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' HydroGeo Consultants microorganisms that would otherwise be oxygen limited While MTBE is not considered to be readily biodegradable, several recent studies have noted MTBE attenuation rates ' higher than those anticipated by dispersion alone (Baker et al, 1998) Several case studies have seen reductions in MTBE from 25% to 77% ' Even though aerobic biodegradation may slowly work for MTBE, there is a concern for lag time of MTBE bioremediation in the presence of BTEX compounds The oxygen released by magnesium peroxide compound is sometimes depleted by bacteria ' preferentially consuming the BTEX compounds, therefore, significant MTBE remediation may not occur until the BTEX compounds have been biodegraded The bioremediation process may therefore not remediate MTBE in groundwater in a fast time ' frame Additionally, several injections of the oxygen release compound may be required to reduce MTBE concentrations to an acceptable concentration, the cost for these injections may not be economically feasible Also aerobic biodegradation of MTBE may ' produce unwanted bi-products Therefore, we do not believe that this is the best remedial solution for the Site at this time ' 7.3 Air Sparging ' In situ air sparging is not technically feasible at this Site due to the relatively confined nature of the saturated zone (more permeable soil layers at depth ranging from 65 to 70 feet bgs confined by fine grained soil above and below) Aside from air sparging not being ideal for stripping MTBE from the water column, this restrictive soil layer will not allow the sparged air to move upward and strip BTEX and MTBE for capture and treatment by vapor extraction This lower permeability layer could also create a ' preferential horizontal pathway for the injected air that could mobilize the plume in various directions Although the cost for implementing this remedial technology is low to moderate, we do not believe it is technically feasible to implement it at the Site, therefore ' we do not recommend it be implemented 7.4 Groundwater Pumping with Vapor Extraction ' The most practical treatment technology for remediation of the limited dissolved phase TPHg, BTEX, and MTBE groundwater plume at the Site and soil is a combination of ' vapor extraction and groundwater pumping The vapor extraction system would be used to remove the contaminants from the soil within the vadose zone, and the groundwater pumping system would be used to extract and treat the contaminated groundwater and ' provide hydraulic control to prevent off-site migration of the dissolved phase plume Also by lowering the water table through groundwater extraction, it will expose potentially contaminated sediments and allow them to be more expeditiously remediated via vapor ' extraction ' Thrifty Oil TOC#171 Aquifer Test and RAP Rpt Page 13