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Final September 2002 On-Post Quarterly Groundwater Monitoring Report
Section 5 - Summary
An average increase in water levels of 104.50 feet occurred between June 2002 and September 2002. This average increase is calculated from wells screened in separate formations and open boreholes. CSSA had 30.98 inches of rainfall between June 18, 2002 and September 16, 2002. Rainfall events were spread throughout the 3-month period with the majority of rainfall occurring in late June and early July 2002. Seven rainfall events exceeded 1 inch during a 24-hour period.
Two of the three newly installed wells (CS-MW17-LGR and CS-MW19-LGR) had detections of PCE. These concentrations were above the MDL and below the RL.
Well upgrades were preformed on wells CS-MW1-LGR, CS-MW2-LGR, CS-MW16-LGR, and CS-G-LGR before the September 2002 sampling event. The upgrade included new casing, screen, and pumps set at appropriate depths to ensure samples are collected from the Lower Glen Rose formation in these wells.
The groundwater potentiometric surface map for September 2002 indicates a variety of groundwater flow directions and large fluctuations in the potentiometric gradient. In general, the maps indicate groundwater flows in a southerly direction across the post. An exception to this is CS-MW4-LGR where flow appears to be in a northerly direction. Groundwater extraction from both on- and off-post drinking water supply wells, varying rates of recharge from rain events, and differences in well completions all contribute to the complexity of the potentiometric surface at CSSA.
The Lower Glen Rose potentiometric surface map near Building 90 indicates a groundwater flow to the south-southwest in September 2002, a slight change from June 2002 (south). Other potentiometric surface maps near Building 90 show varying groundwater flow directions to the southeast, southwest and the south (Figure 2-8). The flow direction in the Cow Creek Formation, which was to the north in December 2001, reversed direction to flow south in the March and June 2002 events. In the September 2002 event the flow direction has reversed again to the north.
MCLs for PCE and TCE were exceeded in Well CS-MW16-LGR during the September 2002 monitoring event. The concentrations of PCE and TCE in CS-MW16-LGR were 54 mg/L and 61 mg/L, respectively. Historically, Well CS-MW16-LGR concentrations have varied (ranging from 21 �g/L to 509 �g/L) with a decreasing trend observed over the last two years. In July 2002, this well was re-completed with 25 feet of screen in the bottom of the Lower Glen Rose. The impact of this re-completion on the contaminant levels is uncertain at this time, but will be evaluated as monitoring continues.
Well CS-D detections for PCE, TCE, and cis-1,2-DCE exceeded the MCL with concentrations of 170 mg/L, 250 mg/L, and 230 mg/L, respectively. PCE and TCE concentrations have increased over the last three months.
On-post drinking water wells CS-1 and CS-10 each had a detection of PCE above the MDL at concentrations of 0.080 mg/L and 0.086, respectively. No other VOCs exceeded the RL or MCL in CS-1 and CS-10 or the other on-post drinking water well (CS-9).
Of the wells sampled in September 2002, only well CS-MW17-LGR exceeded the MCL for nickel. Analyses of the sample from CS-MW17-LGR had a nickel concentration of 150.0 mg/L, which exceeds the MCL of 100.
In September 2002, CS-MW1-LGR detected PCE at 12.0 �g/L, TCE at 25.0 �g/L, and cis-1,2-DCE at 17.0 �g/L; well CS-MW2-LGR had concentrations for PCE of 2.9 �g/L and TCE of 2.0 �g/L. In the past nine months PCE and TCE trends have been decreasing in these wells.
Figure 5-1, Figure 5-2 and Figure 5-3 are included for evaluation of the concentrations of PCE, TCE, and cis-1,2-DCE over time in wells where these compounds have exceeded the MCL (CS-D, CS-MW16-LGR, CS-MW1-LGR, and CS-MW2-LGR). Concentrations have varied greatly over time with generally increasing trends over the last three months of groundwater monitoring in well CS-D and a slight decrease in well CS-MW16-LGR. Wells CS-MW1-LGR and CS-MW2-LGR concentrations have remained somewhat constant.