SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

Karen Petryna ~ `� Shell Oil Products 880 E. Victor Rd., Stockton, CA Page 3 of 5 closest point of the former UST pit and 70 feet from the most impacted monitoring well, as the delineator of the vertical extent of impacted groundwater is highly dependant on the unknown hydraulic conductivity of the unit and the flow direction — EHD believes it quite possible that this well missed the critical monitoring point. Cambria has demonstrated that a groundwater extraction rate less than 1 gallon per minute will not exert much hydrological control of the plume. If the plume is stable and not migrating as currently interpreted by Cambria, there would not be much need for hydrological control. Cambria attributes the decreasing MTBE concentration and increasing TBA concentrations to mass removal of MTBE by way of the groundwater extraction events and biodegradation of MTBE to TBA. Cambria supplied a copy of a report from a Shell study of MTBE/TBA-impacted sites that isolate biodegradation as the most likely cause of temporary rising TBA concentrations coupled with declining MTBE concentrations. EHD concurs that this is likely to be occurring on the subject site, but does not appear to be a site-wide process. It appears that the bioconversion of MTBE to TBA occurs primarily in the immediate vicinity of the well borings. The analytical data from samples collected from MW-4 during the batch extraction events demonstrate an abrupt decline of TBA and increase of MTBE concentrations. Plots of the data show that at the conclusion of each 5-day extraction event the TBA concentrations are not even close to stabilizing and MTBE concentrations are at least 30,000 ppb. This indicates to EHD that the well bores are probably influencing the surrounding groundwater chemistry and biologic response. Assuming that there is minimal drawdown during the less than 1-gallon-per- minute extraction, that the saturated section against the MW-4 screen is 10 feet thick, and that the soil has a 20% effective porosity, EHD estimates that with a 5,000-gallon extraction event, the last water into the well traveled approximately 10 feet. It would appear that the normal 3 to 4 well volumes commonly utilized for purging is not adequate at this site to get samples representative of the predominant aquifer conditions. Cambria concludes that GWE was effective and EHD believes that it is still effective for mass removal. This is based on a recovered mass of 8 pounds MTBE in nearly 20,000 gallons of water extracted. EHD does not concur with Cambria's conclusion that no known sensitive receptors are threatened and thereby there is no necessity for engineered mass removal or hydraulic control. Intuitively, any migration of impacted groundwater potentially impacts additional