With Contributions From Ethan
Grossman and Jennifer
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
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 the aquifer?
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:
- The Canadian government Contaminated Sites Management Working Group
publishes a Site
Remediation Technologies Reference Manual with much more detailed
- The US Environmental Protection Agency also provides information
on remediation technologies.
- 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
- 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 contaminant?
- Does it dissolve in water?
- How does it react with oxygen, rock, or sediments in the aquifer?
- 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 leaked?
- What is the extent of the aquifer? How deep? How wide? Location of
- 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
- 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,
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 the predictions.
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 factors:
Many remediation methods are used. The more common are:
- Hydrogeologic setting
- Contaminant characteristics
- Physical properties (sink or float)
- Chemical properties (solubility, sorption)
- Subsurface access, land use
- Cost. All are expensive, and some are much more expensive than others.
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.
From 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 permeability zones,
- difficulty operating below 9m (30ft),
- difficulty 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 bacteria.
Carolina Division of Pollution Prevention and Environmental Assistance
- 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
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 to Phytoremediation.
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 waste sites.
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.