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Activity II - Groundwater Investigation Downgradient of Former Disco Tanks This activity is designed to characterize potential offsite migration that may have occurred as a result of shifting groundwater flow directions.Coincident with rising static water elevations in the uppermost aquifer is a shift of groundwater flow from primarily north in 1991 to south in 1995,and currently southeast using the most recent water level measurements. Based on discussions with PHS staff, there is a need to characterize the extent of petroleum hydrocarbons in groundwater in the new downgradient direction. However, due to the location of the Caltrans' Highway 4 "Cross-Town"Freeway,there will be limitations to drilling locations south of the Disco Tanks. Activity II will be conducted according to the same four subtasks described in Activity I: prefield, field activities,laboratory analysis, and reporting. Subtask 1 Prefield Prefield activities for this task are similar to Activity I,requiring: • Updating the well survey in area to identify potential risk receptors,if any. • underground utility clearance, • a drilling permit from PHS,and • an encroachment permit from Caltrans. The existing well survey will be updated to identify potential risk receptors. Based on available information, the well survey will include boring logs, screened intervals, current status,and pumping rates (past,present,and projected future rates and seasonal pumping patterns). If downgradient water-supply wells are identified,the information may be used for locating downgradient sampling activities. The encroachment permit is required because the proposed drilling areas (Figure 1) are offsite and are in a Caltrans right-of-way. Caltrans will have to approve a workplan prior to starting work in their right of way. The Q wo sou re a description of the work,a health and safety plan, a traffic safety plan using Caltrans specifica ' and a maps the locations of the proposed borings. Approval for this work may 6 nths.This workplan should also y� describe a n sample any permanent wells ' �edd in the Caltrans right-of-way for the As�w urpos four arters of monitoring. Provisions must be made to gain access to any h Qperma s in the future. task 2 Field Activities The field activities will start with drilling two borings offsite to determine the lateral extent of downgradient migration of petroleum hydrocarbons.These borings will be drilled and continuously logged to a depth of 10 feet below the top of the current static water elevation. Grab groundwater samples will be collected form both of these borings at the first occurrence of groundwater.Based on the results from these analyses, a possible third boring will be drilled adjacent to one of these shallow borings into a lower portion of the 5