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Groundwater Remediation Steps

Groundwater Remediation Steps

          (http://oceanworld.tamu.edu/resources/environment-book/groundwaterremediation.html)

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 such as gasoline float on water. LNAPL such as gasoline

 

light non-aqueous phase liquids in an aquifer 

Dense Non-Aqueous Phase
Liquids DNAPL such as dry-cleaning solvents sink in water.

 dense non-aqueous phase liquids in an aquifer

 

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.

 excavation of leaking gasoline tank


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 onBiogeochemical
    and Geohydrologic Processes in a Landfill-Impacted Alluvial Aquifer,
    Norman, Oklahoma    .

    monitoring well locations Norman Oklahomamonitoring well locations Norman Oklahoma 


  3. 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.

plume-extent Norman landfill
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.
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