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EXECUTIVE SUMMARY <br /> Six borings were drilled during the first quarter of 2005, and nine new monitor wells were installed <br /> in the 1st, 2nd, and 3rd aquifers in the Riverbank Formation These sand bodies are fluvial channel <br /> deposits that are oriented generally east-west Diesel fuel has been detected at very low <br /> concentrations (1-3 parts per million) in all three during previous phases of drilling, and also in <br /> groundwater samples at concentrations up to about 600 parts per billion Drilling has expanded <br /> outward as the extent of contamination has been traced away from the former underground diesel <br /> storage tank, which is the documented source of contamination <br /> Two of the new wells were screened only within the 1St (uppermost) Riverbank Aquifer in the <br /> interval 60-70 feet below grade One well was screened only in the 3rd aquifer from 115-125 feet <br /> Two wells were screened in both the 1st and 2nd aquifers and one was screened in the 2nd and 3rd <br /> Nine soil and water samples were collected, In addition to the 11 water samples from the previously <br /> installed wells All samples were analyzed for Total Petroleum Hydrocarbons in the diesel range <br /> using EPA method 8015 <br /> In most cases, the thickness of individual aquifers in individual wells differed somewhat from <br /> previous estimates, making it necessary to revise existing thickness maps of each aquifer This <br /> resulted in small changes in the trend of each fluvial channel axis, and in some cases led to the <br /> identification of braided channels with interchannel bars Pebbles and clay (perhaps mud rip-up <br /> clasts from underlying silt or clay beds) occur within basal channel-tag deposits, whereas fine-to- <br /> medium-grained sand without clay or pebbles appears to characterize interchannel bars <br /> Water-depth measurements indicate that static water levels rise by approximately one foot <br /> progressing downward from the I" aquifer to the 3rd aquifer The resulting piezometric surface in <br /> the 1St aquifer slopes northward and implies groundwater flow in that direction, while the <br /> piezometnc surface in both the 2nd and 3rd aquifers slopes eastward and records flow at <br /> approximately a90' angle to that in the 1st aquifer These conditions suggest local confinement and <br /> separation but communication between aquifers at a site-scale or larger <br /> Diesel fuel was detected in three soil samples and 14 of 20 water samples An unexpectedly high <br /> concentration of 4100 parts per million was detected at the base of the Modesto Formation channel <br /> at a depth of 50 feet in the easternmost well, but concentrations in the 2nd and 3rd Riverbank <br /> aquifers were within the historical 1-3 ppm range Groundwater concentrations ranged up to 370 <br /> parts per billion in the 2nd Riverbank aquifer, but were less than 300 ppb in the other two <br /> Groundwater isoconcentration maps depict a narrow northeast-southwest trending groundwater <br /> plume in the 2"d and 3rd aquifers, and a more elliptical east-west trending plume in the 1 st aquifer <br /> l <br /> 1� <br /> 1 <br /> 1 <br /> I� <br /> I <br />