ESTIMATION OF EXCISION VOLUMES FOR AREAS OF SUBSURFACE CONTAMINATION DOE/ERSP TECH NOTE NO. 7.0 DATED: April 1978 AUTHOR: B. Friesen, DRI Introduction Subsurface contamination at activity levels above excision criteria is known to exist on several northern islands in the Enewetak Atoll. Long term planning of cleanup action requires knowledge of both surface and subsurface excision volumes. Surface volumes can be estimated, retaining full view of necessary assumptions, from the combined efforts of soil sampling and in situ 24lam gamma surveys; however, estimation of subsurface volumes is more complex. This tech note is intended to describe the method used to derive a broad-brush first estimate of subsurface volumes to be excised. This exercise was undertaken to produce preliminary results in time for a 3-4 May 78 meeting in Washington, D.C. While the demand for data afforded us an opportunity to step through the procedures, the paucity of data in many areas made estimation of volumes very tenuous and highly unsatisfactory. Data Selection All surface and subsurface soil analysis results from an area on a given island were assembled into one list in order by location. Every type of available data was tabulated. In evaluating this data, preference was given first to chemically determined total transuranies, then to laboratory counted 41am gamma, then to gross alpha determinations, either laboratory or field counted. If gross alpha was available from both backhoe and augerprofiles at the same location, preference was given to the backhoe profile data. In essence, the symbols placed on the estimation maps represent the most accurate data available for each point at each level. Estimation Maps Maps were drawn for each of eight areas: Irene 13-N-1 Area; Irene, Central Area; Janet, Easy/X-ray Area; Janet, Item GZ; Pearl, 5-S-3 Area; Pearl, 1-N-1 Area; Sally, Kickapoo GZ; Sally, Yuma GZ. Each map page contained representations of 3 subsurface depths or "plates." The first page for an area contained plates representing the plane at 0, 20 and 40 em. The second page for an area showed the plane at 60, 80 and 100 em. The intent of this graphic portrayal is to simulate a three-dimensional representation. Each page had grid tick marks on all boundaries to facilitate plotting data symbols, and beach lines were shown where applicable. Date Symbols Four symbols were selected to show different levels of activity with the size or intensity of the symbolincreasing with level of activity as follows: . + less than 40 pCi/g = greater than 39.9 but less than 100 * = greater than 99.99 but less than 400 # = greater than 400 The appropriate symbol was then plotted at the appropriate location on the plate map. Only the highest quality value was plotted when more than one was available from the same location and depth. All of the plate maps are labelled to indicate that the plotted symbols represent gross alpha, pCi/g, when in fact approximately half of the values were of better quality than gross alpha. Alternative labelling would have implied better data quality than existed or would have required a more complex selection of symbols to portray both magnitude and quality of each datum entry. B-7-1