Environmental Programs, Los Alamos National Laboratory, Los Alamos, NM 87545, USA.
Environ Monit Assess. 2011 Jan;172(1-4):135-43. doi: 10.1007/s10661-010-1322-x. Epub 2010 Feb 6.
Hazardous contaminants buried within vadose zones can accumulate in soil gas. The concentrations and spatial extent of these contaminants are measured to evaluate potential transport to groundwater for public risk evaluation. Tritium is an important contaminant found and monitored for in vadose zones across numerous sites within the US nuclear weapons complex, including Los Alamos National Laboratory. The extraction, collection, and laboratory analysis of tritium from subterranean soil gas presents numerous technical challenges that have not been fully studied. Particularly, the lack of moisture in the soil gas in the vadose zone makes it difficult to obtain enough sample (e.g., > 5 g) to provide for the required measurement sensitivity, and often, only small amounts of moisture can be collected. Further, although silica gel has high affinity for water vapor and is prebaked prior to sampling, there is still sufficient residual moisture in the prebaked gel to dilute the relatively small amount of sampled moisture; thereby, significantly lowering the "true" tritium concentration in the soil gas. This paper provides an evaluation of the magnitude of the bias from dilution, provides methods to correct past measurements by applying a correction factor (CF), and evaluates the uncertainty of the CF values. For this, 10,000 Monte Carlo calculations were performed, and distribution parameters of CF values were determined and evaluated. The mean and standard deviation of the distribution of CF values were 1.53 ± 0.36, and the minimum, median, and maximum values were 1.14, 1.43, and 5.27, respectively.
包气带内埋藏的有害污染物会在土壤气体中积聚。为了评估对地下水的潜在迁移风险，需要对这些污染物的浓度和空间分布进行测量。氚是美国核武器设施内众多地点包气带中发现和监测的一种重要污染物，包括洛斯阿拉莫斯国家实验室。从地下土壤气体中提取、收集和实验室分析氚，存在许多尚未充分研究的技术挑战。特别是，包气带土壤气体中的水分缺乏，使得难以获得足够的样本（例如，> 5 g）以提供所需的测量灵敏度，而且通常只能收集少量的水分。此外，尽管硅胶对水蒸气具有高亲和力，并且在采样前进行了预烘烤，但预烘烤的凝胶中仍有足够的残留水分来稀释相对较小量的采样水分；从而显著降低了土壤气体中“真实”氚浓度。本文评估了稀释带来的偏差的大小，提供了通过应用校正因子（CF）来校正过去测量值的方法，并评估了 CF 值的不确定性。为此，进行了 10,000 次蒙特卡罗计算，确定并评估了 CF 值的分布参数。CF 值分布的平均值和标准差分别为 1.53 ± 0.36，最小值、中位数和最大值分别为 1.14、1.43 和 5.27。
