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John and Margaret Marci Pr6perty 17 October 2008 <br /> 2969 Loomis Road Page 2 of 3 <br /> Stockton, California 95205 <br /> • Background barometric pressure measurement were not collected during <br /> the tests, so the results cannot be corrected for that source of natural <br /> variation in the pressure measurements; and <br /> • The reported analytes for the vapor samples collected during the DPE pilot <br /> test included only volatile . organics with naphthalene, trichloroethene, <br /> carbon disulfide, benzene, toluene, ethylbenzene and total xylenes <br /> reported; total petroleum hydrocarbons quantified as gasoline (TPHg) or as <br /> diesel (TPHd) were not analyzed. <br /> These design features of the three field tests introduce uncertainty into the <br /> interpretation of the test results, thereby preventing the EHD from either concurring <br /> or disagreeing with the GTI conclusions and recommendation; therefore the EHD <br /> does not concur with preparation of a CAP at this time. The EHD notes the <br /> following observations regarding the field tests: <br /> The background oxygen level in soil gas prior to initiation of the bioventing test was <br /> approximately 16 to 17%, nearly the atmospheric level of 20.9%, and well above <br /> the 5% needed for aerobic bacterial degradation of hydrocarbons according to the <br /> Guidance. During the test, the oxygen and carbon dioxide concentrations varied <br /> inversely to one another from high to low concentrations, which GTI interpreted to <br /> indicate stimulation of the biological breakdown of the petroleum hydrocarbons due <br /> to the increased oxygen level, from 16% to nearly 21% followed by rapid depletion <br /> of the oxygen, which in turn was followed by a speedy recovery of oxygen level to <br /> near 21% again. The oxygen and carbon dioxide levels fluctuated on a daily basis <br /> during the test. <br /> The experience of the EHD evaluating bioventing pilot test data is limited, but it <br /> doesn't seem reasonable that the addition of 3 or 4 percent oxygen to soil gas that <br /> is already well oxygenated would kick off a biological response that would rapidly <br /> consume nearly all the oxygen over 4 to 6 hours and that the low rate of air <br /> injection would replenish the oxygen levels to near atmospheric concentrations <br /> over the whole site by the next morning. <br /> The relatively high oxygen content of the soil gas suggests that either biological <br /> activity is low or that oxygen replenishment is in equilibrium with biological oxygen <br /> consumption. The guidance indicates that the latter case is rare, therefore the EHD <br /> concludes that there is a low level of biological activity in soil and that injection of <br /> additional oxygen is not likely to be of significant benefit. The EHD will reconsider <br /> this opinion if presented more technical data and other case studies that document <br /> similar biologic response at sites with relatively well oxygenated soil gas. <br /> Although it is a limited data set, the dissolved oxygen measurements recorded <br /> from monitoring well MW-5, do appear to show that biosparging may be effective <br /> for increasing the dissolved oxygen content in the site groundwater. <br /> Bioventing Pilot Test Comment Letter 1008.doc <br />