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EXECUTIVE SUMMARY This report presents the results of drilling that was conducted on August 8 and 9 and groundwater monitoring that was conducted on October 6, 2005 at 3931 Newton Road The drilling was performed to obtain current analytical data regarding diesel fuel concentrations in the Modesto Formation, and the groundwater monitoring was done as part of the regular quarterly sampling program in place for this site Six hollow-stem auger borings were drilled through the Modesto Formation, and soil samples were collected at 5-foot intervals Selected samples were preserved for laboratory analysis, and thirty-two were analyzed for Total Petroleum Hydrocarbons in the diesel range (TPH-d) The borings were drilled to a depth of at least 55 feet, and some were drilled to 65 or 70 feet Diesel was detected in 23 of the 32 samples, at concentrations ranging from 1 part per million in distant borings to 20,200 parts per million in one boring near the contaminant source (the former underground storage tank facility) Because the borings were drilled within the known contaminated area, none were completely free of diesel fuel The laboratory data were combined with the data from many samples that had been collected between 1990 and March 2005, and usdd to construct revised cross sections and diesel tsoconcentration maps at 5-foot intervals from 35 to 70 feet This series of maps illustrates that the hydrocarbon plume becomes longer and wider from 35 to about 50 feet, and then diminishes in size from 55 to 70 feet Maximum concentrations are highest in the 60-70 foot range within the upper part of the Riverbank Formation but are confined to a very small area in the immediate vicinity of the underground tank Due to the small size of the plume at this depth,the total volume and mass of diesel is lower than it is in the lower part of the Modesto Formation between 45 and 55 feet l The soil concentration data were also used to calculate the volume and mass of diesel fuel that remains in place in the soil in the 35-70 foot depth interval We calculate that 3,356 gallons, or approximately 24,000 pounds of diesel is adsorbed to the soil in this depth interval The depth to groundwater was measured in all three aquifers, and groundwater elevation maps were prepared for each All three depict an eastward slope to the piezometne surface, resulting in eastward groundwater flow at a low gradient (0 005 to 0 006 ft/ft) Diesel isoconcentration maps were constructed for the groundwater in each aquifer In the I" Riverbank aquifer, the plume trends northeast-southwest and has a linear central core located east of the UST facility, where concentrations 'exceed 750 parts per billion In surrounding wells, concentrations are in the 100-300 ppb range In the 2°d Riverbank aquifer, the plume is rests acted to a small area in the vicinity of well GT-15, where a concentration of 192 ppb was detected In the 3,d Riverbank aquifer, the plume is centered to the north of the UST facility and it trends north-south Concentrations are in the 100-300 ppb range • 1