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Subsurface transport
contaminants
/A
John
E
McCarthy
Oak
Ridge National Laboratory
Oak
Ridge,
TN
3
7831
-6036
John
M.
Zachara
Battelle Pacifc Northwest Laboratory
Richland,
WA
99352
Contaminants originating from human
activities have entered the subsurface
environment through waste disposal
practices, spills, and land application
of
chemicals. The establishment of effec-
tive disposal and isolation procedures
for chemical wastes, the protection of
public health, and the amelioration of
subsurface contamination rely on the
of
ability to predict
the
velocity at which
contaminants move through the vadose
(unsaturated) and saturated zones.
However, attempts to describe and pre-
dict contaminant transport cannot suc-
ceed if major pathways and mecha-
nisms for transport
are
not defined.
Most approaches to describing and
predicting the movement of contami-
nants treat groundwater as a two-phase
system in which contaminants partition
between immobile solid constituents
and the mobile aqueous phase. Con-
taminants that
are
sparingly soluble in
water and that have a strong tendency
to bind to aquifer media are assumed to
be
retarded (to move much more slowly
than the
rate
at which groundwater
flows) (Figure la).
Many contaminants readily sorb to
immobile aquifer media and therefore
are considered to be virtually immobile
in the subsurface and to present little
danger to groundwater supplies. For
example, in soil and aquifer material,
many metals and radionuclides bind
strongly to mineral components; fur-
thermore, many nonpolar organic con-
taminants tend to bind to particulate or-
ganic matter. Colloids in the solid
phase, however, also may be mobile in
subsurface environments. Because the
composition
of
colloids is expected to
be chemically similar to that of the sur-
faces of immobile aquifer material,
these particles
also
could sorb organic
and inorganic contaminants and stabi-
lize them in the mobile phase. The col-
496
Environ. Sci. Technol..
Vol.
23.
No.
5. 1989
001
3-936x189/0923-0496$01.50/0
0
1989 American Chemical Societv
This article is a U.S. government work, and is not subject to copyright in the United States.
loids act as a third phase that can in-
crease the amount of contaminant that
the flow of groundwater can transport
(see
Figure
1
b)
.
This article calls attention to the po-
tentially critical but poorly understood
role of colloids in facilitating contami-
nant transport. Failure to account for
this mode of transport can lead to seri-
ous underestimates of the distances that
contaminants will migrate. For exam-
ple, at a Defense Programs site at
Los
Alamos National Laboratory, pluto-
nium and americium disposed of at a
liquid seepage site migrated up to
30
m
(I);
predictions that were based on lab-
oratory measurements of radionuclide
binding to immobile subsurface materi-
als and that ignored colloids forecast
that migration would
be
limited to a
few millimeters. At another site at Los
Alamos, not only were plutonium and
americium detected in monitoring wells
over a mile from a liquid waste outfall,
but the transported radionuclides were
shown by ultrafiltration to be present as
colloids
(0.025
to
0.45
pm in diameter)
(2).
Colloids are particles with diameters
less than
10
pm
(Figure 2)
(3).
A vari-
ety of organic and inorganic materials
exist as colloids in groundwater, includ-
ing macromolecular components of
“dissolved” organic carbon (DOC)
such as humic substances, “biocol-
loids” such as microorganisms,
mi-
croemulsions of nonaqueous phase liq- Existing information is sufficient to
uids, mineral precipitates and raise concerns about the potential effect
weathering products, precipitates of of colloids on the mobility of ground-
transuranic elements such as Pluto- water contaminants. Nevertheless, cur-
nium, and rock and mineral fragments. rent approaches to monitoring and pre-
In
this
article, colloidal materials sus- dicting contaminant transport generally
pended in water are referred to as parti- ignore colloid-facilitated transport
cles; the term
media
or matrix refers to mechanisms because little, if any, infor-
the fixed
bed
of porous or fractured mation is available on the abundance
subsurface material through which a and identity
of
colloids in groundwater,
solution may flow.
their tendency to bind contaminants, or
We will discuss the genesis and stabi- their mobility in subsurface systems.
lization of groundwater colloids and the Our understanding of the subsurface
chemical and hydrologic factors con- environment is limited by the tech-
trolling their transport through porous niques we use to characterize it, and
media; we also will assess the evidence nowhere in geochemistry is this more
that contaminants bind to mobile col- evident than in the study of subsurface
loids in groundwater. We then will re- colloids.
view the status of current capabilities to Drilling redistributes material, cre-
incorporate the facilitated transport by ates fine particles, introduces materials
colloids into predictive hydrogeo- (drilling muds, for example) into the
chemical models
as
well as the potential borehole, and provides a conduit for air
application of colloid mobilization and to contact groundwater. Sarqding pro-
deposition to waste management strate- cedures also can introduce artifacts, in-
gies. Finally, we identify research that cluding the removal
of
existing col-
is needed to understand and predict the loids, the creation of colloids during
role of colloids in the subsurface trans- sampling, or changes in the chemical
port
of contaminants. First of
all,
how- and physical properties of the natural
ever, it is important to call attention
to
colloids because of alterations in oxy-
the critical caveat in research on gen and carbon dioxide (C02) content,
groundwater colloids: Are colloids in temperature, pH, Eh (redox potential),
water recovered from a well bore rep- and light as the groundwater is brought
resentative of those present within the to the surface. For example, introduc-
porous media?
tion of oxygen can lead to the produc-
samPm
difficulties
tion of colloids, especially Fe(II1)-
oxides
(4,
5);
the rapid pumping of a
well can force otherwise immobile aq-
uifer solids into the well or can disrupt
fragile colloidal aggregates
(6,
7).
Clearly, the issue of proper sampling
protocols is central to progress in eval-
uating the role of colloids in contami-
nant transport. Critical evaluation of
the occurrence, composition, nature,
and abundance of subsurface colloids
requires testing and validation of
sam-
pling methodologies that correctly
sam-
ple mobile material in groundwater
so
that suspended colloidal particles are
included but immobile particles are ex-
cluded.
Colloidal
material in groundwater
The occurrence of colloids in
groundwater should not
be
surprising;
colloidal-sized substances are known to
be
associated with geologic matrices.
Coarse-grained aquifer material can
contain up to
5%
clay-sized materials
(<2
pm) that may
be
detrital (con-
tained in the original parent geologic
material) or authigenic (formed in situ
through geochemical alteration
of
pri-
mary
mineral solids in groundwater
zones), Detrital colloidal material can
be diverse in nature and is a function of
the depositional environment as well as
of the mineralogic composition
of
the
original source geologic material.
Various layer silicates, as well as iron
and aluminum oxides, can
be
detrital in
subsurface sediments. Authigenic col-
loidal-sized particles composed of sec-
ondary hydrous oxides, aluminosili-
cates, and silica, as well as complex
mixtures and solid solutions of these
phases, also form on the surfaces of
larger mineral grains as a result of the
alteration of thermodynamically unsta-
ble primary minerals. Other solids such
as calcite and iron sulfide have been
observed to form directly as a result of
downgradient changes in groundwater
hydrochemistry. Surface analyses of
sediments typically show various col-
loidal-sized secondary precipitates
coating larger grains.
The presence of a clay-sized fraction
in many aquifer materials suggests that
the colloidal fraction is not intrinsically
mobile. Indeed, the isolation of clay-
sized materials from subsurface sedi-
ments often requires the use of ultra-
sonic or chemical dispersion because
the fines are aggregated or are bound to
larger particles by cementing agents or
electrostatic forces. spical cementing
agents include iron oxides, carbonates,
and silica. Stable aggregates of layer
silicates are promoted by saturation
with divalent ions, such as Ca2+, that
coagulate single crystallites.
A
first but essential step in the gene-
sis of mobile colloids in groundwater is
Environ. Sci. Technol.,
Vol.
23,
No.
5,
1989
497
FIGURE
2
Size spectrum
of
waterborne particles
the formation
of
a colloid suspension in
the pore water (Figure
3).
This step can
involve a number
of
mechanisms in-
cluding the homogeneous nucleation
of
inorganic solids in the fluid phase, the
release of colloidal material from the
geologic matrix, and the translocation
of inorganic and organic substances
from the vadose or recharge zone.
Colloidal-sized precipitates can form
in the aqueous phase if gradients in
geochemical parameters such as
groundwater pH, ion composition, re-
dox potential, or CO, partial pressure
induce supersaturation with respect to
readily precipitable solid phases. Such
gradients may result either from natural
geochemical processes (including mi-
crobiological activity that can reduce
redox potential or influence C02 partial
pressures) or from contaminant infiltra-
tion. Geochemical gradients are com-
mon between contaminant plumes and
associated uncontaminated groundwat-
ers because plumes often are of differ-
ent pH, DOC concentration, and inor-
ganic ion composition.
The precipitates that form can con-
tain both major ions (hydrous oxides
of
iron and manganese, calcium carbon-
ates, iron sulfides) and minor ions
(metal and radionuclide oxides, hy-
droxides, carbonates, and sulfides) in
groundwater. Homogeneous precipita-
tion initially can yield large molecular
clusters and colloidal-sized precipitates
that are suspended in the groundwater.
As
examples, Gschwend and Reynolds
observed the precipitation of ferrous
phosphate solids in groundwater
be-
neath a sewage infiltration basin
(8).
Also,
iron oxide colloids have been ob-
served to precipitate in groundwaters as
a result of both pH changes and oxy-
genation
(9).
Moreover, certain
strongly hydrolyzing radionuclides
form colloids whose particles are
nano-
meter-sized
(10).
Colloidal materials also can
be
re-
leased to groundwater as a result of
geochemical and biologic processes
acting upon larger inorganic or organic
particulate materials
in
the aquifer ma-
trix. Organic macromolecules such as
humic substances can be made soluble
from kerogen, bitumen, or lignitic ma-
terials in the aquifer matrix
as
a result
of microbial activity and abiotic hydrol-
ysis. Silica colloids are released during
the diagenesis of amorphous silica.
Similarly, radionuclide-bearing silica
colloids are released from vitrified
waste under simulated groundwater
leaching conditions
(11).
Bacteria at-
tached to aquifer solids may secrete
various exocellular materials or release
cell fragments to groundwater.
In addition, colloids can
be
released
to groundwater if inorganic cementing
agents that bind colloidal-sized materi-
als to larger mineral grains dissolve or
if stable aggregates are deflocculated.
The reductive dissolution of iron oxide
cements appears to be a mechanism by
which colloids can be mobilized
in
nat-
ural waters or in contaminant plumes
containing levels of DOC high enough
to promote microbial activity and an-
oxic conditions. The dissolution of iron
oxides in oxide-cemented sand under
anoxic conditions liberates layer silicate
clays previously bound to the aquifer
matrix
(7).
Similarly, siliceous colloidal
material is released into groundwater in
a calcareous environment because infil-
tration
of
waters
of
different composi-
tion dissolve carbonate cements
(12).
Attached subsurface bacteria may be-
come unattached under high nutrient
and carbon concentrations associated
with contaminant plumes. Colloidal
material
also
may
be
mobilized through
disaggregation if the ionic strength of
the groundwater is decreased or if the
ion balance is shifted from one domi-
nated by Ca2+ to one dominated by
Na+. Permeability reductions in sub-
surface sediments and sandstones that
accompany groundwater electrolyte
changes have been ascribed to mobili-
zation of colloids.
Colloids also can be moved to
groundwater from the vadose or root
zone (Figure
4). Humic substances can
be
flushed from upper, organic-rich
soil
horizons
to
the vadose and saturated
zones during storms and major periods
of infiltration such as snowmelt
(13).
Significant bacterial populations
(104-
1
O7
microorganisms/g) have been
498
Environ. Sci.
Technol.,
Vol.
23,
No.
5,
1989
found in groundwater zones
(14,
and
their origin has, in part,
been
ascribed
to migration from the upper soil zone.
Although humic substances and
mi-
crobes can adsorb to mineral surfaces,
their movement to a given depth appar-
ently is facilitated by transport through
preferential flow paths in porous media
such as macropores and cracks associ-
ated with soil structure
(15,
16).
Layer
silicate clays also can be mobilized
from the soil zone by large storm
events that lead to the infiltration of low
ionic strength meteoric waters. The ar-
tificial recharge of groundwater by ap-
plication of water to soils can lead to
turbidity in groundwater and permea-
bility reductions in soil, exacerbating
the effects of mobilization of colloidal-
sized material
(1
7).
Stabilization
and
filtration
The preceding discussion has shown
how colloidal-sized material may be re-
leased from, transported to, or formed
in groundwater.
To
be mobile over sig-
nificant lateral distances and thus facili-
tate contaminant transport, suspended
colloidal material must be stable (resist-
ing aggregation with other like parti-
cles) and must not be susceptible to par-
ticle filtration in passing through
porous media (Figure
3).
Whether a
particle will be stable, aggregated,
fil-
tered, or will settle in groundwater de-
pends on a complex combination of
density, size, the surface chemistry, the
water chemistry, and the water flow
rates. The complicated interdepen-
dency of these factors makes it difficult
to predict particle behavior, based on
current understanding and models;
however, general concepts are well es-
tablished for assessing these interac-
tions qualitatively.
According to the theory of Derja-
guin, Landau, Verwey, and Overbeek
(the DLVO theory), the stability of a
homogeneous colloidal suspension is
determined by
van
der
Waals
attractive
forces that promote aggregation and by
electrostatic repulsive forces that drive
particles apart. When electrostatic re-
pulsions are dominant, the particles are
electrostatically stabilized and remain
in a dispersed state. Colloid stabiliza-
tion therefore is influenced by particle
mineralogy and surface chemistry, by
other chemical factors controlling sur-
face charge, and by the extent of the
electrical double layer
(18, 19). These
chemical factors are summarized in Ta-
ble
l.
Particles are stabilized when their
double layers are expanded (by de-
creasing electrolyte concentration and
ionic strength) and when the net parti-
cle charge
(up)
does not equal zero. The
converse promotes coagulation. Coun-
terion valence controls double-layer
thickness, and divalent ions coagulate
colloids at much lower aqueous con-
centrations than do monovalent ions
(Schulze-Hardy rule).
Sorbable species (alkali earth cat-
ions, hydrolyzable metals such as Fe3
+,
and weak acid anions) and natural poly-
electrolytes such as humic substances
form surface complexes on colloidal
material and influence net particle
charge and stability. Strongly sorbing
species may destabilize colloids at low
concentrations and restabilize them at
higher concentrations as their sorption
induces charge reversal and an increase
in
up.
Indeed, even under chemical con-
ditions that should promote stability,
coagulation of colloids in groundwater
may occur if the surface charges on the
suspended colloids are heterogeneous
and permit interparticle electrostatic in-
teractions. For example, the coagula-
tion behavior of layer silicate clays is
complicated because the edges and
faces of the crystallites may carry op-
posite charge, giving rise to premature
aggregation through edge-to-face asso-
ciation.
The destabilization of suspended col-
loids in groundwater as a result of fac-
tors such as increasing ionic strength or
equivalent fraction of Ca2+ over “a+,
or through strong ion adsorption,
does
not necessarily mean that they will be
rendered immediately immobile.
Coag-
ulation kinetics are complex, and elec-
trostatic interactions that occur during
particle collisions are not well under-
stood
(20).
These interactions are a
function of many variables, including
particle concentration, particle size,
and particle size distribution, all of
which can influence the extent of parti-
cle-particle collisions in the ground-
Environ. Sci. Technol.,
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23,
No.
5,
1989 499