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Dan Schreiner - 2 - 15 June 2015 recommends preparing a work plan for further assessment of soil vapors in the vicinity of UPSV-6 and APSV-15, and recommends suspending the quarterly soil vapor monitoring while the work plan is prepared, reviewed, approved, and the work is completed. The report also notes that there is a correlation between elevated methane levels and elevated TPH in soil gas. The report presents the results of methane monitoring that was conducted in 37 locations around the Delta View Apartments in utility vaults, irrigation boxes, and monitoring well vaults. The measured levels were very low (below 60 parts per million by volume [ppmv]) with the exception of one location with 1,300 ppmv which is still below the 5% (50,000 ppmv) lower explosive limit (LEL). The report concludes that the results do not indicate potential methane accumulation at levels indicating an imminent hazard to residents at the apartments, but also recommends that methane monitoring should be conducted during any future sub-surface disturbance. Central Valley Water Board staff has the following comments on the report: 1. We note that there is no longer a screening level being proposed for TPH-g in the soil vapor probes. A screening level for TPH-g is necessary for this site given detections of TPH-g in certain probes where other the other constituents for which proposed screening levels remain (BTEX and naphthalene) are generally not detected. TPH-g in soil vapor at levels above the Region 2 screening level does remain a potential health concern and is also potentially a concern for nuisance odors given its low odor threshold. The previously proposed Region 2 screening level for TPH-g of 295,000 ug/m3 for shallow soil gas (that is based on dividing the residential indoor air criterion for TPH-g by a 0.002 attenuation factor) is acceptable and should be used for this site since there is no other relevant health-based criterion for TPH-g. Without a relevant screening level for TPH-g in shallow soil gas, gasoline constituents could remain at unacceptably high levels and prevent us from finding that human health has been protected in any future request for no further action. 2. We also note that soil vapor probes APSV-9 and APSV-10 have been recommended for elimination from the soil vapor monitoring program because they are not considered to be representative of current vapor intrusion protection measures. We do not find that the reasons stated in the report (and reiterated in this letter, above) are sufficient to warrant ignoring TPH-g concentrations above the screening level in these probes, particularly since they are near the apartment buildings. Soil vapor probes APSV-9 and APSV-10 should be retained in the soil vapor monitoring program and should also be addressed in the proposed work plan since they have TPH-g concentrations well above the 295,000 ug/m3 screening level. 3. We agree with the observation in the report that there is a correlation between high levels of methane and relatively high levels of TPH-g. Although the results of the methane measurements taken at the 37 locations around the site indicate no immediate hazard, we are still concerned about methane levels above the 5% LEL that are caused by degradation of petroleum constituents being a potential threat to the apartment buildings and utility worker under conditions that may not have been anticipated (i.e. changing atmospheric conditions, varying HVAC operations in apartments, etc.). The report recommends methane monitoring during any future sub-surface disturbance; however, this would need to be recorded in a deed covenant on the property to require methane monitoring during any future soil disturbance activities. Since this option is very