include the balance of the vegetation samples. With the inclusion of these samples, a complete examination of 235°240py correlation with 225U was possible. RESULTS AND DISCUSSION Accurate interpretation of the measurement results demands that all reported specific activities be representative of the extent of sample (vegetation) contamination, i.e., necessary corrections for local background have been applied. Since Area 11 is also known as "Plutonium Valley," it was not at all surprising to find that the plutonium atom ratios from the four selected vegetations showed no requirement for background corrections. Such was not the case for the uranium. The observed 738y/235y atom ratios were influenced by the natural uranium background in degrees varying from slight to significant. In order to eliminate these natural uranium influences, mathematical expressions were derived which provided for resolution of the local background (b) and contaminant (c) fractions via known atom ratios. Detailed discussion of this mathematical technique may be found in MCL-TM-76-26 dated April 20, 1976. Briefly, the atom ratio of 2354 in the contaminant to 73°5U in the background was calculated from the following general expression: © 235 235 U, b U - U o -wU c where UL = atom ratio of isotope X to *3°U in the local background. Uy = atom ratio of isotope X to *35U in the observed sample, and U. = atom ratio of isotope X to 235y in the contaminant or extraneous source. Although values of the 235y /235y, atom ratio may be calculated from Eq. (1) for each of the uranium isofopic atom ratios (*38y/235y, 236y/235y, and 234y/235y), only the *38y/235y ratios were used. The reason for this decision was based solely on an inability to accurately define the °36y/235y and 2341/2359 atom ratios of the contaminant. contaminant *38y/235y atom ratio. Such was not the case for the For the type of uranium assumed to have contaminated the sampling area, an uncertainty in the possible range of 238y/235y atom ratios of no greater than 2% can be accurately predicted. the 235y /235y, Once ratios were determined, the atoms of 235y per gram associated with the contaminant, *3°U were determined from 2355 — 233) o 235, 235, (2) 235. +1 235, 81