**1. Introduction**

#### **1.1 Aims and objectives**

The aim of this chapter is to consider the hydrology of the hyporheic zone in the context of the hydrological cycle. The chapter starts, the chapter starts with a description of the definitions of, and reasons for studying, the hyporheic zone, then goes on to consider sampling and measurement techniques followed by a consideration of the assessment and applications of this data and understanding. The results of two case studies provide examples of research approaches and technique. The case studies show how integrating hydrological measurements with geochemistry assists in the elucidation of hyporheic zone functioning.

### **1.2 Definitions and significance**

The hyporheic zone is the term given to the subsurface interface between surface and groundwater bodies. It is most commonly considered in the context of streams (or rivers) interfacing with groundwater. Groundwater in this context is the water that fills the spaces between soil particles and fractured rock that comprises the saturated ground that extends beneath the water table and the overlying unsaturated zone. Cardenas [1] suggests that the hyporheic zone is synonymous with the transient storage zone of earlier literature, i.e. including bank storage within the riparian zone (broadly defined as the interface between land and a river or stream), e.g. Bencala and Walters [2]. As it is a zone of flux between ground- and

**6**

2012

*Hydrology - The Science of Water*

2004. pp. 3-12

**References**

2014

2000;**289**(5477):284-288

[1] Dooge JC. Background to modern hydrology. The basis of civilization– Water science. In: Proceeding of

UNESCO/IAHS Symposium; December 2003. Rome: IAHS Publication 286;

[2] Vörösmarty CJ, Green P, Salisbury J, Lammers RB. Global water resources: Vulnerability from climate change and population growth. Science.

[3] Shahid M, Gabriel HF, Nabi A, Javaid MS. Assessment of effect of land use change on hydrological response and sediment yield for catchment area of Simly Lake, Pakistan. In: Proceedings of 1st International Conference on Emerging Trends in Engineering, Management and Sciences (ICETEMS-2014); 29-30 December 2014; Islamabad, Pakistan.

[4] De Vantier BA, Feldman AD. Review of GIS applications in

1993;**119**(2):246-261

hydrologic modeling. Journal of Water Resources Planning and Management.

[5] Tripathi MP, Panda RK, Pradhan S, Sudhakar S. Runoff modelling of a small watershed using satellite data and GIS. Journal of the Indian Society of Remote Sensing. 2002;**30**(1-2):39

[6] Alagha JS, Said MAM, Mogheir Y. Artificial intelligence based modelling of hydrological processes. In: 4th International Engineering Conference– Towards Engineering of 21st Century.

[7] Singh VP. Hydrologic modeling: progress and future directions. Geoscience Letters. 2018;**5**(1):15

surface water, there are hydrological, ecological and hydrogeological responses that characterise the hyporheic zone. Reflecting the range of perspectives, there are differing definitions of the hyporheic zone. Hydrologically it is conceptualised as the proportion of flow that occurs in permeable streambed deposits upon which channel flow occurs. This can also be seen as the component of flow that cannot be measured using conventional flow monitoring techniques. The sediments in this zone have an important role in influencing the distribution of permeability in the hyporheic zone attributable to grain size distribution, source rock and architecture, as related to topography, river dynamics and climate [3] and modified by biological and chemical processes. Orghidan [4] recognised the ecological significance of the hyporheic zone with the introduction of the term "hyporheic corridor concept". One difficulty for the ecologist is defining the thickness of the hyporheic zone. For example, they can delineate it by the occurrence of hyporheobiont life stages or by the extent of riverine animals [5]. The definition favoured by Brunke and Gonser [6] is that the hyporheic zone is distinguished from groundwater and stream water by demonstrating characteristics of both, with different gradients to each. Clearly the hyporheic zone is dynamic as a consequence of changing hydraulic conditions and seasonality, and this is recognised by ecologists in the term "dynamic ecotone" [5]. The hydrogeologist's view of the hyporheic zone is as part of the groundwater system, because it comprises subsurface water within the saturated zone.

As a concept, the hyporheic zone is important in addressing integrated catchment modelling and management. In Europe the Water Framework Directive (2000) provides the context for an increased research interest in the hyporheic zone [7], because it promotes the management of groundwater bodies and surface water bodies in an integrated way, requiring that (hydraulic) pathways between the two are understood. Analysis of the connectivity of surface and groundwater in conjunction with other protected areas such as designated wetland is a specific requirement of the River Basin Catchment Plan [5]. The ability to assess mass flux across the groundwater-surface water interface, predict attenuation processes in this zone, link hyporheic and benthic chemical conditions and ecological health and develop reliable and transferable conceptual models of flow and attenuation defines the requirements of conceptual understanding.

Soil properties impose a strong influence on the dynamics of the hyporheic zone in terms of transient storage and retention. The transient storage capacity of the hyporheic zone can be important in accounting for apparent losses or gains in water balance calculations, e.g. Lapworth et al. [8], which may inform resource evaluation studies. Improved understanding of the spatial components of the hyporheic zone offers significant potential in terms of understanding the process of flood migration along the length of the stream in the context of catchment scale flood modelling and management. Hydrologically, the hyporheic zone is an important component of some poorly understood karst systems (e.g. turloughs or estavelles), wetlands and lake environments. More recently, in the context of urban environments, the concept of the hyporheic zone has been extended to include the impacts of leaking pipes contributing water of different chemistry to the zone of transient storage or service ducts providing preferential pathways, perched water tables and altered flow and groundwater conditions, e.g. Bricker et al. [9].

Within the hyporheic zone, the geology, hydrology, hydrochemistry and biology exhibit feedbacks and dependencies. Consequently, hydrological understanding is important to the aspects of hyporheic zone research that embrace the ecological and chemical benefits (ecosystem services) of the zone. Broadly, hyporheic faunal communities vary with the environmental conditions, including hydrology, climate, geology, sedimentary architecture, land use and chemical conditions (natural and anthropogenic). The influence of hydrological flux in the hyporheic zone is

**9**

*The Hyporheic Zone*

*DOI: http://dx.doi.org/10.5772/intechopen.85218*

legacy of abandoned mines in the UK [13].

**the hyporheic zone**

particularly important in defining nutrient (carbon) distribution and its upward and downward movement and consequently the distribution of the ecotones that impose structure on the hyporheic communities (hyporheos, including benthic, epigean and phreatic species) and their distribution in the sediment [10, 11]. For example, during periods of environmental stress, typically marked by drought or flood, the hyporheic zone provides a place of refuge for some stream-dwelling species. Other species permanently occupy the hyporheic zone niche. In some streams, the hyporheic zone species extend in excess of 100 m beneath the streambed, e.g. [12]; elsewhere they occur at relatively shallow depths. There have been a number of associated shifts in understanding relating to the discovery of hyporheic invertebrates and the concept of the hyporheos as an indicator of ecological health. Furthermore, the broad range of biological species, including microbial fauna, has an important role in contaminant attenuation, and this defines the zone as an ecosystem service with a potential to mitigate contaminants. Whilst there is an extensive literature associated with this topic, it lies beyond the remit of this chapter. In conjunction with a range of biological processes, the geochemistry of the hyporheic zone provides a valuable natural system for the remediation of a range of contaminants. For example, this zone is particularly important in hosting denitrification processes. There are a number of factors that contribute to the geochemistry of the hyporheic zone, including bedrock geology, superficial geology, water

residence time, oxygen concentrations, the degree of mixing of ground- and surface water, pH conditions and breakdown of contaminants, including plastic and the organic content. Additionally, bedrock textures may be important for the growth of specific precipitates. Its attenuation capacity varies with its thickness and permeability. This potential has been recognised in the context of the remediation of the

**2. Research methods: hydrological measurements and sampling in** 

As with any other system, sampling and monitoring of the hyporheic zone requires a strategy and plan [14], the formulation of which requires a clear understanding of the reason for monitoring and what it aims to achieve. This is particularly relevant in the context of the hyporheic zone where, if it is required, it may be possible, with careful planning, to optimise sampling to derive hydrological, ecological and hydrogeochemical data in conjunction with each other. The second step in the development of a sampling strategy is the completion of a desk-based study of the area of interest. Ideally, this should be undertaken at the catchment scale to understand the broader hydrological context with subsequent more detailed studies at the sub-catchment, reach and project scale. As well as considering the spatial scale of interest, decisions will have to be made regarding the temporal aspects of data collection: how frequently will data be collected and how long will the monitoring continue in order to characterise the flow regime, chemical and biological context? Subsequent decisions will relate to how to undertake the monitoring or sampling and whether it should comprise point methods, averaging methods or distributed methods to provide insight into spatial or temporal variation [14]. Key factors influencing the frequency, duration and type of monitoring include the funding that is available, site access and health and safety considerations as well as the scientific factors, such as the nature of geology proposed for sampling. Streambeds with rock or coarse sediments in their base are inherently more challenging than finer sediment-bedded streams. A selection of potential sampling methods is detailed in **Table 1**. In selecting appropriate sampling and monitoring

#### *The Hyporheic Zone DOI: http://dx.doi.org/10.5772/intechopen.85218*

*Hydrology - The Science of Water*

surface water, there are hydrological, ecological and hydrogeological responses that characterise the hyporheic zone. Reflecting the range of perspectives, there are differing definitions of the hyporheic zone. Hydrologically it is conceptualised as the proportion of flow that occurs in permeable streambed deposits upon which channel flow occurs. This can also be seen as the component of flow that cannot be measured using conventional flow monitoring techniques. The sediments in this zone have an important role in influencing the distribution of permeability in the hyporheic zone attributable to grain size distribution, source rock and architecture, as related to topography, river dynamics and climate [3] and modified by biological and chemical processes. Orghidan [4] recognised the ecological significance of the hyporheic zone with the introduction of the term "hyporheic corridor concept". One difficulty for the ecologist is defining the thickness of the hyporheic zone. For example, they can delineate it by the occurrence of hyporheobiont life stages or by the extent of riverine animals [5]. The definition favoured by Brunke and Gonser [6] is that the hyporheic zone is distinguished from groundwater and stream water by demonstrating characteristics of both, with different gradients to each. Clearly the hyporheic zone is dynamic as a consequence of changing hydraulic conditions and seasonality, and this is recognised by ecologists in the term "dynamic ecotone" [5]. The hydrogeologist's view of the hyporheic zone is as part of the groundwater

system, because it comprises subsurface water within the saturated zone.

the requirements of conceptual understanding.

flow and groundwater conditions, e.g. Bricker et al. [9].

As a concept, the hyporheic zone is important in addressing integrated catchment modelling and management. In Europe the Water Framework Directive (2000) provides the context for an increased research interest in the hyporheic zone [7], because it promotes the management of groundwater bodies and surface water bodies in an integrated way, requiring that (hydraulic) pathways between the two are understood. Analysis of the connectivity of surface and groundwater in conjunction with other protected areas such as designated wetland is a specific requirement of the River Basin Catchment Plan [5]. The ability to assess mass flux across the groundwater-surface water interface, predict attenuation processes in this zone, link hyporheic and benthic chemical conditions and ecological health and develop reliable and transferable conceptual models of flow and attenuation defines

Soil properties impose a strong influence on the dynamics of the hyporheic zone in terms of transient storage and retention. The transient storage capacity of the hyporheic zone can be important in accounting for apparent losses or gains in water balance calculations, e.g. Lapworth et al. [8], which may inform resource evaluation studies. Improved understanding of the spatial components of the hyporheic zone offers significant potential in terms of understanding the process of flood migration along the length of the stream in the context of catchment scale flood modelling and management. Hydrologically, the hyporheic zone is an important component of some poorly understood karst systems (e.g. turloughs or estavelles), wetlands and lake environments. More recently, in the context of urban environments, the concept of the hyporheic zone has been extended to include the impacts of leaking pipes contributing water of different chemistry to the zone of transient storage or service ducts providing preferential pathways, perched water tables and altered

Within the hyporheic zone, the geology, hydrology, hydrochemistry and biology exhibit feedbacks and dependencies. Consequently, hydrological understanding is important to the aspects of hyporheic zone research that embrace the ecological and chemical benefits (ecosystem services) of the zone. Broadly, hyporheic faunal communities vary with the environmental conditions, including hydrology, climate, geology, sedimentary architecture, land use and chemical conditions (natural and anthropogenic). The influence of hydrological flux in the hyporheic zone is

**8**

particularly important in defining nutrient (carbon) distribution and its upward and downward movement and consequently the distribution of the ecotones that impose structure on the hyporheic communities (hyporheos, including benthic, epigean and phreatic species) and their distribution in the sediment [10, 11]. For example, during periods of environmental stress, typically marked by drought or flood, the hyporheic zone provides a place of refuge for some stream-dwelling species. Other species permanently occupy the hyporheic zone niche. In some streams, the hyporheic zone species extend in excess of 100 m beneath the streambed, e.g. [12]; elsewhere they occur at relatively shallow depths. There have been a number of associated shifts in understanding relating to the discovery of hyporheic invertebrates and the concept of the hyporheos as an indicator of ecological health. Furthermore, the broad range of biological species, including microbial fauna, has an important role in contaminant attenuation, and this defines the zone as an ecosystem service with a potential to mitigate contaminants. Whilst there is an extensive literature associated with this topic, it lies beyond the remit of this chapter.

In conjunction with a range of biological processes, the geochemistry of the hyporheic zone provides a valuable natural system for the remediation of a range of contaminants. For example, this zone is particularly important in hosting denitrification processes. There are a number of factors that contribute to the geochemistry of the hyporheic zone, including bedrock geology, superficial geology, water residence time, oxygen concentrations, the degree of mixing of ground- and surface water, pH conditions and breakdown of contaminants, including plastic and the organic content. Additionally, bedrock textures may be important for the growth of specific precipitates. Its attenuation capacity varies with its thickness and permeability. This potential has been recognised in the context of the remediation of the legacy of abandoned mines in the UK [13].

## **2. Research methods: hydrological measurements and sampling in the hyporheic zone**

As with any other system, sampling and monitoring of the hyporheic zone requires a strategy and plan [14], the formulation of which requires a clear understanding of the reason for monitoring and what it aims to achieve. This is particularly relevant in the context of the hyporheic zone where, if it is required, it may be possible, with careful planning, to optimise sampling to derive hydrological, ecological and hydrogeochemical data in conjunction with each other. The second step in the development of a sampling strategy is the completion of a desk-based study of the area of interest. Ideally, this should be undertaken at the catchment scale to understand the broader hydrological context with subsequent more detailed studies at the sub-catchment, reach and project scale. As well as considering the spatial scale of interest, decisions will have to be made regarding the temporal aspects of data collection: how frequently will data be collected and how long will the monitoring continue in order to characterise the flow regime, chemical and biological context? Subsequent decisions will relate to how to undertake the monitoring or sampling and whether it should comprise point methods, averaging methods or distributed methods to provide insight into spatial or temporal variation [14]. Key factors influencing the frequency, duration and type of monitoring include the funding that is available, site access and health and safety considerations as well as the scientific factors, such as the nature of geology proposed for sampling. Streambeds with rock or coarse sediments in their base are inherently more challenging than finer sediment-bedded streams. A selection of potential sampling methods is detailed in **Table 1**. In selecting appropriate sampling and monitoring


**11**

*The Hyporheic Zone*

**Table 1.**

*DOI: http://dx.doi.org/10.5772/intechopen.85218*

Geophysical testing [21] Field-based electrical resistivity

water table

can be used in monitoring contaminant plumes or tracers such as salt, as well as monitoring changes in moisture content and the position of the

rise to artificially high concentrations of zinc.

*A range of potential sampling/monitoring methods.*

techniques, construction materials and their potential to impact results should be considered; for example, whilst robust, the use of galvanised steel products can give

**Method Description Hydrological Ecological Hydrogeochemical**

Y Y

Catchment scale analysis of the topography, land use, geology, hydrogeology and water features, as well as any hydrological data, will provide an evidence base for assessing potential zones of groundwater-surface water interaction and possible access points to the stream. Typically, hydrological studies focus on the flux and determination of the gradients between the surface and groundwater; therefore, they will largely focus on head, seepage, changes in stream discharge, permeability and hydraulic conductivity. This will require measurements in the subsurface, in the hyporheic zone and in the stream. Wherever possible measurement techniques should be selected to provide complimentary data and optimise confidence in the results. Traditionally, flow regimes within a single river system were measured using flow-gauging techniques, typically using impeller or electromagnetic current meters [22]. However, whilst these techniques are valuable in identifying zones of potential loss or gain in discharge for further investigation, they preclude measurement of flow in the hyporheic zone; therefore, different or additional techniques are required for monitoring the hyporheic zone. Three-dimensional temperature has been found to be a valuable parameter for characterising the hyporheic zone [23, 24]. This is because the longer the groundwater is in contact with bedrock, the more it will equilibrate with the bedrock temperature, whereas surface water will tend to equilibrate with atmospheric conditions. The rates of temperature exchange are affected by the depth of the river, the thickness and permeability of the river sediment, the characteristics of the upper part of the bedrock and the specific heat capacities of the bedrock and sediments. These effects vary both seasonally and in response to hydrological events such as flooding or, for example, releases of dam water [24, 25]. Tracing experiments provide the advection and dispersion characteristics of the transient storage zone that are required for modelling [26]. Conservative tracers, such as sodium chloride, sodium bromide and rhodamine WT are well-established techniques in the analysis of hydrological characteristics of streams [27, 28]. Others have included natural tracers such as radon [29]. Additionally, there are a number of emerging, anthropogenically introduced conservative tracers; for example, Möller et al. [30] used gadolinium. Other contaminants can be utilised as tracers to demonstrate the extent of the hyporheic zone or the extent of change in the hyporheic zone, e.g. Ciszewski and Aleksander-Kwaterczak [31] used the concentrations of zinc and cadmium in the sediment and waters of Baila Przemsza River in southern Poland to define the extent of mining-induced alteration of the hyporheic zone. However, consideration will need to be given to the geochemistry of the contaminants if they are non-conservative and are being used to provide information on permeability and flow. Streambed geology and sediment characterisation are required for the effective design of field experiments, hydrological modelling and effective river basin


**Table 1.**

*Hydrology - The Science of Water*

Mini drive-point piezometers [16]

Natural tracers including temperature and electrolytic conductivity

Physicochemical parameters as a proxy for hydrochemical

Diffusive equilibrium in thin films (DET), Byrne et al. [17], or diffuse gradient in thin

zoning

films [18]

Seepage meter [15] Measures exchange of water

Wells and sampling pits Can be used for sediment

interface

screen

sampler

DET gel

Net sampling Can be lowered into wells or

Kick sampling [19] Standard method in qualitative

Pump sampling [20] Another method for qualitative

Kubiena tin or similar Used for undisturbed sampling

properties

For geotechnical characterisation of the alluvium, might include density, grading, porosity, field capacity, hydraulic conductivity, moisture content and electrical resistivity

Automatic sampling Automatic samplers can be

and redox probes

In situ passive sampling of porewater by diffusive equilibrium in thin films. Stainless steel cover containing

natural water bodies

studies of macroinvertebrates

studies of macroinvertebrates

programmed to take periodic samples, e.g. to sample through a weather event such as a storm

of sediment for resin moisture replacement and optical examination of hydrogeological

(see case studies)

across the sediment–water

characterisation as well as constructing access for water sampling. Can also be used for hydraulic testing (e.g. falling or rising head tests; slug tests). The sampling zone of a well is dictated by the positioning of the sample points, e.g. well

Used to enhance understanding of head and hydrochemistry

Measurement of temperature as a tracer of flow paths. Care needed to ensure that readings are not affected by sampling materials, if measured in a piezometer or multilevel

pH, temperature, electrolytic conductivity, dissolved oxygen

**Method Description Hydrological Ecological Hydrogeochemical**

Y

Y

Y

Y

Y Y

Y Y

Y

Y

Y

Y Y

Y

Y

**10**

Geotechnical soil property tests

*A range of potential sampling/monitoring methods.*

techniques, construction materials and their potential to impact results should be considered; for example, whilst robust, the use of galvanised steel products can give rise to artificially high concentrations of zinc.

Catchment scale analysis of the topography, land use, geology, hydrogeology and water features, as well as any hydrological data, will provide an evidence base for assessing potential zones of groundwater-surface water interaction and possible access points to the stream. Typically, hydrological studies focus on the flux and determination of the gradients between the surface and groundwater; therefore, they will largely focus on head, seepage, changes in stream discharge, permeability and hydraulic conductivity. This will require measurements in the subsurface, in the hyporheic zone and in the stream. Wherever possible measurement techniques should be selected to provide complimentary data and optimise confidence in the results.

Traditionally, flow regimes within a single river system were measured using flow-gauging techniques, typically using impeller or electromagnetic current meters [22]. However, whilst these techniques are valuable in identifying zones of potential loss or gain in discharge for further investigation, they preclude measurement of flow in the hyporheic zone; therefore, different or additional techniques are required for monitoring the hyporheic zone. Three-dimensional temperature has been found to be a valuable parameter for characterising the hyporheic zone [23, 24]. This is because the longer the groundwater is in contact with bedrock, the more it will equilibrate with the bedrock temperature, whereas surface water will tend to equilibrate with atmospheric conditions. The rates of temperature exchange are affected by the depth of the river, the thickness and permeability of the river sediment, the characteristics of the upper part of the bedrock and the specific heat capacities of the bedrock and sediments. These effects vary both seasonally and in response to hydrological events such as flooding or, for example, releases of dam water [24, 25].

Tracing experiments provide the advection and dispersion characteristics of the transient storage zone that are required for modelling [26]. Conservative tracers, such as sodium chloride, sodium bromide and rhodamine WT are well-established techniques in the analysis of hydrological characteristics of streams [27, 28]. Others have included natural tracers such as radon [29]. Additionally, there are a number of emerging, anthropogenically introduced conservative tracers; for example, Möller et al. [30] used gadolinium. Other contaminants can be utilised as tracers to demonstrate the extent of the hyporheic zone or the extent of change in the hyporheic zone, e.g. Ciszewski and Aleksander-Kwaterczak [31] used the concentrations of zinc and cadmium in the sediment and waters of Baila Przemsza River in southern Poland to define the extent of mining-induced alteration of the hyporheic zone. However, consideration will need to be given to the geochemistry of the contaminants if they are non-conservative and are being used to provide information on permeability and flow.

Streambed geology and sediment characterisation are required for the effective design of field experiments, hydrological modelling and effective river basin management. In the first instance, this information can come from reconnaissance visits and remote sensing data. However, more in-depth understanding of the processes and sediment sorting patterns is important for hydraulic modelling, because the distribution of sediments influences the boundary roughness and geochemistry. This may require specialist sediment sampling techniques at the site scale or techniques such as airborne LiDAR, in conjunction with geomorphological modelling [32] at the reach or catchment scale. Remote sensing technologies offer the advantage of providing remote access to areas of restricted ground access which potentially offers a new opportunity to start to undertake higher-resolution stream sediment mapping, for example, as undertaken by Miklin and Galia [33]. Geomorphological mapping of this type offers the potential to predict the lateral extent of the hyporheic zone in areas where active channel migration occurs.
