Natural and Enhanced Remediation Systems - Chapter 3 pot

Suthersan, Suthan S. “Monitored Natural Attenuation”

Natural and Enhanced Remediation Systems

Edited by Suthan S. Suthersan
Boca Raton: CRC Press LLC, 2001
©2001 CRC Press LLC

CHAPTER

3
Monitored Natural Attenuation

CONTENTS

3.1 Introduction
3.1.1 Definitions of Natural Attenuation
3.2 Approaches for Evaluating Natural Attenuation
3.3 Patterns vs. Protocols
3.3.1 Protocols for Natural Attenuation
3.3.2 Patterns of Natural Attenuation
3.3.2.1 Various Patterns of Natural Attenuation
3.4 Processes Affecting Natural Attenuation of Compounds
3.4.1 Movement of Contaminants in the Subsurface
3.4.1.1 Dilution (Recharge)
3.4.1.2 Advection
3.4.1.3 Dispersion
3.4.2 Phase Transfers
3.4.2.1 Sorption
3.4.2.2 Stabilization

3.4.2.3 Volatilization
3.4.3 Transformation Mechanisms
3.4.3.1 Biodegradation
3.5 Monitoring and Sampling of Natural Attenuation
3.5.1 Dissolved Oxygen (DO)
3.5.2 Oxidation–Reduction (REDOX) Potential (ORP)
3.5.3 pH
3.5.4 Filtered vs. Unfiltered Samples for Metals
3.5.4.1 Field Filtration and the Nature of
Groundwater Particulates
3.5.4.2 Reasons for Field Filtration
3.5.5 Low-Flow Sampling as a Paradigm for Filtration
3.5.6 A Comparison Study
References

©2001 CRC Press LLC

…natural attenuation (NA) is not a “no action (NA)” alternative. Monitored
natural Attenuation (MNA) defines the required monitoring parameters to dem-
onstrate that the ongoing natural processes will continue to meet the remediation
objectives…

3.1 INTRODUCTION

The term monitored natural attenuation (MNA) refers to an approach to clean up
subsurface contamination, specifically in groundwater, by relying on natural processes
and monitoring. MNA is also referred to as

natural degradation

and

intrinsic

or

passive
remediation

. Natural attenuation processes include a variety of physical, chemical, or
biological processes that, under favorable conditions, act without human intervention
to reduce the mass, toxicity, mobility, volume, and concentration of contaminants in
groundwater. Depending on the geologic conditions, types of contaminants, and con-
taminant mass and distribution at a given contaminated site, MNA could emerge as
the preferred choice of remediation approach. Natural attenuation relies on the

assim-
ilative capacity

of the ecosystem for the reduction of contaminant concentration and
mass. This approach has been utilized by environmental engineers for a long time to
control industrial and municipal wastewater discharges into surface waterbodies and
maintain acceptable water quality standards.

3.1.1DeÞnitions of Natural Attenuation

A variety of organizations have espoused the following definitions of natural
attenuation due to the emerging popularity and preference of MNA as the remedi-
ation method of choice at many contaminated sites across the country.

1

Environmental Protection Agency

2

:

This policy directive defines monitored
natural attenuation as the reliance on natural attenuation process (within the
context of a carefully controlled and monitored site cleanup approach) to achieve
site-specific remediation objectives within a time frame that is reasonable com-
pared to that offered by other more active methods. The “natural attenuation
processes” that are at work in such a remediation approach include a variety of
physical, chemical, or biological processes that, under favorable conditions, act
without human intervention to reduce the mass, toxicity, mobility, volume, or
concentration of contaminants in soil or groundwater. These

in situ

processes
include biodegradation; dispersion; dilution; sorption; volatilization; radioactive
decay; and chemical or biological stabilization, transformation, or destruction of
contaminants.

American Society for Testing and Materials (ASTM)

3
: Its document titled

Standard Guide for Remediation of Groundwater by Natural Attenuation at
Petroleum



Release Sites

defines natural attenuation as the “reduction in mass or
concentration of a compound in groundwater over time or distance from the
source of constituents of concern due to naturally occurring physical, chemical,
and biological processes, such as biodegradation, dispersion, dilution, adsorption,
and volatilization.”

©2001 CRC Press LLC

Air Force

4

:

The first document, published in 1995, defines the process as resulting
“from the integration of several subsurface attenuation mechanisms that are
classified as either destructive or nondestructive. Biodegradation is the most
important destructive attenuation mechanism. Nondestructive attenuation mech-
anisms include sorption, dispersion, dilution from recharge, and volatilization.”
Army
5
: Its report defines natural attenuation as “the process by which contam-
ination in groundwater, soils, and surface water is reduced over time…[via]

natural processes such as advection, dispersion, diffusion, volatilization, abiotic
and biotic transformation, sorption/desorption, ion exchange, complexation, and
plant and animal uptake.”
In the past, the first question to be asked in consideration of the potential for
natural attenuation at a contaminated site was whether biodegradation of the chem-
ical contaminant had been reported. Oftentimes the question was, “Does the bio-
geochemistry exist for ongoing degradation?” due to the assumption that the respon-
sible microorganisms are ubiquitous in the subsurface. However, in this chapter the
term “natural attenuation” will include all the processes that contribute towards the
decrease in contaminant concentrations.

3.2 APPROACHES FOR EVALUATING NATURAL ATTENUATION
Documenting that contaminant concentration has become very low or detectable
in groundwater samples is an important piece of evidence that natural attenuation
is working. However, such documentation is not completely sufficient to show that
natural attenuation is protecting human health and the environment, for three primary
reasons:

• Monitoring of contaminant concentration reductions is not always precise due to
the complex nature of groundwater systems. In some cases the total contaminant
mass may have decreased, but the contaminant may have transformed to another,
more hazardous chemical form.
• In a few instances reactions that initially cause contaminants to attenuate may not
be sustainable until reasonable cleanup goals are achieved.
• Another situation of concern occurs when natural biogeochemical parameters,
such as electron acceptors and electron donors that support attenuation, are used
up before the treatment of contamination is complete.

For these reasons, environmental regulators and others should not rely on simple
rules of thumb (such as maximum contaminant concentration data or trends in these

data over a relatively short time) in evaluating the potential success of natural
attenuation.
The decision to rely on natural attenuation and the confirmation that it will
continue to work depend on linking monitoring data to a site conceptual model and
“footprints” of the underlying mechanisms. Footprints are mappings of concentration
changes in reactants (contaminant(s), electron acceptors, and donors) or products of
the biogeochemical processes (such as Cl

–

ion, dissolved Fe

2+

) that degrade or

©2001 CRC Press LLC

immobilize the contaminants (Figures 3.1a, b, and c). Footprints can be measured
to document that these transformation or immobilization processes are active at the
site. An observation of the loss of a contaminant, coupled to observation of a few
footprints, helps to establish which processes are responsible for the decrease in
contaminant mass and concentrations. The three basic steps to document natural
attenuation are as follows:

1.

Develop a conceptual model of the site

: The model should show where and how

fast the groundwater flows, where the contaminants are located and at what
concentrations, and which types of natural processes could theoretically affect the
contaminants (Figures 3.2a and b).
2.

Analyze site measurements

: Samples of groundwater should be analyzed chemi-
cally to look for footprints of the natural attenuation processes and to determine
whether these processes are sufficient to control the contamination.
3.

Monitor the site

: The site should be monitored until regulatory requirements are
achieved to ensure that documented attenuation processes continue to occur.

Although the basic steps are the same for all sites, the level of effort needed to
carry out these steps varies substantially with the complexity of the site. When site
characteristics or the controlling mechanisms are uncertain, it will be difficult to
develop the site conceptual model; thus, a large amount of data will be required to
document natural attenuation. In these complex situations, computer modeling may
be necessary, and data on footprints and site characteristics will have to be more
than adequate to develop the model.

Figures 3.1a

Initial vinyl chloride plume at a landÞll site in Maryland with radial groundwater
ßow from the center of the landÞll.

©2001 CRC Press LLC

Figures 3.1b

Natural attenuation effects on the vinyl chloride plume. Note: The signiÞcant
reduction in vinyl chloride concentration and mass due to natural attenuation.

Figures 3.1c

Effects of the primary electron acceptor dissolved oxygen on the attenuation of
VC and Mn along a North-South transect through the middle of the landÞll.
Three-dimensional perspective plot
of observed vinyl choride concentrations
in groundwater -1996
Landfill
boundary
500
200
150
100
20
5
1
0
1200
1000
800
600
400
200

0
Dissolved Oxygen, Redox, and
Vinyl Chloride Distribution
Landfill
Vinyl Chloride
Manganese
Fe
Redox
Dissolved Oxygen
2+
Saprolite
Bedrock
Sand/Gravel

Figures 3.2a

A general site conceptual exposure model (adapted from ASTM, 1997).
Current Domestic
Water Supply Well
Future Domestic
Water Supply Well
Confining Layers?
Confining Layers?
Confining Layers?
Dissolved Plume
Utilities
Utilities
Residual NAPL
Abandoned Well?
Current Municipal

Water Supply Well
Shallow Water Table

©2001 CRC Press LLC

Figures 3.2b

Site conceptual exposure models.
Primary
Sources
Secondary
Sources
Transport
Mechanisms
Exposure
Routes
Receptors
Chemical Storage
Piping / Distribution
Operations
Waste Management Unit
Soil or Waste Piles
Lagoons or Ponds
Other
Residential
Commercial/Industrial
Construction Worker
Relevant Ecological Recept
o
Residential

Commercial/Industrial
Construction Worker
Relevant Ecological Recept
o
Residential
Commercial/Industrial
Affected
Subsurface
Soils
(>3 ft depth)
Affected
Surface
Soils
(<3 ft depth)
Dissolved
Groundwater
Plume
Non-Aqueous
Volatilization
and
Atmospheric
Dispersion
AIR
Inhalation of Vapor
or Particulates
GROUNDWATER
Potable Water Use
SOIL
Dermal Contact
or Ingestion

Wind
Erosion and
Atmospheric
Dispersion
Dissolved
Groundwater
Plume
Leaching and
Groundwater
Transport
Mobile

©2001 CRC Press LLC

©2001 CRC Press LLC

3.3 PATTERNS VS. PROTOCOLS
3.3.1Protocols for Natural Attenuation

Within the past few years, many organizations have issued documents providing
guidance on evaluating natural attenuation.

1

Among the 14 documents developed by
a range of organizations from federal and state agencies to private companies and
industry associations, the available technical protocols address two classes of organic
contaminants only: fuel hydrocarbons and chlorinated solvents (with the exception
of the Department of Energy (DOE) document). A large body of empirical evidence
and scientific and engineering studies in recent years has been developed to support

understanding of natural attenuation of these contaminants — mostly fuel hydro-
carbons under certain conditions. However, the natural attenuation of polycyclic
aromatic hydrocarbons, polychlorinated biphenyls, explosives, and other classes of
persistent organic contaminants is not addressed in any protocol.

1

Furthermore,
although the DOE document proposes a method for assessing natural attenuation
processes for inorganic contaminants such as metals, such processes are extremely
complex, and this document does not adequately reflect this complexity.

6

A recent effort was made to compare the guidelines currently available on natural
attenuation against a list of characteristics of a comprehensive protocol.

1

The con-
sensus was that a comprehensive protocol should cover three broad areas:

•

Community concerns

: The protocol should describe a plan for involving the
affected community in decision making, maintaining institutional controls to
restrict use of the site until cleanup goals are achieved, and implementing contin-
gency measures if natural attenuation fails to continue as expected.

•

Scientific and technical issues

: The protocol should describe how to document
which natural attenuation processes are responsible for observed decreases in
contaminant concentrations, how to assess the site for contaminant source and
hydrogeologic characteristics that affect natural attenuation, and how to assess the
sustainability of natural attenuation over the long term.
•

Implementation issues

: The protocol should be easy to follow and should describe
the monitoring frequency and various monitoring procedures, in addition to the
training and expertise required for the personnel carrying out the field implemen-
tation.

None of the current documents fulfills all the criteria defined above.

1

To some
extent, this reflects the various, and sometimes limited, purposes for which these
documents were prepared. Some are detailed technical guides; others are intended
to help ensure consistency in site evaluation within a particular organization (such
as a private corporation or a branch of the military), and others are intended to guide
policy. Nonetheless, key gaps in the existing body of protocols have to be addressed.
The existing protocols provide little or no discussion of when and how to involve
the public in site decisions and when and how to implement institutional controls.

In the few instances where these matters are mentioned, the discussion is typically
brief, almost in passing. Although most environmental regulatory agencies have
separate policies that specify procedures for community involvement and

©2001 CRC Press LLC

institutional controls, these procedures may be inadequate in cases where natural
attenuation is selected as the remedy. Discussion of when and how to implement
contingency plans in case natural attenuation does not work is also inadequate in
many of the protocols. Further, the protocols do not provide sufficient guidance on
when and how engineered methods to remove or contain sources of contamination
benefit natural attenuation.
A major shortcoming of some of the protocols relates to scoring systems used for
initial screening to determine whether a site has potential for treatment by natural
attenuation. Such scoring systems yield a numeric value for the site in question. If this
value is above a certain level, the site is judged an eligible candidate for natural
attenuation. Frequently, such scores are used inappropriately as the key factor in
deciding whether natural attenuation can be a successful remedy at the site. Moreover,
these scores often lead to erroneous conclusions about whether natural attenuation will
or will not succeed, due to the complexity of the processes involved and the tendency
of scoring systems to oversimplify them. In addition, the scoring systems developed
for evaluating natural attenuation at petroleum sites are erroneously used to evaluate
sites with chlorinated solvents by many practitioners of remediation.
In summary, the existing body of natural attenuation protocols is limited in
several important areas.

1

Where and how existing protocols can be used to meet
regulatory requirements for documenting site cleanup — and whether such protocols

are required at all — is also unclear. Guidance on the use of natural attenuation for
remediation has to be developed to cover topics not addressed in existing protocols
and to provide for the use of protocols in regulatory programs.

3.3.2 Patterns of Natural Attenuation

Instead of relying on protocols and scoring systems, an educated screening tool
should be to observe the

patterns

in reduction of contaminant concentrations. Nat-
urally attenuating contaminant plumes can take a variety of forms: they might be
expanding, stable, or shrinking, depending on the trends in the spatial variations of
contaminant concentrations with time (Figures 3.3a, b, and c). Common patterns in
all attenuating plumes are a decline in the dissolved contaminant mass with time,
and a decline in contaminant concentrations downgradient from the source. Once
these patterns are observed initially, the following list of questions should be devel-
oped to collect additional data to develop a platform demonstrating that MNA is an
ongoing and continuing process to meet the site cleanup objectives:

•What chemical, physical, and biological processes are in effect to support natural
degradation of the site-specific contaminants?
•What site biogeochemical conditions are needed for these chemical, physical, and
biological processes to work? Which types of site conditions are optimal? Which
conditions inhibit natural attenuation?
•What level of information is needed to characterize the site fully?
•What breakdown products that may be more toxic, persistent, or mobile are created
when the contaminants degrade? How does one prove that contaminants are
degrading into harmless substances?

©2001 CRC Press LLC
•What kinds of specific monitoring and testing are needed to determine that the
site and the contaminants are suitable for natural attenuation? Is extensive mon-
itoring necessary?
•How long is it reasonable to monitor to ensure that natural attenuation is working?
•How viable are institutional controls? Can they be enforced?
• Is stabilization by natural attenuation irreversible for metals or other substances?

3.3.2.1 Various Patterns of Natural Attenuation

Removal of Contaminant Sources:

At most contaminated sites, the bulk of the
contaminant mass is in what remediation professionals call “source zones.” Examples
of source zones include landfills, areas of chemical spills, buried tanks that contain
residual chemicals, deposits of tars, etc. Some of these sources can be easily located

Figures 3.3a

Expanding plume.
Cross Sectional View
Contaminated
Zone
Plan View
MW-3
MW-2
MW-1
t
1

t
0
t
2
"Contaminant plume is continuing to grow and move downgradient from the source area"
Monitoring
Well

©2001 CRC Press LLC

and complete or partial removal or containment may be possible. However, other
common types of sources often are extremely difficult to locate and remove or
contain. One example of a source in this category is chemicals that have sorbed to
soil particles but have the potential to dissolve later into groundwater that contacts
the soil. Another extremely important example is the class of organic contaminants
known as “nonaqueous-phase liquids” (NAPLs). There are two types of NAPLs:
those that are more dense than water (dense nonaqueous-phase liquids, or DNAPLs),
and those that are less dense than water (light nonaqueous-phase liquids, or
LNAPLs). When released to the ground, these types of fluids move through the
subsurface in a pattern that varies significantly from that of the water flow because
NAPLs have different physical properties than water. As shown in Figure 3.4a,
b,
and c, LNAPLs can accumulate near the water table, DNAPLs can penetrate the
water table and form pools along geologic layers, and both types of NAPLs can
become entrapped in soil pores. These NAPL accumulations contaminate the

Figures 3.3b

Stable groundwater plume.
Cross Sectional View

Plan View
MW-3 MW-2 MW-1
t
1
t
0
t
2
Contaminant plume is almost stationary over time and concentrations at points
within the plume are relatively constant over time with a slight declining trend.
Contaminated
Zone
Monitoring
Well

©2001 CRC Press LLC

groundwater that flows by them by slow dissolution. Common LNAPLs include
fuels (gasoline, kerosene, and jet fuel) and common DNAPLs include industrial
solvents (trichloroethene, tetrachloroethene, and carbon tetrachloride) and coal tar.
Once they have migrated into the subsurface, NAPLs are often difficult or impossible
to locate in their entirety. Normally, the total mass of a contaminant within source
zones is significantly larger compared to the mass dissolved in the plume. Therefore,
the source usually persists for a very long time. The rate at which contaminants
dissolve from a typical NAPL pool is so slow that many decades to centuries may
be needed to dissolve the NAPL completely by dissolution without any intervention.
The potential for success of natural attenuation of various dissolved organic and
inorganic compounds is presented in Table 3.1.
Given the persistent nature of contaminant sources, removing them would seem
like a practical way to speed natural attenuation of the contaminant plume (Figure

3.4). In many cases, environmental regulators require source removal or containment

Figures 3.3c

Shrinking groundwater plume.
Cross Sectional View
Plan View
MW-3
MW-2
MW-1
t
1
t
0
t
2
Contaminant plume is receding back toward the source area over time and the
concentrations at points within the plume are declining over time.
Contaminated
Zone
Monitoring
Well

©2001 CRC Press LLC

Table 3.1 The Potential for Success of Natural Attenuation for Various Compounds

(adapted from NCR, 2000)
Contaminant Type Dominant Attenuation Processes
Likelihood of

Success

Or
ganic

Hydrocarbons

BTEX Biotransformation High
Gasoline, fuel oil Biotransformation Moderate
Nonvolatile aliphatic compounds Biotransformation, immobilization Low
PAHs Biotransformation, immobilization Low
Creosote Biotransformation, immobilization Low

Oxygenated Hydrocarbons

Low-molecular-weight Biotransformation High
Alcohols, ketones, esters
MtBE Biotransformation Moderate

Chlorinated Aliphatics

PCE, TCE, carbon tetrachloride Biotransformation Moderate
Tr ichloroethane (TCA) Biotransformation, abiotic
transformation
Moderate to
High
Methylene chloride Biotransformation High
Vinyl chloride (VC) Biotransformation Moderate to
High
Dichloroethene (DCE) Biotransformation Moderate

Chlorinated Aromatics

Highly chlorinated PCBs,
pentachlorophenol,
multichlorinated benzenes
Biotransformation, immobilization Low
Less chlorinated PCBs, dioxins Biotransformation Low
Monochlorobenzene Biotransformation High

Nitroaromatics

TNT, RDX Biotransformation, abiotic
transformation, immobilization
Low

Inorganic

Metals

Ni Immobilization Moderate
Cu, Zn Immobilization Moderate
Cd Immobilization Low
Pb Immobilization Moderate
Cr Biotransformation, immobilization Low to
Moderate
Hg Biotransformation, immobilization Low

©2001 CRC Press LLC

as part of a natural attenuation remedy. Although requiring source control or removal
is good policy for many sites, expert opinions conflict on whether source removal
is advisable when using natural attenuation as a remedy, even when such removal
is technically feasible.

Nonmetals

As Biotransformation, immobilization Low
Se Biotransformation, immobilization Low

Oxyanions

Nitrate Biotransformation Moderate
Perchlorate Biotransformation Moderate

Figures 3.4

Various possibilities of source zone contamination.

Table 3.1 The Potential for Success of Natural Attenuation for Various Compounds

(adapted from NCR, 2000) (continued)
Contaminant Type Dominant Attenuation Processes
Likelihood of
Success
a)
Not Enough Mass Spilled to Form an NAPL
t
1
t

3
t
4
t
2
b)
LNAPL Release
DNAPL Release
M
NAPL
M
DISS
t
1
t
3
t
4
t
2
M
N
> M
D
Adsorbed
DNAPL
DNAPL Pool
c)
M
NAPL

M
DISS
t
1
t
3
t
4
t
2
M
N
> M
D

©2001 CRC Press LLC

Goals of source removal should be the following:

• Remove as much contaminant mass as practical to reduce the mass flux of con-
taminants emanating from the source zone, thus reducing the concentration of the
contaminant plume rapidly and also reducing the longevity of the required mon-
itoring period; and
•Avoid any changes that would reduce the effectiveness of natural attenuation, such
as disturbing the natural dissolution equilibrium from an NAPL source by drilling
through it and thus increasing the mass flux.

In theory, if one can delineate the source completely and succeed in removing most
of the mass, then a significant benefit may be achieved. There are many case studies
available in the literature even for compounds like polycyclic aromatic hydrocarbons

(PAHs) plumes in which it appears that, after removal of the source, the plumes
attenuated rapidly. However encouraging this example might be, this kind of success
may not always be realized. Particularly, DNAPL sources in fractured bedrock envi-
ronments cannot be delineated completely and/or cannot be removed to any significant
degree at a reasonable cost. Hence, source removal options may be rejected because
none are anticipated to be able to warrant the expense and risks of the removal effort
by removing all of the source mass without leaving a significant level of residual mass.
In some cases, source removal efforts may directly and adversely affect natural
attenuation. Most of the negative impacts will be caused mainly by the disturbance
of the equilibrium between the moving groundwater and the quiescent mass of
NAPL, particularly DNAPL. As a precautionary measure, an outside-in approach to
investigating the source zone is recommended in contrast to an inside-out approach.
Consideration should be given when looking at removal of the source of one
type of contaminant which may adversely affect natural attenuation of another type
and thus result in minimal or no overall benefit. A good example is the removal of
a petroleum hydrocarbon source zone serving as a nutrition source for microbes
involved in degrading a chlorinated solvent plume. Such an action could slow down
or completely shut off natural attenuation of the chlorinated solvent.

Natural Attenuation Capacity (NAC):

The manner in which natural attenuation
and active remediation measures (such as source removal, pump and treat, chemical
oxidation, or enhanced bioremediation) are combined depends on the

natural atten-
uation capacity

(NAC) of the system. If the NAC is small, for example, active
remediation measures will need to remove or degrade a high proportion of the

contaminant source to protect downgradient receptors. Conversely, if the NAC is
large, less source removal may be required to protect downgradient receptors. In
either case, it is necessary to quantify the NAC of the biogeochemical system to
combine contaminant source-removal methods with natural attenuation effectively.
Natural attenuation capacity is a concept that refers to the capacity of a bio-
geochemical system to lower contaminant concentrations along aquifer flow paths.
The NAC of groundwater systems depends on hydrogeologic (dispersion and advec-
tion) and biological (biodegradation rates) factors for organic contaminants and
precipitation potential also for heavy metals.

©2001 CRC Press LLC

The concept of NAC is useful because it illustrates those characteristics and
parameters of a groundwater system that affect the efficiency of natural attenuation.

7

For example, if the biodegradation rate constant is small (

@

0.001 d

–1

) relative to
the groundwater velocity (~3 ft/day) and aquifer dispersivity (30 feet), the NAC of
the system also will be small. Because of this small NAC, contaminants will be
transported relatively long distances downgradient of the source area (Figure 3.5a).
Conversely, if the biodegradation rate is high relative to groundwater velocity and

aquifer dispersivity, the NAC will be proportionally higher, and the transport of
contaminants will be restricted closer to the source area.
Quantitative mathematical techniques in addition to empirical methods are avail-
able to estimate NAC. In addition to NAC, the distance that contaminants are
transported in a groundwater system also depends on the contaminant concentrations
at the source area (Figure 3.5b).

Figure 3.5a

The effect of natural attenuation capacity on contaminant transport.

7

Figure 3.5b

The effect of source area concentrations on the distance required to reach
cleanup standards.
High NAC
Moderate NAC
Concentration
Very Low NAC
Distance Along Flow Path
Distance Along Flow Path
High concentration, low NAC
Concentration
Lower concentration, low NAC
High concentration, higher NAC
Lower concentration, higher NAC
Cleanup Standards

©2001 CRC Press LLC

3.4 PROCESSES AFFECTING NATURAL ATTENUATION OF
COMPOUNDS
3.4.1Movement of Contaminants in the Subsurface

Even in the absence of biotic and/or abiotic transformations of a contaminant,
the contaminant always is subject to transport processes — meaning that physical
processes cause it to move. All important transport processes for subsurface con-
taminants can be categorized as dilution, advection, dispersion, or “phase transfer”
(from one type of physical medium to another, such as from an NAPL to groundwater
or from water to the soil matrix).

3.4.1.1 Dilution (Recharge)

Recharge is the amount of water entering the saturated zone of the water table
at the water table surface, made available mainly by precipitation events. In recharge
areas, flow near the water table is generally downward. Recharge defined in this
manner may therefore include not only precipitation that infiltrates through the
vadose zone, but also water entering the groundwater system via discharge from
surface water bodies. Where a surface water body is in contact with or is part of the
groundwater system, the definition of recharge is stretched slightly. However, such
bodies often are referred to as

recharging lakes

or

streams

.

8

The recharge of the
water table aquifer has two effects on the natural attenuation of a dissolved contam-
inant plume: 1) additional water entering the system due to infiltration of precipita-
tion or from surface water will contribute to dilution of the plume and 2) the influx
of relatively fresh, electron-acceptor-charged water will alter the geochemical pro-
cesses and in some cases, facilitate additional biodegradation.

8,9
Recharge from infiltrating precipitation is the result of a complex series of
processes in the unsaturated zone. Description of these processes is beyond the scope
of this chapter; however, it is worth noting that the infiltration of precipitation through
the vadose zone brings the water into contact with the soil and thus may allow the
introduction of electron acceptors (such as NO

3
–

and SO

4
2–

) in addition to the DO in
the recharge water and also dissolved organic carbon (electron donor). Infiltration
therefore provides fluxes of water, inorganic species, and possibly organic species
into the groundwater. In the case of surface water it may be connected as part of

the groundwater system, or it may be perched above the water table. In either case,
the water entering the groundwater system will not only aid in dilution of a con-
taminant plume, but it may also add electron acceptors and possible electron donors
to the groundwater.
An influx of electron acceptors will tend to increase the overall assimilation
capacity of the groundwater system. In addition to the introduction of electron
acceptors that may be dissolved in the recharge (e.g., dissolved oxygen, nitrate, or
sulfate), the infiltrating water may also foster biogeochemical changes in the aquifer.
For example, Fe

2+

will be oxidized back to Fe

3+

and will be precipitated out. This
reprecipitation of Fe

3+

could be again available for reduction by microorganisms.
Such a shift may be beneficial for biodegradation of contaminants utilized as electron

©2001 CRC Press LLC

donors, such as fuel hydrocarbons or vinyl chloride. However, these shifts can also
make conditions less favorable for reductive dechlorination.
Evaluating the effects of recharge can be difficult. The effects of dilution might
be estimated if one has a detailed water budget for the system in question. However,

if a plume has a significant vertical extent, it cannot be known with any certainty
what proportion of the plume mass is being diluted by the recharge. In addition,
separating the effects of dilution from other processes of mass reduction may be
difficult. After recharge, the effects of the addition of electron acceptors may be
apparent due to elevated electron acceptor concentrations, differing patterns in elec-
tron acceptor consumption, or by-product formation in the area of recharge. How-
ever, the effects of short-term variations in such a system (which are likely due to
the intermittent nature of precipitation events in most climates) may not easily be
quantified. Where recharge is from surface water, the influx of mass and electron
acceptors is more steady over time. In this scenario, quantifying the effects of dilution
may be less uncertain, and the effects of electron acceptor replenishment may be
more easily identified (although not necessarily quantified).
In some cases the effects of recharge-diluting contaminant plumes can be esti-
mated with a simple relationship based on the specific discharge of groundwater
passing through the point of interest and the amount of recharge entering the plume
area. It is imiportant to note that at most sites, recharge will not actually mix with
groundwater in an aquifer but will form a stratified layer on top due to the very low
amount of vertical dispersion characteristic of aquifer systems. Mixing can be
assumed in some cases, such as a very thin, unconfined aquifer: the aquifer dis-
charges into a surface water body, and the groundwater associated with the recharge
is assumed to be mixed with the original groundwater flowing past a source zone.

8-10

The relationship for estimating the amount of dilution caused by recharge is
(3.1)
Eliminating the width and rearranging gives:
(3.2)
where
C

L

= concentration at distance L from origin assuming complete mixing of
recharge with groundwater (mg/L)
C

0

= concentration at origin or at distance L = 0 (mg/L)
R=recharge mixing with groundwater (ft/yr)
W=width of area where recharge is mixing with groundwater (ft)
L=length of area where recharge is mixing with groundwater (ft)
dc
dt
dc
dt

©2001 CRC Press LLC

V

D

= Darcy velocity of groundwater (ft/yr)
T

h

=thickness of aquifer where groundwater flow is assumed to mix com-

pletely with recharge (ft)

3.4.1.2 Advection

Transport of a contaminant molecule occurring with the groundwater movement
is called advection or convection or bulk flow. Advection occurs in any moving fluid.
Thus, contaminants can advect when they are in air in soil pores or in a moving
NAPL, as well as in water. Advection transport is illustrated simply by considering
a contaminant that does not react biotically or abiotically (also known as conservative
compound or tracer) in the subsurface and that moves at the average velocity of the
groundwater. Figures 3.6a and b describe this phenomenon. The contaminant moves
at exactly the same velocity as the water and does not change from its initial
concentration of C

0

¢

at the injection point.

9

Figure 3.6a

Dispersion of a pulse of a tracer substance in a sand column experiment.

Figure 3.6b

Concentration curves showing plug ßow with an instantaneous source from
advection only and from a combination of advection, dispersion, and sorption.

Concentration (C)
Distance (x)
t
0
1
t
2
t
time=
t
time=
t
1
time=
t
2
Q
Advection and
Dispersion
Advection,
Dispersion,
and Sorption
Advection
Only
Advection
Only
Initial
Contaminant
Slug
Distance From Source

0
0.5
1.0
C
/
C
O

©2001 CRC Press LLC

The mass flux rate at which a dissolved contaminant moves across a vertical
plane in the subsurface is the product of the contaminant concentration and the
velocity of groundwater. Groundwater velocity is governed by three key factors
specific to each site:

• The

hydraulic gradient

includes gravity and pressure components and is the
driving force for water movement. Water always moves in the direction of higher
hydraulic head (which can be thought of qualitatively as elevation) to lower head.
•

Hydraulic conductivity

is the ability of porous rocks or soil sediments to transmit
fluids and is measured from field tests or samples. Hydraulic conductivity values
for common rocks and sediments vary over ten orders of magnitude from almost
impermeable crystalline rocks to highly permeable gravels; the hydraulic conduc-

tivity values for fractured rocks, sand, and clay are between these extremes. A
contaminant plume moving with the groundwater will travel faster through sand
layers, which have high hydraulic conductivity, than through clays of low hydrau-
lic conductivity, under the same hydraulic head gradient.
•

Porosity

is a measure of the volume of open spaces in the subsurfaces relative to
the total volume. Like hydraulic conductivity, it depends on the type of geologic
material present and can be determined from field tests or samples.

The equation for describing the rate of groundwater flow from one location to
another is known as Darcy’s equation:
(3.3)
where
K

H

=hydraulic conductivity (in units of distance per time)
=hydraulic gradient
V

D

= Darcy velocity (in units of distance per time)
To determine the seepage velocity of a contaminant that travels at the same speed
as the groundwater, the Darcy velocity must be divided by the effective porosity

e

:
(3.4)
K

H

and

e

can be estimated using various field test methods or laboratory evaluations
of cores taken from the subsurface. Uncertainty is inherent in all such measurements,
and this uncertainty must be acknowledged by developing a range of possible flow
scenarios.
VK
h
X
DH
=-
D
D
D
D
h
X
V
V
D

=
e

©2001 CRC Press LLC

3.4.1.3 Dispersion

Spreading of contaminants from the main direction of groundwater flow takes
place as the groundwater moves, altering concentrations from those that would occur
if advection were the only transport mechanism. This mixing is called hydrodynamic
dispersion. The mechanisms causing dispersion within the plume include molecular
diffusion, different water velocities within individual pores, different water velocities
between adjacent pores, and tortuosity of the subsurface flow path (Figure 3.7).
Mixing caused by local variations in velocity is also known as mechanical dispersion.
Groundwater scientists quantify the combined mixing effect using a hydrodynamic
dispersion coefficient D

H

. Except at very low water velocities, D

H

increases linearly
with the average speed of groundwater.
The curve labeled “dispersion” in Figure 3.6 a and b illustrates the effects of
dispersion for a conservative contaminant that travels precisely with the water mol-
ecules. The solute is detected at the observation well before it would be if advection
were the only process affecting its movement. Dispersion causes the solute to spread,
rather than moving as an unchanged “plug.”

Molecular Diffusion:

Molecular diffusion takes place as a result of the contam-
inant gradients created within the zones of contamination. It is significant only when
the groundwater velocities are low, and the diffusive flux of a dissolved contaminant,
at steady state, can be described by Fick’s first law.
(3.5)
where
F=mass flux of solute per unit area of time
D=diffusion coefficient
C=solute concentration
= concentration gradient

Figure 3.7

Seemingly random variations in the velocity of different parcels of groundwater
are caused by the tortuous and variable route the water must follow.
Flow Direction
Average Water
C'
B'
A'
C
B
A
FD
dc
dx
=-

dc
dx

©2001 CRC Press LLC

For systems where the dissolved contaminant concentrations are changing with
time, Fick’s second law must be applied. The one-dimensional expression of Fick’s
second law is
(3.6)
where, is the change in concentration with time.
The process of diffusion is slower in porous media than in open water because
the contaminant molecules must follow more tortuous flow paths. To account for
this, an effective diffusion coefficient D* is used. Fetter estimates a range of 1

¥

10

–9

to 2

¥

10

–9

m

2

/S for D* has been estimated.

9(a)

The effective diffusion coefficient is expressed quantitatively as
D* = wD (3.7)
where w is the empirical coefficient determined by laboratory experiments. The
value of w ranges greatly from 0.01 to 0.5.

9

Mechanical Dispersion:

Mechanical dispersion occurs due to variations in flow
velocity because of varying pore throat sizes and tortuosity caused by variations in
flow path lengths. An additional cause of mechanical dispersion is variable friction
within an individual pore, thus allowing the groundwater flowing in the center of
the pore to move faster than groundwater flowing next to the soil particle itself.
The component of hydrodynamic dispersion contributed by mechanical disper-
sion can be described as:
mechanical dispersion =

µ

x
V(3.8)
where
µ

x
=dispersivitiy
V=seepage velocity
Advection dispersion equation: The advection-dispersion equation, which
includes hydrodynamic dispersion, can be described as:
8,9
(3.9)
where
c=contaminant concentration
t=time
D
H
=hydrodynamic dispersion
x=distance along flow path
V=seepage velocity
dc
dt
D
d
dx
c
=
2
2
dc
dt
∂
∂
=
∂

-
∂
∂
c
t
D
c
Ox
V
c
x
H
2
2
©2001 CRC Press LLC
3.4.2 Phase Transfers
Contaminants will be added or removed from the groundwater when they transfer
between phases. The relevant phases in the subsurface are groundwater (dissolved),
soil grains (adsorbed), NAPLs (liquid), and soil gas (air) in the vadose zone. Phase
transfers can increase or decrease the contaminant concentration within the ground-
water plume, depending on the transfer mechanism, the contaminant, and the
geochemistry. Although the basic concepts of phase transfer are straightforward,
quantification of these transfers often is not easy.
3.4.2.1 Sorption
Many contaminants, including chlorinated solvents, BTEX and dissolved metals,
are removed from solution by sorption onto the aquifer matrix, thus slowing the
movement of contaminants. This slowing of contaminant transport is called retardation
of the contaminant relative to the average seepage velocity of groundwater and results
in a reduction in dissolved organic concentrations in groundwater. Sorption can also
influence the relative importance of volatilization and biodegradation. Figure 3.6b

illustrates the effects of sorption on an advancing dissolved contaminant front.
Sorption is a dynamic and reversible reaction; thus, at a given solute concentration,
some portion of the contaminant is partitioning out of solution onto the aquifer matrix,
and some portion is desorbing and reentering solution. As solute concentrations change,
the relative amounts of contaminant that are sorbing and desorbing will change. For
example, as solute concentrations decrease due to other factors such as biodegradation
and dilution, the amount of contaminant reentering solution will probably increase.
The affinity of a given compound for the aquifer matrix will not be sufficient to isolate
it permanently from groundwater, although for some compounds the rates of desorption
may be so slow that the adsorbed mass may be considered as permanent residual
within the time scale of interest. Sorption, therefore, does not permanently remove
solute mass from groundwater; it merely retards migration.
The various mechanisms that cause sorption effects to take place within the
aquifer matrix are described in detail in Chapter 2. Because of their nonpolar
structure, hydrocarbons most commonly exhibit sorption through the process of
hydrophobic bonding. When the surfaces comprising the aquifer matrix are less
polar than the water molecule, as is generally the case, there is a strong tendency
for the nonpolar contaminant molecules to partition from the groundwater and sorb
to the aquifer matrix. This phenomenon, referred to as hydrophobic bonding, is an
important factor controlling the fate of many organic pollutants in soils. As described
in Chapter 2, two components of an aquifer have the greatest effect on sorption:
organic matter and clay minerals. In most aquifers, the organic fraction tends to
control the sorption of organic contaminants.
Sorption Models and Isotherms: Regardless of the sorption mechanism, it is
possible to determine the amount of sorption to be expected when a given dissolved
contaminant interacts with the materials comprising the aquifer matrix. Bench-scale
experiments are performed by mixing water-contaminant solutions of various con-
centrations with aquifer materials containing various amounts of organic carbon and
©2001 CRC Press LLC
clay minerals. The solutions are then sealed with no headspace and left until equi-

librium between the various phases is reached. (True equilibrium may require hun-
dreds of hours of incubation, but 80 to 90% of equilibrium may be achieved in one
or two days.) The amount of contaminant left in solution is then measured.
The results are commonly expressed as a plot of the concentration of chemical
sorbed (mg/g) vs. the concentration remaining in solution (mg/L). The relationship
between the concentration of chemical sorbed (C
a
) and the concentration remaining
in solution (C
s
) at equilibrium is referred to as the sorption isotherm because the
experiments are performed at constant temperature (Figure 2.11). Sorption isotherms
generally exhibit one of three characteristic shapes, depending on the sorption
mechanism: the Langmuir isotherm, the Freundlich isotherm, and the linear isotherm
(a special case of the Freundlich isotherm).
Retardation: As mentioned earlier, sorption tends to slow the transport velocity
of contaminants dissolved in groundwater. When the average velocity of a dissolved
contaminant is less than the average seepage velocity of the groundwater, the con-
taminant is said to be retarded. The coefficient of retardation, R, is used to estimate
the retarded contaminant velocity. The variation between the velocity of the ground-
water and that of the contaminant is caused by sorption and is quantified by the
coefficient of retardation, defined as:
(3.10)
where
R=coefficient of retardation
V=average seepage velocity of groundwater parallel to groundwater flow
V
c
=average velocity of contaminant parallel to groundwater flow
The ratio (V/V

c
) describes the relative velocity between the groundwater and the
dissolved contaminant. When K
d
= 0 (no sorption), the transport velocities of the
groundwater and the solute are equal (V/V
c
). If it can be assumed that sorption is
described adequately by the distribution coefficient (valid when the fraction of
organic carbon (f
oc
) > 0.001), the coefficient of retardation for a dissolved contam-
inant is described by the following equation:
9
(3.11)
where
R=coefficient of retardation
r
b
=bulk density of aquifer
K
d
=distribution coefficient
n=porosity
R
V
V
c
=
R

K
n
bd
=+1
r