[Home] [Master Table of Contents]
Well Installation Report - Wells CS-MW3 through CS-MW10, August 2003
Section 5 - Conclusions and Recommendations
Fifteen groundwater monitoring wells were installed between November 1999 and September 2001 at CSSA under two DOs. The effort resulted in eight LGR wells, two BS wells, and five CC wells. Soil/rock samples collected from retrieved core and groundwater samples from open holes and discrete intervals were analyzed to monitor for any environmental impact due to past hazardous waste operations. Each location was geophysically logged and injection packer testing was performed. Subsequently, wells were installed in the production intervals of the Middle Trinity Aquifer (and the BS). Low-flow bladder pumps were installed in each of the new wells, and transducers were installed in four wells. Observations for the drilling program led to the following generalized conclusions:
Based on soil sampling results, the data indicate that contamination in the deep bedrock materials of the Middle Trinity Aquifer appears to be minimal within the rock matrix itself. No soil/rock samples submitted for analyses yielded contaminant results above their respective RLs. Very low levels only slightly above MDLs were reported in any of the borings for toluene and methylene chloride. These results may suggest that the bulk of subsurface soil/rock contamination is likely limited to and residing in secondary porosity volumes such as faults, fractures, and karst features not represented by the submitted samples. Due to the vertical orientation of faults and fractures, some contaminants have apparently migrated downward to significant depths. Evidence for contamination at depth is seen during drought conditions, where detectable quantities of contaminants are found even when groundwater elevations are quite low.
To better characterize the hydrologic profile in the vicinity of a known source area, discrete interval groundwater samples were obtained from wells near AOC-65 (Building 90 and vicinity). These included wells CS-MW7-LGR, CS-MW7-CC, CS-MW8-LGR, CS-MW8-CC, and CS-MW10-CC. Results indicate that increased concentrations of PCE, TCE, cis-1,2-DCE, and toluene were present in upper portions and/or perched waters of the LGR Limestone. Concentrations up to 57 �g/L of PCE, 20.5 �g/L TCE, 0.57 �g/L of cis-1,2-DCE, and 14.2 �g/L of toluene were encountered at three or more intervals at the CS-MW8 cluster location. Lesser concentrations of the same compounds were also encountered at CS-MW10 and CS-MW7 locations. A single detection of MEK (15 �g/L) and two detections of acetone (50 �g/L) were also reported at the CS-MW8 location.
The discrete interval packer test data strongly indicate that much of the residual contamination occurs in the upper 300 feet of the LGR Limestone. It was demonstrated consistently across the southwest portion of the facility that contaminant levels generally decreased to less than 1 �g/L below 300 feet bgs once the main production zone of the aquifer was penetrated. These results indicate that contaminants may be attenuated naturally by dilution and dispersion in the basal 60 feet of the LGR production zone. It is hypothesized that, regionally, within the LGR, the basal portion of the limestone yields most of the groundwater available from the formation. Depending on recharge conditions, upper water bearing zones in VOC source areas may also contribute significant well discharge at greater contaminant concentrations. Hence, well construction factors (e.g., casing depth) may play a critical role in the overall contaminant concentration present in a well.
Qualitative data regarding the character of the aquifer was obtained by performing injection packer tests in each member of the Middle Trinity Aquifer. As expected, results for the tests conducted indicate that the BS is quite impermeable (averaging 6 x10-7 ft/second), with the LGR and CC members being six to 11 times more permeable, respectively. However, it should be noted that the Middle Trinity Aquifer can be quite variable, and therefore it would be inappropriate to apply these generalizations area-wide.
While the two BS wells installed produced groundwater, the low permeability values suggest the unit acts as a confining layer for both the LGR and CC Limestones at CSSA. This position is further substantiated by direct observation of lithologic coring, the lack of groundwater produced during drilling, the inability to either inject or withdraw groundwater during packer tests, and very slow recharge during well development. However, hydraulic communication between the LGR and CC Limestones cannot be completely ruled out because of the potential for �leakage� due to structural features or open borehole well completions. Long-term monitoring at well cluster CS-MW9 has demonstrated a propensity for the CC to exhibit a lower hydraulic head than the LGR and BS, possibly indicating the potential of a small downward component of groundwater flow.
As expected, the uppermost water-bearing unit, the LGR, is the primary portion of the aquifer impacted by past management practices of hazardous materials. To date, routine groundwater sampling of BS and CC wells show minimal, if any, groundwater contamination within these lower strata. As learned by the packer tests, not all portions of the aquifer yield water. While the LGR tends to behave under a water table condition, a significant portion of carbonate matrix is not saturated, or does not freely yield groundwater. Zones of secondary porosity (dissolution of fossils and vugs) or structural features, transmit the majority of available groundwater. The CC has the characteristic of being more permeable than the LGR, and is also under confining conditions. However, the overall water quality of the CC tends to be less desirable then the LGR because of increased sodium, chlorides, and sulfates.
This phase of the groundwater investigation provided valuable insight into the character of the Middle Trinity Aquifer and the behavior and occurrence of contaminated groundwater. However, additional work is recommended to further understanding of the complexities of carbonate aquifers. The following paragraphs identify current data gaps or concerns that should be considered for future work.
Upgrading older existing monitoring wells and inactive agricultural wells to better fit the monitoring approach is recommended. Depending on the location, some older wells are in need of surface re-completions, low-flow sampling pumps, and/or casing and screen. Multiple existing wells have open-hole completions open to large portions of the aquifer, or have no record of completion method or depth. It is recommended that wells CS-MW1 and CS-MW2 be re-completed with 3-inch casing and screen to be consistent with other CSSA monitoring wells. Wells CS-D, CS-2, CS-3, and CS-4 do not have sufficient annular space to be upgraded. Inactive agricultural wells require technical data and rehabilitation. Both CS-G and CS-H should be geophysically logged and surface pads replaced. CS-G should be upgraded with a low-flow pump for sampling, and CS-H can be rehabilitated to provide groundwater for agricultural use.
The BS and CC portions of Well CS-16 should be plugged and abandoned to prevent further cross-contamination between water-bearing units. At this time, it is recommended that the remaining open hole be completed with the existing 200 feet of surface casing already present. This will allow for the reinstallation of both the high capacity and low-flow pumps that have been used at CS-16.
Recommended well upgrades are currently being conducted under task order TO0042 of contract F41624-00-D-8024. The effects of the on-post upgrades may reveal vital information regarding the character of contaminants in the aquifer, and provide insight as to the effect that well upgrades may have at off-post locations.
5.2.2 Installation of New Wells
Parsons is recommending installation of new wells at locations that either complete an existing well cluster (or pair) and at portions of the post that require further horizontal delineation of the contaminant plume(s). In addition to the CS-16 upgrade, a CC well at that same location should be considered. A CC well is also recommended at CS-MW1 and CS-MW2 to obtain more structural and hydrologic data. Additional plume delineation between CS-16 and the potable water well field (CS-9, CS-10, and CS-11) could be accomplished by an additional three-well cluster (LGR, BS, and CC) to the west of the H&I munitions storage area. Data gaps with respect to the horizontal delineation of the northern plume area (Plume 1) and data gaps identified by preliminary CSM efforts can be filled by additional LGR wells within the Inner Cantonment and East Pasture. Finally, off-post wells in the vicinity of RFR-10 can address the vertical extent of contamination detected in off-post wells in the vicinity of
Under TO0042 (contract F41624-00-D-8024), 16 additional wells are being installed (including four multi-level wells). Six additional wells are being installed in the Glen Rose Limestone, two additional wells are being installed in the BS, and four additional wells are being installed in the CC Limestone. Three multi-level wells will be completed within the Glen Rose Limestone, and one multi-level well will be completed throughout the entire thickness of the Middle Trinity Aquifer. At the conclusion of the TO0042 drilling activities, there will be 45 wells in CSSA�s monitoring network.
5.2.3 Vertical Profiling of Contaminants/Recharge Study
Current data in the vicinity of Building 90 (Plume 2) indicate that vertical delineation of contaminant occurrence in the upper portion of the aquifer (0 to 300 feet bgs) requires additional profiling and study. The use of multi-level sampling wells can be used to collect long-term vertical profiling with respect to contaminant concentration and hydraulic head variations within the aquifer. This study could also attempt to address structural control and how that relates to contaminant migration. Implementation of this technology will address the impact of contaminants to off-post potable wells.
The multi-level sampling wells should be incorporated into an extended recharge study driven by precipitation events. A flux of groundwater and contaminants can be monitored as they are pushed through the aquifer by recharging meteoric waters. A set of guidelines can be established which dictate when and where samples and pressure readings will be obtained during recharge or drought conditions.
These recommended actions are currently underway as part of work being conducted under TO0042 and TO0058 (contract F41624-00-D-8024). Four multi-level wells are being constructed; three in the vicinity of Plume 2 near AOC-65, and the fourth will be off-site west of AOC-65. A recharge study which incorporates data from the Building 90 weather station/rain gauge will be conducted in the vicinity of AOC-65 as part of TO0058.
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. Several vendors offer downhole probes that monitor various parameters including groundwater pressure head and conductivity data. The automatic recording and logging of these data over a long-term period would be beneficial for monitoring the behavior of the aquifer units below the post. Recording units with telemetry capability offer real-time data acquisition and downloading from remote locations which allow both contractor and client to obtain and use the data.
Transducers with automatic data recorders are currently being installed in 14 wells under TO0042.
5.2.5 Collection of Off-post Data
Continued public outreach fosters goodwill, trust, and invaluable data regarding the aquifer. It is recommended that off-post monitoring for contaminants be continued using the existing public/domestic well network already developed around CSSA. Furthermore, the government should encourage the public to inform CSSA when routine well maintenance is being performed. Offering to perform services such as video and geophysical logging at no expense to the owner would be beneficial. In addition, any water level measurement apparatus (gauging tube or air line) that can be installed in off-post wells would be greatly appreciated by the government.
As discussed in Section 5.2.3, one multi-level well will be installed off-site under TO0042, and the off-post groundwater monitoring program currently continues in accordance with the Off-Post Monitoring Program Response Plan.