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Former Beacon Station No. 12474 March 18, 2002 Stockton, California Problem Assessment Reno rt/C orrecti ve Action Plan 8.2.3 In-Situ Bio-Remediation This remediation alternative involves the stimulation of microflora or bacteria facilitating biodegradation of petroleum hydrocarbons. Stimulation of the microbes is achieved by introduction of nutrients and oxygen into the saturated zone through existing wells or trenches. 8.3 Dual—Phase Soil and Groundwater Remediation Dual-Phase Extraction(DPE) is an in-situ technology that uses an applied vacuum to remove impacted groundwater and hydrocarbon vapor from the subsurface. Extracted liquids and vapor are collected, separated and treated aboveground using conventional treatment equipment. The high vacuum (greater than 300 inches of water column) applied to the subsurface by the DPE creates vapor-phase gradients toward the vacuum well. These vapor-phase pressure gradients are also transmitted directly to the subsurface liquids present, and those liquids existing in a continuous phase will flow toward the vacuum well in response to the imposed gradient. The higher the applied vacuum, the larger the hydraulic gradients that can be achieved in both vapor and liquid recovery rates. These recovery rates are typically greater than those achieved using convention(low vacuum) technologies. The relatively high vacuum achievable with DPE is generally effective in extracting liquids from relatively uniform soil with permeability as low as 10"11 cm2. DPE is difficult to apply at sites where the water table is located less than 3 feet below surface grade (bsg). This difficulty is due primarily to the high potential for airflow short-circuiting due to large vertical airflow rates in the immediate vicinity of the extraction well. Currently, groundwater at this site is approximately 55 feet bsg. 474PARCAP3-02.doc 17 Project No. 1474.23 HORIZON ENVIRONMENTAL INC.