Commentary on Studies of 36Cl in the Exploratory Studies Facility at Yucca Mountain, Nevada

ABSTRACT36Cl data from the Exploratory Studies Facility (ESF) at Yucca Mountain, Nevada, reported by LANL researchers [1,2,3,4] provide important empirical constraints on the velocity and pathways for water flow under natural conditions in the unsaturated zone at Yucca Mountain. It has been concluded that some data exhibiting exceptionally high 36Cl/Cl ratios (i.e., greater than 1250×10−15 [3]) indicate unambiguous bomb pulse contamination and fast groundwater travel times to depths of the proposed waste emplacement horizon. Several lines of evidence indicate that some ESF samples with 36Cl/Cl ratios greater than 900×10−15 to 1000×10−15 are also affected by bomb pulse contamination. The distribution of 36Cl/Cl data can be well represented by a mixture of two normally distributed populations, which are hypothesized to represent uncontaminated and bomb pulse contaminated sets. The transition between these groups occurs at 36Cl/Cl between 600×10−15and 1000× 10−15, and the hypothesized contaminated samvles comprise 20 to 25 percent of the total. Shapiro-Wilk statistics confirm that when samples with 36Cl/Cl ratios greater than about 1000×10−15 are included, the distribution deviates from normality. Variation in the Earth's magnetic field, which is a primary mechanism for variation in natural 36Cl production, also appears to be normally distributed, supporting the hypothesis that natural, relatively undecayed 36Cl/Cl ratios are normally distributed. Further evidence that samples with 36Cl/Cl greater than 950×10−15 to 1000×10−15 are likely to have a bomb pulse component is provided by the spatial correlation of these samples with those containing unambiguous contamination at values greater than 1250×10−15. Zones in which elevated 36Cl/Cl ratios are localized constitute about 23 percent of the length of the ESF at the level of the proposed waste emplacement horizon.