Decontamination of Groundwater Bioremediation and Natural Attenuation

Bioremediation is the term applied to the decontamination of water or soil using biochemical rather than chemical or physical processes. Recently there has been interesting progress reported in using bioremediation to cleanse water of chlorinated ethene solvent contamination.

The biodégradation of chloroethenes by aerobic bacteria becomes less and less efficient as the extent of chlorination increases, so it is ineffective for perchloroethene. However, under anaerobic conditions, the reductive biodégradation of PCE and TCE proceeds more quickly, particularly if an easily oxidized substance such as methanol is added to supply electrons for the reduction processes. Unfortunately, the stepwise dechlorination of these compounds proceeds through vinyl chloride, CH2=CHC1, a known carcinogen. Recently, a bacterium has been discovered that removes all the chlorine from organic solvents such as TCE and PCE.

Owing to the high cost and limited effectiveness of many groundwater cleanup technologies, the inexpensive process of natural attenuation— allowing natural biological, chemical, and physical processes to treat groundwater contaminants—has become popular. Indeed, it is now used at more than 25% of Superfund program sites in the United States and is the leading method to remedy the contamination of groundwater from leaking underground storage sites.

However, there is great controversy about whether or not natural attenuation is an appropriate strategy for managing groundwater contamination: Many environmentalists feel that it is a cheap way for industry to avoid expensive cleanup costs. The U.S. National Research Council in 1997 appointed a committee to determine which pollutants could be treated successfully by this technique. Table 14-2 summarizes their results.

Likelihood of Success of Groundwater Remediation

by Natural Attenuation for Various Substances

Dominant Attenuation

Likelihood of Success Given

1 Chemical Class

Processes Current Level of Understanding |

Organic Compounds

Hydrocarbons

BTEX

Biotransformation

High

Gasoline, fuel oil

Biotransformation

Moderate

Nonvolatile aliphatic

Biotransformation,

compounds

immobilization

Low

PAHs

Biotransformation,

immobilization

Low

Creosote

Biotransformation,

immobilization

Low

Oxygenated hydrocarbons

Low-molecular-weight

Biotransformation

High

alcohols, ketones, esters

MTBE

Biotransformation

Low

Hcdogenated aliphatic*

PCE, TCE, carbon tetrachloride

Biotransformation

Low

TCA

Biotransformation,

abiotic transformation

Low

Methylene chloride

Biotransformation

High

Vinyl chloride

Biotransformation

Low

Dichloroethylene

Biotransformat ion

Low

Halogenated aromatics

Highly chlorinated

PCBs, tetrachlorodibenzofuran,

Biotransformation,

pentachlorophenol,

immobilization

Low

multichlorinated benzenes

Less chlorinated

PCBs, dioxins

Biotransformation

Low

Monochlorobenzene

Biotransformation

Moderate

inorganic Substances

Metals

Ni

Immobilization

Moderate

Cu, Zn

Immobilization

Moderate

Cd

Immobilization

Low

Pb

Immobilization

Moderate

TABLE 14-2

Likelihood of Success of Groundwater Remediation by Natural Attenuation for Various Substances

Dominant Attenuation Likelihood of Success Given Chemical Class Processes Current Level of Understanding

Cr Hg

Biotransformation, immobilization Biotransformation, immobilization

Low to moderate Low

Nonmetals As

Se

Biotransformation, immobilization Biotransformation, immobilization

Nitrate

Perchlorate

Biotransformation Biotransformation

Moderate Low

Source: Adapted from J. A. Macdonald, "Evaluating Natural Attenuation for Groundwater Cleanup," Environmental Science and Technology (1 August 2000): 346A,

Source: Adapted from J. A. Macdonald, "Evaluating Natural Attenuation for Groundwater Cleanup," Environmental Science and Technology (1 August 2000): 346A,

Only three pollutants are highly likely to be successfully treated by natural attenuation:

• BTEX hydrocarbons (i.e., BTX hydrocarbons plus ethylbenzene),

• low-molecular-weight oxygen-containing organics, and

• methylene chloride.

In all three cases, biotransformation is the dominant process by which attenuation occurs. Notice that neither MTBE nor highly chlorinated organics, including TCE and PCE, are usually successfully treated in this way, nor is mercury or perchlorate ion.

Continue reading here: Decontamination of Groundwater In Situ Remediation

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