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i <br /> Del Monte Disco Site Page 2 <br /> 110 N. Filbert Street February 25, 2008 <br /> Stockton, CA 95206 <br /> that residual contaminants do not pose a significant risk to the environment or to human <br /> health. The EHD believes that additional remedial effort is needed at this site to qualify it j <br /> for NFA consideration; to better evaluate the need for further remedial action and I <br /> suitability of the site for future closure consideration, the EHD requires the following: <br /> A site conceptual model (SCM) that includes a detailed hydrogeological model. <br /> Cross sections depicting site 'lithology, monitoring and vapor extraction well <br /> screen intervals, soil boring traces, air sparge well and ozone sparge well screen <br /> intervals, and the distribution of sorbed and dissolved contaminants should be <br /> part of the SCM. <br /> A report on the remediation system design, noting all air sparge, ozone sparge <br /> wells utilized, the screen intervals.and injection rates. <br /> • Updated estimates of dissolved and sorbed contaminant masses: <br /> Please include in the SCM a map accurately showing the locations of all borings, wells, <br /> and the former UST system, tables with all soil and grab groundwater analytical data, <br /> and a rose diagram showing the groundwater gradient directions since cessation of <br /> pumping from the local water supply wells. Plots for a few core area wells showing the j <br /> principle contaminant concentrations over time, the thickness of the water column over <br /> the tops of the well screen and periods of remediation system operation may be useful. <br /> If additional soil sampling on site is necessary to estimate residual contaminant mass <br /> left at the source area submit a work plan to acquire the needed data. <br /> In addition to the needed items noted above, the EHD has concerns about the following <br /> items that must be addressed: <br /> • It appears to the EHD that the air sparge wells utilized are screened at depths <br /> that range from 87 feet to as much as 138..5'feet below surface grade (bsg), with <br /> one well screened at 78 feet bsg. With depths to water ranging from 37 feet to 45 <br /> feet bsg, approximately 33 to 50 feet of water overlies the shallower air sparging <br /> screens. The EHD is concerned that the sparged air may not be able to migrate <br /> vertically to the vadose zone for stripped hydrocarbons to be recovered by the <br /> SVE system, and therefore air sparging may actually be diluting the contaminant <br /> concentrations by inducing lateral migration. Evaluate this situation. <br /> Rising groundwater has 'drowned' the shallow groundwater monitoring wells; <br /> water currently is 15 to 20 feet above the tops of the shallowest monitoring well <br /> screened intervals and has saturated soil impacted by sorbed hydrocarbon <br /> contaminants. As there are no wells that monitor shallowest groundwater, <br /> propose additional monitoring wells to adequately characterize shallowest <br /> groundwater on your site in and surrounding the area of known impacted soil. <br /> The ozone injection system was placed into operation at this site prior to the <br /> CVRWQCB's requirement for Waste Discharge Requirement (WDR) evaluation, <br /> therefore the CVRWQCB has requested that as part of the next monitoring event metal <br /> and mineral analyses be conducted on the ground water samples in the plume area <br /> likely to be affected by ozone injection; for specific appropriate chemical analyses refer <br /> 110 Filbert.Nlt9 CIVIL 2-21-08 - - <br />