Implications of groundwater-surface water connectivity for nitrogen transformations in the hyporheic zone

地下水-地表水连通性对潜流带氮转化的影响

基本信息

批准号：
NE/F006063/1
负责人：
Ann Louise Heathwaite
金额：
$ 49.05万
依托单位：
Lancaster University
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2009
资助国家：
英国
起止时间：
2009 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=NE%2FF006063%2F1
关键词：
Implications groundwater surface water connectivity

项目摘要

Rivers have (rather controversially) been described as 'simply outcrops of groundwater'. Many of the rivers in the UK are supplied mainly from groundwater sources, especially during the summer months when rainfall is characteristically low. The hyporheic zone is a critical interface between surface and subsurface waters in groundwater catchments. Here, the mixing of groundwater and surface water and the resulting biological and chemical reactions, may exert a lot of control on the water quality of the river and also its ecology: so much so that the hyporheic zone has been ascribed pollutant attenuating properties by some. Groundwater abstraction, effluent disposal and diffuse nutrient pressures - especially nitrogen - may all compromise the capacity of the hyporheic zone to influence the water quality of a river. Although quite a few researchers have recognised that the hyporheic zone has some special control on the river habitat, most have looked at it only from the perspective of the relationship between river water and the upper few centimetres of the sediments of the riverbed. They have ignored the fact that as well as downward flux from the river into the sediments of the riverbed there will also be upward flows from groundwater through the hyporheic zone and into the river. We are especially interested in what happens to the chemistry of groundwater as it moves through the hyporheic zone. We will look in detail at the relationship between different nitrogen species, such as nitrate and ammonium and chemical reactions known collectively as 'redox' or reduction-oxidation reactions. Redox reactions use electron acceptors other than oxygen for organic carbon oxidation as the amount of oxygen in the riverbed sediments is exhausted. These reactions and their relationship with nitrogen are important because the hyporheic zone has been proposed as a zone in which nitrogen attenuation occurs. This has led to the proposition that the movement of groundwater through this zone will reduce the concentration of nitrogen reaching the river water. In this project, we will investigate further the claim that the hyporheic zone can attenuate groundwater contaminants such as nitrate. We want to look much more carefully at the pattern of flow from groundwater through the hyporheic zone. We propose that groundwater flux is influenced by the permeability of the river bed and this is in turn influenced by the physical structure and topography of the riverbed. We believe that where the permeability of the riverbed is high and flux from groundwater towards the river is high, we will find different patterns of biogeochemical activity in the hyporheic zone compared to where the permeability is low. We like to think of the riverbed rather like a cheese grater with fast and slow flow pathways corresponding to 'holes' in the riverbed. We expect these holes to be quite dynamic as winter storms change the superficial topography of the riverbed sediments and rearrange the patterns of pool-riffle and fast-slow flow features in the underlying sediments of a river. The reason why these flow pathways are important is they may allow 'hotspots' of biogeochemical activity within the hyporheic zone that could be important controls on the ecology of groundwater-fed rivers because they either release or transform nitrogen through processes such as nitrification or denitrification. The latter converts nitrate, which can damage the ecology of a river where it is present at high concentrations, into nitrogen gas, which is harmless. If we are able to show clearly how important the hyporheic zone is in influencing the water quality in rivers that are groundwater-fed, we will be able to provide evidence that can be used to protect this zone, and can also be used in helping the UK meet the requirements of critical European legislation such as the Water Framework Directive.

河流（颇有争议）被描述为“只是地下水的露头”。英国的许多河流主要来自地下水源，特别是在降雨量较低的夏季。潜流带是地下水集水区地表水和地下水之间的关键界面。在这里，地下水和地表水的混合以及由此产生的生物和化学反应，可能会对河流的水质及其生态产生很大的控制：以至于一些人认为潜流带具有污染物衰减特性。。地下水抽取、污水处理和扩散营养物压力（尤其是氮）都可能损害潜流带影响河流水质的能力。尽管不少研究者认识到潜流带对河流栖息地具有某种特殊的控制作用，但大多数只是从河水与河床上部几厘米沉积物之间的关系角度来看待它。他们忽略了这样一个事实：除了从河流向下流入河床沉积物的流量之外，还会有从地下水通过潜流带向上流入河流的流量。我们对地下水流经潜流带时的化学变化特别感兴趣。我们将详细研究不同氮物种（例如硝酸盐和铵）与统称为“氧化还原”或还原氧化反应的化学反应之间的关系。随着河床沉积物中的氧气耗尽，氧化还原反应使用氧以外的电子受体进行有机碳氧化。这些反应及其与氮的关系很重要，因为潜流区已被认为是发生氮衰减的区域。由此得出的结论是，地下水流过该区域将减少到达河水中的氮浓度。在这个项目中，我们将进一步调查潜流带可以减弱地下水污染物（例如硝酸盐）的说法。我们想要更仔细地观察地下水流经潜流带的流动模式。我们认为地下水通量受河床渗透性的影响，而河床渗透性又受河床物理结构和地形的影响。我们认为，在河床渗透性高且地下水流向河流的通量高的地方，我们会发现与渗透性低的地方相比，潜流带中的生物地球化学活动模式不同。我们喜欢把河床想象成一个奶酪刨丝器，有与河床上的“洞”相对应的快流和慢流路径。我们预计，随着冬季风暴改变河床沉积物的表层地形，并重新排列河流底层沉积物中的水池浅滩和快慢流特征的模式，这些洞将非常动态。这些流动路径之所以重要，是因为它们可能会在潜流区内形成生物地球化学活动的“热点”，这可能对地下水供水河流的生态起到重要的控制作用，因为它们通过硝化或反硝化等过程释放或转化氮。后者将硝酸盐转化为无害的氮气，硝酸盐在高浓度下会破坏河流的生态。如果我们能够清楚地表明潜流带对影响地下水源河流的水质有多么重要，我们将能够提供可用于保护该区域的证据，也可用于帮助英国满足《水框架指令》等关键欧洲立法的要求。