INVITED COMMENTARYHYDROLOGICAL PROCESSESHydrol. Process. 14, 2797–2798 (2000)

Hyporheic zone hydrological processes

Kenneth E[ Bencala

US Geolo`ical Survey\ Menlo

Park\ CA 83914

Correspondence to]

Kenneth E[ Bencala\ MS 328\

Research*Water\ US Geological

Survey\ 234 Middle_eld Road\

Menlo Park\ CA 83914[

E!mail] kbencalaÝusgs[gov

Hyporheic zones

Hyporheic zones in~uence the biogeochemistry of stream ecosystemsby increasing solute residence times\ and more speci_cally solute con!tact with substrates\ in environments with spatial gradients in dis!solved oxygen and pH[ That is a one!sentence summary of theenvironmental signi_cance of hyporheic zones[ In more depth\ thebiogeochemical implications from 19 years of study of solute trans!port\ retention\ and transformations associated with hyporheic zonesare well!summarized and well!placed in broader frameworks of sur!face waterÐground water interactions in the recent book edited byJones and Mulholland "1999#[

Hyporheic zones\ at whatever scale\ are part of a continuum ofstreamÐcatchment connections between stream water and soil water\root!zone water\ riparian water\ quick!~ow\ delayed!~ow\ macropore~ow\ and so on to including base ~ow[ Drawing further distinctionsto de_ne hyporheic zones would seem obfuscatory\ if not excessive\ ina hydrological framework of surface waterÐground water interactionssuch as presented by Winter et al[ "0887#[ However\ in the frameworkof biogeochemical function\ the matter of scale confounds any attemptat a single de_nition[ In studying the in!stream dynamics of solutes\speci_cally nutrients and metals\ the de_nition that I prefer of ahyporheic zone is {{a subsurface ~owpath along which water {recently|from the stream will mix with subsurface water to {soon| return to thestream|| "Harvey and Bencala\ 0882#[ For studying solute input:output:retention:transformation budgets along the length of a stream\a less hydrologic de_nition\ {{the subsurface zone exchanging at least09) stream water with subsurface water||\ is most commonly given"Triska et al[\ 0878#[ When studying the catchment as the source ofsolutes to a stream\ the hyporheic zone is simply {{the subsurfaceinterface between stream water and subsurface water||\ e}ectively amembrane\ often reactive\ of no physical dimension "Schindler andKrabbenhoft\ 0887#[

Hydrological processes

Continuing advances in knowledge of the hydrological processes ofhyporheic zones are critical to quantitative analysis of stream eco!systems[ I accepted the opportunity to write this Commentary as ameans to encourage further studies to meet this need[ Across the scalesof streamÐcatchment connections there are challenges[ In consideringeach of the three scales mentioned above\ I think of the following asbroad challenges for hydrology relevant to hyporheic zone bio!geochemistry[

This article is a US government work and is

in the public domain in the United States 2797

K. E. BENCALA

, Solute dynamics] The water within a hyporheic

~owpath is an evolving mixture of stream and sub!

surface waters[ Each of the waters contribute dis!

solved gases\ dissolved solutes\ and possibly

colloidal materials[ Can we determine the time

scales of the physical mixing and transport< Do

these time scales control the extent of reactions<

, Stream transport] The scale of the individual\ de_n!

able hyporheic ~owpath will typically be far too _ne

to be the scale used to parameterize the simulation

of stream solute transport[ What physical and

hydrometric properties of a streamÐcatchment sys!

tem determine the characteristics of transport

within a hyporheic zone and can be routinely mea!

sured "or {mapped|# along a stream at distances of

tens of metres\ hundreds of metres\ or a kilometre<

, Catchment sources] The mechanisms of stream~ow

generation "the quantitative speci_cation of how

and when water travels from a catchment to a

stream# remain elusive[ The questions of how and

when solutes travel from a catchment to a stream

are compounded by the potential for reactions

across the hyporheic interface[ We need to further

our understanding of ~ow in the hyporheic zone

from the catchment perspective as well as from that

of the stream[

Hydrol. Process. 14, 2797–2798 (2000)2798

Hyporheic zones form a connection of the streamto its catchment in manners distinct from those of astream cutting through an aquifer from which groundwater discharges into the stream much as water froman array of pipes[ Over a substantial span of scale\ thecommon element in the challenges I have suggested isthat the transport of solutes * by water * betweenbiogeochemical environments is fundamental to thefunction of hyporheic zones[

References

Harvey JW\ Bencala KE[ 0882[ The e}ect of streambed topography

on surfaceÐsubsurface water exchange in mountain catchments[

Water Resources Research 18"0#\ 78Ð87[

Jones JB\ Mulholland PJ\ Eds[ 1999[ Streams and Ground Waters[

Academic Press\ New York\ 314 pp[

Schindler JE\ Krabbenhoft DP[ 0887[ The hyporheic zone as a

source of dissolved organic carbon and carbon gases to a temperate

forest stream[ Bio`eochemistry\ 32\ 046Ð063[

Triska FT\ Kennedy VC\ Avanzino RJ\ Zellweger GW\ Bencala

KE[ 0878[ Retention and transport of nutrients in a third!order

stream in northwestern California] Hyporheic processes[ Ecolo`y\

69"5#\ 0782Ð0894[

Winter TC\ Harvey JW\ Franke OL\ Alley WM[ 0887[ Ground

Water and Surface Water * A Sin`le Resource\ US Geological

Survey Circular 0028\ 68 pp[ http]:water[usgs[gov:pubs:circ:

circ0028: