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Three-Tiered Long Term Monitoring Network Optimization Evaluation

Section 3 - Long-Term Monitoring Program at CSSA

The 2004 groundwater monitoring program at CSSA was examined to identify potential opportunities for streamlining monitoring activities while still maintaining an effective monitoring program. The 2004 monitoring program at CSSA is reviewed in the following subsections.

3.1 - Description of Monitoring Program

The CSSA groundwater monitoring program contains 97 wells, including on-post, off-post and Westbay^� (WB)-equipped wells. The WB wells have ports at multiple depths across the LGR, BS, and CC zones; the four wells have 46 distinct sampling locations that are considered separately for the LTMO analysis. Thus, the monitoring program examined in this 3-tiered LTMO evaluation includes 139 sampling locations (49 on-post wells, 44 Off-post wells, and 46 WB sampling locations). The objectives of the monitoring program at CSSA are presented in both the Data Quality Objectives for the Groundwater Contamination Investigation, (November, 2003) and in the CSSA Off-post Groundwater Monitoring Response Plan (June, 2002) and include, in part:

The CSSA Groundwater Monitoring Program wells and their associated current (2004) monitoring frequencies were identified from the Quarterly Groundwater Monitoring Reports available in Volume 5 of the CSSA Environmental Encyclopedia and subsequent review by site hydrogeologist Scott Pearson. Well information is listed in Table 3.1, including the hydrogeologic zone (as described in Section 2.2), current sampling frequency (as of December 2004), the first and most recent sampling events, well zone and well classification. Wells are classified into the following groups for the statistical analyses:

The 97 site wells are shown on Figures 3.1 and 3.2 classified by type of well. The most recent COC concentrations (December 2004) for each well are shown for zones LGR, BS, and CC in Figures 3.3 through 3.5, respectively. The on and off-post open boreholes are grouped into the LGR zone for this LTMO analysis. The typical well construction for the open borehole wells includes an open borehole completion through the LGR, BS, and CC portions of the aquifer with minimal surface casing. Historical results from on-post cluster wells indicate where COCs are detected in the LGR, the corresponding BS and CC wells are typically non-detect. Detections of COCs are generally confined to the LGR with the exception of the source area. Therefore, on and off-post open boreholes should be evaluated as LGR zones. The WB wells and area AOC-65 wells are considered separately from the LGR, BS, and CC zones because the data from these wells are �screening level� and not considered comparable to the validated chemical data from the other wells considered in the analysis. The AOC-65 and WB wells are analyzed separately in a vertical cross-section analysis. The location of the two vertical cross sections (north to south and west to east) are shown on Figure 3.2. Figures 3.6 and 3.7 display the vertical distribution of the most recent COC concentrations for wells in the north to south and west to east cross sections, respectively, along with their most recent sampling event.

3.2 - Summary of Analytical Data

In general, the CSSA groundwater plume is well-characterized both laterally and vertically. The groundwater monitoring program for this plume was evaluated using results for sampling events performed from 1991 through December 2004. The database was processed to remove duplicate data by retaining the maximum result for each duplicate sample pair. As discussed in Section 2.3, the COCs identified for CSSA include TCE, PCE, and cis-1,2-DCE. Table 3.3 presents a summary of the occurrence of potential COCs in groundwater based on data collected from CSSA wells for all the sampling data. Tables 3.4 through 3.8 show the summary statistics by well classification: LGR, on-post Open Boreholes (OPBH), CC, BS, Off-Post Open Borehole (OffBH), and Westbay^�/AOC-65 wells, respectively. Tables 3.3 through 3.8 confirm that TCE, PCE, and cis-1,2-DCE are the main contaminants in groundwater beneath CSSA based on their widespread and relatively high (compared to their respective MCL) concentrations. Although it has been sampled less frequently than the primary COCs, lead (Pb) is of potential concern because of the relatively high percentage of and number of wells with detections. Other chemicals of potential concern include bromoform (TBME) and bromodichloromethane (BDCME) because of their action levels of zero. Toluene (BZME) detections occurred in screening level samples collected during discrete interval groundwater sampling during well installations and sporadically among definitive sampling events. Vinyl chloride (VC) has also been detected and is an indicator that degradation of larger-chain chlorinated hydrocarbons is occurring. Although no wells have had exceedances of BZME or VC, both chemicals are of potential concern at CSSA, and are included in the temporal statistical analysis.

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