Corrective Action Decision Document/Corrective Action Plan for Corrective Action Unit 447: Project Shoal Area, Subsurface, Nevada, Rev. No.: 3 with Errata Sheet

This Corrective Action Decision Document/Corrective Action Plan (CADD/CAP) has been prepared for Corrective Action Unit (CAU) 447, Project Shoal Area (PSA)-Subsurface, Nevada, in accordance with the ''Federal Facility Agreement and Consent Order'' (FFACO) (1996). Corrective Action Unit 447 is located in the Sand Springs Mountains in Churchill County, Nevada, approximately 48 kilometers (30 miles) southeast of Fallon, Nevada. The CADD/CAP combines the decision document (CADD) with the Corrective Action Plan (CAP) and provides or references the specific information necessary to recommend corrective actions for CAU 447, as provided in the FFACO. Corrective Action Unit 447 consists of two corrective action sites (CASs): CAS 57-49-01, Emplacement Shaft, and CAS 57-57-001, Cavity. The emplacement shaft (CAS-57-49-01) was backfilled and plugged in 1996 and will not be evaluated further. The purpose of the CADD portion of the document (Section 1.0 to Section 4.0) is to identify and provide a rationale for the selection of a recommended corrective action alternative for the subsurface at PSA. To achieve this, the following tasks were required: (1) Develop corrective action objectives. (2) Identify corrective action alternative screening criteria. (3) Develop corrective action alternatives. (4) Perform detailed and comparative evaluations of the corrective action alternatives in relation to the corrective action objectives and screening criteria. (5) Recommend a preferred corrective action alternative for the subsurface at PSA. The original Corrective Action Investigation Plan (CAIP) for the PSA was approved in September 1996 and described a plan to drill and test four characterization wells, followed by flow and transport modeling (DOE/NV, 1996). The resultant drilling is described in a data report (DOE/NV, 1998e) and the data analysis and modeling in an interim modeling report (Pohll et al., 1998). After considering the results of the modeling effort, the U.S. Department of Energy (DOE) determined that the degree of uncertainty in transport predictions for PSA remained unacceptably large. As a result, a second CAIP was developed by DOE and approved by the Nevada Division of Environmental Protection (NDEP) in December 1998 (DOE/NV, 1998a). This plan prescribed a rigorous analysis of uncertainty in the Shoal model and quantification of methods of reducing uncertainty through data collection. This analysis is termed a Data Decision Analysis (Pohll et al., 1999a) and formed the basis for a second major characterization effort at PSA (Pohll et al., 1999b). The details for this second field effort are presented in an Addendum to the CAIP, which was approved by NDEP in April 1999 (DOE/NV, 1999a). Four additional characterization wells were drilled at PSA during summer and fall of 1999; details of the drilling and well installation are in IT Corporation (2000), with testing reported in Mihevc et al. (2000). A key component of the second field program was a tracer test between two of the new wells (Carroll et al., 2000; Reimus et al., 2003). Based on the potential exposure pathways, two corrective action objectives were identified for CAU 447: Prevent or mitigate exposure to groundwater contaminants of concern at concentrations exceeding regulatory maximum contaminant levels or risk-based levels; and Reduce the risk to human health and the environment to the extent practicable. Based on the review of existing data, the results of the modeling, future use, and current operations at PSA, the following alternatives have been developed for consideration at CAU 447: Alternative 1--No Further Action; Alternative 2--Proof-of-Concept and Monitoring with Institutional Controls; and Alternative 3--Contaminant Control. The corrective action alternatives were evaluated based on the approach outlined in the ''Focused Evaluation of Selected Remedial Alternatives for the Underground Test Area'' (DOE/NV, 1998b). Each alternative was assessed against nine evaluation criteria. These criteria include overall protection of human health and the environment; compliance with appropriate requirements; long-term effectiveness; reduction of the toxicity, mobility, or volume of contamination; short-term effectiveness; implementability; cost; state acceptance; and community acceptance. Based on the results of this evaluation, the preferred alternative for CAU 447 is Alternative 2, Proof-of-Concept and Monitoring with Institutional Controls. The preferred corrective action alternative was chosen for its technical implementability, focusing on performance, reliability, feasibility, safety, and cost. Alternative 2 was judged to meet all requirements for the technical components evaluated and will control inadvertent exposure to contaminated groundwater at CAU 447