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

河流（颇有争议地）被描述为“地下水的露头”。英国的许多河流主要由地下水供应，特别是在降雨量低的夏季。潜流带是地下水集水区地表水与地下水之间的重要界面。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，可能对河流的水质及其生态产生很大的控制作用：以至于有些人把潜流带归因于污染物衰减特性。地下水抽取、污水处理和扩散营养物压力——尤其是氮压力——都可能损害潜流带影响河流水质的能力。虽然相当多的研究人员已经认识到，隐隐带对河流的栖息地有某种特殊的控制作用，但大多数人只是从河水与河床上几厘米沉积物之间的关系的角度来看待它。他们忽略了这样一个事实，即除了河流向下流入河床沉积物的通量外，也会有地下水通过暗流带向上流入河流。我们特别感兴趣的是，当地下水在潜流带中移动时，其化学成分会发生什么变化。我们将详细研究不同种类的氮之间的关系，如硝酸盐和铵，以及统称为“氧化还原”或还原氧化反应的化学反应。当河床沉积物中的氧气耗尽时，氧化还原反应使用电子受体而不是氧气来氧化有机碳。这些反应及其与氮的关系是很重要的，因为已提出的隐渗区是氮衰减发生的区域。这就导致了这样一个命题，即地下水通过该带的运动将降低到达河水的氮的浓度。在这个项目中，我们将进一步调查暗沉带可以减弱地下水污染物如硝酸盐的说法。我们想要更仔细地观察地下水流经潜流带的模式。我们提出地下水通量受河床渗透率的影响，而渗透率又受河床的物理结构和地形的影响。我们认为，在河床渗透性高、地下水流向河流的通量高的地方，我们将发现与渗透性低的地方相比，潜流带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磨碎器，河床上的“洞”对应着快速和缓慢的流动路径。我们预计，当冬季风暴改变河床沉积物的表层地形，并重新排列河流下垫沉积物的池纹和快-慢流特征时，这些孔是相当动态的。这些流动路径之所以重要，是因为它们可能允许在潜流区内的生物地球化学活动的“热点”，这可能是对地下水河流生态的重要控制，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，而硝酸盐在高浓度时会破坏河流的生态。如果我们能够清楚地表明潜流带对地下水河流水质的影响有多重要，我们将能够提供证据来保护这个区域，也可以用来帮助英国满足重要的欧洲立法的要求，如水框架指令。

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.