Hydraulic Conductivity for Each Core

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“Spatial variability in physical and geochemical properties of sediments affecting nitrogen attenuation in the hyporheic zone of the River Tern”

Dissertation submitted as part requirement for the degree of Master of Sciences in Contaminant Hydrogeology

Hannah Wooldridge

Supervisor:

Dr Stephen Thornton

The University of Sheffield

Department of Civil and Structural Engineering

3 September 2010

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Declaration of originality

Hannah Wooldridge certifies that all the material contained within this document is all her own work except where it is clearly referenced to others.

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Abstract

Six riverbed cores from the River Tern, Shropshire were assessed for a range of physical and chemical properties, at a core and spatial scale, in order to determine the nitrogen attenuation potential. Acid digestions, wet and dry sieving and the Malvern Mastersizer 1E 2000 was used in order to determine the particle grain size distributions and average grain size for each core. The Hazen empirical formula was applied in order to derive hydraulic conductivity for each core. The fraction of organic carbon was determined using the modified Walkley Black method, along with pore water chemistry data to outline the chemical properties of each core. Depth profiles of both the physical and chemical properties were produced and statistically analysed using ANOVA in order to deduce correlations between the physical and chemical properties and to highlight areas of enhanced denitrification. Correlations were found between decreasing grain size and decreasing hydraulic conductivity and increased weight percentage organic carbon. From the statistical analysis hydraulic conductivity was found to be the physical property that exerts the most influence on the nitrate concentrations. The denitrification potential for each core was assessed at a spatial scale, concluding that at certain depths and at geologies with certain physical characteristics nitrate concentrations can successfully be reduced to below the 50 mg/L drinking water limit, as highlighted by The European Union and World Health Organization. These findings demonstrate the potential for nitrate attenuation within the hyporheic zone, and on this basis should be included in environmental risk assessments in order to assess and manage nitrate pollution in both groundwater and surface water.

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Acknowledgements:

I would like to thank Dr Steve Thornton for all the help, support and advice in the execution and production of this research.

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Contents

1 Introduction 5-11 2 Study Site 12-13 3 Literature Review 3.1Biogeochemistry 16-18 3.2 Flow 18-20 3.3 Sedimentology 20-25 4 Aims and Objectives 25 5 Methodologies 5.1Particle Grain Size Analysis 25-27 5.2 Fraction of Organic Carbon 27-28 5.3 Pore Water Chemistry 28 6 Results 6.1 Sediment Core 2.2- 29-34 6.2- Sediment Core 2.4- 34-38 6.3- Sediment Core 2.5- 38-42 6.4- Sediment Core 2.6- 41-44 6.5- Sediment Core 2.7- 44-48 6.6- Sediment Core 2.8- 48-51 6.7 Spatial Scale 51-57 7 Conclusions 58-60 8 References 61-66 9 Appendices 67

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List of Figures

Figure 1.1 Illustrative hydrogeological conceptual model of the 9 GW/SW interface and hyporheic zone (after Smith et al 2005). Figure 1.2 Illustrative sedimentological conceptual model of the 9 GW/SW interface and hyporheic zone. Figure 1.3 Denitrification reaction chain (Buss et al 2005) 12 Figure 1.4 Nitrate reduction half equation (Rivett et al 2008) 12 Figure 1.5 Denitrification (Rivett et al 2008) 12 Figure 2.1- Site location (www.ordenancesurvey.co.uk) 15 Figure 2.2- River Tern Study Site and Core Locations 15 Figure 6.1- Hyporheic sediment cores stratigraphy 28 Figure 6.2- Sediment Core 2.2- Hydraulic Conductivity (K) profile 29 Figure 6.3- Sediment Core 2.2- wt% Organic Carbon profile 29 Figure 6.4- Sediment Core 2.2- Grain Size profile 29 Figure 6.4- Sediment Core 2.2- Chloride profile 30 Figure 6.5- Sediment Core 2.2- Nitrate and Acetate profile 30 Figure 6.6- Sediment Core 2.2- Alkalinity profile 34 Figure 6.7- Sediment Core 2.4- Hydraulic conductivity (K) profile 35 Figure 6.8- Sediment Core 2.4- Carbon profile 35 Figure 6.9- Sediment Core 2.4- Grain Size profile 35 Figure 6.10- Sediment Core 2.4- Chloride Profile 36 Figure 6.11- Sediment Core 2.4- Nitrate and Acetate profile 36 Figure 6.12- Sediment Core 2.4- Alkalinity profile 38 Figure 6.13- Sediment Core 2.5- Hydraulic Conductivity (K) profile 38 Figure 6.14- Sediment Core 2.5- Carbon profile 39 Figure 6.15- Sediment Core 2.5- Grain size profile 39 Figure 6.16- Sediment Core 2.5- Chloride profile 39 Figure 6.17- Sediment Core 2.5- Acetate and Nitrate profile 40 Figure 6.18- Sediment Core 2.5- Alkalinity profile 40 Figure 6.19- Sediment Core 2.6- Hydraulic Conductivity (K) profile 42 Figure 6.20- Sediment Core 2.6- Carbon profile 42 Figure 6.21- Sediment Core 2.6- Grain Size profile 43 Figure 6.22- Sediment Core 2.6- Chloride profile 43 Figure 6.23- Sediment Core 2.6- Nitrate and Acetate profile 43

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Figure 6.24- Sediment Core 2.6- Alkalinity profile 44 Figure 6.25- Sediment Core 2.7- Hydraulic Conductivity (K) profile 45 Figure 6.26- Sediment Core 2.7- Carbon profile 46 Figure 6.27- Sediment Core 2.7- Grain Size profile 46 Figure 6.28- Sediment Core 2.7- Chloride profile 46 Figure 6.29- Sediment Core 2.7- Acetate and Nitrate profile 47 Figure 6.30- Sediment Core 2.7- Alkalinity profile 47 Figure 6.31- Sediment Core 2.8- Hydraulic Conductivity (K) profile 49 Figure 6.32- Sediment Core 2.8- Carbon profile 49 Figure 6.33- Sediment Core 2.8- Grain Size profile 50 Figure 6.34- Sediment Core 2.8- Chloride profile 50 Figure 6.35- Sediment Core 2.8- Acetate and Nitrate profile 50 Figure 6.36- Sediment Core 2.8- Alkalinity profile 51 Figure 6.40 Nitrate Distribution Contour Plot 54 Figure 6.41 River Tern Study Site and Core Locations 57

List of Tables

Table 3.1 Classification table for the Coefficient of sorting and grain 24 size (Fetter et al 2001) Table 6.71 ANOVA results for grain size, hydraulic conductivity, 52 wt% organic carbon and depth Table 6.72 ANOVA results for grain size, hydraulic conductivity, wt% 53 organic carbon and depth parameters Table 6.73 – Spatial variability of denitrification 56

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1. Introduction

The hyporheic zone is part of the groundwater-surface water interface that is established within saturated fluvial sediments, beneath or adjacent to a river or stream, where there is interaction and exchange between the two water bodies (The hyporheic network, www.hyporheic.net) (Smith et al 2005) (Figure 1.1). There are distinct differences between the geochemical, sedimentological, hydrogeological and ecological properties of the aquifer and the hyporheic zone, with the hyporheic zone being fairly rich in organic carbon and microbial populations. The hyporheic zone is both temporally and spatially dynamic and frequently displays continuous fluctuation in both chemical and physical conditions (The Hyporheic Handbook 2009).

The Hyporheic zone is defined as the section of fluvial sediments in which there is an exchange of water from the stream to the riverbed sediments (geological media) and back to the stream, with this occurring within the order of days to months (The Hyporheic Handbook 2009). This definition alters within differing disciplines which can use various indicators such as hyporheic ecology or biogeochemically active zones as the basis for characterization. Throughout the zone there are frequently chemical and temperature gradients which influence the behaviour of both chemicals and organisms at the interface, and in the neighbouring aquifer and stream environments (The Hyporheic Handbook 2009).

Figure 1.1 is outlining the GW/SW interface and hyporheic zone in a hydrogeological context. From figure 1.1 it is apparent that the nature and extent of the hyporheic zone is highly variable, with the principle controlling factor being the localised heterogeneity and geology of the site. This variation results in differences in hydraulic gradients within an individual reach over a time period, resulting in varying extents of GW/SW interaction and the direction of water movement, with the aquifer discharging or recharging. Further exchange processes also take place involving the down welling of river water into the sediment and re-emerging back into the stream; at times the re-emerging water can contain both groundwater and stream water. The driving force behind this exchange is pressure variations driven by Geomorphological features such as pool and riffle sequences or variations in slope and riverbed profiles (Kalbus et al 20006). The hyporheic zone can also be subdivided into the surface and interactive zones as a result of depth and contact

090116876 faces. Figure 1.2 highlights the fact that the hyporheic flow/zone can extend through topographical features such as river bends and meanders.

Figure 1.2 is illustrating the GW/SW interface and hyporheic zone in a sedimentological setting. The fluvial sediments, where the interface is established, are controlled by the sediment parent and bedrock geology as well as the local Geomorphological controls on sedimentation and erosion. The fluvial sediments are established over time due to the varying conditions, resulting in different depths of sediment having different sediment structures. For example river braiding depositional bars will establish areas of coarse deposits.

Differences in sediment structures, such as grain size, particle sorting, packing, bedding and jointing and bed form shape can result in various hydraulic conductivities throughout the fluvial sediments. Generally clays will display a reduction in hydraulic conductivity and fine grained gravel and silt, typically in the form of riffles and pools will display enhanced hydraulic conductivity. The localised production of biofilms and sediment reworking by macroinvertebrates can both reduce and increase local hydraulic conductivity. The GW/SW interface interaction can be temporarily stopped due to the production of a colmatage, which is the deposition of fine-grained sediments from filtering of sediment-containing downwelling stream water by the porous sediments of the streambed and sedimentation, which can blind the riverbed. As a result of these variations in conductivity and sedimentation there is also variation in the potential for pollution retardation and degradation throughout the hyporheic zone.

The Hyporheic zone has a distinct and varied ecology, with the zones physical and geochemical properties and abiotic conditions all affecting the ecological communities and locations within the zone. Due to the varying conditions of the hyporheic zone to the surface waters there is the potential for the hyporheic zone to act as a refuge for benthic fauna (The Hyporheic Handbook 2009).

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Figure 1.1 Illustrative hydrogeological conceptual model of the groundwater/ surface water interface and hyporheic zone (after Smith et al 2005).

Figure 1.2 Illustrative sedimentological conceptual model of the groundwater/ surface water interface and hyporheic zone.

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Bencala et al 2006 suggests that the overall exchange flow within the hyporheic zone is occurring at a small scale, in comparison to the length and volumetric transport characteristics of a river. The hyporheic zone is a zone thought to have a range of enhanced properties compared to subterranean and surface aquatic environments. These features include the cycling of carbon, energy and nutrients, providing a sink and source for sediment within a river channel, moderating river water temperature and the flux of water exchange between the stream and groundwater (The Hyporheic Handbook 2009). The hyporheic zone also provides a natural attenuation potential for various pollutants by chemical and biological transformation processes, sorption, (through governing the distribution of contaminants in the aqueous phase, solid phase, gas phase and nonaqueous phase liquids), and mixing (Bencala et al 2006).

Smith et al 2009 has highlighted the fact that the enhanced properties of this zone have the potential for pollutant cycling and biochemical reactions. Bencala 2006 further outlined the influence that the hyporheic zone can exert on nutrient dynamics and concentrations of major-ions and metals in stream-catchment systems. The quantity and quality of organic matter in sediments affects the partitioning and bioavailability of sediment-associated contaminants, whilst the extent of the microbial populations will affect the degree of microbially mediated transformations of the pollutants. Variations within the sedimentology and hydrogeology of the hyporheic zone will affect the flow rates, path and residence time of water within the zone, significantly influencing the rate of attenuation. On this basis the hyporheic zone has the potential to act as a buffer, altering the flux of groundwater pollutants discharging into rivers or surface waters and vice versa, with this occurring through both degradation and retardation processes (Smith et al 2009). In response to this buffering effect it is necessary to assess the processes occurring within the hyporheic zone when estimating and quantifying water and contaminant fluxes throughout a catchment (The hyporheic handbook).

In recent years the hyporheic zone has been a site of intense interest, with it now being thought that the groundwater- surface water interface offers significant potential for the attenuation of pollutants migrating through the interface zone (Smith et al 2008). These findings have highlighted the fact that there is the need to quantify the efficiency and the significance of the attenuation processes within the hyporheic

090116876 zone, gaining further understanding of the processes and relationships. Through an enhanced understanding of the zone it should be possible to accurately predict the overall attenuation capacity of the zone, meaning that the natural attenuation efficiency of the zone can be incorporated as part of the groundwater pollution risk assessment (Smith et al 2005). Clarifications of the natural attenuation efficiency will also aid the new integrated treatment and management of groundwater and surface water catchments, helping to achieve the good status set out by the Water framework directive (Smith et al 2005). Further understanding will also help the development of new integrated catchment management approaches for the treatment of catchment wide pollutant transfer and ecological health (The Hyporheic Handbook 2009).

The EU water framework directive, which was established in 2000, has highlighted that the nutrient nitrogen as a major pollution source for poor chemical and ecological status in water catchments (The Hyporheic Handbook 2009). Nitrate is a significant environmental groundwater pollutant; it can impose severe impacts to the environment by inducing eutrophication in surface waters as well as posing human health concerns such as methaemoglobinaemia and cancer (Rivett et al 2008). The application of nitrogen based fertilizers and effluence to land, atmospheric deposition, along with leaking sewers, pipes and septic tanks has resulted in both diffuse and point source nitrogen contamination to U.K. surface waters, and through water exchange, ground waters. Rivett et al 2008 highlights the fact that ~70-80% of nitrate in English surface and ground waters is derived from agricultural activities. However the leaching of nitrate from fertilizers is dependent upon soil texture, rainfall and irrigation and the nitrogen source (Wakida et al 2005). The European Union and World Health Organization have set regulatory guidelines for drinking - water to be 11.3mg nitrogen (N) per litre and 50mg per litre nitrate (NO3 ) for all natural freshwaters. Although it has also been recognised that eutrophication may be induced at much lower nitrogen concentrations. The typical baseline concentrations of nitrate in groundwater beneath natural grassland in the U.K. are typically below 2mg/L (Wakida et al 2005).

Advection and Dispersion are physical methods which can reduce nitrate concentrations during migration through the hyporheic zone (Wakida et al 2005). There are a range of biogeochemical nitrate depletion mechanisms that can transform nitrate into differing species, resulting in a reduction of both concentration

090116876 and mass of nitrate. These involve denitrification, dissimilatory nitrate reduction to ammonium and assimilation of nitrate into microbial biomass. Denitrification converts nitrate, through a series of microbial reduction reactions to nitrogen gas (Figure 1.3), and under abiotic reactions it can be reduced to nitrite and nitrous oxide.

Nitrate ions Nitrite ions Nitric oxide Nitrous oxide Dinitrogen gas

Figure 1.3 Denitrification reaction chain (Buss et al 2005)

- + + Figure 1.4 Nitrate reduction half equation (Rivett et al 2008)

- - 5CH2O + 4NO3 → 2N2 + 4HCO3 +CO2 +3H2O Figure 1.5 Denitrification (Rivett et al 2008)

Figure 1.4 outlines the nitrate reduction reaction, which uses electron donors to complete the transformation process. The final endpoint of the denitrification process is nitrogen gas which is a stable species, however the transformation processes can cease at any of the stages outlined in figure 1.3. This can result in a range of problems, in particular if transformation ceases at the nitrite stage, which is more toxic than nitrate, and nitrogen oxides which are environmentally destructive gases. Throughout the denitrification process oxygen is released at each step, often in - 2- the form of bicarbonate (HCO3 ) or sulphate (SO4 ) (Figure 1.5 and Rivett et al 2008). However in the subsurface the biotic denitrification is much more prominent (Buss et al 2005). Denitrifying bacteria are principally facultative anaerobic heterotrophs, using natural organic carbon, from the oxidation of organic compounds as their energy source. Denitrifying bacteria can also be autotrophs, obtaining their energy from the oxidation of inorganic species- reduced sulphur or iron.

The dissimilatory nitrate reduction process is another nitrate depletion mechanism, involving a further anaerobic reduction reaction, with the availability of organic matter dictating whether this mechanism or denitrification occurs. Ammonium or nitrite is generated through this process, which is subsequently released back into an aerobic environment, and either oxidised back into nitrate or

090116876 taken up by vegetation. Dissimilatory nitrate reduction is the preferential mechanism when nitrate (electron acceptor) supplies are limiting and denitrification is preferred when carbon (electron donor) supplies are limiting (Buss et al 2005).

A range of heterotrophic micro-organisms can also assimilate both nitrate and ammonium into their biomass, utilising the compounds for growth. Although in the presence of both compounds ammonium is preferentially assimilated, it has been demonstrated that biomass is an important mechanism for nitrogen uptake (Buss et al 2005).

The wide extent and differing sources of nitrate contamination mean that there is frequently a need for engineered remediation systems, with considerable financial and practical implications, in order to reach compliance levels. The natural attenuation potential of the hyporheic zone could substantially reduce the need for these systems and reduce the financial constraints that these pose, by replacing systems for monitoring programmes. The need for comprehension of the processes and ideal conditions of the zone are vital if the hyporheic zone is to be successfully integrated in the reduction of nitrate concentrations in both ground and surface water. This reduction of nitrate concentrations can occur through a variety of transformational processes, as highlighted by Rivett et al 2008 or through other biodegradation processes such as sorption, dilution and dispersion.

The uncertainty relating to the extent and processes of denitrification within the hyporheic zone have led to the overall rationale of this project, which is to assess a range of differing physical and geochemical properties of sediments within the hyporheic zone of the River Tern, Shropshire. From this work it should be possible to detail a range of parameters and relationships that can be used in assessing the overall natural attenuation capabilities, which can then be included in ground and surface water nitrate pollution risk assessments.

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2. Study site

The study site for this research is the River Tern, which is a shallow-gradient (0.11%), midland River, located near Stoke on Tern, Shropshire, U.K., (Figure 2.1). The River Tern is part of the River Severn basin (59,119 ha), spanning for about 30 miles (43km)and being fed by the River Meese, and the River Roden, with the source thought to originate in the grounds of Maer Hall, Staffordshire. The River Tern is a groundwater dependant river that is flowing over a major aquifer, consisting of Permian Bridgnorth Sandstone bedrock geology.

3 - b, The river Terns base flow (Q95) has been determined to be 1.65 (m S 1) with the study site found to be relatively small in width, with a mean channel gradient of 0.052%. The altitude of both the headwater and the catchment outlet are 289 and 99m above sea level (Smith et al 2008). The annual average rainfall has been determined to be 715 mm, which is evenly distributed throughout the year, but with an increasing trend of drier summers and wetter winters.

The dominant local land use uses near the study site are dairy, arable and equestrian. Due to agricultural operations and the application of surface fertilisers and animal effluents there is a diffuse source of nitrate into the surface river water and groundwater (Smith et al 2008).

Figure 2.2 highlights the study site and the core locations, cores 2, 6, 4,5,7,8 (highlighted in green) are the cores that I am going to be analysing throughout this study. These cores were chosen as they offer a good spatial distribution of the study site.

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Figure 2.1- Site location (www.ordenancesurvey.co.uk)

Figure 2.2- River Tern Study Site and Core Locations

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3. Literature Review

3.1 Biogeochemistry The Hyporheic zone has repeatedly been shown to be a biogeochemically active zone, which possesses a range of pollutant cycling processes that can modify the flux of pollutants moving through the zone (Smith et al 2009). The rate, magnitude and properties of natural attenuation within the hyporheic zone are governed by a range of both contaminant- specific properties and environmental properties. Smith et al 2009 highlights these to be pollutant sorption, recalcitrance, sediment geochemistry, hydrochemistry, transport velocity and residence times within the reactive zones.

The Hyporheic handbook outlines that the hyporheic zone acts as a buffer zone for both groundwater and surface water, due to transformational processes. However the success of these processes is typically dependant on the presence of organic matter, microbial activity and the presence of steep redox gradients. The transformation efficiency is determined by the pollutants present and residence times along with the hyporheic zones specific redox conditions, as this will determine the reaction type and kinetics (The Hyporheic Handbook 2009).

Denitrification is widely accepted as the principle mechanism of nitrate attenuation within groundwater (Rivett et al 2008), as nitrate generally is not sorbed within the subsurface (Buss et al 2005). The presence of denitrifying bacteria within the subsurface is generally omnipresent, meaning that the limiting factors for denitrification are generally the oxygen and electron donor concentrations (such as organic carbon for heterotrophic denitrification) and their availability. Secondary influences are also exerted on the denitrification process by factors such as nitrate concentrations, nutrient availability, pH, temperature, presence of toxins and microbial acclimation (Rivett et al 2008).

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Buss et al 2005 highlights the fact that within the fluvial sediments of the hyporheic zone there are sharp and defined dissolved oxygen boundaries, in response to varying extents of oxygen migration. In the upper level of the zone oxygen is able to diffuse in and out, but with increasing depth diffusion is no longer possible, resulting in a sharp and defined redox boundary between the aerobic benthic sediments and the underlying anoxic sediments. This zone can be extensive and transitional, depending on the rivers flow regime and fluvial sediment conditions.

The sediment geochemistry is of utmost importance in the natural attenuation of nitrates as it provides a source of electron donors and determines the donor concentrations (Rivett et al 2008), which are vital in the denitrification process. Work carried out by Smith et al 2005 outlined the importance of the fraction of organic carbon (foc) and the content of reducible iron, carbonate and manganese in the denitrification process. Rivett et al 2008 highlights the fact that there are a range of electron donors that can be used for denitrification, including organic carbon, sulphur and iron, with organic carbon being the preferential donor.

The sediment geochemistry is determined by the autochthonous source materials and influenced by a range of Geomorphological processes such as deposition, erosion, and sorting. Further variation can subsequently occur due to physical, chemical and biological processes, and allochthonous import of materials (Allen-King et al 2002). Throughout the hyporheic zone the locations and quantities of organic carbon and other electron donors are widely heterogeneous, with the sediment geochemistry and the Geomorphological and depositional regimes affecting the locations, concentrations and availability.

Natural attenuation protocols have often used measurements of the total organic carbon as an indicator of sorption; however there have been only a few studies which examine the geochemical properties to determine the attenuation capacity of the hyporheic sediments, with Smith et al 2009 being a prominent recent study. The environment agency report by Smith et al 2005 again showed the importance of sediment geochemistry as a feature of natural attenuation for the reduction of nitrates. Within this work it was shown that that there is a requirement to relate mineralogy and attenuation capacity to source-rock terrain, depositional

090116876 environment and sedimentary architecture. Smith et al 2008 found that there was a general correlation with the fraction of organic carbon increasing and mean dominant grain size decreasing.

The determination of the fraction of organic carbon, and therefore the availability of electron donors within the hyporheic zones fluvial sediments has widely been determined using the modified Walkley Black method (Smith et al 2008). The modified Walkley Black method involves the wet combustion of the organic matter with sulphuric acid and potassium dichromate. The residual dichromate is then titrated against ferrous sulphate, allowing the proportion of carbon within the sample to be determined (Van Reeuwijk 2002). The modified Walkley Black method offers a quick and simple process to determine the organic carbon content of relatively small sample sizes.

Hill et al 1998 indicated how it is possible to use a combination of hydrometric data and pore water chemistry concentrations of conservative ions in surface and groundwater to delineate the hyporheic zone size. As well as delineation it is also possible to compare the concentrations and patterns of the reactive elements within the hyporheic zone to assess transformation/ attenuation processes.

3.2 Flow The flow regime into and out of the hyporheic zone is a principle area of interest, and can strongly influence both the water and nutrient exchanges, and subsequently the pore water chemistry within the vertical and lateral zone (Smith et al 2005). Hill et al 1998 demonstrated that at a localised scale the exchanges of surface and ground water can significantly influence the solute transport and nutrient cycling in stream ecosystems.

Flow and mixing within the hyporheic zone is a result of a range of influences, such as the catchment and localised geomorphology, stream water level, groundwater discharge and hydraulic conductivity (The Hyporheic Handbook 2009). Smith et al 2008 elaborated further on these findings, suggesting that the bed form shape, sediment grain size, mineralogy and permeability distributions are all contributing to the overall Geomorphological influence of flow and mixing within

090116876 the hyporheic zone. Sophocleous 2002 outlined the fact that areas with high hydraulic conductivity and short residence times, such as paleo-channels, offer preferential subsurface flow paths. These paths provide routes for the transport of water from near the surface, with surface chemical signatures, into deeper alluvial layers.

Convective flow into the fluvial sediments, from the surface water can be induced by water flowing over small obstacles and local bed form features such as riffles and pools and channel sinuosity (Hill et al 1998). These bed form features, and any other irregularities in the riverbank and bed morphology can induce drag and flow recirculation, leading to horizontal pressure differences that can induce exchange between the two water zones (The Hyporheic Handbook 2009). Hill et al 1998 highlighted patterns of downwelling into the fluvial sediments at the upstream face of the roughness element, and upwelling from the fluvial sediments at the downstream face of the roughness element. It is also possible to establish convective exchanges across the river bed by larger scale roughness elements such as rapids and steps.

The exchange of surface and groundwater within the hyporheic sediments occurs at a more localised and smaller scale along stream and river banks. The process is again driven by horizontal pressure differences, which are derived from small scale concave-convex features along the banks. These features can take the form of alternating unit bars, mid-channel transverse bars or channel sinuosity, and can result in flow occurring in fluvial sediments outside of the surface stream or river (The Hyporheic Handbook 2009).

To coincide with the riverbed features it has also been suggested that the variations in the river-sediment interface can induce flow in the absence of pressure gradients. In a uniformly permeable, smooth sediment water interface, interstitial flow within the sediments would be parallel to the interface. In reality this is never the case and as such any variability in permeability due to alterations in the topography or changes in the composition of alluvium materials can result in flow paths deflecting away to toward the rive-sediment interface (The Hyporheic Handbook 2009).

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Sophocleous 2002 outlined a classification system for stream and aquifer systems, based on three differing types of flow throughout the hyporheic zone. The underflow-component dominated classification is assigned when groundwater flux is moving in a parallel direction to the surface water. The baseflow-component classification is assigned when groundwater flux from the surface water is either effluent or influent, with the water moving perpendicular to the surface water. The third classification is mixed, with water from groundwater and surface water completely interspersed with each other. Sophocleous 2002 further outlined that these classifications can frequently be inferred from Geomorphological data such as the Channel slope, river sinuosity and the character of the fluvial sediment deposits.

There are a range of differing techniques that can be employed in order to assess and quantify the magnitude and spatial distribution of groundwater/ surface water interactions. Hill et al 1998 and Rogers et al 2004 employed the use of subsurface concentration patterns of conservative ions, such as chloride, that are present only as a result of the exchange of ground and surface water, to deduce flow regime within the fluvial sediments. Keery et al 2007 demonstrated that the use of in-stream solute tracers is also a widely established practice to characterise groundwater-surface water exchanges within fluvial sediments. A new technology employing the use of temperature time series was proposed as a new and efficient method for calculating vertical water flux across riverbed sediments.

3.3 Sedimentology The sedimentology, and the resultant hydrogeology and flow regime within the hyporheic zone can strongly influence the lateral and vertical extent of the zone, along with the attenuation potential. The principle control of this feature is the localised and site specific geology, as this will affect the degree of transport, weathering, erosion, deposition, sorting, sediment maturity and heterogeneity of the fluvial sediments (The Hyporheic Handbook 2009).

Both active and buried land units will affect the sediment grain size distribution, as relict river features such as oxbow lakes, and riverbed sediments from braided rivers and differing conditions will be deposited with unique sediment

090116876 signatures. Smith et al 2005 highlighted the fact that heterogeneity and sediment geochemistry will also be reflective of the sedimentology regime. Heterogeneity is likely to be prolific throughout the hyporheic zone with stark contrasts occurring at each meter of fluvial sediment, as well as at the reach scales (The Hyporheic Handbook 2009).

Sediment grain size distributions, including grain size and shape distributions, sediment unit weight and bedrock outcropping, throughout the zone are a vital parameter, which influences the hydraulic conductivity (K) of the sediments. (The hyporheic network, www.hyporheic.net). This results in the sediment grain distribution controlling the flow path, residence times and therefore the exchange and mixing of surface and groundwater within the hyporheic zone. Landon et al 2001 highlights the fact that the estimation of hydraulic conductivity (K) can facilitate the estimation of the magnitude and spatial distribution of groundwater and surface water interactions. It was proposed that there will be areas of fluvial sediments which will have lower hydraulic conductivities than the aquifer and other areas of sediments, resulting in a restriction of flow/ fluxes of groundwater and surface water.

A decrease in hydraulic conductivity within the hyporheic zone is typically related to a decrease in the mean sediment grain size. The Environment Agency’s Hyporheic Handbook 2009 highlights the fact that fine bed sediments play an important role in the either the temporary storage or the fate of pollutants. Smith et al 2009 found that the attenuation potential increased in response to areas of the hyporheic zone that have decreased mean sediment grain size and therefore permeability/hydraulic conductivity (K), due to the subsequent reduction in flow rates and increase in pollutant residence time. Based on this premise there is a great need to estimate the various flow rates in the heterogeneous hyporheic zone, allowing areas of reduced flow and therefore potential attenuation to be defined.

As highlighted above fine sediment can play a role in the temporary storage or fate of nutrients and contaminants, and has been found to be advantageous in nitrate attenuation rates. However the deposition of fine sediment infiltration from river water into the hyporheic zone can also prove inhibitory. During periods of low river flow, fine sand material can settle out of the river water column and settle close

090116876 to the surface. This deposition of fine material will form a surface seal, siltation, limiting the interaction between surface and groundwater. This seal is temporary, and can be removed once river flow rates increase and shear forces have been exceeded, as the underlying pore spaces will dilate, allowing the fine sediment to settle deeper into the bed. This variation in interaction between surface and groundwater can affect the temporal rates of attenuation (The hyporheic handbook 2009).

Work by Scanlon et al 2002 outlined a range of techniques that were applicable to the measurement of groundwater discharge to streams and recharge through the stream beds, which were also valid to estimate hydraulic conductivity (K) of fluvial sediments. Simultaneously Landon et al 2001 offers a comparison of methods for measuring hydraulic conductivity, with the overall aim being to establish the most appropriate techniques.

Values of hydraulic conductivity (K) were determined using a range of techniques that quantified hydraulic conductivity over a range of scales. Landon et al 2001 tested four differing methods, which included slug and pumping tests, analysed using Hvorslev and Bouwer and Rice formulas, sediment grain size analysis and appropriate empirical relationships, and permeameter and seepage meter tests employing the Darcy equation. From these tests it was revealed that Slug and Pump tests yielded values of hydraulic conductivity significantly greater than values derived from sediment grain size analysis. This discrepancy was attributed to use of empirical relationships which neglect the fact that Hydraulic conductivity is a complex function of packing, sediment structure, heterogeneity and other key factors, suggesting that the pumping tests should in theory be more accurate. Permeameter tests, which utilise the Darcy equation in order to determine hydraulic conductivity values resulted in variable results. From the Landon et al 2001 trails 40% of the tests failed to return a result, in part due to the coupled seepage meters and hydraulic gradient measurements that used in the hydraulic conductivity trials, which both have high failure rates. Due to the unreliability of the results this technique is unfavoured in the determination of hydraulic conductivity values at the stream scale.

The variation and success of the techniques can in part be attributed to the scale dependency of the techniques; Kalbus et al 2006 surmised that this is due to the

090116876 heterogeneity of the samples, with areas of high-conductivity coexisting with areas of low conductivity, rather than the actual techniques. On this basis techniques that take in point measurements, at the local scale, such as the grain size analysis and permeameter tests will provide a good estimate of hydraulic conductivity at the specific point of sampling. This is in comparison to pumping and slug tests which operate on a much larger scale, with the proportion of sampled area volume being much larger, on the scale of meters-kilometres. This results in hydraulic conductivity values relevant to a much larger area of fluvial sediments (Kalbus et al 2006). This would appear to make point measurement techniques ideal for the quickly and inexpensively measuring differing sediment structures in fluvial sediments, over a range of locations.

Sediment grain size analysis has been extensively used in the study of fluvial sediments, for example work by Smith et al 2008. This analysis involves the wet and dry sieving of samples, with fractions smaller than 63μm being determined using techniques such as hydrometers within sediment suspensions, which can be time consuming, or Malvern Mastersizer which enables a rapid analysis. Once the sediment grain size distribution has been established the hydraulic conductivity (K) can be derived by applying various empirical equations to the resulting sediment particle distribution data. Odong et al 2007 advocates that there are seven empirical formulae that can reliably estimate K within the known ranges from the soil samples. The type of formula used depends upon the type of soil, as there is the potential for great variation between the derived hydraulic conductivity (K) values, due to the difficulty in including all of the possible variables within the porous media. Through testing a wide range of soil types and empirical equations Odong et al 2007 found that a range of the empirical formulae either under or over estimated the hydraulic conductivity values. Odong et al 2007 suggested that the optimum empirical equations were the Kozeny-Carman and the Hazen formula. The Hazen formula is applicable for soil types with an effective grain size range ( of 0.1mm to 3mm (Weight et al 2001).

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The Hazen formula derives K from

g= acceleration due to gravity, v= kinematic viscosity, n = porosity, d10= effective grain size (cm)

Fetter 2001 outlines the Hazen method to derive K from the particle grain size

2 distribution curves to be: K = C (d10)

K = hydraulic conductivity (cm/s), d10= effective grain size (cm), C is a coefficient of sorting and grain size

The degree of sorting is estimated from the uniformity coefficient Cu (Cu= and

grain size is determined by evaluating the median grain size from a grain size distribution curve. From determining sorting and grain size C can be assigned, based on table 3.1 (Fetter et al 2001).

Very fine sand, poorly sorted 40-80 Fine sand with appreciable fines 40-80 Medium sand, well sorted 80-120 Coarse sand, poorly sorted 80-120 Coarse sand, well sorted, clean 120-150 Table 3.1 Classification table for the Coefficient of sorting and grain size (Fetter et al 2001)

The Hazen formula can be utilised on the sand to gravel range that has a uniformity coefficient of less than 5 and an effective grain size of between 0.1 and 3mm (Fetter 2001). These characteristics are ideal for the River Tern site, and when combined with the success of the Hazen formula in the Ogden 2007 trials, it appears to be the ideal formula to use to estimate hydraulic conductivity from the particle grain size distribution, even though determination is based solely on the d10 particle size.

As demonstrated in the literature the geochemistry, flow regimes and the hyporheic sediment structure are key parameters of the hyporheic zone in relation to the attenuation of nitrate potential. It is of key importance to understand and quantify the exchange and attenuation processes and pathways within the hyporheic zone, especially when assessing contamination, risk and remediation schemes. In response

090116876 to these findings the overall rationale of this work is to assess various parameters relating to sedimentology, geochemistry and flow regimes within the hyporheic zone of the River Tern, Shropshire. Through this work it should be possible to define areas of enhanced nitrogen attenuation within the hyporheic zone and at a spatial scale within the study area. Through this enhanced understanding it should then be possible to consider the natural attenuation potential of the hyporheic zone as part of the risk assessment for nitrate pollution in both ground and surface waters.

4. Aims and objectives

The aim of this proposed research is to characterise the spatial variability in the sedimentological, hydrogeological and geochemical properties, that influence nitrogen attenuation in the hyporheic zone of the River Tern, Shropshire U.K. I intend to achieve this through analysing six river sediment cores from the River Tern, deducing spatial correlations between the physical and geochemical properties. I will assess the flow properties of the core by analysing the sediment grain size distribution and applying empirical relationships to deduce areas of high and low hydraulic conductivity (K). The geochemical properties of the cores will be assessed by determining the weight percentage of organic carbon, using the modified Walkley Black method and through analysing the pore water chemistry data. Once obtained, the data will be statistically analysed in order to assess linkages between the physical and geochemical processes and to deduce spatial variations of flow and nitrate attenuation.

5. Methodology

5.1Particle Grain Size Analysis

In 2005 a representative sampling coring regime took place along a reach of the river bed of the River Tern, Shropshire (figure 2.2), providing a range of unconsolidated samples. The cores ranged in depth from 0-80cm, and were each separated out into differing fractions based on the stratigraphy and varying geology throughout the cores. A range of investigative methods were conducted on the six riverbed sample cores.

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The cores were initially air dried for a few days in the laboratories prior to any investigative methods being carried out. Each of the separate core fractions were then separated out to allow different analysis to be performed. Half of each core fraction sample was separated for the grain size analysis, a quarter was separated for the analysis of the fraction of organic carbon (FOC), and the remaining quarter of each sample was preserved for reference and stored in a cold store.

The Particle grain size analysis of the cores was undertaken by using wet and dry sieving, employing a set of British standard sieves, and laser diffraction analysis using the Malvern, UK, Mastersizer1E 2000 for the grain size fractions of <63 μm. The overall principle of this procedure is to separate the primary mineral parts of the sediment cores into differing size fractions, and to determine the proportions of mass within these fractions. On this basis it is then possible to determine the predominant grain size classification for each separate core fraction.

Initially an acid digestion was carried out for all of the sediment grain size analysis samples in order to pre-treat the sample and obtain complete dispersion of the primary particles. The basis of the acid digestion is to remove cementing materials which limit the dispersion of the primary particles. The cementing materials are often of secondary origin such as organic matter and calcium carbonate.

The acid digestion was implemented using Hydrogen peroxide (30%), and a dispersing agent comprised of sodium hexametaphosphate (4%) and soda (1%). Each sample is weighed out into a beaker, with 50:50 diluted hydrogen peroxide added to cover the sample. The samples were then placed onto hotplates at C and left to react, with frothing being typical. Additional hydrogen peroxide solution is added throughout the reaction, with the reaction been complete when there is no more effervescence. Once this stage is met deionised water is added to make the solution up to 250ml and left to boil for an hour.

The solution is subsequently decanted into Erlenmeyer flasks, and 25ml of dispersing agent is added. The flasks are then placed on a shaking table overnight that is moving at150 rev/min. The contents of the flasks are then passed and washed, with deionised water, through a <63μm sieve. The >63μm fraction is then collected in a beaker and dried to residue on the hot plate and the <63μm fraction collected into a metal tray and placed in a fan assisted oven to dry.

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When dry, the >63μm fraction were dry sieved using a set of British standard sieves ranging from >2mm to <63μm, stacked in descending size order. The sieves were placed on a mechanical shaker and shook for a twenty minute time period. After shaking the contents in each sieve were collected, with the aid of a metal wired brush and weighed, with the weights being used to generate particle grain size distribution curves, allowing grain size analysis to be carried out. The fraction collected that is <63μm for the dry sieving, was subsequently added to the corresponding sample of <63μm fraction that was derived from the wet sieving.

Once dried the <63μm fraction is weighed and combined with the corresponding sample <63μm fraction from the dry sieving. A small quantity of each combined sample was placed in the Malvern, UK, Mastersizer1E 2000. The Malvern uses laser diffraction in order to determine the grain size distribution within this size fraction, which is tabulated and printed off. The accuracy of the Malvern was verified through the use of glass microsphere standards 45-53μm in size from the Whitehouse scientific company.

5.2 Fraction of Organic Carbon

The fraction of organic carbon (foc) analysis was carried out using the modified Walkley-Black method. This involved grinding ~5g of each sample so that it would pass through a 0.25mm sieve. ~1g of each sample was then weighed and placed in a flask, and 10ml of potassium dichromate standard solution (0.1667M) was added, followed by 20ml of concentrated sulphuric acid (96%) after which the flask is allowed to stand for 30 minutes in a fume cabinet. Once 30 minutes has passed 250 ml of deionised water was added along with 10ml of concentrated orthophosphoric acid (85%). 1ml of the indicator barium diphenylamine sulphonate (0.16%) solution was then added and titrated with Ferrous sulphate solution (1M), whilst being continuously stirred with a magnetic mixer. At the end point there is a sharp colour change from brown/purple to bright green, and the volume of ferrous sulphate solution added is then recorded. For each core, two repeat measurements and two blank measurements using just the solutions were carried out and recorded in order to verify the molarity of the ferrous sulphate and the precision of the solutions (Van Reeuwijk 2002). NCS certified reference stream sediment from the China national analysis centre for iron and steel 2004 were also tested in order to verify the success of the modified Walkley Black method. The results from each core

090116876 sample and the blanks were then used in order to calculate the weight (wt) percentage of carbon within the individual sample.

5.3 Pore Water Chemistry

Pore water chemistry data, which was collected and analysed in the laboratory at the time of coring. The collected pore water was analysed for a range of parameters including acetate, nitrate, alkalinity and total dissolved organic carbon. This data will be used as a secondary source alongside the physical and chemical properties derived from the laboratory experimentation in order to quantify spatial variability of nitrate within the River Tern.

6. Results

Figure 6.1- Hyporheic sediment cores stratigraphy

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6.1 Sediment Core 2.2-

Hydraulic Conductivity (K) 2.2- 50000 40000 30000

20000 K (m/d) K 10000 0 0 5 10 15 20 25 30 35 40 Depth (cm)

Figure 6.2- Sediment Core 2.2- Hydraulic Conductivity (K) profile

Carbon 2.2- 1.20 1.00 0.80 0.60 0.40

wt%Carbon 0.20 0.00 0 5 10 15 20 25 30 35 40 Depth (cm)

Figure 6.3- Sediment Core 2.2- wt% Organic Carbon profile

Grain Size 2.2- 2 1.5 1 0.5 0 0 5 10 15 20 25 30 35 40 Depth (cm) Average Grain Size (mm)

Figure 6.4- Sediment Core 2.2- Grain Size profile (Determined from d50 particle grain size analysis curves- appendices 3)

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Chloride 2.2- 50.00 48.00 46.00 44.00 42.00 40.00

0 5 10 15 20 25 30 35 40 Chloride ConcentrationChloride (ppm) Depth (cm)

Figure 6.4- Sediment Core 2.2- Chloride profile

Nitrate and Acetate 2.2- 150.00 1.50

100.00 1.00

50.00 0.50

0.00 0.00 0 5 10 15 20 25 30 35 40

Depth (cm)

NitrateConcentration (ppm) Acetate ConcentrationAcetate (ppm) Acetate Nitrate

Figure 6.5- Sediment Core 2.2- Nitrate and Acetate profile

Alkalinity 2.2- 1000.00 800.00 600.00 400.00 200.00 0.00 0 5 10 15 20 25 30 35 40

Concentration CaCo3Concentration mg/l Depth (cm)

Figure 6.6- Sediment Core 2.2- Alkalinity profile

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Figure 6.2 outlines the varied nature of the hydraulic conductivity (K) values throughout the sediment core 2.2- profile. There is a near linear increase in K values from 25,000 to 42,000 m/d, from the beginning of the core to ~15cm depth within the sediment core. This correlates to the peaty clay layer that is the dominant geology at the near surface layer of the core (Figure 6.1), with variations in the formation regime of this layer resulting in the heterogeneous increasing K values. Song et al 2010 outlines how the upper layers of the hyporheic sediments can be a site of lower hydraulic conductivities, due to clogging layers. The sediments within these layers are frequently tightly packed with a compact texture and low pore space, resulting in a reduced hydraulic conductivity (K). The initial low value of 25,000 m/d demonstrated at the top of the sediment core 2.2- profile suggests that a clogging feature, along with the peaty clay geology, could be contributing to the initial low hydraulic conductivity value. From 15-27cm depth there is a general decrease in the K values, followed by a levelling off at 28,000 m/d at 27-32 cm depth, and a final increase at the lower level of the core to 33,000 m/d. Again this variation in K values throughout the core profile is corresponding to the varying stratigraphy outlined in Figure 6.1, which outlines a layer of fine sand and gravel at ~27-32cm depth followed by a layer of grit and pebbles at the lowest level of the core.

Smith et al 2009 highlighted the fact that there is wide acknowledgement that hydraulic conductivity (K) values are highly linked to mean sediment grain size, with decreasing grain size resulting in a lower hydraulic conductivity (K) value. In the case of sediment core 2.2- there seems to be a definite correlation between average grain size, as determined from the d50 fraction of the sediment particle distribution curves (Appendices 3) and hydraulic conductivity (K). Figure 6.4 outlines a gentle increase in the average grain size from the beginning to core to 10cm, reaching an average value of 0.35mm, this is followed by a sharp linear increase to 1.7mm at 15cm depth, which is the largest grain size value throughout the core. Both the largest grain size value and the largest hydraulic conductivity (K) value both occurred at 15cm depth. After 15cm depth there is a rapid decrease in grain size to 0.2mm at 20cm depth followed by a gentle continuing reduction in size and a levelling off for the remainder of the core. This variation in grain size for the lower depths of the core is reflective of the general decrease in hydraulic conductivity as outlined in Figure 6.2, verifying the correlation between the two physical properties.

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However the variation in grain size profile (Figure 6.4) shows a more distinct and pronounced variation, between the different depths of the profile, than the hydraulic conductivity (k) profile (Figure 6.5). This could potentially be as a result of employing the Hazen empirical relationship, which does not encompass every porous media property when determining hydraulic conductivity values.

The hydraulic properties of the hyporheic sediments will influence the route and the rate that both surface and groundwater will take through the zone, along with the extent of mixing between the two water sources, which can be both vertical and horizontal in nature (Hill et al 1998). The subsurface- surface water mixing and exchange, transports a range of substances and regulates physicochemical conditions within the zone (Mutz et al 2003), and as such is extremely important in the assessment of nitrate attenuation. Both groundwater and surface water have distinct chemical signatures, Baskaran et al 2009 outlines that near surface water will have a higher chloride signature than groundwater. The chloride profile (Figure 6.4) is outlining a distinct area of low chloride concentration at 27cm depth, with chloride values reaching as low as 42ppm. The area of highest chloride value occurs at 15cm depth, with a value of 49ppm, suggesting that this area has significant proportions of surface water. There is a varying range of chloride concentrations between 15-27cm depth, with chloride concentration values being below the highest inferred surface water concentrations, and greater than the inferred groundwater values, suggesting that at this depth mixing of the two water bodies is occurring. Due to the conservative nature of chloride it can also be used to assess anticipated and actual routes of nitrate and concentrations throughout the hyporheic zone.

Figure 6.3 outlines the organic carbon profile of sediment core 2.2-, which is cited as the preferential electron donor in the denitrification process (Rivett et al 2008). Through varying deposition regimes, the hyporheic sediments receive differing quantities of organic carbon, which is subsequently involved in a range of biogeochemical processes (Schindler et al 1998). Rivett et al 2008 found that it was the availability of dissolved organic carbon (DOC) that was primarily responsible for the rate of the denitrification process; however no DOC data is available for sediment core 2.2-. In response to these findings the assessment of the carbon properties of the

090116876 geochemistry within the hyporheic zone, is very important in the assessment for the natural attenuation of nitrates.

Figure 6.3 initially shows a profile from 4-16cm which has a small increase in organic carbon, ranging from 0-16%, followed by a large rapid linear increase form 15-21cm depth with carbon content increasing to 00.96%. From 21 cm depth to the end of the core there is a slight decrease in the carbon content, however the content remains fairly high at 0.6% carbon content. From this profile it would suggest that from 15- 36cm depth there is a higher carbon content, and if available to microorganisms in the form of dissolved organic carbon, then this area should be a site of enhanced denitrification as there is ample electron donor availability. From comparison with figure 6.2 and 6.4 it is apparent that where there is small grain size and small hydraulic conductivity (K) along with high weight percent content of organic carbon at this depth.

Figure 6.5 and 6.6 are outlining the nitrate, acetate and alkalinity chemical profiles for sediment core 2.2-. From assessing the nitrate profile, and comparing it to the chloride profile, which can be taken as the background concentration due to chlorides conservative nature, there is evidence of nitrate depletion. Depths of 15 and 25 and 35cm are demonstrating greatly depleted nitrate profiles in comparison to the chloride profile (Figure 6.3). On this basis denitrification appears to be prominent at depth, suggesting that the process is prevalent under anoxic conditions, as it is anticipated that there will be substantially less oxygen at depth due to diffusion gradients. As outlined by Buss et al 2005 most biological denitrification processes rely on heterotrophic microorganisms, which require a carbon source to carry out denitrification. Within the River Tern the dissolved organic carbon, principally from the geology is the dominant carbon source. The end product of anaerobic degradation of organic matter is acetate (Rulik et al 2000). Figure 6.5 is outlining sediment core 2.2- nitrate and acetate profiles, from this profile there is evidence at depths 15, 25 and 35 cm of the acetate profile increasing, with depth 25cm displaying a dramatic acetate concentration increase to 90 ppm. This is further suggesting that denitrification is taking place at depths 15, 25 and 35cm, with an enhanced rate occurring at 25cm depth.

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Valett et al (1996) found that increased nitrate attenuation was correlated with longer water residence times within the hyporheic Zones, which is related to reduced hydraulic conductivity (K) values and reduced grain size. Sediment core 2.2- appears to strongly follow these trends at 25 cm depth where there is evidence of nitrate attenuation along with the low values of hydraulic conductivity (K), areas of decreased grain size and high organic carbon content. Figure 6.6 is showing a reduction in alkalinity throughout the core, with depth. During denitrification there is a step wise removal of oxygen, which entails the removal of a carbon source (CH2O) - and forming HCO3 , as seen in equation 6.1 (Rivett et al 2008) - - Equation 6.1: 5CH2O + 4NO3 → 2N2 + 4HCO3 +CO2 +3H2O On this basis the overall reduction in alkalinity that is displayed in Figure 6.6 is in contrast to the expected trend for denitrification. However through analysis chloride, nitrate and acetate it appears that denitrification is occurring at depths 15, 25 and 35cm.

6.2- Sediment Core 2.4-

Hydraulic Conductivity (K) 2.4- 200000

150000

100000 K (m/d) K 50000

0 0 20 40 60 80 100 Depth (cm)

Figure 6.7- Sediment Core 2.4- Hydraulic conductivity (K) profile

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Carbon 2.4- 4 100 3 80 60 2 40 1

20 DOC mg/L wt%Carbon 0 0 0 20 40 60 80 100 Depth (cm)

wt% organic carbon DOC

Figure 6.8- Sediment Core 2.4- Carbon profile

Grain Size 2.4- 1.2 1 0.8 0.6 0.4 0.2 0

0 20 40 60 80 100 Average Grain Average (mm) Size Depth (cm)

Figure 6.9- Sediment Core 2.4- Grain Size profile (Determined from d50 particle grain size analysis curves- appendices 3)

Chloride 2.4- 50.00 40.00 30.00 20.00 10.00 0.00 0 20 40 60 80 100

Chloride ConcentrationChloride (ppm) Depth (cm)

Figure 6.10- Sediment Core 2.4- Chloride Profile

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Nitrate and Acetate 2.4- 400.00 60.00 50.00 300.00 40.00 200.00 30.00 20.00 100.00 10.00 0.00 0.00 0 20 40 60 80 100

Depth (cm)

NitrateConcentration (ppm) AcetateConcentration (ppm) Acetate Nitrate

Figure 6.11- Sediment Core 2.4- Nitrate and Acetate profile

Alkalinity 2.4- 2000.00

1500.00

1000.00

500.00

0.00 0 20 40 60 80 100

Concentration CaC03 CaC03 Concentration(mg/L) Depth (cm)

Figure 6.12- Sediment Core 2.4- Alkalinity profile

Figure 6.7- is displaying the hydraulic conductivity (K) profile for sediment core 2.4. Throughout the core profile there is a general decrease in K values with depth, with the highest value of 19,000 m/d occurring at the start of the core, (16cm depth) which has an initial geology of coarse sand (Figure 6.1). Song et al 2010 outlines how hyporheic flow can induce hyporheic processes, resulting in larger K values at the top of the hyporheic zone by increasing the pore space in the stream bed. Song et al 2010 further outlined the fact that sediment deposited in the lower layer of the hyporheic sediments may have been subjected to pressure from the upper sediment layer. This pressure could induce compaction of the lower sediments, resulting in part, to the lower hydraulic conductivities demonstrated at depth in the sediment profile (Figure 6.7). There is a slight divergence from the overall

090116876 decreasing trend of hydraulic conductivity with depth at 48cm depth, where there is a small increase in K values 17,000 m/d. This variation is corresponding to an alteration in the geology of the sediment core from a grit horizon to gravel and grit, with the properties of the two media altering the hydraulic conductivity (Figure 6.1).

Figure 6.9 outlines the grain size profile for sediment core 2.4-, and is displaying a general correlation between hydraulic conductivity (K) and grain size, with a general decrease in grain size throughout the core. However there is an exception to this trend, with a sharp peak of increased grain size at ~32cm depth, which corresponds to a change in the sediment cores geology from sand to gravel (Figure 6.1). This variation in grain size fails to be reflected within the hydraulic conductivity, this could potentially be as a result of the limitations of the use of empirical equations to determine hydraulic conductivity (K) values or other unforeseen physical characteristics.

The organic carbon profile, outlined in Figure 6.8 is displaying a diverse pattern of weight percentage (wt%) carbon content, with depths of 24 and 68cm being areas with extremely low wt % carbon content. These areas are comprised of coarse sand and brown gravel and grit geologies, and the carbon content will be in part reflective of the deposition regime. From figure 6.8 there appears to be little correlation between the weight % of organic carbon and hydraulic conductivity and grain size. However the dissolved organic carbon content (DOC), which has an initial low value of 0.95 mg/L is following the general decreasing trend displayed in both the grain size and hydraulic conductivity profiles. This pattern appears to suggest that as there is less carbon electron donors available with depth; denitrification could be limited with depth, however the DOC values are consistently greater than the wt% organic carbon values. Figure 6.8 is displaying a slight increase in DOC concentrations at 40cm depth; this is reflecting the increase also displayed by the hydraulic conductivity profile at 40cm depth, which facilitates enhanced carbon dissolution.

The chloride profile Figure 6.10 is again generating a profile that correlates to the hydraulic conductivity and grain size profiles, with high values towards the top of the profile and a gradual decrease in values with depth. This signature is reflective of the fact that surface water generally has higher chloride

090116876 values than groundwater, suggesting that groundwater is prevalent towards the base of the core. Figure 6.10 displays a lower chloride value than anticipated at 40cm depth; this is suggestive that this is an area of mixing between ground and surface water.

Figure 6.11 displays three significant areas of rapid nitrate depletion at 4, 32 and 64 cm depth, when compared to the conservative chloride profile (Figure 6.10). This is suggestive of denitrification occurring at an enhanced rate within these three sites, which have geologies of black clay, grit and brown gravel and grit. Figure 6.11 is also displaying three sites of enhanced acetate concentration, which occur at 4, 32 and 64 cm depth and are corresponding to areas of enhanced nitrate depletion. This is again suggesting that denitrification has been prevalent within these areas as acetate is produced as the main anaerobic degradation products of organic carbon (Rulik et al 2000). Figure 6.12 is displaying the alkalinity profile for sediment core 2.4-, which has an overall decreasing trend with depth. However there are small increases at depths 32 and 64cm, which are generated through the production of alkalinity during the denitrification process, further supporting the conclusion that denitrification is prevalent at these depths.

6.3- Sediment Core 2.5-

Hydraulic Conductivity (K) 2.5- 150000

100000

K (m/d) K 50000

0 0 10 20 30 40 50 Depth (cm)

Figure 6.13- Sediment Core 2.5- Hydraulic Conductivity (K) profile

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Carbon 2.5- 2.5 100 2 80 1.5 60 1 40

0.5 20 mg/L DOC wt% Carbonwt% 0 0 0 10 20 30 40 50 Depth (cm)

wt% organic carbon DOC

Figure 6.14- Sediment Core 2.5- Carbon profile

Grain Size 2.5- 0.8

0.6

0.4

0.2

Average Grain Average (mm) Size 0 10 20 30 40 50 Depth (cm)

Figure 6.15- Sediment Core 2.5- Grain size profile (Determined from d50 particle grain size analysis curves- appendices 3)

Chloride 2.5- 30.00 25.00 20.00 15.00 10.00 5.00 0.00 0 10 20 30 40 50

Chloride ConcentrationChloride (ppm) Depth (cm)

Figure 6.16- Sediment Core 2.5- Chloride profile

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Acetate and Nitrate 2.5- 250.00 60.00 200.00 50.00 150.00 40.00 30.00 100.00 20.00 50.00 10.00 0.00 0.00 0 10 20 30 40 50

Depth (cm)

Nitrate Concentration (ppm)ConcentrationNitrate Acetate ConcentrationAcetate (ppm) Acetate Nitrate

Figure 6.17- Sediment Core 2.5- Acetate and Nitrate profile

Alkalinity 2.5- 350.00 300.00 250.00 200.00 150.00 100.00 50.00 0.00

ConcentrationCaCo3 mg/L 0 10 20 30 40 50 Depth (cm)

Figure 6.18- Sediment Core 2.5- Alkalinity profile

Figure 6.13 is outlining the hydraulic conductivity (K) profile of sediment core 2.5-, which highlights three distinct areas of reduced hydraulic conductivity at depths 7, 16 and 34 cm, with black peaty clay and wood geologies. There is wide variation between the hydraulic conductivity values, with 3, 16 and 34 cm all having values of 105,000 m/d, and values below 36 cm depth substantially reduced to 30,000 m/d. The hydraulic profile is very reflective of the sediment grain size distribution profile between the depths of 15-35cm, which is outlined in Figure 6.15. However both the top and bottom of the cores are displaying grain size profiles that are significantly different to the hydraulic conductivity profile, with the range of values for depths 7-15cm being much smaller, with a variance of 0.06mm. Figure 6.15 is also displaying further variance from the hydraulic conductivity profile at

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42cm depth, which has a wood geology. In this instance the grain size value sharply increases to 0.7mm, which is in stark contrast to the hydraulic conductivity profile which is at a constant low value of 30,000 m/d at this depth.

The weight percentage (wt%) of organic carbon (Figure 6.14) is displaying two distinct peaks of elevated carbon content, located at 7 and 47 cm depth. At these depths there is a rapid increase and then decrease of wt% carbon values, which rise to 2 and 1.7%, with the remainder of the profiles weight percentage being under 1%. The elevated wt % organic carbon value at 7cm has a geology of black peaty clay, this elevated organic carbon value would suggest that at 7cm there is a high proportion of peat in the sediment, as peat is known to have a high organic carbon content. The dissolved organic carbon (DOC) (Figure 6.14) is displaying a profile that has a sharp decrease to ~0 mg/L DOC from the beginning of the core to 10cm depth, followed by a gradual step increase for the remainder of the core. This is suggesting that little denitrification will be occurring at this depth as a carbon source will be limiting.

The chloride profile (Figure 6.16) is displaying a gently undulating profile, with a small decrease in concentration at 10 and 46cm depth and the greatest concentration occurring at 26cm depth. Through comparing nitrate concentration profiles with acetate concentration profiles it has been possible to highlight two areas of nitrate depletion, which are not displayed within the chloride profile. These are located at the beginning of the core at 2cm depth, with a nitrate concentration of 8ppm and from 32cm depth on wards, with a nitrate concentration of 2ppm. At 10cm depth figure 6.16 is displaying a reduction in chloride concentration to 21ppm, this reduction is mirrored within the nitrate profile suggesting that little denitrification is occurring at this depth. From comparing the weight percentage organic carbon and DOC for 10cm depth, it is apparent that there is little carbon available to act as an electron donor, therefore limiting denitrification. Figure 6.17 is outlining the acetate concentration profile for sediment core 2.5-, which is displaying enhanced acetate concentration values that are greater than nitrate at depths 2 and 32cm. This is further suggesting that denitrification is occurring at these depths as acetate is produced as a result of the degradation of organic carbon, which is utilised in the denitrification process (Rivett et al 2008). Figure 6.18 is displaying the alkalinity profile, which is showing enhanced alkalinity concentrations at 2 and 32 cm depth, further supporting

090116876 the conclusion that denitrification is occurring at these depths as the denitrification process generates alkalinity.

6.4- Sediment Core 2.6-

Hydraulic Conductivity (K) 2.6- 60000 50000 40000 30000

K (m/d) K 20000 10000 0 0 5 10 15 20 25 30 35 40 Depth (cm)

Figure 6.19- Sediment Core 2.6- Hydraulic Conductivity (K) profile

Carbon 2.6- 4.00 100 3.00 80 60 2.00 40 1.00

20 DOC mg/L wt% Carbonwt% 0.00 0 0 5 10 15 20 25 30 35 40 Depth (cm)

wt% organic carbon DOC

Figure 6.20- Sediment Core 2.6- Carbon profile

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Grain Size 2.6- 0.8

0.6

0.4

0.2

Average Grain Average (mm) Size 0 5 10 15 20 25 30 35 40 Depth (cm)

Figure 6.21- Sediment Core 2.6- Grain Size profile (Determined from d50 particle grain size analysis curves- appendices 3)

Chloride 2.6- 35.00 30.00 25.00 20.00 15.00 10.00 5.00 0.00 0 5 10 15 20 25 30 35 40

Depth (cm) Chloride ConcentrationChloride (ppm)

Figure 6.22- Sediment Core 2.6- Chloride profile

Nitrate and Acetate 2.6- 400.00 2.00 300.00 1.50 200.00 1.00 100.00 0.50 0.00 0.00 0 5 10 15 20 25 30 35 40

Depth (cm) NitrateConcentration (ppm) AcetateConcentration (ppm) Acetate Nitrate

Figure 6.23- Sediment Core 2.6- Nitrate and Acetate profile

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Alkalinity 2.6- 500.00 400.00 300.00 200.00 100.00 0.00 0 5 10 15 20 25 30 35 40

Concentration CaCo3Concentration (mg/L) Depth (cm)

Figure 6.24- Sediment Core 2.6- Alkalinity profile

Figure 6.19 is displaying the hydraulic conductivity (K) profile for sediment core 2.6-. There is an initial high K value of 52,000 m/d which rapidly decreases to 24,000 m/d at 8cm depth, this is reflective of the geology which consists of mud and pebbles (Figure 6.1), and large grain sizes (Figure 6.21). From 8cm depth the overall profile displays a low and continuous K profile, with two symmetrical areas of slightly elevated K of 34,000 m/d at 14 and 31 cm depths, comprised of a wood and a mud & wood geology. This variation in K values is partially reflected in the grain size profile Figure 2.6-, which initially decreases from 0.7 to 0.1mm as the geology alters from mud and pebbles to predominantly wood (Figure 6.1). For the remainder of the profile the grain size values remain fairly low at ~0.15mm until 27cm depth where there is a small increase to 0.25mm.

Figure 6.21 outlines the carbon profile for sediment core 2.5-, which is displaying an area of high weight percentage organic carbon from ~8-30cm depth, which reflects the wood and mud & wood geology of the sediment core. The two areas of low wt% carbon occur at the top and bottom of the sediment cores, and are again reflecting the mud and pebbles, and grit and pebbles geology. The dissolved organic carbon DOC profile is displaying a general decreasing trend with depth throughout the profile. From depth 4-14 the DOC ranges from 50-80 mg/L and is consistently higher than the wt% organic carbon. From 14-33 cm depth the DOC mg/L concentration is lower than the wt% organic carbon profile; this depth correlates to an area of reduced hydraulic conductivity (Figure 6.19), which suggests that the reduced flow is leading to reduced carbon dissolution from the sediment to

090116876 the water, which could limit the denitrification process. From depth 33cm there is an increase in the DOC concentration, to levels greater than wt% carbon. This pattern is reflective of the grain size profile (Figure 6.21) and is suggesting that from 33cm depth to the end of the core may be an area where denitrification is occurring.

Figure 6.22 is demonstrating the chloride concentration profile throughout the sediment core. There is a general decrease in chloride concentration with depth, with a more prominent decrease occurring at 13cm depth, following the hydraulic conductivity pattern. This is suggestive of surface water being prominent towards the top of the core, and an area of mixing between ground and surface water at 13cm depth. From the comparison of the chloride and nitrate profiles (Figures 6.22 and 6.23) there appears to be variations in the nitrate and conservative chloride profile at 25cm depth, with nitrate displaying a low concentration value of 0.60ppm., and at 35 cm depth (1.25 ppm), suggesting that denitrification is occurring. Further support for denitrification occurring at this depth is apparent from the acetate profile and the alkalinity profile (Figure 6.24), where both these profiles display an increase in acetate and alkalinity, from this depth, which are both generated during the denitrification process (Equation 6.1).

6.5- Sediment Core 2.7-

Hydraulic Conductivity (K) 2.7- 60000 50000 40000 30000

K (m/d) K 20000 10000 0 0 10 20 30 40 50 60 Depth (cm)

Figure 6.25- Sediment Core 2.7- Hydraulic Conductivity (K) profile

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Carbon 2.7- 1 300 200 0.5

100 DOC mg/L DOC

wt% Carbonwt% 0 0 0 10 20 30 40 50 60 Depth (cm)

wt% organic carbon DOC

Figure 6.26- Sediment Core 2.7- Carbon profile

Grain Size 2.7- 0.5 0.4 0.3 0.2 0.1 0

0 10 20 30 40 50 60 Average Grain Size (mm) Depth (cm)

Figure 6.27- Sediment Core 2.7- Grain Size profile (Determined from d50 particle grain size analysis curves- appendices 3)

Chloride 2.7- 100.00 80.00 60.00 40.00 20.00 0.00 0 10 20 30 40 50 60

Chloride ConcentrationChloride (ppm) Depth (cm)

Figure 6.28- Sediment Core 2.7- Chloride profile

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Acetate and Nitrate 2.7- 150.00 3.00

100.00 2.00

50.00 1.00

0.00 0.00 0 10 20 30 40 50 60

Depth (cm)

NitrateConcentration (ppm) Acetate ConcentrationAcetate (ppm) Acetate Nitrate

Figure 6.29- Sediment Core 2.7- Acetate and Nitrate profile

Alkalinity 2.7- 1000.00 800.00 600.00 400.00 200.00 0.00

0 10 20 30 40 50 60 Concentration CaC03 CaC03 Concentration(mg/L) Depth (cm)

Figure 6.30- Sediment Core 2.7- Alkalinity profile

The hydraulic conductivity profile for sediment core 2.7- (Figure 6.25) is displaying three areas of elevated K rates, with the overall trend being a gradual gentle increase in K rates with depth. The three areas of elevated K occur at 9, 20, and 38 cm depth, with the largest increase occurring at 9cm depth, increasing from 34,000 to 52,000 m/d. The overall decreasing trend in hydraulic conductivity values with depth is similar to the overall decrease in grain size with depth, displayed in Figure 6.27. However there is variation within the profile, with an area of increased grain size, to 0.38mm, occurring at 18-28cm depth, which is reflected in the hydraulic conductivity (K) profile as an area of elevated K but to a smaller extent. Both the hydraulic conductivity and grain size profiles (Figure 6.25 and 6.27) are

090116876 showing decreased values at 12cm depth, further supporting the correlation between grain size and hydraulic conductivity.

The weight percentage organic carbon profile (Figure 6.26) is displaying three distinct peaks at 10, 20 and 45 cm depth and an area of low organic carbon at 28-38 cm depth. The areas of elevated wt% carbon at 10 and 20 cm depth are correlating to areas of high conductivity (Figure 6.25) but relatively small grain size of 0.32 mm. This is unreflective of the general trend of elevated organic carbon with reduced grain size and hydraulic conductivity, however the geological profiles are outlining that both 10 and 20 cm depths are areas of wood and brown sand, suggesting high organic content at the time of deposition. The dissolved organic carbon profile (DOC) is reflecting a differing pattern to wt% carbon, which is closely mirroring the average grain size profile, suggesting that at areas of larger grain size there is an increased ability for carbon to dissolve and to act as electron donors for the denitrification process.

The chloride profile (Figure 6.28) is again showing a general declining trend with depth. This is with the exception of 20cm depth where there is a sharp increase and then decrease in concentration to 85 ppm, suggesting that there is to be a large proportion of surface water within this area. By comparing the conservative chloride profile with the acetate and nitrate profiles (figure 6.29) there is suggestion that denitrification is occurring throughout the profile. Depths 10 and 20 cm are closely mirroring the chloride sharp rise and fall concentration pattern, suggesting that little denitrification is occurring here. However at 15 and 25 cm depth both alkalinity (Figure 6.30) and acetate (Figure6.29) are displaying increases suggesting that some denitrification is occurring at these depths. At 35cm depth the nitrate concentrations are substantially greater than the corresponding chloride profile. From Figure 6.26 it is revealed that there is an extremely low wt% organic carbon and DOC content at this depth, suggesting that denitrification is not operating at a high rate at this depth due to the limited carbon electron donors. Figure 6.29 is also displaying further nitrate variation from the chloride profile at depths 4cm and 46- 50cm, where nitrate levels are lower than anticipated, and suggesting further denitrification. Denitrification at 4cm depth is further supported by the alkalinity profile, which displays an elevated alkalinity concentration of 800.00 mg/L, although there is little increase in acetate concentrations. Depth 46-50 cm displays a reduction

090116876 in nitrate concentration, from 1.0- 0.8 ppm. This reduction is mirrored in both the acetate (Figure 6.29) and the alkalinity profiles (Figure 6.30), but there is an increase in the chloride profile at this depth (Figure 6.28), suggesting a chemical signature of surface water. These variations suggest that the reduction in nitrate concentrations could be a result of mixing and dispersion instead of denitrification.

6.6- Sediment Core 2.8-

Hydraulic Conductivity (K) 2.8- 500000 400000 300000

200000 K (m/d) K 100000 0 0 10 20 30 40 50 60 70 Depth (cm)

Figure 6.31- Sediment Core 2.8- Hydraulic Conductivity (K) profile

Carbon 2.8- 2 60 1.5 40 1 20

0.5

DOC DOC mg/L wt% Carbonwt% 0 0 0 10 20 30 40 50 60 70 Depth (cm)

wt% organic carbon DOC

Figure 6.32- Sediment Core 2.8- Carbon profile

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Grain Size 2.8- 0.8 0.6 0.4 0.2 0 0 10 20 30 40 50 60 70

Average Grain Average (mm) Size Depth (cm)

Figure 6.33- Sediment Core 2.8- Grain Size profile (Determined from d50 particle grain size analysis curves- appendices 3)

Chloride 2.8- 40.00

30.00

20.00

10.00

0.00 0 10 20 30 40 50 60 70 Depth (cm) Chloride ConcentrationChloride (ppm)

Figure 6.34- Sediment Core 2.8- Chloride profile

Acetate and Nitrate 2.8- 200.00 60.00 150.00 40.00 100.00 50.00 20.00 0.00 0.00 0 10 20 30 40 50 60 70 Depth (cm)

Acetate Nitrate NitrateConcentration (ppm) Acetate ConcentrationAcetate (ppm)

Figure 6.35- Sediment Core 2.8- Acetate and Nitrate profile

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Alkalinity 2.8- 400.00 300.00 200.00 100.00 0.00 0 10 20 30 40 50 60 70

Depth (cm) Concentration CaC03 CaC03 Concentration(mg/L)

Figure 6.36- Sediment Core 2.8- Alkalinity profile

Figure 6.31 is outlining the hydraulic conductivity (K) profile of sediment core 2.8-, which is displaying an overall low value of ~50,000 m/d and two areas of increased K at 20-35 and 58 cm depth. The hydraulic conductivity profile and the depths with elevated hydraulic conductivity are closely correlated with the grain size profile (Figure 6.33), which is outlining areas of increased grain size at the same depths as areas of elevated K values. At depth 20-35 cm the predominant geology is dark sand and organics, with a transition to peat and wood. At 58 cm depth the dominant geology is fine sand and organics.

The wt% organic carbon profile (Figure 6.32) is outlining a distribution that is very similar to both hydraulic conductivity and grain size, with an elevated area of carbon occurring at 28-28 cm depth. This is in contrast to the general trend of decreased grain size, hydraulic conductivity and increased organic carbon; however the geology at this depth is dark sand and organics and peat and wood (Figure 6.1), confirming a deposition regime high in organic carbon. The dissolved organic carbon profile (DOC) is displaying an extremely varied profile throughout the sediment core. Depths 8-14 and 36 cm are displaying very low DOC ~0 mg/L, despite having higher wt% carbon. This is suggesting that little carbon is able to dissolve at these sites and therefore there will be little denitrification occurring at these depths as carbon isn’t available to act as an electron donor. Within the DOC profile there are also two areas of high concentration carbon at 26 and 44-60, with carbon values of ~40 mg/L, therefore providing a site of high electron donor availability.

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Figure 6.34 is displaying a chloride profile that has a general increase in concentration with depth, with the highest concentration of 34 ppm being reached at 58cm depth. There are small fluctuations in the chloride profile with a small increase at 12cm depth and a small decrease at 20cm depth. Through comparison with the chloride profile (Figure 6.34) and nitrate concentrations (Figure 6.35), it is apparent that there are areas within the nitrate profile that are lower than anticipated, suggesting that denitrification has occurred. There are two key areas that are displaying lower nitrate concentrations than expected, these are occurring at 20 and 52-58cm depth, which have a geology of dark brown sand and organics, and fine sand and organics. These depths are both displaying evidence of enhanced acetate concentrations (Figure 6.35) with values of 150 and 190 ppm. As acetate is considered the main product of anaerobic degradation of organic matter (Rul et al 2000), this would further suggest denitrification is occurring at these depths. Figure 6.36 is displaying the alkalinity profile, which is displaying two areas of increased alkalinity at depths 20 and 52-58. Since denitrification is an alkalinity generating process, this further supports the conclusion that denitrification is prominent at depths 20 and 52-58 cm in sediment core 2.8-.

6.7 Spatial Scale

Through assessing the individual sediment cores it is clear that there are advantageous areas within the core profiles that facilitate the reduction in nitrate concentrations, through the denitrification process. Table 6.73 outlines the location of nitrate depletion for each core, and the physical and geochemical properties that correspond to each core and depth of nitrate depletion. Throughout the analysis of each individual core, correlations between the physical and geochemical properties were repeatedly displayed. In order to verify and establish the strength of these relationships between the sedimentological and geochemical properties, Geostatistical analysis was carried out using analysis of variance (ANOVA), to a 90% confidence level.

From Table 6.71 it is apparent that grain size, hydraulic conductivity, wt% organic carbon and depth are all collectively contributing to the variability in the denitrification process. The significance F value (p-value) is extremely low at 0.014,

090116876 verifying the fact that the control variables (physical and chemical parameters) and their relationships are significantly influencing the depletion of nitrate concentrations, through the denitrification process.

Table 6.71 ANOVA results for grain size, hydraulic conductivity, wt% organic carbon and depth parameters

Table 6.72 ANOVA results for grain size, hydraulic conductivity, wt% organic carbon and depth parameters

Figure 6.72 is displaying the p-values for the individual control variables, which are all displaying low values, supporting the findings of Figure 6.71 that the control variables (physical and chemical parameters) and their relationships are significantly influencing the depletion of nitrate concentrations, through the denitrification process. However Figure 6.72 is also outlining how there is variation within the differing parameters, suggesting that different parameters are exerting

090116876 differing amounts of influence on the nitrate concentrations and denitrification process. Hydraulic conductivity is displaying the lowest p-value of 0.011, followed by grain size at 0.57, wt% organic carbon 0.84 and depth at 0.85. Based on the differences in the order of magnitude of the results, it appears that hydraulic conductivity is the parameter that exerts the most influence on the nitrate concentrations within the hyporheic zone.

Figure 6.40 Nitrate Distribution Contour Plot

Figure 6.40 is displaying the nitrate distribution throughout the hyporheic sediments, drawn using the spatial location of the cores along the river, core depths and nitrate concentrations for the corresponding depths. From Figure 6.40 it is apparent that the areas of greatest nitrate concentrations are occurring at depths 0.5-

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0.6 m below the river bed, with the greatest values occurring up stream at ~5m, although the concentrations are universally high throughout this depth. The geologies at this depth are predominantly sands, gravels and grits, which have fairly high hydraulic conductivity values. Table 6.73 outlines the location of nitrate depletion for each core, and the physical and geochemical properties that correspond to each core and depth of nitrate depletion. Figure 6.40 displays the spatial locations of each core, and through combining the results displayed in Table 6.73, it is apparent that, with the exception of core 2.7-, which is located downstream at 10.2 m, there are no cores indicating denitrification at this depth.

From Figure 6.40 it is possible to identify several spatial zones of low nitrate concentrations throughout the hyporheic sediments, which suggest areas of denitrification. These areas of lowest nitrate concentration appear to be occurring at depths of 0-0.4m and are most prominent within the mid reach of the river at 5-10m (Figure 6.41). Table 6.73 further confirms these findings, with all of the six cores highlighting areas of denitrification within this depth range. Sediment Cores 2.4, 2.5 and 2.7, which are spatially centred in the mid reach of the river (Figure 6.4) are all displaying areas of low nitrate concentration, and evidence of denitrification at depths 0-5cm. The geology at this depth, for all these cores has a clay component, and hydraulic conductivities range from 32,000 to 150,000.

Sediment cores 2.2, 2.4, 2.5 and 2.6 are further supporting the conclusion of enhanced denitrification at depths of 0-0.4m, by displaying evidence of denitrification at depths of 30-35cm. These cores are spread predominantly in the up and mid-stream areas of the river and have geologies comprised of pebbles and grit (Figure 6.41 and Table 6.73). Sediment cores 2.7 and 2.8 are located in the downstream area of the river, and are demonstrating evidence of denitrification (Figure 6.41) at the lower depths of 46-58 cm. The dominant geology for both cores at this depth is sand, with the nitrate concentrations being 1.0 and 42 ppm.

On the basis of this evidence it is apparent that when assessing denitrification it is essential to look at the position within the river, as well as the depth and geology and other physical and chemical properties such as organic carbon content and hydraulic conductivity. By assessing the spatial variability of Table 6.73 and Figure

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6.40 it has been revealed that there are three prominent depths and geologies where denitrification is occurring within this reach of the River Tern, Shropshire. At the upstream to midstream location this relates to depths of 0-5cm and geology of sands gravels and grits. At the midstream area of the river denitrification is principally occurring at 30-35cm depth with geology of pebbles and grit. Within the downstream reach of the river the denitrification is prevalent within the lower depths, 46-58cm, with a sand geology.

Throughout all the areas of denitrification highlighted in table 6.73, nitrate concentrations had been reduced to a level well within the 50mg/L drinking water guidelines highlighted by The European Union and World Health Organization (Wakida et al 2005). Within cores 2.2, 2.6 and 2.7 have been reduced to levels below 2 mg/L which are typical of the U.K. baseline concentrations in groundwater beneath natural grassland in the U.K (Wakida et al 2005). On this basis it would appear that at certain depths, geologies and physical and chemical parameters denitrification in the hyporheic zone is very effective at reducing nitrate concentrations.

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Depth Geology Nitrate Wt % Dissolved Average Hydraulic Core depleted Concentration Organic organic grain conductivity (cm) (ppm) carbon carbon % size (K) (m/D) (mm) 2.2- 15 Peaty 0.45 0.15 - 2.7 42,000 clay 25 Pebbly 0.50 1.0 - 0.12 30,000 gravel 35 Grit & 0.45 0.7 - 0.1 32,000 pebbles 2.4- 4 Black 0.30 2.0 0 - 150,000 clay 32 Grit 25.0 2.2 55.0 1.0 140,000 64 Brown 26.0 0.6 25.0 0.24 130,000 gravel & grit 2.5- 2 Black 12.0 1.0 70.0 - 105,000 peaty clay 32 Coarse 8.0 0.3 40.0 0.44 105,000 sand & pebbles 2.6- 25 Mud & 0.6 3.2 45.0 0.13 26,000 wood 35 Grit & 1.25 1.0 55.0 0.24 34,000 pebbles 2.7- 4 Peaty 0.9 0.2 210.0 200.0 32,000 black clay 46-50 Brown 1.0-0.75 0.9 0 0 30,000- sand 24,000 2.8- 20 Dark 28 0.3 1.3 0.44 100,000 brown sand & organics 52-58 Fine sand 42 0.1 1.4 0.48 80,000 & organics

Table 6.73 – Spatial variability of denitrification

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Figure 6.41 River Tern Study Site and Core Locations

7. Conclusions

A range of laboratory techniques, including the modified Walkley Black method and wet and dry sieving were carried out in order to assess the grain size analysis distribution and the organic carbon content of six sediment cores from the River Tern, Shropshire (Figure 6.41). Through producing particle grain size analysis graphs and applying empirical equations and calculations the hydraulic conductivity, average d50 grain size and weight percentage of organic carbon was determined for each core. The physical and chemical parameters, determined from the laboratory, were analysed and compared individually for each core. A range of correlations between each the physical and geochemical parameters were derived, which confirmed relationships highlighted by Smith et al 2009 linking decreasing grain size to lower hydraulic conductivity values, and small grain size and hydraulic conductivity values to high weight percentage organic carbon. Analysis of variance (ANOVA) statistical analysis was carried out in order to statistically assess the relationships between the cores physical properties and organic carbon and nitrate concentrations. This statistical analysis confirmed the importance of both the physical and organic carbon properties, and enforced the particular significance of the hydraulic conductivity property on nitrate concentrations.

Depth profiles for these physical and chemical properties, along with pore water chemistry data, which outlined a range of chemical parameters, were created,

090116876 and further data analysis was carried out in order to assess denitrification at each separate fraction of the sediment cores. Baskaran et al 2009 outlined the distinct chemical signatures of both groundwater and surface water, and how this can be used to indicate both the mixing of the water bodies within the hyporheic sediments and as a conservative tracer. Through comparing nitrate profiles with chloride profiles, areas of mixing and denitrification were outlined for each core. Denitrification was highlighted when concentrations of nitrate profile were significantly lower than the corresponding chloride profile. The areas of anticipated denitrification were further examined and contrasted with the dissolved organic carbon (DOC) and alkalinity profiles. This was carried out in response to findings be Rivett et al 2008, which found that the during denitrification there is a step wise removal of oxygen, which entails the removal of a carbon source (CH2O), as an electron donor, and the - - - formation of HCO3 , Equation 7.1: 5CH2O + 4NO3 → 2N2 + 4HCO3 +CO2 +3H2O

Areas of high DOC and increasing alkalinity, which is generated throughout the denitrification process, were used in order to verify the areas of anticipated denitrification. Further confirmation of denitrification was obtained from acetate profiles, as acetate is generated as a degradation product of organic matter (Rulik et al 2000).

Spatial assessment of the denitrification potential throughout the catchment, using nitrate distribution contour maps (Figure 6.40) and tabulated denitrification data (Table 6.73) was carried out. It has been found that there are three prominent depths and geologies where denitrification is occurring within this reach of the River Tern, Shropshire. From the spatial assessment it has become apparent that the position within the river affects the locations and depths where denitrification occurs. At the upstream to midstream location this relates to depths of 0-5cm and geology of sands gravels and grits. At the midstream area of the river denitrification is principally occurring at 30-35cm depth with geology of pebbles and grit. Within the downstream reach of the river the denitrification is prevalent within the lower depths, 46-58cm, with a sand geology.

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Through the spatial assessment of denitrification it has been possible to prove the success of natural attenuation within the hyporheic zone, as the nitrate concentrations within the highlighted areas of denitrification (Table 6.73) all have concentrations below the 50 mg/L drinking water limit, as highlighted by The European Union and World Health Organization (Wakida et al 2005). Within cores 2.2, 2.6 and 2.7 nitrate concentrations have been reduced to levels below 2 mg/L which are typical of the U.K. baseline concentrations in groundwater beneath natural grassland in the U.K (Wakida et al 2005).

These findings verify the success of denitrification, at certain depths and geologies with certain physical characteristics, to naturally attenuate and reduce concentrations of nitrate, within the hyporheic zone of the River Tern, Shropshire to regulatory limits. On this basis it should be possible to assess the hyporheic zone for that will naturally attenuate nitrate pollution, and to implement targeted remediation strategies to areas of the zone with high nitrate concentrations, that will not be naturally attenuated. Further work is required to quantify the success of these findings on a larger scale and within different geological and river settings. However as a result of these findings the natural attenuation of the hyporheic zone should be included in the environmental risk assessment, management and treatment of nitrate pollution for both groundwater and surface water.

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090116876 statistical analysis using national-scale nitrate data. Journal of Hydrology 369 p392- 402

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Chapter Five

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Chapter Six

Baskaran S., Ransley T, Brodie R.S., Baker P (2009) Investigating groundwater-river interactions using environmental tracers. Australian journal of earth sciences 56 p 13-19

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Chapter Seven

Baskaran S., Ransley T, Brodie R.S., Baker P (2009) Investigating groundwater-river interactions using environmental tracers. Australian journal of earth sciences 56 p 13-19

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Appendices list

Data provided in electronic format- see attached CD

1) Grain size Analysis Sieving Fraction Weights and Walkley Black wt% Organic Carbon 2) Grain Size Analysis Data 3) Sediment Particle Grain Size Analysis diagrams 4) Walkley Black wt% Organic Carbon spread sheet 5) Hydraulic Conductivity and Average Grain Size Data 6) The River Tern Site Two Chemical Data