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1433 North Market Boulevard•Suite.1 •Sacramento,CA 95834-1943•(916)928-3300•Fax(916)928-3341 <br /> June 29, 1999 <br /> Project 16038-700.000 <br /> Mr. Wes Johnson <br /> County of San Joaquin <br /> Department of Public Works <br /> PO Box 1810 <br /> Stockton, California 95201 <br /> Re: Landfill Gas and Groundwater Interactions at the Corral Hollow Landfill, <br /> San Joaquin County, California <br /> Dear Mr. Johnson: <br /> Volatile organic compounds (VOCs) have been detected in both landfill gas (soil vapor) <br /> and groundwater at the Corral Hollow Landfill. The purpose of this letter is to evaluate <br /> whether the source of the VOCs detected in groundwater are from landfill gas or <br /> leachate. Evaluation of data from the site provides a strong indication that landfill gas <br /> appears to be the source of the VOCs detected in the groundwater and the soil vapor. <br /> If VOCs are present in groundwater, then some VOCs will diffuse into the air just above <br /> the water table. If the vapor and aqueous phases have sufficient contact time, <br /> equilibrium will be established, and the theoretical concentration in the groundwater can <br /> be calculated given the measured concentration in the soil vapor. The slower the <br /> groundwater moves and the more constant the VOC concentrations are in the soil vapor, <br /> the greater the chances that equilibrium will be reached between the two phases. <br /> If equilibrium has been established, the theoretical concentration of VOCs in the <br /> groundwater (Cw) based on soil vapor concentration should correlate to the measured <br /> values of Cw, especially if soil vapor samples are collected close to the water table: If <br /> calculated values of Cw are higher, then either equilibrium has not been established or <br /> the groundwater is not the source. The theoretical equilibrium concentrations were <br /> calculated as follows: <br /> SAC:V:\PROJECTS\PJ6\60387000.AS-99\cbell:I EMCO17 <br /> 16038-700.000 <br /> Rev.O06/29/1999 <br />