SITE INFORMATION AND CORRESPONDENCE_FILE 2

Home Browse Search

Jimco Trick Plaza -4 - 5 September 2012 1022 E. Frontage Rd. (Hwy. 99) Ripon, San Joaquin County Water Quality Goals in a reasonable time frame. Sensitive receptors are not threatened by the two releases. No significant risks exist from Site soil, soil vapor or groundwater to human health or the environment. Sensitive Receptor Survey: A 2,000' :sensitive receptor survey, conducted in 2003, identified one (1) public supply well (750' to the southwest across Highway 99) and three (3) private/domestic supply wells (one onsite well within the USTs plume, one RFS well 175' to the southeast, and one domestic well 500' to the southeast). Groundwater monitoring of the Site supply well (used for process water at the truck wash) and the RFS supply well from 2001 to 2011, along with the well-defined extent of the residual plumes, showed no threat from the two releases to the sensitive receptors. The nearest surface water body, the Stanislaus River, is 9,000' south of the Site. Site Geology and Hydrogeology: Soils beneath the Site consist of laterally discontinuous beds of sands, silts, and clays from surface grade to 104' below ground surface (bgs), the maximum extent of the investigations. Less to moderately permeable clays, sandy clays, silts, and sandy silts are present from surface to 15' bgs 35' to 42' bgs, 58' to 66' bgs, and 78' to 90' bgs. Poorly graded (highly permeable) sands and silty sands are present between 15' to 35' bgs, 42' to 58' bgs, 66' to 78' bgs, and 90' to 104' bgs. Depths to groundwater from 1999 to 2011 have ranged from 15' to 27' bgs. The flow direction of groundwater has varied in all directions, and was most recently towards the southeast in August 2011. The groundwater gradient varied from 0.0001 ft./ft. to 0.001 ft./ft. Residual Mass Estimate: The consL.ltant estimated residual mass in soil as 34,463.5 lbs. of TPHd, 3,317 lbs of TPHg, and 3 lbs of MTBE. Groundwater residual mass is estimated as less than 1 Ib of TPH. Groundwater was estimated to reach Regional Board Water Quality Goals (WQGs) by 2023 for all constituents. Risk Assessment Analysis: The consultant addressed soil vapor risk by stating the majority of residual soil contamination is over 100' ':rom the Site buildings (except for the open bay truck wash), the Site is covered by pavement the low permeability of shallow soil inhibits vapor intrusion, and that groundwater is currently at 22' bgs. Residual soil concentrations failed the San Francisco Bay Regional Board EnvironmE!ntal Screening Levels for commercial usage for direct contact and gross contamination for TPHd at 5.5' ' to 6' bgs in the dispenser area, at an unspecified depth at the former waste oil UST, and at 2.5' bgs at waste oil line sample location WO4. The same soil ESLs werE! also exceeded for TPHg and TPHd at 16' bgs near the former gasoline/diesel USTs and at 10' bgs at boring B6. The consultant addressed direct contact risk by stating Site use (truck stop) is not anticipated to change, and that the majority of the TPHd and TPHg mass is located just above and below the water table between 18' and 36' bgs. The residual soil contamination beneath the subject Site will continue to attenuate in place, and poses a minimal risk to sensitive receptors. The Site private supply well is used only for the truck wash, as public water is used for the remaining Site buildings. Groundwater monitoring from 2001 to 2011 shows no sunniv wells are threatened by the Site residual groundwater plumes.