a » 0 NN below 40 pCi/g. 2. Reduce the average concentration of the "defined regicn’’ to some lower number which shall be determined by cost-benefit considerations, but will usually not be belowlocal background. (4) Condition D: An assay area whose average Pu concentration is any 5 ca thickness of soil below the surface layer when measured® (at the 67 percent confidence level) to exceed 400 pCi/g will be excavated end measured iteratively until its average Pu concentration. in the new S cm layer is found by measurement (at the 50 percent confidence level) to be reduced in the defined region to some lower number which shall be determined by cost - benefit considerations, but will | usually not be below local background. Footnotes: A Assay Area. The field of view of the in situ detector in its normal operating position; typically a 28 meter diameter circle of 3 - 5 cm in cepth. Scattered measurement can be used to estimate average concentrations between such measurements by means of a linear estimator progran known as "'Krigging." P Statistically, two-thirds of the time the actual concentration will be below the guide number. One-third of the time the actual concentration may exceed the number by some percentage which must be empirically deter- mined (up to 20-30 percent, as an estimate). — This is similar to using a - ~ 50 percent confidence level with a mmerical guide 20-30 percent (estimated) lower. If a 90 percent confidence level were used with the numerical guide, the equivalent guide at a 50 percent confidence level would