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December 2001 Off-Post Quarterly Groundwater Report
Section 2 - 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. 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.
No VOC contamination was reported in the four off-post wells sampled in 1995 (RFR-3, JW-30, RFR-8 and OFR-2), which are 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. Methylene chloride is not detected in wells consistently over time. Earliest detections of methylene chloride were in November 1992, with sporadic detections in September 1999 and again in December 2001. The December 2001 detections of methylene chloride are discussed in Section 4.
Well LS-7 was sampled for the first time 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 in conjunction with its on-post quarterly 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 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, since linear regression statistics from the March 2000 through July 2001 sampling events indicated an exceedance of the MCL would occur near 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.
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 split into a separate report in order to present this large amount of data in a more organized and concise fashion. This information 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 PRE, LS-7 PRE, 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, exceeding the MCLs of 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. These results are discussed later in this report.
Cumulative summary tables of all off-post analytical groundwater monitoring results are provided in Volume 5, Groundwater, Introduction to Groundwater Monitoring Program, Table 8 of the Environmental Encyclopedia. Table C-1 includes all analytical data for VOCs collected from off-post wells by CSSA. Metals data collected from off-post wells are presented in Table C-2. 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).