Hyporheic Zone in Urban Streams: A Review and Opportunities for Enhancing Water Quality and...
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Hyporheic Zone in Urban Streams: A Review and Opportunities for Enhancing Water Quality and Improving Aquatic Habitat by Active Management Justin E. Lawrence, 1,2, * Magnus E. Skold, 1,3 Fatima A. Hussain, 1,4 David R. Silverman, 1,3 Vincent H. Resh, 1,2 David L. Sedlak, 1,5 Richard G. Luthy, 1,4 and John E. McCray 1,3 1 Engineering Research Center for Re-Inventing the Nation’s Urban Water Infrastructure (ReNUWIt), National Science Foundation, Stanford, California. Departments of 2 Environmental Science, Policy, & Management and 5 Civil & Environmental Engineering, University of California, Berkeley, California. 3 Department of Civil & Environmental Engineering, Colorado School of Mines, Golden, Colorado. 4 Department of Civil & Environmental Engineering, Stanford University, Stanford, California. Received: June 15, 2012 Accepted in revised form: May 6, 2013 Abstract Tremendous opportunities exist for enhancing water quality and improving aquatic habitat by actively managing urban water infrastructure to operate in conjunction with natural systems. The hyporheic zone (HZ) of streams, which is the area of active mixing between surface water and groundwater, is one such system that is overlooked by many water professionals, because the state of the science on this topic has not been transferred into practice. As a biogeochemically active zone, the HZ offers great potential to provide natural treatment of organic compounds, nutrients, and pathogens in urban streams, which are often strongly impacted by flow modifications and water pollution. Reliable treatment is most likely in streams in which the majority of flow occurs through the HZ, the flow is aerated, and sufficient residence times occur, which may be limited to specific channel morphologies and seasons. Integration of the HZ into stream management plans could also provide quality habitat in a landscape with increasingly depauperate biodiversity. Here, we review current knowledge on hydrological, chemical, and biological aspects of the HZ, with a focus on urban settings, and include a set of examples drawn from the literature of low-flow, effluent-dominated streams in which there is significant hyporheic flow and potential for contaminant attenuation. The HZ can be incorporated much more effectively into urban water manage- ment, including stream restoration efforts, by understanding the surface and subsurface features conducive to HZ flow and the water-quality and biodiversity improvements that can be gained in the HZ without posing unreasonable risk. The main barriers to implementation of HZ considerations include lack of information, absence of established metrics for evaluating success, small number of controlled HZ experiments in urban settings, and concern over risks to both public health and aquatic organisms. A combination of field studies, laboratory experiments, and model development that consider hydrological, chemical, and biological inter- actions in the HZ can overcome these barriers. Key words: biodiversity; dry season flows; micropollutants; natural systems enhancement; nutrients; subsurface flow; urban water infrastructure; wastewater effluent Introduction U rban streams and rivers have a significant role to play in the reinvention of urban water infrastructure. For example, through implementation of appropriate manage- ment tools, it may be possible to create systems in which streams flowing out of cities are cleaner than streams flowing into them (Daigger, 2007), and key components of the natural flow regime are nearly intact (Poff et al., 2010). Urban stream channels are commonly incised, lowering the ground water level, which can reduce base flow and accelerate channel erosion and sediment production. They also sometimes have impermeable channel linings, which reduces interaction be- tween surface and ground waters. Hydromodification of stream channels, runoff from impervious surfaces, and input of low quality, wastewater effluents are some of the many variables associated with urban growth and development *Corresponding author: Department of Environmental Science, Pol- icy, and Management, University of California, Berkeley, CA 94720- 3114. Phone: 510-642-6315; Fax: 510-642-7428; E-mail: jlawrence@ berkeley.edu ENVIRONMENTAL ENGINEERING SCIENCE Volume 30, Number 8, 2013 ª Mary Ann Liebert, Inc. DOI: 10.1089/ees.2012.0235 480
Transcript of Hyporheic Zone in Urban Streams: A Review and Opportunities for Enhancing Water Quality and...
Hyporheic Zone in Urban Streams:A Review and Opportunities for Enhancing Water Quality
and Improving Aquatic Habitat by Active Management
Justin E. Lawrence,1,2,* Magnus E. Skold,1,3 Fatima A. Hussain,1,4 David R. Silverman,1,3
Vincent H. Resh,1,2 David L. Sedlak,1,5 Richard G. Luthy,1,4 and John E. McCray1,3
1Engineering Research Center for Re-Inventing the Nation’s Urban Water Infrastructure (ReNUWIt),
National Science Foundation, Stanford, California.
Departments of 2Environmental Science, Policy, & Management and 5Civil & Environmental
Engineering, University of California, Berkeley, California.3Department of Civil & Environmental Engineering, Colorado School of Mines, Golden, Colorado.
4Department of Civil & Environmental Engineering, Stanford University, Stanford, California.
Received: June 15, 2012 Accepted in revised form: May 6, 2013
Abstract
Tremendous opportunities exist for enhancing water quality and improving aquatic habitat by actively managingurban water infrastructure to operate in conjunction with natural systems. The hyporheic zone (HZ) of streams,which is the area of active mixing between surface water and groundwater, is one such system that is overlookedby many water professionals, because the state of the science on this topic has not been transferred into practice. Asa biogeochemically active zone, the HZ offers great potential to provide natural treatment of organic compounds,nutrients, and pathogens in urban streams, which are often strongly impacted by flow modifications and waterpollution. Reliable treatment is most likely in streams in which the majority of flow occurs through the HZ, the flowis aerated, and sufficient residence times occur, which may be limited to specific channel morphologies and seasons.Integration of the HZ into stream management plans could also provide quality habitat in a landscape withincreasingly depauperate biodiversity. Here, we review current knowledge on hydrological, chemical, andbiological aspects of the HZ, with a focus on urban settings, and include a set of examples drawn from theliterature of low-flow, effluent-dominated streams in which there is significant hyporheic flow and potentialfor contaminant attenuation. The HZ can be incorporated much more effectively into urban water manage-ment, including stream restoration efforts, by understanding the surface and subsurface features conducive toHZ flow and the water-quality and biodiversity improvements that can be gained in the HZ without posingunreasonable risk. The main barriers to implementation of HZ considerations include lack of information,absence of established metrics for evaluating success, small number of controlled HZ experiments in urbansettings, and concern over risks to both public health and aquatic organisms. A combination of field studies,laboratory experiments, and model development that consider hydrological, chemical, and biological inter-actions in the HZ can overcome these barriers.
Key words: biodiversity; dry season flows; micropollutants; natural systems enhancement; nutrients; subsurfaceflow; urban water infrastructure; wastewater effluent
Introduction
Urban streams and rivers have a significant role to playin the reinvention of urban water infrastructure. For
example, through implementation of appropriate manage-ment tools, it may be possible to create systems in which
streams flowing out of cities are cleaner than streams flowinginto them (Daigger, 2007), and key components of the naturalflow regime are nearly intact (Poff et al., 2010). Urban streamchannels are commonly incised, lowering the ground waterlevel, which can reduce base flow and accelerate channelerosion and sediment production. They also sometimes haveimpermeable channel linings, which reduces interaction be-tween surface and ground waters. Hydromodification ofstream channels, runoff from impervious surfaces, and inputof low quality, wastewater effluents are some of the manyvariables associated with urban growth and development
*Corresponding author: Department of Environmental Science, Pol-icy, and Management, University of California, Berkeley, CA 94720-3114. Phone: 510-642-6315; Fax: 510-642-7428; E-mail: [email protected]
ENVIRONMENTAL ENGINEERING SCIENCEVolume 30, Number 8, 2013ª Mary Ann Liebert, Inc.DOI: 10.1089/ees.2012.0235
480
that impair urban streams, and these stresses are expected tointensify in the future with additional population growth andclimate change (Grischek et al., 2002).
To restore the ecology and aesthetics of urban streams,municipal wastewater effluent and urban runoff should beenvisioned as resources, rather than wastes (Bischel et al., 2013).However, contaminants in these water sources have potentialto affect drinking-water supplies and degrade aquatic life. Forexample, pharmaceuticals (e.g., acetaminophen), personal-careproducts (e.g., triclosan), and consumer products (e.g., per-fluorinated surfactants) can reach aquifers through the hy-porheic zone (HZ). In the HZ, surface water and groundwateris exchanged, chemical transformations of various types occur,and microorganisms and animals live (Boulton et al., 2010;Bianchin et al., 2011). The HZ is increasingly recognized for itspotential to attenuate contaminants (Biksey and Gross, 2001).As a result of this function, it has been metaphorically called theriver’s liver (Fischer et al., 2005).
The HZ provides interrelated ecosystem services related towater quality and aquatic habitat. For example, transforma-tions of redox-active chemicals occur in the HZ, which arepartly facilitated by the suitable conditions that an intact HZprovides (Boano et al., 2010a). When relatively long residencetimes occur in HZs (e.g., hours to days), significant attenuationof contaminants and buffering of water temperature can oc-cur, which can be considered a valuable ecosystem-servicebenefit associated with their management and preservation.Furthermore, microorganisms and animals use the HZ as botha primary and a refuge habitat at different life stages, createhydrologic flow paths that enhance permeability, and alsohave the potential to remove nutrients and trace organicchemicals (Clinton et al., 2010; Wood et al., 2010).
Most studies conducted on the HZ have taken place eitherin pristine headwaters or in agricultural areas rather than inurban areas, which exhibit site-specific dynamics that may notnecessarily be generalized to all streams (Malcolm et al., 2010;Lewandowski et al., 2011). Urban streams are often located inlowlands (where cities are) and can have less turbulent,though higher-velocity (as a result of channelization for floodcontrol) and higher-volume (as a result of flashiness fromimpervious surfaces) flows than mountain streams, and theyoften receive elevated sediment inputs and have reducedcanopy coverage as a result of human development (Walshet al., 2005). Furthermore, the turbulence level would alsodepend on whether the stream is located in a forest (and thusmore likely to be composed of riffle-steps-pools) or in mead-ows (smooth meanders). The discharge of urban streams thatare small or which exist in arid or semi-arid climates may bedominated by effluent from municipal wastewater treatmentplants (WWTPs). In streams receiving municipal wastewatereffluent, the HZ may contain relatively high concentrations ofnutrients and trace organic contaminants, as well as highwater temperatures relative to what is natural for the region(Tufenkji et al., 2002).
The objective of this article is to review the processes in theHZ that influence the fate of nutrients and trace organic con-taminants, as well as the structure and function of ecologicalcommunities in the HZ, and to suggest practical HZ manage-ment strategies for enhancing both water quality and aquaticbiodiversity. We identify (1) gaps in the current understandingof hyporheic processes in urban streams; (2) research needs forachieving contaminant attenuation and ecological improve-
ment; and (3) barriers to the implementation of HZ manage-ment projects. The concepts examined are illustrated with acollection of examples, primarily drawn from low-flow,effluent-dominated streams in urban areas in the westernUnited States and other developed countries in which thereis high population growth and increasing water-demandpressures.
Discussion
Hydrology and the HZ
Many variables have been used to define the HZ, includingthose based on a fixed distance within or from the stream (e.g.,a prescribed depth), ecology (e.g., shifts in floral and faunaldistributions), morphology (e.g., changes in substrate type),chemistry, hydrology, residence time, and combinations ofthese variables (Williams, 1989; Boulton et al., 2010; Gooseff,2010; Bianchin et al., 2011). The definition selected can dra-matically affect the defined size of the HZ (White, 1993). Forexample, the depth of the HZ can range from several centi-meters when bedrock is close to the surface to > 10 m whenstreams flow over a sand or alluvial substrate (Chen, 2011;Stelzer et al., 2011). Moreover, the lateral extent of the HZcan range from just a few centimeters in bedrock-confinedchannels to multiple kilometers in large river systems withwide alluvial floodplains (Lapworth et al., 2009; Sawyer et al.,2009).
In this article, we define the HZ using the hydrologicaldefinition in which flowpaths originate and terminate at thestream, including the concept of residence time (Gooseff,2010). For example, a 24-h HZ would refer to a region inwhich hyporheic water takes 24 h to pass from the streamthrough the subsurface zone of active mixing of surfacewater and ground water and then back to the stream. We usethe term hyporheic exchange in the sense of Harvey et al.(1996) to refer to exchanges of water between channels andthe subsurface at small scales (centimeter to meter). The sizeof the HZ under this hydrological definition can be measuredin the field using tracers (such as NaCl, bromide, or tem-perature), and it fluctuates through space and time as thereservoirs of surface water and groundwater expand andcontract. High-resolution distributed temperature sensingcan be used to quantify spatiotemporal variability in verticalhyporheic flux (Briggs et al., 2012). The residence-time con-cept allows efficient linkage of the HZ to chemical transfor-mation (Fig. 1).
A selection of case studies of low-flow, effluent-dominatedurban streams in which there is documented pollutantattenuation and the HZ is likely a key factor was made toillustrate the range of settings, hydrology, pollution, attenu-ation, and implications for HZ management that are likely tobe encountered in urban areas (Table 1), and additional ex-amples are provided throughout the text to illustrate thatother sites can exhibit similar characteristics. In Upper SilverCreek and Coyote Creek (Table 1), for example, residencetimes in the HZ were observed to range from 15 to 40 minalong the *5 km reach examined. Under these conditions ofshort residence time, little degradation of dissolved trace or-ganic contaminants was expected. In contrast, confirmation ofsignificant natural attenuation of contaminants combinedwith low flows, and presumably long residence times, wasobserved in the Santa Cruz River (Table 1).
ACTIVE MANAGEMENT OF HYPORHEIC ZONES IN URBAN STREAMS 481
Hydrology has been widely recognized to be the domi-nant driver of ecological structure and function in streamecosystems (Resh et al., 1988; Power et al., 1995), and itcertainly affects processes in the HZ (Boulton et al., 2010;Robertson and Wood, 2010). The connection of the HZ withthe stream is best thought of conceptually in four dimen-sions, that is, the three spatial dimensions and time (Ward,1989). As with stream management that is based on thenatural flow regime principle (Richter et al., 1997; Poff et al.,1997; Tharme, 2003), the magnitude, frequency, duration,timing, and rate of change of flows through the HZ are allimportant components that should be considered alongwith water quality for a management program to be trulyeffective in meeting a stream’s ecological needs (Hawkinset al., 2010; Poff et al., 2010).
The hyporheic flow regime can be altered by a variety ofhuman activities, including those that change the streamflowregime or alter the streambed surface or subsurface (Boanoet al., 2010b; Maier and Howard, 2011). For example, thestreamflow regime in the Salt and Gila Rivers is altered byelevated storm runoff from impervious surfaces, upstreamdam releases, and wastewater effluent discharge (Table 1).
The associated reduction in retention time in many urbanstreams (because urban runoff is associated with high veloc-ities and high volumes) can result in reduced infiltration intothe HZ, giving rise to lower relative volumes and residencetimes (Grimm et al., 2005). Channelization, culverting, ar-moring of banks with riprap, and sediment inputs are somecommon activities in urban streams that would also alter thenatural flow regime from surface water to the HZ (Brownet al., 2009; Stranko et al., 2012). Finally, the hydrology of astream has a major impact on the processes in the associatedHZ that affect water quality, including the establishment ofappropriate redox conditions (Lewandowski et al., 2011).
Water quality and the HZ
The HZ forms a unique and dynamic ecosystem that is bothinfluenced by and can significantly influence neighboringstream-water and groundwater quality. The cycling of ele-ments, organic compounds, nitrogen (N), and phosphorous(P) in the HZ has been studied in depth (Findlay et al., 1993;Crenshaw et al., 2010; Lapworth et al., 2011). An under-standing of the fate and transport of these solutes, which is
FIG. 1. Simplified conceptual overview of: (A) hyporheic zone (HZ) management in urban streams encompassing thehydrological, ecological, and water quality considerations associated with wastewater effluent in urban streams and in-cluding the concept of residence time (RT); and (B) microbially mediated pathways for transformation of nutrients andbiodegradation of trace organic contaminants (TrOCs). This figure does not cover all possible reactions in the HZ.
482 LAWRENCE ET AL.
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MD
Fas
low
as0.
04m
3/
s,th
ou
gh
this
can
incr
ease
by
afa
cto
r10
0–10
00d
uri
ng
rain
even
ts.k
MA
Fd
ata
are
un
avai
lab
leat
US
C.
MA
F(1
999–
2010
)in
CC
atM
ilp
itas
,C
A,
was
1.3
m3/
sw
ith
MD
Fas
low
as0.
2m
3/
s(U
SG
Sg
aug
ing
stat
ion
#11
1721
7).
US
Can
dC
Cd
on
ot
curr
entl
yre
ceiv
eW
WT
Pef
flu
ent,
bu
tm
icro
po
llu
tan
tsar
est
ill
det
ecte
din
the
riv
ersy
stem
.k,l
PF
Cs
wer
ed
etec
ted
insu
rfac
ew
ater
,h
yp
orh
eic
wat
er,
and
the
un
der
lyin
gaq
uif
er,
and
neg
lig
ible
atte
nu
atio
nw
aso
b-
serv
ed.k
,lIt
was
hy
po
thes
ized
that
the
stu
dy
reac
hw
asto
osh
ort
tod
etec
tat
ten
uat
ion
.k,l
Th
eH
Zw
asta
ken
asth
ezo
ne
imm
edia
tely
un
der
-ly
ing
the
riv
erb
edw
her
ew
ater
flo
wed
bet
wee
nth
est
ream
and
sub
surf
ace
wit
hre
sid
ence
tim
esin
the
ord
ero
f15
–60
min
asm
easu
red
by
tim
ese
ries
of
riv
eran
dg
rou
nd
wat
erte
mp
erat
ure
.k,l
Rec
alci
tran
tP
FC
sar
ere
adil
ytr
ansp
ort
edfr
om
surf
ace
wat
erin
tou
nd
erly
ing
aqu
ifer
s,k
,lb
ut
the
exte
nt
tow
hic
hth
eyar
eat
ten
uat
edin
the
HZ
rem
ain
sto
be
det
erm
ined
.U
SC
and
CC
may
serv
eas
ah
igh
lysu
itab
lete
stb
edfo
rH
Zat
ten
-u
atio
nst
ud
ies
bec
ause
of
the
var
ied
stre
amb
edm
orp
ho
log
yan
dth
ed
ocu
men
ted
mic
rop
oll
uta
nt
con
tam
inat
ion
.T
he
San
taC
lara
Val
ley
Wat
erD
istr
ict
(San
Jose
,C
A)
con
sid
ered
aug
men
tin
gU
SC
and
CC
flo
ww
ith
tert
iary
trea
ted
was
te-
wat
er,
bu
tth
ep
roje
ctw
asd
isco
nti
nu
edd
ue
toco
nce
rns
ov
erW
WD
C.k
,l
(con
tin
ued
)
483
Ta
bl
e1.
(Co
nt
in
ue
d)
Sit
eS
etti
ng
and
hy
dro
log
yP
ollu
tion
and
atte
nu
atio
nIm
pli
cati
ons
for
HZ
man
agem
ent
Bo
uld
erC
reek
(BC
),B
ou
lder
,C
OB
Cd
rain
sa
1,16
0-k
m2
wat
ersh
ed(5
5cm
/y
ear
mea
nra
infa
ll)
and
isp
eren
nia
l.m
Inth
eci
tyo
fB
ou
lder
,W
WT
Pef
flu
ent
can
acco
un
tfo
rap
-p
rox
imat
ely
77%
of
the
stre
amfl
ow
.M
AF
inB
Cb
elo
wth
e75
thS
tree
tW
WT
P(1
984–
2011
)w
as2.
6m
3/
sw
ith
MD
Fas
low
as0.
025
m3/
sd
uri
ng
the
dry
seas
on
(US
GS
gau
gin
gst
atio
n#
0673
0200
).T
he
75th
Str
eet
WW
TP
dis
char
ges
on
aver
age*
0.65
m3/
so
fef
flu
ent.
m
41W
WD
Ch
ave
bee
nd
etec
ted
inB
C.m
,n,o
Th
een
do
crin
e-d
isru
pti
ng
det
erg
ent
com
po
un
d4-
NP
was
com
ple
tely
min
eral
ized
ino
xic
mic
roco
sms
pre
par
edfr
om
BC
sed
imen
tfr
om
abo
ve
the
WW
TP
,w
hil
em
iner
aliz
atio
nw
assi
gn
ifica
ntl
yd
imin
ish
edin
ox
icm
icro
cosm
sp
rep
ared
wit
hB
Cse
dim
ent
fro
mb
elo
wth
eW
WT
P.p
Nei
ther
mic
roco
sms
pre
par
edw
ith
sed
imen
tsfr
om
abo
ve
no
rb
elo
wth
eW
WT
Psh
ow
edsi
gn
ifica
nt
min
eral
izat
ion
of
4-N
Pw
hen
incu
bat
edu
nd
eran
ox
icco
nd
itio
ns.
pT
he
HZ
was
no
td
irec
tly
stu
die
d.
BC
can
serv
eas
anil
lust
rati
ve
con
tras
tto
eph
em-
eral
stre
ams
inth
ear
idW
est
that
rece
ive
WW
TP
effl
uen
t.H
Zm
anag
emen
tp
ract
ices
for
ast
ream
sim
ilar
toB
Cw
ith
turb
ule
nt
flo
wan
dm
any
recr
eati
on
alu
sers
du
rin
gth
esu
mm
erm
ayd
iffe
rg
reat
lyfr
om
tho
seo
fan
eph
emer
alar
id-l
and
stre
am.
BC
sed
imen
th
assh
ow
nth
eca
pac
ity
toae
rob
ical
lyb
iod
egra
de
mic
rop
oll
uta
nts
,su
chas
4-N
P;
ho
wev
er,
incr
ease
dB
OD
and
low
erD
Ofr
om
WW
TP
effl
uen
td
ecre
ases
effi
cien
cy.p
So
uth
Pla
tte
Riv
er(S
PR
),D
env
er,
CO
Th
eS
PR
dra
ins
ab
asin
of
63,0
00k
m2
(40
cm/
yea
rm
ean
rain
fall
)w
ith
>10
0W
WT
Ps.
qT
he
stre
am-
bed
con
sist
so
fw
ell-
sort
ed,
med
ium
-gra
ined
san
d,
and
hy
po
rhei
cex
chan
ge
isfa
st.r,
sM
AF
(198
3–20
11)
inth
eS
PR
bel
ow
the
city
of
Den
ver
was
7.4
m3/
sw
ith
MD
Fas
low
as0.
34m
3/
s(U
SG
Sg
aug
ing
#06
7115
65).
WW
TP
effl
uen
tac
cou
nts
for
>95
%o
fb
asefl
ow
.q,t
Ho
url
yv
aria
-ti
on
sin
WW
TP
dis
char
ge
rate
sca
use
mea
sura
ble
var
iati
on
sin
riv
erst
age
for
>35
km
do
wn
-st
ream
.t
Ho
url
yfl
uct
uat
ion
sin
effl
uen
td
isch
arg
era
tes
fro
mD
env
er’s
larg
est
WW
TP
dir
ectl
yaf
fect
stre
amst
age,
and
hen
ceh
yp
orh
eic
exch
ang
eo
na
60-k
mst
retc
hd
ow
nst
ream
.tH
igh
erh
yp
orh
eic
exch
ang
era
tes
led
toin
crea
sed
up
tak
eo
fD
Oin
the
HZ
.t
‘‘Hy
po
rhei
cex
chan
ge’
’w
asd
efin
edas
flo
wac
ross
the
sed
imen
t-w
ater
inte
rfac
ean
dw
asd
eter
min
edfr
om
the
ver
tica
lh
yd
rau
lic
gra
die
nt
(bet
wee
nsu
rfac
ew
ater
and
dep
tho
f30
cm)
and
hy
dra
uli
cco
nd
uct
ivit
yo
fb
ott
om
sed
imen
ts.t
DO
sam
ple
sin
the
HZ
wer
eta
ken
ata
dep
tho
f30
cm.t
DO
up
tak
era
tes
by
stre
amb
edse
dim
ent
wer
e2–
5ti
mes
hig
her
than
that
req
uir
edb
ym
od
els
wh
ich
sim
ula
teS
PR
DO
dy
nam
ics,
sug
ges
tin
ga
mo
resi
gn
ifica
nt
role
of
the
HZ
inco
ntr
oll
ing
riv
erD
Od
yn
amic
sth
anp
rev
iou
sly
tho
ug
ht.
tO
ther
stu
die
sh
ave
do
cum
ente
dth
eb
iod
egra
dat
ion
of
end
ocr
ine-
dis
rup
tin
gch
emi-
cals
inS
PR
sed
imen
tm
icro
cosm
s.p
,uT
hes
est
ud
ies
sho
wed
that
4-N
P,
17b
-est
rad
iol,
estr
on
e,an
dte
sto
ster
on
ew
ere
atte
nu
ated
tov
ary
ing
deg
rees
inm
icro
cosm
sp
rep
ared
fro
mS
PR
sed
-im
ent
fro
mab
ov
ean
db
elo
wth
eW
WT
Pw
ith
bio
deg
rad
atio
np
lay
ing
asi
gn
ifica
nt
role
.
Wat
eru
tili
ties
wer
eu
nk
no
win
gly
affe
ctin
gh
yp
or-
hei
cex
chan
ge
pro
cess
esw
ith
ho
url
yfl
uct
uat
ion
so
fW
WT
Pef
flu
ent
dis
char
ge
rate
s.tU
nin
ten
tio
nal
anth
rop
og
enic
effe
cts
on
HZ
pro
cess
essh
ou
ldb
eb
ette
rch
arac
teri
zed
toin
tell
igen
tly
man
age
thes
eco
mp
lex
syst
ems.
As
we
con
tem
pla
teH
Zm
an-
agem
ent,
we
sho
uld
reco
gn
ize
that
man
agem
ent
pra
ctic
esm
ayh
ave
con
flic
tin
gre
sult
s.F
or
in-
stan
ce,
effo
rts
toim
pro
ve
DO
dy
nam
ics
inef
flu
ent-
do
min
ated
riv
ers
mig
ht
con
sid
erre
du
c-in
gh
yp
orh
eic
exch
ang
e.t
Wh
ile
red
uci
ng
hy
po
r-h
eic
exch
ang
em
ayp
rov
ide
ad
esir
edo
utc
om
efo
rD
Od
yn
amic
s,re
du
ced
exch
ang
ew
ou
ldre
sult
inu
nfa
vo
rab
leco
nd
itio
ns
for
the
atte
nu
a-ti
on
of
man
ym
icro
po
llu
tan
ts.
Up
per
Wil
lam
ette
Riv
er(U
WR
),C
orv
alli
s,O
R
Th
eU
WR
dra
ins
ab
asin
of
31,0
00-k
m2
(114
cm/
yea
rm
ean
rain
fall
)an
dfl
ow
so
ver
ap
oro
us
sub
stra
teth
atis
con
du
civ
eto
hy
po
rhei
cex
-ch
ang
e.v
Th
eC
orv
alli
sW
WT
Pd
isch
arg
es8–
10M
GD
into
the
riv
ery
ear
rou
nd
.wT
he
MA
F(2
011)
inth
eU
WR
atC
orv
alli
s,w
as44
4m
3/
sw
ith
MD
Fas
low
as15
0m
3/
s(U
SG
Sg
aug
ing
stat
ion
#14
1716
00).
TM
DL
s,th
eC
ity
of
Co
rval
lis
isco
nsi
der
ing
furt
her
trea
tmen
to
fth
eir
effl
uen
tin
aco
nst
ruct
edw
etla
nd
wit
hsu
bse
qu
ent
SE
Din
toth
eU
WR
via
the
HZ
.wIn
ast
ud
yco
mp
arin
gp
rop
osa
lsto
mee
tT
MD
Ls,
itw
asco
ncl
ud
edth
atad
dit
ion
altr
eat-
men
to
fm
icro
po
llu
tan
ts,
tem
per
atu
re,
and
nu
-tr
ien
tsw
ou
ldo
ccu
rw
ith
pas
sag
eth
rou
gh
the
HZ
,w
hic
hw
asq
ual
itat
ivel
yd
efin
edas
the
zon
eo
fac
tiv
esu
rfac
ew
ater
and
gro
un
dw
ater
ex-
chan
ge
inth
eri
ver
bed
.w
Th
eC
ity
of
Co
rval
lis’
SE
Dal
tern
ativ
eis
ap
ract
ical
exam
ple
of
ho
wn
atu
ral
HZ
pro
cess
esth
atat
ten
uat
eW
WD
Cca
nb
eu
sed
inm
anag
emen
t.If
imp
lem
ente
d,
this
exam
ple
wil
lse
rve
asa
mo
del
for
futu
reH
Zm
anag
emen
tp
roje
cts.
(con
tin
ued
)
484
Ta
bl
e1.
(Co
nt
in
ue
d)
Sit
eS
etti
ng
and
hy
dro
log
yP
ollu
tion
and
atte
nu
atio
nIm
pli
cati
ons
for
HZ
man
agem
ent
Pin
eR
iver
(PR
),A
ng
us,
On
tari
o,
Can
ada
Th
eP
Rd
rain
sa
348-
km
2b
asin
(70
cm/
yea
rm
ean
rain
fall
).x
A60
m-w
ide
PC
Eg
rou
nd
wat
erp
lum
eex
ten
ds
195
md
ow
ng
rad
ien
tfr
om
aD
NA
PL
sou
rce
of
PC
Elo
cate
db
enea
tha
dry
-cle
anin
gfa
cili
ty.x
Th
eM
AF
is2
m3/
s,an
dsu
mm
erb
ase-
flo
ws
can
dro
pto
1m
3/
s.x
,y
Th
en
ear-
stre
amzo
ne,
wh
ich
incl
ud
esth
eH
Z,
stro
ng
lym
od
ified
the
dis
trib
uti
on
,co
nce
ntr
atio
n,
and
com
po
siti
on
of
the
PC
Ep
lum
eb
efo
reb
ein
gd
isch
arg
edin
toth
eri
ver
.xE
xte
nsi
ve
anae
rob
icb
iod
egra
dat
ion
inth
eto
p2.
5m
of
the
stre
amb
edw
aso
bse
rved
,ev
enth
ou
gh
no
bio
deg
rad
atio
no
fth
eP
CE
plu
me
occ
urr
edin
the
up
gra
die
nt
aqu
ifer
.x
Infi
ltra
tio
no
fco
nta
min
ated
gro
un
dw
ater
into
stre
ams
isco
mm
on
inu
rban
area
s.z
Th
eP
Rsc
enar
iosu
gg
ests
that
HZ
man
agem
ent
can
red
uce
con
cen
trat
ion
so
fg
rou
nd
wat
erco
nta
mi-
nan
tsb
efo
reth
eyen
ter
into
riv
ers
inad
dit
ion
toin
crea
sin
gat
ten
uat
ion
of
surf
ace-
app
lied
WW
DC
.
Riv
erE
rpe
(RE
),G
erm
any
Th
eR
Ed
rain
sa
wat
ersh
edar
eao
f21
1k
m2
(55
cm/
yea
rp
reci
pit
atio
n).
Th
est
ream
bed
ism
ade
up
of
clo
gg
edan
dan
aero
bic
fin
e-sa
nd
yse
dim
ents
wit
hh
igh
con
ten
tso
fo
rgan
icca
rbo
nan
dn
utr
i-en
ts.*
,{It
rece
ives
effl
uen
tfr
om
sep
tic
tan
ksp
illw
ays
and
sev
eral
pri
vat
ean
dm
un
icip
alW
WT
Ps.
*T
he
larg
est
WW
TP
dis
char
ges
0.5
m3/
s,w
hic
hco
mp
rise
sas
mu
chas
81%
of
the
stre
amfl
ow
.*T
he
MA
Fis
1.2
m3/
sw
ith
MD
Fas
low
as0.
62m
3/
s.*,{
Mic
rop
oll
uta
nts
,in
clu
din
gcl
ofi
bri
cac
id,
nap
rox
en,
ket
op
rofe
n,
bez
afib
rate
,ib
up
rofe
n,
dic
lofe
nac
,an
din
do
met
hac
inh
ave
bee
nd
etec
ted
inth
eR
E,
and
con
cen
trat
ion
sd
ecre
ased
wit
hd
epth
inth
eH
Zto
ag
reat
erex
ten
tth
anco
uld
be
acco
un
ted
for
by
dil
uti
on
,su
gg
esti
ng
deg
rad
atio
n.*
Sam
-p
les
wer
eta
ken
toa
dep
tho
f1
mb
elo
wth
eri
ver
bed
,an
dth
ed
ilu
tio
nef
fect
was
qu
anti
fied
usi
ng
con
serv
ativ
etr
acer
s(b
ora
tean
dan
thro
-p
og
enic
gad
oli
niu
m).
*
Att
enu
atio
no
fW
WD
Cin
the
HZ
of
anef
flu
ent-
do
min
ated
,u
rban
stre
amw
assu
gg
este
db
yin
situ
mea
sure
men
tso
fd
egra
dat
ion
rate
sin
alo
wla
nd
stre
am.*
Fu
ture
stu
die
sth
atq
uan
tify
atte
nu
atio
no
fW
WD
Cin
the
HZ
wil
lal
soh
ave
toac
cou
nt
for
the
dil
uti
on
fact
or
fro
mth
em
ixin
go
fri
ver
wat
eran
dg
rou
nd
wat
er.
Ref
eren
ces
cite
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485
related strongly to redox conditions, pH, and temperature, iscritical to determine appropriate management practices forwater-quality managers.
Hyporheic flow enhances mass transfer of dissolved or-ganic compounds, nutrients, trace organic contaminants, andterminal electron acceptors [e.g., O2, manganese(IV), iron(III),and SO4
2 - ] to the subterranean habitats of both microorgan-isms and macroorganisms, thereby influencing transforma-tion rates (Harvey and Fuller, 1998; Bernhardt and Likens,2002; Gandy et al., 2007). HZ flow also results in contaminantscontacting subsurface sediments and the integuments ofaquatic biota, such as invertebrates and fish (Smith et al.,1998). Both transformation and sorption can be enhanced inthe HZ under the right conditions (Triska et al., 1993; Paria,2008).
Carbon. The HZ plays a paramount role in carbon cycling(Hendricks, 1993; Fellman et al., 2009). For example, riparianflora and fauna produce dissolved organic carbon (DOC),which supports biochemical processes that are vital to thehealth of streams (Fig. 1). DOC concentration in most unim-paired streams typically ranges up to 5 mg/L, while effluentfrom WWTPs typically contains between 5 and 30 mg/L (Imaiet al., 2002). Thus, organic matter from municipal effluent mayact as an additional source of DOC (Andersen et al., 2004).
Biofilms, that is, compilations of autotrophic and hetero-trophic microorganisms which coat the surfaces of both waterand substrate, in the HZ need DOC for their metabolism,causing the HZ to act as a DOC sink because of carbon uptakeand sequestration that occurs through resident bacteria andinvertebrates (Barlocher and Murdoch, 1989). For example,Findlay et al. (1993) found that the metabolic activity of hy-porheic organisms removed *50% of the DOC in interstitialwater in the HZ underneath a gravel bar of Wappinger Creek,an impacted urban stream in New York. Efficient water ex-change between the stream and the HZ accelerates the me-tabolism and increases consumption of DOC in the hyporheicwater. More information on biofilms is provided in the ecol-ogy section.
Several studies have shown that hyporheic ecosystems areheterotrophic (i.e., microbes use DOC as a C source in additionto fixing CO2 to sustain growth). Thus, the HZ serves as a linkbetween the river, which primarily consumes DOC, and thesurrounding ecosystem that provides the C. A comparisonbetween two small, pristine streams in Tennessee and NorthCarolina showed that relative to gross primary production(GPP), respiration was about 2.4 times greater in the streamwith the larger HZ; the value of GPP in the stream with thelarger HZ was 0.21 g O2/m2 per day compared with 0.32 inthe other stream (Mulholland et al., 1997). The authors con-cluded that metabolism was enhanced in the HZ comparedwith the stream. Heterotrophic conditions have also beenobserved in 4th- and 5th-Strahler stream order rivers in Mi-chigan (Uzarski et al., 2004), wherein an increase in nutrientand O2 supply associated with hyporheic flow was assumedresponsible for respiration.
In contrast, a much lower hyporheic respiration rate rela-tive to GPP was observed in the River Lahn in Germany, inwhich municipal wastewater effluent comprised 30% of baseflow (Ingendahl et al., 2009). At this site, the observed con-tribution of the HZ to riverine ecosystem respiration rates wasonly 14%, compared with HZ respiration rates from 40% to
96% observed in other studies (Naegeli and Uehlinger, 1997;Fellows et al., 2001; Battin et al., 2003). The respiration rates inthe upper 20 cm of sediment were approximately one order ofmagnitude greater than in sediments between 20 and 40 cmdepths. The authors suggested that the eutrophic state of theRiver Lahn enhanced algal primary production in the streamand decreased respiration in the HZ. Wagner and Beisser(2005) investigated the effect of addition of nutrients in a 2nd-Strahler stream order gravel stream, the Oberer Seebach inAustria, on biofilms and hyporheic biota. Both the quantityand quality (i.e., the ability to support diverse communities)of the biofilm increased during the 8 month experiment, andthe response of animals varied and was attributed to thequality of the biofilm; the living space for certain animalsincreased, and some exhibited increased reproductive suc-cess, while the mobility of others decreased.
Consequently, the composition and concentration of or-ganic C can influence the biological activity in the HZ.However, different sites can exhibit contrasting relationships.For example, biofilm growth in hyporheic sediments in StonyCreek in New Brunswick, Canada, was not related to DOCconcentration (Barlocher and Murdoch, 1989). In contrast,Findlay et al. (2003) observed a positive correlation betweenthe bacterial activity in biofilms and DOC concentration inWappinger Creek in New York. These conflicting findings arelikely related to whether C is a limiting nutrient in the eco-system.
The nature of organic carbon can also affect the ability ofthe microbial community to transform trace organic contam-inants, which are described in a later section. For example,microorganisms metabolizing on cattail and duckweed ex-tracts exhibited a better ability to remove certain recalcitrantpharmaceuticals in microcosms compared with microorgan-isms metabolizing wastewater effluent (Lim et al., 2008). Thehighest increase in degradation rate was observed for gemfi-brozil (a drug used to reduce lipid levels in blood), which wasnot degraded in DOC from wastewater effluent. However, itwas almost completely transformed after 6 days in plant ex-tracts, which is a residence time that could occur along deepflow paths in the HZ (e.g., residence times > 10 h were ob-served in a bedrock constrained reach of Lookout Creek inOregon (Kasahara and Wondzell, 2003). Differences betweenthe wastewater-derived DOC and the plant-derived DOC forthe existing communities were assumed responsible for thedifferent attenuation rates. Consideration of plants in additionto microbes may, thus, be important for optimizing contam-inant attenuation in the HZ.
Hydrologic modification can influence the concentrationand composition of DOC in streamwater and hyporheic wa-ter. For example, Westerhoff and Anning (2000) found thatstreams with a natural flow regime had higher DOC con-centrations compared with streams with regulated flow re-gimes characterized by damped temporal variability relativeto natural high and low flows. They also found that DOC instreams dominated by wastewater effluent was composed ofC molecules with lower molecular weight and less complexstructure, which provides poorer substrate for biofilms, andconsequently reduced food quality for the invertebrates andother organisms that feed on these biofilms. Although theorganic C in the HZ was not investigated, the hyporheic watermay be influenced by the stream water, especially alongdownwelling reaches with minimal groundwater exchange.
486 LAWRENCE ET AL.
Consequently, the diversity of the microorganisms and mac-roorganisms in the HZ of urban streams may be limited.
Nutrients. Excess nutrients are a major concern forstream-water quality and ecosystem health, and can result ineutrophication and anoxic conditions in the HZ. Nutrientbalance can be disturbed by discharges from WWTPs, returnflow from agricultural fields, and storm runoff (Withers andJarvie, 2008; Carey and Migliaccio, 2009). Both biotic andabiotic processes in the HZ attenuate N and P and mayprovide an especially important ecosystem service duringdry-season flows when urban streams are subject to thehighest concentrations of these nutrients (Withers and Jarvie,2008).
Denitrification occurs primarily by activities of the anaer-obic microbial community residing within the lower HZlayers (Fig. 1). It is controlled by relative rates in advectionand diffusion of N and O2 from the overlying stream waterinto the bed, with greater denitrification rates occurring underconditions of low oxygen flux (Claessens et al., 2010a, 2010b).Increased NO3
- levels in streams can increase denitrificationrates in sediments if residence times are long enough and thesediment zones are sufficiently voluminous, making the HZ asignificant nitrate sink (Cirmo and McDonnell, 1997). The HZmay develop anoxic conditions that promote denitrification inshallow, deoxygenated sediments of effluent-dominated ur-ban streams that are supplied by municipal wastewater ef-fluent and urban runoff (Schipper et al., 1993; Mayer et al.,2010; Lewandowski et al., 2011). In addition, groundwaterpassing through the HZ may provide significant NO3
- inputsto some surface waters, especially during base flow. Deni-trification rates in the HZ depend highly on the amount oflabile organic C available to serve as a microbial energysource, and, thus, N cycling is linked to C cycling (Holmeset al., 1996; Bernhardt and Likens, 2002; Strauss et al., 2002;Groffman et al., 2005; Birgand et al., 2007).
The efficiency of NO3- removal rates is highly variable and
depends on both the relative amount and the residence time ofstream water flowing into the HZ, and other variables, in-cluding seasonality, water temperature, pH, and riparianvegetation (Cirmo and McDonell, 1997; Groffman et al., 2005;Geza et al., 2010a; Zarnetske et al., 2011). The lowest NO3
-
concentrations typically occur in the summer months whenplant growth is greatest and there is relatively lower flow aswell as reduced nutrient loading (Birgand et al., 2007).Growing plants with their roots in the HZ take up N into theirtissues and thus act as a nutrient sink, but they can subse-quently release the N back into the water column on theirdeath or senescence (Dosskey et al., 2010). In addition to suchbotanic processes, heterotrophic immobilization or assimila-tion of N can decrease nitrification rates, while at the sametime consuming O2 and promoting denitrification (Mulhol-land et al., 2000; Baker et al., 2001; Groffman et al., 2005).
N transport in the HZ is closely linked to hydrologic flowpaths (Cirmo and McDonnell, 1997; Pinay et al., 2009). Thus,information on hydraulic conductivity, grain-size distribu-tion, streambed geometry, and stream velocity is needed toanalyze hyporheic N exchange (Salehin et al., 2004). Althoughlow O2 and increased NO3
- levels promote denitrification inthe HZ (Hill et al., 1998; Kasahara and Hill, 2006), the extent towhich denitrification occurs is determined principally byhydrology. High hydraulic conductivity not only promotes
large exchange volumes, but also limits residence time in theHZ, which is necessary for N removal (Grimaldi and Chaplot,1999). Surface-water storage capacity can also be a main de-termining factor of residence time in N attenuation zoneswithin the HZ (Hill et al., 1998), and denitrification rates arealso influenced by surface features, such as riffle steps, gravelbars, and meanders (Kasahara and Hill, 2006).
P activity is affected by a variety of conditions. For ex-ample, under reducing conditions commonly encountered inO2-depleted zones deep in the HZ of pristine streams, or atshallower depths in effluent-dominated urban streams, ironand manganese oxides dissolve and the metal ions migrateupward until conditions become oxic where they precipitateas hydroxides and oxides ( Jarvie et al., 2008). These freshlyprecipitated minerals, sometimes called the ‘‘oxic cap,’’ havelarge surface areas and can sequester P from the hyporheicwater within the HZ (Lijklema, 1980), thereby acting as abarrier for P release into the water column ( Jarvie et al., 2008).A combination of these mechanisms may be responsible forthe P attenuation observed downstream of a WWTP alongthe River Vene in France where 25% of the P in the effluentwas sequestered in the HZ (David et al., 2011). Conversely,release of P stored in hyporheic sediments of effluent-dom-inated urban streams has also been observed (Haggard et al.,2005).
A decrease in the thickness and the effectiveness of the oxiccap in attenuating PO4
3 - concentrations can result in degra-dation of organic matter in effluent, can consume O2 in hy-porheic sediments, and can cause metal oxides to reductivelydissolve at shallower depths ( Jarvie et al., 2008). For example,growth of sewage fungus caused reducing conditions and Prelease in shallow sediments in a small stream in easternEngland called the Lone Pine Pasture Stream (Palmer-Felgateet al., 2010); more than 30 mg/L P was observed in the HZ ofthis stream.
Biological uptake by bacteria and plants in the HZ has beenshown to remove or transform bioavailable PO4
3 - to lessavailable mineral forms. For example, the proportion of dis-solved to total P in both surface water and groundwater was‡ 90% in a chalk stream, the River Lambourn, in England,while in the HZ of this stream this proportion was < 70% as aresult of creation of particulate and colloidal forms of P(Lapworth et al., 2011). Similarly, greater PO4
3 - uptake wasobserved in a pristine 2nd-Strahler stream order with a largeHZ in North Carolina compared with a similar type streamwith a smaller HZ in Tennessee (Mulholland et al., 1997). Theuptake was, at least partially, related to biological activitywithin the HZ.
Trace organic contaminants. A wide range of these che-micals, which are synonymously referred to as emergingcontaminants, microcontaminants, trace pollutants, and mi-cropollutants, are derived from domestic and industrialwastewater and enter the urban water cycle via both WWTPsand storm water (Gobel et al., 2007; Ternes et al., 2008). Theyare typically defined to include minerals and chemicals thatare present in the environment in trace amounts, which have apotentially deleterious effect on public health or aquatic eco-systems, and they are not typically regulated with regard towater-quality criteria (Sedlak et al., 2000). Their concentrationsrange from ng to lg/L (Plumlee and Reinhard, 2007; Monteiroand Boxall, 2010). Common examples include pharmaceutical
ACTIVE MANAGEMENT OF HYPORHEIC ZONES IN URBAN STREAMS 487
compounds and household-cleaning products. These sub-stances can lead to toxicity in some aquatic biota despite beingpresent at very low concentrations, which presents a real butpoorly understood challenge to water management (Conn etal., 2010; Brar, 2011).
A few reports of unregulated trace organic contaminants inthe HZ are available (Hoehn et al., 2007), but a limited numberof studies report the attenuation of certain regulated, organicwastewater-derived pollutants in the HZ, which may behavein a similar manner, including fuel oxygenates (Landmeyeret al., 2010), tetrachloroethene or perchloroethylene (Conantet al., 2004), and volatile organic compounds (Durand et al.,2007). In general, the few published studies have found thatthe HZ has a high capacity for degradation of organic con-taminants, but that the volume of stream water entering theHZ is small relative to stream flow (Hoehn et al., 2007; Le-wandowski et al., 2011). Thus, long travel distances would beneeded to achieve sufficient residence time to provide water-quality enhancements.
Wastewater effluent is not the only source of trace organiccontaminants to streams and their HZs (Rayne and Forest,2009). For example, storm runoff from impermeable surfacesis a recognized contributor of such contaminants to urbanstreams (Oram et al., 2008; Zushi et al., 2008; Cousins et al.,2011; Grebel et al., 2013). Agricultural activities can releasepesticides, such as pyrethroids and organophosphates, tostreams, and these can flow into urban areas and have nega-tive impacts on biodiversity (Liess and von der Ohe, 2005;Martin, 2009). Nonetheless, wastewater effluent is the pri-mary source of most trace organic contaminants in urbanstreams. Clearly, more research in the area of organic con-taminant interactions with the HZ, particularly in urban oragricultural streams, is warranted.
Managers of water utilities are concerned about potentialrisks, including the possibility of contaminated streamwaterinfiltrating through the HZ to underlying aquifers and nega-tively impacting drinking-water sources. For example, thecommonly used wastewater-indicator compounds ethylene-diaminetetraacetic acid (EDTA) and naphthalene dicarboxylicacid (NDC) were found in drinking water production wellslocated at 1.8 km and 2.7 km downgradient from the SantaAna River in California and also in a monitoring well adjacentto the river (Ding et al., 1999) (Table 1); EDTA concentrationsmeasured in this study ranged from 0.1 to 6.3 lg/L, and NDCconcentrations ranged from undetectable to 1.5 lg/L. Incontrast, other commonly used indicators such as nitriloaceticacid and alkylphenoxy ethoxylate carboxylic acids were at-tenuated more rapidly in a soil-aquifer-treatment system andwere, thus, not encountered in the production wells in thisstudy. Concerns about aquifer contamination through the HZarising after the detection of perfluorochemicals prevented aplanned water reuse project in Coyote Creek, California (Ta-ble 1). However, some aquifers underlying urban areas arealready contaminated from legacy impacts, and downwellingof highly treated recycled water may actually improve water-quality conditions (Bischel et al., 2013).
Laboratory experiments have shown that biodegradationof trace organic contaminants occurs in hyporheic systems,and biodegradation may have been due to faster exchange,larger HZ area, longer residence time (e.g., due to deeper HZcaused by greater pressure head), or some combination ofparameters. Under aerobic conditions in a batch experiment
composed of water and sediment collected from the RoterMain, a river in Germany, biodegradation and sorption at-tenuated pharmaceuticals with half-lives ranging from 3 to328 days, suggesting that biodegradation in HZ sediments isan important pharmaceutical-attenuation mechanism (Loffleret al., 2005). In another study, laboratory flume experimentssimulating flow in this river found significant degradation ofbezafibrate, gemfibrozil, and ibuprofen when the exchange ofsurface water and pore water in the upper 20 cm of sedimentswas experimentally enhanced (Kunkel and Radke, 2008).Increases in the flow velocity in the flume created more oxi-dizing conditions and enhanced advective transport of con-taminants into the sediments, which accelerated degradation.Half-lives ranged from 1.2 to 6.9 days, and rate limitedtransfer was observed at low streamflow.
Apparently conflicting results obtained from laboratorystudies and field studies may be related to the difference ingeochemical conditions in the HZ. For example, in a follow-upfield study of pharmaceuticals in the Roter Main, only limitedattenuation was observed over a reach representing a traveltime of 6–30 h under anoxic conditions (Radke et al., 2010),which is a marked difference from the outcomes of the labo-ratory studies that were conducted. In situ measurements inthe field component of this study showed that photolysis wasof limited importance, and naproxen was the only pharma-ceutical observed to photodegrade with a half-life of *4 days.
Reactive tracers were used to assess attenuation of phar-maceuticals in the Sava Brook in central Sweden. Six com-pounds were studied, and only ibuprofen and clofibric acidwere attenuated with half-lives of < 0.5 day and 2.5 days,respectively (Kunkel and Radke, 2011). Due to the limited HZexchange through the fine-grained sediments in the stream,the authors hypothesized that the observed degradation re-sulted from biofilms growing on submerged macrophytesand sediment in the uppermost (*7 cm) HZ layer. In theRiver Erpe, which consists of *80% effluent, some attenua-tion of pharmaceuticals was observed in the top 100 cm of theHZ (Table 1), but the details of this attenuation were difficultto elucidate because of varying surface water composition.
The variability in attenuation rates of trace organic con-taminants observed among stream studies indicates largedifferences between streams in surface, hyporheic, andgroundwater conditions, illustrating the importance of site-specificity, and perhaps explaining some of the variability inthe concentrations of trace organic contaminants observedamong various aquatic reservoirs. The presence of O2 as anelectron acceptor appears to be very important for the bio-degradation of many trace organic contaminants, which oftendegrade aerobically (Meakins et al., 1994). Thus, degradationmay not be optimal in some effluent-dominated urbanstreams that have a relatively large O2 demand. The differentresults observed also highlight the lack of mechanistic un-derstanding of attenuation in both surface water and HZs.Aerobic and anaerobic degradation rates, and the rate ofsolute transport in the HZ, are poorly characterized.
Redox conditions. Oxygen depletion leading to localizedanoxia in the HZ occurs for multiple reasons, both abiotic andbiotic. These can include reductions in O2 concentrations in thesupplying stream water, reductions in infiltration through thestreambed because of reduced permeability (i.e., through sil-tation or channelization), switch from a downwelling to an
488 LAWRENCE ET AL.
upwelling condition as a result of changes in the groundwaterhead, increases in ambient temperatures that reduce O2 disso-lution, high levels of nutrients or biological O2 demand leadingto enhanced biological production and subsequent O2 deple-tion, and changes in the rates or pathways of microbe-mediatedchemical transformations because of alterations in environ-mental conditions (Heffernan et al., 2008; Nogaro et al., 2010). Ofparticular concern in urban streams are impermeable concreteflood-control channels that completely disconnect streams fromtheir HZ, thereby entirely eliminating the O2-supplying func-tion of stream water (Bernhardt and Palmer, 2007).
The redox conditions in the HZ represent a dynamic bal-ance between the influx of O2 as well as other electron ac-ceptors, and DOC, which consumes O2 as it undergoesbiodegradation. For example, turbulent flow aerates streamwater, resulting in O2 concentration near saturation, 8–10 mg/L, depending on water temperature. Stream waterDOC concentrations typically range from 1 to 20 mg/L(Worrall et al., 2004). In contrast, groundwater usually con-tains dissolved O2 concentrations around 1 mg/L, and DOC<1 mg/L (Malard and Hervant, 1999; Chapelle et al., 2012).The rate of organic C degradation depends on the chemicalstructure of the DOC, and highly aromatic organic matter ismore persistent (Thurman, 1985). Stream water is said to havea net positive O2 demand if the O2 demand from DOC exceedsthe dissolved O2 level. Continuous aeration of stream waterfrom turbulent mixing can keep O2 levels high, and turbulentmixing is enhanced by higher streamflows. Therefore, therelative mixing of the surface water and groundwater in theHZ largely determines the redox conditions, because a greatersurface water contribution typically results in more aerobicconditions.
The upper strata of hyporheic sediments is typically aerobicbecause of infiltration of aerated surface water, while bio-logical degradation consumes O2 and creates reducing con-ditions in the deeper sediments. Oxidation of DOC isthermodynamically most favorable with O2 as the primaryelectron acceptor followed by NO3
- , manganese oxides, iron(oxy)-hydroxides, and sulfate, in order of decreasing energyreleased during reaction. When O2 in the sediments is con-sumed, NO3
- is converted to N2 gas, followed by reductivedissolution of manganese and iron oxides and consumption ofsulfate. In pristine streams, the oxygenated area of the HZtypically extends to greater depths than in lowland streams,which exhibit less turbulence and aeration, and typically havefiner grained sediments (i.e., sands, silts, and clays) that aremore prone to clogging and thus less hyporheic exchange(Findlay, 1995; Boulton et al., 2011). Hill et al. (1998) observedthat O2 concentrations declined with depth within the HZ ofan agricultural stream in Ontario, Canada; the concentrationsmeasured were greatest at the head of riffles where streamwater infiltrated. Similarly, other researchers have observedchanges in O2 concentration in the HZ caused by down-welling stream water (Hancock et al., 2005). In anotherlowland stream that receives a high wastewater supply, O2
consumption within the upper millimeters of the stream-bed sediments created reducing conditions in the HZ (Le-wandowski et al., 2011).
Redox conditions vary significantly in the HZ, and stronglyaffect biological activity. Given the appropriate redox condi-tions for a particular contaminant, efficiency of biodegrada-tion in the HZ is enhanced by microbes and potentially
facilitated by burrowing animals, such as benthic macro-inverterbrates. Biodegradation rates can be much faster forsome pharmaceuticals under aerobic than under anaerobicconditions (Groning et al., 2007; Kujawa-Roeleveld et al., 2008;Musson et al., 2010). In laboratory experiments, faster atten-uation rates of selected pharmaceuticals that degrade aero-bically have been observed in the upper part of the HZ(Kunkel and Radke, 2008). Respiratory and hydrolytic activityof bacteria is strongly linked to O2 consumption, and thisbacterial activity is generally greater within oxygenated sed-iments, which has been demonstrated in slow-filtration col-umn experiments (Mermillod-Blondin et al., 2005). In a studyof the White Clay Creek in Pennsylvania, hyporheic respira-tion accounted for 41% of whole ecosystem respiration withmost of the biological activity documented in the upper 30 cmof the sediments in reaches where HZ exchange was observed(Battin et al., 2003). Burrowing animals in the HZ createpreferential flow channels that can enhance infiltration,thereby extending the aerobic zone to deeper sediments(Boulton et al., 1998).
Generally, redox conditions in the HZ change along astream reach, and stream water will access both aerobic andanaerobic compartments. Thus, favorable conditions for bio-degradation can exist for any contaminant provided residencetimes in the appropriate redox zones are sufficient. However,most trace organic contaminants degrade more efficiently inaerobic conditions (Meakins et al., 1994; Angelidaki et al.,2000), which tend to occur in the HZ during higher streamflows or under stream channels with surface features such aslarge boulders and large woody debris that promote turbulentmixing. During high flows, the volume percent of streamflowthrough the HZ can be small, which would limit attenuation.Thus, reliable removal of contaminants is more likely instreams in which both the majority of flow occurs through theHZ and the flow is aerated, which may be limited to specificmorphologies (e.g., course grained substrates, riffle-poolsequences, channel meanders, and other complex surfaceelements).
Metal concentrations in the HZ are also primarily con-trolled by redox conditions. For example, under reducingconditions, manganese oxides and iron (oxy)hydroxides dis-solve, releasing manganese and iron to the stream; however,manganese and iron oxides have a strong affinity to bindother metals, and, thus, they can be co-transported (Dzombakand Morell, 1990; Harvey and Fuller, 1998; Smedley andKinniburgh, 2002). In contrast, infiltration of aerated surfacewater causes dissolved manganese and iron to oxidize andprecipitate as oxide and hydroxide minerals with other metalsco-precipitating or sorbing to freshly oxidized surfaces.
The HZ may prevent groundwater contamination becauseof its redox chemistry. For example, in mining impacted areas,the HZ has provided a barrier to metals (Gandy et al., 2007).At the Silver Bow Creek in western Montana, acidic, metal-contaminated groundwater mixed with stream water andiron formed oxides in the HZ to a depth of 80 cm, whilemanganese precipitated in the shallower, more highly oxi-dized sediments (Benner et al., 1995). In this study, Benneret al. (1995) suggested that the geochemical boundary of ironcorresponded to the interface between the groundwater andthe deeper HZ, while the geochemical boundary of manga-nese corresponded to the boundary between the water col-umn and the shallower HZ.
ACTIVE MANAGEMENT OF HYPORHEIC ZONES IN URBAN STREAMS 489
Acidity. The acidity in the HZ, which is typically re-presented as the pH, is a relatively easy-to-measure mastervariable that affects chemical and biological processes. Forexample, metal sorption to sediment tends to decrease withpH (Stumm and Morgan, 1996). The pH of the HZ is a func-tion of the relative acidities of the contributing groundwaterand surface water, and it is also correlated to other parame-ters, such as O2 and DOC. Groundwater is typically wellbuffered near neutral pH, although mining impactedgroundwater can be more acidic, and, therefore, the pH of thehyporheic water in mining areas may be lower (Gandy et al.,2007).
Oxidative precipitation of metal hydroxides consumes O2
and hydroxide ions, thus lowering the pH. In the Pinal CreekBasin in Arizona, for example, manganese-contaminated,acidic groundwater continually mixed with oxygenatedneutral stream water, removing 20% of the manganese load inthe HZ (Harvey and Fuller, 1998). The increase in O2 and pHwithin the HZ of this system controlled the microbially en-hanced process of manganese oxidation. In contrast, reductivedissolution of metal oxides can increase pH. Petrunic et al.(2005) found that pH increased as microbially mediated re-ductive dissolution of manganese oxides occurred in columnexperiments that were designed to simulate conditions ofaquifer recharge by rivers.
The pH can also be lowered in the HZ by production oforganic acids by microbes during their processing of organicmatter. For example, in a temperate forested stream innorthern Wisconsin that receives large seasonal inputs of leaflitter, the pH decreased from 8.5 to below 7.0 within the HZ(Schindler and Krabbenhoft, 1998). This drop in pH wascorrelated to increases in methane and DOC, and to the bio-chemical activity of microbes processing the leaves.
Temperature. Water temperature is an important deter-minant of environmental conditions in the HZ because of itsdirect correlation with both O2 levels and biochemical reac-tion rates and pathways (Hatch et al., 2006; Schmidt et al.,2006; Webb et al., 2008). Emerging technologies for high-res-olution measurement of water temperatures in the HZ includedistributed sensors that rely on fiber optics (Briggs et al., 2012).The temperature of hyporheic water reflects the relative con-tributions of both surface water and groundwater (Krause etal., 2009; Rau et al., 2010). Surface-water temperatures followambient air temperatures with a relatively short time lag, andthe temperature of smaller streams is more responsive to localatmospheric microclimates than the temperature of largerrivers because of their lower thermal mass (Nelson and Pal-mer, 2007). In contrast, groundwater temperatures remainrelatively constant near the mean annual air temperature;however, smaller perched groundwater reservoirs may beinfluenced by stream temperature fluctuations (Chu et al.,2008).
The temperature regime of the HZ is, thus, responsive tohuman alterations to stream-water temperatures, which oc-cur through a variety of mechanisms at different spatio-temporal scales, including WWTP discharges, dam releases,and climate change (Kaushal et al., 2010). Wastewater efflu-ents are typically warm relative to natural rainfall, snow-melt, or groundwater and can, thus, raise streamtemperatures (Kinouchi et al., 2007). The temperature ofwater released from dams depends largely on whether it is
released from the bottom or the top of the impounded res-ervoir (Olden and Naiman, 2010). Climate change will likelyincrease regional streamwater and, consequently, hyporheicwater temperatures, with consequent effects on ecologicalcommunities (Lawrence et al., 2010). Further, climate-induced changes in the western United States may have ledto widespread forest infestations that influence the temper-ature, hydrology, and ecology of these urban corridors(Mikkelson et al., 2013).
Ecology and the HZ
The HZ is an important habitat for many freshwater or-ganisms, which are often referred to as the hyporheos whenthey make use of this subterranean zone (Boulton et al., 1998;Wood et al., 2010). Some of these organisms only use the HZtransiently as a refuge when the conditions on the surfacebecome unfavorable, such as during floods, droughts, or pe-riods of impaired water quality or high predation pressure(Stubbington et al., 2011). Others use the HZ exclusively, andthese have been observed to exhibit a higher level of ende-mism relative to biota in surface-water habitats because of therelatively lower level of disturbance (Gibert and Deharveng,2002). In some cases, the ecological connection between thesurface water and the HZ has been found to be very weak (i.e.,the aquatic organisms do not appear to migrate between thesetwo reservoirs), but often this is not the case. The reason forthis lack of connectivity is not well understood, but it may berelated to the highly variable nature of subsurface flow pathsand also to the unique adaptations of biota, which in manylocations are highly site specific (Dole-Olivier, 2011; Younget al., 2011).
The HZ is also an important component of the life cycle ofmany of these biota, including fishes, macroinvertebrates, andamphibians (Lopez-Rodrıguez et al., 2009; Resh and Rosen-berg, 2010; Williams et al., 2010). For example, many salmonidfishes deposit their eggs in the HZ, and egg survival dependson localized downwelling of streamwater to both meet oxy-gen requirements and provide nutrients during early matu-ration (Soulsby et al., 2009). Downwelling may expose eggs tochemical contaminants in wastewater-effluent-impactedstreams, which is a significant ecological concern. Manyaquatic insects use the HZ during their young larval stagesand before emerging from both the HZ and the stream tooccupy terrestrial habitats during their adult life stages(Reynolds and Benke, 2012). Amphibians, such as salaman-ders, may use the HZ to forage, escape predation, obtainrefuge from low streamflows, and also for nesting (Feral et al.,2005).
To survive in the HZ, biota must have specialized biolog-ical traits, including the ability to burrow through the sub-strate, navigate in the absence of light, and obtain sustenancefrom the relatively limited food sources available, such asmicrobes and partially decayed organic matter (Bonada et al.,2007; Mueller et al., 2011; Datry, 2012). For example, macro-invertebrates with burrowing ability often have relativelystrong forearms to dig through sediments, and those thatspend most of their time in the HZ often have reduced eyes orlack of eyes altogether as a result of the light limitation(Walters, 2011). Small body size is another common trait forhyporheic organisms as an adaptation to both the low flowand the low food availability (Boulton, 2007).
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The hyporheic fauna are sampled using a variety of de-vices, and the device selected affects the densities measuredand also the proportion and size of the taxa observed. Forexample, hyporheic sampling devices can include nonfrozenor frozen sediment corers, colonization chambers filled withartificial substrates, and pumps or bailers attached to stand-pipes or wells (Fraser and Williams, 1997). Since the numbersof organisms are often extrapolated and then expressed in cm3
or m3, the densities reported can be very high. Microorgan-isms are also an important component.
Biofilms serve an important function in water-quality en-hancement through natural processes (Cardinale, 2011; Hsuet al., 2011; Writer et al., 2011). They have a three-dimensionalmatrix that supports interconnected communities of algae,bacteria, and fungi (Lear and Lewis, 2009; Augspurger et al.,2010; Pohlon et al., 2010). The maintenance of the biofilm isperformed naturally by grazers, typically macroinvertebrates,which keep the constituent communities in an active growthstate that provides optimal conditions for processing bothorganic and inorganic compounds as well as other chemicalconstituents in the aqueous environment, such as trace metals(Sabater et al., 2002).
The important role of biofilms in WWTPs and in con-structed wetland systems is widely recognized (McBride andTanner, 1999; Toet et al., 2003; van den Akker et al., 2011;Jasper et al., 2013), but their role in regulating processes in theHZ is not nearly as well understood. For example, the com-munities comprising biofilms are responsible for many of theimportant redox reactions that result in the transformation oftrace organic contaminants (Gadd and White, 1993; Hockinand Gadd, 2003). Similar processes likely occur in the HZ;however, the community composition of biofilms in the HZmay be different than in surface waters.
Oxygen depletion can have dramatically negative effectson the life cycles of the hyporheos (Malard et al., 2002; Sabaterand Tockner, 2010; Tomlinson and Boulton, 2010). For ex-ample, salmonid eggs, which are buried in the HZ in struc-tures commonly known as redds, will fail to develop properlyin the absence of O2, resulting in reduced fecundity of salmonpopulations (Tonina and Buffington, 2009). In addition,macroinvertebrates in the HZ are highly sensitive to O2 in theinterstitial water, and their populations can be dramaticallyreduced or even completely extirpated if the local O2 supply ishighly curtailed or eliminated (Olsen et al., 2010).
Excessive deposition of fine sediment is a byproduct ofhuman development and has had dramatic negative effects onthe hyporheos, as well as on the surface-water biota, of manyurban streams (Paul and Meyer, 2001; Coleman et al., 2011).Increased siltation is often associated with conversion of for-est lands to other uses or with construction of roads andbuildings in urban areas. Fishes, macroinverterbrates, andamphibians have been observed to be adversely affected bysiltation effects (Yamada and Nakamura, 2009; Larsen andOrmerod, 2010; Sternecker and Geist, 2010; Sullivan andWatzin, 2010; Louhi et al., 2011). Increased sediment instreams from urban development may also lead to increasedP levels in stream waters (Geza et al., 2010b).
The deleterious effects of siltation result largely from theclogging of interstitial spaces between sand grains, gravels,and cobbles on the streambed (Arnon et al., 2010; Song et al.,2010). With pore spaces clogged, streamwater is no longerable to downwell into the HZ, which can cause the HZ to
become water stressed (Kasahara et al., 2009). In addition, finesediment can coat the gill surfaces of organisms, reducing O2-uptake efficiency and compounding the already deleteriouseffects of the associated reductions in O2 delivery (Kemp et al.,2011). However, many invertebrates and other organisms canrestore permeability in the streambed through the process ofbioturbation, which describes the formation of preferentialflow paths and the mixing of sediment that occurs as a resultof burrowing activities (Nogaro et al., 2010).
Many freshwater biota have physiologically optimal tem-perature ranges and if the local conditions are outside of thisrange, then the biota will be stressed or perhaps even elimi-nated (Li et al., 2011). Moreover, if water temperatures areoutside of the range of natural variability for a sufficientlylong time, nonnative species can overtake native species andreductions in biodiversity can occur (McDermott et al., 2010).Consequently, Olden et al. (2006) argue that watershed man-agers should aim at restoring not just natural flow regimes,but natural thermal regimes as well.
Active management of the HZ
Effluent-dominated, low-flow urban streams. These streamspresent a great opportunity to evaluate the efficacy of the HZto improve water quality or serve as a barrier against aquifercontamination. Streams during low-flow conditions, whichcan have long residence times and a high percentage of thetotal streamflow passing through the HZ, appear more likelyto exhibit measureable attenuation via sorption and biodeg-radation, provided redox conditions are suitable (e.g., suffi-cient O2 is available for trace organic contaminants thatundergo aerobic degradation).
Consequently, we believe that there is promise for water-quality enhancements for effluent dominated streams in aridenvironments, particularly during the late summer, fall, andearly winter, when streamflow is relatively low and HZ flowsmay be more significant. For example, recycled water addedto dry urban streambeds, which are often dry because of an-thropogenic activities such as ground-water pumping, mayenable revitalization of the ecology of these areas while en-hancing water quality of urban streams. We also believe thatengineering HZs in reconstructed urban streams may havepromise for providing water-quality enhancement.
Examples from the literature of low-flow, effluent-dominated urban streams with documented pollutant atten-uation illustrate implications for active management in a va-riety of geographic settings and under a range of scenariosrepresenting different HZ characteristics (Table 1). In theseurban basins, mean annual rainfall ranged from 33 to 114 cm,watershed areas varied from 48 to 63,000 km2, and mean an-nual streamflows were measured between 0.084 and 444 m3/s(Table 1). The measurements collected at these sites includedextensive hydrogeologic surveys and/or intensive analyses ofnutrients and trace organic contaminants.
Stream restoration. Restoration projects in streams areincreasingly being implemented to counteract the escalatingnegative effects of urbanization (Kondolf and Micheli, 1995;Ebersole et al., 1997; Palmer et al., 2009), but they typicallytarget surface features of streams without directly consid-ering the effects on the underlying HZ in the project designs(Hester and Gooseff, 2010). These projects often includechannel reconfigurations (i.e., installing meanders),
ACTIVE MANAGEMENT OF HYPORHEIC ZONES IN URBAN STREAMS 491
establishment of riffle-pool sequences, placement of struc-tures to increase physical-habitat complexity (i.e., installingboulders and large wood), selective planting of riparianvegetation, re-stocking of imperiled aquatic species, gravelreplenishment, and gravel cleaning operations (Sarriquetet al., 2007; Meyer et al., 2008; Bernhardt and Palmer, 2011).Gravel cleaning operations in the form of managed high-flow releases are intended to cleanse the bed of fine sedimentand may be especially relevant to WWTP-effluent projects assuggested by the example of the Santa Cruz River in Arizona(Table 1). Some of these practices may also have serendipi-tous benefits to contaminant removal because of the aerationfunction they provide.
Many of these restoration techniques that are applied at thesurface can directly affect the HZ. For example, installation ofboulders and large wood will promote localized increases inthe water-surface elevation (i.e., will effectively create smallwater-storage reservoirs), which will cause greater infiltration(potentially of oxygenated water) into the HZ provided thestreambed is sufficiently permeable (Lautz and Fanelli, 2008;Scordo and Moore, 2009; Boulton et al., 2010). Many types ofriparian vegetation will also enhance infiltration of water intothe HZ, because their roots create macropores that can act assubsurface flow paths (Dosskey et al., 2010), but these sameplants can also withdraw large volumes of water throughevapotranspiration and thereby reduce the total volume of thehyporheic reservoir (Pollen-Bankhead and Simon, 2010).However, evapotransporation may also potentially enhancedownwelling, because water removed by plants may be re-placed by stream water.
Benthic macroinvetebrates are also recognized as playingan important bioturbation role in the HZ and can, thus, en-hance permeability. They can break down particulate organicmatter into smaller pieces through their food-collection ac-tivities (which can be then be more easily consumed by mi-crobes) (Collins et al., 2007), and they can serve as a usefulindicator for the success of ecosystem recovery and restora-tion (Del Rosario and Resh, 2000). Numerous structural andfunctional metrics are available for determining the biologicalintegrity of the ecosystem based on macroinvertebrate com-munity composition (Sponseller et al., 2010; Wesener et al.,2011). Riverine wetlands and the lateral components of theHZ into the floodplain are important sources and sinks formany macroinvertebrates (Paillex et al., 2009).
Nutrients and trace organic contaminants are a concern forhuman health and biodiversity that should be considered instream restoration, and the HZ can play an important role(Higgins et al., 2010; Williams et al., 2010; Biksey et al., 2011;Lewandowski et al., 2011). For example, contaminated streamwater can pass through the HZ to groundwater aquifers,which are often used for drinking water (Krause et al., 2011).In contrast, contaminated water can also travel fromgroundwater to streams, and the example of the Pine River inOntario, Canada, where extensive anaerobic biodegradationwas observed as occurring in the top 2.5 m of the streambed,suggests that the HZ can provide an effective layer of pro-tection (Table 1). The management outcomes could be par-ticularly relevant to urban streams located near leaking(exfiltrating) underground sewers, which are common inmany older cities. Public health concerns exist related to traceorganic contaminants (although the risk has been found to bevery low in most cases examined), and there is some evidence
of potential harm to fishes and other freshwater biota that areexposed to such contaminants (Glassmeyer et al., 2005; Vajdaet al., 2008). In Boulder Creek, Colorado (Table 1), antide-pressants were measured in native white suckers (Catostomuscommersoni) and in HZ sediments (Schultz et al., 2010). Thiscreek also serves as a heavily used recreational area for thelocal human community, and public health is thus highlyrelevant.
Management opportunities. A variety of HZ manage-ment techniques for water reuse for stream restoration areenvisioned that require a more thorough understanding of theinteraction between the hydrology, chemistry, and ecology ofthe HZ in urban-stream systems. The stream is not just a pipeconveying water to a larger pipe, as is sometimes conceptu-alized by wastewater managers (Bencala et al., 2011). It hasboth ecological and aesthetic value. In water-stressed streamsand their associated HZs, reclaimed water can serve as awater supply for flow augmentation to provide furthertreatment or restore ecological condition (Fig. 1), particularlygiven the unlikely availability of alternative supplies of sur-face water or groundwater in the future.
The reclaimed water could be percolated throughstreambeds, stream banks, or floodplains (Fig. 1), dependingon the site-specific conditions, to achieve objectives that areimportant to local stakeholders. Surface features, such asriffle-pool sequences and large-wood installations, could beengineered to increase residence time. If sufficient energy isavailable at a low cost, water could be pumped from shallowwells downstream and injected back into the system up-stream to achieve greater residence time over a shorterstream reach. The example of the Willamette River in Cor-vallis, Oregon, suggests that subsurface effluent dischargeinto the HZ, preceded by treatment in constructed wetlands,can provide additional treatment and thus an additionallayer of protection before flows reach the river (Table 1).Engineered elements could be incorporated in the streamchannel to enhance aeration, and effluent discharges couldbe tailored to match ecologically significant components ofthe water-quality and flow regimes. Lastly, the relationshipof the HZ with physical channel modifications, such as checkdams that raise ground water levels, reduce channel erosion,and reduce local stream gradient, as well as the removal orreplacement of impermeable channel linings, should beconsidered. Such physical modifications are already beingimplemented for ecological, recreational, and other purposes(e.g., Strawberry Creek in Berkeley, California) (Charbon-neau and Resh, 1992).
As a potential engineered natural system to complementWWTPs, water could be routed from a treatment facility to astream channel where the substrate composition of HZs couldbe designed as originally suggested by Vaux (1968) and re-cently revisited by Ward et al. (2011), possibly using recycledconstruction debris or gravel imported from an externalsource. Alternatively, the water could be delivered to an un-derutilized channel or a paleochannel (i.e., an ancient, cur-rently inactive stream channel) where the substratecharacteristics are amenable to water-quality enhancement. Inaddition, carbon filters with higher hydraulic conductivitythan the surrounding substrate could potentially be added toHZs to enhance treatment and could be designed so that theycould be removed periodically for cleaning or replacement,
492 LAWRENCE ET AL.
although this technique has not been tested. Manipulationsthat promote favorable HZ redox conditions should also beconsidered in addition to these structural applications. Wepropose that engineering enhanced contaminant removal inartificial or modified HZs is analogous to the accepted prac-tice of reclaimed water treatment in constructed wetlands,providing both water-quality improvement and habitat en-hancement.
Barriers and research or logistical needs. To optimizeimplementation of active HZ management strategies, fur-ther knowledge is needed of how the associated flow aug-mentations or substrate modifications might affect drinking-water reservoirs and the diverse ecological communities inthe subsurface (fungal, microbial, floral, vertebrate, and in-vertebrate). The ecological communities in the HZ have notreceived as much attention scientifically as their counter-parts on the surface, but they deserve equal attention inbiodiversity conservation efforts. This idea should be in-troduced into the public consciousness. These ecologicalcommunities certainly play a paramount role in the regula-tion and maintenance of hydrologic flow paths, chemicalreaction rates, and contaminant fate in the subsurface HZenvironment.
The concept of active HZ management has been applied inonly a very limited number of cases in urban areas, and datacollection from these cases was often minimal, thus limitingour ability to evaluate the success of these projects (Table 1). Itappears that the focus of research in the near term should beon effluent-dominated, low-flow streams in which attenua-tion of nutrients and degradation of trace contaminants islikely to be reliably high. A related issue is that metrics formonitoring conditions in the HZ are not well established, and,thus, appropriate metrics of both water quality and ecologyshould be developed. Moreover, research will need to addresslegitimate concerns that exist over the potential for bothstreamwater and groundwater contamination which couldoccur if HZs are intentionally made more permeable or filledto higher capacity. In addition to the contaminants described,the HZ is also involved in trafficking of pathogenic bacteria,which could have public-health implications (Grant et al.,2011).
New laboratory experiments and field studies should in-form future model development. A wide variety of modelshave been adapted and applied to the HZ (Runkel, 1998;Lautz and Siegel, 2006; Kasahara and Hill, 2008; Claessensand Tague, 2009; O’Connor et al., 2010; Ward et al., 2011), butthese models need further development and testing to bewidely applied in active management scenarios. Develop-ment and refinement of such models requires tracer studiesto calibrate residence times, and preliminary studies haveoccurred in urban streams (Ge and Boufadel, 2006; Ryan andBoufadel, 2006a, 2006b; Ryan and Boufadel, 2007; Ryan et al.,2010; Ryan et al., 2011; Toran et al., 2012). Conservativetracers are often used to visualize breakthrough curves,which are analyzed using the reactive-solute-transport andtransient-storage concepts (Choi et al., 2000; Briggs et al.,2009; O’Connor et al., 2010). However, some subsurface-flowpaths may be too long to be detected by such tracer experi-ments (Bencala et al., 2011), and turbulence through inter-stitial spaces is difficult to capture mathematically (Grantand Marusic, 2011).
Moreover, increased model sophistication has not gener-ally led to greater reliability (Wondzell et al., 2009), andmodels based entirely on geomorphic data (as opposed todata from tracer experiments) may provide reasonable esti-mates of residence times in the HZ (Cardenas et al., 2004;Cardenas and Wilson, 2007; Cardenas, 2008). However, state-of-the-art models for the HZ have not been sufficiently linkedmechanistically with hydrochemical processes, bioturbationfrom macrofauna, such as fishes and macroinvertebrates, orvegetation uptake-processes. These variables can clearly in-fluence much of the variability in the biogeochemistry of theHZ, and, subsequently, pollutant attenuation, which as pre-viously discussed is more likely in streams in which both themajority of flow occurs through the HZ and the flow isaerated.
Conclusion
Under appropriate conditions, HZ management offers agreat and largely unexamined opportunity for improvingwater quality and supporting biodiversity in a rapidly ur-banizing world that is facing the uncertainty of global climatechange and continually changing cultural institutions andvalues. The HZ will inevitably respond to wastewater dis-charges whether water managers are aware of it or not, andincreased awareness will enable these natural processes to bemanaged more holistically. This awareness issue for watermanagers is illustrated clearly by the example of the SouthPlatte River in Colorado in which water utilities were un-knowingly affecting hyporheic exchange processes withhourly fluctuations of their effluent discharge rates (Table 1).We envision that water utilities of the future will include HZmanagement of effluent-dominated streams in their portfolioof projects intended to provide public value, and that waterwill flow out of cities cleaner than it flows in, and in a formwhich will benefit the ecology. However, barriers should beovercome before this vision can become a reality, and prog-ress will most likely be made by engaging multiple stake-holders. These stakeholders should include the general publicwho are the rate-payers, as well as the water professionalsworking in affiliated utilities, nonprofits, government, andacademic institutions.
Acknowledgments
This work was made possible through funding providedby the National Science Foundation Engineering ResearchCenter for Re-inventing the Nation’s Urban Water Infra-structure, ReNUWIt (NSF EEC-028968). The authors thank M.Plumlee for reviewing all or portions of this article, and K.Bencala and J. Zarnetske for providing resources andthoughtful discussion.
Author Disclosure Statement
No competing financial interest exists.
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