR-1, OFR-3, RFR-10, RFR-11, RFR-12, FO-J1, HS-2, HS-3, I10-2, I10-4, JW-9, LS-1, LS-2, LS-3, LS-4, LS-5, and LS-6 pre-GAC. Concentrations ranged from 0.12 mg/L to 12.44 mg/L for PCE, 0.15 mg/L to 5.85 mg/L for TCE, and 0.26 mg/L to 0.33 mg/L for cis-1,2-DCE. Due to the increasing concentration trends and concentrations approaching the MCL for PCE in wellhead samples from municipal supply wells LS-2 and LS-3, and the presence of PCE and TCE in distribution system EP samples, installation of a GAC treatment system for these wells was expedited and completed in April 2002. Figure 2.3 shows the PCE and TCE concentration trends in wells LS-2 and LS-3 and Figure 2.4 shows the PCE and TCE concentration trends in LS-2/LS-3 EP and OFR-3.
Cumulative summary tables of all off-post analytical groundwater monitoring results are provided in Volume 5, Groundwater, Introduction to Groundwater Monitoring Program, Table 8 and Table 9 of the Environmental Encyclopedia. Historical cation and anion data for selected off-post wells are located in Volume 5, Section 4.0, Table 4.1 of the Off-Site Well Survey (Parsons 2001).