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A petroleum hydrocarbons, the rate is much slower than with aerobic bacteria. Depleted levels of oxygen appear to be the primary limiting factor for aerobic bacterial activity. Two common methods of increasing dissolved oxygen in groundwater are injection of hydrogen peroxide a n d one-time application of Oxygen Releasing Compound (ORC). Advantages for this type of remediation include (a) it is very low cost, (b) it is a passive, unintrusive method for groundwater remediation, (c) there is little or no equipment to maintain, and (d) it often works very quickly. Disadvantages include (a) it is not effective at all sites since it is very dependent on groundwater flow rates, (b) soil remediation is also required using these methods, (c) in-situ bioremediation is not typically as effective on MTBE as on other hydrocarbons, and (d) additional applications may be required if using ORC. Although bioremediation may be an effective remedial option for the site, its success will be limited by the remaining hydrocarbon source in the unsaturated zone. Bioremediation is generally only effective in soils with high moisture content, and the unsaturated soil in the vadose zone will r-eillain untreated. For this reason, ASE is not considering the use of' in- situ bioremediation as a primary remediation option for -the, site at this time. 9.5 In-Situ Chemical Oxidation In-situ chemical oxidation/reduction has been considered as a remedial option for the site. This method involves injecting an oxidant to the subsurface, which will destroy organic hydrocarbons. The three most common oxidants are peroxide, permanganate and ozone. Oxidation using liquid hydrogen peroxide in the presence of ferrous iron (native or supplemental) produces Fenton's Reagent, which yields free hydroxyl radicals, which is a strong oxidizer. These strong oxidants can rapidly degrade a variety of organic compounds. Permanganate can participate In numerous complex reactions to destroy organic compounds. However, using either peroxide or permanganate requires the injection of liquid into the water bearing zone, which in this case appears to be a thin, non- obvious zone of higher permeability within thick non-permeable clayey zones. The low permeability clay-rich soils at the site will not accept these liquids. This would make this remediation using these methods difficult at this site. There would also be very little effect on the vadose zone still leaving a source of hydrocarbons in [lie unsaturated lane. Ozone, however, is injected as a gas into sparging wells. Ozone c a n oxidize contaminants directly or through the formation of hydroxyl 574 West Grant Line Road CAP — January 2002 -19-