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Three-Tiered Long Term Monitoring Network Optimization Evaluation
Section 7 - Summary of Three-Tiered Long-Term Monitoring Optimization Evaluation
The 139 sampling points at CSSA were evaluated using qualitative hydrogeologic information, temporal statistical techniques, and spatial statistics. As each tier of the evaluation was performed, monitoring points that provide relatively greater amounts of information regarding the occurrence and distribution of COCs in groundwater were identified, and were distinguished from those monitoring points that provide relatively lesser amounts of information. In this section, the results of the evaluations are combined to generate a refined monitoring program that could potentially provide information sufficient to address the primary objectives of monitoring, at reduced cost. Monitoring wells not retained in the refined monitoring network could be removed from the monitoring program with relatively little loss of information. The results of the qualitative, temporal, and spatial evaluations are summarized in Table 7.1, along with the final recommendations for sampling point retention or exclusion and final sample frequency. Figure 7.1 shows the final recommendations for the LGR zone wells. The results of the evaluations were combined and summarized in accordance with the following decision logic:
Each well retained in the monitoring network on the basis of the qualitative hydrogeologic evaluation is recommended to be retained in the refined monitoring program.
Those wells recommended for removal from the monitoring program on the basis of all three evaluations, or on the basis of the qualitative and temporal evaluations (with no recommendation resulting from the spatial evaluation) should be removed from the monitoring program.
If a well is recommended for removal based on the qualitative evaluation and recommended for retention based on the temporal or spatial evaluation, the final recommendation is based on a case-by-case review of well information.
If a well is recommended for retention based on the qualitative evaluation and recommended for removal based on the temporal and spatial evaluation, the recommended sampling frequency is based on a case-by-case review of well information.
It should be noted, as stated in number four above, the final recommended monitoring frequencies shown in Table 7.1 are not, in all cases, the same as those recommended as a result of the qualitative evaluation (Table 4.3). In the CSSA qualitative evaluation, few wells were recommended for exclusion from the monitoring network, while many were recommended for reduced sampling frequency. Thus, the temporal and spatial statistical evaluation results were primarily used to confirm or adjust qualitative monitoring frequency recommendations. The justification for these modifications is provided in the �Rationale� column in Table 7.1, and fall into the following general categories:
| Temporal and/or spatial statistical results confirm the sampling frequency recommendations from the qualitative evaluation. For example, well CS-D is recommended for retention by both the temporal and spatial statistical results; thus, the statistics confirm the semi-annual sampling frequency. Likewise, well CS‑MWH-LGR is recommended for exclusion from the network by both the temporal and spatial statistical results; thus, the statistics confirm the low biennial sampling frequency. | |
| Decrease sampling frequency due to statistics results. For example, well CS‑2 is recommended for semi-annual sampling in the qualitative evaluation; however, the well was recommended for exclusion/reduction in the temporal evaluation because it has had only trace PCE since 1999 and was determined to be of relatively little importance in the spatial evaluation. Therefore, annual sampling is recommended in the summary evaluation. | |
| Qualitative factor overrides statistics recommendations. Well CS-10 is similar to CS-2, in that the statistical evaluations showed it to be contributing limited temporal and spatial information to the monitoring network. However, the qualitative annual sampling recommendation was due to the fact that the well is a drinking water supply well, which was not considered by the statistics, so the summary recommendation remains at an annual frequency. Similarly, analysis of several WB sampling points resulted in statistics that would support less frequent monitoring (e.g., CS‑WB01‑LGR-04, CS-WB01-LGR-05), yet it was determined to maintain semi-annual sampling at all of the WB wells due to qualitative considerations and to continue plume characterization in the immediate area of Building 90. | |
| Increase sampling frequency due to statistical recommendations. For example, well CS-11 was recommended for biennial sampling in the qualitative evaluation; however, the temporal evaluation revealed increasing lead concentrations trends downgradient. Therefore, the well was recommended for annual sampling in the summary evaluation. |
In addition to the above situations, it should be noted that spatial statistical results obtained during the two vertical cross section analyses (shown with a � or � in Table 7.1) were only applicable to the Westbay� and AOC-65 wells, and did not influence the summary result of the BS and CC wells included in the analysis.
The results of the qualitative hydrogeologic and spatial analyses were also used to suggest locations for possible future monitoring wells. Based on the evaluations made in this study, eleven possible locations are shown on Figure 7.2. Based on qualitative review additional wells may be necessary to investigate groundwater level mounding along Salado Creek. Additional wells based on spatial analysis evaluation may be necessary for further plume delineation in areas with high spatial uncertainty. Figure 7.2 categorizes the possible well locations as related to plume definition or to evaluation of groundwater levels or for a combination of both purposes.
A breakdown of the final well and frequency recommendations is shown in Table 7.2, along with the original 2004 sampling breakdown (shown in parentheses) for the on-post, off-post, AOC-65 area and WB wells.
Table 7.2 - Summary of Revised and Original Monitoring Program
| Type of Well | Monitoring Frequency | Total Sampling Points | |||||
| Not Sampled | Biennial | Annual | Semi-Annual | Quart-erly | After Rain Event | ||
| On-post | 1 (1) | 13 | 11 | 16 | (40) |
| 40 (40) |
| Off-post |
| 20 | 17(18) |
| 7(26) |
| 44 (44) |
| AOC-65 | 6 |
|
|
| (8) | 2 | 2 (8) |
| Westbay� |
|
|
| 46 |
| 46 (46b/) | 46 (46) |
| Total Wells | 7 (1) | 33 | 28 (18) | 52 | 7 (74) | 48 (46)c/ | 132 (138) |
| a/ 2004 sampling frequency corresponding to Table 3.1 shown in parentheses. b/ WB wells previously sampled monthly & after a rain event. c/ WB wells sampled after rain event also sampled periodically not included in total sampling points. | |||||||
The LTMO analysis supports the exclusion of well CS-3 that is currently not included in the monitoring program. In addition, the AOC-65 area piezometers are recommended for exclusion from the monitoring program, and the AOC-65 area monitoring wells are recommended to be sampled only after major rain events. All 46 WB sampling points are recommended to be reduced from monthly sampling to semi-annual sampling. The temporal and vertical cross section spatial analysis results could potentially be used to evaluate the importance of continued sampling within subzones.
For the on-post and off-post wells, the LTMO results indicate that a refined monitoring program consisting of the same 88 wells sampled less frequently (33 wells sampled biennially, 28 sampled annually, 16 sampled annually, and 7 sampled quarterly) would be adequate to address the two primary objectives of monitoring listed in Section 1. This refined on and off-post monitoring network would result in an average of 104.5 (49.5 on-post and 55 off-post) well-sampling events per year, compared to 242 (120 on-post and 122 off-post) well-sampling events per year under the 2004 monitoring program. Reducing WB sampling from monthly to semi-annually would reduce the number of sampling events from 528 to 88 events per year. A well-sampling event is defined as a single sampling of a single well. Implementing these recommendations for optimizing the LTM monitoring program at CSSA would reduce the number of on- and off-post well-sampling events per year by approximately 57 percent and the WB sampling events per year by approximately 88 percent.