Optimizing Sampling, Sample Processing and Analysis Methods for Radon (SUP222/SUPRn) in Water by Liquid Scintillation Counting

摘要

Throughout the United States, laboratories use different sampling methods (“Direct Fill” vs. “Submerged Bottle” methods), sample preparations (“Simultaneous Drawing” vs. “Separate Drawing”), scintillators (“Mineral Oil” vs. “Opti-Fluor”), volume of water plus scintillator in the cocktail (“8 mL plus 8 mL” vs. “10 mL plus 10 mL”), and liquid scintillation counting assays (“Full Spectrum: 0 - 2000 keV” vs. “Region of Interest: 130 - 700 keV”) for analyzing radon (222Rn) in water. We compared these and few other variables on the recovery of radon from two “Proficiency Test (PT)” samples and four “Household Well Water” samples from Georgia. The “130 - 700 keV” assay had significantly higher radon recovery than the “0 - 2000 keV” assay. The “Direct Fill” sampling produced significantly lower radon recovery than the “Submerged Bottle” sampling. “Simultaneous Drawing” of both scintillator and water sample yielded higher radon recovery than “Separate Drawing”. Air bubble in the samples resulted in significant loss of radon gas;and such loss became greater as the air bubble was larger. A “10 mL scintillator + 10 sample” combination appeared better than “8 mL scintillator + 8 mL sample”. Mixing scintillator and sample in the laboratory, when compared with doing it on-site, was found superior for better results and practicality of testing radon in private well waters. “Mineral Oil” scintillator provided higher radon activity than “Opti-Fluor”. However, in 10 consecutive measurements of the two proficiency test (PT) samples at 60 days interval (i.e., with full ingrowing), “Mineral Oil” overestimated the radon activity compared to the predicted/assigned value in most cases, whereas “Opti-Fluor” invariably produced results close to the predicted/assigned value. There were noticeable temporal variations in both radon and uranium concentrations in the study wells;nevertheless, uranium and radon concentrations had good positive correlation. Despite this, the use of uranium concentration over 30 ppb (the MCL of uranium in drinking water) as a trigger for recommending test for radon in well water remains questionable because there may be the safe level of uranium but unsafe level of radon in a well and vice versa.