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Former Discovery Chevrolet June 5, 1998 Tracy,California Corrective Action Plan 5.2.2.2 In-Situ Bioremediation This remediation alternative involves the stimulation of microflora or bacteria to degrade hydrocarbons. Groundwater systems generally have a diverse population of native microorganisms, some of which are predisposed to consuming hydrocarbon compounds. If the native micro-organisms have a plentiful food source of petroleum hydrocarbons, the species of micro-organisms best adapted to use the hydrocarbon food source will multiply to a population that can be supported by the food source. Stimulation of the microbes is achieved by introduction of nutrients and oxygen through existing wells or directly into the excavation at the site. Groundwater would be pumped and recirculated to minimize migration of the dissolved petroleum constituents and to distribute oxygen and nutrients into the groundwater. Groundwater samples would be collected from the groundwater after biotreatment for a 4rfj predetermined amount of time. After laboratory analysis, a determination would be made as to whether the groundwater may need additional biotreatment. This method would be fairly cost-efficient, but bioremediation rates in the shallow groundwater are unknown, and realistically would require some sort of time-consuming pilot testing evaluation and groundwater monitoring for effectiveness evaluation. This method of treatment would be fairly economical; however, the time duration and effectiveness for this method at the site is unknown. Assuming approximately 1,000 gallons of groundwater to be treated, the preliminary costs for the permitting, nutrients, onsite biotreatment, and monitoring of groundwater are estimated to be approximately $4,000 to $5,000. 5.3 Conclusions In Horizon's opinion, the remediation alternatives to be utilized for the most expeditious and cost-effective approach at the site would be a combination of soil excavation, along with a limited amount of groundwater removal during the soil excavation process. Either 3001-21.cap 15 HORIZON ENVIRONMENTAL INC.