## 5 Conclusions and Recommendations  
The results obtained from this ILC of Site groundwater and surface water samples are generally consistent with the findings from the prior 2011 study [6] with respect to observed variability in \({}^{226}\)Ra compared to \({}^{228}\)Ra. The current ILC indicate that \({}^{228}\)Ra data reported by the existing contract laboratory (Lab A) are biased high relative to those reported by Lab B, Lab C, Lab D, and this may be due to differences in laboratory skill and technique among the various laboratory technicians [6]. However, the resulting upward trends in \({}^{228}\)Ra/\({}^{226}\)Ra ratios (observed in both impacted wells and the background location) after the \({}^{228}\)Ra method was changed from 904.0 to Ra-05 are more difficult to explain.  
Although the trends were not entirely consistent, the results from this study show a tendency for total Ra to be higher than dissolved Ra, especially for \({}^{228}\)Ra. For samples where the majority of the laboratories reported low \({}^{228}\)Ra concentrations (<2 pCi/L), it appears that the method and techniques used by Lab A for total \({}^{228}\)Ra is contributing to higher apparent \({}^{228}\)Ra concentrations. Therefore, it was recommended that \({}^{226}\)Ra and \({}^{228}\)Ra analyses (in additional to uranium and other radionuclides) be conducted on a filtered sample for Site compliance evaluations. Analysis for dissolved concentrations would also then be consistent with remaining metals analyses which are also analyzed as dissolved.  
For \({}^{226}\)Ra, it was recommended that M903.0 be replaced with M903.1 which is specific to \({}^{226}\)Ra. Method 903.0 includes all alpha emitters and therefore can result in over-reporting of \({}^{226}\)Ra concentrations. The primary additional alpha emitter which could contribute to overestimation of \({}^{226}\)Ra is \({}^{224}\)Ra, which originates from \({}^{232}\)Th. Although \({}^{232}\)Th concentrations were very low to non-detect in the Site samples, the presence of \({}^{232}\)Th in the solid phase of aquifer materials could release \({}^{224}\)Ra into solution. Even though this study did not indicate that M903.0 results in consistent over-reporting of \({}^{226}\)Ra concentrations, use of M903.1 would better ensure that the appropriate analysis for \({}^{226}\)Ra is being conducted. Alternatively, it could be requested that the laboratory employ a 14-day waiting time which will effectively reduce the contribution of \({}^{224}\)Ra to the reported \({}^{226}\)Ra [6].  
For \({}^{228}\)Ra, this study indicates that specific laboratory techniques, rather than the method (Ra-05) used by Lab A are potentially contributing to excessively elevated valuesbeing reported for \({}^{228}\)Ra at the Site. It should be noted that even though standard methods [5] are being used, many laboratories have developed their own customized Standard Operating Procedures (SOPs) which are often proprietary and difficult to obtain. However, it is recommended that any location whose reported \({}^{226}\)Ra + \({}^{228}\)Ra concentrations exceed groundwater or surface water standards in the future be immediately re-sampled and re-analyzed by the existing laboratory, with a sample split also being collected and sent to an outside laboratory for additional confirmatory analysis.  
This research received no external funding.  
Not applicable.  
The data are contained within the article.  
Thanks to [PERSON] (Worthington Miller Environmental, LLC) for her efforts with laboratory coordination and data validation.  
The author declares no conflict of interest.