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<br /> Site Investigation Work Plan
<br /> Nella Oil Station#21,40052 Highway 41, Oakhurst, California
<br /> 2.0 SITE CONCEPTUAL MODEL
<br /> 2.1 Source Area Evaluation
<br /> Site investigation work conducted by El indicates the primary source area for the petroleum
<br /> hydrocarbons in groundwater is centered around monitoring well MW-4, southeast of the former
<br /> - underground storage tanks (USTs) and northeast of the former diesel USTs. Site investigation
<br /> work consisted of soil samples from soil borings B-1 through B-7, installation of monitoring wells
<br /> MW-1 through MW-4, and groundwater analysis collected from the four wells from 1999-2006.
<br /> More recent groundwater analysis collected from groundwater samples during Mecember CICAL;'Y
<br /> 2007 site investigation consisted of CPT borings P-1 through P-4 and the collection of 13 �12f 5 .S bcr° i
<br /> samples. Based on current and historical data, Total Petroleum Hydrocarbons as Gasoline , o2 c 7P*-A
<br /> (TPH-G) in site soil and groundwater is the primary constituent of concern (COC). However, the d `7PrGmc
<br /> fuel oxygenate Di-isopropyl Ether(DIPE) was also detected in low-high concentrations in one
<br /> CPT boring. ` !}I s o I/ z�VIiii
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<br /> TPH-G and benzene were detected in only P-2 with concentrations of 1,680 parts per billion
<br /> (ppb) and 30.7 ppb, respectively and both at a depth of 35 feet bgs. Similarly, DIPE was also
<br /> detected only in P-2 at concentrations of 837, 4.04, 6.68, and 26.4 ppb at 35, 57, 90, and 115
<br /> feet bgs.
<br /> Diesel range organics (DRO) concentrations have historically been persistent and fluctuating in
<br /> wells MW-1 and MW-2, and June 2007 values were recorded at 1,720, and 11,200 ug/I,
<br /> respectively. These concentrations reflect a substantial DRO concentration reduction in MW-1
<br /> but a substantial DRO concentration increase in MW-2. Gasoline range organics (GRO) have
<br /> been persistent and fluctuating in MW-4, and the June 2007 value was recorded at a historic
<br /> high of 26,300 ug/I. Benzene concentrations have historically been persistent and fluctuation in
<br /> well MW-2 and MW-4, and first quarter 2008 monitoring values were reported at 7.79 and
<br /> 1,6,,z ug/I, respectively. Well MW-4 has had the highest concentrations of benzene, toluene,
<br /> ethylbenzene, and total xylenes (BTEX) and reduction has only been substantial for toluene, all
<br /> other gasoline constituents have remained relatively high in concentration (first quarter
<br /> monitoring resulted in detections of BTEX at1, 0, 179, 1,410, and 5,516 ug/I, respectively).
<br /> Quarterly monitoring graphs for gasoline const'.. nts and fuel oxygenates vs. time are attached
<br /> in the Figures section of this report.
<br /> 2.2 Fate and Transport Analysis I
<br /> 2.2.1 Geology
<br /> According to the United States Department of Agriculture (USDA) Soil Conservation Service
<br /> Current Soil Survey of San Joaquin County, the soil at the site is the Jacktone-Urban land
<br /> complex. This soil is found on basin floors in the Great Basin and Central Valley area at
<br /> elevations of 10 to 40 feet above mean sea level.
<br /> At the CTL location the soil is 35% Urban land, 50% Jacktone, 6% Hollenbeck, 5% Stockton,
<br /> and 4% coarse-fine texture overwash. The soil is typically 20 to 40 inches deep, and a typical
<br /> profile for the area is clay to 34 inches bgs, indurated clay to 37 inches bgs, stratified sandy to
<br /> clay loam to 46 inches bgs, and cemented sandy to clay loam to 60 inches bgs.
<br /> According to the Department of Water Resources' Bulletin 118, Groundwater Basin Number: 5-
<br /> 22.01, the Eastern San Joaquin Subbasin aquifer system geology consists of Holocene alluvium
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