SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

Former Chevron Station 9-5775 - 4 - 12 May 2009 301 W Kettleman Lane, Lodi the shallow zone varies from northwest to southeast at an average hydraulic gradient of 0.01 feet/feet. During operation of the GWP&T system on the site, localized groundwater appeared to flow radially inwards towards the extraction wells. In the B -zone, the groundwater direction also varied from northwest to southeast with an average hydraulic gradient of about 0.01 feet/feet. The C zone (105 to 120 ft. bgs) and the S zone (205 to 210 ft. bgs.) did not contain enough monitoring wells to determine apparent groundwater direction, however regional groundwater flow direction at depth in Lodi has been reported in the Lodi drycleaners TCE/PCE plume investigation as towards the south. That plume does not extend to the Site. Residual Mass Calculation: The consultant estimated that the amount of contaminant mass remaining in soil, prior to remediation in 2000, was: TPHg, 3044 Ibs.; Benzene, 64.5 lbs.; and MtBE, 260.8 lbs. Upon completion of the remediation, the estimated percent mass removed by the SVE system exceeded the initial mass estimate, and was calculated at 4,132 lbs (136%) for TPH-g, 85.7 lbs (133%) for benzene, and 443.8 lbs (170%) for MtBE. The groundwater pump and treat system (GWP&T) removed approximately 99.9% of the original estimated MtBE mass in the groundwater. The amount of post remediation MtBE mass remaining in groundwater was calculated as 0.0074 lbs. Based on the post -remediation data for MtBE (no other constituent is currently detected in groundwater) reported on August 2006, the calculated time for the MtBE to reach Water Quality Objectives (WQOs) was 1.97 years. Monitoring data since August 2006 confirms that WQOs have been reached for all constituents, including MtBE. The residual MtBE plume is currently localized at one well (EX -3), has not migrated beyond EX -3 since remediation was completed, and the MtBE concentration has not rebounded since July 2007. As such, the residual MtBE in soil and groundwater does not pose a threat to human health and safety or water quality. Groundwater Fate and Transport Modeling In January 2005, prior to shutting down the GWP&T system, the consultant submitted a groundwater fate and transport model for the residual contaminants in groundwater. Groundwater modeling showed that residual MtBE would not reach any sensitive receptors, including City of Lodi Well No. 12. Quarterly groundwater monitoring since January 2005 has also confirmed the results of the groundwater modeling, in that the MtBE plume has not migrated offsite. Summary and Recommendations: Chevron has complied with all of the requirements in the Regional Board CAO No. 5-00-704. Concentrations of petroleum hydrocarbons, including MtBE, have decreased significantly since groundwater monitoring started in 1999 and soil and groundwater remediation began in 2000. During the most recent monitoring event, 1st Quarter 2008, groundwater sample results showed MtBE concentrations as non -detect (ND) with the exception of EX -3 (5 ug/L). Over the past year 17 wells have been sampled and only EX -3 has contained any detectable constituents of concern (MtBE). Further, the residual MtBE pollution at the site, in EX -3 is isolated to the A zone near the location of the former UST system. The plume has been