Notes on Groundwater Contamination
Contaminants must often be removed from groundwater before the groundwater reaches wells. The removal of pollutants is called remediation.
Releases may contain volatile contaminants like benzene, MTBE or chlorinated solvents that can intrude into inhabited spaces.
Suppose you discover that a tank has leaked gasoline, diesel, engine oil, or the like; where did the leaking fluids go?
In the illustration below:
NAPL = Non-Aqueous Phase Liquid (a liquid that is not water)
LNAPL = Light NAPL (a liquid that is not water and lighter than water)
LNAPL includes petroleum products, like gasoline, diesel, engine oils or lubricants
Commentary:
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Smear Zone is the up-down-up-down-up…, vertical zone where groundwater table fluctuates. Fluctuations come from seasonal recharge by rainfall, from tides, or local pumping. In this way, petroleum that rests on the water table gets “smeared” unto the soil particles.
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Dispersion and dilution diminish the presence of the contaminant in the groundwater. |
- Dispersion is the action of separating and moving in different directions. The picture shows an invasive weed dispersing its seed with the power of the wind.
- Dilution is the action of making thinner or less concentrated. In the case of dispersing plume, particles of the plume disperse from the main mass into the surrounding groundwater where they get diluted.
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As the LNAPL percolates down the soil column, it leaves behind masses trapped in the pores; it’s like the pores become little containers holding the fluids in space.
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Part of the LNAPL gets “sorbed” to the faces of the soil particles. The word “sorb” is used generically to mean “absorbed” or “adsorbed” which in essence are same phenomenon. When a fluid particle is “sorbed” it sticks to the surface of the soil particle by reason of electrical forces of attraction on the molecular level.
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In the case of discharge of a substance that is heavier than water (for example, chlorinated solvents such as used in dry cleaning), the contaminants will sink to the bottom of the aquifer.
In the illustration below:
DNAPL = Dense NAPL (a liquid heavier than water)
Commentary:
- An Aquiclude is an impermeable bed of geologic material that hinders or prevents groundwater movement. Aquicludes separate between upper and lower aquifers.
- Shallow, unconfined aquifers are prone to getting polluted from surface spills; therefore, most groundwater nowadays is obtained from lower aquifers that are separated by confining aquicludes. However, this protection can, at times, be breached through improper man-made piercing of the confining layer or inherent discontinuities of the layer. Sinking chlorinated solvents can thus reach and impair deeper aquifers through breaches in aquicludes.
Acknowledgment:
Materials in this newsletter have been borrowed from these sources:
Oceanography Department, Texas A&M University; http://oceanworld.tamu.edu/resources/oceanography-book/groundwaterremediation.html
http://www.johnston-independent.com/groundwater_recharge.html
http://www.cees.no/?option=com_research&project=69
http://www.univie.ac.at/qccd/hpsc/2003/Podloucky/Teil1/index.html
http://www.groundwater.org/gi/gwglossary.html |
