Natural Attenuation of Groundwater Contaminant Plumes in Riverbeds: Control of Hyporheic Zone Mixing

1437021 hester地下水污染物羽流在河床中的自然衰减：潜流带混合控制受污染的地下水最终进入地表水，包括溪流、河流和河口，对人类和生态健康构成威胁。事实上，美国环境保护署发现，已确定的有害废物场所中有一半会影响地表水。在农业区，过剩的养分也经常流入河流。当地下水污染物向河流移动时，它们最终会穿过河流下方和附近的潜流带，在那里，浅层沉积物中的地表水和地下水混合产生的条件往往比上覆的地表水或更深的地下水更具反应性。在上梯度含水层中几乎没有降解的污染物，一旦到达低渗带（低渗自然衰减），降解率可达100%。然而，上涌的污染物通常需要与潜流区地表水中的反应物混合，以便利用潜流自然衰减的巨大潜力。该项目的结果将改变地下水修复，风险评估和总最大日负荷，以解决由于地下水污染与地表水相交而造成的生态，人类健康和人类娱乐风险和损害。了解潜在的自然衰减电位如何随水文和地貌条件变化，将有助于计算特定地点预期的潜在自然衰减量。拟议的项目还将导致后续的项目，解决特定的污染物和工程方法，以加强低密度自然衰减，如通过河岸重新造林的碳修正。这些实验室实验将成为PI的地表水-地下水课程的优秀工具，使学生以其他方式无法实现的方式可视化潜带过程。PI将与工程学院的工程多样性增强中心合作，招募代表性不足的本科生参与项目研究。对这种混合的控制几乎没有受到重视，但最近的工作表明，这种混合对水文条件高度敏感。为了挖掘潜流自然衰减的潜力，有必要更好地了解这种对潜流混合的控制，并更好地将混合依赖反应与稀释等相关过程区分开来。这将有助于预测在不同水文、气候和生物地球化学条件的河流中，低潜自然衰减是如何变化的；在不同类型的污染物中；跨越时间尺度，如风暴和季节；作为对工程改进的回应。该项目将是第一个研究上升流污染物的行为如何受到潜流带实际复杂的水文流动路径的影响的项目。因此，这项研究的示踪剂和生物地球化学部分都是全新的。例如，该项目将首次测量当地的分散性，并首次估计受实际潜流带水流条件影响的河流沉积物的微生物生长参数。这些参数需要广泛的生物地球化学反应模拟的潜带过程。该项目还将在方法论上具有开创性，包括首次在实验室模拟示踪剂上升流，并与潜流带平流的地表水混合。通过关注广泛相关的过程，如示踪剂、氧和碳的运输和转化，该项目将揭示亚隐区各种污染物的转化，包括金属、作为电子受体的有机污染物（如氯化溶剂）和作为电子供体的有机污染物（如石油碳氢化合物）。这项工作将支持未来评估其他电子受体（如硝酸盐、铁）、特定污染物和夹带特定碳的研究，以及将这些实验扩展到更大的实验室实验（流动水槽）和现场。

1437021HesterNatural Attenuation of Groundwater Contaminant Plumes in Riverbeds: Control by Hyporheic Zone MixingContaminated groundwater eventually exits to surface water including streams, rivers, and estuaries, posing a threat to human and ecological health. In fact, the US EPA found that half of identified hazardous waste sites impact surface water. Excess nutrients also frequently well-up into rivers in agricultural areas. As groundwater contaminants move toward rivers, they eventually cross through the hyporheic zone beneath and adjacent to rivers, where mixing of surface water and groundwater in shallow sediments creates conditions that are often far more reactive than in overlying surface water or deeper groundwater. Contaminants that have degraded little in up-gradient aquifers can degrade up to 100 % once they reach the hyporheic zone (hyporheic natural attenuation). Yet upwelling contaminants often require mixing with reactants from surface water in the hyporheic zone in order to capitalize on this great potential for hyporheic natural attenuation. The results of this project will transform groundwater remediation, risk assessment, and total maximum daily loads to address ecological, human health, and human recreational risks and impairments due to groundwater contamination intersecting surface water. Knowing how hyporheic natural attenuation potential varies with hydrologic and geomorphic conditions will allow calculating the amount of hyporheic natural attenuation expected at specific sites. The proposed project will also lead to subsequent projects addressing specific contaminants and engineering approaches to enhance hyporheic natural attenuation, such as carbon amendment by riparian reforestation. These lab experiments will serve as excellent tools for students in the PI's surface water-groundwater class to visualize hyporheic zone processes in ways that are otherwise not possible. The PI will work with the School of Engineering's Center for the Enhancement of Engineering Diversity to recruit under-represented undergraduates to participate in project research.Controls on such mixing have received almost no attention, yet recent work shows that such mixing is highly sensitive to hydrologic conditions. To tap the potential for hyporheic natural attenuation, it is necessary to better understand such controls on hyporheic mixing, and better distinguish mixing-dependent reactions from related processes such as dilution. This will allow prediction of how hyporheic natural attenuation varies among rivers of differing hydrologic, climatic, and biogeochemical conditions; among different types of contaminants; across time scales such as storms and seasons; and in response to engineered enhancements. This project will be the first to examine how the behavior of upwelling contaminants is affected by realistically complex hydrologic flow paths in the hyporheic zone. Both the tracer and biogeochemical portions of this study are therefore fundamentally novel. For example, this project will produce the first measurements of local dispersivities and first estimates of microbial growth parameters for riverine sediment subject to realistic hyporheic zone flow conditions. Such parameters are required for a wide range of biogeochemical reaction modeling of hyporheic zone processes. This project will also be groundbreaking methodologically, including the first laboratory simulation of upwelling of tracer and mixing with surface water advecting through the hyporheic zone. By focusing on broadly relevant processes like transport and transformation of tracers, oxygen, and carbon, this project will shed light on transformations of a wide range of pollutants in the hyporheic zone, including metals, organic contaminants that act as electron acceptors (e.g., chlorinated solvents), and organic contaminants that act as electron donors (e.g., petroleum hydrocarbons). This work will support future studies that evaluate other electron acceptors (e.g., nitrate, iron), specific contaminants, and entrained particular carbon, as well as extension of these experiments to larger lab experiments (flowing flume) and field sites.