Performance Summary for Seven Projects

MTBE Concentration Range

Greater than or equal to 10,000 ng/L but less than 100,000 ^g/L Greater than or equal to 1000 ng/L but less than 10,000 ^g/L Greater than or equal to 100 ng/L but less than 1000 ^g/L Greater than or equal to 50 ng/L but less than 100 ^g/L Less than 50 ng/L

Number of Projects Reporting Initial MTBE Concentrations

Number of Projects with Last Reported MTBE Concentrations

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.

24.6.7 Factors That Affect the Performance and Cost of Oxygenate Treatment Using MPE

When MTBE or other oxygenates are present and must be remediated at a site, MPE, either alone or in combination with other technologies, may be a suitable remediation approach. MPE affects mass removal by volatilization, dissolution, and advective transport. In general, if both SVE and groundwater pump-and-treat are potentially applicable technologies, then MPE may be considered as a remedial alternative. The performance of MPE is governed, primarily, by media properties and, to a lesser extent, by contaminant properties. MPE is most applicable to fine-grained formations in the fine sand to silty sand range (K = 10-3-10-5 cm/s) with low transmissivity <2.5m3/d/m (200gallons/d/ft).

A typical result of conventional pumping in low conductivity and transmissivity formations is increased, and sometimes rapid, drawdown with steep gradients, with corresponding low recovery rates. This condition limits the influence of the conventional pumping well. MPE overcomes this limiting factor with the application of a vacuum. The vacuum enhancement of MPE also can overcome the capillary forces that can trap contaminants within the capillary zone. This allows better recovery of LNAPL, which tends to accumulate in the capillary zone at the air-water interface.

In addition to the technology-specific factors described above, additional factors may also affect the performance and cost of any MPE system. These factors include

1. The concentration, mass, and distribution of contaminants in the soil and groundwater.

2. Geology, hydrogeology, and heterogeneity of the subsurface; cleanup goals.

3. Requirements for air emissions and water discharges.

These factors affect the number and type of extraction wells, vacuum level, pumping rate, type of aboveground-water and off-gas treatments, and length of time required for treatment.34

24.6.8 Advantages and Limitations

The advantages of applying MPE are as follows46:

1. Increase in groundwater recovery rates compared with conventional pumping practices in equivalent settings.

2. Increase in ROI of individual groundwater recovery wells.

3. Recovery of free product or other LNAPL.

4. Remediation of the capillary fringe and smear zone.

5. Simultaneous remediation of soil vapors and groundwater.

6. Effective on lower-permeability soil sites.

The limitations to applying MPE are as follows46:

1. Greater aboveground treatment requirements as a result of NAPL emulsions and VOC-laden vapors.

2. Initial startup methods and adjustment period may be longer compared with conventional practices.

3. Potentially higher capital costs compared with conventional pumping approaches.

4. Depth limitations to some MPE configurations.

24.6.9 Example Projects

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