SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

Stockton Metropolitan Airport - 4 - 4 January 2016 5000 S. Airport Way, Stockton San Joaquin County • The groundwater gradient has primarily been to the northeast. • Well #2 is completed to a total depth of 425' bgs with a screened interval from 199' to 415' bgs (Table 8). • Petroleum concentrations in groundwater are naturally decreasing. • Benzene concentrations are predicted to meet water quality objectives by 2020. Rydberg Creek (tributary of Walker Slough and the San Joaquin River) is approximately 1,200 feet (up-gradient) to the southwest (Figure 1). Rydberg Creek is not threatened by the release due to the distance (1,200 feet), location (up-gradient) and declining TPH concentrations. 14)The Site appears to meet the State Water Board's LTCP for Petroleum Vapor Intrusion to Indoor air. • A soil gas investigation (Table 9) conducted by REI did not detect petroleum hydrocarbons in shallow soil. Soil was excavated down to 10.5' and down to 18' bgs during tank removals. 15)The Site appears to meet all of the State Water Board's LTCP criteria for Direct Contact and Outdoor Air Exposure. • Maximum concentrations are less than or equal to the Commercial/Industrial and Utility Worker limits in the State Water Board's LTCP Table 1. • In addition, excavation of impacted soil down to 10.5' and down to 18' bgs is beyond the typical depths for construction worker exposure. 16) Public participation through distribution of a Fact Sheet was performed on 15 July 2015. The public participation period ran from 15 July 2015 through 15 September 2015 with no comments received. 17) Site wells MW-1 through MW-8 were properly destroyed on 18 November 2015 prior to issuance of a No Further Action Required letter. Recommendations Based on available data, we concur with REI's recommendation for case closure. No remedial actions beyond excavation during tank removals down to 10.5' to 18' bgs were required by the regulatory agency. REI did not estimate the mass of impacted soil removed or remaining at the Site. Groundwater concentrations appear to be stable to declining. Water quality goals should be reached in five years. Eight (8) Site monitoring wells were properly destroyed on 18 November 2015 prior to case closure. Attachments: Figure 1 Site Location Map Figure 2 Site Plan Figure 3 Groundwater Surface Map April 2014 Figure 4 Groundwater Iso-concentration Map, TPHg, April 2014 Figure 5 Benzene Trend for Monitoring Well MW-1 Figure 6 Benzene Trend for Monitoring Well MW-4 Figure 7 Benzene Trend for Monitoring Well MW-6 Figure 8 Drinking Water Wells Map Table 1 List of Former USTs with Dates Removed Table 2 USTs Removal Confirmation and Investigation Soil Concentrations