Impacts of river bed gas on the hydraulic and thermal dynamics of the hyporheic zone
Research Highlights
► Trapped gas in river beds may significantly alter thermal and hydraulic properties. ► River bed gas observed up to 14% by volume in the River Tame, Birmingham, UK. ► Gas increases modelled GW–SW exchange flux in the channel centre. ► Diurnal temperature changes propagate deeper due to gas.
Introduction
The hyporheic zone (HZ), herein defined in the sense of Krause et al. [32] as the zone of groundwater–surface water mixing, has become an important and quickly evolving area of interdisciplinary research as its ecological significance and role in controlling the fate and transport of contaminants is being increasingly recognized [5], [50]. The HZ is often characterized by a range of redox conditions and associated bacterial activity with anaerobic conditions potentially induced by the presence of labile organic matter, e.g. decaying vegetation and microbiota. Reducing conditions may support denitrification [40], [46] and even methanogenesis [21], [28], [44] and may generate biogenic gases in the HZ. The importance of biogenic gas formation due to denitrification and methanogenesis in groundwater and its influence on flow and transport has been recognized in other hydrogeological settings, for example the contamination of groundwater by biodegradable hydrocarbon fuels [2] or implementation of bioremediation technologies [25]. However, we were unable to find any studies to date on the potential feedbacks of biogenic gas production on the hydraulic and thermal dynamics of the HZ.
Multi-phase flow within subsurface porous media has been examined in various studies on, for example, unsaturated zone flow, transport of immiscible non-aqueous phase liquids, air entrapment and migration in the shallow groundwater — capillary fringe [22] and air-based remediation technology, for example air-sparging [12]. Of greater relevance here though is work conducted on the formation and influence of biogenic gas bubble formation associated with contaminant biodegradation in groundwater systems, albeit not the hyporheic zone [2], [25]. In the context of the hyporheic zone, the volume of gas present within the pore space will be determined by a complex interplay of factors including the rate of gas production and potential sites for bubble nucleation [35], rates of dissolution, and the degree of advective transport of the gas phase. Unless present in large quantities, gas is likely to be predominantly immobile within the hyporheic zone held by capillary forces. This is because considerable pressure is needed to force a bubble through a pore space and to overcome the resistance to flow offered by detached gas bubbles in capillary conduits [19], [42]. Thus, although some movement of bubbles may occur if capillary forces are overcome by viscous and/or buoyancy forces, at high rates of water flow or at times of high gas production, the gas will not flow as a separate phase until the gas content is higher than the trapped gas saturation threshold. This threshold depends on, amongst other factors, the viscosity ratio, wettability, and permeability as well as the geometry of the pore space, with more poorly sorted sediments commonly having higher residual saturations [18]. The maximum residual saturation of a trapped non-wetting phase can be large, for example Fry et al. [18] summarise previous literature values indicating that trapped gas may fill over 40% of the pore space in some cases. They then demonstrate experimentally that the mechanism of gas emplacement is a significant factor in determining residual saturation. Their results indicate that exsolution due to supersaturation may lead to greater values of trapped gas than direct emplacement of gas.
The literature describing gas accumulation within soft sediments is also relevant and shows that growing bubbles, rather than simply filling existing pore space, may also deform the sediments. A useful summary is given by Boudreau [3] indicating that, although the mechanics of uncemented soft sediments during bubble growth are not widely understood, bubbles within muddy cohesive sediments are likely to grow either by fracturing or by re-opening existing fractures. Within soft sandy sediments bubbles tend to be spherical suggesting that the sand acts fluidly or plastically in response to growth stresses, and that bubble rise in such sediments as a result of buoyancy forces can be accomplished by sediment displacement [3].
Although large accumulations of gas are not found in all river beds, the volume of gas present may be significant in some cases and is likely to be highly variable spatially and temporally. This paper begins by developing the theory necessary for understanding the effects of such accumulated gas on the hydraulic and thermal dynamics of a river bed. It then introduces a study site in which accumulations of river bed gas have been observed. The final section tests three hypotheses through data analysis and modelling. The three hypotheses are as follows: Hypothesis 1 Accumulations of biogenic gas may increase the specific storage and reduce the hydraulic conductivity of the river bed significantly enough to lead to more prolonged flow reversals during storm events, and hence may enhance HZ mixing. Hypothesis 2 The effective porosity of the river bed may be reduced such that the unreactive transport of solutes through the HZ may be significantly modified. Hypothesis 3 The thermal properties of the river bed may be altered to such an extent by the presence of gas that the propagation of daily and annual temperature cycles is significantly enhanced.
Section snippets
Theoretical development
Theoretical aspects concerning the effect of gas on hydraulic and thermal properties of porous media are now outlined based on existing literature, and extended in relation to the dynamic setting of the hyporheic zone. We examine the effects on specific storage, relative hydraulic conductivity, effective porosity and thermal diffusivity.
Background
The study site is located in an industrial area of north Birmingham, within a 7 km long reach of the River Tame which drains the unconfined Triassic sandstone aquifer underlying the city (Fig. 3). This reach has been the subject of both assessment of urban contaminated baseflow discharges to the Tame at the city scale [13], [15] and modelling of groundwater–surface-water flow interactions at various scales [14]. The study site is set within the most urbanised basin in the UK [33] towards the
Model setup
A series of 2-D transect models have been developed to test how changes in Ss and K due to gas accumulation in the river bed may affect the hydrodynamics of storm events. Previous work has shown the importance of including the unsaturated zone within such models in order to adequately reproduce observed head responses in river bank piezometers [14]. Hence, a three dimensional variably saturated flow and transport code, FAT3D-UNSAT [37] which also has the capacity for transient particle
Concluding discussion
This paper has outlined some of the physical mechanisms whereby the accumulation of gas within a river bed may affect its hydraulic and thermal regime. Using observational data from a short reach of the urban River Tame, UK, and a range of numerical and analytical models we have tested a series of hypotheses in order to quantify some of these effects for the site.
Gas present in quantities up to around 14% by volume has been observed at the study site, and demonstrated to be present to at least
Acknowledgements
This work was funded by the European Union through the UNESCO SWITCH sustainable urban water project with co-funding from the Environment Agency of England and Wales. The authors are grateful to Simon Shepherd, Martin Hendrie, Dominic Kisz and Liam Mackay for supporting various aspects of data collection in the field, Liz Hamilton and Gilles Pinay for providing the N2O analyses, and the landowners for allowing access to the fieldsite. The manuscript benefitted from comments by several anonymous
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- 1
Present address: UMR 8148 IDES, Bât 504, Faculté des sciences, Université Paris Sud 11, 91405 ORSAY CEDEX, France.
- 2
Present address: Lancaster Environment Centre, Lancaster University, Bailrigg, Lancaster, LA1 4YQ, UK.