lectures 17 groundwater contamination and remediation · remediation methods and examples ......
TRANSCRIPT
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Lectures 17
Groundwater contamination and remediation
Reading: chapter 6
Today
1. organic chemicals in the subsurface
2. organic contaminants
3. Remediation methods and examples
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Groundwater ContaminationToday we look at some contaminants and remediation (clean-up)
techniques.
Treatment of pollution problems require consideration of their
environmental introduction pathway, exit pathways, and
contaminant stability (how long-lived it is).
Recall that contaminants can enter the environment in 2 forms:
� point-source (localized)
� Dispersed
from 2 causes:
� accidental discharge
� purposeful application (e.g., pesticides, fertilizers, etc..)
Chemicals that find their way into the subsurface can
contaminate groundwater.
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Organic Contaminants in Groundwater
Typical Sources
� dispersion of pesticides/insecticides/herbicides/fungicides
purposefully applied in one area, which then later infiltrate a
groundwater system (described in the textbook).
� point-source contamination by hydrocarbon components
leaking from subsurface fuel storage or transfer vessels.
� other organic materials leaking from storage tanks at
industrial sites that lead to point source leaks.
� atmospheric dispersion and rainwater washout of organic
particulates and volatile organic compounds -VOC
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Rate and extent of dispersion
Contaminant migration in the subsurface environment will depend
on:
� the physical characteristics of the ground water flow in the area
(how permeable and anisotropic is the substrate?)
� how retarded this material is in this particular substrate
(how slow does it move relative to the water)
� chemical and/or biochemical reactivity
� Aqueous solubility
Organic Contaminants in Groundwater
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This and similar figures in this lecture modified from http://maven.gtri.gatech.edu/ward
Vadose
Saturated
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Dealing with contaminants
Groundwater is a major source of urban and rural drinking water
globally and organic contaminants are a pervasive problem.
After identification of elevated contaminant levels, treatment
considerations involve:
� solubility in H2O
� density (for non-aqueous dissolved contaminants)
� reactivity (decomposition, particle sorbability, Oxic/Anoxic
biodegradability)
� groundwater flow characteristics
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solubility in H2O and Density
1. Contaminants rich in O, N and S bearing functional groups
and with relatively low molecular weights tend to be more
water-soluble.
2. Contaminants with limited water solubility can still interact
with and be transported by ground water systems as a
function of density.
3. Low molecular weight-non-substituted or halocarbons tend
to be volatile (VOC).
Organic contaminants are usually divided upon into:
� volatile
� water soluble
� non-water soluble classes:
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solubility in H2O and Density
DNAPL -
“dense non-
aqueous
phase
liquids”.
These sink
to the base
of an aquifer
(like pcbs)
Many contaminated subsurface regions can have one or more separate organic
phases, plus an aqueous phase with dissolved organics.
NAPL - “non-aqueous phase liquids”, by definition density < water, so materials
float (e.g., many petroleum constituents).
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Reactivity and MobilityChemicals designed for environmental application are sometimes designed to persist for long periods of time.
If they migrate to unintended areas they can become very problematic.
A hazardous and persistent pollutant should be dealt with differently than a labile one
during contaminant treatment.
Some very reactive pollutants decompose rapidly, which an be either "good" or "bad",
depending on the decomposition pathway and decomposition product formed.
Reactivity includes inorganic and microbially mediated decomposition.
Contaminant mobility is a function of molecular interaction with substrate molecules
Greater retention of a contaminant by substrates means greater retardation relative to
ground water flow.
NAPL and DNAPL contaminants can also have high retention and be held in soil or
deeper substrate pore spaces for very long time periods.
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Some organic contaminants in the environment
Chapter 7 of your text describes many different types of organic
contaminants, including their sources and reactivity.
We will discuss a few types today but I also encourage you to
read through this part of the chapter.
There are so many potential man-made organic pollutants in the
world (most of which are given trade names, rather than standard
nomenclature names) that few people memorize even a small
subset of them.
However, you should be able to say something about a material's
affinity for solids in aqueous systems and retardation in
groundwater given its structural formula.
Next....
Some pollutant compounds by class:
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Benzene and derivatives (BTEX)
BTEX chemicals = Benzene, Toluene, Ethylbenzene, and Xylenes.
• Volatile monoaromatic hydrocarbons
• common in crude petroleum and petroleum products such as gasoline.
• aromatic, volatile, and carcinogenic
• not too hard to decompose.
megatons/year production as:
• industrial solvents
• starting materials for
o pesticides,
o plastics,
o synthetic fiber manufacture.
Major pollution causes:
• widespread occurrences of leakage from underground petroleum storage
tanks. Some people estimate that 35% of the 1.4 million gasoline storage
tanks in the United States are leaking.
• Spills at petroleum production wells, refineries, pipelines, distribution
terminals.
http://umbbd.ahc.umn.edu/BTEX/BTEX_map.html
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http://umbbd.ahc.umn.edu/BTEX/BTEX_map.html
BTEX degradation:
BTEX components degrade
at different rates.
O2 rapidly decreases in
environments where
aeration is limited.
Chemical oxygen demand
(“COD”) remains high
downflow of the first anoxic
point.
Fe and SO4 reducers can become established in the waste
stream and continue to degrade the contaminant DOC.
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Sequential biodegradation of BTEXcomponents from a gasoline leak.
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The class includes pesticides (e.g., DDT and lindane), and PCBs (used in the
electronics industry as large capacitor dielectrics and in the manufacture of
paints, adhesives and flame retardants).
Some were originally intended for purposeful introduction into the environment
(e.g., DDT) but have now been banned in some countries due to their
persistence in the environment and low retardation in groundwater. Others not
intended for dispersal were introduced into the environment accidentally.
Many pesticide halocarbons were designed to be volatile, water insoluble,
resistant to attack and easily absorbed by organisms.
Polychlorinated hydrocarbons:commonly as pesticides, solvents & lubricants. They are widespread
environmental pollutants. Nonpolar forms tend to bioaccumulate, and
organohalides are in general toxic.
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Fun Fact about DDT:DDT has the same lethal dose weight LD50 as Aspirin (25g, which for Aspirin is
about 76 - 333mg tablets).
Aspirin is water soluble and degradable DDT is not water soluble and only
somewhat degradable
You could take 0.5 to 1 g/day for 25-
50 days with no ill effects because it
is excreted by the body
The same dose of DDT would be lethal
because it accumulates rather than being
excreted
aspirin DDT
Cl
OH
Cl
CCl3
C-OCH3
O HC
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Electronegative halides
make the carbon
backbone relatively
oxidized compared to
many hydrocarbon
compounds.
More effectively
biodegraded in
reducing environments,
although there are also
oxidative degradation
mechanisms.
Organohalide degradation:
Catechol(benzene diol)
degradation intermediate
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Tetrachloroethane
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PAH:
These non-water-soluble materials are very reaction-resistant in
the environment. They are produced during combustion and are
common in soot of various types. They are produced by the
petroleum industry, bakeries, automobiles, coal burning, forest
fires, and other things. Typically immobile and non water soluble.
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Cyclic Ethers (both chlorinated and unchlorinated types):
These relatively resistant molecules are produced as side
products during organic matter combustion and are
manufactured as insecticides and herbicides (e.g., Dieldrin, a
"drin“, and tetrachlorodibenzo dioxin).
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Organophosphorous compounds:
This class of insecticides and pesticides was introduced in the 70's and 80's following the discovery of the destructive effects of some organo-chlorine pesticides.
Like the latter they are also lipophilic.
These organo P compounds are also fairly volatile but somewhat less resistant to attack than organo-chlorine compounds.
These polyfunctional molecules can be highly retarded in groundwater, unlike low retardation organo-chlorine compounds.
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Pyrethrin insecticides and related molecules:This class of naturally-occurring molecules (and synthetic analogues) are
sometimes considered safer because they are naturally-produced. Pyrethrin
itself is actually two closely related cyclopropane esters.
Can you identify areas of these molecules that you think would be succeptible
to chemical attack or substitution?
It is their
polyfunctional
but easily
degraded
structures (by
oxidation) that
make them
favorable over
other pest-
ridding agents.
Not a pyrethrin, but a structural cousin
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Organic pollutants and the biosphere
Plots show the relationships between lipophilicity (measured by solubility in n-
octanol), water solubility and bioaccumulation for some organic pollutants:
Besides drinking water sources, we have a responsibility to clean up organic
chemical spills because some can enter the food chain.
Most organic contaminants are fat soluble. Lipophilicity (desire to dissolve in lipids) is
a measure of how they will act in the biosphere.
As we move up the food chain, we can find even relatively dispersed pollutants
bioaccumulated and biomagnified (passed on in higher concentration at each step).
The contaminants can reach toxic levels in the food chain (discussed next week).
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The new materials produced in the biosphere
can be more or less toxic than the original,
more or less bioaccumulated and more of
less retarded in groundwater systems.
Minor changes in materials related to DDT by
substitution (both Rs = Cl is DDT) that reduce
lipophilicity, or increase water solubility or
degradability can significantly reduce
bioaccumulation.
Different R groups affect the π electrons of
the aryl rings, resulting in predictable trends:
the more basic the π electrons are, the less
bioaccumulated the molecules are (i.e.,
replacing Cl- with CH3O- decreases
bioaccumulation and putting in CH3- reduces
it further still).
Some organic contaminants can be modified but not entirely decomposed to other
materials in the biosphere.
Scientists can also modify materials synthetically in hopes of finding safer alternatives
for release in the environment.
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Remediation of organic and inorganic
contaminants in groundwater
Strategies for cleaning up an area contaminated with organics
depend on reactivity and toxicity, including:
� in-situ (in the ground) decomposition
� on-sight decomposition of pumped water (good for VOCs)
� intact removal to a remote site (e.g., by extracting them into a
solvent they prefer more than liquid water).
� removal to a remote site of contaminant plus substrate for
incineration or containment and burial (extreme cases).
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The 5 general techniques for removing
persistent organic gw contaminants are:1. Natural Attenuation - “wait and see”
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2. "pump and treat" - Water is removed by pumping from the
ground, treated by chemical means and reintroduced.
3. in-situ degradation - chemicals reactants are introduced into
the groundwater system to decompose and/or dissolve the
contaminant. This is desirable when practical because it can
be inexpensive and less disruptive
♦ Oxidants
♦ Reductants
♦ Steam
♦ Surfactants
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4. bioremediation - in-situ or above-ground decomposition of degradable pollutants by
organisms. Common types involve:
electron acceptors
♦ Bioventing
♦ Air O2 biosparging
♦ Cometabolism with inducers (toluene, methane)
♦ Oxygen releasing compounds
electron donors
♦ H2 biosparging
♦ H2 releasing compounds
5. "remove and treat" - used mostly for low-solubility and/or high retention pollutants
adsorbed to substrate particles, or pollutants floating above or settled below an aquifer.
Material is taken from the ground and destroyed elsewhere.
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Many problematic contaminants are organic:
Halocarbon, btex, heavy metal
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barriers (PRBs) in contaminated subsurfaces.
Metallic Fe "chemical barriers" are effective at
decomposing dispersed organic contaminants and are
most effective when installed near a point source of
contamination.
The wall rapidly decomposes persistent contaminants
(e.g., polychlorinated hydrocarbons) which, due to their
volatility, are difficult to pump and treat safely.
These barriers are inexpensive and can theoretically last
decades.
A newish in-situ decomposition technology involves installation of permeable reactive
PRB
PRB
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Why Fe(0) in PRBs
• Reduced character of Fe(0) makes electrons readily available - stored chemical energy.
• This energy can fuel redox reactions directly or can fuel microbial activity that can immobilize contaminants.
• FeOOH precipitates can immobilize metal contaminants via sorption and/or co-precipitation.
• Other metals besides Fe, or Fe-X alloys are also sometimes used.
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Organic halide molecules are decomposed in pH
dependent reactions with reduced Fe by analogy to
the oxidation of Fe by O2:
Fe0 + ½O2 + 2H+ ⇆ Fe2+ + H2O
becomes... Fe0 + RX + 2H+ ⇆ Fe2+ + RH + X-
(X is a halogen)
Fe is oxidized as the carbon atom bound to a
halogen becomes reduced (i.e., C-Cl becomes C-H).
Besides the above, two other reactions that may
contribute to the decomposition of RX on or near the
barrier are:
1. Fe0 +H2O ⇆ Fe2+ +OH- +H2
followed by H2 +RX ⇆ RH +X-
2. Fe2+ + H+ + RX ⇆ Fe3+ + RH + X-
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Note the reactivity (measured as
half-life) of
chlorinated ethylene follows
substitution type:
• trans isomers more reactive
than cis ones
• 1,2 dichloro isomers are more
reactive then 1,1 dichloro
isomers.
Others:
• tetrachloro isomers are
degraded more slowly than
dichloro ones;
• tetrabromo isomers are more
reactive than tetrachloro ones.
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Other uses…
PRBs also work for
• effluents of textile mills (containing organic dyes)
• pesticides such as toxaphene (a polychlorinated
campher derivative).
Other metals may be more efficient than Fe at removing a
specific contaminant.
For instance, in the aforementioned study, palladized iron,
Fe(Pd) removed 1,2 dichloroethene in a few hours, where
as the Fe barrier alone took over a month to do the same
job.
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Trenching a PRB for
TCE (tri-cholorethylene)
& Cr remediation
“ZVI” = Fe filings to
back fill trench
http://www.science.uwaterloo.ca/research/ggr/PermeableReactiveBarriers/Cr-TCE_Treatment/Cr-TCE_Treatment.html
PRBs work for inorganic
contaminants
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Such PRBs can reduce other inorganic wastes such as
This PRB application
demonstrates that
organic and inorganic
contaminants can be
degraded by these
types of “walls”.
� electroplating waste: Cr+4 (soluble) � Cr+3 (insoluble)
� radioactive waste: Tc+2 � Tc
� agricultural runoff: NO3- � NO2
-
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The plume BEFORE….
(plan view)
The plume
AFTER
(side view)
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Permeable Fe(0) barrier - highly reducing. Cr
reduction, also O2 reduction, perhaps Fe+2 formed
too, recall that Fe+2 is highly soluble.http://www.science.uwaterloo.ca/research_groups/ggr/TechnologyApplicationInquiries.html
Oxidizing, low pH
plume w/ Cr(VI)
An aside: Cr reduction by Fe metal PRB
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Some more about Permeable Reactive Barriers(see http://www.doegjpo.com/newsinfo/perspective/7-98/index.htm)
This technology involves placing a reactive material underground as a barrier to intercept and react with a contaminant plume in groundwater. Typically, PRBs are emplaced by replacing soils with reactive material in a trench cut through a contaminated groundwater aquifer.
These barriers can also be installed as a subsurface layer in a landfill or disposal cell or through injection wells directly into an aquifer. Selection of material for the barrier is based on results of treatability studies.
The material in the barrier is permeable, which allows the groundwater or contaminant plume to flow through the barrier. When the targeted contaminant encounters the reactive material in the barrier, a chemical reaction occurs with the barrier material that results in adsorption, mineral precipitation, or degradation to a harmless compound. Because the barriers do not incorporate motors or mechanical devices, the technology is considered passive treatment.