TABLE 247 continued

Technology

Pump-and-treat

Phytoremediation

In situ thermal treatment

Potential Benefits

Causes minimal disturbance to site operations

Can be a cost-effective alternative to treat an aquifer or to provide hydraulic containment for sites contaminated with fuel oxygenates

Can be a cost-effective alternative for remediating or containing relatively low concentration, shallow, and widespread soil or groundwater plumes

Can be a cost-effective alternative for preventing the migration of contaminated groundwater plumes

Can be a cost-effective alternative for remediating source areas in soil or groundwater

Tends to remove oxygenates when used to treat other petroleum contaminants (such as petroleum hydrocarbons)

Potential Limitations

Special precautions may be needed to protect worker health and safety during operation

Oxidation reactions may form toxic by-products in the groundwater or in off-gases, and off-gas may require capture and treatment

May require an extended operation and maintenance period Cost of constructing, operating, and maintaining the treatment system can be relatively high

Biofouling of extraction wells can reduce system performance The typical design of common above-ground treatment systems may not be effective for oxygenates Limited information available about the specific processes used in phytoremediation that reduce concentration of fuel oxygenates Typically lengthy startup period Phytoremediation generally less applicable to higher concentrations or deeper groundwater plumes May affect natural groundwater flow gradients at a site, potentially resulting in lateral or vertical migration of the contaminant plume

Requires a high degree of engineering for design and installation May not be cost effective for use at small sites such as service stations

Requires a high degree of engineering for design, installation, and operation

Source: Adapted from U.S. EPA, Technologies for Treating MTBE and Other Fuel Oxygenates, EPA 542-R-04—009, United States Environmental Protection Agency, Washington, DC, May 2004.

24.4.2 Effect of the Properties of MTBE and Other Oxygenates on Treatment

In an air sparging system, the primary mechanism for contaminant removal is by the transfer of contaminants from the dissolved to the vapor phase. The extent to which this transfer can take place during air sparging depends on the Henry's law constant, which is an indication of the extent to which each will partition between the dissolved state and the vapor state under equilibrium conditions. A contaminant with a greater Henry's law constant is more readily stripped from groundwater by air sparging than one with a lower Henry's law constant.

Figure 24.3 shows the Henry's law constants for the common fuel oxygenates. As shown in the figure, all of the common oxygenates (with the possible exception of DIPE) have Henry's law

TABLE 24.8

Types of Technologies Used to Treat MTBE and Other Fuel Oxygenates

In Situ or

Treatment Technology

Groundwater Soil Ex Situ

Description

Air sparging

• In situ

Injection of air into the groundwater

to strip out VOCs

SVE

• In situ

Application of a vacuum to the soil

to extract VOCs and treatment

using aboveground processes

MPE

• • In situ

Simultaneous extraction of VOCs

from soil and free product/

groundwater and treatment using

aboveground processes

In situ bioremediation

• • In situ

Addition of oxygen or other

amendments to stimulate and

enhance biodegradation

ISCO

• • In situ

Injection of chemicals such as ozone,

hydrogen peroxide, or

permanganate into the subsurface to

oxidize contaminants

Groundwater extraction

• Ex situ

Extraction of contaminated

for pump-and-treat

groundwater for treatment prior to

and drinking water

use or disposal

treatment

Aboveground treatment

• Ex situ

Treatment of extracted groundwater

technologies for

using ex situ processes such as air

extracted groundwater

stripping, adsorption, biological

reactors, or oxidation

PRBs

• In situ

Placement of a reactive zone that

treats contaminants as groundwater

flows through the zone

Phytoremediation

• • In situ

Use of trees and other higher plants

to remove or destroy contaminants

In situ thermal

• • In situ

Use of heat to mobilize or destroy

treatment

contaminants

Source: Adapted from U.S. EPA, Technologies for Treating MTBE and Other Fuel Oxygenates, EPA 542-R-04—009, United States Environmental Protection Agency, Washington, DC, May 2004.

constants that are lower than those for BTEX, which range from 0.22 for benzene to more than 0.3 for xylene. Because of this, an air sparging system designed to remediate BTEX may not adequately address oxygenates. Research has shown that the removal of MTBE requires 5-10 times more airflow than would have been used for BTEX alone.38 In addition, the ether-based oxygenates have Henry's law constants that are about two to three orders of magnitude greater than those for alcohol-based oxygenates, suggesting that ether-based oxygenates, such as MTBE, can be removed more readily using air sparging than alcohol-based oxygenates, such as TBA. However, alcohol-based oxygenates may be more readily biodegraded or may have less stringent (higher concentration) cleanup goals at some sites than ether-based oxygenates. Thus, it is possible that air sparging could be used to remediate sites contaminated with both alcohol- and ether-based oxygenates.

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