SITE INFORMATION AND CORRESPONDENCE_2001-CURRENT

Home Browse Search

Montgomery Wards / Unocal Station #5098 - 5 - 12 April 2012 5400 / 5606 Pacific Avenue Stockton, San Joaquin County benzene (6.57 mg/kg) at Wards. And prior to remediation by over-excavation in 1988 at Unocal, residual soil contamination failed the ESLs for TPHg (700 mg/kg) and benzene (9.2 mg/kg) for gross contamination and direct exposure at 49' bgs at Unocal. Prior to upgrades to the SVE/AS in 2008, soil at 50' bgs (below water table) failed the ESLs for TPHg (gross contamination and direct exposure), (gross contamination and direct exposure), benzene (direct exposure), toluene (direct exposure), ethylbenzene (direct exposure), and xylenes (gross contamination and direct exposure). Declining groundwater concentrations since 2008 show that leaching, if occurring, is not exceeding the rate of natural attenuation of the plume. Both sites residual soil contamination are below normal construction worker exposure depths, and Wards residual soil contamination is below the water table (34' bgs in 2010). Post remediation confirmation soil sampling was not conducted. In addition to declining groundwater concentration trends, natural attenuation of petroleum hydrocarbons was confirmed by specific groundwater analyses (nitrate, sulfide, sulfate, ferrous iron, total inorganic carbon, dissolved oxygen, and oxidation-reduction potential) conducted during routine monitoring in July 2008. Residual petroleum hydrocarbons in Site soil, soil vapor, and groundwater pose a minimal threat to human health or the environment. Land use (commercial) is not expected to change in the foreseeable future. Sensitive Receptor Survey In 2001, the RIW well (approximately 350' southwest of Unocal, and 200' west of Wards) was properly abandoned and relocated over 3,300 feet from the commingled plume. A subsequent 2001 sensitive receptor survey identified no additional supply wells located within 2000' of the site. No wells are threatened by the petroleum hydrocarbons release at this site. Site Geology and Hydrogeology Site Iithology consists of clay, silt, and sand to 110' bgs, the maximum extent of investigations. Between 1993 and 2010, depth to water varied from 28' bgs to 60' bgs. Groundwater flow direction varied from southwest to southeast, and the gradient varied from 0.002 ft/ft/ to 0.004 ft/ft. Residual Mass Calculation The consultant did not estimate the mass removed by over-excavation at Unocal. The initial mass of TPHg in soil was approximately 12,000 pounds for Wards. SVE/AS removed 11,638 lbs of TPHg from soil. Approximately 356 lbs of TPHg remain in soil and groundwater at Wards. Groundwater will reach Water Quality Goals (WQGs) in 2039.