Remedial Action Plan and final design for stabilization of the inactive uranium mill tailings at Green River, Utah. Volume 1, Text, Appendices A, B, and C: Final report

Abstract

Dear Mr. Matthews: be placed in the bottom of the disposal cell to retard the migration of any tailings contamination downward to the water table. The combination of these design features will enable the soil layers of the disposal cell to operate together at a net infiltration rate of below 2 x 10-8 cm3/cm2s (see Section E.3.2). With the exception of the relic groundwater plume, the standards for cleanup of the site under Subpart B of 40 CFR 192 will be satisfied with the proposed remedial action plan. Cleanup of the tailings pile, ore storage area, vicinity properties, and windblown tailings materials will be accomplished by consolidating the materials into the disposal cell. The DOE will verify that cleanup to standards has been accomplished. Cleanup of the relic groundwater plume will be addressed In a separate process after the proposed EPA groundwater standards have been finalized. Groundwater monitoring A groundwater performance monitoring program will be fully developed and discussed In the Green River Surveillance and Maintenance Plan. The monitoring program will include disposal cell moisture monitoring and a network of monitor wells in the saturated bedrock surrounding the disposal cell. Monitoring in the disposal cell will consist of neutron access holes into the infiltration/radon barrier and tailings to determine changes in moisture content. This will constitute an early detection monitoring mechanism for the site. Background monitor wells and monitor wells at the point of compliance will be sampled to compare changes in groundwater quality. Further explanation of the monitoring program is found in Section E.3.4. Design changes Changes in the disposal cell design since release of the January 1989 Remedial Action Plan (RAP) have been proposed for two reasons. First, the NRC requested changes in the design specifications (i.e., six percent bentonite added to the infiltration/radon barrier soils). Second, the topslopes of the disposal cell were changed from five percent to 20 percent to accommodate additional quantities of contaminated materials. A Justification of the disposal cell design and responses to the NRC comments and agreement issues have been added in this Executive Summary and in Section 4.0. Major design changes from the February 1988 RAP that are included in this RAP are: 1. An increase in the thickness of the sodium bentonite amended infiltration/radon barrier from 12 inches to 36 inches. (The DOE proposed increasing the thickness from 12 to 18 inches in a letter to the NRC dated August 19, 1988.) 2. The elimination of the select fill layer for frost protection. However, the thicker infiltration/radon barrier will still result in a 15-inch thickness of infiltration/radon barrier below the calculated maximum frost depth of 39 inches. 3. The placement of a six-foot-thick select fill soil layer beneath the tailings on top of the exposed bedrock at the bottom of the cell. This layer will increase the leachate travel time from the tailings to the point of compliance (POC). 4. A revision of th...