SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

New West Petroleum • - 4 - • 26 November 2012 6437 Banner Street Lodi, San Joaquin County supply well, drilled in 2009, is located approximately 1,500 feet north of the Site. None of the wells are threatened by the release. Site Geology and Hydrogeology Soils beneath the Site consist of laterally discontinuous beds of sands, silts, and clays from surface grade to 30 feet bgs. Less permeable sandy silts and sandy clays are present from 30 to 40 feet bgs, 50 to 70 feet bgs, and 80 to 90 feet bgs. Poorly graded sands and silts are present between 40 and 50 feet bgs, 70 and 80 feet bgs, 90 and 110 feet bgs and at depths greater than 120 feet bgs. Depths to groundwater between 2005 and 2011 have ranged from 10' to 22' bgs. The flow direction of groundwater, when not influenced by the GWETS, has been predominantly towards the east- southeast with a gradient variance from 0.001 ft/ft to 0.003 ft/ft. During the operation of the GWETS, ground water flow was measured and depicted graphically as radially inward toward the operating extraction wells in an overlapping series of cones of depressions, indicative of hydraulic control over the groundwater plume. The flow direction of groundwater measured since completion of remediation in May 2011 and April 2011 was towards the east-southeast at a gradient of approximately 0.001 ft/ft. Residual Mass Estimate The consultant estimated 260.5 lbs. of dissolved MtBE existed in groundwater prior to batch extraction and GWETS remediation. Currently approximately 22.15 lbs. of absorbed MtBE remain in soil at the Site. Approximately 0.62 lbs. of dissolved MtBE remains in groundwater. The consultant estimated groundwater will reach water quality goals (WQGs) in 2012. Based on the MtBE concentration linear trends presented in the 10 October 2011 No Further Action Required (NFAR) Report, Regional Board staff estimates groundwater concentrations will periodically spike up above WQGs without significantly affecting plume stability and MtBE will finally remain below WQGs by 2021. Risk Assessment Analysis The November 2007 Tier 2 Human Health & Environmental Risk Evaluation Report evaluated risks to human health and the environment. Soil and soil vapor results onsite and at the neighboring businesses passed the San Francisco Bay Regional Board Environmental Screening Levels for commercial usage. The consultant stated that risk to workers and the public were insignificant based on a cancer cumulative risk of 9.2 x 10-8 and a non-cancer hazard quotient of 1.25 x 10-3. MtBE in groundwater has been removed to the most practical extent, and the groundwater plume is stable and declining. Nearby CSA-31-2 is not threatened by the Site residual groundwater plume.