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s: <br /> U11VINR�i�MENTAL HEALTH <br /> �I-1M N�, SOR10CE <br /> Chem/�isk 913 JUL 2 r PM 1: 5'4 <br /> A Division of McLareniHart <br /> Environmental Engineering <br /> 100 North First Street, Suite 210 <br /> Burbank, CA 91502 <br /> (818) 841-0606 <br /> r FAX (818) 841-0896 <br /> July 19, 1993 <br /> Diane M. Hinson, REHS <br /> Supervising Environmental Health Specialist <br /> San Joaquin County Public Health Services <br /> Environmental Health Division <br /> 445 North San Joaquin Street <br /> Stockton, California 95201 <br /> SUBJECT: ADDITIONAL INFORMATION REQUEST REGARDING THE RISK <br /> ASSESSMENT FOR THE SANTA FE RAILWAY COMPANY SITE IN <br /> STOCKTON, CALIFORNIA <br /> Dear Ms. Hinson: <br /> f <br /> ChemRisk®understands that you are completing your review of the risk assessment for the <br /> Santa Fe Railway Company Site located at 1033 East Scotts Avenue and is pleased to <br /> answer any questions you may have. Discussed in this letter are responses to your questions <br /> during our telephone conversation on Friday,July.16, 1993. <br /> Your first question was regarding the soil pH used in the modeling and its effect on the <br /> migration of organic chemicals. ChemRisk® used a neutral pH value of 7. The value was <br /> not site-specifically determined because of its minimal impact on the fate and transport of <br /> the petroleum hydrocarbons in the SESOIL modeling. <br /> For organic chemicals, the pH value is used in SESOIL's.hydrologic cycle to estimate the <br /> hydrolysis rate constant, which is the rate at which an organic molecule reacts with water <br /> to form a new chemical. It specifies any hydrolysis reactions that may be catalyzed by the <br /> ' presence of hydrogen ions (H+) and/or hydroxide ions (OH). The hydrolysis results in a <br /> ,loss of total mass in the soil column that is available for migration. ChemRisk® has <br /> maintained a conservative approach by specifying that none_of the petroleum hydrocarbons <br /> is able to undergo any acid/base hydrolysis. Thus, the value specifying the pH for the soil <br /> is not used. This conservative procedure was performed by specifying that the acid and base <br /> hydrolysis constants in the chemical input file be zero. <br /> i <br />