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Three-Tiered Long Term Monitoring Network Optimization Evaluation
Section 4 - Qualitative LTMO Evaluation
An effective groundwater monitoring program will provide information regarding contaminant plume migration and changes in chemical concentrations through time at appropriate locations, enabling decision-makers to verify that contaminants are not endangering potential receptors, and that remediation is occurring at rates sufficient to achieve remedial action objectives (RAO) within a reasonable time frame. The design of the monitoring program should therefore include consideration of existing receptor exposure pathways, as well as exposure pathways arising from potential future use of the groundwater.
Performance monitoring wells located within and downgradient from a plume provide a means of evaluating the effectiveness of a groundwater remedy relative to performance criteria. Long-term monitoring (LTM) of these wells also provides information about migration of the plume and temporal trends in chemical concentrations. Groundwater monitoring wells located downgradient from the leading edge of a plume (i.e., sentry wells) are used to evaluate possible changes in the extent of the plume and, if warranted, to trigger a contingency response action if contaminants are detected.
Primary factors to consider when developing a groundwater monitoring program include at a minimum:
| Aquifer heterogeneity; | |
| Types of contaminants; | |
| Distance to potential receptor exposure points; | |
| Groundwater seepage velocity and flow direction(s); | |
| Potential surface-water impacts; and | |
| The effects of the remediation system. |
These factors will influence the locations and spacing of monitoring points and the sampling frequency. Typically, the greater the seepage velocity and the shorter the distance to receptor exposure points, the more frequently groundwater sampling should be conducted.
One of the most important purposes of LTM is to confirm that the contaminant plume is behaving as predicted. Graphical and statistical tests can be used to evaluate plume stability. If a groundwater remediation system or strategy is effective, then over the long term, groundwater-monitoring data should demonstrate a clear and meaningful decreasing trend in concentrations at appropriate monitoring points. The CSSA Groundwater Monitoring Program is conducted under the provisions of the Off-post Groundwater Monitoring Program Response Plan (June 2002) and the Data Quality Objectives for the Groundwater Monitoring Program (November 2003). The current groundwater monitoring program at CSSA was evaluated to identify potential opportunities for streamlining monitoring activities while still maintaining an effective performance and compliance monitoring program.
4.1 - Methodology for Qualitative Evaluation of Monitoring Network
The LTMO evaluation included 139 sampling locations located on and off CSSA. These wells, their associated hydrogeologic zones, and the 2004 monitoring frequencies are listed in Table 3.1, and their locations are depicted on Figures 3.1 and 3.2. As shown in the table, the LTMO evaluation included on-post, off-post, and WB wells.
Multiple factors were considered in developing recommendations for continuation or cessation of groundwater monitoring at each well. The CSSA monitoring network was evaluated to determine any data gaps where information was needed to further characterize the plumes. Recommendations of areas of greater spatial uncertainty were given as locations for future monitoring wells. In some cases, a recommendation was made to continue monitoring a particular well, but at a reduced frequency. A recommendation to discontinue monitoring at a particular well based on the information reviewed does not necessarily constitute a recommendation to physically abandon the well. A change in site conditions might warrant resumption of monitoring at some time in the future at wells not currently recommended for continued sampling. Typical factors considered in developing recommendations to retain a well in, or remove a well from, a LTM program are summarized in Table 4.1. Typical factors considered in developing recommendations for monitoring frequency are summarized in Table 4.2.
Table 4.1 - Monitoring Network Optimization Decision Logic
| Reasons for Retaining or Adding a Well to the Monitoring Network | Reasons for Removing a Well From Monitoring Network |
| Well is needed to further characterize the site or monitor changes in contaminant concentrations through time | Well provides spatially redundant information with a neighboring well (e.g., same constituents, and/or short distance between wells) |
| Well is important for defining the lateral or vertical extent of contaminants. | Well has been dry for more than 2 yearsa/ |
| Well is needed to monitor water quality at compliance point or receptor exposure point (e.g., water supply well) | Contaminant concentrations are consistently below laboratory detection limits or cleanup goals |
| Well is important for defining background water quality | Well is completed in same water-bearing zone as nearby well(s) |
| a/ Periodic water level monitoring should be performed in dry wells to confirm that the upper boundary of the saturated zone remains below the well screen. If the well becomes re-wetted, then its inclusion in the monitoring program should be evaluated. | |
Table 4.2 - Monitoring Frequency Decision Logic
| Reasons for Increasing | Reasons for Decreasing |
| Groundwater velocity is high | Groundwater velocity is low |
| Change in contaminant concentration would significantly alter a decision or course of action | Change in contaminant concentration would not significantly alter a decision or course of action |
| Well is necessary to monitor source area or operating remedial system | Well is distal from source area and remedial system |
| Cannot predict if concentrations will change significantly over time | Concentrations are not expected to change significantly over time, or contaminant levels have been below groundwater cleanup objectives for some prescribed period of time |
4.2 - Results of Qualitative LTMO Evaluation
Results of the qualitative evaluation of wells at CSSA are described in this subsection. The evaluation included the 139 on-post, off-post and WB-equipped monitoring points listed in Table 3.1. The evaluation grouped the wells into these three classifications. The qualitative LTMO evaluation considered historical analytical results, whether the well was necessary for plume definition, and the primary use of the well (i.e. drinking water or monitoring). All COCs from historical monitoring were considered for the qualitative evaluation but special consideration was given to PCE, TCE and cis-1,2-DCE concentrations.
Table 4.3 includes recommendations for retaining or removing each well, the recommended sampling frequency, and the rationale for the recommendations. On and off-post LGR zone wells qualitative evaluation results are displayed in Figure 4.1. Overall, drinking water wells both on and off-post with results consistently below the MCL were recommended for annual sampling. Drinking water wells located off-post with historical detections exceeding the MCL for any COC were recommended for sampling on a quarterly schedule. On-post monitoring wells were recommended for various retention or removal states including removal from sampling, semi-annual, annual or biennial sampling frequencies.
4.2.1 On-post Wells
A total of 49 on-post monitoring wells were considered during the LTMO process for CSSA. In accordance with project DQOs, when four quarters of non-detections occur a well can be sampled less frequently. Recommendations for on-post wells included three wells recommended for annual sampling, 20 recommendations for semi-annual sampling (two per year), 17 recommendations for biennial sampling (one every two years), one well recommended for removal, and eight wells recommended for sampling as needed, based on precipitation. The recommendations and accompanying rationale for on-post wells are summarized in the following paragraphs.
Three on-post drinking water wells were recommended to be retained on an annual sampling frequency. Historical detections have been below the reporting limit or non-detect and annual sampling will ensure that on-post drinking water will continue to meet drinking water standards in the future. This recommendation applied to wells CS-1, CS‑9, and CS-10.
Twenty wells were recommended for sampling on a semi-annual basis. For CS-2, CS‑4, and CS-D, sampling has been conducted since 1991 and 1992. Well CS‑16‑LGR was sampled since 1991 and in 2002 the well was upgraded from an open borehole well to a monitoring well open in the LGR. On-post monitoring wells CS-MW1-LGR and CS-MW2-LGR were sampled since their installation in 1997. These wells were also re-completed in 2002 and sampled quarterly through December 2004. Wells constructed in and sampled since 2001 include CS‑MW3-LGR, CS-MW4-LGR, CS-MW5-LGR, CS-MW6-LGR, CS-MW7-LGR, CS‑MW8‑LGR, CS-MW9-LGR and CS-MW10-LGR. Wells CS-MW12-LGR, CS-MW18-LGR and CS-MW19-LGR were constructed in 2002 and sampled from installation through December 2004. Finally, of the wells recommended for semi-annual sampling, CS-MW11A-LGR, CS‑MW11B‑LGR, and CS-MW16-CC were installed in 2003 and sampled from installation to December 2004. Reducing the sampling frequency to semi-annual for this group of 20 wells will provide continued plume characterization information.
Seventeen wells were recommended to be sampled biennially, or every 2 years. This group includes 12 wells completed in the BS and CC formations. Groundwater monitoring at CSSA has consistently demonstrated that the BS and CC formations are not impacted by COCs. Well CS-MW16-CC is located near the source area and is the one exception. Detections of PCE, TCE and cis-1,2-DCE in CS-MW16-CC are the only detections in a CC well above the reporting limit. The wells completed in the LGR and recommended for biennial sampling (CS-11,
CS-G-LGR, CS-I, CS-MWH-LGR, and CS-MW17-LGR) are not necessary for plume definition and can be retained on the reduced frequency of biennially.
Well CS-3 was recommended to be removed from the sampling program. This well was last sampled in December 1999 and prior to that was sampled for 14 events beginning in November 1992. Both CS-2 and CS-3 are completed as open boreholes in the LGR(E) and LGR(F) zones and the two wells are located less than 200 feet apart. Sampling of CS-2 was recommended for a semi-annual basis and will provide plume characterization in this area. Sampling of CS-3 would be redundant.
Eight wells were recommended for retention to be sampled as needed. AOC‑65‑MW1‑LGR and AOC-65-MW2A were installed to characterize the upper zones of the LGR immediately surrounding the Building 90 area and have been sampled infrequently or following periods of precipitation. Wells AOC-65-PZ01-LGR, AOC-65‑PZ02-LGR, AOC-65-PZ03-LGR, AOC-65-PZ04-LGR, AOC-65-PZ05-LGR, and AOC-65-PZ06-LGR were installed for a treatability study and recharge study and were sampled after weather station data indicated that more than 1-inch of precipitation had occurred over a 24 hour period. Future sampling should also be linked to precipitation events.
4.2.2 Off-post Monitoring Wells
A total of 44 off-post drinking water wells were considered during the LTMO evaluation for CSSA. Of the 44 evaluated wells, seven are recommended to be retained on a quarterly sampling schedule and 37 are recommended to be sampled at a reduced frequency of once per year. Under the DQOs currently in effect for the CSSA Groundwater Monitoring Program, the sampling frequency can be reduced as needed at selected wells based on cumulative analytical results.
The seven off-post drinking water wells to be retained on a quarterly sampling frequency have had concentrations exceeding the MCL for PCE and have been equipped with GAC water treatment systems to ensure drinking water for residents meets EPA and TCEQ drinking water standards. These wells will be retained on a quarterly schedule to continue plume characterization and include LS-2, LS-3, LS-6, LS-7, OFR-3, RFR-10, and RFR-11.
The remaining 37 off-post drinking water wells evaluated have been sampled previously either quarterly or annually. Historical results show consistent concentrations so that sampling frequencies for the quarterly sampled wells can be reduced to annually as provided in the DQOs. Wells previously sampled annually but with low or no detections should be retained for annual sampling to ensure that privately owned off-post drinking water wells continue to meet drinking water standards in the future.
4.2.3 Westbay�-equipped Monitoring Wells
A total of 46 zones from four WB-equipped monitoring wells were considered during the LTMO process for CSSA. There are three WB wells installed on-post and one installed off-post. WB01, WB02 and WB03 are installed near Building 90 on-post and are completed in zones UGR-01 and LGR zones 01 through 09. WB04 is installed off-post near drinking water well RFR-10 and is complete in zones UGR-01, LGR zones 01 through 11, BS-01, BS-02, CC-01, CC-02 and CC-03. These wells are equipped with the Westbay� MP38 system which allows hydraulic pressure data collection and groundwater sampling of each zone using the Westbay� MOSDAX sampling probe.
All WB zones which contained water have been sampled since September 2003 on a monthly basis with additional sampling events occurring after rainfall events of more than 1 inch over a 24-hour period. Certain zones (CS-WB04-LGR-01, CS‑WB04‑LGR-03, and CS-WB02-LGR-02) have occasionally been dry and could not be sampled. Other zones are always dry and have been sampled less than four times since September 2003 (CS-WB04-LGR-02, CS-WB02-UGR-01, CS-WB01-UGR-01, CS-WB03-LGR-01, CS-WB03-UGR-01, CS-WB04-UGR-01 and CS-WB03-LGR-02). These zones only contain water following rainfall of more than 1 inch in duration.
Due to the historical sampling results collected monthly since September 2003 concentrations in the LGR zones are well documented. As a result of the qualitative evaluation, the sampling frequency for all LGR zones was recommended to be reduced to semi-annually. For the BS and CC zones of the WB wells the recommended sampling frequency can also be reduced. The BS and CC zones of the WB wells are not impacted by CSSA activities and sampling can be conducted biennially.
4.2.4 Laboratory Analytical Program
For on-post and off-post wells in the CSSA monitoring program, groundwater samples currently are analyzed for VOCs using method SW8260B for the full list and method SW8260B for the short list of VOCs. On-post drinking water wells are analyzed for the full list of VOCs. The majority of historical sampling events for on-post monitoring wells have been analyzed for the short list of VOCs which includes 1,1-DCE, bromodichloromethane, bromoform, chloroform, cis-1,2-DCE, dibromochloromethane, dichlorodifluoromethane, methylene chloride, naphthalene, PCE, TCE, toluene, trans-1,2-DCE, and VC. Metals are sampled once annually in the on-post monitoring wells and quarterly in the on-post drinking water wells. Metals are analyzed using methods SW6010B (barium, chromium, copper, nickel, zinc), SW6020 (arsenic, cadmium and lead), and SW7470A (mercury). All on-post and off-post drinking water and monitoring wells sampled quarterly receive data validation and verification in accordance with the AFCEE QAPP and the CSSA QAPP. Data packages are submitted to AFCEE chemists for review and approval.
For the WB-equipped wells groundwater samples are analyzed for the VOCs PCE, TCE, cis-1,2-DCE, trans-1,2-DCE, isopropanol, acetone and toluene using method SW8260B. Laboratory data packages from the Westbay samples receive an internal data validation and review but are not subject to review and approval by AFCEE.
The LTM program recommendations summarized in Table 4.3 are based on available data regarding current (and expected future) site conditions. Changing site conditions (e.g., periods of drought or excessive rainfall) could affect plume behavior. Therefore, the LTM program should be reviewed if hydraulic conditions change significantly, and revised as necessary to adequately track changes in plume magnitude and extent over time.