SITE INFORMATION AND CORRESPONDENCE

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Stockton Metropolitan Airport - 3 - 4 January 2016 5000 S. Airport Way, Stockton San Joaquin County 6) Groundwater concentrations are stable and declining (Table 6). Groundwater is defined laterally by the groundwater monitoring well network (Table 7) and vertically in the source area by CPT-1 grab groundwater sample, advanced to 104' bgs in May 2007 (Figure 4). TPHg, 131 ug/L and ethylbenzene, 3.1 ug/L were detected in grab sample CPT-1-GW. Groundwater flow direction has varied from northwest to northeast (Figure 3). The regulatory agency did not require cleanup at this Site. 11) REI predicted WQOs for Benzene at MW-1 will be met by 2020. 12)The Site appears to meet all of the general criteria of the State Water Board's Low-Threat Underground Storage Tank Case Closure Policy (LTCP) as follows: • The unauthorized release is within the service area of a public water system maintained by the City of Stockton. • The unauthorized release consists only of petroleum. • The unauthorized release has been stopped by the USTs removal. • Free product has not been found at the Site. • A Site Conceptual Model (SCM) was developed for the Site based on groundwater flow direction (northeast). The SCM evaluated the nature, extent and mobility of the release. • Secondary source has been removed by soil excavation during USTs removal as evidenced by decreasing concentration trends. • Soil and groundwater were tested for MTBE. Soil was tested for MTBE in June 1997 in the area of the dispensers and groundwater has continuously been sampled for MTBE. • Nuisance as defined by Water Code section 13050 does not exist at the Site. 13)The Site appears to meet all but one of the Groundwater media specific criteria for scenario 2 as follows: • The groundwater pollution that exceeds water quality objectives is stable or decreasing in areal extent, as evidenced by Benzene trends for MW-1, MW-4 and MW-6 (Figures 5, 6 & 7). • The contaminant plume that exceeds water quality objectives is less than 250 feet in length. • Benzene concentrations are less than 3,000 ug/L. • Free product is not present at the Site. • This case does not meet the criteria for this scenario because one water supply well is located within 1,000 feet of the plume (Figure 8). Therefore, Central Valley Water Board staff have determined the Site poses a low threat to human health and the environment to meet Scenario 5 based on the following: Well #1 - Located 350 feet up gradient (to the southwest) of the Site, is not threatened by the release due to the following: • The groundwater gradient direction has primarily been to the northeast since monitoring was initiated in 2002, • Well #1 is completed to a total depth of 365' bgs,(Table 8), • Petroleum concentrations in groundwater are naturally decreasing. • Benzene concentrations are predicted to meet water quality objectives by 2020. Well #2 - Located cross-gradient, approximately 1,800 feet (to the northwest) of the Site, is not threatened by the release due to the following: