## 1 Introduction  
An interlaboratory comparison (ILC) is an exercise carried out by a group of laboratories to compare their performance or to assess a measurement standard, and is typically used to (1) assess random variation in measurements across a population of laboratories, (2) determine the systematic differences in results among a fixed set of laboratories, or (3) determine the value of a physical property of an artifact or a population of artifacts [1]. An ILC was conducted to evaluate the variability of \({}^{226}\)Ra and \({}^{228}\)Ra activity concentrations (hereafter referred to as concentrations) in groundwater and an adjacent surface water input/ment (Impoundment) at a former U mine and mill (Site) located in Converse County, WY, USA. Assessment of \({}^{226}\)Ra + \({}^{228}\)Ra trends at the Site compliance well and selected groundwater monitoring wells has shown that \({}^{226}\)Ra + \({}^{228}\)Ra concentrations are highly variable, and there is some uncertainty regarding the quality of isotopic Ra data reported for the Site. The most common isotopes of Ra are \({}^{226}\)Ra (produced by radioactive decay of \({}^{238}\)U) and \({}^{228}\)Ra (produced during radioactive decay of \({}^{232}\)Th) [2, 3]. At the Site, \({}^{228}\)Ra is the dominate Ra isotope in groundwater, whereas \({}^{226}\)Ra is the dominant Ra isotope in the adjacent Impoundment. At the Site compliance well (Well 176), the historic concentrations of \({}^{226}\)Ra + \({}^{228}\)Ra have periodically exceeded the compliance standard (USEPA Maximum Contaminant Level, MCL) of 5 pCi/L; since approximately 2005, however, the concentrations have shown an increasing trend and with more frequent exceedances of the MCL. In addition to a downgradient well (Well 179), an unexpected increasing trend and high variability in \({}^{226}\)Ra + \({}^{228}\)Ra concentrations have also been measured in the unimpacted Sitebackground well (Well 182) since 2005 (Figure 1). The implication of \({}^{226}\)Ra + \({}^{228}\)Ra concentrations exceeding the MCL at the compliance location (Well 176) is that the Site boundary may potentially need to be expanded to demonstrate that downgradient groundwater \({}^{226}\)Ra + \({}^{228}\)Ra concentrations will remain protective of human health and the environment. A preliminary internal evaluation of \({}^{226}\)Ra + \({}^{228}\)Ra results for Site groundwater indicated that increasing trends and high variability could be the result of analytical error by the contract laboratory based on several key observations: (1) increases in both \({}^{226}\)Ra + \({}^{228}\)Ra and \({}^{228}\)Ra/\({}^{226}\)Ra ratios occurred subsequent to a change in the \({}^{228}\)Ra analytical method from Method 904.0 (M904.0) to Method Ra-05 (Ra-05) in 2005 (Figures 1 and 2), (2) ratios of \({}^{225}\)Ra/\({}^{226}\)Ra have been increasing in the non-impacted Site background Well 182 concurrently with several other monitoring wells and the Impoundment (Figure 2), and (3) re-analysis of samples with unusually elevated \({}^{228}\)Ra has demonstrated at least one occurrence of poor reproducibility and significant analytical error in \({}^{228}\)Ra measurements. For example, the reported \({}^{228}\)Ra value for Site Well 174 (3 rd Quarter in 2016) was 8.6 pCi/L, while the field duplicate value was reported as <1.3 pCi/L. Re-analysis results for these two samples were reported as 2.8 pCi/L \({}^{228}\)Ra for the primary sample and 2.7 pCi/L \({}^{228}\)Ra for the duplicate sample. In addition to the poor reproducibility of the field duplicate, additional \({}^{228}\)Ra results appeared to be inaccurate. For instance, \({}^{228}\)Ra for the unimpacted background well (Well 182) was reported as 27 pCi/L which is not consistent with much lower historical results for the background location. Reanalysis of the sample from Well 182 produced a revised result of 2.0 pCi/L.