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Reduction of Morvtonng Frequency <br />. 3725 Tracy Boulevard Tracy California <br /> Sod 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 /> i)ao e, Awew- <br /> • Duane W Garrabrant <br /> Staff Geologist <br /> DWG/mfg/law <br /> Attachment <br /> cc Dan Kirk, Shell Oil Company <br /> 90-089A LTR 2 <br />