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Corrective Action Plan Page 13 Other remediation technologies, beyond those discussed above, were considered in the context of their applicability to the subject site These included bio-venting, where air is drawn through contaminated soils, either by injecting it into, or drawing it from, an array of wells that penetrate the vadose zone The flow of air removes VOCs from the affected soil and introduces oxygen that can stimulate biodegradation of hydrocarbons by indigenous biota However, in field trials of practical duration, bio-venting has not been shown to be capable of reducing contaminant concentrations to levels below those of regulatory concern at sites where contaminated soil is in contact with groundwater Its applicability is therefore generally restricted to site-specific situations where it can be used to supplement other remediation methods At the 152 East I Ith Street, site bio-venting could not be used to supplement a primary remediation system unless the water table was lowered by at least S ft over a wide area and, as has been previously discussed, this would require groundwater to be pumped at an impractical high rate and the discharge from a suitable treatment system would exceed the Iimiting rate set by the CRWQCB With these constrains and the inability to install the necessary wells in any but a limited portion of the area affected by contamination, bio- venting would not be cost-effective in this case Other in-situ remediation techniques that might otherwise be used to supplement groundwater recovery and treatment (such as active bio-remediation by injection of nutrients and oxygen to stimulate microbial digestion of hydrocarbons) would treat only a small fraction of the most severely affected area of the subsurface and/or would require groundwater to be pumped at such a rate that they would be either impractical or not cost effective under the site-specific conditions PROPOSED REMEDIATION SYSTEM For the reasons presented above, groundwater recovery and treatment has been selected as the most practicable and cost effective method for remediating the subject site Groundwater recovery wells will be installed so that contaminated water can be expeditiously extracted from the most severely affected areas, including areas under the public streets, and so that the plume of contamination can be hydraulically contained while less severely affected groundwater is drawn into the wells from further afield This can be achieved within the regulatory limits set for discharges of treated effluents to surface water and uncontrolled emissions to the air Remediation will be accelerated by operating the system so as to repeatedly flood zones of contaminated soil to flush them of hydrocarbons and stimulate natural biodegradation of contaminants by indigenous microbial biota 1