[Meeting Minute Index]

Technical Interchange Meeting (TIM) #6

For AOC-65

January 20-21, 2003 - Meeting Minutes


January 20-21, 2003


1:30 - 5:00 pm (1/20) and 8:30 am - 5:00 pm (1/21)


CSSA, Boerne, TX


TIM #6 to current status of Area of Concern (AOC)-65 soil vapor extraction (SVE) treatability test; discuss the methods for evaluating the EpiKarst in the vicinity of AOC-65; review Data Quality Objectives (DQOs) for shallow groundwater sampling at AOC-65; and, discuss the results of Parsons' evaluation of borehole flowmeters.

The meeting was  on January 20-21, 2003.  The following were in attendance:




Brian K. Murphy, CSP

CSSA, Environmental Officer

(210) 698-5208

Jeff Aston*


(210) 295-7451

Chris Beal*

Portage Environmental

(210) 295-7417

Ross Miller


(801) 572-5999

Brian Vanderglas


(512) 719-6059

Julie Burdey

Parsons ES, contractor

(512) 719-6062

Scott Pearson


(512) 719-6087

Kyle Caskey


(210) 805-6229

Gary Cobb**


(512) 719-6011

* Jeff Aston and Chris Beal were present on January 21, 2003 only. 

** Minutes prepared by Gary Cobb, Parsons.

Meeting Agenda

These meeting minutes are organized in the order discussed.

Current Status of SVE Treatability at Building 90/AOC-65

The meeting was opened with a discussion of the current status of the soil gas treatability study at Building 90 and AOC-65.  In November 2002, Parsons initiated the startup and testing of the Building 90 Subslab Soil Vapor Extraction (SVE) system and the AOC-65 SVE system and preliminary results were discussed. 

Startup of the Building 90 subslab system was initiated on December 3, 2002.  The system was operated for eight days before being shut down so that the recovery of volatile organic compounds (VOCs) beneath the floor slab could be monitored.  Soil gas samples were collected on January 14, 2003 and screened with field instruments to assess the concentrations of VOCs  that have accumulated in the soils beneath the building for comparison to VOC concentrations from previous testing events.

Results of the Building 90 subslab startup tests indicated a decrease in the removal rate over time during the initial eight days of operation.  The only extraction wells where concentrations of VOCs recovered to near prior concentrations were located in the vicinity of the former vat.  Elsewhere beneath the building the VOC levels recovered much less.  CSSA will consider limiting the extraction for the SVE system to the vat area where elevated concentrations were observed in the recovery test to maximize VOC removal rates.   Additionally, given the limited recovery of VOCs beneath the building observed, continued treatment of the effluent from the subslab system with granular activated carbon (GAC) was determined to unnecessary.

Startup of the AOC-65 SVE system was also initiated on December 3, 2002.  However, due to high water levels in the extraction wells employed for the initial SVE testing,  the system overheated and shut down.  Currently the concentrations observed in the extraction wells and the vapor monitoring points are quite low, while the flow rates are high.  Positive pressures have been observed in the wells screened to the deeper intervals and are believed to be the result of overpressure within the bedrock due to rising groundwater levels in those intervals.   Water in the shallow, near surface screened intervals of the extraction wells influences the effectiveness of the system.  Parsons will do further evaluation of the SVE effectiveness once the groundwater drains from the shallow bedrock material.  If significant levels of contaminants are not observed in the shallow bedrock, then Parsons will evaluate the need to add SVE extraction wells to remove soil gas to the water table.       

Groundwater Sample Results

The results of the Monitored Natural Attenuation (MNA) groundwater sampling conducted in September 2002 were reviewed.  Results of the dissolved oxygen (DO) analysis revealed extremely low DO concentrations in the vicinity of SWMU B-3 and AOC-65.  Given the fractured nature of the bedrock unit, the DO concentration in fractured rock units would normally be expected to be high due to rapid recharge and movement of groundwater.  Low DO concentrations may be an indication of biological activity and may suggest that biodegradation of dissolved contaminants is occurring.  Based on the DO concentrations it appears that there is natural biological activity occurring at SWMU B-3 and AOC-65 which suggests that NMA may be an appropriate remedial strategy for CSSA.   A true representative background well is required to determine what the naturally occurring DO levels should be for the groundwater present in the aquifer underlying CSSA.

Groundwater sample results for the other MNA parameters were reviewed and indications of biological activity were not as evident as with the DO results.  Correlations with the DO results were inconsistent and did not provide clear indications as to the biological degradation processes occurring at the base.  Parsons currently plans to add field monitoring for dissolved oxygen during future groundwater sampling activities.  Parsons will also evaluate the benefit of analyzing groundwater samples for ethane and ethane in future sampling events, and will consider evaluating an appropriate location for a future background well location.

Applicability of Results to CSSA Restoration Program

The results of the site investigations, SVE treatability tests and MNA sampling were discussed with respect to the applicability to CSSA’s base-wide restoration program.  The purpose of this assessment was to identify areas where additional efforts may prove beneficial to CSSA’s program. 

Based on the groundwater sample results, MNA may be an applicable remedial strategy for the post.  Biological activity appears to be occurring in the vicinity of B-3 and AOC-65.  In order to demonstrate that MNA is an effective remedial approach CSSA must prove that the groundwater plumes are not expanding.  CSSA’s current approach of providing well-head treatment to the impacted off-post wells is an effective means of managing the off-post groundwater plume and minimizing potential exposures.  Remediation of source zones will prove helpful in stabilizing the dissolved groundwater plumes.  The regulatory community will require CSSA to demonstrate that the contaminant plume is stable and/or decreasing.  A significant amount of data that is scheduled to be collected for the quarterly groundwater monitoring program can be used to provide evidence to support this demonstration along with MNA parameters.

Based on the recent SVE observations, the levels of contamination identified in the shallow bedrock material at AOC-65 do not appear consistent with the anticipated contaminant levels based on the size of the groundwater plume attributed to this unit.  The current distribution of VOCs in the groundwater migrating from AOC-65 may be explained if DNAPLs have migrated to the groundwater.  In order for the source zone remedial activities at AOC-65 to be effective, additional investigation may be warranted to assess if DNAPLs have migrated to groundwater and determine the depth of migration.  Based on the discussions, it is anticipated that this additional investigation can be accomplished with one boring placed in the vicinity of the former vat at AOC-65.   

SWMU B-3 was identified as a additional potential source area for VOCs in the groundwater associated with the plume in the central portion of the post.  The background of B-3 was reviewed and a visit to the unit was conducted to familiarize attendees with the surface topography and layout of the trenches relative to pertinent drainage features.  Removal efforts were initiated at this unit to address impacted soils and fill material from the former disposal area, but were discontinued after encountering more soils characterized as hazardous for TC-VOCs than anticipated.  Based on the review of site conditions, it was decided that additional remedial efforts utilizing SVE on the fill material and fractured bedrock may result in significant reductions in the levels of VOCs entering the groundwater regime and could also render contaminated soil scheduled for removal nonhazardous.  Limited investigations (soil gas screening and possibly surface geophysical techniques) should be preformed prior to the installation of the SVE system and should be designed to assess the vertical distribution of VOCs in the trenches and fractured rock, and to obtain a more specific geologic description of the formation underlying SWMU B-3.  

Evaluation of Borehole Flowmeters

Parsons conducted a desktop evaluation of borehole flowmeters that could be used to monitor groundwater flow rates and directions at specific subsurface locations near AOC-65.  Parsons contacted professionals at the University of New Mexico and the U.S. Geological Survey in Storr’s Connecticut to obtain direct information regarding the flowmeters.  Based on the information obtained, the heat-pulse flowmeter and the electromagnetic flowmeter are best suited for use at the base.  Additional flowmeters types were identified, but were determined to be unsuitable for the site.  The heat-pulse flowmeter is capable of measuring very low flows and has a lower measurement range than the electromagnetic flowmeter.  Given the potential for high flows through faults and Karstic zones, the electromagnetic flowmeter were recommended if deemed necessary by development of shallow groundwater DQOs.

Flow measurements using the electromagnetic flowmeter can be conducted in an open borehole before the well material is installed to log the flow conditions in the vicinity of the borehole.   Electromagnetic flowmeter logging can also be conducted in the screened interval of installed wells to determine inflow into the well.  No information was found regarding permanent placement of electromagnetic flowmeters in the subsurface to provide real-time long term monitoring of flow rate and direction. However, if measurements are desired over an extended time interval, Parsons anticipates that this can be accomplished using inflatable packers in an open borehole.

Another method identified for measuring inflow into to borehole is hydrophysical logging.  In this method, the borehole is filled with deionized water and the inflow of formation water is monitored using a set of conductivity probes.  The rate of inflow into the borehole from individual zones can be estimated using the conductivity measurements and simple mixing equations.  This method has been proven to allow assessment of flow rates in discrete interval of the borehole, but it does not allow determination of flow direction.

The purpose and use of the borehole flowmeter measurements was discussed in context of the overall scope of the CSSA regulatory program.  Borehole flowmeters and hydrophyscial testing provide information regarding the location, rate and direction of discrete zones of groundwater flow associated with fractures and conduits intersected by the borehole.  However, since CSSA’s current approach to managing contaminated groundwater consists of well-head treatment and source zone remedies, it was determined that the borehole flow measurements will not provide information that will change CSSA’s current approach.  Therefore, it was decided that the use of borehole flowmeters at the base is not justified in light of the current regulatory strategies, but may be re-evaluated for possible use in the conceptual model if deemed necessary.

Review of DQOs for AOC-65 Treatability Study

CSSA prepared draft DQOs to address shallow groundwater sampling activities conducted as part of the overall groundwater investigation at AOC-65.  Shallow groundwater has accumulated in the shallow extraction wells west of Building 90.  CSSA decided to sample this shallow groundwater to assess the distribution and migration of contaminants through this material. The draft DQOs prepared by CSSA presents suggested protocols (triggers, frequency, test parameters, etc) for the sampling of the shallow groundwater from the SVE extraction wells. 

The draft DQOs were reviewed and it was decided that selecting the appropriate time to collect shallow groundwater samples following a recharge event (i.e., rainfall) is crucial to gathering meaningful data.  To determine the appropriate sampling time, it was decided that conductivity probes will be installed in select piezometers/monitoring wells to evaluate conductivity changes in different geologic zones following a recharge event.  This information will then be used to develop a schedule for collection of groundwater samples to assess the maximum concentration of VOCs partitioning from the rock matrix and transported to the water table during recharge events.  Parsons currently is contracted to provide water quality monitoring probes under TO 0042, so Parsons will evaluate whether  these probes can be utilized for the conductivity study at AOC-65 prior to be deployed under TO 0042.  Additionally, Parsons will evaluate procurement one telemetry system to be connected to the AOC-65 weather station so that can be downloaded during a rainfall event to determine if conductivity monitoring should be initiated.

Action Items

Parsons will evaluate the possibility of limiting the SVE extraction beneath Building 90 to those extraction wells in the vicinity of the former vat. 

Parsons will evaluate the effectiveness of the AOC-65 SVE system when the shallow groundwater levels decline and determine if additional investigations are warranted to assess the presence of DNAPLs.

Parson will procure water quality probes under TO 0042 earlier than required so that they may be utilized in an evaluation of groundwater conductivity changes following recharge events to aid in the determination of appropriate sampling schedules for the shallow groundwater at AOC-65.  Additionally, Parsons will evaluate and make a recommendation for procurement of a telemetry system for the AOC-65 weather station to facilitate downloading of rainfall and water level data during rainfall events.