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2.3.2 Injection Pressure and Air/Oxygen Flow Rate <br /> The gas injection pressure must be sufficient to overcome the combined effects of the porous <br /> media entry pressure and the hydrostatic pressure but below the threshold that could cause <br /> fracturing of the soil or failure of the well seal. Typical injection pressures are targeted at about <br /> 10 pounds per square inch (psi) above hydrostatic pressure. For shallow IAS applications, <br /> rotary vane compressors are typically used. As the required injection pressure increases, more <br /> expensive compressors such as reciprocating piston or rotary screw compressors are <br /> required. <br /> Another process to consider when designing IAS systems is the relationship between air <br /> channel density and relative permeability. Increasing the air injection rate increases the <br /> number and density of air channels which should theoretically improve treatment efficiency. <br /> However, as the air channel density (i.e. air saturation) increases the relative permeability is <br /> reduced which can impede the flow of groundwater across the IAS biobarrier (Ahlfeld et al., <br /> 1994; Rutherford and Johnson, 1996; Salinitro et al., 2000). A balance between air channel <br /> density and permeability reduction must be established for optimum biobarrier performance. If <br /> the injection flow rates are too high, the relative permeability to groundwater could be reduced <br /> to the point where groundwater begins to flow around the sparging barrier and channeling of <br /> air flow would also occur, limiting the distribution. Similar to estimating air channel density, <br /> quantifying relative permeability reductions as a function of air injection rates in heterogeneous <br /> systems is fairly complex. Guidance documents typically suggest flow rates that are thought to <br /> represent an appropriate balance between air channel density and the relative permeability to <br /> water. ESTCP (2002) recommends a target injection rate of 20 cubic feet per minute (cfm); <br /> NAVFAC, 2001 recommends an injection rate between 6 and 20 cfm; USEPA (1995) <br /> recommends an injection rate between 3 and 25 cfm. <br /> 3.0 TECHNOLOGY EVALUATION <br /> 3.1 TREATMENT AREA <br /> The evaluation below focuses on the feasibility of biosparging for treating dissolved phase <br /> volatile organic compounds (VOCs) in groundwater near the City of Ripon WWTP Lagoons. A <br /> potential biobarrier alignment is considered here as a possible design; the conceptual <br /> alignment is depicted in plan in Figure B.1-1. The water table in this depiction is approximately <br /> 25 feet below ground surface in the WWTP area. For cost estimating purposes, the barrier <br /> length is assumed to be 4,500 feet and the target treatment zone is assumed to be from <br /> approximately -25 feet mean sea level (msl) (--75 feet feet below ground surface (bgs)) to the <br /> base of the Intermediate aquifer, which is located at -150 feet msl (- 200 feet bgs). The total <br /> vertical thickness of the treatment area is approximately 125 feet. Based on existing site <br /> characterization data, groundwater in the upper portion of the Upper aquifer (25 feet MSL to - <br /> 25 feet msl) is not affected by VOCs and no treatment is assumed to be required. Full scale <br /> AMEC Geomatrix, Inc. <br /> \\oad-fs1\doc_safe\9000s\9837.006\4000 REGULATORYTS Assessment_Apx B_012711\Attachment B.1\Attach B1.doc 131-5 <br />