defining hyporheic zones – advancing our conceptual and operational definitions of where stream...

Defining Hyporheic Zones – Advancing Our Conceptualand Operational Definitions of Where Stream Water andGroundwater Meet

Michael N. Gooseff*Department of Civil & Environmental Engineering, Pennsylvania State University

Abstract

There is a growing recognition of the importance of connections between streams and adjacentalluvial aquifers. The exchange of water, mass, and energy between these locations influencesstream ecosystem structure and function by facilitating nutrient cycling, respiration, stream temper-ature buffering, and survival of macroinvertebrates. Near-surface aquifers that interact with surfacewater have been termed ‘hyporheic zones’, yet there are several accepted definitions of this term(biological, geochemical, and hydrological definitions), which do not necessarily agree with eachother. Biologists refer to it as the subsurface inhabited by hyporheos (‘stream’ macroinvertebratesobserved in the subsurface). Hydrologists support a conceptual model that suggests a ‘flow-through’ subsurface region containing flowpaths that originate and terminate at the stream. Geo-chemical definition suggests that the hyporheic zone is a mixing zone between surface water anddeep-sourced groundwater, with intermediate conditions between these end members. Temporaland spatial scales over which we seek to define hyporheic zones constrain our ability to do so. Itis recognized that stream water exchanges through the subsurface over many timescales. Proposedhere is a definition based on the timescale of flow through the hyporheic zone, in which resi-dence times of interest are explicitly distinguished and referred to – e.g. ‘the 24-h hyporheiczone’. Thus the ‘component’ of the hyporheic zone that is of interest can be related to specificrates of processes of interest (e.g. denitrification rate). Adoption of this approach will reduce ambi-guity in hyporheic science and hopefully contribute to interdisciplinary scientific advancements.

Introduction

Hyporheic zones are generally described as parts of alluvial aquifers (i.e. in the subsurface)that are directly adjacent to streams (both vertically and laterally), which accommodate amixture of stream water and groundwater. The concept of the hyporheic zone is similar tothat of the perirheic zone, which is identified as the surface location of mixing of local andfloodwaters on floodplains (Mertes 1997). Research on the importance of processes thatoccur in hyporheic zones and the role of hyporheic processes in stream ecosystem func-tion, water quality, etc., has increased greatly in the past two decades, enhancing thenotion that streambeds and banks are loose and porous boundaries to the movement ofwater, energy, and solutes. However, many of these hyporheic studies have been con-ducted with different underlying definitions of what constitutes a ‘hyporheic zone’ bothconceptually and operationally. This mixture of definitions limits our ability to substantiallyadvance hyporheic science (and in the case of stream restoration, engineering channels thatenhance hyporheic exchange, i.e. hyporheic engineering), with sufficient interdisciplinaryefforts to generate conclusions and theoretical advances that are broadly applicable. In thisarticle, the varying definitions of hyporheic zones are explored and a more unified

Geography Compass 4/8 (2010): 945–955, 10.1111/j.1749-8198.2010.00364.x

ª 2010 The AuthorJournal Compilation ª 2010 Blackwell Publishing Ltd

approach to defining hyporheic zones is proposed with the goal of advancing a more uni-fied definition that can be used by scientists from a variety of backgrounds (e.g. biologists,geochemists, hydrologists) to significantly advance hyporheic science and engineering.

Biological Underpinning

The roots of hyporheic science are in biology. Orghidan (1959) coined the term hyporheosdefining it as the community of meiofauna (invertebrates that generally inhabit thestreambed) existing beneath a stream, within the gravels and sediments below the stream-bed. The species of meiofauna encountered in the subsurface were found in surface envi-ronments of stream channels. Thus, their presence in the subsurface indicated thatconditions in the subsurface (i.e. dissolved oxygen, nutrient availability) must be similarto those at the surface to accommodate and support these fauna beyond the channel bed.Several studies were conducted in the following decades that further demonstrated theimportance of the subsurface as a habitat for numerous species of invertebrates (e.g. Cole-man and Hynes 1970; Danielpol 1976; Godbout and Hynes 1982; Hynes 1974; Stanfordand Gaufin 1974; Stanford and Ward 1988; Williams and Hynes 1974).

The extent to which surface water had an influence in the subsurface, and thereforeprovided conditions appropriate for hyporheos was referred to the hyporheic zone. Becauseof the necessity that conditions be somewhat similar to those at the surface, the hyporheiczone is often considered a ‘location of mixing’ of stream water and groundwater, and inan ecological context, has become recognized as an ecotone, intermediate between streamand groundwater ecosystems. In the subsurface, the biological and ecological significanceof mixing with stream water provides distinct habitat. Hence, the distribution of ‘streamwater-like’ conditions and hyporheos were expected to be coincident and were character-ized in two-dimensional conceptual views of the channel–aquifer system (e.g. Figure 1b).

Geochemical Approach

The geochemical interpretation of the hyporheic zone and hyporheic processes buildsdirectly upon the implications of the biological development of the concept of hyporheiczones. In recognition of the mixture of two general end member water types (surface waterand groundwater), geochemical approaches to characterizing hyporheic zones and processesare often in the context of extensive sampling of the subsurface for water quality and chem-ical differences. Often, natural tracers can be used to perform a geochemical characteriza-tion of the hyporheic zone (e.g. specific conductance, stable isotope signatures, ionconcentrations). Studies of biogeochemical cycling in streams led Triska et al. (1989) todefine the extent of the hyporheic zone as a subsurface location at which at least 10%, butless than 98% of the water present came from the stream. This arbitrary guidance is deter-mined by end-member mixing analysis of solute concentrations between groundwater andsurface water signatures either for natural tracers (dissolved ion species concentrations thatwould be substantially different between stream water and groundwater) or introducedtracers from stream tracer addition experiments using conservative dissolved ion species.

Hydrological Conceptual Model

The hydrological conceptual model of the hyporheic zone is defined by the exchange ofstream water through (that is, both into and out of) a portion of alluvial aquifers, generallydeveloped as a set of flowpaths that begin and terminate at different locations of a stream

946 Defining hyporheic zones

ª 2010 The Author Geography Compass 4/8 (2010): 945–955, 10.1111/j.1749-8198.2010.00364.xJournal Compilation ª 2010 Blackwell Publishing Ltd

(Figure 1a and c). These may occur over a complex set of space and time scales. Harveyet al. (1996) define hyporheic zone by relating it to hyporheic exchange: ‘The delineatingcharacteristic of the hyporheic zone is the recharge of channel water to the subsurface andmixing with groundwater that has not yet reached the channel.’ They further definehyporheic exchange as ‘small-scale (centimeter to meter) exchanges of water betweenchannels and the subsurface’. Winter et al. (1998) define the hyporheic zone as ‘The sub-surface zone where stream water flows through short segments of its adjacent bed andbanks’. Storey et al. (2003) also define the hyporheic zone in the context of hyporheicexchange:

The hyporheic zone is the zone of saturated sediment beneath and lateral to a stream channelthat receives input of stream surface water… defined by the extent to which surface waterinvades the subsurface beneath and lateral to a stream and returns to the stream surface fartherdownstream in a pattern known as exchange flow.

Whereas these hydrologic definitions are not necessarily incongruent with the biologicalor geochemical definitions described above, they do have a common underlying assump-tion that stream water exchanging through the hyporheic zone necessarily moves andmixes with groundwater along flow paths that begin and end at the stream. This is animportant distinction of the hydrologic definition because it necessarily follows that theprocesses occurring within the hyporheic zone that could modify hyporheic water chem-istry, for example, would have an influence on stream water in the channel because ofthe flow-through requirement of this definition. Indeed this connotation of the hyporheic

(a)

(b)

(c)

Fig. 1. Typical conceptual diagrams of the hyporheic zone in (a) plan view, (b) lateral cross-section, and (c) longitu-dinal cross-section views.

Defining hyporheic zones 947


zone as a ‘biological reactor’ or ‘geochemical reactor’ that can substantially influencesurface water quality has been extensively articulated in several contexts: acid mine drain-age ⁄ pollutant buffering (e.g. Fischer et al. 2005; McKnight et al. 2002), nutrient cycling(e.g. Baker et al. 1999; Bohlke et al. 2004; Pinay et al. 2008), and water temperaturemoderation (e.g. Arrigoni et al. 2008; Hester et al. 2009).

Hyporheic Synergy?

Despite much attention on the near-stream portions of alluvial aquifers that are influencedby stream water, the three above-mentioned definitions are based on independent criteria.The commonalities among the three definitions are: (i) that stream water enters thesubsurface, (ii) that there is some degree of mixing of stream and groundwater in the subsur-face, and (iii) that stream water is influencing the conditions of a portion of the aquifer thatis near the channel. Only the hydrologic definition suggests that the processes occurring inthe alluvial aquifer should influence the stream. In the strictest sense, the biological and geo-chemical definitions could be applied to locations of the alluvial aquifer that were connectedto much deeper aquifers and thus the water leaving the stream would not necessarily returnto the channel. Movement of water back into the stream from the alluvial aquifer is not arequisite of the biological and geochemical definitions, though it is not necessarily omittedfrom such studies. Hence it is difficult to translate findings from studies that incorporatedthe biological definition with others that strictly used the hydrological definition, for exam-ple. Further complicating our understanding of the hyporheic zone is the fact that its loca-tion, adjacent to stream channels, is dynamic in both time and space (Arntzen et al. 2006;Malcolm et al. 2006). Potential for exchange of surface water through the hyporheic zone isstrongly controlled by channel form and the geologic and hydrologic settings of the channel(Cardenas and Wilson 2007; Harvey and Bencala 1993; Stanford and Ward 1993; Wrob-licky et al. 1998); a full review of these controls is however beyond the scope of this article.

In a perspective commentary on hyporheic zone science, White (1993) noted:

Despite considerable use of the term [‘hyporheic’], there still is no single conceptual definitionor framework that would convey a common and concise meaning to biologists, hydrologists,geomorphologists, and aquatic chemists alike. (p. 61)

This challenge persists, as there still has been no broad advance since the publication ofthis article, and that of Palmer (1993), who also advocates for a more uniform conceptualmodel that would enhance our ability to perform experimental science in hyporheiczones. Our varying conceptual models of what the hyporheic zone is have hindered ourability to develop well-posed hypotheses and theories that are generally transferrableamong stream systems. There are enough commonalities of the connotation of ‘hyporheiczone’ that the general use of the term communicates studies of interactions of streamwater and groundwater in near-stream subsurface environments. However, while localcontrols will always have some influence on hydrology, biogeochemistry, and biology ofstreams and connected aquifers, hyporheic science will likely remain underdeveloped andsite specific until a transferrable conceptual model of the influence of stream water–groundwater interactions near streams is resolved, articulated and applied.

Characterizing Hyporheic Zones and Exchange in the Field

One of the challenges to making advances in hyporheic science is that field methods areoften poorly constrained and ⁄or are informing incomplete conceptual models (Palmer



1993). This is partly due to the fact that our field methods and abilities to acquire infor-mation about field settings (e.g. hydraulic conductivity variability in the subsurface) havebeen limited. The concept and the characterization processes are so closely linked in hyp-orheic studies that the concept is rarely challenged because the characterization processesare a direct outcome of a conceptual model (e.g. biological characterization of hyporheoshabitat as the hyporheic zone). Thus, a feedback has developed that has partly limited ourability to develop a unified conceptual model. Besides the sampling schemes noted above(i.e. sampling the subsurface for meiofauna, and using end member mixing analysis todistinguish contributions of stream water and groundwater), the extent of stream waterpenetration into alluvial aquifers and associated processes have been investigated throughtwo general approaches (often conducted together): (i) field study of movement of anintroduced tracer, and (ii) numerical modeling. The goals these approaches are to identifyflow paths that link stream water to the subsurface and ⁄or to characterize the net influ-ence of these interactions on water and solute transport at the scale of a stream reach.

In field studies of tracer movement, a conservative dissolved tracer (and sometimes alsoa reactive tracer) is released either in the stream or into a point location of the alluvialaquifer adjacent to the stream. Down-gradient sampling locations are selected to suit theobjectives of the study, and typically, sampling or measurements are made through time.When sampling the down stream locations for tracer added to the stream, it is oftenexpected that one will use such data to determine how much potential retardation of thetracer signal that occurred along a stream reach that could be attributed to stream–aquiferexchange (Harvey et al. 1996).

The scale of observations often relied upon to characterize the hyporheic zone, hypor-heic exchange and associated effects is often much smaller than the scale to which impli-cations ⁄estimates are to be extrapolated. Point samples of water or measurements of waterquality are expected to indicate the integrated effects of processes occurring upgradient ofthe sampling location, those already experienced by the parcel of water that is sampled.However, each of these locations is unique in its representation of the broader system ofwhich it is a very small part. The characterization and conclusions that come fromsampling one or more such places is a function of a few (or less) flowpaths that intersectthe discrete sampling location. Harvey et al. (1996) and Wondzell (2006) demonstrate thisin their attempts to corroborate reach-scale interpretations of stream solute transportmodels and observations of tracer at discrete points in the adjacent alluvial aquifers.

Numerical modeling approaches in hyporheic studies are generally of two kinds: (i) 1Dsimulation of solute transport in the stream channel that includes some location of tran-sient storage of solute, and (ii) 2D or 3D simulation of groundwater flow where theboundary conditions at the stream are defined by pressure variations that are eitherhydrostatic or hydrodynamic (Kaser et al. 2009); additional simulations of particle ordiffusive solute transport can be added (Gooseff et al. 2006; Lautz and Siegel 2006). One-dimensional solute transport models have been used extensively to simulate ‘reach-repre-sentative’ transport processes (i.e. advection, dispersion, exchange) from one point toanother along a stream reach and attempt to explain the general skew observed in obser-vations of stream tracer concentrations with time (e.g. Bencala and Walters 1983). Suchmodels rely completely on information collected from the stream, yet their application isoften in the context of informing our understanding of stream water exchange with thesubsurface (Haggerty et al. 2002; Valett et al. 1996). Two-dimensional and 3D ground-water flow models simulate flow fields in the subsurface. This modeling approach yields aprecise (though not necessarily accurate) distribution of flow paths, residence times, andextent of stream water movement into the subsurface that is spatially distributed. Such



models often rely upon spatially sparse information about hydraulic conductivity andactual hydraulic head data from the subsurface (Wondzell et al. 2009). However, thesemodels can be combined with stream tracer experiment data to better characterize tracerarrival times at particular locations in the subsurface, for example (Zarnetske et al. 2008).

Be they informed by field studies or measurements of field conditions, our conceptualand numerical models have proven to be too simple in the past to account for the influ-ences of boundary conditions, which are often dictated by assumption. In field studies, itis difficult to characterize both the ‘region of interest’ and the ultimate limit of streaminfluence in the subsurface by sampling numerous discrete points in the subsurface. Inmodeling studies, the extent to which our simulations predict surface water to exchangethrough the subsurface is often very uncertain. Recently, Payn et al. (2009) have pro-vided an updated conceptual model of stream channel water balance including the con-cept of gains and losses of stream water beyond what is generally considered to be part ofthe hyporheic zone. In all stream settings, the stream channel and connected alluvial aqui-fer are both connected to broader flowpaths that link to hillslopes and deeper aquifers.The value of this conceptual model is its definition of the scale at which exchange pro-cesses are detectable from, in that case, stream solute tracer experiments, which is referredto as the ‘window of detection’ by Harvey and Wagner (2000). This conceptual model isfurthered in Figure 2, which represents a broad array of potential flowpaths from a singlestream reach (0, the origin). In a given stream system, the extent and array of flowpathsthat connect a channel with the alluvial aquifer are broad. We expect that only a fractionof those can be detected using typical approaches (outline A in Figure 2), and that streamwater has a broader spatial (and temporal) influence in the continuum of hydrologic flow-paths that connect to the stream (outline B in Figure 2). Wondzell (2006) notes (subse-quent to several days of constant tracer addition to the stream) several observations ofincreasing stream tracer concentrations in riparian wells within several meters of a smallmountain stream after the in-channel concentrations at a downstream sampling locationhave diminished to a point that they can no longer be detected. Are these riparian loca-tions truly ‘hyporheic’? Are flowpaths iv–vii in Figure 2 ‘hyporheic’? Wondzell’s riparianwells are clearly connected to the stream because stream tracer is observed at these loca-tions. However, the flowpaths that connect these locations to the stream do not respondon a timescale that is coincident with the tracer ‘bleed out’ from the hyporheic zone that

Fig. 2. Conceptual model of hyporheic exchange at an arbitrary scale where outline (A) represents the ‘window ofdetection’ of exchange flowpaths for a stream tracer experiment starting at point 0, and (B) the potential set oftrue flowpaths originating from the same location with increasing transport time and ⁄ or length scales from i to vii.



was observed at a downstream sampling location in the stream channel. Hence, these dis-tal locations that are clearly connected to the stream might best be considered to be in azone that is ‘influenced by the stream’, but not necessarily rapidly exchanging (orexchanging at all, perhaps) with the stream.

Discriminating between Hyporheic Exchange Zones and Zones of Stream Influence

That the hyporheic zone should be defined by the extent of hyporheic exchange ofstream water in the subsurface (hydrologic definition from above) provides an unnecessarylimit on the versatility of the concept. One reason that hyporheic zones have been closelyassociated with hyporheic exchange is the general expectation that all water that leavesthe channel returns to the channel. In a strictly hydrologic perspective, this is a narrow(and often convenient) fundamental assumption that may be true only when one considersvery long space (km or more) and time (years or more) scales. Consider the case of astrongly losing ephemeral stream reach in an arid landscape. In such a situation, the streamwater moving into the subsurface may mix with subsurface water in near-stream alluvialaquifers that was previously held in tight suction (i.e. unsaturated conditions) such that lessthan 98% of water in the subsurface was subsurface in origin. Despite this qualification tobe hyporheic water under the Triska et al. (1989) definition, the general infiltration ofthis water into the subsurface may provide local hyporheic exchange (Cardenas andWilson 2007), but generally losing stream reaches or segments, and thereby contribute togeneral groundwater recharge. Is this subsurface water that carries the surface water signa-ture, and is essentially infiltrating and likely recharging a distal aquifer, really hyporheic?

Consider another example, of simple stream–aquifer systems disconnected from largergroundwater aquifers. The streams of the McMurdo Dry Valleys, Antarctica are underlainby permafrost at a depth of less than 1 m below the surface (Conovitz et al. 2006; Gooseffet al. 2003). In these streams, the exchange of water between the channel and thesubsurface has been observed indirectly through simulation of stream tracer experiments(Gooseff et al. 2002; 2004; McKnight et al. 2004; Runkel et al. 1998), and directly duringstream tracer experiments (Gooseff et al. 2004). Thus, the conceptual model of the extentof hyporheic exchange in these streams is defined by a ‘hard’ vertical boundary (perma-frost) and a ‘soft’ lateral boundary, defined by adjacent topography and sediment type anddistribution (Figure 3a). Because of the extremely dry conditions across this landscape, thenear-stream soils and sediments are wetted with water found there is completely derivedfrom the stream (Figure 3b). However, not all water that reaches these ‘wetted streammargins’ is necessarily hyporheic. In the context of the biological definition, and the geo-chemical mixing definition, the extent of the wetted zone may be considered ‘hyporheic’.However, from the hydrologic perspective, the water at the distal extent of these wettedzones is not actively exchanging with the channel, rather it is evaporating or movingdown gradient parallel to the channel, and its presence at the outer limits of the wettedmargin is due to capillary action that pulls the water to the fringe. Hence, the completezone of wetted sediment adjacent to the channel contains a zone of active exchange withthe channel, which is a subset of the larger zone of surface water influence in the subsur-face (Figure 3a). Whereas this is a rather extreme example of stream–aquifer dynamics,analogous processes are occurring in typical temperate streams where stream water is ‘lost’to the locally exchanging system (i.e. Payn et al. 2009). Stream water that leaves thechannel does not move in complete parcels in the subsurface. Hence, water that entersthe subsurface has the opportunity to pursue a variety of flowpaths, some returning to thestream, some recharging larger groundwater reservoirs, etc.



The term hyporheic zone should continue to refer to the locations in the subsurface inwhich an observable influence of surface water occurs, consistent with the genesis of theterm. The term hyporheos should continue to refer to meiofauna associated with thehyporheic zone (i.e. no suggested change here). However, recognizing that there is acontinuum of flow paths and associated residence times, researchers should strive to spe-cifically address in which part of the hyporheic zone their research is being conducted.Thus, it is important to explicitly characterize the time scale of stream water interactionwith the hyporheic zone. This approach will still yield condition-specific results, but willsignificantly advance our ability to compare the influence of residence time on hyporheicprocesses. The ‘rapid’ exchange zone (i.e. flow paths with short residence times) willlikely be easier to characterize than more distal, longer residence time flow paths, in partbecause it is difficult to resolve their connection to streams, and they may not activelyexchange with the channel (e.g. Figures 3a and 4) over temporal scales that are relevantto processes of interest (i.e. ‘losing’ flowpaths indicated by Payn et al. 2009). One mightrefer to the ‘24-h hyporheic zone’, which would be defined as the spatial extent to whichstream water traveled into the subsurface and returned to the stream channel (i.e. flushed outof specific locations) within 24 h. This framework would allow for the use of currenttechniques in a more structured approach to delineating the strength of connection

(a)

(b)

Fig. 3. Stream–hyporheic interactions in the McMurdo Dry Valleys, Antarctica, (a) conceptual model of an activehyporheic exchange zone immediately adjacent to the channel, through which stream water exchanges on timescales of minutes to days, and a more distal zone that is influenced by the stream, but not actively exchanging, infact water is lost from the upper parts of this zone through evaporation from the wetted soil surfaces; (b) image ofPriscu Stream in Taylor Valley, Antarctica. Note the obvious extent of capillary fringe adjacent to the stream channel.



between locations of interest in the subsurface and the stream channel. Further, thisapproach more specifically links the time scale of exchange to the time scale of a processof interest (i.e. denitrification rate, metabolism rate, etc.). This timescale of exchangeapproach will promote some unification among the interdisciplinary hyporheic interests.For example, biological studies of hyporheos rarely account for the flowpath dynamic thatis fundamental to the hyporheic conditions. Following this approach would provide addi-tional information about the residence times and histories of water parcels that reachparticular locations in the subsurface. This approach would, however, require that eithernumerical modeling of flowpaths or stream tracer experiments become more commontools for the proposed informative characterization of the subsurface. The greatest advan-tage of this framework is that comparisons across flow conditions and locations could bemade in a meaningful way, based on the timescales of exchange through the hyporheiczone. This potential for comparison will promote critical advances in hyporheic researchas we seek to articulate the role of the hyporheic zone and its importance to streamecosystems.

Short Biography

Michael Gooseff’s research is focused on the influence of hydrology on ecosystems,particularly in the context of stream–groundwater interactions, climate change, and in polarregions. His field research sites span the world from arctic Alaska to the McMurdo DryValleys of Antarctica. Current interdisciplinary research projects are addressing permafrost

4 hr exchange zone

8 hr exchange zone

Groundwater

Watercolumn

(a)

(b)

(c)

Fig. 4. Conceptual model of proposed division of the hyporheic zone for a hypothetical alluvial aquifer, (a) in planview, (b) in lateral cross-section, and (c) in longitudinal cross-section.



degradation in response to a warming climate in the Arctic, responses of nutrient cycling inarctic river networks in response to changing seasonality, controls of discontinuous snowpack on soil microbial communities and associated biogeochemical cycling in Antarctica,modeling ecosystem processes in Antarctica, and the influence of valley floor hydrology onstream–groundwater interactions during annual stream baseflow recession. Before movingto Penn State University, Gooseff taught at the Colorado School of Mines and Utah StateUniversity. He earned a Bachelors of Civil Engineering at Georgia Tech and MS and PhDdegrees in Civil Engineering at the University of Colorado.

Note

* Correspondence address: Michael N. Gooseff, Department of Civil & Environmental Engineering, PennsylvaniaState University, University Park, PA 16802, USA. E-mail: [email protected].

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defining hyporheic zones – advancing our conceptual and operational definitions of where stream...

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