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Title: CORRECTIVE ACTION DECISION DOCUMENT/CLOSURE REPORT FOR CORRECTIVE ACTION UNIT 527: HORN SILVER MINE, NEVADA TEST SITE, NEVADA

This Corrective Action Decision Document/Closure Report (CADDKR) has been prepared for Corrective Action Unit (CAU) 527: Horn Silver Mine, Nevada Test Site (NTS), Nevada, in accordance with the Federal Facility Agreement and Consent Order (1996). Corrective Action Unit 527 is located within Area 26 of the NTS and consists of CAS 26-20-01, Contaminated Waste Dump No.1. This CADDKR refers to the site as CAU 527 or the Horn Silver Mine (HSM). This CADDKR provides or references the specific information necessary to support the closure of this CAU. Corrective action investigation activities were performed from November 12,2003 through January 21,2004. Additional sampling of liquid obtained from HSM-3 was conducted on May 3,2004. Corrective action investigation activities were performed as set forth in the Corrective Action Investigation Plan for Corrective Action Unit 527 (NNSAiNV, 2002a). Assessment of the data generated from investigation activities identified the explosive nitrobenzene as a contaminant of concern (COC) on the floor of the 500-foot drift (HSM No.2). No other COCs were identified in the rock samples collected during the investigation activities. The air samples collected from borings HSM-1, HSM-2, and HSM-3 showed volatile organic compounds (primarily gasoline-related contaminants) to be present above the acceptable residential exposure criteriamore » in the boreholes. A conservative modeling effort demonstrated that these concentrations would not migrate to the surface at concentrations that will present an unacceptable risk to future land users. However, other COCs are assumed to exist based on historical documentation on the types of waste placed in the shaft; therefore, the mine including the 300- and 500-foot drifts is considered to be contaminated above action levels. Current results of the field investigation show there are no active transport mechanisms or exposure routes for the contaminants identified in the 500-foot drift. The analytical data did not show the migration of COCs beyond the floor of the 500-foot drift or from the air within the drift. On a conservative basis, the subsurface volume of the zone of contamination is limited to a depth from 150 ft to a maximum of 670 feet below ground surface extending to a radius of 300 feet from the mineshaft. Based on these data, a use restriction will be established for this volume of soil. In addition, the security of the mineshaft is maintained and does not allow unauthorized personnel to enter the vicinity of the mineshaft. Since the removal of the contaminants is not feasible, the close in place with administrative controls corrective action alternative is appropriate because it will prevent inadvertent contact with the subsurface COCs and meets all applicable state and federal regulations for closure of the site. Post-closure monitoring will be conducted for one year. This monitoring will include using the lysimeter at HSM-3 and the data logger to measure precipitation-induced vadose zone moisture flow through the rock beneath the waste shaft at the Horn Silver Mine. Results of the monitoring will be documented in a letter report at the end of one year, anticipated in June 2005. A copy of this report will be submitted to the Nevada Division of Environmental Protection. After one year of monitoring, a determination will be made by the Nevada Division of Environmental Protection and U.S. Department of Energy, National Nuclear Security Administration Nevada Site Office if future monitoring is needed or if use restriction boundaries need to be adjusted. If a large enough pulse of water moves into the lysimeter, a sample will he collected for laboratory analysis. If there is not sufficient volume of liquid collected for a sample or if no COCs are detected in collected samples at the end of this time period, it is recommended that the monitoring wells at the HSM be sealed in accordance with the State of Nevada regulations.« less
Publication Date:
OSTI Identifier:
901887
Report Number(s):
DOE/NV--989
TRN: US200717%%20
Resource Type:
Technical Report
Research Org:
DOE/NNSA NEVADA SITE OFFICE
Sponsoring Org:
USDOE
Country of Publication:
United States
Language:
English
Subject:
54 ENVIRONMENTAL SCIENCES; MINES; SILVER ORES; EXPLOSIVES; LYSIMETERS; NEVADA TEST SITE; NITROBENZENE; ORGANIC COMPOUNDS; REMOVAL; SAMPLING; REMEDIAL ACTION; PLANNING; MONITORING