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Final Well Installation Report
Section 5 - Summary and Recommendations
Between November 2001 and October 2003, 17 groundwater MWs were installed, and eight existing wells were upgraded under TO42 at CSSA. The effort resulted in 11 LGR, two BS, and four CC conventional MWs, three multi-port LGR wells, and one multi-port LGR/BS/CC well. Minor upgrades enhanced water level data collection at three existing CSSA supply wells and improved production at a remote agricultural well. Relevant hydrogeological data were gathered through supplementary work at three off-post private wells. One existing agricultural well was abandoned. Five permanent benchmarks were installed to facilitate post wide surveying control.
Groundwater samples collected from open boreholes and discrete intervals were analyzed for possible environmental impacts due to past waste operations. Each well location was geophysically logged. Video logs were made at eight new wells. Conventional MWs were completed with screens emplaced within or overlapping into the major water-bearing zones of the Middle Trinity Aquifer, and in two cases, exclusively the BS. The multi-port MWs provide isolated monitoring intervals from the base of the UGR through most of the LGR, and at one location, to the base of the CC. Low-flow bladder pumps were placed in 13 of the new MWs. Electrical submersible pumps were installed in three of the new conventional MWs. Transducers were installed in 16 wells. No impacts to off-post wells resulting from TO42 well construction activities were observed or reported. No hazardous waste was generated by the TO42 field effort. As of the fourth quarter of 2003, there are 41 conventional wells included in the CSSA on-post quarterly monitoring network, four WB System wells that are monitored monthly, and six piezometers and four shallow MWs in AOC-65 that are monitored on an event-driven basis.
Observations from the TO42 well installation program lead to the following general conclusions:
The data reinforce and enhance the conclusions formulated after the RL83 work. Most of the subsurface contains many dry, hard, and impermeable zones or layers that greatly impede or prevent downward contaminant migration. These layers are breached by faults in the middle and southern sections of CSSA. Within affected areas, some contaminants have apparently migrated downward from the more heavily impacted middle and upper LGR into the underlying BS via faults and fractures, and possibly via former open borehole wells (such as CS-16).
After TO42 borehole and discrete sampling, no VOCs were detected in the CC except at MW16-CC and on the RFR-10 property outside the southwest corner of the post. Both the affected CC areas are very near or are downgradient of plume source areas, and both lie within the northeast-southwest trending fault zones, and are near old �open borehole� wells. The sub-vertical faults may provide openings through impermeable layers. It is postulated that CC contamination has occurred at these locations primarily due to cross-connection associated with the open borehole construction of RFR-10 and former CS-16. The faults also appear to be facilitating the lateral movement of groundwater contamination off-post in a general southwest direction, and they may locally allow groundwater to move laterally at a greater rate than through unfaulted subsurface matrix.
Discrete interval groundwater samples were obtained from 14 new CSSA MW boreholes and from one existing off-post private well. The upgraded wells, CS‑MW17‑LGR, and CS‑MWH-LGR, were not selected for DIGW sampling in the SOW because they were considered to be in low-impact areas of CSSA. Laboratory analysis of the groundwater samples indicates that within the plumes, the majority of VOC contamination is present in upper portions and perched waters of the LGR Limestone. Sections of the BS that underlie contaminated LGR have been penetrated by VOC contamination. Contaminants were detected in the CC Limestone at CS-MW16-CC, CS‑WB04, and RFR-10. Contaminant concentrations generally decrease with distance downgradient from the source areas of both known CSSA plumes.
Discrete samples collected from new wells installed near source areas showed significant concentrations of PCE, TCE, and cis-1,2-DCE throughout the LGR, BS, and CC at those locations. Drinking water MCLs for PCE and TCE were exceeded in many samples from the WB wells and from CS-MW16-CC. The CS-MW16-CC (Plume 1 area) sample results show concentrations of PCE and TCE at 48.4 �g/L and 131 �g/L, respectively, from the lower CC. Concentrations of cis-1,2-DCE show an increasing trend with depth in CS‑MW16‑CC, from 23 �g/L in the upper LGR to 139 �g/L in the lower CC. The only trans‑1,2‑DCE detections reported for TO42 were from CS‑MW16-CC discrete samples, and range from 0.29 �g/L in the LGR to 3.51 �g/L in the lower CC. The trans-1,2-DCE and cis‑1,2-DCE results indicate that degradation (reductive dehalogenation) of the original contaminating solvents is occurring near the source areas.
Similarly, CS-WB03 (Plume 2 area) results show a maximum PCE concentration of 767 �g/L, and 197 �g/L of TCE in the middle LGR. Most interval samples from CS-WB04 and RFR-10 also contained PCE, TCE, and cis-1,2-DCE, although at lower concentrations than above. However, at the RFR-10 private property wells, the highest VOC levels were found in the middle LGR, at shallower depths than in the wells closer to source areas. The PCE, TCE, and cis-1,2-DCE levels in CS‑WB04 and RFR-10, in addition to other nearby private wells, and their location adjacent to a fault zone, suggest AOC-65 plume advancement to the southwest from CSSA Building 90.
Acetone and toluene are sporadically reported throughout the TO42 laboratory results, and often without corresponding PCE or TCE detections. Acetone was detected in samples from CS‑MW1-BS, CS‑MW12-CC, CS‑MW18-LGR, CS‑MW16‑CC, and CS‑MW2-CC. Reported concentrations of acetone range from 13 �g/L (LGR in CS‑MW16-CC) to 3,610 �g/L (BS portion of CS‑MW2-CC). Sampling results of IDWstrongly suggest that occurrences of acetone are likely related to the use of drilling foam as a cuttings removal agent. While IPA is listed as the major component of the foaming agent, a chemical/metabolic reaction is believed to result in the production of acetone. Drilling foam which has escaped into the aquifer (i.e., not circulated back to ground surface) can be expected to persist for several months.
Toluene concentrations fluctuate from below laboratory detection limits (MDL <1 �g/L) to minor detections (≤1.9 �g/L) at CS‑MW1-BS, CS‑MW2‑CC, CS‑MW18-LGR, and CS‑WB01, and as much as 298 �g/L at CS‑MW11B‑LGR. The source of toluene is unknown, but could either result from the sampling method involving the packer system, exhaust from the rig and generators, or actually present in the groundwater. Toluene is a common groundwater contaminant associated with the widespread use of fuels and motor oils, but in these cases is usually associated with benzene, ethyl benzene, and xylenes contamination.
The inconsistent acetone and toluene detections may be related to the well construction process rather than to a historical subsurface contaminant source; however, actual acetone and toluene groundwater contamination cannot be ruled out at this time. Toluene is consistently detected at levels below the RL during quarterly groundwater monitoring activities. Detections are reported in both newly installed MWs and pre-existing on-post agricultural and supply wells. Acetone and toluene did not exceed MCLs in any DIGW sample collected during the investigation.
This phase of the groundwater investigation provided invaluable insight into the regional character of the Middle Trinity Aquifer. Additional work is recommended to improve the understanding of the local complexities of the carbonate aquifer, the local occurrence and movement of contaminated groundwater therein, and better define the horizontal and vertical delineation of the spreading contaminant plumes. A more detailed understanding is crucial to the decision-making process regarding responses to CSSA groundwater contamination. The following paragraphs identify current data gaps and concerns that should be considered for future work.
CSSA is implementing a three-pronged approach to control and reduce VOC contaminant migration into the underlying aquifer and toward possible receptors. The first step has included the partial excavation and removal of continuing source media (buried debris and contaminated soils) at SWMUs B-3 and O-1, and AOC-65. Additional excavation of remaining VOC source materials is expected in 2004-2005. The second tier of remedial activities will include the expansion and enhancement of existing soil vapor extraction (SVE) systems at both B-3 and AOC-65. The intention of the SVE systems is to remove the bulk of contaminant residuals in the complex vadose intervals of underlying fractured bedrock. A potential final step toward source removal at the selected sites is to design and implement an innovative approach to sustain and possibly enhance the biological attenuation characteristics within the plumes.
An increased rate of biologic attenuation may be attempted through a combination of fast-acting and slow-release carbon sources, which would add the necessary substrate to sustain or enhance the natural biological degradation that is already occurring in the bedrock and groundwater regimes. A substrate could be applied through an initial flush of the system, such as lactate to rapidly energize the system, and then additional substrate could be applied by backfilling the excavated areas with organic mulch blended with vegetable oil, or other food-grade oils. During recharge events, the backfilled substrate would passively deliver dissolved carbon into the underlying fractures and ultimately into the aquifer.
5.2.2 Installation of New Wells
Parsons recommends installation of 8 to 10 new MWs. The majority of the new MW locations would be within CSSA, and possibly three MWs off-post. LGR monitoring points are recommended to fill data gaps in the North and East Pastures, the northwest section of the Inner Cantonment, the central portion of CSSA and along Salado Creek, and in the southeast part of the post. Also advised is an off-post well between CSSA and Jackson Woods, and possibly multiple wells outside the southwest corner of CSSA in the Leon Springs area/Interstate 10 area. Geophysical logging and optical televiewing of any new boreholes are strongly recommended.
Past well installation experience is showing that the use of cement-based annular seals can be problematic such that the pH of surrounding groundwater is significantly altered. It is postulated that excessive cement is lost to voids and fractures that surround the wellbore, and result in a continuing source of high pH groundwater. As the grout slowly cures, the chemical reaction between the groundwater and grout result in alkaline groundwater with a pH range between 10 and 13. Remedies to alleviate the high pH of groundwater include prolonged and repeated well development to stimulate the chemical reactions between the groundwater and Portland cement are complete, or the implementation of alternative methods annular seals.
Prolonged well development has been performed at several wells during TO42 and has yielded mixed results. Those wells (CS-MW1-BS, CS-MW2-LGR, CS-MW12-BS, CS-MW12-CC, CS-MW18-LGR, CS-MW19-LGR) with elevated pH in groundwater resulting from the cementation of the well seal or nearby cluster well appear to only be temporarily relieved of extreme pH conditions. Parsons is recommending that an alternative sealing method which would involve the use of a bentonite-based slurry (Volclay�) in lieu of a cement-based slurry. More time and cost is expected to be incurred from the alternative sealing method, but would mitigate the elevated pH condition within wells.
5.2.3 Multi-Port Monitoring Systems
Ideally, four multi-port LGR monitoring systems, with up to 11 monitoring zones each, are recommended. The use of multi-level sampling wells would be used to collect long-term vertical profiling with respect to contaminant concentration, differential recharge, and hydraulic head variations within the aquifer. This study could also attempt to address structural control and how that relates to contaminant migration. Areas that should be considered for multi-port monitoring systems include the CS-MW16 area, Salado Creek, and in the fault blocks at the southern portion of CSSA. Multi-port wells are also ideal for off-post monitoring, since they provide full vertical profiling with a minimal drilling footprint.
5.2.4 Automated Data Collection
An eventual numerical model calibration will require accurate water level and water quality data collected in a short �snapshot� timeframe. An expanded program of automatic recording and logging of these data over a long-term period would be beneficial for monitoring the behavior of the hydrogeologic units underlying the CSSA area. Recording units with telemetry capability offering real-time data acquisition and downloading from remote locations should be sought. The system should allow both contractor and client quick access to the data, and be compatible with other existing and planned data collection systems.
5.2.5 Collection of Off-post Data
It is recommended that off-post monitoring for contaminants be continued using the existing public/domestic well network already developed around CSSA. Residents with new wells should be invited into the network. The government should continue to encourage the public to inform CSSA of water supply quantity or quality problems, and when well improvements or routine well maintenance is performed. Offering to perform services such as video and geophysical logging at no expense to the owner could be very beneficial to CSSA. In addition, when possible, a water level measurement apparatus installed at off-post locations is desirable when appropriate opportunities present themselves.
The off-post groundwater monitoring program should continue in accordance with the Off-Post Monitoring Program Response Plan.