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SAP Addendum Specific to Field Sampling Plan for AOC-65 Treatability Study Under AETC TO 0058
Section 4 -Groundwater Recharge Study
A groundwater recharge study will be performed at AOC-65 to assess the nature, rate, and extent of groundwater recharge and flow in the fractured limestone underlying the site. The study will also evaluate how groundwater flow affects the migration of contaminants in the area and how groundwater fluctuations may impact SVE effectiveness. The groundwater study will include the installation of a weather station and nested piezometers, and the continuous monitoring of rainfall and groundwater levels. Additionally, groundwater samples will be collected for natural attenuation parameters to assess the potential for natural biological degradation of the chlorinated compounds. Additional information regarding the groundwater recharge study is presented in the following sections. Detailed information about this study is presented in Section 3.0 of the AOC-65 Treatability Test Plan, Parsons, April 2002.
The groundwater recharge study will consist of drilling and installing piezometers to assess the magnitude and rate of precipitation infiltration in the unsaturated zone beneath the site. The piezometers will be installed at different depths along primary migration pathways identified during the geophysical surveys, to assess the lateral and vertical infiltration of groundwater in the area. The piezometers will be installed in the primary migration pathways; fractures, faults, and karst features, identified from the results of the geophysical surveys. Groundwater samples will be collected from the piezometers during rainfall/infiltration events to assess influence of groundwater percolation in the unsaturated zone on the migration of dissolved constituents.
Piezometer locations will be determined by geophysical results as discussed in Section 2.0. Proposed piezometer locations are presented in Figure 3.1 of the AOC-65 Treatability Test Plan, Parsons, April 2002.
The piezometers will be constructed with 25-foot screened intervals set at varying intervals to monitor the upper 150 feet of unsaturated material at the site. The wells will consist of 4-inch diameter slotted PVC screen material and extended to land surface with 4-inch, schedule 80 PVC riser pipe. Each piezometer surface completion will be flush-mounted and include an accommodation for a down-hole transducer, which will be connected to the aboveground weather station. Information regarding piezometer installation and construction is included in Section 3.0 of the AOC-65 Treatability Test Plan, Parsons, April 2002. Procedures for the installation of the piezometers are included in Section 7.5 of this addendum.
4.2 - Baseline Groundwater Sampling
Natural attenuation of chlorinated hydrocarbons is likely occurring at AOC-65 given the presence of PCE and TCE breakdown products detected in groundwater and in soil gas samples collected near Building 90. To assess the potential contribution of natural biological reduction, groundwater samples will be collected from all piezometers, VEWs and nearby shallow monitoring wells with a sufficient groundwater volume. Groundwater samples will be collected for natural attenuation indicator parameters for chlorinated compounds including ethene, ethane, and methane. The natural attenuation sample results will serve as a baseline for comparison to subsequent sampling events to assess the contribution of natural attenuation to contaminant removal. Additionally, groundwater samples will be collected from the piezometers for VOC analysis to assess the concentration and distribution of dissolved constituents in the shallow groundwater.
The groundwater analyses to be performed during the baseline sampling are included in Table 1.1. Detailed information regarding this sampling event is included in Section 3.0 of the AOC-65 Treatability Test Plan, Parsons, April 2002. Sample collection procedures are described in Section 7.3 of this addendum.
4.3 - Post-Rainfall Groundwater Sampling
Groundwater samples will be collected from the piezometers following a significant rainfall event (greater than 1 inch) to assess the impacts of infiltration on the migration of contaminants. Groundwater samples will be collected at approximately 24, 48, and 96 hours after the rainfall event begins. The groundwater samples will be analyzed for VOCs as indicated in Table 1.1. Sample collection procedures are included in Section 7.3 of this addendum. Additional information regarding the post-rainfall groundwater sampling is included in Section 3.0 of the AOC-65 Treatability Test Plan, Parsons, April 2002.
A weather station equipped with a data recorder will be installed near AOC-65 to monitor rainfall events. The weather station will be equipped with a tipping-bucket rain gauge and will record incremental and cumulative rainfall amounts. The weather station will also be equipped with sensors to monitor temperature, wind speed and barometric pressure. The weather station will be connected to a solar-powered data logger to record and store the data. Installation, calibration, and maintenance of the weather station equipment will be performed in accordance with the manufacturer’s recommendations. Additional information regarding the rainfall measurements is presented in Section 3.0 of the AOC-65 Treatability Test Plan, Parsons, April 2002.
4.5 - Groundwater Elevation Measurements
To assess the infiltration and movement of groundwater fractures, faults, and karst features at AOC-65, groundwater elevation measurements will be collected from the six piezometers. Water level measurement devices will be installed in each of the piezometers to provide continuous groundwater level data. The water level devices will be connected to the data logger at the weather station for data recording and storage. Installation, calibration, and maintenance of the water level devices will be performed in accordance with the manufacturer's recommendations. Additional information regarding the groundwater level measurements is presented in Section 3.0 of the AOC-65 Treatability Test Plan, Parsons, April 2002.
4.6 - Groundwater Tracer Study
A groundwater tracer study may be performed in the AOC-65 area to provide more specific information regarding the travel time, retardation, and flush time of the chlorinated compounds in the fractured limestone unit. 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, George Veni and Associates (GVA), and the University of Texas at San Antonio (UTSA). A revision to this SAP Addendum will be prepared to include the tracer test scope.