ARCHIVED REPORTS_REPORT ON SOIL VAPOR SAMPLING AND HUMAN HEALTH RISK ASSESSMENT DEC 2010

Home Browse Search

AECOM Report on Soil Vapor Sampling and Human Health Risk Assessment 2-2 detector was used to ensure that the air inside the chamber contained approximately 16 to 43% helium during sampling.Laboratory analysis for helium was used to assess if leakage occurred during Sampling. 2.2.3 Purging Prior to collecting a soil vapor sample,the sampling tubes and vapor well were purged using a purge canister to ensure that the vapor samples collected were representative of actual soil vapor concentrations. Purge:volumes were calculated based on the dimension specifications of all above- ground gauges, tubing,sampling equipment, and below-ground tubing.The volumes of the well screen and sand pack were not included in the purge volume calculation since they are assumed to be in equilibration with soil vapor in the subsurface. The flow rate for purging was the same as that used for sampling (< :200 ml/min). Three tubing volumes were purged before sampling, based on the guidance documents referenced above. Calculated purge volumes and durations were recorded on the vapor sampling field sheets included as Attachment A. 2.2.4 Sample Collection Sample collection from each of the soil vapor wells was started immediately after purging. Leak testing was performed using helium, concurrent with sampling, as described above. To begin sampling, the valve on the Summa®canister was opened and the time and initial pressure were documented.As the canister filled, the pressure gauge on the flow controller was observed to ensure that the vacuum in the canister was decreasing over time. Each canister was allowed to fill for one half hour or, if possible, until the canister vacuum had decreased to minus five inches Hg. Once the samples were collected, the Summa®canisters were dosed and sealed using brass caps supplied by Air Toxics. Samples were labeled following standard chain-of-custody(COC)protocols, including noting the final canister vacuums and the serial numbers of all canisters and flow controllers. AECOM documented the sampling activities, such as sampling times and conditions, in the field sheets included in Attachment A. Samples were delivered directly to the analytical lab under COC protocols within 24 hours of sampling.AECOM delivered the vapor samples to Air Toxics, a California state certified laboratory,for chemical analyses. 2.3 September 9, 2010, Event Concern about the validity of the May sample results for SV-1 prompted the additional September sampling of SV-1 (only),to confirm the accuracy of the laboratory analytical results. During the May sampling event, no significant(>_0.5 inches)rainfall had occurred during or immediately prior to (within 24 hours)sampling; however, surface soils in the vicinity of the SV-1 location were damp, with some small pools of water observed.This condition was presumed to have been a result of automatic sprinklers.Additionally,a vacuum of 5510 inches Hg (CalEPA, 2003)was not obtained in the Summa canister. Soil moisture at depth is a potential contributor to low permeability during sample collection. However, observation of surface moisture is a subjective indicator of potential soil moisture at depth (i.e., 5 feet bgs) and mayor may not reflect the actual state of soil moisture at depth.There is no direct correlation between the soil moisture observed at the surface and the condition at depth. [No December 15,2010 60146576-1010