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Site History 14000 Highway 88 !_ockeford CA Time Oil Property No 04049 Between the First and Second Quarters of 1996, gasoline and BTEX concentrations decreased slightly in all of the wells The observed fluctuations in hydrocarbon concentrations may be a result of a general increase in the groundwater elevation or a result of natural attenuation A copy of SECOR's "Quarterly Monitoring Report, April - June 1996" dated July 22, 1996 was forwarded to the appropriate regulatory agencies on July 31, 1996 Additional Site Inv s i ation and Third Quarter 1996 Groundwater Sampling Results-October 1996 On July 15 through 18 and July 31, 1996, six groundwater monitoring wells were installed onsite and offsite to assist in defining the extent of hydrocarbon impacts As in previous site activities, soils encountered during drilling consisted of fine silty sands to approximately 15 feet below ground surface (bgs), this strata was underlain by fine to coarse sands to approximately 30 feet bgs Clayey silt and silty clay were generally encountered at 30 feet to 35 feet while fine to coarse sands and gravelly sands were encountered to 70 feet bgs Silty clay with sandy and gravelly stringers was encountered from 70 feet bgs to the extent of each boring Groundwater was first encountered at depths of approximately 75 feet to 80 feet bgs Each of the wells was installed as follows Casing Total Screened Well No. Diameter(m ) Depth (ft ) Interval (ft) Purpose MW-5 2 100 35 HC delineation to the NW of former tanks(sidegradient) MW-6 2 100 35 HC delineation to the NE of former tanks(upgradient) MW-7 2 100 35 HC delineation to the NE of former tanks(uptsidegradient) MW-8 2 100 35 HC delineation to the E of former tanks(sidegradient) MW-9 2 100 35 HC delineation to the SE of former tanks(sidegradfent) MW-10 2 975 35 HC delineation to the SW of former tanks(downgradient) The casing diameter, well depth, and screened interval of each well was selected by a representative of SECOR based on the proposed well use and the estimated depth to groundwater at each drilling location Each of the wells was constructed with 0 020-inch slotted casing Soil samples collected during the dnlling of each of the wells were submitted to an analytical laboratory Where possible, capillary fringe samples were submitted, although shallower soil samples were also analyzed Although the sample collected from MW-8 at a depth of 75 feet bgs contained trace concentrations of hydrocarbons (0 064 ppb benzene, 0 005 ppb toluene, and 0 007 ppb xylenes), none of the other soil samples contained detectable concentrations of hydrocarbons On July 19, 1996, samples were collected from MW-1 through MW-9 by a representative of SECOR MW-10 was sampled on August 1, 1996 Prior to purging, the depth to water in each well was measured to evaluate elevation, flow direction, gradient, and free product thickness Groundwater with a southwesterly flow direction and a gradient of 0 03 ft/ft were encountered in the wells at depths of 67 7 feet to 84 07 feet below ground surface Free product was not encountered in any of the wells Since the previous sampling event, groundwater elevation decreased in MW-3 by 0 07 feet and increased in wells MW-1, MW-2, and MW-4 by 0 15 feet to 0 33 feet, resulting in an overall average increase in groundwater elevation of 0 18 feet across the site Groundwater flow direction changed from southeast to southwest and gradient decreased from 0 15 ft/ft 0 03 ft/ft The variations in groundwater elevation, flow 9