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5.3 Time to Achieve Water Quality Objectives As discussed in Section 4.0,the proposed water quality objective for TPH-d of 210 µg/L has already been achieved. Although the more conservative USEPA water quality objective for TPH-d of loo pg/L has not yet been achieved, calculations of TPH-d degradation demonstrate that the 140 pg/L value will be achieved in a very short time period(within months). Figure 8 shows the projected degradation of TPH-d based on available data for MW-5 and a linear regression analysis (Sara, 2003). 6.o IMPACTS TO SENSITIVE RECEPTORS AND BENEFICIAL USES 6.1 Groundwater Water Supply Wells During July 2010,TRC conducted a sensitive receptor survey to determine the proximity of supply wells and other surface waters located within a 1/2 mile radius of the Site. Groundwater monitoring wells and remediation wells were not included in the survey as they are not considered sensitive receptors. According to the Department of Water Resources(DWR), five (5) active water supply wells are identified within a 1/2 mile radius of the Site.The locations of these wells are depicted in the attached Figure 9. The only well identified within the path of local groundwater flow is located approximately 1,96o feet northeast of the Site. According to the driller's report,the well is screened from 200 to 56o fbg. The other four wells are not located within the approximate path of local groundwater flow. Details of the wells located within a 1/2 mile radius of the Site are summarized in Table 6. Given the significant depth interval between the impacts reported in Site wells and the deeper screened intervals for wells located within a 1/2 mile radius, the overall low magnitude of these impacts and the distance these wells are situated from the Site, it is unlikely that these wells are at risk from Site-related groundwater impacts. Furthermore, dissolved-phase petroleum hydrocarbon plumes, especially those consisting primarily of less mobile constituents(primarily petroleum hydrocarbons in the diesel and motor oil range), do not typically extend well beyond their source area and, according to findings from the Lawrence Livermore National Laboratory(LLNL) plume study(Rice et, al., 1995) most plumes are less than 26o feet in length. Therefore, it is considered unlikely that petroleum hydrocarbon impacted groundwater, currently defined within a small area onsite, could have migrated a sufficient distance offsite to present any threat to a public supply well located nearly 2,000 feet from the Site and screened well below a depth that would be expected for shallow groundwater TPH impacts to have migrated vertically. 6.2 Surface Water The nearest surface water is the Stockton Diverting Canal,located approximately 1 mile north of the property, as indicted in Figure 9. However,based on the distance from the Site, the canal is unlikely to be impacted and is therefore not considered a potential sensitive receptor. 9