SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

River City Petroleum - 5- 15 April 2014 Connell Motor Lift Truck 2211 North Wilson Way Stockton, San Joaquin County (IAS-1 through IAS 14) and four shallow and four deep SVE wells (VW-16 A through VW-19A, and VW- 16B through VW-19B) were installed. Monitoring wells MW-1, MW-15, and MW-16 were also used as SVE wells. The SVE operated from November 2004 to January 2011. AS was implemented from July 2008 to January 2012. SVE removed approximately 41,097 lbs. (6,227 gal.) of TPH from soil. Four groundwater rebound tests were conducted, in July 2008 (shutdown SVE/AS), January 2009 (shutdown SVE/AS), January 2011 (shutdown SVE with AS continuing), and January 2012 (shutdown AS). The first three rebound tests resulted in significant concentration rebound in groundwater monitoring wells, requiring restarting of the remediation system. The final rebound test did not result in significant rebound in concentration levels (See Trend Maps for perimeter wells screened at 60' bgs, 100' bgs, 150' bgs and 248' bgs, Figures 12 through 26). Groundwater monitoring ended in February 2012 with Staff approval. The July 2012 closure report determined that, after 8 years of remediation and 13 years of groundwater monitoring, additional monitoring of the declining and shrinking commingled plume was not necessary. Perimeter wells MW-18 and MW-20 showed a declining trend during active remediation from 2004 to 2008 when last sampled. Air Sparging continued until 2012 and the source area data support that the plume is stable. The pollutant plume is predicted to remain onsite and no sensitive receptors identified, are threatened. Estimated date to reach water quality goals (WQGs) is 2037. Risk Assessment The residual soil contamination is limited in extent to approximately 3,000 square feet and does not threatening human health from direct contact of outdoor air inhalation. The only building overlying the residual pollution is the shop building. Due to the large open door of the shop building when people are working in this space and open storage area of the yard, Staff did not require a vapor intrusion study. A RISC 4 Fate and Transport Model for residual 1, 2-DCA determined that detectable concentrations (above CA Maximum Contaminant Level) within the plume are not predicted to travel offsite, and will not threaten any identified sensitive receptors. Land use (commercial) is not expected to change in the foreseeable future. Sensitive Receptor Survey A sensitive receptor survey (Figure 27) conducted in 2012 identified one inactive agricultural supply well 1,000' to the west, one municipal supply well 1,500' east of the site, and one small water supply well (trailer park water supply) 1,750' northeast of the site. An onsite domestic supply well 350' north of the USTs was tested for petroleum hydrocarbons. No detectable impacts were found however, the well was properly destroyed in 2010. The Site is currently connected to municipal water supply. None of the identified water supply wells are threatened by the residual petroleum hydrocarbons at this Site. Site Geology and Hydroqeolopy Site lithology consists of fine grained soil (clay to silty clay) with discontinuous lenses of sand from surface to 248' bgs, the total depth investigated. Sand lenses were logged from 25' to 30' bgs, 75' to 85' bgs; 100' to 110' bgs 150' to 155' bgs, 165' to 185' bgs and 240' to 248' bgs. Although laterally continuous clay was present at 120' bgs, groundwater was impacted by the 1, 2-DCA down to at least 150' bgs across most of the Site, and as deep as 248' bgs in select wells. Depth to groundwater varied from 25' bgs (February 2007) to 63' bgs (September 2004). Currently depth to water is 39' bgs (February 2012). Historically groundwater depths in east Stockton declined to nearly 90' bgs in the early 1990's, which may explain the depth of the 1,2-DCA groundwater plume (150' to 248' bgs) and soil contamination down to 85' bgs. Groundwater flow direction has varied from northeast to northwest. The groundwater gradient varied from 0.005 ft/ft to 0.015 ft/ft.