SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

Joe Lentini 4- 5 November 2010 Former Pipelines, Tracy pipelines in the vicinity of Trench 3 has been delineated. However, the groundwater data obtained from B-77 and B-78 in October 2008 show that the lateral extent of the pipeline release in the Trench 3 area has not been delineated. Figure 3b in the Work Plan shows that TPHc has been detected at 570 Ng/L about 250 feet east of Trench 3 at a depth of 146 feet bgs. This pollution could have migrated with the flow of deep zone groundwater because no contaminants of concern were detected above 100 feet bgs in borings B-77 and B-78. The flow direction of groundwater in aquifers underlying the shallow zone has not been determined. Additional investigation and monitoring wells are needed in this area to delineate the lateral extent of COCs below the shallow zone. 3. Based on the analytical results which show that elevated concentrations of TPH are present in the shallow zone in MW-2, we concur that the lateral and vertical extent of petroleum hydrocarbons has not been delineated in the Trench 2 area. Additional investigation and monitoring wells are needed in this area to delineate the extent of CDCs in the shallow zone to the west of Trench 2. 4. The additional investigation to delineate the lateral extents in the Trench 2 and 3 areas should include the depth discrete groundwater sampling proposed in the 6 November 2009 Risk Assessment Scope of Work, Former Shell Central Valley Crude Oil Pipeline, Hansen Road Near Schulte Road, Cordes Ranch (RA Work Plan) for the MW-1 through MWA 1 well installation phase of the investigation. Groundwater sampling prior to well installation would more accurately characterize the vertical extent of pollutants and finalize the total depths of future monitoring wells in at Trenches 2 and 3. 5. The Investigation Report does not provide any indication that the vadose zone soil sampling for the analysis of physical properties, as described in the RA Work Plan was conducted. Shell proposed to analyze the physical properties to support a proposed assessment of soil vapor intrusion pathways. The soil vapor sampling was conducted on April 2007 in areas containing the highest levels of TPH in soil and groundwater in Trenches 2, 3, and 4. The results of soil sampling and the performance of a Johnson and Ettinger (J&E) Model evaluation were summarized in Shell's August 2007 Site Investigation Report. The J&E Model results indicated that a detection of benzene in boring B-72 at 30 feet bgs at a concentration of 390,000 micrograms per cubic meter (Ng/m3) posed an incremental cancer risk that exceeded the de minimis standard of one-in-a-million. Based on these results, Shell stated that permanent soil vapor probes would be installed near the highest levels of soil and groundwater pollution to facilitate additional soil vapor sampling. 6. The soil vapor contaminant concentrations and indoor air intrusion modeling results indicate that cleanup will be necessary to prevent further groundwater degradation and protect human health. Cleanup levels to protect groundwater quality from soil vapors may be evaluated using modeling software, such as VLEACH, which is available on the internet at http://wvvw.epa.gov/ada/csmos/models/vleach html. Contaminant concentrations remaining after cleanup to protect groundwater can be re-modeled using J&E Model. The Department of Toxic Substances Control's latest version of the model and guidance for using it are available on the internet at hftp://www.dtsc.ca..clov/assessin.grisk/humanrisk2.cfm.