Evaluation of sample preservation methods for analysis of selected volatile organic compounds in groundwater at the Idaho National Laboratory, Idaho

During 2020, water samples were collected from 25 wells completed in the eastern Snake River Plain aquifer and from 1 well completed in perched groundwater above the aquifer at the Idaho National Laboratory to determine the effect of different sample-preservation methods on the laboratory determinations of concentrations of volatile organic compounds. Paired-sample sets were collected at each well. One sample in each set was preserved with hydrochloric acid, and one sample without. Both samples were chilled after collection and during shipping to the laboratory for analysis. The samples were analyzed for 61 volatile organic compounds at the U.S. Geological Survey National Water Quality Laboratory in cooperation with the U.S. Department of Energy. A comparison of the reproducibility of the analyses of co-located unpreserved and preserved samples by a relative percent difference method determined that all sample pairs were statistically equivalent. Using a normalized absolute difference method, 81 percent of the analyses were found to be statistically equivalent. This study confirms that the results of analyses of historical collected samples, which were preserved by chilling only, are statistically comparable to the analyses of samples being currently collected and preserved by both hydrochloric acid and chilling, and thus are valid for use in future geochemical evaluations.