Parker Beth L, Chapman Steven W, Guilbeault Martin A
School of Engineering, University of Guelph 50, Stone Road East, Guelph, Ontario, Canada N1G 2W1.
J Contam Hydrol. 2008 Nov 14;102(1-2):86-104. doi: 10.1016/j.jconhyd.2008.07.003. Epub 2008 Jul 15.
This paper concludes that back diffusion from one or a few thin clayey beds in a sand aquifer can cause contaminant persistence above MCLs in a sand aquifer long after the source zone initially causing the plume is isolated or removed. This conclusion is based on an intensive case study of a TCE contaminated site in Florida, with the processes evaluated using numerical modeling. At this site, the TCE DNAPL zone formed decades ago, and was hydraulically isolated by means of an innovative system performing groundwater extraction, treatment and re-injection. Treated water is re-injected in a row of injection wells situated a short distance downgradient of the extraction wells, creating a clean-water displacement front to efficiently flush the downgradient plume. This scheme avoids the creation of stagnation zones typical of most groundwater pump-and-treat systems, thereby minimizing the time for aquifer flushing and therefore downgradient cleanup. The system began operation in August 2002 and although the performance monitoring shows substantial declines in concentrations, detectable levels of TCE and degradation products persist downgradient of the re-injection wells, long after the TCE should have disappeared based on calculations assuming a nearly homogenous sand aquifer. Three hypotheses were assessed for this plume persistence: 1) incomplete source-zone capture, 2) DNAPL occurrence downgradient of the re-injection wells, and 3) back diffusion from one or more thin clay beds in the aquifer. After careful consideration, the first two hypotheses were eliminated, leaving back diffusion as the only plausible hypothesis, supported by detailed measurements of VOC concentrations within and near the clay beds and also by numerical model simulations that closely represent the field site hydrogeologic conditions. The model was also used to simulate a more generalized, hypothetical situation where more thin clayey beds occur in a sand aquifer with an underlying aquitard. While there is no doubt that DNAPL source mass reduction can eventually improve downgradient groundwater quality, the magnitude and time scale over which the improvement occurs is the major uncertainty given current characterization approaches. This study shows that even one thin clay bed, less than 0.2 m thick, can cause plume persistence due to back diffusion for several years or even decades after the flux from the source is completely isolated. Thin clay beds, which have a large storage capacity for dissolved and sorbed contaminant mass, are common in many types of sandy aquifers. However, without careful inspection of continuous cores and sampling, such thin clay beds, and their potential for causing long-term back-diffusion effects, can easily go unnoticed during site characterization.
本文得出结论，在砂质含水层中，来自一层或几层薄黏土层的反向扩散可能会导致污染物在源区最初形成羽流后很久仍持续存在于砂质含水层中，且浓度高于最大污染物浓度限值（MCL），即便最初产生羽流的源区已被隔离或清除。这一结论基于对佛罗里达州一个受三氯乙烯（TCE）污染场地的深入案例研究，其中使用数值模型对相关过程进行了评估。在该场地，TCE重质非水相液体（DNAPL）区域形成于数十年前，并通过一套创新的系统进行水力隔离，该系统包括地下水抽取、处理和回注。处理后的水被回注到位于抽水井下游不远处的一排注水井中，形成一个清水置换前沿，以有效冲洗下游的羽流。该方案避免了大多数地下水抽提处理系统中典型的滞水区的形成，从而最大限度地缩短了含水层冲洗时间以及下游清理时间。该系统于2002年8月开始运行，尽管性能监测显示污染物浓度大幅下降，但在回注井下游仍可检测到TCE及其降解产物，而根据假设砂质含水层近乎均质的计算，TCE此时本应已消失。针对这种羽流持续存在的现象评估了三种假设：1）源区捕获不完全；2）回注井下游存在DNAPL；3）含水层中一层或多层薄黏土层的反向扩散。经过仔细考虑，前两种假设被排除，反向扩散成为唯一合理的假设，这得到了黏土层内及附近挥发性有机化合物（VOC）浓度的详细测量结果以及紧密反映现场水文地质条件的数值模型模拟的支持。该模型还被用于模拟一种更具普遍性的假设情况，即在一个下方有隔水层的砂质含水层中存在更多薄黏土层。虽然毫无疑问，减少DNAPL源质量最终可以改善下游地下水质量，但鉴于当前的表征方法，改善发生的幅度和时间尺度是主要的不确定性因素。本研究表明，即使是一层厚度小于0.2米的薄黏土层，也可能由于反向扩散导致羽流在源通量完全隔离后的数年甚至数十年内持续存在。薄黏土层对溶解态和吸附态污染物质量具有较大的储存能力，在许多类型的砂质含水层中都很常见。然而，如果没有对连续岩芯进行仔细检查和采样，这种薄黏土层及其导致长期反向扩散效应的可能性在场地表征过程中很容易被忽视。
