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SAP Addendum Specific to Field Sampling Plan for AOC-65 Treatability Study Under AETC TO 0058
Section 5 - AOC-65 SVE System
An SVE treatability study will be performed at AOC-65 to evaluate the effectiveness of SVE as a remedial alternative to address the contamination at the site. The study will be designed to assess the effective radius of influence of the VEWs installed in the vicinity of Building 90. The treatability study will also be used to quantify the estimated mass of PCE and TCE that can be removed by an SVE system installed at the site.
The SVE system will consist of up to six VEWs and up to six multi-depth VMPs. An extraction packer test will be performed on the deepest borehole advanced for the groundwater recharge study piezometers to provide data for the optimal depth intervals for the VEWs. Off-gas treatment of the emissions from the SVE system will be managed using vapor-phase carbon treatment units. Following construction of the system, an initial system test, and a radius of influence test will be performed to evaluate the system performance and determine optimal operational parameters. Continued operation of the SVE system will be conducted through an operation and maintenance (O&M) program. Additional information regarding the AOC-65 SVE system is included in Section 3.0 and 4.0 of the AOC-65 Treatability Test Plan, Parsons, April 2002.
Six VEWs will be installed around Building 90 near the building�s former vat and associated drainage ditch. Five of the VEWs are planned for the west side of the building where soil gas data suggest the contamination is located, and one is planned for the east side of the building. The VEWs will be located in areas of faults, fractures, and karst features if identified by geophysical surveys conducted at AOC-65. Exact VEW locations, depths, screened interval placement, and design will be determined based on geophysical data, lithologic observations, soil gas packer testing, and other site conditions noted as field activities progress. Preliminary locations for the VEWs are included in Figure 3.1 of the AOC-65 Treatability Test Plan, Parsons, April 2002.
An extraction packer test will be performed in the borehole advanced for the installation of the deepest piezometer during the groundwater recharge study. The extraction test will be performed in the completed borehole prior to installation of the piezometer well material. Soil gas samples will be collected from up to eight different depth intervals in the borehole and submitted for laboratory analysis. Data generated during the packer test will provide information regarding the vertical distribution of volatile contaminants in the fractured limestone unit. Table 1.1 includes sampling to be conducted during the soil gas extraction packer test. Procedures for conducting the extraction packer test are included in Section 7.7 of this addendum.
The VEWs will be installed in boreholes advanced with an air rotary drill rig. Each location will be continuously cored to the desired well depth and then over drilled to allow for completion of the well. The extraction wells will be completed using 4.0-inch diameter, schedule 40 PVC well material and will include a 25 foot screened interval. The top of the VEWs will include 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 used as an extraction well. Each well will be connected to the SVE manifold system with PVC pipe buried for protection. VEW construction procedures are presented in Section 7.6 of this addendum. Detailed information on the VEWs is presented in Section 3.0 of the AOC-65 Treatability Test Plan, Parsons, April 2002.
Up to six multi-depth VMPs will be installed in the Building 90 area and will be designed to monitor multiple depths at each location. The multi-depth VMPs will include up to four distinct nested screened intervals per boring and will extent to a maximum depth of 100 feet below grade. The VMPs will be installed on both sides of Building 90, with most installed on the west side near the building�s suspected vat and the associated drainage ditch. The VMPs will be installed in faults, fractures, and karst features if identified in the geophysical surveys. The final locations of the VMPs will be selected based on geophysical testing results and locations of VEWs. Preliminary locations for the VMPs are depicted on Figure 3.1 of the AOC-65 Treatability Test Plan, Parsons, April 2002.
Each of the six VMPs will be constructed with 1-inch, schedule 40 PVC screen and �-inch inside diameter, schedule 80 PVC riser. Each VMP will consist of four nested wells installed within one 8-inch borehole and will completed at land surface with a flush-mount well box. Each of the four wells within each VMP will include a 5-10 screened interval that is separated from the zone above and below by a minimum of five feet of bentonite. No soil samples are proposed for collection during well installation activities. Procedures for construction of the VMPs are included in Section 7.6 of this addendum. Additional information regarding the multi-depth VMPs is included in Section 3.0 of the AOC-65 Treatability Test Plan, Parsons, April 2002.
After construction of the AOC-65 SVE system, treatability tests will be conducted to evaluate the potential effectiveness of the SVE. The treatability test will include an initial system check and a radius of influence test. During the initial system check, soil gas samples will be collected from the VEWs and VMPs prior to system startup, and at specific locations and times after the system us started. Radius of influence tests will be performed by selecting various configurations of extraction wells and monitoring the vacuum response at other points to assess the radius of influence of the VEWs and the interconnectivity of the fractures in bedrock. Data collected during the treatability tests will be used to verify the effectiveness of the activated carbon soil-gas treatment unit. In addition, a soil gas tracer test may be performed to evaluate travel times, flush times and retardation rates for VOCs in the fractures. Additional information for the treatability test is included in Section 4.0 of the Treatability Test Plan, Parsons, April 2002. A summary of the treatability test activities is presented in the following sections.
The initial system check of the AOC-65 SVE system will be performed at the start-up of the extraction systems. Sampling of all VMPs will be conducted utilizing soil gas collection techniques described in Section 7.4 of this addendum. Sampling of VEWs will utilize in-line sampling ports installed at the wellhead and the blower manifold unit.
Prior to system start-up, all VEWs and VMPs will be field monitored for oxygen, carbon dioxide, and total volatile hydrocarbons (TVH) levels to assess static conditions. The extraction well with the highest TVH level and carbon dioxide, and lowest oxygen level, indicating the highest degree of contamination, will be selected as the extraction well for the start-up test. Soil gas samples will be collected from at least six VEWs for VOC analysis. By comparing the field screening results to the analytical soil gas results a correlation will be developed which can be used to estimate VOC concentrations based on field screening results in subsequent field tests.
The SVE blower will then be turned on and a vacuum will be pulled on the selected extraction well. The system will run continuously for four to eight days and air samples will be collected for VOC analysis. Air samples will be collected from the selected extraction well after 1-hour, 8-hours, 24-hours, 48-hours, four days, and possibly eight days of operation to assess changes in VOC removal rates over time after start-up. Pressure responses and field screening parameters will be measured from all available monitoring points including: VMPs, nearby monitoring wells, subslab VEWs, and piezometers. The pressure response and field screening parameters will be measured at one, two, four, and eight days after start-up to assess soil gas changes with time and to evaluate the radius of influence for the selected extraction well.
Table 1.1 includes a summary of the sample parameters and testing requirements for the soil gas samples to be collected during the initial system check. Soil gas sample will be collected in a ordance with the procedures presented in Section 7.4 of this addendum. Detailed information about the initial system check is presented in Section 4.0 of the AOC-65 Treatability Test Plan, Parsons, April 2002.
5.3.2 Radius of Influence Testing
Following the initial system check, the system will be shut down and subsurface conditions allowed to equilibrate. Radius of influence tests will then be performed on some or all of the five remaining extraction wells. Radius of influence testing of the AOC-65 SVE system will include the measurement of soil gas changes and pressure responses to assess the depth and lateral influence of each VEW.
Different configurations of extraction using the Building 90 and AOC-65 VEWs will be tested by shutting off air flow from one or more VEWs and assessing the pressure response from each VMP and existing monitoring well at the site. The results will provide initial data on preferential pathways and interconnections in the subsurface soils beneath Building 90 and at AOC-65. Following at least two weeks of continuous extraction, airflow measurements and VOC concentrations will again be measured at each of the VEWs. This data will be used to assess the relative impact the extraction exerted on soil gas levels by observing contaminant concentration changes at each VEW.
Soil gas samples will be collected during the radius of influence test for the parameter indicated in Table 1.1. Soil gas sample will be collected in a ordance with the procedures presented in Section 7.3 of this addendum. Detailed information about the radius of influence testing is presented in Section 4.0 of the AOC-65 Treatability Test Plan, Parsons, April 2002.
Tracer tests may be performed to obtain more specific information regarding the travel time, retardation, and flush time of VOCs in the fractured limestone beneath the site. The tracer test results will determine the flush times from the point of tracer injection to the SVE and, in turn, how big the area of influence is and how often the fractures are flushed with air. The test will also help determine NAPL, organic matter, and water content along those flow paths and enable the forecast of TCE retardation due to flow in the fractures. If the decision is made to perform the groundwater tracer test, a detailed scope of work will be developed through collaboration between CSSA, Parsons, a project karst expert, GVA, and the UTSA. A revision to this SAP Addendum will be prepared to include the tracer test scope.