Commentary on Studies of 36Cl in the Exploratory Studies Facility at Yucca Mountain, Nevada
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INTRODUCTION 3CI
is a radioactive isotope with a half life of 301,000 y. It is generated in nature primarily by
cosmic radiation effects on stable Cl and Ar in the atmosphere and occurs naturally at trace concentrations. 36C1 is a useful tracer for evaluations of the behavior of groundwater. Chloride is soluble in groundwater, and dissolved chloride is relatively unaffected by gas-water-rock interactions in most groundwater systems. Except for its radioactivity, 3Cl behaves in geochemical systems as does stable Cl, so a conventional concentration scale for 'lC is the ratio of Cl to total Cl. The natural 36CI/C1 ratio of material deposited by precipitation and dry fall at the ground surface is variable, primarily because variations in the Earth's magnetic field affect cosmic radiation shielding and the 3CI production rate, and because the rate of stable Cl deposition varies with climatic conditions and geographic setting. Also, systems that have been isolated from sources of 3Cl production (e.g., the stratosphere) for time periods that are comparable or long relative to the 3Cl half life have diminished 3Cl concentrations because of radioactive decay. The age of a closed quantity of groundwater can be bounded using 3CI data on time scales comparable to the 3CI half life. 3CI was also produced at relatively high concentrations by testing of nuclear bombs, particularly in the Pacific Proving Grounds during the 1950s. Groundwater containing 3CI concentrations that are above natural levels is likely to contain at least some fraction of water that is less than 50 years old. Yucca Mountain, Nevada, is under investigation to host a high level radioactive waste repository. The Exploratory Studies Facility (ESF) at Yucca Mountain is an 8 km tunnel that descends to the level of the proposed nuclear waste emplacement horizon. Draft reports on 3CI studies at Yucca Mountain were released recently by Los Alamos National Laboratory (LANL) [1,2,3]. The former report [1] is an update of a previously published LANL report [4]. Most 36Cl data presented in earlier references [1,2,4] have been corrected, in some cases significantly, and reported in [3], which is the source of data for the analyses presented here. 407 Mat. Res. Soc. Symp. Proc. Vol. 506 01998 Materials Research Society
'Cl data from the ESF provide important empirical constraints on the velocity and pathways for water flow under natural conditions in the unsaturated zone at Yucca Mountain. The object of this commentary is to provide insight on these flow characteristics with the aid of original statistical analyses of the 36C1 data. A key challenge and focus of this study is identification of those samples that are contaminated with bomb pulse 6C1. Variability of natural background 3CV/CI ratios and radioactive decay of 3C1 preclude unambiguous screening for contamination for all samples. However, it has been concluded that some data exhibiting exceptionally high 3C1/Cl ratios indicate unambiguous bomb pulse contamination [1,4]. Statistical analyses provide additiona
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