Implications of groundwater-surface water connectivity for nitrogen transformations in the hyporheic zone
Lead Research Organisation:
Queen Mary University of London
Department Name: Geography
Abstract
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.
Publications
Yvon-Durocher G
(2012)
Linking community size structure and ecosystem functioning using metabolic theory.
in Philosophical transactions of the Royal Society of London. Series B, Biological sciences
Yvon-Durocher G
(2015)
Temperature and the biogeography of algal stoichiometry
in Global Ecology and Biogeography
Yvon-Durocher G
(2012)
Reconciling the temperature dependence of respiration across timescales and ecosystem types.
in Nature
Ullah S
(2012)
In situ measurement of redox sensitive solutes at high spatial resolution in a riverbed using Diffusive Equilibrium in Thin Films (DET)
in Ecological Engineering
Ullah S
(2013)
Influence of emergent vegetation on nitrate cycling in sediments of a groundwater-fed river
in Biogeochemistry
Trimmer M
(2012)
River bed carbon and nitrogen cycling: state of play and some new directions.
in The Science of the total environment
Reuman DC
(2014)
A metabolic perspective on competition and body size reductions with warming.
in The Journal of animal ecology
Perkins D
(2012)
Consistent temperature dependence of respiration across ecosystems contrasting in thermal history
in Global Change Biology
Lansdown K
(2015)
The interplay between transport and reaction rates as controls on nitrate attenuation in permeable, streambed sediments
in Journal of Geophysical Research: Biogeosciences
Lansdown K
(2012)
Characterization of the key pathways of dissimilatory nitrate reduction and their response to complex organic substrates in hyporheic sediments
in Limnology and Oceanography
Description | 1. Through incremental method development and by integrating new and existing tools for quantification of physical hydrology and nutrient biogeochemistry we show the capacity for a nested scaling approach (1cm to 200m) to quantify fluxes at small scales that may have a significant impact on nutrient transport at larger scales. 2. We provide evidence that the fate of nitrate in the hyporheic zone (HZ) depends on the depth it enters the riverbed; and we found significant transformations of nitrate occur in the riverbed below the depth of hyporheic exchange flows. These data highlight critical interlinkages between hydrological pathways and biogeochemical transformations in the HZ of groundwater rivers. 3. Our data demonstrate uniquely that integrated measurements of hydrological and biogeochemical fluxes are needed to understand the functioning of the HZ. 4. Our findings indicate there are opportunities for managing nitrate in groundwater rivers through the lens of the HZ but this can only be achieved through coupled understanding of physical hydrology and biogeochemistry. As policy continues to evaluate the extent and management of nitrate vulnerable zones in catchments, our work illustrates the critical importance of process understanding beneath the riverbed. |
Exploitation Route | We have quantified the extent of nitrate removal from the hyporheic zone in a gaining reach of a groundwater-fed river. Our data helps us understand the variability in geochemical conditions and the extent of nitrate removal in a 'typical' reach-scale setting. Such data could be used by policymakers and practitioners who are interested in the extent to which nitrate can be attenuated in river environments, and in the variability in the magnitude of nitrate removal in the hyporheic zone, for the purposes of meeting the requirements of the Water Framework Directive. |
Sectors | Agriculture, Food and Drink,Education,Environment,Government, Democracy and Justice |
Description | The scientific findings from the research project are still being assimilated and it is too early to translate these into impact beyond the immediate research community developing and advancing the science in this area. |
Description | The role of lateral exchange in modulating the seaward flux of C,N,P |
Amount | £1,016,144 (GBP) |
Funding ID | NE/J012106/1 |
Organisation | Natural Environment Research Council |
Sector | Public |
Country | United Kingdom |
Start | 10/2012 |
End | 10/2015 |