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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 EI 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 9'4 ecember Cjd-D
<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 , e1 o 7P*-A
<br /> (TPH-G) in site soil and groundwater is the primary constituent of concern (COC) . However, the d `7Prdmc
<br /> fuel oxygenate Di-isopropyl Ether (DIPE) was also detected in low-high concentrations in one
<br /> CPT boring . ` !} Iso I/ z� _
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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 /> wellp, and MW-4 , and first quarter 2008 monitoring values were reported at 7 . 79 and
<br /> 1 , 6,A 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 constitu 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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