A total of 908 historical nuclear detonations occurred in shafts or tunnels at the NTS. They are categorized into 878 CASs assigned to the UGTA. These CASs are grouped into five CAUs. CASs in each CAU are located near each other, and CAUs are geographically distinct. CAUs have distinctly different contaminant source, geologic, and hydrogeologic characteristics related to their location.
3.1 Corrective Action Units
The CAUs, shown in Figure 3-1, are listed below:
 Frenchman Flat CAU consists of 10 CASs located in the northern part of Area 5 and the Southern part of Area 11.  These events were conducted in both vertical emplacement holes and mine shafts. The events in Frenchman Flat were located in alluvium of great depth. The deeper geology is not well known. Lateral transport in the alluvium is very slow due to the low lateral gradient.
Western Pahute Mesa CAU consists of 18 CASs along the western edge of Area 20. These events were all conducted in vertical emplacement holes. This CAU is separated from Central Pahute Mesa by the Boxcar Fault and is distinguished by the relative abundance of tritium. Transport of contaminants on and from Western Pahute Mesa involves groundwater flow in both welded and vitric tuffs, both in the rock matrix and in the fracture system.
Central Pahute Mesa CAU consists of 64 CASs in Areas 19 and 20 on Pahute Mesa. These events were all conducted in vertical emplacement holes. Transport of contaminants on and from Central Pahute Mesa involves groundwater flow in fractures and the rock matrix, in welded and vitric tuffs, and lava flow aquifers. The influence of the large-scale block faulting is not well known.
Yucca Flat/Climax Mine CAU consists of 717 CASs located in Areas 1, 2, 3, 4, 6, 7, 8, 9, 10, and 3 CASs located in Area 15. These events were conducted in vertical emplacement holes and tunnels. Contaminant transport in Yucca Flat/Climax Mine may involve alluvium, both welded and vitric tuffs, fractured granite, and carbonate rocks.
Rainier Mesa/Shoshone Mountain CAU consists of 60 CASs on Rainier Mesa and 6 CASs on Shoshone Mountain, located in Areas 12 and 16. These events were all conducted above the water table in tunnels constructed in bedded and non-welded vitric and zeolitized volcanic tuffs.
The process outlined in Section 1.3 was used for the initial prioritization of UGTA CAUs. The first three CAUs are in priority order, but the priority of the remaining CAUs may change as they become specifically addressed in the planning process.
3.2 Corrective Action Strategy
The corrective action strategy for UGTA is based on the complex corrective action process. The objective of the CAI process is to define boundaries around each UGTA CAU to establish areas that contain water that may be unsafe for domestic and municipal use. Any ambiguity resulting from different language used in this subpart of Appendix VI versus the body of the FFACO Agreement shall be resolved in favor of terms and conditions found in the body of the FFACO Agreement.
The UGTA Corrective Action Strategy was developed to address the contamination created by the testing of nuclear devices in shafts and tunnels at the Nevada Test Site. The objective of the strategy is to analyze and evaluate each UGTA CAU through a combination of data and information collection and evaluation, and modeling groundwater flow and contaminant transport. This analysis will estimate the vertical and horizontal extent of contaminant migration for each CAU in order to predict contaminant boundaries. A contaminant boundary is the model-predicted perimeter, which defines the extent of radionuclide-contaminated groundwater from underground testing above background conditions exceeding the Safe Drinking Water Act (SDWA) standards. The contaminant boundary will be composed of both a perimeter boundary and a lower hydrostratigraphic unit boundary. The computer model predicts the location of this boundary within 1,000 years and must do so at a 95% level of confidence. Additional results showing contaminant concentrations and the location of the contaminant boundary at selected times will also be presented. These times may include the verification period, the end of the five-year proof of concept period, as well as other times that are of specific interest.
From the contaminant boundary predicted by the computer model, a compliance boundary will be negotiated between NDEP and DOE. The compliance boundary will define the area within which the radiological contaminants above the SDWA standards relative to background are to remain. DOE will be responsible for ensuring compliance with this boundary. The compliance boundary may or may not coincide with the contaminant boundary. If the predicted location of the contaminant boundary cannot be accepted as the compliance boundary, an alternate compliance boundary will be negotiated by both parties.
An initial assumption is that contaminant control will not be required. After establishing a compliance boundary for each CAU, an evaluation of remedial alternatives and a monitoring Corrective Action Plan will be developed. A 5-year proof of concept period will follow using groundwater wells in a monitoring network to determine if the monitoring network design will provide adequate CAU surveillance. If the monitoring network is found acceptable, a closure plan will then be developed, followed by implementation of a long-term closure monitoring program.
The long-term closure monitoring program will address any contamination left in place in a closed CAU. This program consists of all activities necessary to ensure protection of human health and the environment following the completion of corrective actions at a CAU. These activities will include periodic analysis of monitoring results, determining optimum performance indicators, evaluation of monitoring performance criteria, locating new monitoring wells and replacing existing monitoring wells to support performance criteria evaluation at timed intervals of interest within the 1,000-year time period.
A model of regional flow encompassing the NTS and the groundwater flow systems extending to downgradient discharge has been completed. Regional modeling is a cross-cutting activity, supporting the entire UGTA program, which provides the initial basis for assessing flowpaths from CAUs, determining potential receptors, evaluating isolation or interaction of CAUs, and creating a consistent hydrogeologic framework across all the CAUs. Regional transport modeling provides the initial basis for determining the magnitude of risk from the source to potential receptors and for scaling individual CAU work.
The second phase of the CAI process will focus on refining CAU boundaries through CAU-specific models that include CAU-specific data. The CAU-specific modeling objectives are to estimate movement of contaminants utilizing the acquisition and evaluation of CAU-specific hydrogeologic data and define boundaries that encompass the extent of contamination. If CAU-specific modeling is not successful in achieving CAU objectives, this strategy will be evaluated to determine whether it will allow the objectives to be reached. If it is not possible or feasible to achieve the objectives, it may be necessary to reevaluate and consider alternative approaches.
Figure 3-2 is a diagram of the generalized decision process leading to the closure of CAUs. The process contains five major decision points where data and/or data analysis are reviewed and consensus reached before proceeding with the next phase of corrective action activities. The first of these major decisions is the determination of data adequacy prior to developing the CAU flow and contaminant transport model. If the data are not adequate, alternatives will be evaluated, and the second major decision point, a decision on whether the UGTA strategy can be achieved, will be reached. If the strategy can be achieved, an addendum to the CAIP will then be developed. If the strategy cannot be achieved, a new strategy will then be proposed. If the data are adequate, the CAU flow and transport model will be developed.
The third major decision concerns the acceptability of the CAU flow and transport model. If the CAU flow and transport model is not acceptable, the alternatives will be evaluated and, again, the second major decision point, a decision on whether the UGTA strategy can be achieved, will be reached. If the strategy can be achieved, an addendum to the CAIP will then be developed. If the strategy cannot be achieved, a new strategy will then be proposed. If the CAU model is acceptable, the CAU boundaries will be defined.
The model results, along with the results of the CAI, will be utilized for an evaluation of remedial alternatives and a proposed remedial action. The fourth major decision is whether contaminant control is required. If contaminant control is required, then a corrective action plan will be developed and implemented. If contaminant control is not required, then a monitoring corrective action plan will be developed and a five-year proof of concept monitoring program will be initiated. The fifth and final major decision occurs after a review of the monitoring results. If DOE and NDEP are confident of the results, the closure process will begin. If the results at any of these decision points are not acceptable, then contingency activities will be initiated and evaluated, as appropriate, to correct the deficiencies.
For saturated conditions, a flow model of each CAU will be constructed to provide local three-dimensional flow, to evaluate the range of flow conditions in the CAU that may be important in determining maximum extent of transport, and to provide boundary conditions for modeling transport. Saturated conditions are planned to be modeled for Frenchman Flat, Yucca Flat/Climax Mine, Western Pahute Mesa, and Central Pahute Mesa CAUs.
For CAUs where unsaturated groundwater conditions prevail (Rainier Mesa/Shoshone Mountain CAU), saturated zone flow and transport modeling results, based on field data, will be evaluated to determine if the saturated zone has been impacted. If the saturated zone has been impacted, then the need for further examination of the unsaturated zone will be evaluated.
CAU models utilizing tritium as the source term will be used to establish the contaminant boundary for each CAU. The boundary will be composed of a perimeter boundary and a lower hydrostratigraphic unit boundary. The perimeter boundary will define the aggregate maximum extent of contamination transport at or above the concentration of concern for the CAU. The lower hydrostratigraphic unit boundary will define the lowest aquifer unit affected by the contamination. Long-lived radionuclides, besides tritium, will be included to evaluate the relative extent of migration of different radionuclides in the future. If it is predicted that another radionuclide will migrate farther than tritium at concentrations of concern, the contaminant boundary will include that prediction.
Figure 3-3 illustrates how modeling uncertainty can be expressed as confidence levels. Each contour reflects an increased level of confidence
that no contaminants exceeding a given regulatory concentration will ever cross that boundary. As confidence increases, the distance from the CAU increases. The confidence levels could lead to the development of different contaminant boundaries, depending on the degree of certainty decision makers need to select appropriate controls.
Monitoring compliance with the CAU boundaries will be accomplished through measurement of appropriate physical and chemical parameters in wells within the modeled region. Appropriate physical and chemical parameters remaining within the range of measurements used in the flow model will be an indication that the conditions have not significantly changed. Sensitivity analysis of parameters relevant to the groundwater will indicate the extent that appropriate physical and chemical parameters can vary before the acceptable confidence limit for the model is exceeded.
3.3 Implementing Corrective Action Investigations and Corrective Actions
Work elements expected to be required to conduct the CAI and corrective action process for each of the UGTA CAUs are identified in the Process Flow Diagram for the Underground Test Area Corrective Action Units (see Figure 3-2), and are described below. These descriptions form the basis for establishing due dates for milestones and deadlines for these CAUs. If activities other than those described herein are determined to be necessary to achieve closure of the CAUs, the milestones and schedule will be reevaluated in accordance with the terms and conditions defined in the Agreement. As of the effective date of this Agreement, no specific, proven cost-effective technologies, as known by the parties individually, have been previously demonstrated to either remove radioactive contaminants from the groundwater, stabilize them, or remove the source of the contaminants at the CASs that are subject to that Agreement. Such technologies may be perfected in the future, which may perhaps alter the choice of corrective action at that time.
The following dictionary sets forth the meaning of each block/step of the Process Flow Diagram for Underground Test Area Corrective Action Units (see Figure 3-2) for achieving the UGTA Corrective Action Strategy. The dictionary is presented in tabular form identifying each of the steps developed to achieve the Strategy. The table presents the process section that each block/step is in, the descriptor, or name, of each block/step, and a definition of the block/step.
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