Meeting Summary
Monitoring and Assessment of the SVE System at SWMU B-3
Camp Stanley Storage Activity
F11623-94-D0024/RL74
Parsons ES 734521.05

Date:  24 August 2000

Time:  2:30 P.M. - 4:00 P.M.

Place:  Camp Stanley Storage Activity, Boerne, Texas

Subject:  SVE System Operation Findings and Recommended Changes

            Attendees:

This summary was prepared by Brian Vanderglas, Parsons ES.

Introduction

    The meeting commenced at 2:30 P.M. in the CSSA conference room. The primary discussion topics included (1) condition of 4 vapor extraction wells (VEWs) that appeared to be short-circuiting to the surface, (2) planned percolation testing with the Hermit datalogger, (3) change in operations and maintenance routine to increase flow rates at the main exhaust line, (4) important measurements and unusual readings measured during the bi-weekly system checks, and (5) comparison of results to SVE treatability study results.

Discussion Topics

Condition of Short-circuiting VEWs

    Free flow conditions were discovered from four VEWs (VEW-8, VEW-11, VEW-14, and VEW-18) during the past two system checks (July 27 and August 10, 2000).  After completing the August 24 system check, Brian Vanderglas and Kyle Caskey attempted to identify the source of the short-circuit by flooding the surface around each of the VEWs with approximately 100 gallons of water while injecting air into the affected VEW.  Water penetrated VEW-08 and VEW-14 immediately, indicating that the short-circuit is located around the well casing and annulus at these locations.  Kyle indicated that he attempted to re-seal VEW-08 by adding another bag of bentonite and slurry during the resampling event, but was obviously unsuccessful.  The depression noted around the top of the borehole around VEW-08 suggests that the seal and slurry may be lost to subsurface voids within the landfill trench.  Although not considered to be directly related, it is conspicuous that the drilling of the resample borings were completed at the site shortly before these problems were observed. 

    Also of note, the three VEWs on the south side of the system layout have all historically maintained high vacuum pressures on their respective VEWs and produced relatively low flows.  It was also considered that the high vacuum, coupled with the hot and dry conditions experienced this summer, could be a factor in the initiation of a short-circuit.  No air bubbles were observed in VEW-11 and VEW-18 within a radius around each well of at least 20 feet, or in the vicinity of the adjacent resampling borehole (grouted).  The three VEWs on the south side of the trench are also adjacent to each other, such that the cause of the short-circuits may be geographical.  One suggestion tendered was that exposed subsurface fractures or voids that were previously sealed were opened to the screened interval of the VEWs by a combination of heat, drilling within 5 feet of each VEW, and high extraction vacuum pressure exerted on the formation.

    Due to the discovery of the short-circuited VEWs, the recommendation in the Interim Report to shut down flow from VEW-07 and VEW-18 was modified.  Instead, flow from all four short-circuiting VEWs will be closed, and checked during the bi-weekly checks to monitor changes on soil gas and to determine whether pressure changes can be observed if rainfall ever begins to fall again, and moisture content in the soil increases appreciably.

Planned Percolation Tests

    The planned percolation test will be adjusted to include two of the closed VEWs rather than VEW-07 and VEW-18.  A two channel Hermit data logger will be used to record water level information within the landfill trench at SWMU B-3 after CSSA receives some significant rainfall this fall.  There was considerable discussion about whether it was more important to gather data from inside the trench, or to attempt to gather data from VEW-04/VEW-05 which are screened entirely across the limestone.  There is some concern that it would not be possible to shut down one of the two limestone VEWs without experiencing significant pressure interference from the apparent subsurface connections during the percolation test.  Due to this concern, we are leaning toward monitoring two of the VEWs with short-circuiting.  The recommended VEWs are VEW-18 and VEW-08.  The selection of VEWs for the percolation testing is still open for discussion, as this activity will not be completed until significant moisture content returns to the site’s soil. 

    The percolation test was included in the SVE study to collect relatively inexpensive data regarding precipitation recharge while performing the bi-weekly system monitoring.  The data will be presented in an appendix of the final report, but will not be discussed or evaluated in any great detail in the report.  This data will have more relevance to groundwater modeling efforts, and planning for future data collection needs than to the SVE system.  The only area where water levels in the trench may affect SVE performance would be the reduction in the effective screened intervals for each of the VEWs.  As appropriate, this relationship will be evaluated in the report, assuming it rains in the next 4 months.

Adjustments to Operations and Maintenance (O&M) Protocols

    One of the major observations noted in the interim report was the general decrease in airflow rates over time observed at several of the VEWs and at the main exhaust line.  The Standard Air Exemption allows flow rates as high as 100 cfm, so flow rates were increased at each VEW to approximately 500 feet per minute, which improved the flow rate at the main exhaust to 4,500 feet per minute (or approximately 98 cfm).  During each bi-weekly system check, the flow rates at each VEW will be adjusted to 500 feet per minute, or maximum attainable rate prior to collecting readings from each VEW.  This should result in greater contaminant removal rates during the final few months of operation under Delivery Order RL83.

Important Measurements and Unusual Readings

    The achievable flow rate is limited by the Standard Exemption for air emissions allowed from the SVE system.  The maximum removal rates observed were 0.0256 pounds per hour (lb/hr) compared to the 16.875 lb/hr that are permitted under the exemption.  The maximum levels of vinyl chloride were measured at 0.0006 lb/hr compared to the exempted level of 0.25 lb/hr.  Assuming uninterrupted operation of the SVE, the total TCE removed from the trench would be approximately 68 pounds over a one-year period.    The bottom line was that the air emissions data collected through the first two sampling events confirms that our emissions are significantly below the allowable rates established in our registration.

    Some discussion also focused on Table 3.4 in the interim report, which details results from monthly soil gas measurements.  Of particular interest were the fluctuations of the oxygen readings over time.  As anticipated, most of the anaerobic (oxygen less than 5%) points saw increases in oxygen levels after startup of continuous SVE as air displacement causes soil air with higher oxygen levels to migrate toward the extraction wells.  In some cases, the oxygen levels actually increased, then decreased, with no pattern noted through all VEWs.  The fluctuations could be related to changes in the flow rate, and hence, changes in the area influenced by each VEW.  Rainfall that occurred in June could have also affected the area of influence around each VEW by adding soil moisture to the available volume of pore space. 

Comparison of O&M Results to SVE Treatability Study Results

    The results from the first five months of O&M were consistent with the findings from the pilot test and the expanded SVE treatability study.  The greatest removal rates continue to be from VEW-01 in the center of the trench and from the two VEWs screened in the native limestone adjacent to the trench.  A review of soil gas survey maps for the area showed the highest TCE levels in the general area represented by VEW-01.  The continued effectiveness of removal from the VEWs screened in the limestone provides optimism for use of SVE to contribute to removal of VOCs located within limestone fractures underlying the trench.  A lot of unanswered questions remain about the effectiveness of removal of VOCs from the limestone, but none of the data planned for the next few months of O&M will likely provide any answers. 

    One of the biggest questions is determination of a mass balance for contaminants disposed in the trench.  Based on soil data collected at the beginning of the expanded treatability study, the removal rates observed from the VEWs should have already removed most VOCs from the soil.   Yet the rate of removal appears to be fairly constant, suggesting that removal is occurring from both the trench and from the surrounding limestone.   Once a sharp drop in the removal rate is observed, we may be able to conclude that most of the VOCs not tightly bound to soil or debris have been removed from the trench, and that the bulk of the removal is related to vapors from the fractured limestone.  This would require more data (statistically significant) on the extent and concentrations of contamination remaining in the trench over a defined treatment period.  Of course, the mass of contaminants that are desorbed into solution during high water periods in the trench would also need to be determined to estimate the amount of contamination entering the underlying limestone.  Bottom line:  Lots of additional data would be necessary to accurately estimate the amount of time needed to remove the trench area as a potential continuing source of contamination to the groundwater.  This additional data collection is not in the current RL74 or RL83 budgets for SVE operations and maintenance at SWMU B-3.

Action Items

    Based on the discussions from the meeting, the following actions items were identified: