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Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved. Colloid Movement in Unsaturated Porous Media: Recent Advances and Future Directions Nicole M. DeNovio, James E. Saiers, and Joseph N. Ryan* ABSTRACT (Buol and Hole, 1961; McKeague and St. Arnaud, 1969; Matlack and Houseknecht, 1989). Investigations of colloid movement through geologic materials are driven by a variety of issues, including contaminant transport, soil- How do these colloids become suspended in pore- profile development, and subsurface migration of pathogenic micro- water? Are they readily transported through the vadose organisms. In this review, we address recent advances in understanding zone? How rapidly are these colloids deposited back of colloid transport through partially saturated porous media. Special onto soil surfaces? These questions can be addressed, emphasis is placed on features of the vadose zone (i.e., the presence in part, by examining processes of colloid deposition of air–water interfaces, rapid fluctuations in porewater flow rates and chemistry) that distinguish colloid transport in unsaturated media and mobilization in saturated porous media (McDowell- from colloid transport in saturated media. We examine experimental Boyer et al., 1986; Ryan and Elimelech, 1996), but in studies on colloid deposition and mobilization and survey recent devel- this review, we focus on what is known about these opments in modeling colloid transport and mass transfer. We conclude processes in unsaturated porous media. with an overview of directions for future research in this field. Three key features of the vadose zone play a critical role in colloid movement: (i) the presence of air–water interfaces, (ii) transients in flow and chemistry, and (iii) M obile colloids are ubiquitous in the porewaters soil structure and heterogeneity (Fig. 1). First, the unsat- of vadose zone soils. Concentrations in excess of urated nature of the vadose zone introduces a third phase, 1gL 1 have been reported during simulated and natural air, which affects colloid partitioning between water and rainfall events (Table 1). The colloids include mineral soil. Colloids of many types associate with the air–water fragments, microbes, and plant decay debris, with min- interface (Wan and Wilson, 1994b; Sirivithayapakorn eral fragments being the most plentiful in typical soils. and Keller, 2003), and the movement of these colloids The mineral fragments are derived mainly from the soil is affected by the movement of air bubbles (Gomez- itself, which contains a great abundance of particles in Suarez et al., 1999; Gomez-Suarez et al., 2001; Saiers et the colloidal size range (Wu et al., 1993; Grout et al., al., 2003). Second, porewater flow and chemistry are 1998; Posadas et al., 2001). The colloidal size range is highly transient in unsaturated porous media. Flow tran- about 10 nm to 10 m, with the smallest colloids being sients, generated by rainfall and snowmelt events inter- those that are just larger than dissolved macromolecules, spersed by drying periods, can promote very rapid col- and the largest colloids being those that resist settling loid mobilization (El-Farhan et al., 2000). Chemical once suspended in soil porewaters. transients, often produced by the introduction of low Colloid movement in the vadose zone is of concern ionic-strength rainwater into the vadose zone, result in for four major reasons: destabilization of colloidal aggregates in soils and mobili- 1. The movement of mobile colloids may facilitate zation of colloids (e.g., Kaplan et al., 1993; Ryan et al., the transport of some contaminants (Amrhein et 1998). Third, the soils of the vadose zone are usually al., 1993; de Jonge et al., 1998; Ryan et al., 1998; structured or physically heterogeneous to some extent. McGechan and Lewis, 2002). For example, macropores promote preferential flow 2. The movement of pathogenic microbes (“biocol- that has the potential to augment colloid mobilization loids”) during wastewater reclamation and aquifer and reduce colloid deposition. Soil layering often inhib- recharge presents a public health risk (Hurst, 1980; its colloid movement by enhancing deposition of col- Powelson et al., 1993; Redman et al., 2001). loids mobilized in the upper soil horizons (Bond, 1986). 3. The deposition of mobile colloids may reduce soil In this review, we emphasize processes that control permeability (Quirk and Schofield, 1955; Frenkel the transfer of inorganic colloids between immobile et al., 1978; Baveye et al., 1998). phases of unsaturated porous media and moving pore- 4. The movement of colloids through the vadose zone water. Microbes and particulate organic matter are not (illuviation) is an important process in soil genesis considered in detail, nor are the effects of solution com- position, soil composition, biota, and soil aggregate structure on the dispersion and stability of soil colloids. N.M. DeNovio and J.N. Ryan, Department of Civil, Environmental, and Architectural Engineering, University of Colorado at Boulder, These factors have been studied extensively (e.g., Ren- 428 UCB, Boulder, CO 80309-0428; J.E. Saiers, School of Forestry gasamy et al., 1984; Pojasok and Kay, 1990; Brubaker and Environmental Studies, Yale Univ., Sage Hall, 205 Prospect et al., 1992; Oades, 1993; Le Bissonnais, 1996), but usu- Street, New Haven, CT 06511. Received 22 Jan. 2004. Special Section: ally in batch systems that do not elucidate the mass- Colloids and Colloid-Facilitated Transport of Contaminants in Soils. *Corresponding author ([email protected]). transfer processes that occur during flow. We begin by examining colloid deposition and mobilization in “ideal” Published in Vadose Zone Journal 3:338–351 (2004). soils, or unsaturated porous media composed of grains Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA of uniform size and shape (Table 2), and survey the 338

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Page 1: Colloid Movement in Unsaturated Porous Media: Recent Advances and

Rep

rodu

ced

from

Vad

ose

Zon

e Jo

urna

l. P

ublis

hed

by S

oil S

cien

ce S

ocie

ty o

f Am

eric

a. A

ll co

pyrig

hts

rese

rved

.

Colloid Movement in Unsaturated Porous Media:Recent Advances and Future Directions

Nicole M. DeNovio, James E. Saiers, and Joseph N. Ryan*

ABSTRACT (Buol and Hole, 1961; McKeague and St. Arnaud,1969; Matlack and Houseknecht, 1989).Investigations of colloid movement through geologic materials are

driven by a variety of issues, including contaminant transport, soil- How do these colloids become suspended in pore-profile development, and subsurface migration of pathogenic micro-water? Are they readily transported through the vadoseorganisms. In this review, we address recent advances in understandingzone? How rapidly are these colloids deposited backof colloid transport through partially saturated porous media. Specialonto soil surfaces? These questions can be addressed,emphasis is placed on features of the vadose zone (i.e., the presencein part, by examining processes of colloid depositionof air–water interfaces, rapid fluctuations in porewater flow rates and

chemistry) that distinguish colloid transport in unsaturated media and mobilization in saturated porous media (McDowell-from colloid transport in saturated media. We examine experimental Boyer et al., 1986; Ryan and Elimelech, 1996), but instudies on colloid deposition and mobilization and survey recent devel- this review, we focus on what is known about theseopments in modeling colloid transport and mass transfer. We conclude processes in unsaturated porous media.with an overview of directions for future research in this field. Three key features of the vadose zone play a critical

role in colloid movement: (i) the presence of air–waterinterfaces, (ii) transients in flow and chemistry, and (iii)

Mobile colloids are ubiquitous in the porewaters soil structure and heterogeneity (Fig. 1). First, the unsat-of vadose zone soils. Concentrations in excess of urated nature of the vadose zone introduces a third phase,

1 g L�1 have been reported during simulated and natural air, which affects colloid partitioning between water andrainfall events (Table 1). The colloids include mineral soil. Colloids of many types associate with the air–waterfragments, microbes, and plant decay debris, with min- interface (Wan and Wilson, 1994b; Sirivithayapakorneral fragments being the most plentiful in typical soils. and Keller, 2003), and the movement of these colloidsThe mineral fragments are derived mainly from the soil is affected by the movement of air bubbles (Gomez-itself, which contains a great abundance of particles in Suarez et al., 1999; Gomez-Suarez et al., 2001; Saiers etthe colloidal size range (Wu et al., 1993; Grout et al., al., 2003). Second, porewater flow and chemistry are1998; Posadas et al., 2001). The colloidal size range is highly transient in unsaturated porous media. Flow tran-about 10 nm to 10 �m, with the smallest colloids being sients, generated by rainfall and snowmelt events inter-those that are just larger than dissolved macromolecules, spersed by drying periods, can promote very rapid col-and the largest colloids being those that resist settling loid mobilization (El-Farhan et al., 2000). Chemicalonce suspended in soil porewaters. transients, often produced by the introduction of lowColloid movement in the vadose zone is of concern ionic-strength rainwater into the vadose zone, result infor four major reasons: destabilization of colloidal aggregates in soils and mobili-

1. The movement of mobile colloids may facilitate zation of colloids (e.g., Kaplan et al., 1993; Ryan et al.,the transport of some contaminants (Amrhein et 1998). Third, the soils of the vadose zone are usuallyal., 1993; de Jonge et al., 1998; Ryan et al., 1998; structured or physically heterogeneous to some extent.McGechan and Lewis, 2002). For example, macropores promote preferential flow

2. The movement of pathogenic microbes (“biocol- that has the potential to augment colloid mobilizationloids”) during wastewater reclamation and aquifer and reduce colloid deposition. Soil layering often inhib-recharge presents a public health risk (Hurst, 1980; its colloid movement by enhancing deposition of col-Powelson et al., 1993; Redman et al., 2001). loids mobilized in the upper soil horizons (Bond, 1986).

3. The deposition of mobile colloids may reduce soil In this review, we emphasize processes that controlpermeability (Quirk and Schofield, 1955; Frenkel the transfer of inorganic colloids between immobileet al., 1978; Baveye et al., 1998). phases of unsaturated porous media and moving pore-

4. The movement of colloids through the vadose zone water. Microbes and particulate organic matter are not(illuviation) is an important process in soil genesis considered in detail, nor are the effects of solution com-

position, soil composition, biota, and soil aggregatestructure on the dispersion and stability of soil colloids.N.M. DeNovio and J.N. Ryan, Department of Civil, Environmental,

and Architectural Engineering, University of Colorado at Boulder, These factors have been studied extensively (e.g., Ren-428 UCB, Boulder, CO 80309-0428; J.E. Saiers, School of Forestry gasamy et al., 1984; Pojasok and Kay, 1990; Brubakerand Environmental Studies, Yale Univ., Sage Hall, 205 Prospect

et al., 1992; Oades, 1993; Le Bissonnais, 1996), but usu-Street, New Haven, CT 06511. Received 22 Jan. 2004. Special Section:ally in batch systems that do not elucidate the mass-Colloids and Colloid-Facilitated Transport of Contaminants in Soils.

*Corresponding author ([email protected]). transfer processes that occur during flow. We begin byexamining colloid deposition and mobilization in “ideal”Published in Vadose Zone Journal 3:338–351 (2004).soils, or unsaturated porous media composed of grains Soil Science Society of America

677 S. Segoe Rd., Madison, WI 53711 USA of uniform size and shape (Table 2), and survey the

338

Page 2: Colloid Movement in Unsaturated Porous Media: Recent Advances and

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www.vadosezonejournal.org 339

Tab

le1.

Exa

mpl

esof

stud

ies

onco

lloid

mob

iliza

tion

,dep

osit

ion,

and

tran

spor

tin

natu

rals

oils

duri

ngsi

mul

ated

rain

fall

inla

bora

tory

colu

mns

and

infi

eld

expe

rim

ents

.The

follo

win

gva

riab

les

are

used

inch

arac

teri

zati

onof

the

expe

rim

ents

:vol

umet

ric

disc

harg

e(Q

),sp

ecif

icdi

scha

rge

(q),

infl

uent

collo

idco

ncen

trat

ion

(C),

ioni

cst

reng

th(I

),m

oist

ure

cont

ent

(�),

spec

ific

cond

ucta

nce

(SC

),is

oele

ctri

cpo

int

(pH

iep)

,ele

ctro

phor

etic

mob

ility

(EM

),an

dze

tapo

tent

ial

(�).

Wat

erC

ollo

idC

ollo

idR

efer

ence

Por

ous

med

ium

chem

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yW

ater

flow

natu

reco

ncen

trat

ion

Maj

orre

sult

s

mg

L�

1

Pilg

rim

and

•in

situ

fiel

dex

peri

men

t•

natu

ral

rain

fall

•tw

ora

infa

llev

ents

•na

tura

lco

lloid

s10

0–53

00•

collo

idco

ncen

trat

ion

incr

ease

dw

ith

tim

edu

ring

rain

fall

Huf

f(1

983)

•G

avio

talo

am,

(9,2

5m

m)

•si

ze4–

8�

m•

mac

ropo

res

allo

wed

tran

spor

tof

larg

erco

lloid

sA

ltam

ont

clay

loam

•ir

riga

tion

spri

nkle

rs•

orga

nic

frac

tion

10%

•m

acro

poro

us(q

�10

.7m

mh�

1 )K

apla

net

al.

•3

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�1.

5m

tank

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pw

ater

•ra

insi

mul

ator

•na

tura

lco

lloid

s:ka

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ite,

300–

1700

•co

lloid

conc

entr

atio

nan

dsi

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crea

sed

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crea

sing

(199

3)pa

cked

wit

hso

il•

SC�

6�

Scm

�1

•q

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1cm

h�1

quar

tz,v

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site

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sand

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6.1

•du

rati

on2

hfe

rric

oxid

es•

EM

decr

ease

dw

ith

incr

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ngfl

owra

te(A

pan

dE

hori

zons

)•

size

0.24

–0.4

2�

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min

eral

cont

ent

incr

ease

dan

dm

etal

oxid

eco

nten

t•

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��

2.4

to�

3.4

�m

s�1

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ease

dw

ith

incr

ease

infl

owra

tecm

V�

1

Bid

dle

etal

.•

insi

tufi

eld

expe

rim

ent

•na

tura

lra

infa

ll•

two

natu

ral

rain

fall

•na

tura

lco

lloid

s:ill

ite,

13–2

90•

favo

rabl

eas

sess

men

tof

capi

llary

wic

kly

sim

eter

sfo

rso

il(1

995)

•tw

oso

ils:A

quic

even

tsof

sam

eka

olin

ite,

verm

icul

ite

colle

ctio

nH

aplo

xera

lf,M

ollic

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tion

and

End

oaqu

alf

inte

nsit

yJa

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enet

al.

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tact

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sin

•ta

pw

ater

•ra

insi

mul

ator

•ill

ite:

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�2

�m

,��

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mV

20–5

50•

part

icle

size

inef

flue

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crea

sed

wit

hti

me

(199

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bora

tory

colu

mns

•SC

�30

0�

S•

q�

11an

d30

mm

h�1

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rich

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rved

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rate

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g27

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ffer

ence

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rved

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etr

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ort

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posi

t);

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ite

and

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ated

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Hap

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lf•

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incr

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dw

ith

squa

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tim

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)K

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natu

ral

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olin

ite,

•co

lloid

surf

ace

char

gew

ashi

ghly

nega

tive

(199

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cked

wit

hso

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es,g

ibbs

ite,

•co

lloid

sor

igin

ated

from

surf

ace

hori

zons

•U

ltis

olqu

artz

,ver

mic

ulit

e•

smal

ler

collo

ids

mor

eab

unda

nt•

size

�1

�m

•ka

olin

ite,

ferr

icox

ide,

gibb

site

wer

een

rich

edin

smal

ler

•or

gani

cm

atte

r1%

collo

ids

•la

rger

part

icle

sw

ere

pref

eren

tial

lyre

mov

eddu

ring

tran

spor

tR

yan

etal

.•

loam

,Ari

dic

Arg

iust

oll

•ta

pw

ater

•ra

insi

mul

ator

•na

tura

lco

lloid

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atio

nw

asob

serv

edbe

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nco

lloid

conc

entr

atio

n(1

998)

•m

acro

poro

us•

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30�

S•

q�

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cmh�

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tufi

eld

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rim

ent

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1•

dura

tion

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2h

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rocl

ine,

ferr

ic•

collo

ids

mob

ilize

dde

crea

sed

wit

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rain

falls

atox

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roxi

de5–

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rval

s•

size

1–20

�m

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chan

gew

asob

serv

edin

part

icle

size

wit

hfl

owve

loci

ty•

pHie

p3–

4•

noco

rrel

atio

nw

asob

serv

edbe

twee

nco

lloid

sm

obili

zed

and

soil

grai

nsi

zeor

com

posi

tion

Læ

gdsm

and

et•

inta

ctco

res

in•

synt

heti

cra

in•

rain

sim

ulat

or•

natu

ral

collo

ids

3–45

•co

lloid

conc

entr

atio

nin

crea

sed

wit

hde

crea

sein

SCal

.(19

99)

labo

rato

ryco

lum

nsw

ater

•q

�1.

6–6.

5m

mh�

1•

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nic

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acti

on0.

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%•

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nic

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onde

crea

sed

wit

hti

me

•sa

ndy

loam

,Alf

isol

,•

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�S

cm�

1•

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idm

obili

zati

onra

tein

crea

sed

wit

hin

crea

sein

Typ

icH

aplu

dalf

•pH

4.6

outf

low

rate

•m

acro

poro

us•

mas

sof

mob

ilize

dco

lloid

sin

crea

sed

wit

hth

esq

uare

root

ofti

me

(sug

gest

ing

diff

usio

n-co

ntro

lled

mob

iliza

tion

kine

tics

)

Con

tinu

edne

xtpa

ge.

Page 3: Colloid Movement in Unsaturated Porous Media: Recent Advances and

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rodu

ced

from

Vad

ose

Zon

e Jo

urna

l. P

ublis

hed

by S

oil S

cien

ce S

ocie

ty o

f Am

eric

a. A

ll co

pyrig

hts

rese

rved

.

340 VADOSE ZONE J., VOL. 3, MAY 2004

Tab

le1.

Con

tinu

ed.

Wat

erC

ollo

idC

ollo

idR

efer

ence

Por

ous

med

ium

chem

istr

yW

ater

flow

natu

reco

ncen

trat

ion

Maj

orre

sult

s

mg

L�

1

El-

Far

han

etal

.•

insi

tufi

eld

expe

rim

ents

•�

0.1

mM

NaC

l•

pond

ing

•na

tura

lco

lloid

s7–

265

•m

ass

ofco

lloid

sm

obili

zed

decr

ease

dw

ith

(200

0)•

Fre

deri

ck,L

odi

solu

tion

•5–

20cm

init

ial

dept

h•

size

253–

270

nmsu

cces

sive

infi

ltra

tion

even

tssi

ltlo

am,T

ypic

•co

lloid

mas

sfl

uxw

asst

eady

amon

gin

filt

rati

onH

adlu

dult

even

ts•

high

lyst

ruct

ured

•pe

akco

lloid

conc

entr

atio

nsan

dfl

uxw

ere

obse

rved

duri

ngri

sing

and

falli

nglim

bsof

wat

erfl

ux(s

ugge

stin

gm

ovin

gai

r–w

ater

inte

rfac

esca

used

mob

iliza

tion

)N

oack

etal

.•

siev

edfr

acti

ons

pack

ed•

dist

illed

wat

er•

rain

sim

ulat

or•

Mul

oori

naill

ite,

size

0.07

�m

C�

100

•co

lloid

brea

kthr

ough

incr

ease

dw

ith

incr

ease

in(2

000)

inla

bora

tory

colu

mn

•SC

�10

�S

•q

�50

mm

h�1

•F

ithi

anill

ite,

two

size

frac

tion

s,m

oist

ure

cont

ent

•O

xiso

l,cl

ayte

xtur

ecm

�1

•�

�0.

28–0

.59

0.08

–0.2

,1–2

�m

•co

lloid

brea

kthr

ough

incr

ease

dw

ith

incr

ease

in•

Spod

osol

,san

dycl

ay•

pH5.

5•

soil

frac

tion

com

pose

dof

illit

e,ab

unda

nce

ofla

rger

pore

size

sin

poro

usm

ediu

mte

xtur

eka

olin

ite,

size

0.08

–0.2

�m

•co

lloid

brea

kthr

ough

incr

ease

dw

ith

decr

ease

inco

lloid

size

•co

lloid

surf

ace

char

gean

dca

tion

satu

rati

onha

dlit

tle

effe

cton

collo

idbr

eakt

hrou

ghG

amer

ding

er•

labo

rato

ryce

ntri

fuge

•de

ioni

zed

wat

er•

flow

driv

enby

•po

lyst

yren

ela

tex

C�

4.5

•in

crea

sed

collo

idbr

eakt

hrou

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www.vadosezonejournal.org 341

Fig. 1. Processes affecting colloid movement in unsaturated porous media. Colloid deposition mechanisms include attachment to grains byphysicochemical filtration, attachment to immobile air–water interfaces (water flow is around bubble trapped in a pore), attachment bystraining in water-saturated pores, and entrapment in thinning water films during draining. Colloid mobilization mechanisms include colloiddispersion by chemical perturbation, expansion of water films during imbibition, air–water interface scouring during imbibition and drainage,and shear mobilization (soil profile from Tarbuck and Lutgens, 1997).

grain surface and on the probability that a colloid collisiondevelopment of mathematical models that describe col-with the mineral grain will succeed in attachment. Colloidsloid transport in these ideal soils. We then explore theare transported from the bulk fluid to the mineral grains byapplication of our understanding of colloid depositionBrownian diffusion, interception, and sedimentation (Yao etand mobilization in ideal soils to “nonideal” soils, oral., 1971). The transport rates due to these three mechanismsnatural and intact soils that are physically and geochemi-can be calculated for water-saturated media as functions ofcally heterogeneous (Table 1). We conclude with recom- the physical properties of the porous medium–water–colloid

mendations for future research. system, including colloid diameter and density, grain size, andflow velocity (Yao et al., 1971; Rajagopalan and Tien, 1976;Logan et al., 1995; Tufenkji and Elimelech, 2004). An analo-COLLOID MOVEMENT IN IDEALgous theory for water-unsaturated media is unavailable. ItsPOROUS MEDIAdevelopment relies on improvements in models for air–water

Colloid Transport and Deposition configuration in variably saturated porous media and, for natu-ral systems, on consideration of the effects of irregularities inMost experimental studies in ideal porous media have beenthe shapes of the mineral grains and colloids.conducted under conditions of uniform moisture content and

Attachment of colloids that strike the mineral grains issteady porewater velocity and have focused on elucidatingdetermined from the net-interaction potential, which can befactors that influence colloid deposition. The experimentalcalculated from DLVO theory as the sum of the electrostaticresults reveal that colloid deposition rates are sensitive todouble-layer force, the van der Waals force, and short-rangeseveral physical and chemical properties, including volumetricsolvation or steric forces (Derjaguin and Landau, 1941; Ver-moisture content, flow rate, porewater ionic strength, andwey and Overbeek, 1948; McDowell-Boyer et al., 1986; Ryancolloid size and composition (Wan and Wilson, 1994b; Wanand Elimelech, 1996). The magnitude and direction of theseand Tokunaga, 1997; Jewett et al., 1999; Gamerdinger andforces depend on the chemical and physical characteristicsKaplan, 2001; Saiers and Lenhart, 2003a). The variations inof the colloid and soil-grain surfaces and, for the electricalcolloid deposition rates with changes in these properties have

been attributed to interactions among three deposition mecha- double-layer force, the chemical composition of the porewater.nisms: mineral-grain attachment, air–water interface capture, At low ionic strength and for similarly charged colloids andand film straining (Fig. 1). soil grains, the net-interaction potential exhibits a repulsive

The kinetics of colloid deposition on mineral grains depends maximum that hinders the attachment of colloids that ap-on the rate of colloid transport from the bulk fluid to the proach the mineral-grain surface. With increasing ionic strength,

Page 5: Colloid Movement in Unsaturated Porous Media: Recent Advances and

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342 VADOSE ZONE J., VOL. 3, MAY 2004

Tab

le2.

Exa

mpl

esof

stud

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nco

lloid

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spor

t,de

posi

tion

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iliza

tion

inid

ealp

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iaun

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nsat

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edco

ndit

ions

.The

follo

win

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riab

lesa

reus

edin

char

acte

riza

tion

ofth

eex

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men

tsan

dex

peri

men

talm

ater

ials

:flo

wve

loci

ty(v

),vo

lum

etri

cdi

scha

rge

(Q),

spec

ific

disc

harg

e(q

),in

flue

ntco

lloid

conc

entr

atio

n(C

),io

nic

stre

ngth

(I),

moi

stur

eco

nten

t(�

),an

del

ectr

opho

reti

cm

obili

ty(E

M).

Stud

ies

cond

ucte

dun

der

stea

dyfl

owfo

cus

onco

lloid

depo

siti

on,

whi

leth

ose

cond

ucte

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der

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lloid

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iliza

tion

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eren

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loid

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and

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on(1

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Rep

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from

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www.vadosezonejournal.org 343

eter exceeded about one-twentieth to one-tenth the diameterof the porous media grains (Sakthivadivel, 1966; Herzig et al.,1970). More recent studies motivated by the need to betterunderstand the removal of protozoan cysts during riverbankfiltration have explored the pore straining of polystyrene mi-crospheres in uniform and poorly sorted porous media (Brad-ford et al., 2002, 2003). This recent work showed that porestraining can be modeled as first-order removal with a ratecoefficient that depends on the depth and mean grain diameterof the porous media. Pore straining may also contribute tocolloid immobilization within small, water-filled pore spacespresent within unsaturated porous media. In partially satu-rated pores with dimensions that exceed those of the colloids,film straining may remove colloids from the mobile phase.According to Wan and Tokunaga (1997), colloid immobiliza-tion by film straining depends on the probability of pendularFig. 2. Negatively charged latex colloids (0.95 �m) deposited prefer-

entially onto an air bubble trapped in a pore body of a porous- ring discontinuity and on the ratio of colloid size to film thick-medium micromodel (from Wan and Wilson, 1994a). ness (a pendular ring is water held by surface tension near

the contacts of adjacent mineral grains). The probability ofthe repulsive barrier decreases in magnitude, which increasespendular ring discontinuity increases from zero to unity asthe probability that a colloid-grain collision will succeed inthe capillary pressure decreases (i.e., as the porous mediumcolloid attachment. The repulsive barrier is absent for oppo-drains). As pendular rings disconnect, an increasing propor-sitely charge colloids and soil grains, in which case the deposi-tion of water flow and colloid transport is relegated to thetion rate is controlled by the rate at which colloids are trans-adsorbed films of water that envelop the mineral grains. Whenported from the pore fluid to the mineral-grain surface.film width is greater than colloid diameter, straining does notPredictions of colloid deposition that are based on DLVOoccur. When film width is similar to or less than the colloidtheory have not been published for water-unsaturated sys-

tems, but DLVO theory has been tested against measurements diameter, however, surface tension retains colloids against theof colloid deposition in water-saturated porous media. These mineral grain surfaces.evaluations show that theoretically determined deposition rates The relative importance of soil-grain attachment, air–watersubstantially underestimate corresponding measured values interface capture, and film straining to colloid deposition iswhen repulsive barriers exist between the colloids and mineral not constant, but varies as a function of porewater chemistry,grains (Elimelech et al., 1995). Agreement between DLVO- moisture content, and colloid characteristics. The work of Wanbased and laboratory-measured deposition rates has been im- and Tokunaga (1997) and Lenhart and Saiers (2002) suggestsproved through recent modifications to theory that account that film straining represents the most important depositionfor complexities associated with surface-charge heterogeneity, mechanism for hydrophilic colloids under conditions of lowgrain-scaled surface roughness, and deposition within the sec- ionic strength (�10�3 M) and low to intermediate moistureondary minimum of the net-interaction energy profile (Bhatta- content. As moisture content and ionic strength increase, thecharjee et al., 1998; Hahn and O’Melia, 2004). These modifi-

leading colloid deposition mechanism may transition from filmcations, although designed to improve descriptions of colloidstraining to air–water interface capture or soil grain attach-deposition in water-saturated media, should also be applicablement, depending on the surface characteristics of the colloidsfor quantifying colloid deposition reactions on mineral-grainand mineral grains (Saiers and Lenhart, 2003a).surfaces present within unsaturated porous media.

Like the soil surfaces, air–water interfaces present withinunsaturated porous media can serve as collectors of colloidal Modeling Colloid Transport and Depositionparticles (Fig. 1 and 2). Colloids that are transported to the

The observations reviewed above have been instrumentalair–water interface are retained by either capillary or electro-in guiding the development of mathematical models for colloidstatic forces; therefore, colloid capture at air–water interfaces

depends on pH, ionic strength, and colloid surface properties. transport and deposition within homogeneous granular mate-Increases in ionic strength reduce the magnitude of the repul- rials. Most of these transport and deposition models are basedsive energy barrier between the negatively charged air–water on the assumption of steady porewater flow and conceptualizeinterface and like-charged mineral colloids, leading to progres- the unsaturated porous medium as a three-component systemsively more favorable conditions for attachment and faster consisting of air, water, and mineral grains (e.g., Sim andrates of air–water interface capture (Wan and Wilson, 1994a; Chrysikopoulos, 2000). Colloids are transmitted through theSaiers and Lenhart, 2003a). Hydrophobic colloids, such as cer- water-filled sections of the porous medium by advection andtain bacteria, exhibit a greater affinity for air–water interfaces dispersion and are removed from the porewater by straining,than mineral colloids, which have comparatively hydrophilic air–water interface capture, and deposition onto soil–watersurfaces (Wan and Wilson, 1994b; Schafer et al., 1998; Lenhart interfaces. Film straining and air–water interface capture areand Saiers, 2002). Among clay-mineral colloids, the affinity treated as irreversible mass-transfer processes, a suitable ap-for the air–water interfaces depends on the colloid shape and

proximation provided that flow and porewater chemistry re-surface-charge distribution and varies inversely with colloidmain steady (Corapcioglu and Choi, 1996; Wan and Tokunaga,cation-exchange capacity. Kaolinite partitions more strongly1997). Colloid release from soil–water interfaces is often ac-to the air–water interface than illite, while bentonite and mont-commodated in unsaturated transport models, but is generallymorillonite exhibit negligible partitioning (Wan and Tokunaga,slow in the absence of hydrologic and chemical perturbations2002).(Schafer et al., 1998; Chu et al., 2001).Straining occurs within mobile-water conduits that are too

The advection–dispersion equation describes the move-narrow to permit colloids to pass (Fig. 1). Early studies onment of porewater colloids. The one-dimensional form of thisthe removal of colloids by pore straining in water-saturated

porous media showed that colloids were retained if their diam- equation is given by

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344 VADOSE ZONE J., VOL. 3, MAY 2004

the attachment probability can be accurately determined on�C�t

��STR

�t�

c

Sw�fair

�AWI

�t� fsoil

�SWI

�t � � AL v�2C�z2

� v�C�z

[1]a theoretical basis, discernible trends between the magnitudeof kAWI and some system properties have been identified. In

where C is the porewater colloid concentration; STR, AWI, and particular, values of kAWI that quantify silica-colloid attach-SWI are immobile-phase colloid concentrations for removal by ment vary proportionately with the one-third power of thefilm straining (STR) air–water interface capture (AWI), and porewater velocity (kAWI � v1/3) (Lenhart and Saiers, 2002) andsoil–water interface deposition (SWI); t is time; c is the ratio increase linearly with porewater ionic strength (Saiers andof colloid mass to its effective cross-sectional area; Sw is water Lenhart, 2003a).saturation; fair is the air–water interfacial area per unit void The reciprocal of �AWI (�AWI

�1) defines the maximum attain-volume; fsoil is the soil–water interfacial area per unit void able surface coverage at the air–water interface. Estimates ofvolume; AL is the longitudinal dispersivity; v is the average

�AWI�1 increase with ionic strength because of a reduction in

porewater velocity; and z is the coordinate parallel to flow. repulsive electrical double layer forces between colloids. EvenThe concentration of strained colloids (STR) is expressed in at elevated ionic strengths, maximum surface coverages forterms of colloid mass per volume of porewater, while AWI both biocolloids and mineral colloids are low. For example,and SWI are expressed in terms of normalized surface cover- Abdel-Fattah and El-Genk (1998) reported �AWI

�1 values forages (i.e., area of attached colloids per area of interface). hydrophobic microsphere ranging from 0.012 to 0.08 for ionicSolution of Eq. [1] requires specification of the kinetics expres- strengths between 0.001 and 1 M, while Saiers and Lenhartsions for film straining, air–water interface capture, and depo- (2003a) reported �AWI

�1 values for silica colloids ranging fromsition onto soil–water interfaces. 0.001 to 0.03 for ionic strengths between 2 � 10�4 and 0.2 M.

Wan and Tokunaga (1997) quantified colloid straining in- The parameter �AWI�1 likely depends on hydrodynamic forces

side thin films with a first-order kinetics expression: in addition to forces between colloids (Ko and Elimelech,2000). Because hydrodynamic forces vary with position along�STR

�t� kSTR C [2] the air–water interface, colloid surface coverages are undoubt-

edly nonuniform, with some areas of the air–water interfacecompletely devoid of colloids (even at maximum surface cov-where kSTR, the rate coefficient for film straining, varies ac-erages), while other areas collect colloids in high concentra-cording totions. Estimates of �AWI

�1, then, should be regarded as a spatialaverage over the entire air–water interface.kSTR � P( )�d

w��

Nv(1�h) [3]Methods for quantifying soil–water interface reactions in

unsaturated media are largely based on approaches derivedIn Eq. [3], P( ) is the probability of pendular ring discontinu- from studies conducted in water-saturated systems. Severality (expressed as a function matric potential, ), d is the investigators have adopted a second-order reversible rate lawcolloid diameter, set w is the film thickness. h, N, and � are to describe colloid mass-transfer reactions with the solid phaseempirical parameters. Wan and Tokunaga (1997) employed (Corapcioglu and Choi, 1996; Schafer et al., 1998; Chu et al.,Eq. [2] and [3] to describe film straining rates in a suite of 2001):column experiments that were conducted at matric potentialsranging from �0.05 to �0.5 m and with microspheres ranging c

Sw

fsoil�SWI

�t� kSWI�SWI C �

c

Sw

fsoil kRSWI [5a]in diameter from 0.014 to 0.97 �m.Colloids traveling within relatively large water channels

(e.g., interconnected pendular rings) are not affected by film�SWI � 1 � �SWISWI [5b]straining, but they may diffuse to the air–water interface where

electrostatic or capillary forces retain them. A second-orderwhere kSWI is a rate coefficient for colloid deposition onto thekinetics expression has been invoked to describe the attach-mineral grains, kR is a rate coefficient for colloid release, andment of microspheres, bacteria, viruses, and mineral colloids�SWI is an excluded area parameter. Application of this kineticsat air–water interfaces present within porous media (Corapci-formulation to data on microsphere, virus, and bacteria trans-oglu and Choi, 1996; Schafer et al., 1998; Chu et al., 2001).port indicate that kR is small or zero, at least for conditionsThe formulation of this rate law varies slightly depending onof constant flow and porewater chemistry. Like their air–waterwhether the captured colloid mass is normalized by the volumeinterface counterparts, kSWI and �SWI are sensitive to porewaterof air or by air–water interfacial area. For the case of normal-chemistry, soil composition, and colloid type (Corapciogluization by interfacial area, the rate law is expressed byand Choi, 1996; Schafer et al., 1998; Chu et al., 2001). Thedeposition rate coefficient (kSWI) should exhibit an additionalc

Sw

fair�AWI

�t� kAWI�AWI C [4a] dependence on volumetric moisture content because changes

in air–water configuration that accompany variation in mois-where kAWI is a rate coefficient for air–water interface capture ture content will affect colloid trajectories around (and theand �AWI is a blocking function. The blocking function declines transport rate to) the mineral-grain surfaces.linearly as AWI increases: Equations [1], [2], and [4a] to [5b] with unknowns C, STR,

AWI, and SWI are suitable for simulating colloid transport,�AWI � 1 � �AWIAWI [4b] film straining, air–water interface capture, and mineral-grain

attachment in unsaturated, homogeneous porous media. Pub-where �AWI is an excluded area parameter equivalent to thelished models that incorporate one or more of these threeratio of blocked air–water interfacial area to the projectedmass-transfer mechanisms have successfully reproduced datacross-sectional area of the colloid. Inspection of Eq. [4a] andfrom laboratory experiments on the transport of both inor-[4b] shows that colloid capture rates vary linearly with C andganic and organic colloids in ideal porous media. Though verydecline as colloids accumulate on the air–water interface.encouraging, these results should not be taken as evidence thatThe magnitude of kAWI depends on the rate of colloid trans-the colloid-transport problem has been solved. The publishedport from the bulk fluid phase to the air–water interface andsimulations rely on adjustment of model parameters that can-on the probability that a colloid collision with the interface

will result in attachment. While neither the transport rate nor not be determined on a theoretical basis and hence the favor-

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www.vadosezonejournal.org 345

able model-data agreement should not be considered defini- measuring the concentrations of colloids in water samplescollected in lysimeters installed within the soil profile (fortive proof of positive identification of the mechanisms that

govern colloid mass transfer. Alternative interpretations of field experiments) or at the base of the core (for laboratoryexperiments). Results of these studies have been instrumentalthe experimental observations are possible.in improving our understanding of factors that control themobilization of naturally occurring soil colloids.Colloid Mobilization

A salient characteristic of these field and intact soil labora-Few experimental or theoretical studies on colloid mobiliza- tory experiments is the consistent occurrence of a pulse of

tion within ideal unsaturated media are available. On the basis colloids at the beginning, and sometimes at the end, of aof studies with saturated porous media, we anticipate that rainfall event with an interlude of relatively steady colloidperturbations in porewater chemistry will promote colloid re- mobilization (e.g., Kaplan et al., 1993; Jacobsen et al., 1997;lease (Fig. 1). Ionic-strength reductions and pH increases are Ryan et al., 1998; El-Farhan et al., 2000). The colloid pulsesthe most common chemical perturbations that mobilize col- during imbibition and draining can be attributed to the effectloids in saturated systems (McDowell-Boyer, 1992; Ryan and of flow transients on colloid mobilization. The relatively steadyGschwend, 1994; Grolimund and Borkovec, 1999) and are colloid mobilization during the rainfall event can be attributedlikely to play an important role in colloid mobilization within to the gradual propagation of chemical (and perhaps someunsaturated systems. physical) perturbations through the soil column.

Physical perturbations in flow that characterize typical infil- The best example of colloid mobilization pulses coincidingtration events also drive colloid mobilization. Several mecha- with the beginning and end of a simulated rainfall event isnisms for this flow-induced mobilization have been proposed provided by the field experiments conducted by El-Farhan et(Fig. 1). Colloids trapped in narrow porewater conduits (by al. (2000). Infiltrating water was applied as water ponded onstraining) may be released into the pore fluid when these flow the soil surface. Peak colloid concentrations (up to 265 mgpaths expand during soil imbibition (Fig. 3; Saiers and Lenhart, L�1) were recorded in the first few and last few samples of2003b). Moving air–water interfaces associated with wetting water taken from zero-tension lysimeters at 25-cm depths (Fig.and drying fronts may scavenge colloids from mineral-grain 4). These peak concentrations were attributed to the passagesurfaces and facilitate their transport through the porous me- of colloid-scavenging air–water interfaces during imbibitiondium (Gomez-Suarez et al., 1999, 2001; Saiers et al., 2003). and draining. The experiments conducted by Saiers et al.Increases in shear stress that accompany porewater-velocity (2003) in ideal porous media reinforce this interpretation forincreases may cause colloids to roll along the surface to which the draining. In addition, some of the pulse of colloid mobiliza-they are attached, and these colloids may be released into the tion that occurs at the beginning of a rainfall event can beporewater upon encountering surface roughness that reduces attributed to the release of colloids into expanding of waterthe DLVO adhesion force (Hubbe, 1985). films (Saiers and Lenhart, 2003b).

Following the pulse of colloid mobilization typically ob-COLLOID MOVEMENT IN NONIDEAL served during imbibition, colloid concentrations are often rela-

tively steady (El-Farhan et al., 2000) or they gradually decreasePOROUS MEDIAwith time (Kaplan et al., 1993; Jacobsen et al., 1997; Ryan et al.,Findings from ideal systems have been used to identify 1998; Schelde et al., 2002). The colloid mobilization behaviorkey mechanisms that influence colloid-deposition kinetics in observed during steady rainfall infiltration has frequently beennatural vadose-zone environments and to define, at least quali- interpreted as control of colloid mobilization kinetics by col-tatively, how colloid mobility in soils and sediments responds loid diffusion. Colloid mobilization can be viewed as a two-to changes in measurable properties, such as moisture content, step process involving (i) detachment of colloids from soilporewater chemistry, and flow velocity. However, natural geo- grain and aggregate surfaces and (ii) diffusion of colloids fromlogic environments are more heterogeneous than ideal sys- the detachment site to the mobile porewater. The diffusiontems. Although the soils of some vadose-zone systems exhibit step may be envisioned as diffusive transport through a layera narrow distribution in pores sizes and are characterized by of immobile water in which diffusive transport of colloids isweak structure, abiotic and biotic processes lead to the cre- more important than advective transport (Ryan and Gschwend,ation of macropores (e.g., root channels, worm borrows, desic- 1994). The diffusion step can also be viewed as diffusioncation cracks) and aggregation of primary mineral particles through two regions, one being a soil “crust” representing soilin many near-surface soils. This soil structure complicates de- aggregates or soil matrix, and the other being the immobilescriptions of colloid transport because it produces nonunifor-water layer (Schelde et al., 2002).mity in the velocity of infiltrating water (Beven and Germann,

In nonideal porous media, there are indications that the1982; Selker et al., 1999). Therefore, mathematical modelsdetachment step is promoted by various chemical and physicaldeveloped for ideal porous media that are based on the as-perturbations (e.g., decreasing ionic strength, increasing pH,sumption of uniform flow cannot be used without modificationshear stress), with the addition of another chemical perturba-to quantify colloid movement through macroporous or aggre-tion, the detachment of colloids by dissolution of mineralgated soils. In addition to heterogeneity in porous-mediumcements that bind together various soil constituents (e.g., Har-physical properties, the geologic solids of real vadose-zoneris et al., 1987; Weisbrod et al., 2002). Despite these indica-environments exhibit substantial geochemical heterogeneity.tions, experiments in nonideal porous media have not yieldedConsequently, the distribution in the rates of colloid mass-much insight into detachment mechanisms because it is highlytransfer reactions may be broader than those measured inunlikely that the detachment kinetics would be the rate-lim-experiments with ideal porous media.iting step in an experiment in which a measurable amount ofcolloids were mobilized. Instead, most of these experimentsExperimental Findings show that kinetics of colloid mobilization during steady infil-tration appears to be limited by the diffusion step (JacobsenColloid movement through nonideal porous media has beenet al., 1997; Lægdsmand et al., 1999; Schelde et al., 2002).measured in small-scale field experiments and in laboratory

The key experimental result that supports an interpretationexperiments with intact soil cores. These experiments mostoften involve applying water to the surface of the soil and of diffusion-limited kinetics for colloid mobilization is a linear

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346 VADOSE ZONE J., VOL. 3, MAY 2004

Fig. 3. Model-computed results and those measured in duplicate experiments on silica-colloid mobilization from columns of quartz sand: (A,B)measured specific discharge at column boundaries, (C,D) measured moisture content (symbols) and modeled moisture content (lines) forthree positions along the 32-cm-long columns (z � 0 at column top), and (E,F) colloid breakthrough pulses generated by successive increasesin flow rate (from Saiers and Lenhart, 2003b).

relationship between the cumulative mass of mobilized col- may be dominating colloid mobilization. At high flow rates,loids and the square root of time (Fig. 5) following shear stress may affect colloid mobilization kinetics. In model

systems of spherical colloids attached to flat plates, the forceof hydrodynamic shear (FH) is proportional to the flow velocityMt � 4M∞l �Dct

�[6]

VR at the height of the colloid radius R and the radius of thewhere Mt is the cumulative mass of mobilized colloids as a colloids (O’Neill, 1968):function of time t, M∞ is the total mass of colloids that can be

FH � (1.7)6��RVR [7]mobilized in a sheet of thickness l, and Dc is the diffusioncoefficient of the colloid (Crank, 1975). Such linear relation-

where � is the dynamic viscosity of the fluid. The shear forceships were observed by Jacobsen et al. (1997), Lægdsmand etis opposed by an adhesive force, which is described by DLVOal. (1999), and Schelde et al. (2002) for intact soils in labora-interactions. Kaplan et al. (1993) and Lægdsmand et al. (1999)tory columns.found support for mobilization by shear in positive correla-Under some conditions, the linear relationship betweentions between mobilized colloid concentrations and flow ratecumulative mass and the square root of time has not been ob-by assuming that the velocity of infiltrating water is propor-served. For example, both Jacobsen et al. (1997) and Lægdsmandtional to flow rate and the concentration of colloids is propor-et al. (1999) noted deviations from the linear relationship fortional to the shear force. Similarly, Weisbrod et al. (2002)early time (during imbibition) and for high flow rates. Thesereported a power law relationship between the flow rate anddeviations indicate that processes other than diffusion maythe amount of colloids mobilized from a fractured chalk for-control colloid mobilization kinetics under these conditions.

During imbibition, colloid scavenging by air–water interfaces mation.

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www.vadosezonejournal.org 347

that approximates the average behavior of the actual macro-Modeling Colloid Mobilization in Nonideal Mediapore network. Water in the partially saturated macropore is

Efforts are just beginning to build a modeling framework assumed to occur as a thin film with mobile- and immobile-appropriate for describing the mobilization and transport of water portions. Colloids are generated from a “crust layer”colloids in nonideal, unsaturated porous media (Jarvis et al., near the macropore edge. These colloids presumably diffuse1999; Schelde et al., 2002). These colloid-transport models, across the stagnant portion of the water film and enter itslike those developed for ideal systems, ignore the effects of mobile-water portion, where flow is steady and the colloids arebiological processes (e.g., growth, decay, predation, and inacti- transported by advection and dispersion. Although Schelde etvation) and thus are most appropriately applied to the move- al. (2002) developed this model in the context of macroporousment of inorganic colloids. soils, it could be applied to describe colloid transport and mass

Schelde et al. (2002) developed a model capable of simulat- transfer in aggregated soils by conceptualizing the water ining the mobilization and transport of natural mineral colloids the aggregates as immobile water and the water in the interag-within macroporous soils cores (Fig. 6). This model is similar gregate pore spaces as the mobile water.in form to dual-porosity, mobile–immobile models for solute The model of Jarvis et al. (1999) shares the two-domaintransport in structured and aggregated porous media (Coats conceptualization embodied in the model of Schelde et al.and Smith, 1964; van Genucthen and Cleary, 1979; Nkedi- (2002), but accounts for transient porewater flow in both the

macroporous and microporous regions of the soil. This modelKizza et al., 1984). It accounts for an equivalent macropore

Fig. 4. Colloid mass flux (filled circles) and porewater flow rates (solid lines) measured during two ponded infiltration experiments. Colloidconcentrations peak during the passage of both wetting and drying fronts (from El-Farhan et al., 2000).

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348 VADOSE ZONE J., VOL. 3, MAY 2004

While progress has been made toward developing a capabil-ity to simulate the unsaturated transport of colloids in nonidealsystems characterized by porous-medium heterogeneity, thereis clearly a long way to go. Available models are very simpleand incorporate only a subset of the mass-transfer processesthat combine to influence colloid mobility in the vadose zone.Additional testing of models over a broader range of experi-mental conditions is needed. These model-data evaluationswill lead to model refinement by illuminating gaps in ourunderstanding of processes and will help to define quantitativerelationships between model parameters and measurable sys-tem properties.

FUTURE DIRECTIONS

To better understand colloid movement through the unsatu-rated zone, five major areas of research should be emphasized:(i) improved visualization of unsaturated flow and colloidtransport phenomena, (ii) continued investigation of transientflow (wetting and drying) conditions, (iii) further examina-tion of the effects of soil structure on colloid mobilizationand transport, (iv) better quantification of pore straining ofcolloids and its effect of soil clogging, and (v) assessmentof colloid mobilization under extreme conditions present at

Fig. 5. Cumulative colloid mobilization as a function of the square waste sites.root of time during leaching through intact macroporous soil cores.Using tools like light transmission through transparent mi-The linear relationship between these variables suggests that the

cro- and meso-models (Wan and Wilson, 1994a; Sirivithaya-kinetics of colloid mobilization were controlled by diffusion (frompakorn and Keller, 2003), magnetic resonance imaging, andJacobsen et al., 1997).X-ray computed tomography, efforts are underway to improveour understanding of flow and colloid transport in the un-is based on the assumptions that mineral colloids are only

mobilized at the soil surface, not within the soil profile, and saturated zone (Darnault et al., 2002; Nestle et al., 2002;Wildenschild et al., 2002; Weisbrod et al., 2003). As the resolu-that colloid deposition in both porewater domains can be

described by simple first-order kinetics expressions. Calcula- tion and capabilities of these visualization systems improve,it will be possible to test hypotheses regarding proposed mech-tions of the model of Jarvis et al. (1999) agree reasonably well

with colloid concentrations measured over an 80-d period in anisms of colloid mobilization and deposition, as well as toidentify new mechanisms that cannot readily be inferred fromsoil water samples collected from a tile-drained silty clay soil

in Sweden. analysis of column experiments. Visualization experiments

Fig. 6. Representation of a single equivalent macropore with colloid mass transfer between three phases: mobile water, immobile water, andcrust. The horizontal arrows indicate colloid diffusion between phases, and the vertical arrows indicate advective colloid transport (fromSchelde et al., 2002).

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that permit air–water interface reactions to be unambiguously 1987; Liang et al., 1993; Schemel et al., 2000), must be assessedin vadose zones subject to these hazardous-waste environments.distinguished from solid–water interface reactions should be

particularly useful in guiding the development of mechanisticmodels for colloid deposition and mobilization. ACKNOWLEDGMENTS

Transients in flow conditions—the wetting and drying cyclesThis work was supported by National Science Foundationof soils—have recently been identified in field and laboratory

grants EAR-9909553 (JNR) and EAR-9909508 (JES) and De-experiments as key factors governing the mobilization of soilpartment of Energy grants DE-FG07-02ER63492 (JES) andcolloids (El-Farhan et al., 2000; Saiers and Lenhart, 2003b;DE-FG07-02ER63491 (JNR).Saiers et al., 2003). This transient flow–induced mobilization

is particularly complex because it is governed by multiplemechanisms, including (but not limited to) thin-film expan- REFERENCESsion, air–water interface scour, and fluid shear. Additional

Abdel-Fattah, A.I., and M.S. El-Genk. 1998. Sorption of hydrophobic,field and laboratory studies on bulk soils, combined with betternegatively charged microspheres onto a stagnant air/water inter-visualization techniques, are needed to evaluate the responsesface. J. Colloid Interface Sci. 202:417–429.of these mechanisms for the range of physical and chemical Amrhein, C., P.A. Mosher, and J.E. Strong. 1993. Colloid-assisted

conditions encountered in real vadose-zone environments. transport of trace metals in roadside soils receiving deicing salts.These observations are required to advance theory appro- Soil Sci. Soc. Am. J. 57:1212–1217.priate for quantifying colloid mobilization in near-surface Baveye, P., P. Vandevivere, B.L. Hoyle, P.C. DeLeo, and D.S. desoils, where transient-flow regimes predominate. Lozada. 1998. Environmental impact and mechanisms of biological

clogging of saturated soils and aquifer materials. Crit. Rev. Environ.Soil structure (i.e., preferential flow paths, aggregates) playsSci. Technol. 28:123–191.an important role in infiltration processes and thus in the

Beven, K., and P. Germann. 1982. Macropores and water flow in soils.mobilization and transport of colloid-sized particles. Disinte-Water Resour. Res. 18:1311–1325.gration of soil aggregates leads to the release of clay particles.

Bhattacharjee, S., C.-H. Ko, and M. Elimelech. 1998. DLVO interac-Although observations of this phenomenon are available tion between rough surfaces. Langmuir 14:3365–3375.(Rengasamy et al., 1984; Pojasok and Kay, 1990; Brubaker et Biddle, D.L., D.J. Chittleborough, and R.W. Fitzpatrick. 1995. Fieldal., 1992; Oades, 1993; Le Bissonnais, 1996), additional re- monitoring of solute and colloid mobility in a gneissic sub-catch-search is needed to examine the relationship between colloid ment, South Australia. Appl. Clay Sci. 9:433–442.dispersion in the typical batch system and in intact soils. A Blume, T., N. Weisbrod, and J.S. Selker. 2002. Permeability changes

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