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July 17,2002 <br /> Job Number:LG02-136 <br /> Page 6 <br /> r <br /> In the early 1900's, natural levels of nitrate in ground water were measured in forty-three <br /> (43) wells throughout the Sacramento Valley by Bryan 2 It was thought that ground water at that <br /> r time was close to "natural"conditions. Based on the work by Bryan, it is estimated that under <br /> "natural"conditions, ground water concentration containing nitrate is no more then about 13.5 <br /> NO3 (3 mg/L-N). Areas having 24.75 mg/L NO3 (5.5 mg/L-N)or more are those in which nitrate <br /> concentrations may be increasing.' <br /> DBCP was not detected in the water sample. DBCP is a nematocide and soil fumigant <br /> for vegetables, and grapes. It is thought to cause health problems consisting of kidney damage, <br /> liver damage, and cancer. The use of DBCP was discontinued in 1979. The Environmental <br /> Protection Agency(EPA) set the maximum contaminate limit(MCL) at 0.2 parts per billion <br /> (ppb) because the EPA believes that given present technology and resources, this is the lowest <br /> level to which water systems can reasonably be required to remove this contaminate should it <br /> occur in drinking water.' <br /> r <br /> VI. GEOLOGY, SOILS,AND PERCOLATION TEST <br /> The surface geology at the site is shown on Atwater's maps and is composed of upper and <br /> lower members of the Pleistocene Modesto Formation(Plate 6). The upper and lower members <br /> are composed of arkosic alluvium. The alluvium forms Mokelumne River terraces and alluvial <br /> fans. The alluvium is chiefly sand and is probably glacial outwash. The lower member differs <br /> from the upper member and contains additional gravel and silt. <br /> 2 Bryan,K., 1923,Geology and ground-water resources of Sacramento Valley of California:U.S. <br /> Geological Survey Water-Supply Paper 495,285p. <br /> 3 Hull,L.C., 1984,Geochemistry of Ground Water in the Sacramento Valley,California:U.S. Geological <br /> Survey Professional Paper 1401-B. <br /> 4 EPA, 1999,Drinking water and health,National Primary Drinking Water Regulations,consumer fact <br /> sheet on:DIBROMOCHLOROPRPOANE. Internet:http://www.epa.gov./OGWDW/dwh/c-soc/dibromoc.html <br /> S Atwater,B.F., 1982,Geologic Maps of the Sacramento-San Joaquin Delta,California:U.S.Geologic <br /> Survey Miscellaneous Field Survey Map MF-1401,21 Sheets,Scale 1:24,000. <br /> eaeso <br /> s <br /> e <br />