Smart tracers and distributed sensor networks for quantifying the metabolic activity in streambed reactivity hotspots

监管机构和河流管理者主要关注的是，在河流与周围含水层连接良好的地方，受污染的地下水上涌可能对地表水质量和河流的生态条件构成严重威胁，并造成重大的经济损失。为了评估与污染地下水排放到地表水有关的环境风险，监管机构和河流管理者迫切需要地下水含水层和含水层-河流界面自然衰减潜力的详细信息。由于缺乏有机碳和微生物活动，地下水含水层的自然衰减潜力往往有限，但地下水和河床地表水的混合可以提供更高浓度的有机碳，从而刺激代谢活动增强，增加自然衰减。研究者的初步研究表明，低地河流的含水层-河流界面可能在河床泥炭和粘土结构下的缺氧、高碳袋中存在代谢活动热点。该项目旨在确定这些地点并量化这些河床反应性热点的代谢活动，以便为监管者和河流管理者提供评估含水层-河流界面自然衰减潜力迫切需要的知识。因此，本研究采用光纤分布式温度传感技术（FO-DTS），沿着铺设在河床上的光纤电缆，对河床温度模式进行高细节监测（精度0.01℃，空间分辨率2米）。在夏季，当地下水和地表水的温度差异为6 - 10℃时，它将识别出地下水上涌强烈的地区（冷点）和地下水上涌抑制的地区（暖点）。在确定的暖点将安装河床多层微型压力表，在这些暖点，预计冷地下水上涌的延迟将表明潜在的反应热点，河床泥炭结构下的代谢活动增加。为了进行比较，还将在地下水密集上涌造成的确定的冷点安装压力表，这表明地下水与地表水的连通性高，停留时间短，自然代谢活性可能较低。一种智能示踪剂，结合了反应性（雷唑脲）和非反应性（NaCl）化合物，将被注入压力计底部（150厘米深），并沿着多层压力计每隔15厘米分析突破曲线。NaCl突破曲线将用于计算地下水上涌速率和停留时间，而再氮脲还原为强荧光物质间苯二酚将用于反应性衰变模拟，以指示相应深度的微生物代谢活性。将代谢活动和停留时间与压计位置的硝酸盐浓度、氧化还原电位、溶解氧和有机碳含量进行比较，以证明代谢活动热点增加了自然衰减电位。该项目的结果将为监管机构和流域管理者提供迫切需要的了解微生物活性增强的河床反应热点的自然衰减潜力。该项目提供的知识对一系列相关科学学科以及监管机构和流域管理者都非常重要。热条件是水文系统生态系统健康的一个组成部分。提高对河床热流的认识将直接有利于水生生态学家和生态水文学家，而对代谢活动热点的深入了解将极大地有利于未来含水层-河流界面多污染物反应输送和生物地球化学循环的研究。

There are major concerns amongst regulators and river managers that at locations where rivers are well connected to the surrounding aquifer, the up-welling of contaminated groundwater can represent a severe risk to the surface water quality and the ecological conditions in the river and cause significant economical damage. For assessing the environmental risks associated with the discharge of contaminated groundwater into surface waters, regulators and river managers urgently require detailed information of the natural attenuation potential of groundwater aquifers and aquifer-river interfaces. While the natural attenuation potential in groundwater aquifers is often limited due to a lack organic carbon and microbial activity, the mixing of groundwater and surface water in streambeds can provide higher concentrations of organic carbon and thus, stimulate enhanced metabolic activity and increase natural attenuation. Initial research of the investigator indicated that aquifer-river interfaces of lowland rivers may contain hotspots of metabolic activity in anoxic, high carbon pockets underneath peat and clay structures in the streambed. This project is designed to identify such locations and quantify the metabolic activity of these streambed reactivity hotspots in order to provide regulators and river managers with the knowledge urgently required for assessing the natural attenuation potential of aquifer-river interfaces. This study therefore applies fibre-optic Distributed Temperature Sensing (FO-DTS) to monitor streambed temperature patterns in high detail (0.01 C accuracy, 2 m spatial resolution) along a fibre-optic cable deployed in the streambed. It will identify locations with intensive groundwater up-welling (cold spots) against areas with inhibited groundwater up-welling (warm spots) during summer months when groundwater and surface water temperatures differ by 6 - 10 C. Streambed multi-level mini-piezometers will be installed in identified warm spots, where the delayed up-welling of cold groundwater is expected to indicate potential reactivity hotspots with increased metabolic activity underneath streambed peat structures. For comparison, piezometers will also be installed in identified cold spots caused by intensive groundwater up-welling, indicating high groundwater-surface water connectivity with low residence times and potentially low natural metabolic activity. A smart tracer, combining the reactive (resazurine) and non-reactive (NaCl) compounds will be injected at the bottom (150 cm depth) of the piezometers and breakthrough curves will be analysed at 15 cm intervals along the multi-level piezometers. NaCl breakthrough curves will be used to calculate groundwater up-welling rates and residence times whereas the reduction of resazurine to the strongly fluorescent substance resorufin will be used in a reactive decay simulation to indicate the microbial metabolic activity at the respective depths. Metabolic activity and residence times will be compared to nitrate concentrations, redox-potential, dissolved oxygen and organic carbon content at the piezometers locations to proof that metabolic activity hotspots increase the natural attenuation potential. The result of this project will provide regulators and river basin managers with the urgently required understanding of the natural attenuation potential of streambed reactivity hotspots of enhanced microbial activity. The knowledge provided by this project is of great importance for a range of related scientific disciplines as well as regulators and river basin managers. Thermal conditions are an integral part of the ecosystem health of hydrological systems. The improved understanding of heat flow in streambeds will directly benefit aquatic ecologists and eco-hydrologists while advanced knowledge on metabolic activity hotspots will greatly benefit future research of multi-contaminant reactive transport and biogeochemical cycling at aquifer-river interfaces.