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I r . <br /> • <br /> Ms. Lori Duncan <br /> January 11,2012 <br /> RE:2911 E.Fremont St, Stockton,CA uv c <br /> Page 3 of 4 ��,oyr — ✓1r1F %µdibr 1 k,+beJv <br /> equations provided in the Cal/EPA-DTSC guidance document Final Guidance for the Evaluation and <br /> Mitigation of Subsurface Vapor Intrusion to Indoor Air, October 2011. The only parameters that were <br /> modified were exposure parameters,changing the default parameters for a residential scenario to those for <br /> the indoor worker,which were taken directly from the Cal/EPA-DTSC document. The changed parameters <br /> were: 1) exposure time changed from 24 hours per day (residential) to 8 hour per day(commercial); 2) <br /> exposure frequency changed from 350 days per year(residential)to 250 days per year(commercial);and 3) <br /> exposure duration from 30 years(residential)to 25 years(commercial). These modifications were noted in <br /> the report (Page 11) and the guidance document was also referenced at the end of the report. <br /> 4.L&P, in the conclusions and recommendations section ofthe report, states that petroleum hydrocarbon <br /> vapor persists in the vadose zone at the site; however, no vadose zone vapor samples were collected that <br /> could substantiate that statement. This may be important as pea gravel was utilized to backfill the <br /> excavation,potentially providing a highly permeable vertical pathway between residual impacted soil at <br /> depth and the concrete floor of the building. <br /> L&P Response: EHD is correct that no vadose zone vapor samples were collected to quantify the presence <br /> of petroleum hydrocarbon vapors in the vadose zone. However,it is a reasonable assumption that residual <br /> petroleum hydrocarbon vapors indeed exist, given the presence of volatile organic compounds in soil <br /> samples collected from the floor and sidewalls of the excavation. L&P,EHD,and the property owner were <br /> aware prior to excavation activities that site conditions would prohibit complete removal of known <br /> petroleum hydrocarbon contamination and that some residual petroleum hydrocarbons would consequently <br /> remain at the site. Since the contaminants are known volatile chemicals,it follows that hydrocarbon vapors <br /> OV, so`T would persist in the subsurface.Although the pea gravel backfill has greater permeability than native soil <br /> s � beneath the site (predominantly silt),the native soil also provides a vertical pathway for vapor migration <br /> p, zL-0`u� over time. The purpose of the air sampling and evaluation was to determine if the chemical make up of <br /> 7vta1:P indoor air differed substantially from the chemical make up of ambient air and,if so,if the concentrations <br /> of chemicals in air at the site pose undue risk to site workers. L&P concluded that there are observable <br /> differences in the chemical make up of indoor air when compared to ambient air. However, L&P <br /> concluded that regardless of the source,the indoor air in warehouse#2,as well as ambient air at the site, <br /> contain levels of volatile organic compounds at levels that are marginally above the threshold considered <br /> best case scenario,but well within the range of acceptable risk for a commercial setting. <br /> As presented in our November 15,2011 report,L&P concludes that potential vapor intrusion at the site has <br /> been adequately investigated using methodologies recommended by the U.S.EPA and Cal/EPA and that <br /> chemical concentrations in air at the site are within the lower range of what is considered acceptable in a <br /> commercial setting. <br /> Pe- 61105(s Awr,wJ, CARS4 lypy P C.AilaMSr.A< i4 S,,/ e7;It l9af y J[�vs/ 4ir <br /> rip ) ,y' r p�� / <br /> r j tt eil'� 2/�,w,,e,4 ap�y/ 4 JOI I'M �!jAJr 0"? C 4,6,,04<r�r,V' <br /> 10759-LTR Response <br />