The primary objective of this project was to demonstrate propane biosparging for in situ remediation of methyl tert-butyl ether (MTBE) to below the 5 micrograms per liter (μg/L) California Department of Health Services regulatory limit. Other objectives included: (1) evaluating the microorganisms' ability to degrade MTBE; (2) selecting and characterizing a field demonstration site; (3) designing, constructing and operating the field demonstration; (4) evaluating the system during a 10-month period; and, (5) evaluating the costs associated with full-scale application.

This project evaluated propane biosparging at the National Environmental Technology Test Site in Port Hueneme, CA. Propane biosparging was implemented as an extension of conventional sparging techniques, where air is injected directly into the saturated subsurface to either volatilize contaminants for treatment or biodegrade contaminants via oxygen stimulation.

MTBE concentrations decreased in the test and control plots, although concentrations were reduced to below 5 μg/L in only 3 of the 10 test plot monitoring wells and none of the control plot wells. MTBE concentrations were reduced to below the upper end of the U.S. Environmental Protection Agency-recommended Health Advisory Level of 40 μg/L in 8 of the 10 test plot deep monitoring wells and 7 of the 10 control plot deep monitoring wells. In general, most MTBE degradation occurred near the oxygen injection points. MTBE concentrations decreased from 62 to 88 percent in shallow test plot wells and from 86 to 97 percent in shallow control plot wells. Similar, but slightly higher, MTBE reduction ranges were observed in the deep monitoring wells in both the test and control plots.

The advantages of propane biosparging over existing MTBE remediation technologies include: (1) in situ application to treat MTBE and tertiary butyl alcohol (TBA) without waste product generation; (2) lower costs than conventional pump-and-treat; (3) ability to add the technology to new or existing systems with minimal additional equipment costs because it is complementary to air sparging; and (4) establishment of propane biosparging treatment zones to treat benzene, toluene, ethylbenzene, and xylenes (BTEX) and other hydrocarbons. Propane biosparging can safely and inexpensively lower concentrations of MTBE. In this project, MTBE concentrations were reduced overall but not to the stated objective level of 5 μg/L. Limiting conditions include low groundwater pH (i.e., less than 5) and low saturated zone permeability. For low pH conditions, an alkaline solution could be injected to achieve the ideal pH range of 6-9. For low permeability soils, pneumatic or hydraulic fracturing could be implemented to increase permeability. (Project Completed - 2003)