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Canepa's Car Wash(Pacific Avenue) Problem Assessment Report and Corrective Action Plan Page 12 lie there is no deficiency, then nutrient addition would not increase microbial activity). Oxygen could be introduced through air sparging or chemical addition (an oxygen releasing compound such as peroxide ORC®, or ozone). Ozone can oxidize contaminants directly or through the formation of hydroxyl radicals, and can enhance aerobic microbial activity to produce decomposition. However, effective injection would require closely-spaced injection wells as noted in the air sparging discussion below. IAlternative 3, soil vapor extraction, involves the installation of perforated well casings into the vadose zone above the groundwater level, and use of a vacuum to remove the soil vapor with subsequent ex situ treatment (as necessary). As noted in Section 3.0, this site has been undergoing relatively consistent soil vapor extraction since 1999. The decrease in soil vapor contaminant concentrations is an indication that the majority of contaminated soil vapor has been removed, and the likely source of soil vapor I contaminant concentrations is off-gassing from the groundwater. Soil vapor extraction is not an effective method for removing contaminants from the groundwater. Consequently, further soil vapor extraction will be considered only in combination with other treatment methods as appropriate. Alternative 4, air sparging, involves the direct injection of air into the groundwater at or below the depth of contamination. Air sparging can provide an oxygen supply, as discussed in Alternative 2, above. However, the primary purpose of air sparging is to strip or volatilize contaminants that are then released into the soil vapor above the groundwater. Air sparging would normally be used in conjunction with soil vapor extraction. Air sparging also tends to cause additional groundwater circulation, further dispersing dissolved oxygen for greater enhanced biodegradation. Air sparging can be severely limited by soil characteristics. If the soil is sufficiently dense, then the injected air does not reach far from the injection well, and a large number of closely-spaced injection points are necessary. Also, if the soil allows for the development of preferential pathways, then again the injected air would not reach a wide area. The soil on site is heterogeneous (highly variable with interlaced silts, sands, and clays). Consequently, in a effort ' to maximize the effectiveness of air sparging, a large number of injection wells would be required. ' Alternative 5, ex situ groundwater treatment (the pump and treat alternative), can be effective; however, removal of the hydrocarbons can be very slow because the saturated soils may release the hydrocarbons trapped in blind pore space very slowly. In this alternative, groundwater is pumped from the aquifer and treated on the surface by any one of several treatment systems. Consequently, groundwater pump and I treat systems often require an inordinately long time to complete remediation. A major benefit of pump and treat is that migration of the plume of contaminated water can be captured and directed toward the pumping well. ' Alternative 6, is the use of combinations of Alternatives 2 through 5. The purpose of teaming the alternatives is gain synergy from the different alternative attributes. Pump and treat has the advantages of ' plume capture and removal of the contaminated groundwater in addition to lowering the water table exposing formerly saturated soil, allowing for vapor extraction from that soil. The air sparging system couples aeration and circulation of the groundwater with air lift to extract contaminated groundwater in each of the air sparging wells. A vapor extraction system can be used extract the contaminant-containing air resulting from the air sparging, or without air sparging a vapor extraction system can remove the contaminants adhering to the soil above the lowered water table. In any scenario the addition of oxygen, ' nutrients, or chemical oxidizers can potentially increase in situ contaminant degradation. 7.2 COMPARISONS OF REMEDIATION ALTERNATIVES The analytical data from initial groundwater monitoring through current monitoring has not indicated sufficient natural biodegradation. Enhanced in situ bioremediation with or without chemical oxidation is a 1 ' �a CONDOR