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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.