ARCHIVED REPORTS_QUARTERLY GROUNDWATER MONITORING REPORT 1ST QUARTER 2016

Home Browse Search

L&M Operable Unit 19 December 2011 San Joaquin County - 2 - * Aquifer Test and DPE System Feasibility Study In November 2010, the L&M Group performed an aquifer test and DPE system feasibility study for the groundwater collection trenches at the Unocal site. Two groundwater extraction wells and piping were installed to connect the Unocal trenches with the existing DPE remediation compound located at the adjacent L&M Group site. The results of the aquifer test and DPE system feasibility study will be used to develop a conceptual design for expanding the current DPE system to incorporate additional flows from the Unocal site. Based on the results of the aquifer test and DPE system feasibility study, we concur with the L&M Group's finding that it is feasible to extract and treat groundwater and soil vapor utilizing the existing DPE system on the L&M Group site. Central Valley Water Board staff concurs with the L&M Group that additional pilot tests will be needed to evaluate how to optimize above-ground groundwater treatment at the Unocal site, especially with regard to tert-butyl alcohol. These tests will help guide the design and modification of the L&M Group's existing DPE system. MRP Revision During our August 2011 meeting, Central Valley Water Board staff agreed to revise the L&M Group's existing MRP to reflect the understanding of the groundwater conditions in the region. Current MRP R5-2004-0820 has been in effect for approximately seven years. Central Valley Water Board staff finds that sufficient data has been collected under the current MRP to merit a decrease in frequency of groundwater well sampling and constituent analysis. We have prepared a draft revised MRP for the L&M Group's review and will forward it to the group in November 2011. DPE Rebound Test On 29 September 2011, the L&M Group submitted a request and work plan to perform a rebound test on the existing DPE system. Only a portion of the system is proposed to be shut- off. All wells located in the apartment building complex west of the site will remain operational. The work plan proposes to shut-off 13 wells located around the existing remediation compound. Central Valley Water Board staff concur that removal rates of gasoline, benzene, toluene, ethyl-benzene, and xylene (BTEX) have steadily decreased and the DPE rebound test is warranted. We require that the wells be shut-off for a minimum of 45 days to allow site soil gas to equilibrate. In an effort to provide confirmation of adequate remedial system coverage, the L&M Group needs to propose the additional collection of soil gas samples from at least two new locations onsite. The samples from the new locations shall be collected during confirmation sampling. The L&M Group needs to submit a report of findings that contains the results of these confirmation samples. Conclusion • Central Valley Water Board staff concur with the following Report findings: