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CALIFORNIA GEOPHYSICAL GROUP , INC . <br /> P . O . Box 578341 , Modesto, CA 95357 Tel : (209) 527- 1247 <br /> Fax: (209) 527-4810 <br /> May 4, 2004 ECF8VLD <br /> Mr. Nuel Henderson MAY 0 5 2004 <br /> Enviro mentalA Health <br /> Dd Floor ENVIRONMENT HEALTH <br /> 304 E . PER-411T/SERVICES <br /> Stockton , CA 95202 <br /> Your letter dated April 13 , 2004 requested us to supply you with evidence that we made in a <br /> statement about an MtBE plume sinks about 1 foot per 20 feet in length . Attached is a Well and <br /> Borings Location plot and two sheets from the data base, where we developed this statement. <br /> In October 2003, we drilled two monitoring wells MWj9 and MWa10 on a site we are working . <br /> During the drilling we took water samples every 10 feet below the water table . The water table <br /> was located at was located at 27 feet bgs . The distance between MWa9 and MW410 is 396 feet in <br /> the direction of the water gradient. <br /> The water in well MW?9 was contaminated from 30 feet to 100 feet. The water in MWa10 was <br /> contaminated from 5.0 feet to 100 feet. The source of the contamination is located near MW9. <br /> The first contamination in MW49 is higher than the first contamination in well MW410. The <br /> estimated level difference is about 18 feet. <br /> We calculated the sinking rate to be about 1 foot per 20 feet, by dividing 18/396 = 0. 045 feet/foot, <br /> This converts to 1 foot per 22.22 feet. When rethinking this calculation we found we had not taken <br /> into account the yearly rainfall in Modesto. This is about 1 foot per year. If all of the rainfall <br /> penetrated to the water table then the calculation was wrong . If we reduce the difference between <br /> MW+9 and M* 10 we find that the difference is 8 feet. We divided 8/396 = 0. 020 feet per foot. <br /> This converts to 1 foot per 49 . 5. feet. We believe the decent rate of the plume is between 22 . 22 <br /> and 49. 5. feet per foot. <br /> You will note that the MtBE is also present at 100 feet in well MW4'9 closest to the source. This <br /> decent is not due to geology since the source is the geology is the same at both locations. <br /> I trust this explanation of the calculation is understandable. <br /> Yours truly, <br /> 1 -7 <br /> John Cussen <br /> RG 6506 O�ocSTER <br /> NNNHE�cF� <br /> G �O <br /> Enclosures : N0 660sZ <br /> 1 Plot B <br /> 2 Data Sheets <br /> �F�R4FORNIP <br />