Fitting statistical frequency distributions to baseline environmental monitoring data for the Waste Isolation Pilot Plant

A preoperational environmental monitoring program has been active at the Waste Isolation Pilot Plant (WIPP) since July 1985 to determine the levels of radioactivity in various environmental media prior to the first receipt of waste. Ideally, statistical frequency distributions based on preoperational data can be found to accurately model the probability of obtaining a particular result during operations, assuming preoperational processes are intact. This paper presents a preliminary evaluation of four families of probability distribution functions (normal, lognormal, Weibull, and gamma) for their relative fit to 96 weeks of preoperational gross alpha and beta activity data from eight low-volume continuous air sampler locations. The results indicate that for gross beta counts, the lognormal gave the best fit to most data sets, with the gamma, normal, and Weibull following in that order. For gross alpha counts, the Weibull gave the best fits overall, with the gamma, normal and lognormal following