[Home] [Master Table of Contents]

[Table of Contents] [Next Section]

AOC 65 Treatability Test Plan

Section 3 - Study Design and Construction

3.1 - Groundwater Recharge Study

A groundwater recharge study will be conducted at AOC 65 to assess the nature, rate, and extent of groundwater recharge and flow in the unsaturated zone beneath the site. The groundwater recharge study data will assist in the determination of potential contaminant flow paths and migration rates in soil and rock beneath AOC 65 and determine temporal variations important to SVE effectiveness.

3.1.1   Piezometer Design

Six nested piezometers will be constructed as part of the groundwater recharge study to be conducted at AOC 65. Results from geophysical surveys conducted at AOC 65 and around Building 90 were used to assist in locating piezometers along groundwater flow features. Proposed locations of the piezometers to be installed as part of the groundwater recharge study are shown in Figure 3.1.

Each piezometer will be completed with a discrete 25-foot screened interval selected to provide complete coverage of the entire upper 150-foot profile of the unsaturated formation. The piezometer casing will be constructed of 4-inch diameter, 0.020-inch PVC slotted screen with 4-inch diameter PVC riser. Each piezometer will extend approximately 1.5 feet above land surface and include a 12-inch diameter protective cover. The piezometers will also include a 1-inch diameter, below-grade PVC pipe to accommodate installation of electrical monitoring equipment. The pipe will house the piezometers’ transducer cable and connect the piezometer to the weather station.

To avoid potential cross-contamination between saturated and unsaturated intervals, piezometers deeper than 100 feet will only be installed if the water table is deeper than 150 feet below grade during drilling. Each borehole will be continuously cored and lithologically logged. To assist in the determination of optimum screened intervals for piezometers and VEWs to be installed as part of the AOC 65 SVE System, an injection packer test will be performed in the open borehole during installation of the deepest piezometer (150 feet bgl). The AOC 65 SVE System is discussed in Section 3.2.2 of this report.

3.1.2   Piezometer Completion Details

The annular space of each piezometer will be filled with number 8-16 silica sand from the base of the borehole to a height of 2 feet above the top of the screened interval. The sand filter pack will be emplaced via tremie pipe from the base of the borehole to the top of the designated screened zone. The height of the filter pack will be monitored continuously during emplacement using a weighted measuring device.

A 100 percent sodium bentonite seal approximately 2 to 3 feet thick will be placed above the filter pack in each borehole. The bentonite will be dropped into the borehole from the surface at a rate sufficient to avoid clumping and bridging. The bentonite seal will be fully hydrated per the manufacturer’s specifications before grouting activities commence.

Beginning with small lifts, a Portland cement/bentonite grout mixture will be slowly pumped into the annular space using a side-discharge tremie pipe. The grout mixture will be mixed in the following proportions: 94 pounds of neat Type I Portland or American Petroleum Institute (API) Class A cement, not more than 3 to 5 pounds of 100 percent sodium bentonite powder, and not more than 7 gallons of potable water. A properly mixed batch of grout should yield 10.8 gallons per 94-pound sack of Portland at a density of 14.5 pounds per gallon. This proportion of grout yields approximately 1.45 cubic feet per sack of Portland cement. The volume of grout will be calculated prior to emplacement. The slurry will be injected until grout flows freely at the surface. The annular space will be checked periodically for settlement, and will be topped off as needed to no greater than 2 feet bgl. The grout will be allowed to cure for at least 48 hours prior to well development.

Figure 3.2 illustrates a typical construction detail of the nested piezometers to be installed as part of the groundwater recharge study. Table 3.1 includes piezometer identification numbers and proposed screened intervals for each of the six nested piezometers to be installed as part of the groundwater recharge study conducted at AOC 65.

Table 3.1 - Groundwater Recharge Study/Proposed Borehole Completion Summary

3.1.3   Weather Station Design and Installation

A weather station will be set up on the west side of Building 90 near the nested piezometers to continuously monitor weather and hydrologic conditions in the groundwater recharge study area. The weather station will be fitted with a rain gauge, and be connected to downhole transducers in each of the six nested piezometers. A solar powered data recorder capable of transmitting real-time data to the CSSA Environmental Office, either through telemetry or telephone lines, will be secured at the weather station to record readings from the rain gauge and each transducer. Data collected from the weather station will measure rainfall intensity and assess migration patterns and rates of water flow into the various screened intervals of each nested piezometer. The data will also assist in assessing the rate of vertical groundwater flow (percolation rate) relative to subsurface retention capabilities.

3.1.4   Tracer Testing

Tracer tests may be performed to obtain more specific information regarding travel time, retardation, and flush time of VOCs in the fractured limestone beneath the site. The University of Texas at San Antonio (UTSA) proposes to conduct SVE performance tracer tests to assess PCE/TCE migration following rainstorms.

The tracer test design will include placement of a canister containing tracers in one shallow monitoring well. For the next couple of months the adjacent monitoring wells will be bailed and sampled weekly to determine if the tracers have moved from the injection well to the adjacent well(s). A non-volatile, non-toxic tracer that sorbs to soil at about the same levels as TCE will be utilized. The tracer can be measured at extremely low levels (near 1 part per trillion) and will be tagged with the non-radioactive hydrogen isotope deuterium, which can also be detected at extremely low levels.

3.2 - Soil Vapor Extraction Systems

Two SVE pilot systems are proposed for installation at AOC 65 to evaluate the potential for SVE to reduce the levels of contaminants detected in soils at the site. One system will consist of installing extraction wells beneath the floor of Building 90 and will comprise the Building 90 subslab ventilation system. The second SVE system will consist of six VEWs and six multi-depth VMPs outside the building for use in the treatability study conducted at AOC 65. Building 90 subslab ventilation will be conducted in conjunction with the removal actions planned for AOC 65. Removal actions planned for AOC 65 are detailed in the Environmental Cleanup Plan for AOC 65, Parsons, April 2002.

3.2.1   Building 90 Subslab Ventilation System

Twelve borings will be installed inside Building 90 near the former metal vat, and in other portions of the building as determined from results of the soil gas survey conducted in 2001. The objective of these borings is to reduce the contaminant mass underlying the building without conducting excavation activities. However, if results from the Building 90 subslab ventilation system indicate that the system is not adequately removing contaminants, excavation of the Building 90 floor and the contaminated soils beneath it may be required.

Each subslab ventilation boring will be drilled through the concrete floor and into soils beneath the building prior to commencement of any potential excavation activities. The borings will be drilled using a drill rig equipped with solid stem augers and advanced until auger refusal is encountered. Each boring will be completed as a combination VEW/VMP that can be used to remove vapors from beneath Building 90 or serve as a soil gas sampling point.

Building 90 excavation activities will also include installation of up to 12 soil borings in areas outside Building 90. The exterior boring locations will be focused on the drainage ditch, near an abandoned drain outfall west of the building. Exterior soil boring data will determine the volume of soil within the ditch requiring excavation. The proposed soil boring locations and depths were based on soil gas survey results. The proposed locations of the Building 90 subslab ventilation system wells and exterior soil boring locations are depicted in Figure 3.1.

3.2.1.1   Combination VEW/VMP Design

Each of the 12 borings drilled through the Building 90 floor will be completed as a combination vapor monitoring point/subslab extraction well. The dual functionality of these wells will allow them to be used as extraction points for the subslab ventilation system as well as points for monitoring soil gas concentrations or pressure responses during subslab ventilation activities of the AOC 65 SVE treatability study conducted outside Building 90. The VMPs may also be used to evaluate the impacts of the ventilation system on preventing soil vapors from entering Building 90. To reduce potential exposures to CSSA employees working inside the building, VEW/VMP drilling activities will be scheduled during evenings and/or weekends. Additionally, air samples will be collected for VOC analysis during the drilling and subslab ventilation system installation activities inside the building to ensure that the work activities do not create potential exposure concerns for CSSA employees.

3.2.1.2   Combination VEW/VMP Construction

Each of the 12 subslab combination VEW/VMPs will be completed with 1-inch diameter, schedule 40 PVC casing and 1-inch diameter, schedule 40 PVC, 0.020-inch factory slotted screen. The screened interval will be 6 to 12-inches in length depending on the depth of the boring. Each combination VEW/VMP will be completed above grade and include a riser to the Building 90 ceiling rafters, and will be mainfolded to an extraction blower located on the east side of the building. The extraction wells will be located adjacent to support posts within Building 90 and covered with a metal plate so riser piping is protected from damage. Each subslab combination VEW/VMP will be completed per the procedures outlined in Section 3.1.2.

An additional access port will be included on each combination VEW/VMP to enable ventilation (extraction) or injection of amendments to enhance contaminant removal from beneath the slab. Well casings will be plumbed with access along the western wall of Building 90 or completed through the western wall to the exterior loading dock. In the event of a removal action inside Building 90, horizontal screen will be installed in the base of the excavation prior to backfilling and restoring the site. The horizontal piping will be plumbed to the same manifold as the subslab ventilation system. Horizontal well designs and construction detail will be developed if removal actions are deemed necessary.

The combination VEW/VMPs will be manifolded together with individual control valves to turn on and off the vacuum to each well. Each combination VEW/VMP will also be constructed with a pressure monitoring and sampling port so it can be used to measure pressure response and collect vapor samples when not being utilized as an extraction well. The flexibility of the system design will allow extraction from any or all of the wells and collection of data from the wells not involved in extraction. A typical combination VEW/VMP construction detail for Building 90 subslab ventilation system wells is shown in Figure 3.3.

3.2.1.3   Piping, Blower, and Installation

The Building 90 subslab ventilation system will consist of 12 combination VEW/VMPs, piping, one moisture separation vessel, and one blower. Final piping location, specification, and design will be determined during final installation of VEWs and based on site conditions encountered during field activities. The proposed location for piping associated with the Building 90 subslab ventilation system is included in Figure 3.4. A typical blower system instrumentation diagram for the Building 90 subslab ventilation system is included as Figure 3.5.

Each of the 12 combination VEW/VMPs will include a 1-inch PVC riser completed to the ceiling rafters, which will be manifolded to an extraction blower located on the east side of the building. Parsons will salvage the CSSA Gast R6 regenerative blower removed from the SVE System at SWMU B-3 for the Building 90 subslab ventilation system. The regenerative blower will be installed on the west loading dock of the building near an existing electrical panel.

The vacuum blower system will contain a moisture separation vessel, a pressure relief valve, and air emissions treatment. The moisture separation vessel will include a drain system and secondary containment. The exhaust from the blower will be connected to the exhaust from the AOC 65 system blower before passing through a 500-gallon vapor-phase carbon treatment unit. The treatment unit will be used to ensure that VOCs extracted from contamination beneath Building 90 are not discharged into the air. A cinder-block shelter (housing) will be constructed on the west loading dock of Building 90 to house the blowers and piping manifolds for the subslab ventilation and AOC 65 systems to protect the components and allow maintenance personnel easy access to the blower’s gauges and measurement ports.

3.2.2   AOC 65 SVE System

Six VEWs and six multi-depth VMPs will be installed for use in the AOC 65 SVE Treatability Study. The VEWs will serve multiple purposes in addition to their use as vapor extraction wells. The VEWs may validate findings from geophysical testing and groundwater sampling, or used to collect soil gas/pressure response data when not actively removing soil gas. In addition, VEWs may also provide additional insight on contaminant and groundwater movement into the lower saturated zones of the formation.

3.2.2.1   Extraction Well Design

Six VEWs are proposed for installation around Building 90 near the building’s former metal vat, drainage piping, and the associated drainage ditch on the west side of the building where soil gas data suggest the contamination is located. The VEWs will be located in areas of faults, fractures, and karst features as identified by the geophysical surveys. The proposed locations of the extraction wells are depicted on Figure 3.1 along with the locations of geophysical anomalies. However, exact VEW locations, depths, screened interval placement, and design will be determined based on geophysical data, lithologic observations, soil gas packer testing (Section 3.3.3), and other site conditions noted as field activities progress.

3.2.2.2   Extraction Well Construction

Each of the six VEWs will be constructed with 4-inch diameter PVC, 0.050 slot screen and 4-inch diameter PVC riser. Screened interval lengths will be limited to 25 feet, and each borehole will be continuously cored and lithologically logged. Each VEW will extend approximately 1.5 feet above grade and will include a 12-inch diameter lockable protective cover for security purposes. Each VEW will also include below grade PVC piping that connects the VEW to the SVE blower. Each VEW will be completed per the procedures outlined in Section 3.1.2. Table 3.2 includes the proposed well identification number and screened intervals for each of the six VEWs to be installed as part of the SVE system at AOC 65.

Each VEW will be constructed with a pressure monitoring and sampling port, so that it can be used to measure extraction flow rates and pressure response plus collect vapor samples when not being utilized as an extraction well. The flexibility of the system design will allow extraction from any or all of the VEWs and collection of data from the VEWs not involved in the extraction. A typical VEW construction detail for AOC 65 SVE System wells is shown in Figure 3.6.

Table 3.2 - AOC 65 SVE System/Proposed Borehole Completion Summary for VEWs

3.2.2.3   Piping, Blower, and Instrumentation

VEWs will be connected to the AOC 65 SVE System with 1.5-inch, schedule 80 PVC piping. Final piping material, diameter, length, and design will be determined during final installation of VEWs and based on site conditions encountered during field activities. The proposed location for piping associated with the AOC 65 SVE System is included in Figure 3.4. A typical blower system instrumentation diagram for the AOC 65 SVE system is included as Figure 3.7.

The vacuum blower system will include a moisture separation vessel, pressure relief valve, and air-emissions treatment. The moisture separation vessel will include a drain system and secondary containment. The blower exhaust piping will be connected to the piping for the exhaust from the subslab ventilation blower prior to entering a 500-gallon vapor-phase carbon treatment unit. The carbon absorption unit will be shared by the two system and will ensure that VOCs extracted from contaminated soils are not discharged to the air. A cinder-block shelter (housing) will be constructed on the west loading dock of Building 90 to house the blowers, piping manifolds and carbon absorption unit for the two systems to protect the components and allow maintenance personnel easy access to the blower’s gauges and measurement ports.

3.2.2.4   Vapor Monitoring Point Design

Six multi-depth VMPs will include up to four distinct nested screened intervals per boring and will extend to a maximum depth of 100 feet bg. The six VMPs will be installed on both sides of Building 90, with most installed on the west side near the building’s former metal vat, drainage piping, and the associated drainage ditch. The VMPs will be installed in faults, fractures and karst features identified in the geophysical surveys and through core evaluation. The planned locations for the VMPs are depicted on Figures 3.1 and 3.4. Final locations of VMPs will be selected based on geophysical testing results, field conditions, and final locations of VEWs.

3.2.2.5   Nested Vapor Monitoring Point Construction

Each of the VMPs will be constructed with 5 to 10-foot lengths of 1-inch diameter, 0.020-inch slotted, schedule 40 PVC screen and ¼-inch inside diameter, schedule 80 PVC riser. Each borehole will be continuously cored and lithologically logged. Each 8-inch diameter VMP borehole will contain four screened intervals within a flush-mounted, 8-inch diameter water tight well box. Each screened interval will be completed with a sampling port at the top of casing for monitoring soil gas from a discrete depth interval. Each of the four screened intervals will be separated from the zone above and below by a minimum of one foot of number 8-16 silica sand and five feet of bentonite. Each VMP will be completed per procedures outlined in Section 3.1.2.

Table 3.3 includes the proposed well identification number and screened intervals for each of the 24 VMPs to be installed as part of the SVE system at AOC 65. A typical VMP construction detail for AOC 65 SVE System wells is shown in Figure 3.8.

Table 3.3 - AOC 65 SVE System/Proposed Borehole Completion Summary for VMPs

3.3 - Analytical Sampling

3.3.1   Groundwater Recharge Study

Natural attenuation of chlorinated hydrocarbons is likely occurring at AOC 65 given the presence of PCE and TCE breakdown products in groundwater and in soil gas samples collected near Building 90. To assess the potential contribution of natural biological reduction, groundwater samples from each of the six piezometers will be tested for natural attenuation indicating parameters. The piezometers may also be sampled for ethane, ethene, and methane by analytical method RSK-175 to establish baseline groundwater conditions.

After a baseline is established, groundwater will be collected from each piezometer following significant rainfall events (at least greater than 1.0 inch or as indicated by transducer data) and analyzed for VOCs by SW-846 method 8260. Samples are initially proposed for collection approximately 24 hours after the rainfall event begins, and then again after 48 hours and 96 hours. The proposed sampling intervals will be modified if appropriate based on previous testing results, rainfall intensities, etc. If the opportunity arises, VOC samples may also be collected from AOC65 VEWs over several rainfall events to further assess groundwater recharge during longer duration rainfall periods versus shorter duration rainfall periods.

No well development or purging will be conducted prior to groundwater sample collection to ensure that samples are representative of real-time groundwater recharge conditions. Groundwater samples will be collected with dedicated or disposable miniature bailers equipped with weights. It is assumed that no more than one significant rain event will occur per month. Additional information on analytical sampling to be conducted as part of the Groundwater Recharge Study is included in the Draft AOC 65 Treatability Study Sampling and Analysis Plan Addendum, Parsons, April 2002.

3.3.2   Building 90 Subslab Ventilation System

Analytical sampling conducted during installation of the Building 90 subslab ventilation system will include collection of one soil sample from each subslab ventilation boring for analysis of VOCs by SW-846 method 8260. These analyses will be used to determine if future removal actions beneath Building 90 are necessary and the extent of any such action. Additional information on analytical sampling to be conducted during VEW installation is included in the Draft AOC 65 Treatability Study Sampling and Analysis Plan Addendum, Parsons, April 2002.

3.3.3   AOC 65 SVE System

To assist in determining appropriate screened intervals for VEWs to be installed as part of the AOC 65 SVE system, soil gas samples will be collected during the injection packer test performed on the deepest groundwater recharge study piezometer (150 feet deep). This data will be used to identify the location and depth of soil intervals containing higher contaminant levels. The packer interval tested will be based on geophysical testing results and lithologic descriptions of core samples, with the objective of collecting samples from discrete and significant features or fractures. Injection packer testing will include collection of soil gas samples from up to eight separate depth intervals for analysis of VOCs by SW-846 analytical method TO-14.

To assess the potential contribution of natural biological reduction, groundwater samples from the VEWs will be tested for attenuation indicating parameters prior to initiating any extraction. In addition, if sufficient volumes are present, groundwater samples may be collected from each of the six VEWs for ethane, ethene, and methane by analytical method RSK-175 to establish baseline conditions.

Up to six soil gas samples are proposed for collection from VEWs for analysis of VOCs by method TO-14 before and following vapor extraction. The operating SVE system is likely to decrease breakdown products detected in soil gas samples and considerably slow the natural attenuation process. However, natural attenuation parameters will continue to be monitored to determine the impact of extraction on contaminant levels in the subsurface. Additional information on analytical sampling to be conducted during VEW installation is included in the Draft AOC 65 Treatability Study Sampling and Analysis Plan Addendum, Parsons, April 2002.

[Next Section]