Groundwater Remediation

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 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 problem.

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 problem.

Online Information
Remediating groundwater contamination is not easy or cheap. Several organizations have published useful web pages on remediation:

1. The Canadian government Contaminated Sites Management Working Group publishes a Site Remediation Technologies Reference Manual with much more detailed information.
2. The US Environmental Protection Agency also provides information on remediation technologies.
3. The US Geological Survey, through their Toxic Substances Hydrology Program provides useful information.

Remediation Steps

1. Discovery and Source Determination

What do we know about the source and the contaminant?

1. What contaminant(s) are leaking into the aquifer?
   1. 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).
   2. How long has the source leaked contaminants?
2. What is the spatial extent of the source?
   1. Is it contained to a relatively small area such as a leaking gasoline tank?
   2. Is it spread over many acres, such as a leaking landfill?
   3. Is it spread over many square kilometers such as a military installation or large mine?
3. What are the physical properties of the contaminant?
   1. Density (is it heavier or lighter than water)?
   2. Viscosity?
4. What are the chemical properties of the contaminant?
   1. Does it dissolve in water?
   2. How does it react with oxygen, rock, or sediments in the aquifer?
5. 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.
6. Toxicology.
   1. What is the effect of the toxic contaminant on plants, animals, humans or an ecosystem?
   2. 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?

1. What is the extent of the aquifer? How deep? How wide? Location of aquatards?
2. What are the physical properties of the aquifer?
   1. Pore size?
   2. Sediment or rock type?
   3. Hydraulic diffusivity?
   4. How fast does the water flow through the aquifer?
3. What are the chemical properties of the rock and sediment within the aquifer?
   1. 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.
   2. What gases are disolved in the aquifer. For example, how much oxygen is in the water?
      

4. Impact Evaluation

1. What has happened to the the contaminant within the aquifer?
   
   1. How far has has the contaminant spread?
   2. Has the chemical composition changed due to natural remediation?
   3. Answers to these questions comes mostly from a multiple well drilled into the aquifer.
2. 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" through pinholes.
   
   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.
   

5. Modeling

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.

6. Remediation

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.

Remediation Methods

The method for remediation depends on the several factors:

1. Hydrogeologic setting
2. Contaminant characteristics
3. Physical properties (sink or float)
4. Chemical properties (solubility, sorption)
5. Subsurface access, land use
6. Toxicity-risk
7. Cost. All are expensive, and some are much more expensive than others.

1. Pump-and-treat
   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.
   1. The effectiveness depends on the geology of the aquifer and the type of contaminant.
   2. It is slow, taking decades to centuries to remove contaminated water yet it often fails to remove all contaminated water.
   3. It is very costly.
   4. 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.
      
      
2. 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.
   
   
3. Air Sparging/Soil Vapor Extraction
   The limitations include:
   1. difficulty flushing in low permeability zones,
   2. difficulty operating below 9m (30ft),
   3. 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.
      From North Carolina Division of Pollution Prevention and Environmental Assistance
      
      
4. 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.
   
   
5. 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 Highlights.
   
   
   
6. Phytoremediation
   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
   
   
7. 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.
   
   
8. 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.
   USGS Toxic 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).
   From Center for Biofilm Engineering at Montana State University-Bozeman.