WORK PLANS - Laserfiche WebLink

Home Browse Search

it 1 CONCLUSIONS There are no sensitive receptors within 2,000 feet downgradient of the Site. The CWS municipal well (DWR Well ID No. 24090) is located crossgradient to the Site and is reportedly only used for emergency backup purposes. In addition, the municipal well is screened from 209 to 426 feet, significantly deeper than the site wells and separated from shallow groundwater by a 53-foot hard clay aquitard., Our estimates of 538 kilograms of TPHg in soil, 0.31 gallon of TPH9,in groundwater,and 4.95.gallons of TPHd in groundwater are conservative estimates based on maximum concentrations. The MODFLOW results show that TPHg (as benzene)' and TPHd,(as naphthalene) in groundwater are not projected to migrate beyond site boundaries, even if the public water supply emergency backup well is pumped continuously at 2,000 gpm.Based on the MT313MS results,TPHg and TPHd concentrations beneath the Site are projected to decrease to below 100 µg/1 within 3 and 20 years, respectively. The MODPATH results show that inert particles in groundwater beneath the Site would reach the emergency backup well in 29 years,again assuming the well is pumped continuously at 2,000 gpm.for the entire duration. The fate and transport modeling results demonstrate that residual TPHg and TPHd concentrations in shallow groundwater beneath the Site do not pose a risk to the public water supply emergency backup well. The model results also demonstrate that residual TPHg and TPHd concentrations in groundwater will not migrate beyond site boundaries and will decrease to below 100 pg/1 within reasonable time frames. The model results are predicated on the assumption that no additional contaminant mass is introduced into shallow groundwater. RECOMMENDATIONS Based on the limited mass of petroleum hydrocarbons remaining in soil and groundwater, the lack of sensitive receptors within 2,000 .feet downgradient of the Site, and the results of the fate and transport modeling that show the emergency backup well is unlikely to be affected by petroleum hydrocarbons from the Site, even if the well is pumped continuously, active soil and groundwater remediation at the Stockton Yard Maintenance Station does not appear to be warranted and the Site may be suitable for closure. 'v Stockton Yard Maintenance Station,TO No.1 B Caltrans Contract No.06A1895,EA No.43-910206 Geocon Project No.S9800-01-01B -9- September 30,2013