Transcript Slide 1

Groundwater Pollution
150506 GW 10 Monitored Natural
Attenuation
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• These lectures were adopted from
“ENHANCED ATTENUATION: A
REFERENCE GUIDE ON APPROACHES
TO INCREASE THE NATURAL
TREATMENT CAPACITY OF A SYSTEM”
August 2006 Washington Savannah
River Company. Prepared for the U.S.
Department of Energy
• www.cluin.org/download/contaminantfocus
/tce/DOE_EA_doc.pdf
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• Monitored natural attenuation
(MNA) and enhanced attenuation
(EA) are two environmental
management strategies that rely on
various processes to degrade or
immobilize contaminants and are
used at sites where contaminant
plumes have low risk and are not
growing.
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Monitored Natural Attenuation
The term ‘monitored natural attenuation’ …refers
to the reliance on natural attenuation processes
(within the context of a carefully controlled and
monitored clean-up approach) to achieve sitespecific remedial objectives within a time frame
that is reasonable compared 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 mass, toxicity, mobility,
volume, or concentration of contaminants in soil
or groundwater.
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Monitored natural attenuation
A strategy for in situ remediation.
MNA relies on the naturally
occurring physical, chemical, and
biological processes.
MNA can lessen concentrations of
certain contaminants in
groundwater, enough to protect
human health and the
environment.
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Monitored natural attenuation
The changes in contaminant
concentrations are monitored
through wells that are placed
throughout the contaminated
groundwater zone.
These show the level of
contamination over time and its
movement in the subsurface.
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The important initial step in MNA
includes some form of primary
source treatment to reduce source
mass (and mass flux). Both modeling
and field investigations indicate that
reducing source mass leads to
decreases in the mass flux feeding a
plume.
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However, the magnitude of reduction in
mass flux attained by source
treatment depends on a combination
of site specific factors including
source contaminant architecture and
site specific hydrogeologic conditions.
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Rao (2001) looked at dense nonaqueous phase liquid (DNAPL)
source reduction and suggested
the following:
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It is proposed that not exceeding a
threshold contaminant flux across a
control plane, rather than contaminant
concentration at a monitoring point or
contaminant mass reduction, should be
used as the basis for evaluating the
effectiveness of source-zone remediation.
The threshold contaminant flux should be
set equal to the natural attenuation
capacity within the dissolved plume.
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This statement makes the
important connection among
source treatment, mass flux
reduction, and the goal of
achieving a balance between flux
and natural attenuation capacity.
Attaining this objective ensures a
stable plume (i.e. one that does
not expand over time).
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The impact of MNA on a cVOC
plume can be illustrated in Figure
2-1. As indicated, the resultant
flux at the control plane exceeds
the regulatory limit showing that
the attenuation capacity of the
system is insufficient for MNA to
be a viable treatment alternative.
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2.1 NATURAL ATTENUATION
PROCESSES
The natural attenuation capacity of a
hydrogeologic system results from the
combined impact of several natural
processes.
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Examples include physical, chemical, and
biological mechanisms operating within
hydrogeologic systems by which
contaminants are either diluted, degraded
to innocuous byproducts, or their rate of
migration is retarded. All of these
processes operate without human
intervention.
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Organic versus inorganic contaminant plume.
Natural processes are active within the groundwater aquifer.
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2.1.1 Physical Attenuation
Processes
Mass transfer of contaminants to
groundwater in the source area
creates a dissolved phase plume
that transports contaminants by
advection.
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In addition, a vapor phase plume
will develop for cVOC
contaminants in the vadose zone.
Vapor phase contaminants can
outgas to the atmosphere and be
transferred to groundwater by
entrainment in infiltrating
precipitation or by diffusion.
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These are important physical
attenuation mechanisms for
cVOCs that operate in the vadose
zone.
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As shown in Figure 2-1, as the
contaminant plume migrates it will
be influenced by various
attenuation processes. Each
process contributes to the overall
attenuation of contaminants.
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In fact, what happens is a reduction
of the amount of contaminant
mass passing through a control
plane per unit time (mass flux).
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The location of the control plane
may be defined by a compliance
agreement or may be associated
with discharges to the receptor.
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Dispersion and diffusion are
examples of physical attenuation
mechanisms that occur in
groundwater. Hydrodynamic
dispersion leads to physical
dilution of contaminants in
groundwater resulting in an
increase in the size (volume) of
the plume.
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Diffusion results in contaminants
migrating into low permeability
parts of the aquifer where they
are sequestered in the small
pores present in clay-rich material
and porous bedrock and only
released slowly into groundwater
by back-diffusion.
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Eventually a steady state condition
will exist for diffusion and
dispersion. The combination of
processes reduces both the
concentration and mass flux of
contaminants.
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2.1.2 Chemical Attenuation
Processes
Sorption includes both physical
(absorption) and chemical
(adsorption) attenuation processes by
which cVOCs are partitioned into the
sorbing medium (e.g., soil organic
material) or attach to the surfaces of
certain solid phases, respectively.
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Sorption of cVOCs can occur in
both the vadose and saturated
zones. Sorption causes a
reduction in the rate of migration
of contaminants in aquifers
resulting in a reduction in mass
flux. For convenience, both
mechanisms are treated in this
section.
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In equilibrium sorption a steady
state will be reached in which the
rate of sorption becomes equal to
the rate of re-release resulting in
no further changes in mass flux.
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For some contaminants under
certain conditions sorption is
sometimes referred to as
“irreversible”. Irreversible sorption
is when a chemical species is
more strongly bound to the
sorbing medium during desorption
than during initial sorption.
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The term sometimes appears to be
used to describe a situation
where once sorbed, the
contaminant is essentially
permanently removed from the
plume and remains associated
with the sorbing aquifer (or
vadose zone) material.
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The net result of sorption is to
reduce the mass flux of
contaminants crossing the control
plane.
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2.1.3 Biological and Abiotic
Degradation Attenuation
Processes
Both biological and abiotic
processes degrade contaminants
into a variety of products.
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Aerobic and anaerobic bacteria
may metabolize the contaminants
or may reduce sulfate into sulfide,
which, in turn, can combine with
Fe(II) to form sulfide minerals
having the capability of
reductively dechlorinating cVOCs
to non-toxic products.
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Plant-based processes can result
in in situ destruction of cVOCs in
the root zone, uptake, storage,
metabolism, or translocation to
the atmosphere. In every case,
contaminants are removed from
the plume leading to a reduction
of mass flux.
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This and the previous slide show an example of a
network design for performance monitoring,
including target zones for monitoring effectiveness
with respect to specific remedial objectives.
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An EPA site for Monitored Natural
Attenuation is at
http://www.epa.gov/ada/gw/mna.ht
ml
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