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Stockton Promenade—N Edisof treet USTs& Utility Corridors • Page 18 Underground Storage Tank Removal Report June 8,2009 IWKA No, 7345.09 8.0 RECOMMENDATIONS UST removal and assessment results identified the presence of residual petroleum hydrocarbon affected soil and groundwater beneath and in the vicinity of the former N. Edison Street UST area. Overlapping historical land use activities and the physical constraints of the adjacent seawall and WOTOA parking lot complicates source identification of petroleum hydrocarbon ' release(s) and their distribution. ' As part of the Stockton Promenade construction project, petroleum-impacted soil was identified and removed along a portion of the Tuleberg Levee Road and former N. Edison Street UST area 1 in preparation for the installation of underground utilities. Due to the potential risk of destabilizing the structural integrity of the nearby seawall, and/or undermining the stability of the WOTOA parking lot, some petroleum-impacted soil was left in place. As a result of these logistical constraints and the planned continued commercial use of the Promenade, the City of Stockton is requesting approval from the RWQCB to leave the remaining petroleum impacted ' soil at the former N. Edison Street UST area in-place under a deed restriction. This area will permanently be covered with a paved walkway and access road. This mitigation approach is consistent with RWQCB approved remediation actions implemented at other portions of the Promenade property. Delineation of historical petroleum release source(s)may not be clearly defined due to the complexity of the following long-term and overlapping land uses in close proximity of the former N. Edison Street UST area: • The prior use of the four USTs, one of which was identified as having been installed and ' owned by Standard Oil Company as of 1918. • The presence and prior use of previous underground petroleum piping running adjacent to the former UST area, and likely connecting wharf docks along the Stockton Channel to bulk petroleum facilities south of Weber Avenue. • Previous grain milling operations and a large wooden petroleum storage reservoir west of ,- the N. Edison Street UST area The close proximity to the Stockton Channel and unknown condition of the nearby seawall immediately north of the former N. Edison Street UST area likely influences local groundwater ' conditions beneath the former N. Edison Street UST area. Stantec is currently monitoring groundwater conditions in the nearby WOTOA parking lot downgradient and south of the site. The presence of the seawall, the adjacent WOTOA parking lot, and recently installed