PNNL

Abstract

The Hanford Reach National Monument (HRNM) was created by presidential proclamation in 2000. It is located along the Columbia River in south central Washington and consists of five distinct units. The McGee Ranch-Riverlands and the North Slope units are addressed in this report. North Slope refers to two of the HRNM units: the Saddle Mountain Unit and the Wahluke Slope Unit. The Saddle Mountain and Wahluke Slope Units are located north of the Columbia River, while the McGee Ranch-Riverlands Unit is located south of the Columbia River and north and west of Washington State Highway 24. To fulfill internal U.S. Department of Energy (DOE) requirements prior to any radiological clearance of land, the DOE must evaluate the potential for residual radioactive contamination on this land and determine compliance with the requirements of DOE Order 5400.5. Authorized limits for residual radioactive contamination were developed based on the DOE annual exposure limit to the public (100 mrem) using future potential land-use scenarios. The DOE Office of Environmental Management approved these authorized limits on March 1, 2004. Historical soil monitoring conducted on and around the HRNM indicated soil concentrations of radionuclides were well below the authorized limits (Fritz et al. 2003). However, the historical sampling was done at a limited number of sampling locations. Therefore, additional soil sampling was conducted to determine if the concentrations of radionuclides in soil on the McGee Ranch-Riverlands and North Slope units were below the authorized limits. Sixty-seven soil samples were collected from the McGee Ranch-Riverlands and North Slope units. A software package (Visual Sample Plan) was used to plan the collection to assure an adequate number of samples were collected. The number of samples necessary to decide with a high level of confidence (99%) that the soil concentrations of radionuclides on the North Slope and McGee Ranch-Riverlands units did not exceed the authorized limits was determined to be 27. Additional soil samples were collected from areas suspected to have a potential for accumulation of radionuclides. This included samples collected from the riparian zone along the Columbia River, Savage Island, and other locations across the North Slope and McGee Ranch-Riverlands units. The 67 soil samples collected from the McGee Ranch-Riverlands and North Slope units all had concentrations of radionuclides far below the authorized limits established by the DOE. Statistical analysis of the results concluded that the Authorized Limits were not exceeded when total uncertainty was considered. The calculated upper confidence limit for each radionuclide measured in this study (which represents the value at which 99% of the measurements reside below with a 99% confidence level) was lower than the Authorized Limit for each radionuclide. The maximum observed soil concentrations for the radionuclides included in the authorized limits would result in a potential annual dose of 0.23 mrem assuming the most probable use scenario, a recreational visitor. This potential dose is well below the DOE 100-mrem/year dose limit for members of the public. Furthermore, the results of the biota dose assessment screen, which used the RESRAD biota code, indicated that the sum of fractions is less than one. This assumed soil concentrations equal to the maximum concentrations of radionuclides measured on the McGee Ranch-Riverlands and North Slope units’ in this study. Since the sum of fractions was less than 1, dose to terrestrial biota will not exceed the recommended biota dose limit for the soil concentrations measured in this study.