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r w tt ,, <br /> 01.;eb/49 1 ani I-AA 9J6 : Z U4 ATTACHMENT ] wicOz,UU-0 <br /> 1} <br /> Department of Toxic Substances Control <br /> Jesse R. Huff, Director <br /> 400 P Street,4th Floor, P.O. Box 806 <br /> Sacramento, California 95812-0806 <br /> Gray Davis - Winston H.Hickox <br /> Governor Secretary for <br /> January 26, 1999 Environmental <br /> Mr. Kit Custis Protection <br /> California Regional Water Quality Control Board <br /> Central Valley Region <br /> 3442 Routier Road, Suite A <br /> Sacramento, California 95827-3003 <br /> In response to a request from the California Regional Water Quality Control Board, Central <br /> Valley Region (CVRWQCB), the Department of Toxic Substances Control (DTSC) Human and <br /> Ecological Risk Division has reviewed the technical merits of the document Health Risk <br /> Assessment, Corral Hollow acrd Grant Line Roads-Site,-Tracy,-Crilijornid.'This document was <br /> prepared on behalf of Chevron Pipe Line Company by Geomatrix Consultants of San Francisco. <br /> The problem in question concerns the potential health and environmental impacts as a <br /> consequence of an accidental release of refined petroleum product from a pipeline during sewer <br /> construction at the intersection of Corral Hollow and Grant Line Roads in Tracy, California. <br /> Based upon the information supplied in the report under review, the difference between the <br /> cstimated volume of material released and that recovered is approximately 50 gallons of product. <br /> This residual mass estimate is important for reasons which will be explained below. <br /> In the risk assessment, exposure point concentrations were calculated using measured <br /> concentrations in soil and ground water. Where a chemical of potential concern went undetected, <br /> one-half of the detection limit for that chemical was substituted in the estimate of the total mass <br /> of material, expressed as the 95% upper confidence of the mean of the measured sample values_ <br /> These estimates were then used in standard U.S. EPA and DTSC.models for emission of the <br /> COPC into the air. Since the default DTSC approach was chosen, the emission was modelcd <br /> assuming the source was at a steady state, that is, concentrations which are undiminished over <br /> time.!'i� For cancer causing chemicals such as benzene, these emissions are as to occur, at <br /> steady state, for 30 years_ <br /> I <br /> Given the nature of the model, the estimate of human exposure and calculation of human risk and <br /> hazard in the document under review is highly conservative. Refined product concentrations in <br /> soil and water does diminish significantly over time, due to natural processes of dispersion, <br /> dilution, and biodegradation. In many cases where models are used, the estimated amount <br /> emitted7over the 30 year period exceeds the size of the source itself-, especially-if the-sourcc term <br /> was calculated using half of the detection limit for a chemical. As a consequence, all risks and <br /> hazards reported in the document should be considered highly conservative upper bounds. <br /> Califomia Environmental Protection Agency <br /> i <br /> ® Printed on Recycled Paper <br /> I <br /> I <br /> I <br />