There are several key data gaps in our understanding of Natural Source Zone Depletion (NSZD) processes at sites impacted by light nonaqueous phase liquid (LNAPL), and quantifying NSZD rates can be challenging due to the inherent differences in measurement methods. In this study, four different NSZD measurement methods (oxygen influx measured by the Gradient Method, long‐term carbon dioxide efflux measured with Carbon Traps, instantaneous carbon dioxide efflux measured with Dynamic Closed Chambers (DCC LI‐COR), and the long‐term heat flux from biodegradation measured by Thermal NSZD monitoring), as well as LNAPL composition and dissolved gas sampling, were applied at a site in Southern California. These techniques were used to evaluate key questions such as: (1) how do different NSZD rate measurement methods compare, and what causes variability in NSZD results?; (2) to what extent NSZD processes are occurring in LNAPL within the saturated zone?; and (3) how is NSZD related to LNAPL composition change over time? Carbon Traps and Thermal NSZD monitoring measurement methods provided the most consistent NSZD data at this geologically heterogeneous site, with two location average NSZD rates of 540 and 480 gal/acre/year, respectively. Overall, comparisons of NSZD rates between methods were challenging due to different measurement timeframes, significant temporal and spatial heterogeneity, and operational challenges with two of the NSZD methods. Finally, samples of subsurface LNAPL were collected for analysis in 2007 and 2016; results indicated that diesel‐range constituents were already very degraded and anaerobic degradation of gasoline‐range constituents was ongoing. A LNAPL depletion model (Douglas et al. 1996) applied to the measured LNAPL composition change appeared to greatly overestimate the amount of LNAPL depletion compared to the measured NSZD rate, but did provide an independent semiqualitative line of evidence that LNAPL was being depleted by active NSZD processes at the site.

中文翻译：

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四种测量技术在了解LNAPL站点的自然源区耗竭过程中的应用

在我们对受轻非水相液体（LNAPL）影响的地点的自然源区耗竭（NSZD）过程的理解中，存在几个关键的数据缺口，由于测量方法的固有差异，量化NSZD率可能会带来挑战。在这项研究中，我们使用了四种不同的NSZD测量方法（通过梯度法测量的氧气流入量，通过碳阱测量的长期二氧化碳排放量，通过动态封闭腔室（DCC LI-COR）测量的瞬时二氧化碳排放量以及长期的通过热NSZD监测测量的生物降解产生的热通量，以及LNAPL成分和溶解气体采样，都在南加州的一个地点应用。这些技术用于评估关键问题，例如：（1）如何比较不同的NSZD速率测量方法，是什么导致NSZD结果的差异？（2）在饱和带内的LNAPL中，NSZD过程发生到什么程度？（3）NSZD与LNAPL组成如何随时间变化？碳捕集阱和热NSZD监测测量方法在该地质异质站点提供了最一致的NSZD数据，两个位置的平均NSZD率分别为540和480 gal /英亩/年。总体而言，由于不同的测量时间框架，明显的时间和空间异质性以及两种NSZD方法的操作挑战，因此比较方法之间的NSZD率具有挑战性。最后，在2007年和2016年收集了地下LNAPL样本进行分析；结果表明，柴油范围成分已经非常降解，汽油范围成分的厌氧降解正在进行中。与测量的NSZD速率相比，应用于测量的LNAPL组成变化的LNAPL损耗模型（Douglas等，1996）似乎大大高估了LNAPL损耗的量，但确实提供了独立的半定性证据，表明LNAPL被活性物质消耗现场的NSZD流程。