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 (CO₂) and oxygen (O₂) 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 O₂ and CO₂ concentration profiles. In this work, we proposed a new method that relies on calculating the O₂ and CO₂ 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.

天然源区耗竭率 (NSZD) 越来越多地用于辅助补救决策和石油释放地点的轻质非水相液体 (LNAPL) 寿命估计。当前的 NSZD 估计方法基于对二氧化碳 (CO ₂ ) 和氧气 (O ₂ ) 土壤-气体浓度梯度的分析（“梯度法”），假设浓度分布随深度呈线性。这种假设可能会低估浓度梯度，尤其是高于 LNAPL 源的浓度梯度，这些源通常以曲线或半曲线 O ₂和 CO ₂浓度分布为特征。在这项工作中，我们提出了一种依赖于计算 O ₂和 CO ₂的新方法使用一级反应模型的浓度梯度。该方法需要对扩散反应长度的估计，这可以很容易地从土壤气体浓度数据中推导出来。提供了应用新方法的简单分步指南。还开发了列线图以促进方法应用。使用已发表文献中的现场数据应用列线图表明，如果不考虑包气带中的反应性，NSZD 率可能会被低估近一个数量级。新方法有助于改进 NSZD 速率估计并改进某些石油污染场地的基于风险的决策。