John and Willie Leone Family Department of Energy and Mineral Engineering, The Pennsylvania State University, University Park, PA 16802, USA.
Department of Civil and Environmental Engineering, The Pennsylvania State University, University Park, PA 16802, USA.
Sci Total Environ. 2018 Jul 15;630:1573-1582. doi: 10.1016/j.scitotenv.2018.02.223. Epub 2018 Mar 7.
Produced or flowback waters from Marcellus Shale gas extraction (MSWs) typically are highly saline and contain chemicals including trace metals, which pose significant concerns on water quality. The natural attenuation of MSW chemicals in groundwater is poorly understood due to the complex interactions between aquifer minerals and MSWs, limiting our capabilities to monitor and predict. Here we combine flow-through experiments and process-based reactive transport modeling to understand mechanisms and quantify the retention of MSW chemicals in a quartz (Qtz) column, a calcite-rich (Cal) column, and a clay-rich (Vrm, vermiculite) column. These columns were used to represent sand, carbonate, and clay-rich aquifers. Results show that the types and extent of water-rock interactions differ significantly across columns. Although it is generally known that clay-rich media retard chemicals and that quartz media minimize water-rock interactions, results here have revealed insights that differ from previous thoughts. We found that the reaction mechanisms are much more complex than merely sorption and mineral precipitation. In clay rich media, trace metals participate in both ion exchange and mineral precipitation. In fact, the majority of metals (50-90%) is retained in the solid via mineral precipitation, which is surprising because we typically expect the dominance of sorption in clay-rich aquifers. In the Cal column, trace metals are retained not only through precipitation but also solid solution partitioning, leading to a total of 75-99% retention. Even in the Qtz column, trace metals are retained at unexpectedly high percentages (20-70%) due to precipitation. The reactive transport model developed here quantitatively differentiates the relative importance of individual processes, and bridges a limited number of experiments to a wide range of natural conditions. This is particularly useful where relatively limited knowledge and data prevent the prediction of complex rock-contaminant interactions and natural attenuation.
马塞勒斯页岩气开采产生的产出水或回注水通常具有高盐度，并含有痕量金属等化学物质，这对水质构成了重大威胁。由于含水层矿物与马塞勒斯页岩气之间的复杂相互作用，地下水对马塞勒斯页岩气化学物质的自然衰减过程了解甚少，这限制了我们监测和预测的能力。在这里，我们结合了流动实验和基于过程的反应传输模型，以了解马塞勒斯页岩气化学物质在石英（Qtz）柱、富方解石（Cal）柱和富蒙脱石（Vrm，蛭石）柱中的保留机制并量化其保留量。这些柱子用于代表砂岩、碳酸盐岩和富蒙脱石含水层。结果表明，不同柱子之间的水岩相互作用的类型和程度有很大差异。尽管众所周知，富蒙脱石介质会阻碍化学物质的迁移，而石英介质会最小化水岩相互作用，但这里的结果揭示了与以往观点不同的见解。我们发现，反应机制远比吸附和矿物沉淀复杂。在富蒙脱石介质中，痕量金属既参与离子交换又参与矿物沉淀。事实上，大部分金属（约 50-90%）通过矿物沉淀保留在固体中，这令人惊讶，因为我们通常期望在富蒙脱石含水层中吸附占据主导地位。在 Cal 柱中，痕量金属不仅通过沉淀而且通过固溶体分配被保留，导致总保留率为 75-99%。即使在 Qtz 柱中，由于沉淀，痕量金属的保留率也出人意料地高（约 20-70%）。这里开发的反应传输模型定量地区分了各个过程的相对重要性，并将有限数量的实验扩展到广泛的自然条件。在相对有限的知识和数据防止预测复杂的岩石污染物相互作用和自然衰减的情况下，这一点尤其有用。
