DISTRIBUTION OF RN-222 IN COMMUNITY GROUNDWATER SYSTEMS - ANALYSIS OFTYPE-I LEFT-CENSORED DATA WITH SINGLE CENSORING POINT

Community groundwater systems serve as the primary source of household water for over 81 million people in the United States. In many reside nces, groundwater may contain significant concentrations of radon-222, serving as a potential source of human exposure. An extensive databas e of measured radon concentrations in community groundwater systems is available from the National Inorganic and Radionuclides Survey (NIRS) . However, estimation of key descriptive statistics and distribution f unctions for these data is complicated by the fact that the data are c ensored, that is, a portion of the NIRS samples have concentrations be low the minimum reporting level of 100 pCi/L. A variety of parameter e stimation techniques were investigated for fitting the NIRS data, incl uding maximum likelihood and regression methods. In general, the NIRS data were found to be well-described by lognormal distributions. The r esulting lognormal distributions, along with quantitative estimates of uncertainty, are presented for five groundwater system sizes and for eight geographic regions.