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Reduction of Monitoring Frequency <br /> y 3725 Tracy Boulevard,Tracy,Callfornla <br /> Soil boring logs prepared in November 1990 by Delta indicate sandy clays, clayey to <br /> sandy silts, and silty to clayey sands at or near the water table beneath the site. Delta <br /> reported an average hydraulic conductivity of 23.2 feet per day based on slug tests <br /> performed on the, three wells ("Preliminary Report and Tank Closure Report," dated <br /> November 28, 1990). Using the most recent estimated hydraulic gradient of 0.003 ft/ft, <br /> and a conservative effective soil porosity of 0.33, the estimated maximum average linear <br /> velocity of groundwater flow is 0.21 foot per day or about 77 feet per year. <br /> The available information indicates petroleum hydrocarbons in groundwater have not, in <br /> the past 2 years, migrated from the former tank basin. Based on the relatively low <br /> hydraulic conductivity of the soils at the water table, and the relatively low flow rate, <br /> annual monitoring is expected to be adequate for this site. Aegis will schedule no <br /> additional groundwater monitoring at this site until your response to this letter is received. <br /> Sincerely, <br /> AEGIS ENVIRONMENTAL, INC. <br /> Duane W. Garrabrant <br /> Staff Geologist <br /> DWG/mjg/law <br /> Attachment <br /> cc: Dan Kirk, Shell Oil Company <br /> C2- <br /> 90-089A.LTR -2- <br />