Laboratory of Environmental Engineering, Department of Civil Engineering and Computer Science Engineering, University of Rome Tor Vergata, Via del Politecnico 1, 00133 Rome, Italy.
Laboratory of Environmental Engineering, Department of Civil Engineering and Computer Science Engineering, University of Rome Tor Vergata, Via del Politecnico 1, 00133 Rome, Italy.
J Contam Hydrol. 2021 Aug;241:103807. doi: 10.1016/j.jconhyd.2021.103807. Epub 2021 Mar 27.
Rates of natural source zone depletion (NSZD) are increasingly being used to aid remedial decision making and light non-aqueous phase liquid (LNAPL) longevity estimates at petroleum release sites. Current NSZD estimate methods, based on analyses of carbon dioxide (CO) and oxygen (O) soil-gas concentration gradients ("gradient method") assume linear concentration profiles with depth. This assumption can underestimate the concentration gradients especially above LNAPL sources that are typically characterized by curvilinear or semi-curvilinear O and CO concentration profiles. In this work, we proposed a new method that relies on calculating the O and CO concentration gradient using a first-order reaction model. The method requires an estimate of the diffusive reaction length that can be easily derived from soil-gas concentration data. A simple step-by-step guide for applying the new method is provided. Nomographs were also developed to facilitate method application. Application of the nomographs using field data from published literature showed that NSZD rates could be underestimated by nearly an order of magnitude if reactivity in the vadose zone is not accounted for. The new method helps refine NSZD rates estimation and improve risk-based decision making at certain petroleum contaminated sites.
自然源区衰减（NSZD）速率越来越多地被用于辅助补救决策和轻质非水相液体（LNAPL）在石油泄漏点的寿命估计。当前的 NSZD 估计方法基于二氧化碳（CO）和氧气（O）土壤气体浓度梯度的分析（“梯度法”），假设浓度随深度呈线性分布。这种假设可能会低估浓度梯度，特别是在 LNAPL 源上方，其通常具有曲线或半曲线 O 和 CO 浓度分布。在这项工作中，我们提出了一种新的方法，该方法依赖于使用一阶反应模型计算 O 和 CO 浓度梯度。该方法需要估计扩散反应长度，该长度可以很容易地从土壤气体浓度数据中得出。提供了一个简单的分步指南，用于应用新方法。还开发了图表以方便方法的应用。使用来自已发表文献的现场数据应用图表表明，如果不考虑包气带中的反应性，NSZD 速率可能会被低估近一个数量级。该新方法有助于改进某些石油污染场地的 NSZD 速率估计和基于风险的决策。
