Natural gas production from the Marcellus Shale formation has significantly changed energy landscape in recent years. Accidental release, including spills, leakage, and seepage of the Marcellus Shale flow back and produced waters can impose risks on natural water resources. With many competing processes during the reactive transport of chemical species, it is not clear what processes are dominant and govern the impacts of accidental release of Marcellus Shale waters (MSW) into natural waters. Here we carry out numerical experiments to explore this largely unexploited aspect using cations from MSW as tracers with a focus on abiotic interactions between cations released from MSW and natural water systems. Reactive transport models were set up using characteristics of natural water systems (aquifers and rivers) in Bradford County, Pennsylvania. Results show that in clay-rich sandstone aquifers, ion exchange plays a key role in determining the maximum concentration and the time scale of released cations in receiving natural waters. In contrast, mineral dissolution and precipitation play a relatively minor role. The relative time scales of recovery τrr, a dimensionless number defined as the ratio of the time needed to return to background concentrations over the residence time of natural waters, vary between 5 and 10 for Na, Ca, and Mg, and between 10 and 20 for Sr and Ba. In rivers and sand and gravel aquifers with negligible clay, τrr values are close to 1 because cations are flushed out at approximately one residence time. These values can be used as first order estimates of time scales of released MSW in natural water systems. This work emphasizes the importance of clay content and suggests that it is more likely to detect contamination in clay-rich geological formations. This work highlights the use of reactive transport modeling in understanding natural attenuation, guiding monitoring, and predicting impacts of contamination for risk assessment.

中文翻译：

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天然水能“记住”马塞勒斯页岩水事件的时间：使用反应输运模型的一阶近似

近年来，马塞勒斯页岩地层的天然气生产大大改变了能源格局。意外泄漏，包括马塞勒斯页岩的回流，渗漏和渗漏，以及采出水会给天然水资源带来风险。在化学物质的反应性运输过程中，由于存在许多竞争过程，因此尚不清楚哪个过程占主导地位，并控制着Marcellus页岩水（MSW）意外释放到天然水中的影响。在这里，我们进行数值实验，以利用城市固体废弃物中的阳离子作为示踪剂来探索这一很大程度上未被利用的方面，重点是从城市固体废弃物与天然水系统释放的阳离子之间的非生物相互作用。利用宾夕法尼亚州布拉德福德县的天然水系统（含水层和河流）的特征建立了反应性运输模型。结果表明，在富含粘土的砂岩含水层中，离子交换在确定接收天然水体中释放的阳离子的最大浓度和时间尺度方面起着关键作用。相反，矿物的溶解和沉淀作用相对较小。恢复的相对时间标度τrr（无量纲数字，定义为恢复到本底浓度所需的时间与天然水的停留时间之比），对于Na，Ca和Mg在5到10之间，在10到20之间变化代表Sr和Ba 在河流和含有微不足道的粘土的砂砾石含水层中，τrr值接近1，因为阳离子在大约一个停留时间被冲洗掉。这些值可以用作自然水系统中已释放MSW时间尺度的一阶估计。这项工作强调了粘土含量的重要性，并表明它更有可能检测出富含粘土的地质构造中的污染。这项工作强调了反应性运输模型在理解自然衰减，指导监测和预测污染影响以进行风险评估中的用途。