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ARCHIVED REPORTS XR0011755
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ARCHIVED REPORTS XR0011755
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Entry Properties
Last modified
11/19/2024 10:19:08 AM
Creation date
9/4/2019 11:10:25 AM
Metadata
Fields
Template:
EHD - Public
ProgramCode
3500 - Local Oversight Program
File Section
ARCHIVED REPORTS
FileName_PostFix
XR0011755
RECORD_ID
PR0544801
PE
3528
FACILITY_ID
FA0003210
FACILITY_NAME
TEXACO TRUCK STOP
STREET_NUMBER
7500
Direction
W
STREET_NAME
ELEVENTH
STREET_TYPE
ST
City
TRACY
Zip
95378
APN
25015018
CURRENT_STATUS
02
SITE_LOCATION
7500 W ELEVENTH ST
P_LOCATION
03
P_DISTRICT
005
QC Status
Approved
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EHD - Public
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Evaluation of Natural Attenuation_ 7500 West Eleventh Street,Tracy, CA. Page 26 <br /> concentrations of TPHg, TPHd, BTEX, and MTBE against distance down-gradient. As is <br /> shown on those Figures, the rapid reduction of concentrations of the analytes with <br /> =' distance down-gradient from well MW-7 results in a highly non-linear curve, even when <br /> i plotted in the logarithmic format. However, all the slopes of the linear regression line <br /> remain as would be expected - negative - and the RZ values are greatly improved <br /> I compared to those computed for the simple plots of analyte concentrations versus <br /> distance. <br /> The distributions of concentrations of analytes of concern with distance down-gradient <br /> ?- that are formally detailed above can be easily seen by an experienced engineer by simple <br /> inspection of the data in Table 2 without benefit of sophisticated log-normal plots. Such <br /> simple observations and are clear primary evidence of the progress of natural attenuation <br /> at the site. <br /> Although the relationship between concentrations of analytes of concern and distance in <br /> the orthogonal direction to the axis of the plume is not direct evidence of natural <br /> attenuation, to complete the discussion, Figures 18 and 19 are presented to show the <br /> distribution of BTEX concentrations versus distance along sections B-B' and C-C" (see <br /> Figure 3 for locations). As can be seen in those Figures, the concentrations of the analytes <br /> are at a peak where the axis of the plume crosses the sectional lines and fall away to east <br /> and west as the plume perimeter is approached. <br /> 7.1.2 Attenuation of Analytes of Concern with Time <br /> In addition to demonstration that concentrations of analytes of concern are reduced with <br /> distance from the source of the release, the evidence that those concentrations have a <br /> downward trend with time is also of primary importance for demonstrating that natural <br /> attenuation is in progress in the source area of a site. <br /> Figures 20-23 arep lots of the natural logarithm of, respectively, TPHg, TPHd, BTEX, <br /> and MTBE in groundwater with time at monitoring well MW-3. As is shown on Figure 2, <br /> Monitoring Well MW-3 is a few feet down-gradient from the pump island area of the <br /> former fueling station that was located on the 7500 West Eleventh Street site. That area <br /> was the principal location at which fuels were leaked into the subsurface. Groundwater in <br /> L- that well is representative of the conditions at the source of the plume of contaminants <br /> -a <br /> emanating from the site. <br /> Examination of Figures 20-23 reveals that while there is a strongly declining trend in the <br /> concentrations of analytes of concern in groundwater at Monitoring Well MW-3, there is <br /> considerable fluctuation of the concentrations around the trend line. As will be discussed <br /> in greater detail in section 8.1, those fluctuations occur due to the rise and fall of the <br /> groundwater table through the zone of soil which was most severely affected by the <br /> release of the fuels. That zone, known as the "smear zone," was generated as the <br /> groundwater table rose and fell and fuel hydrocarbons were adsorbed onto the soil <br /> articles dining the period following the release of fuels to the subsurface. However, as is <br /> p g p g <br /> demonstrated by the linear regression lines shown on the Figures, those fluctuations are <br /> sic <br />
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