With Contributions From Ethan
Grossman and Jennifer McGuire
Contaminants must often be removed from groundwater before it reaches
wells used by agriculture and municipal water systems. The removal of
containment of pollutants is called remediation. In this chapter we will
examine the various ways pollutants are evaluated, traced,
and removed from groundwater.
When Is It Needed?
Remediation is required when concentrations
of contaminants exceed or are expected to exceed predetermined levels
for the type of resource that is impacted. For example, lead levels in
drinking water should not exceed the EPA action level of 0.015 mg/L.
What caused the high levels of lead? How can the lead be removed from
Under Subtitle I of the 1984 Resource Conservation and
Recovery Act (RCRA), Congress directed the Environmental Protection Agency
to publish regulations that would require owners and operators of new
tanks and tanks already in the ground to prevent, detect, and clean up
releases. Title XV, Subtitle B of the Energy Policy Act of 2005 (entitled
the Underground Storage Tank Compliance Act of 2005) contains amendments
to Subtitle I of the Resource Conservation and
Recovery Act. This new law
significantly affects federal and state underground storage tank programs,
will require major changes to the programs, and is aimed at reducing
underground storage tank releases to our environment.
EPA's federal underground storage tank (UST) regulations
require that contaminated UST sites must be cleaned up to restore and
protect groundwater resources and create a safe environment for those
who live or work around these sites. Petroleum releases can contain
contaminants like MTBE and other contaminants of concern that can make
water unsafe or unpleasant to drink. Releases can also result in fire
and explosion hazards, as well as produce long-term health effects.
From Cleaning Up
Underground Storage Tank System Releases.
Under Subtitle D of RCRA, Congress directed the EPA to
issue regulation for control and clean up landfill releases. Other federal
rules and regulations cover remediation of superfund sites.
Six Steps to Treating a Contaminated Aquifer
Suppose a gas-station owner discovers that a gasoline tank
has been slowly leaking for many years. Where has the gasoline gone?
Will it cause a problem? How can the leaked gasoline be cleaned up, especially
if it has reached an aquifer? The owner now has a groundwater-remediation
Or suppose a contractor building a new office tower notices
gasoline fumes coming from below the foundation. What is the source?
Can the aquifer below the site be cleaned up so fumes will not leak into
the building after it is built? This too is a groundwater-remediation
Remediating groundwater contamination is not easy or cheap.
Several organizations have published useful web pages on remediation:
Canadian government Contaminated Sites Management Working Group publishes
Remediation Technologies Reference Manual with much more detailed
- The US Environmental Protection Agency also provides information
- The US Geological Survey, through their Toxic Substances
Hydrology Program provides useful information.
1. Discovery and Source Determination
What do we know about the source and the contaminant?
- What contaminant(s)
are leaking into the aquifer?
- Many different types of chemical compounds are released
from landfills, industrial activity, end even and gasoline
tanks. You may thing gasoline is relatively simple, but in
addition to many volatile organic (carbon-based) compounds, gasoline
also has toxic anti-knock compounds such as tetraethyl lead
(up until 1986) and methyl tertiary-butyl ether (MTBE).
- How long has the source leaked contaminants?
- What is the spatial extent of the source?
- Is it contained to a relatively small area such as
a leaking gasoline tank?
- Is it spread over many acres, such as a leaking landfill?
- Is it spread over many square kilometers such as a military
installation or large mine?
- What are the physical properties
of the contaminant?
- Density (is it heavier or
lighter than water)?
- What are the chemical properties of the
it dissolve in water?
- How does it react with oxygen, rock, or sediments in the
- What is their concentration at various locations of an extended
source? Large industrial sites may have multiple leaking underground
tanks and disposal areas scattered over many square kilometers.
- What is
the effect of the toxic contaminant on plants,
animals, humans or an ecosystem?
- Is the toxiticity high
enough to kill people or wildlife?
Density effects how contaminants move through an aquifer. Light Non-Aqueous
Phase Liquids LNAPL such as gasoline float on water.
Dense Non-Aqueous Phase Liquids DNAPL such as dry-cleaning solvents
sink in water.
2. Removal of the Source
Once a source has been found, the most important first step toward
remediation is to remove the source if feasible. Removal often involves
excavation of leaky tanks and contaminated soil. Once the source
is removed, the next step is to clean up contaminated water still
in the ground.
Employees of Cortland Pump and Equipment Company and Sherman Vincent
Associates General Contractors remove the concrete above the gasoline
storage tanks at a gas station in Jacksonville, FL.
3. Site Characterization
What do we know about the geologic and
hydraulic properties of the aquifer into which the contaminants
- What is the extent of the aquifer? How deep? How
wide? Location of aquatards?
- What are the physical properties of the aquifer?
- Pore size?
- Sediment or rock type?
- Hydraulic diffusivity?
- How fast does the water flow through the aquifer?
- What are the chemical properties of the rock and sediment
within the aquifer?
- How pure is the aquifer upstream of the source of contamination?
This helps separate what is introduced by the source
from what is otherwise occuring in the aquifer.
- What gases are disolved in the aquifer. For example,
how much oxygen is in the water?
4. Impact Evaluation
- What has happened to the the contaminant within the aquifer?
- How far has has the contaminant spread?
- Has the chemical composition changed due to natural remediation?
- Answers to these questions comes mostly from a multiple
well drilled into the aquifer.
- The monitoring wells give the extent
of the plume of contaminants and the rate at which they move through
the aquifer. Wells are expensive to drill and operate, so much
care must go into selecting sites for wells. And, because few wells
can be drilled, our ability to visualize the subsurface is less
than perfect. We are "looking"
Here is information from the Norman, Oklahoma landfill that is
leaking contaminants into an aquifer that runs under the site.
It illustrates how one site was evaluated.
The remediation efforts are described by the US Geological Survey's
Report on Biogeochemical and Geohydrologic
Processes in a Landfill-Impacted Alluvial Aquifer, Norman, Oklahoma.
Left: Drilling monitoring wells
at Norman, Oklahoma landfill site. Right:
Location of monitoring wells at Norman, Oklahoma landfill. The
location of the landfill is shown in red and orange.
Click on images for a zoom.
Concentration of non-volatile dissolved organic carbon (DOC) in
the plume of contaminated water from the Norman, Oklahoma landfill
as measured by monitoring wells. Well numbers are at top of graph.
It is not possible to completely monitor conditions within the plume
and to predict future changes. Models are used to help interpolate
conditions between monitoring wells, and to predict possible changes
in the future. Additional wells and monitoring will be needed to test
This involves removing or containing the plume of contaminants within
an aquifer. Many methods have been devised and used to treat the many
types of contaminants in the many types of aquifers. Eight of the more
common remediation methods are discussed below.
The method for remediation depends on the several
Many remediation methods are used. The more common are:
- Hydrogeologic setting
- Contaminant characteristics
- Physical properties (sink or float)
- Chemical properties (solubility,
- Subsurface access, land use
- Cost. All are expensive, and some are much more expensive
23 December, 2008
This involves removing contaminated groundwater from strategically
placed wells, treating the extracted water after it is on the
surface to remove the contaminates using mechanical, chemical,
or biological methods, and discharging the treated water to
the subsurface, surface, or municipal sewer system.
Environment Protection Agency.
The method has several limitations.
- The effectiveness depends
on the geology of the aquifer and the type of contaminant.
- It is slow, taking decades to centuries to remove contaminated
water yet it often fails to remove all contaminated water.
- It is very costly.
- It doesn't always work. Some contaminants stick to soil and
rock (they are adsorbed) and they cannot easily be removed
(desorbed). Non-Aqueous Phase Liquids NAPLs cannot be removed.
- Hydraulic Containment
Pumping water from wells can be done in such
a way that it changes the flow of water through an aquifer in ways
to keep contaminants away from wells used for cities or farms. The
technique works if the flow through the aquifer is relatively simple,
so the plume of contaminated water does not divide into different paths. It
is often used together with pump and treat, and it has the same limitations.
- Air Sparging/Soil Vapor Extraction
The limitations include:
- difficulty flushing in low
- difficulty operating below 9m (30ft),
extracting multicomponent phases.
An example of air sparging. This system was used to remove volatile
trichloroethylene from the soil and an aquifer below the Blaine Naval
Ammunition Depot east of Hastings, Nebraska. At one time during World
War II, the 50,000 acre facility produced 40% of all U.S. Navy munitions.
Later in the remediation air containing natural gas and triethyl phosphate
was pumped into the ground water to improve bioremediation by soil
From North Carolina Division of Pollution Prevention and Environmental
- In-situ Oxidation
This method injects an oxidant such as hydrogen peroxide (H2O2)
into the contaminated aquifer. The contaminant is oxidized,
primarily producing carbon dioxide and water.
An example of injection of chemicals that remove contaminants from
an aquifer. Here a permeable treatment zone is created by reducing
the ferric iron in the aquifer sediments to ferrous iron by injecting
a reducing reagent and appropriate buffers such as sodium dithionite
and potassium carbonate.
Click on the image for a zoom.
From Field Hydrology
and Chemistry Group of the Pacific Northwest National
Laboratory of the US Department of Energy.
- Permeable Reactive Barriers
This methods uses a trench backfilled with reactive material
such as iron filings, activated carbon, or peat, which absorb
and transform the contaminant as water from the aquifer passes through
the barrier. This works only for relatively shallow aquifers.
Left: A Dewind Trencher installs a permeable treatment barrier in a
trench. Click on the image for a zoom. Right: The barrier absorbs contaminants
(in this image VOC is Volatile Organic Carbon) leaving treated water
to flow downstream in the aquifer. Click
on the image to download a 1.3 MByte animation.
Right: From Dewind
One Pass Trenching. Left:
From EPA Research
Some plants accumulate heavy metals and metal like elements, such as
arsenic, lead, uranium, selenium, cadmium, and other toxins such as
nutrients, hydrocarbons, and chlorinated hydrocarbons. Chinese Ladder
fern Pteris vittata, also known as the brake fern, is a highly efficient
accumulator of arsenic. Genetically altered cottonwood trees
suck mercury from the contaminated soil in Danbury Connecticut. And,
transgenic Indian mustard plants to soak up dangerously high selenium
deposits in California. The remediation consists of growing such plants
so their roots tap the groundwater. Then, the plants are harvested
and disposed. The method is limited to remediation of groundwater that
is close enough to the surface that it can be reached by plant roots.
From Environmental Protection Agency: A Citizen’s Guide
A typical plant may accumulate about 100 parts
per million (ppm) zinc and 1 ppm cadmium. Thlaspi caerulescens (alpine
pennycress, a small, weedy member of the broccoli and cabbage family)
can accumulate up to 30,000 ppm zinc and 1,500 ppm cadmium in its shoots,
while exhibiting few or no toxicity symptoms. A normal plant can be
poisoned with as little as 1,000 ppm of zinc or 20 to 50 ppm of cadmium
in its shoots.
From US Department of Agriculture Phytoremediation:
Using Plants To Clean Up Soils
- Natural Attenuation
Sometimes natural processes remove contaminants with no human intervention.
The removal may involve dilution,
radioactive decay, sorption (attachment of compounds to
geologic materials by physical or chemical attraction), volatization,
or natural chemical reactions that stabilize, destroy, or transform contaminants.
- Intrinsic and Enhanced Bioremediation
Biodegradation is the breakdown of carbon-based
contaminants by microbial organisms into smaller compounds. The microbial
organisms transform the contaminants through metabolic or enzymatic
processes. Biodegradation processes vary greatly, but frequently the
final product of the degradation is carbon dioxide or methane. Biodegradation
is a key processes in the natural attenuation of contaminants at hazardous
Substances Hydrology Program.
Bacteria and archaea can metabolize hydrocarbons and other contaminants, converting
them to less toxic products. Some live deep underground, some live in the absence
of oxygen. Specific organisms are injected into the groundwater, and in some
cases, special nutrient are injected with the microbes. The method is especially
useful for remediation of hydrocarbons in groundwater.
Natural bioremediation occurs when naturally occurring bacteria living
in the aquifer degrade toxic contaminants into less toxic compounds.
Natural bioremediation is most effective in aquifers where bacteria
are plentiful, and where contaminant levels are low.
Enhanced bioremediation involves stimulating natural bacteria by injecting
nutrients and/or carbon compounds needed by the bacteria into the aquifer.
Nutrients and carbon compounds are injected into an aquifer (macroscale
sub-image) to stimulate naturally occurring bacteria living in biofilms
on sediment particles (microscale sub-image). The bacteria break
down contaminants such as trichloroethylene TCE into non-toxic compounds
such as carbon dioxide (mesoscale sub-image).
for Biofilm Engineering at Montana State University-Bozeman.