**3. Analysis**

### **3.1 Hydrological characterisation**

Fundamental to researching the hydrology of the hyporheic zone is the development of the conceptual understanding of the host system. A conceptual model is necessary to facilitate targeting of the dominant hyporheic zone processes at the catchment or reach-scale. This requires understanding of the underlying catchment characteristics including land use, the climate and meteorology, geology and geomorphology. Whilst stream hydrology can be considered in terms of catchment recharge, throughflow and discharge (or infiltration, surface run-off, interflow and base-flow), flooding, seasonality and environmental interaction (including anthropogenic factors), extension to the hyporheic zone requires greater consideration of the bed sediments and the hydrological exchange processes therein. For example, in the UK, the Centre for Ecology and Hydrology (CEH) Integrated Hydrological Digital Terrain Model derives outputs for flood modelling based on the range of catchment parameters presented in **Table 2**. Using the CEH approach, it is implicit that the hydrology of soil type (HOST, [34]) parameters embrace the hyporheic zone and that the catchment parameters reflect both the stream hydrology and the


#### **Table 2.**

*The primary catchment parameters used for flood estimation by the Centre for Ecology and Hydrology.*

**13**

*The Hyporheic Zone*

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

stream bedforms at both the catchment and reach scale. However, this model does not allow for geomorphological nuances of the stream sediments and underlying geology that are fundamental to understanding hyporheic exchange. Stream bedforms and their sediments are commonly a product of current, historic and geological climatic conditions. Typically in the UK, drainage patterns and stream sediments reflect the legacy of climatic change throughout the Quaternary, and as a consequence the average bed load grain size may exceed that of the current hydro-

In the UK, sediments deposited in upland catchments are dominated by braided streams that are graded from cobbles in the higher reaches, to gravel and sand in the lower reaches [35, 36]. In lower alluvial floodplains, meandering rivers commonly occupy larger floodplains within which channel migration occurs and the sediments are dominated by sand silt and clay-grade particles. Within each of these environments, stream geomorphology influences the distribution of sediments, e.g. formation of point bars on the inner bank of meander bends. As well as the large-scale lithological variations observed in point bar deposits [37], there are smaller-scale variations, whereby mud-prone sediment is interbedded with sand-prone sediment as inclined heterolithic strata [38]. These complex internal heterolithic variations are particularly important when considering circumstances where chute channels may traverse the point bar deposit because the internal architecture of the hyporheic zone could be more complex and its lateral extent altered. Fine-grained counter point bars may occur on the outer bank of the meander bend where the bend becomes convex in shape [39]. These deposits baffle throughflow due to their fine-grained nature and contrast with eddy-accretion deposits, in turn characterised by thick, sand-prone accumulations [40, 41], which would more easily encourage throughflow. These local variations in permeability affect streambed biogeochemistry and the potential

The hydraulic connectivity of streams and groundwater can also be considered at the catchment scale and conceptualised in terms of losing and gaining stretches of the stream, where a gaining reach is one that is supplemented by groundwater and a losing reach is one where a proportion of the stream water recharges the underlying aquifer [42]. Perennial streams losing stretches of perennial streams are in constant continuity with groundwater, whereas continuity of gaining stretches may vary with groundwater levels, unless continuity is maintained by switching from gaining to losing conditions. Gaining and losing reaches can be defined from the relationship between stream water level and the potentiometric surface of the groundwater or by changes in the stream discharge, unless subject to artificial modification. The zones of recharge and discharge from the streambed are likely to migrate in accordance with the hydraulic conditions, e.g. due to flooding or seasonality. In many streams the area of discharge or recharge is not evenly distributed across the streambed, which may be due to head differences, differences in hydraulic conductivity or the structure and composition of the streambed sediments. Hyporheic zone flow paths can be both diffuse and focused. Diffuse flow will reflect the matrix permeability, whereas focused flow may comprise bypass flow that utilises macropores and pipes in the unsaturated zone [43, 44]. The bedrock geology and sediment source zones are reflected in the properties of the hyporheic zone sediments, whilst the form or geomorphology of the sediments reflects the hydraulic setting and its influence on sediment distribution [45]. Flow rates fluctuate around a meander bend, whereby stronger currents are observed at the outer bank and weaker flow is observed on the inner bank [46]. In meandering fluvial systems where alluvium is accreted onto point bar deposits, the finer-grade alluvium is deposited on the inner bank of the downstream limb of the bend, whereas the coarser-grade alluvium

logical conditions of the river or stream, i.e. stream under-fit.

lateral and vertical extents of the hyporheic zone.

is deposited at, and upstream of, the meander apex [37, 47].

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

*Hydrology - The Science of Water*

**3. Analysis**

**3.1 Hydrological characterisation**

**Catchment parameter Description**

AREA Catchment drainage area

FPEXT Extent of the floodplain LDP Longest drainage path PROPWET A catchment wetness index

SAAR Average rainfall over a standard period

SMD Mean soil moisture deficit for the standard period SPRHOST Standard percentage run-off of each soil type URBEXT Extent of urban and suburban land cover

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.

Fundamental to researching the hydrology of the hyporheic zone is the development of the conceptual understanding of the host system. A conceptual model is necessary to facilitate targeting of the dominant hyporheic zone processes at the catchment or reach-scale. This requires understanding of the underlying catchment characteristics including land use, the climate and meteorology, geology and geomorphology. Whilst stream hydrology can be considered in terms of catchment recharge, throughflow and discharge (or infiltration, surface run-off, interflow and base-flow), flooding, seasonality and environmental interaction (including anthropogenic factors), extension to the hyporheic zone requires greater consideration of the bed sediments and the hydrological exchange processes therein. For example, in the UK, the Centre for Ecology and Hydrology (CEH) Integrated Hydrological Digital Terrain Model derives outputs for flood modelling based on the range of catchment parameters presented in **Table 2**. Using the CEH approach, it is implicit that the hydrology of soil type (HOST, [34]) parameters embrace the hyporheic zone and that the catchment parameters reflect both the stream hydrology and the

BFI HOST A base-flow index derived from the hydrological properties of soils as

DPSBAR Mean drainage path slope as an indicator of catchment steepness

*The primary catchment parameters used for flood estimation by the Centre for Ecology and Hydrology.*

FARL Potential for flood attenuation by reservoirs and lakes

CENTROID; DPLBAR Modelling descriptors: centroid of the catchment; distance between model nodes

detailed in the HOST dataset, which is based on soil types at a 1-km grid [34]

**12**

**Table 2.**

stream bedforms at both the catchment and reach scale. However, this model does not allow for geomorphological nuances of the stream sediments and underlying geology that are fundamental to understanding hyporheic exchange. Stream bedforms and their sediments are commonly a product of current, historic and geological climatic conditions. Typically in the UK, drainage patterns and stream sediments reflect the legacy of climatic change throughout the Quaternary, and as a consequence the average bed load grain size may exceed that of the current hydrological conditions of the river or stream, i.e. stream under-fit.

In the UK, sediments deposited in upland catchments are dominated by braided streams that are graded from cobbles in the higher reaches, to gravel and sand in the lower reaches [35, 36]. In lower alluvial floodplains, meandering rivers commonly occupy larger floodplains within which channel migration occurs and the sediments are dominated by sand silt and clay-grade particles. Within each of these environments, stream geomorphology influences the distribution of sediments, e.g. formation of point bars on the inner bank of meander bends. As well as the large-scale lithological variations observed in point bar deposits [37], there are smaller-scale variations, whereby mud-prone sediment is interbedded with sand-prone sediment as inclined heterolithic strata [38]. These complex internal heterolithic variations are particularly important when considering circumstances where chute channels may traverse the point bar deposit because the internal architecture of the hyporheic zone could be more complex and its lateral extent altered. Fine-grained counter point bars may occur on the outer bank of the meander bend where the bend becomes convex in shape [39]. These deposits baffle throughflow due to their fine-grained nature and contrast with eddy-accretion deposits, in turn characterised by thick, sand-prone accumulations [40, 41], which would more easily encourage throughflow. These local variations in permeability affect streambed biogeochemistry and the potential lateral and vertical extents of the hyporheic zone.

The hydraulic connectivity of streams and groundwater can also be considered at the catchment scale and conceptualised in terms of losing and gaining stretches of the stream, where a gaining reach is one that is supplemented by groundwater and a losing reach is one where a proportion of the stream water recharges the underlying aquifer [42]. Perennial streams losing stretches of perennial streams are in constant continuity with groundwater, whereas continuity of gaining stretches may vary with groundwater levels, unless continuity is maintained by switching from gaining to losing conditions. Gaining and losing reaches can be defined from the relationship between stream water level and the potentiometric surface of the groundwater or by changes in the stream discharge, unless subject to artificial modification. The zones of recharge and discharge from the streambed are likely to migrate in accordance with the hydraulic conditions, e.g. due to flooding or seasonality. In many streams the area of discharge or recharge is not evenly distributed across the streambed, which may be due to head differences, differences in hydraulic conductivity or the structure and composition of the streambed sediments.

Hyporheic zone flow paths can be both diffuse and focused. Diffuse flow will reflect the matrix permeability, whereas focused flow may comprise bypass flow that utilises macropores and pipes in the unsaturated zone [43, 44]. The bedrock geology and sediment source zones are reflected in the properties of the hyporheic zone sediments, whilst the form or geomorphology of the sediments reflects the hydraulic setting and its influence on sediment distribution [45]. Flow rates fluctuate around a meander bend, whereby stronger currents are observed at the outer bank and weaker flow is observed on the inner bank [46]. In meandering fluvial systems where alluvium is accreted onto point bar deposits, the finer-grade alluvium is deposited on the inner bank of the downstream limb of the bend, whereas the coarser-grade alluvium is deposited at, and upstream of, the meander apex [37, 47].

The variation in grain size distribution within the sediments gives rise to variability in flow of water through the deposits; more mud-prone sediment has lower porosity and permeability than more sand-prone sediment. The fluctuating flow rates around a meander bend lead to reach-scale variations in sediment-bedded stream hydrological processes commonly leading to the formation of pool and riffle bedforms. Pools are associated with fast, turbulent flow, and consequently the streambed at the pool is comprised of coarse grains up to pebble and cobble calibre [48], which may make these areas proficient in enabling throughflow of water. There is an inverse relationship between both relative pool depth and distance between pools and increasing channel gradient [49]. Generally, the riffles are points of down welling or infiltration at the upstream or stoss side and discharge at the crest or lee side of the riffle [1, 2, 5, 27]. However, the catchment context can result in exceptions to this, e.g. Magliozzi et al. [50].

Streambed permeability is susceptible to modification by other factors, such as the chemical processes that give rise to dissolution or precipitation [51], physical sedimentation and clogging (colmation) and biofilm formation and modification by the streambed fauna. Chen et al. [52] established that influent groundwater flow paths are associated with fine sediment removal from the sediment matrix, thereby increasing the near-surface hydraulic conductivity, whereas downward entrainment of fine particles resulting in siltation or clogging of the sediment in zones of effluent flow (losing reaches of a stream) is associated with a reduction in permeability. For the Nebraskan example described by Chen et al. [52], the alterations to the streambed permeability extended to depths of 5 m or more.

Bypass flow, also termed "preferential" or "fast" flow, is transmitted at orders of magnitude greater than the Darcian matrix flow. Soil pipes are the largest category of macropore and can form connected networks [53]; they have been defined as macropores that are sufficiently large for water to sculpt their form [54]. Soil erosion by throughflow of water gives rise to conduits for lateral or vertical flow [44]. The maximum diameter that a pipe can reach without collapsing will be determined by the density, structure and grain size of the sediment. Piping is most common in the hyporheic zone in bank sediments, locations where surface water is focused by the topography and also where groundwater is focused in gaining reaches of a stream.

The hyporheic zone is related to the base-flow by the residence time, which can be measured using tracers [55]. Investigating a mountain stream at an "Experimental Forest" in Oregon, Haggerty et al. [55] characterised the residence time from a tracer breakthrough curve with a long-tail, poorly characterised by an exponential model and indicative of a large range of exchange timescales, each associated with different volumes of water. This would seem to be representative of many sediment-bedded streams with ranges in permeability and hydraulic regimes. The hyporheic zone also responds to seasonality and importantly for stream ecology, may extend the wet season of ephemeral stream reaches and continue to provide base-flow after the surface stream appears to have "dried up".

Flooding can affect the hyporheic zone causing a hydraulic response as flood waters extend into it. The response is scale dependent and is greater where the channel is unconstrained [56]. In unconstrained systems where channel changes occur, morphological change in the channel may result in localised steepening of the hydraulic gradient or abandonment of hyporheic zones associated with abandoned channels [56, 57]. As the flooding recedes and base-flow re-establishes, the volume of the hyporheic zone will adjust to the new conditions, which has implications for shifts in the stream ecology. During flooding, the increased velocity of surface water can impact the hyporheic zone in accordance with the Venturi Effect. This comprises a net pressure decrease as a function of water velocity, thereby

**15**

*The Hyporheic Zone*

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

**Factor Characterising data requirements**

mobility

Reynolds number

Hydrological Precipitation; temperature; variation with aspect, temporal and seasonal

Hydrogeological Head, seepages seasonality as indicated by storativity and head change, permeability and transmissivity at all scales Geological Bedrock and superficial lithologies, geological structure, weathering and

Valley type Degree of channel confinement and stream connectivity, upland versus

in-channel bedforms and bank conditions Vegetation Types of vegetation: in-channel and riparian and their influence on the

Urbanisation Culverted drainage inputs, hydrological changes due to sustainable urban

Climate Potential climate change impacts, e. g. impacts on existing conditions [60], and climatic zone analogues, e.g. Peel et al. [61]

Scale Regional, catchment, reach or geomorphological feature

flooding and catchment parameters (**Table 1**) Topographical Slope gradients and lengths, aspect, relationship of sub-catchments to catchments and reach-scale representation

variability (changing spatial and vertical distribution of the hyporheic zone);

erosion history, alluvial sediment thickness and architecture and sediment

lowland channels, longitudinal and lateral valley gradients, distribution of

hydrology in terms of water balance, hydrogeochemistry and hydrology, e.g.

drainage scheme implementation, hard and soft engineering impact on channel characteristics [59] and ecological impacts (biological and chemical)

Chemical and biological characteristics and diversity, nutrient potential,

for the management of flood risk.

*Factors to consider in catchment scale hyporheic zone modelling.*

Biogeochemical and ecohydrological

**Table 3.**

understanding the functioning of the hyporheic zone.

potentially giving rise to a local reversal in the hydraulic gradient and influx of deeper groundwater [58], despite the higher river stage. Understanding exchanges of this type is particularly important in the characterisation of the hyporheic zone

organic matter content and habitats

Hyporheic zone hydrology research scales range from catchment to valley or reach scale. Fully integrated catchment scale modelling and management requires catchment through valley and reach-scale understanding of the hyporheic zone exchange [50]. The key factors underpinning such an analysis correspond with the requirements for conceptual modelling detailed above and summarised in **Table 3**. Each of these factors is subject to scaling issues and time-dependent variability. The range in the scale of hyporheic exchange flows also impacts the attenuating and ecological benefits that are derived from the hyporheic zone in different zones within a catchment. For example, a system dominated by groundwater recharge through coarse sediments may mask the potential hydrochemical benefits afforded by pools and riffles. This complexity is additional to the broader understanding of gaining and losing stretches of the stream or river and is informed by detailed bedform architecture as related to geomorphological processes and described above. Additionally, there are temporal and climatic variations that are fundamental to

There are numerous modelling approaches that reflect the multifaceted nature of the hyporheic zone, including ground- and surface water flux and biological and chemical gradients. Hydrological modelling can incorporate the hyporheic zone as a single storage component (e.g. [62]) with fractal scales of response (ranges of permeability and flow path lengths) accounting for the very long tail in the hydrograph


#### **Table 3.**

*Hydrology - The Science of Water*

in exceptions to this, e.g. Magliozzi et al. [50].

the streambed permeability extended to depths of 5 m or more.

The variation in grain size distribution within the sediments gives rise to variability in flow of water through the deposits; more mud-prone sediment has lower porosity and permeability than more sand-prone sediment. The fluctuating flow rates around a meander bend lead to reach-scale variations in sediment-bedded stream hydrological processes commonly leading to the formation of pool and riffle bedforms. Pools are associated with fast, turbulent flow, and consequently the streambed at the pool is comprised of coarse grains up to pebble and cobble calibre [48], which may make these areas proficient in enabling throughflow of water. There is an inverse relationship between both relative pool depth and distance between pools and increasing channel gradient [49]. Generally, the riffles are points of down welling or infiltration at the upstream or stoss side and discharge at the crest or lee side of the riffle [1, 2, 5, 27]. However, the catchment context can result

Streambed permeability is susceptible to modification by other factors, such as the chemical processes that give rise to dissolution or precipitation [51], physical sedimentation and clogging (colmation) and biofilm formation and modification by the streambed fauna. Chen et al. [52] established that influent groundwater flow paths are associated with fine sediment removal from the sediment matrix, thereby increasing the near-surface hydraulic conductivity, whereas downward entrainment of fine particles resulting in siltation or clogging of the sediment in zones of effluent flow (losing reaches of a stream) is associated with a reduction in permeability. For the Nebraskan example described by Chen et al. [52], the alterations to

Bypass flow, also termed "preferential" or "fast" flow, is transmitted at orders of magnitude greater than the Darcian matrix flow. Soil pipes are the largest category of macropore and can form connected networks [53]; they have been defined as macropores that are sufficiently large for water to sculpt their form [54]. Soil erosion by throughflow of water gives rise to conduits for lateral or vertical flow [44]. The maximum diameter that a pipe can reach without collapsing will be determined by the density, structure and grain size of the sediment. Piping is most common in the hyporheic zone in bank sediments, locations where surface water is focused by the topography and also where groundwater is focused in gaining reaches of a

The hyporheic zone is related to the base-flow by the residence time, which

"Experimental Forest" in Oregon, Haggerty et al. [55] characterised the residence time from a tracer breakthrough curve with a long-tail, poorly characterised by an exponential model and indicative of a large range of exchange timescales, each associated with different volumes of water. This would seem to be representative of many sediment-bedded streams with ranges in permeability and hydraulic regimes.

Flooding can affect the hyporheic zone causing a hydraulic response as flood waters extend into it. The response is scale dependent and is greater where the channel is unconstrained [56]. In unconstrained systems where channel changes occur, morphological change in the channel may result in localised steepening of the hydraulic gradient or abandonment of hyporheic zones associated with abandoned channels [56, 57]. As the flooding recedes and base-flow re-establishes, the volume of the hyporheic zone will adjust to the new conditions, which has implications for shifts in the stream ecology. During flooding, the increased velocity of surface water can impact the hyporheic zone in accordance with the Venturi Effect. This comprises a net pressure decrease as a function of water velocity, thereby

can be measured using tracers [55]. Investigating a mountain stream at an

The hyporheic zone also responds to seasonality and importantly for stream ecology, may extend the wet season of ephemeral stream reaches and continue to

provide base-flow after the surface stream appears to have "dried up".

**14**

stream.

*Factors to consider in catchment scale hyporheic zone modelling.*

potentially giving rise to a local reversal in the hydraulic gradient and influx of deeper groundwater [58], despite the higher river stage. Understanding exchanges of this type is particularly important in the characterisation of the hyporheic zone for the management of flood risk.

Hyporheic zone hydrology research scales range from catchment to valley or reach scale. Fully integrated catchment scale modelling and management requires catchment through valley and reach-scale understanding of the hyporheic zone exchange [50]. The key factors underpinning such an analysis correspond with the requirements for conceptual modelling detailed above and summarised in **Table 3**. Each of these factors is subject to scaling issues and time-dependent variability. The range in the scale of hyporheic exchange flows also impacts the attenuating and ecological benefits that are derived from the hyporheic zone in different zones within a catchment. For example, a system dominated by groundwater recharge through coarse sediments may mask the potential hydrochemical benefits afforded by pools and riffles. This complexity is additional to the broader understanding of gaining and losing stretches of the stream or river and is informed by detailed bedform architecture as related to geomorphological processes and described above. Additionally, there are temporal and climatic variations that are fundamental to understanding the functioning of the hyporheic zone.

There are numerous modelling approaches that reflect the multifaceted nature of the hyporheic zone, including ground- and surface water flux and biological and chemical gradients. Hydrological modelling can incorporate the hyporheic zone as a single storage component (e.g. [62]) with fractal scales of response (ranges of permeability and flow path lengths) accounting for the very long tail in the hydrograph (Haggerty et al., [55]). Alternatively, attempts have been made to model both the relative stationarity of surface water in the main channel and the transient storage (characterised by advection and dispersion) in the stagnant zones separately, e.g. Kazezyilmaz-Alhan and Medina [26].

Further research is required to understand the long-term impacts of environmental change on the hyporheic zone. Undoubtedly the hyporheic zone is susceptible to environmental change (climate and anthropogenic impacts) leading to chemical changes, such as pH change or the addition of contaminants. However, global recognition of the value of the hyporheic zone suggests that, although subject to modification, the value of the hyporheic zone will continue in the context of climate change. For example, it is important in both Arctic streams [27] and tropical streams [63]. Therefore it is likely that the hyporheic zone will be increasingly valued in the context of climate change, because of its buffering capacity (hydrological, chemical and ecological), particularly in the light of forecasts of temperature extremes and higher-intensity rainfall and consequential flooding events.

Increased urbanisation is associated with a decrease in stream base-flow [64]. This is, in part, a consequence of hard engineering, with both consequential reduction in recharge to the hyporheic zone and occlusion of its discharge zones. This in turn, reduces the buffering for flood events, leading to higher flood levels and a need for increased levels of engineering to push flood waters through urban environments as quickly as possible. This urban reduction in stream base-flow reduces the calibre of alluvium accumulating on the streambed, baffling circulation of water between the stream and the hyporheic zone, which in turn reduces the overall water capacity of the system by reducing bank and streambed storage and potentially increasing flood risk. Additionally, increased plastic sedimentation in urban areas may prevent circulation altogether by forming an impermeable barrier. The susceptibility of the ecology of the hyporheic zone to environmental change adds further to the pressure on its hydrological functioning, e.g. algal blooms increase the potential for colmation. The hydrological functioning of the hyporheic zone may also be affected by natural processes such as changes to bedrock weathering and geomorphological processes that affect bank stability. Whilst bedrock susceptibility and geomorphology vary with rock type and hydrological conditions, they tend towards reducing the permeability of the hyporheic zone by the addition of sediment to the streambed, whereas flooding events have the potential to remobilise and transport sediment.

#### **3.2 Hyporheic zones in karst environments**

Karst aquifers warrant separate consideration in the context of their hydrological functioning. It is widely recognised that water-mixing zones can be the focus for karst processes within a karst system [65]; therefore, the hyporheic zone is likely to be an important zone within a karstic system. In practice, in many karst aquifers, the hyporheic zone is particularly difficult to define. This is attributed to (i) the difficulty accessing karst systems owing to the range of pore sizes and the difficulty in predicting their distribution, (ii) the rapid change in contact between surface water and groundwater in karst environment, (iii) the water table is commonly poorly defined and (iv) the complexity of some karst systems, e.g. the interplay of matrix and karst porosity in weakly karstic systems, such as chalk [8]. However, the significance of the hyporheic zone in karst systems should not be overlooked particularly with respect to hydraulic function and because of its vulnerability to contamination, attenuation potential and contribution to the evolution of karst systems [65], as well as its ecological value [66].

**17**

thereby contributing to dilution.

*The Hyporheic Zone*

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

Karst systems primarily develop their porosity and permeability and therefore their flow paths through dissolution. Dissolution is commonly focused on structural [67], lithological or geochemical boundaries in the lithology, e.g. inception horizons [68]. If, over geological timescales, the hydrological conditions change, the relict karst systems may be abandoned or only partially functioning in the context of the current hydrological regime of a karst aquifer. Furthermore karst systems can form as tiers that are hydrologically focused on different catchment scales, e.g. base level of the major catchment and base level of a sub-catchment, i.e. a losing tributary river. This has the potential to extend the hyporheic zone of a karst system to a considerable depth in some karstic aquifers and makes it difficult to define the groundwater table of karstic aquifers. Characteristically karst aquifers exhibit low storage and therefore seasonally larger changes in head- than matrix-dominated aquifer with higher storage; thus, the hydraulic conditions are variable, and it is difficult to monitor and quantify hyporheic zone processes, particularly when they occur over the very short timescales that are characteristic of more "flashy" catchments.

Humid tropical karst is characterised by limestone hills (towers or pinnacles). Tower karst development appears to be related to the presences of massive crystalline limestone and the development of a system of open, steeply dipping joints that have been exploited by meteoric and shallow groundwater associated with either current or past climatic regimes. Twidale [69] argues that the pinnacles are the consequence of subsurface-weathering fronts and that weathering has been achieved by deep phreatic waters retained in a regolith. Associated with the pinnacles are a succession of notches, comprising sub-horizontal, laterally, solutionally enlarged conduit systems. It is suspected that these are indicative of Pleistocene interglacial high sea levels at elevations of tens of metres above the current sea level, and it has been suggested that they are formed by swamp waters and that subsoil solution may be associated with the formation of cliff foot caves. Without being explicit in the literature, these formational processes are clearly associated with the hyporheic zone. Further evidence of the significance of the hyporheic zone in karst processes comes from progressive increases in the base-flow index towards the lower end of karst streams. Whilst, traditionally, speleogenesis has been conceptualised from the hydrogeological perspective of source, throughflow and discharge with the independent flow paths of the unsaturated zone becoming more ordered at the "water table". Diffusely recharged water in karst aquifers with long residence times is quickly saturated by carbonate [65], even with the addition of carbon dioxide derived from vegetation or the pH change attributable to overlying acid-generating soils, suggesting that an additional process is required for dissolutional enlargement at the discharge end of the system. Gulley et al. [70] in their investigation of the less mature karst systems of the Suwannee River in North-Central Florida established that undersaturated floodwater-related dissolution during flow reversal in the hyporheic zone of karst discharge areas likely accounts for a significant component of the dissolutional enlargement at the downstream end of the flow path. Additionally, exchange between different scales of conduit contributes to attenuation of contaminants in this zone. More specifically, flood water-dispersed contaminants will be displaced by distal groundwater as the floodwater recedes,

Flood waters are also responsible for the delivery of sediment via the hyporheic

zone into karst systems, and this has a significant role in the armouring of portions of the karst aquifer and attenuation of contaminants [71], but is prone to disturbance by subsequent flooding. Similarly, algal armouring of conduit surfaces is sustained by nutrient supplied by hyporheic exchange. The processes that take place in the hyporheic zone of karst aquifers occur at a range of scales from that of a conduit-wall topography, e.g. scallops to large-scale conduit networks. Arguably,

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

*Hydrology - The Science of Water*

flooding events.

and transport sediment.

**3.2 Hyporheic zones in karst environments**

systems [65], as well as its ecological value [66].

Kazezyilmaz-Alhan and Medina [26].

(Haggerty et al., [55]). Alternatively, attempts have been made to model both the relative stationarity of surface water in the main channel and the transient storage (characterised by advection and dispersion) in the stagnant zones separately, e.g.

Further research is required to understand the long-term impacts of environmental change on the hyporheic zone. Undoubtedly the hyporheic zone is susceptible to environmental change (climate and anthropogenic impacts) leading to chemical changes, such as pH change or the addition of contaminants. However, global recognition of the value of the hyporheic zone suggests that, although subject to modification, the value of the hyporheic zone will continue in the context of climate change. For example, it is important in both Arctic streams [27] and tropical streams [63]. Therefore it is likely that the hyporheic zone will be increasingly valued in the context of climate change, because of its buffering capacity (hydrological, chemical and ecological), particularly in the light of forecasts of temperature extremes and higher-intensity rainfall and consequential

Increased urbanisation is associated with a decrease in stream base-flow [64]. This is, in part, a consequence of hard engineering, with both consequential reduction in recharge to the hyporheic zone and occlusion of its discharge zones. This in turn, reduces the buffering for flood events, leading to higher flood levels and a need for increased levels of engineering to push flood waters through urban environments as quickly as possible. This urban reduction in stream base-flow reduces the calibre of alluvium accumulating on the streambed, baffling circulation of water between the stream and the hyporheic zone, which in turn reduces the overall water capacity of the system by reducing bank and streambed storage and potentially increasing flood risk. Additionally, increased plastic sedimentation in urban areas may prevent circulation altogether by forming an impermeable barrier. The susceptibility of the ecology of the hyporheic zone to environmental change adds further to the pressure on its hydrological functioning, e.g. algal blooms increase the potential for colmation. The hydrological functioning of the hyporheic zone may also be affected by natural processes such as changes to bedrock weathering and geomorphological processes that affect bank stability. Whilst bedrock susceptibility and geomorphology vary with rock type and hydrological conditions, they tend towards reducing the permeability of the hyporheic zone by the addition of sediment to the streambed, whereas flooding events have the potential to remobilise

Karst aquifers warrant separate consideration in the context of their hydrological functioning. It is widely recognised that water-mixing zones can be the focus for karst processes within a karst system [65]; therefore, the hyporheic zone is likely to be an important zone within a karstic system. In practice, in many karst aquifers, the hyporheic zone is particularly difficult to define. This is attributed to (i) the difficulty accessing karst systems owing to the range of pore sizes and the difficulty in predicting their distribution, (ii) the rapid change in contact between surface water and groundwater in karst environment, (iii) the water table is commonly poorly defined and (iv) the complexity of some karst systems, e.g. the interplay of matrix and karst porosity in weakly karstic systems, such as chalk [8]. However, the significance of the hyporheic zone in karst systems should not be overlooked particularly with respect to hydraulic function and because of its vulnerability to contamination, attenuation potential and contribution to the evolution of karst

**16**

Karst systems primarily develop their porosity and permeability and therefore their flow paths through dissolution. Dissolution is commonly focused on structural [67], lithological or geochemical boundaries in the lithology, e.g. inception horizons [68]. If, over geological timescales, the hydrological conditions change, the relict karst systems may be abandoned or only partially functioning in the context of the current hydrological regime of a karst aquifer. Furthermore karst systems can form as tiers that are hydrologically focused on different catchment scales, e.g. base level of the major catchment and base level of a sub-catchment, i.e. a losing tributary river. This has the potential to extend the hyporheic zone of a karst system to a considerable depth in some karstic aquifers and makes it difficult to define the groundwater table of karstic aquifers. Characteristically karst aquifers exhibit low storage and therefore seasonally larger changes in head- than matrix-dominated aquifer with higher storage; thus, the hydraulic conditions are variable, and it is difficult to monitor and quantify hyporheic zone processes, particularly when they occur over the very short timescales that are characteristic of more "flashy" catchments.

Humid tropical karst is characterised by limestone hills (towers or pinnacles). Tower karst development appears to be related to the presences of massive crystalline limestone and the development of a system of open, steeply dipping joints that have been exploited by meteoric and shallow groundwater associated with either current or past climatic regimes. Twidale [69] argues that the pinnacles are the consequence of subsurface-weathering fronts and that weathering has been achieved by deep phreatic waters retained in a regolith. Associated with the pinnacles are a succession of notches, comprising sub-horizontal, laterally, solutionally enlarged conduit systems. It is suspected that these are indicative of Pleistocene interglacial high sea levels at elevations of tens of metres above the current sea level, and it has been suggested that they are formed by swamp waters and that subsoil solution may be associated with the formation of cliff foot caves. Without being explicit in the literature, these formational processes are clearly associated with the hyporheic zone. Further evidence of the significance of the hyporheic zone in karst processes comes from progressive increases in the base-flow index towards the lower end of karst streams. Whilst, traditionally, speleogenesis has been conceptualised from the hydrogeological perspective of source, throughflow and discharge with the independent flow paths of the unsaturated zone becoming more ordered at the "water table". Diffusely recharged water in karst aquifers with long residence times is quickly saturated by carbonate [65], even with the addition of carbon dioxide derived from vegetation or the pH change attributable to overlying acid-generating soils, suggesting that an additional process is required for dissolutional enlargement at the discharge end of the system. Gulley et al. [70] in their investigation of the less mature karst systems of the Suwannee River in North-Central Florida established that undersaturated floodwater-related dissolution during flow reversal in the hyporheic zone of karst discharge areas likely accounts for a significant component of the dissolutional enlargement at the downstream end of the flow path. Additionally, exchange between different scales of conduit contributes to attenuation of contaminants in this zone. More specifically, flood water-dispersed contaminants will be displaced by distal groundwater as the floodwater recedes, thereby contributing to dilution.

Flood waters are also responsible for the delivery of sediment via the hyporheic zone into karst systems, and this has a significant role in the armouring of portions of the karst aquifer and attenuation of contaminants [71], but is prone to disturbance by subsequent flooding. Similarly, algal armouring of conduit surfaces is sustained by nutrient supplied by hyporheic exchange. The processes that take place in the hyporheic zone of karst aquifers occur at a range of scales from that of a conduit-wall topography, e.g. scallops to large-scale conduit networks. Arguably,

rapid exchange over shorter flow paths [72] is probably the most hydrologically significant type of hyporheic zone impact on karst aquifers, because of the greater potential for dissolution resulting from rapid water exchange. However, this is a relatively new research area, and interplay with delivery of soils gases and carbon storage (e.g. [73, 74]) may place this in a different perspective in the longer term.

Two additional aspects of the hyporheic zone have evolved from karst research. Firstly, the relative independence of karst flow paths has led some authors to consider karst conduits as being analogous to surface streams and therefore suggest that karst conduit hyporheic zones can be characterised (e.g. [75]). Whilst this is undoubtedly true because of the influence on biology, contaminant attenuation, geochemistry and speleogenesis of karst systems, such an approach is likely to lead to unnecessary ambiguity in hyporheic zone research. Secondly, the extensive research on the use of tracers to identify flow paths in karst aquifer provides a rich resource in terms of understanding the availability, benefits and challenges associated with a broad range of techniques, e.g. Smart [28].

### **4. Results from case studies**

The two case studies presented below comprise accounts of research that have been undertaken by the British Geological Survey to further understanding of the hydrogeochemical functioning and value of the hyporheic zone in the context of groundwater contamination. Other British Geological Survey research projects include research on the role of the hyporheic zone in flooding in Oxford [76] and Lambourne [8] in Southern England and Eddleston, Scotland [77].

#### **4.1 Rookhope Burn, wear sub-catchment of Northumbria River Basin District, Northern England**

This case study was focused on the Rookhope Burn, an upland stream forming a tributary of the River Wear in northern England. Here the potential for attenuation in the hyporheic zone was explored because the stream lies within a significantly mine-impacted area of the North Pennines Orefield, UK. Zinc had been identified as the contaminant of concern. Mass balances of in-stream and in-flow (subsurface, historic mine working related, contributions of zinc-contaminated groundwater) chemical loads determined from major and trace elements concentrations and synoptic flow monitoring had identified sinks as well as sources of zinc in the burn. The sources comprise rising, mining-contaminated groundwater [78] with the potential to shift the hyporheic zone [56].

In order to investigate this further, the physicochemical composition of the hyporheic zone, a stream stretch, was characterised at two contrasting flow and temperature regimes [13]. The Rookhope Burn streambed comprises coarse-textured river terrace deposits. The underlying bedrock geology is formed of mineralised Dinantian limestones capped by Namurian sandstones and mudstones. For this catchment vertical element concentration gradients were obtained using multilevel samplers down to a depth of 40 cm below the water-sediment boundary and along a 12-m reach. Additionally, in situ diffuse gradients in thin film (DGT) measurements of surface water and porewater were obtained (**Figure 1**).

The multilevel samplers described by Dearden and Palumbo-Roe [18], like those used at Polmadie Burn and described below, comprised a 12 mm ID 1200 mm long, HDPE pipe, fitted at one end with a stainless steel drive-point to enable penetration of the device into sediments. The pipe had two 4-mm-diameter holes at the base

**19**

*The Hyporheic Zone*

**Figure 1.**

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

to allow the piezometric surface within the hyporheic zone to be monitored. Four discrete 1.6 mm ID Teflon sampling tubes were attached around the central pipe and were terminated such that ports were located at 10-cm intervals. Each of the ports was encased in (45 μm) nylon mesh screen to prevent sediment blockages. Porewater samples were recovered using a low-flow multichannel peristaltic pump that enabled simultaneous sampling of the four ports at an approximate flow rate of 4 mL/min; pH and Eh were measured using a flow-through cell, and Pt electrode Eh-measured values were corrected to the standard hydrogen electrode. The level of the hyporheic zone water was measured relative to the river water level using a mini

Two sampling surveys were undertaken, one in the summer (July 2010) and one in the autumn (October 2010). The hydrological conditions in the catchment were very consistent for at least 10 days before each sampling event. In addition to the multilevel samples, samples of surface water were collected at each of the four locations. Groundwater was also collected from a private well adjacent to the sampling

Near neutral pH and oxidising conditions characterised the hyporheic zone. The upper 15–20 cm was dominated by the chemistry of the overlying water, whilst interactions with the solid phase occurred in the deeper part of the hyporheic zone. Mineralogical analysis of manganese-rich grain coatings from the bed sediment indicated that manganese was being attenuated in the hyporheic zone. Additionally, there was clear evidence for hyporheic porewater enrichment in lead that was unaffected seasonally, whilst zinc concentrations were higher in July. The significance of the observed sediment-scale hyporheic processes on the reach-scale geochemical mass balance was estimated by using surface water geochemical loading calculations. Along a 700-m stream stretch of the burn, a constant loss of manganese stream load and continuous gain of lead stream load with more temporally variable zinc stream loading were measured. This demonstrated that the hyporheic zone of the mineimpacted stream supports steep manganese, lead and zinc gradients. Seasonality in

dipper placed down the central HDPE tube [13].

*DGT placed in the base of the hollow sampler and left for 24 hours.*

site for comparison with the hyporheic porewaters [13].

*Hydrology - The Science of Water*

rapid exchange over shorter flow paths [72] is probably the most hydrologically significant type of hyporheic zone impact on karst aquifers, because of the greater potential for dissolution resulting from rapid water exchange. However, this is a relatively new research area, and interplay with delivery of soils gases and carbon storage (e.g. [73, 74]) may place this in a different perspective in the longer term. Two additional aspects of the hyporheic zone have evolved from karst research.

Firstly, the relative independence of karst flow paths has led some authors to consider karst conduits as being analogous to surface streams and therefore suggest that karst conduit hyporheic zones can be characterised (e.g. [75]). Whilst this is undoubtedly true because of the influence on biology, contaminant attenuation, geochemistry and speleogenesis of karst systems, such an approach is likely to lead to unnecessary ambiguity in hyporheic zone research. Secondly, the extensive research on the use of tracers to identify flow paths in karst aquifer provides a rich resource in terms of understanding the availability, benefits and challenges associ-

The two case studies presented below comprise accounts of research that have been undertaken by the British Geological Survey to further understanding of the hydrogeochemical functioning and value of the hyporheic zone in the context of groundwater contamination. Other British Geological Survey research projects include research on the role of the hyporheic zone in flooding in Oxford [76] and

**4.1 Rookhope Burn, wear sub-catchment of Northumbria River Basin District,** 

In order to investigate this further, the physicochemical composition of the hyporheic zone, a stream stretch, was characterised at two contrasting flow and temperature regimes [13]. The Rookhope Burn streambed comprises coarse-textured river terrace deposits. The underlying bedrock geology is formed of mineralised Dinantian limestones capped by Namurian sandstones and mudstones. For this catchment vertical element concentration gradients were obtained using multilevel samplers down to a depth of 40 cm below the water-sediment boundary and along a 12-m reach. Additionally, in situ diffuse gradients in thin film (DGT) measurements of surface water and porewater

The multilevel samplers described by Dearden and Palumbo-Roe [18], like those used at Polmadie Burn and described below, comprised a 12 mm ID 1200 mm long, HDPE pipe, fitted at one end with a stainless steel drive-point to enable penetration of the device into sediments. The pipe had two 4-mm-diameter holes at the base

This case study was focused on the Rookhope Burn, an upland stream forming a tributary of the River Wear in northern England. Here the potential for attenuation in the hyporheic zone was explored because the stream lies within a significantly mine-impacted area of the North Pennines Orefield, UK. Zinc had been identified as the contaminant of concern. Mass balances of in-stream and in-flow (subsurface, historic mine working related, contributions of zinc-contaminated groundwater) chemical loads determined from major and trace elements concentrations and synoptic flow monitoring had identified sinks as well as sources of zinc in the burn. The sources comprise rising, mining-contaminated groundwater [78] with the

Lambourne [8] in Southern England and Eddleston, Scotland [77].

ated with a broad range of techniques, e.g. Smart [28].

**4. Results from case studies**

**Northern England**

were obtained (**Figure 1**).

potential to shift the hyporheic zone [56].

**18**

**Figure 1.** *DGT placed in the base of the hollow sampler and left for 24 hours.*

to allow the piezometric surface within the hyporheic zone to be monitored. Four discrete 1.6 mm ID Teflon sampling tubes were attached around the central pipe and were terminated such that ports were located at 10-cm intervals. Each of the ports was encased in (45 μm) nylon mesh screen to prevent sediment blockages. Porewater samples were recovered using a low-flow multichannel peristaltic pump that enabled simultaneous sampling of the four ports at an approximate flow rate of 4 mL/min; pH and Eh were measured using a flow-through cell, and Pt electrode Eh-measured values were corrected to the standard hydrogen electrode. The level of the hyporheic zone water was measured relative to the river water level using a mini dipper placed down the central HDPE tube [13].

Two sampling surveys were undertaken, one in the summer (July 2010) and one in the autumn (October 2010). The hydrological conditions in the catchment were very consistent for at least 10 days before each sampling event. In addition to the multilevel samples, samples of surface water were collected at each of the four locations. Groundwater was also collected from a private well adjacent to the sampling site for comparison with the hyporheic porewaters [13].

Near neutral pH and oxidising conditions characterised the hyporheic zone. The upper 15–20 cm was dominated by the chemistry of the overlying water, whilst interactions with the solid phase occurred in the deeper part of the hyporheic zone. Mineralogical analysis of manganese-rich grain coatings from the bed sediment indicated that manganese was being attenuated in the hyporheic zone. Additionally, there was clear evidence for hyporheic porewater enrichment in lead that was unaffected seasonally, whilst zinc concentrations were higher in July. The significance of the observed sediment-scale hyporheic processes on the reach-scale geochemical mass balance was estimated by using surface water geochemical loading calculations. Along a 700-m stream stretch of the burn, a constant loss of manganese stream load and continuous gain of lead stream load with more temporally variable zinc stream loading were measured. This demonstrated that the hyporheic zone of the mineimpacted stream supports steep manganese, lead and zinc gradients. Seasonality in

the hyporheic zone was suspected from the results, but was not fully investigated, and therefore further investigation of seasonality was identified as a necessary future research direction for this catchment. More specifically, this would require periods of continuous monitoring, which is difficult to establish in remote catchments.

#### **4.2 Polmadie Burn, Clyde catchment, Scotland**

The potential for hyporheic zone attenuation of chromium contamination in Polmadie Burn, a minor tributary of the River Clyde in Scotland, UK, was investigated during two monitoring periods in February and September 2012 [79]. A summary of the monitoring techniques and findings are presented here to demonstrate the importance of linking hydrology to geochemical assessments of the context of hyporheic zone.

The Polmadie Burn is an urban steam, in the order of 3 km to the south-east of Glasgow City Centre. It is impacted by hexavalent chromium-rich effluent leached from the landfilled residue from historical (late nineteenth and early twentieth century) processing of chromite ore [80]. The stream responds rapidly to changes in the discharge from its culverted urban drainage catchment. The underlying Carboniferous bedrock comprises cyclic sequences of mudstone, siltstone and sandstone that are overlain by superficial deposits of river terrace deposits capped by low-permeability alluvium. The low permeability of the hyporheic zone contrasts with the coarse streambed investigated at Rookhope.

Following a desk study investigation of the burn, a sampling strategy was designed to integrate hydrology and geochemistry in the context of spatial and temporal change. A specific aspect of the investigation was the attention given to the health and safety issues associated with working in the chromium-contaminated stream with a soft silty streambed (**Figure 2**). Upstream and downstream transects were selected for sampling, which included in situ monitoring via multilevel minipoint piezometers (**Figure 2**), hydraulic testing (falling head slug tests in the bank sediments) as well as sampling and geochemical/mineralogical characterisation of hyporheic water, surface water and streambed sediments. At each transect the multilevel piezometers comprised a number of carefully labelled sampling ports to facilitate hydraulic gradient determination and vertical profile sampling from within the bed sediments at depths of up to 90 cm below bed level. Following a stabilisation period of 12 hours, sampling was undertaken with a low-flow multichannel peristaltic pump. The fieldwork comprised two campaigns to monitor the hyporheic zone processes in operation at different stages of the stream. The February visit allowed sampling of water from the hyporheic zone and stream over a large short-term variation in stream depth, which comprised a low tide during which there was temporary exposure of river bed sediments followed by overnight rainfall and high stream levels. The second field visit, in September, coincided with more constant hydraulic conditions. This allowed synoptic surface water quality sampling to be carried out to provide qualitative evidence of any whole-streamcontaminant attenuation to which the hyporheic zone may contribute.

Detrital grains of historical chromium process residue were found to contribute to the total chromium concentrations (size fraction < 150 μm) that reached 8800 mg kg<sup>−</sup><sup>1</sup> in the streambed sediment. There was a sharp decrease of total dissolved (filtered < 0.45 μm) chromium concentrations at the surface water-sediment boundary in all profiles, from a mean chromium concentration of 1100 μg l<sup>−</sup><sup>1</sup> in the surface water to 5 μg l<sup>−</sup><sup>1</sup> in the porewater. This was associated with an elevated ferrous iron concentration in the porewater (mean concentration 1700 μg l<sup>−</sup><sup>1</sup> ) and chromium (VI) reduction to chromium (III) solids of low solubility. However, the hyporheic zone did not respond to the large short-term changes of stream stage as indicated by

**21**

**Figure 2.**

*pump was deployed.*

**Acknowledgements**

leading to improvements of the manuscript.

*The Hyporheic Zone*

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

the hyporheic zone water composition. It was concluded that the low-permeability alluvial sediments imposed a limit on the effectiveness of the hyporheic zone for enhancing Cr surface water quality at the reach scale. This was also evident in the surface water quality synoptic sampling which showed only moderate to low downstream decreases in surface water chromium concentrations. Thus, the key finding was that although the geochemical potential for hyporheic attenuation of surface water chromium was clearly established, the hydraulic functioning of the hyporheic

*Hyporheic zone multilevel sampling of the bed of Polmadie Burn, Glasgow. Note how, for ease of access, the multipoint sampling tubes were extended from the sampling point to the base station where the peristaltic* 

zone was limited by poor hydrological connectivity with the Polmadie Burn.

This chapter is published with the permission of the Executive Director of the British Geological Survey (UKRI). Stephanie Bricker (British Geological Survey) and an unnamed reviewer are thanked for insightful reviews and suggestions

*Hydrology - The Science of Water*

hyporheic zone.

**4.2 Polmadie Burn, Clyde catchment, Scotland**

with the coarse streambed investigated at Rookhope.

the hyporheic zone was suspected from the results, but was not fully investigated, and therefore further investigation of seasonality was identified as a necessary future research direction for this catchment. More specifically, this would require periods of continuous monitoring, which is difficult to establish in remote catchments.

The potential for hyporheic zone attenuation of chromium contamination in Polmadie Burn, a minor tributary of the River Clyde in Scotland, UK, was investigated during two monitoring periods in February and September 2012 [79]. A summary of the monitoring techniques and findings are presented here to demonstrate the importance of linking hydrology to geochemical assessments of the context of

The Polmadie Burn is an urban steam, in the order of 3 km to the south-east of Glasgow City Centre. It is impacted by hexavalent chromium-rich effluent leached from the landfilled residue from historical (late nineteenth and early twentieth century) processing of chromite ore [80]. The stream responds rapidly to changes in the discharge from its culverted urban drainage catchment. The underlying Carboniferous bedrock comprises cyclic sequences of mudstone, siltstone and sandstone that are overlain by superficial deposits of river terrace deposits capped by low-permeability alluvium. The low permeability of the hyporheic zone contrasts

Following a desk study investigation of the burn, a sampling strategy was designed to integrate hydrology and geochemistry in the context of spatial and temporal change. A specific aspect of the investigation was the attention given to the health and safety issues associated with working in the chromium-contaminated stream with a soft silty streambed (**Figure 2**). Upstream and downstream transects were selected for sampling, which included in situ monitoring via multilevel minipoint piezometers (**Figure 2**), hydraulic testing (falling head slug tests in the bank sediments) as well as sampling and geochemical/mineralogical characterisation of hyporheic water, surface water and streambed sediments. At each transect the multilevel piezometers comprised a number of carefully labelled sampling ports to facilitate hydraulic gradient determination and vertical profile sampling from within the bed sediments at depths of up to 90 cm below bed level. Following a stabilisation period of 12 hours, sampling was undertaken with a low-flow multichannel peristaltic pump. The fieldwork comprised two campaigns to monitor the hyporheic zone processes in operation at different stages of the stream. The February visit allowed sampling of water from the hyporheic zone and stream over a large short-term variation in stream depth, which comprised a low tide during which there was temporary exposure of river bed sediments followed by overnight rainfall and high stream levels. The second field visit, in September, coincided with more constant hydraulic conditions. This allowed synoptic surface water quality sampling to be carried out to provide qualitative evidence of any whole-stream-

contaminant attenuation to which the hyporheic zone may contribute.

iron concentration in the porewater (mean concentration 1700 μg l<sup>−</sup><sup>1</sup>

Detrital grains of historical chromium process residue were found to contribute to the total chromium concentrations (size fraction < 150 μm) that reached

solved (filtered < 0.45 μm) chromium concentrations at the surface water-sediment boundary in all profiles, from a mean chromium concentration of 1100 μg l<sup>−</sup><sup>1</sup>

(VI) reduction to chromium (III) solids of low solubility. However, the hyporheic zone did not respond to the large short-term changes of stream stage as indicated by

in the streambed sediment. There was a sharp decrease of total dis-

in the porewater. This was associated with an elevated ferrous

in the

) and chromium

**20**

8800 mg kg<sup>−</sup><sup>1</sup>

surface water to 5 μg l<sup>−</sup><sup>1</sup>

#### **Figure 2.**

*Hyporheic zone multilevel sampling of the bed of Polmadie Burn, Glasgow. Note how, for ease of access, the multipoint sampling tubes were extended from the sampling point to the base station where the peristaltic pump was deployed.*

the hyporheic zone water composition. It was concluded that the low-permeability alluvial sediments imposed a limit on the effectiveness of the hyporheic zone for enhancing Cr surface water quality at the reach scale. This was also evident in the surface water quality synoptic sampling which showed only moderate to low downstream decreases in surface water chromium concentrations. Thus, the key finding was that although the geochemical potential for hyporheic attenuation of surface water chromium was clearly established, the hydraulic functioning of the hyporheic zone was limited by poor hydrological connectivity with the Polmadie Burn.

## **Acknowledgements**

This chapter is published with the permission of the Executive Director of the British Geological Survey (UKRI). Stephanie Bricker (British Geological Survey) and an unnamed reviewer are thanked for insightful reviews and suggestions leading to improvements of the manuscript.

*Hydrology - The Science of Water*
