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---------------- <br /> Evaluation of Natural Attenuation: 7500 West Eleventh Street, Tracy, CA. Page 24 <br /> r�r� <br /> s- 7.0 NATURAL ATTENUATION OF CONTAMINANTS IN GROUNDWATER <br /> = ' Evidence of natural attenuation in groundwater at sites affected by releases of fuel <br /> hydrocarbons can be demonstrated based on primary lines of evidence that include <br /> attenuation of concentrations of analytes of concern with time and distance from the <br /> source. However, in cases where the plume of affected groundwater is stable, natural <br /> attenuation may be in progress despite limited evidence for spatial and temporal <br /> attenuation of measured concentrations of analytes of concern. In such cases, proof of <br /> natural attenuation can be found through examination of geochemical indicators that <br /> `7 evidence intrinsic remediation due to biological processes. <br /> Following are evaluations of the primary and secondary evidence for ongoing natural <br /> attenuation of analytes of concern at the Navarra Site. <br /> f 7.1 Primary Evidence of Natural Attenuation <br /> The principal line of primary evidence of natural attenuation is based on evaluation of the <br /> changes in the concentrations of analytes of concern with time and with distance down- <br /> gradient from the source of the release of the contaminants. <br /> 7.1.1 Attenuation of Analytes. of Concern with Distance from the Source <br /> To analyze variations in concentrations with distance from the source, it is necessary to <br /> establish a point of origin for a set of coordinates that permit the locations of wells, at <br /> .:..,, :✓ <br /> which the concentrations have been measured, to be defined. For this purpose, SJC <br /> established a point of origin at a location five feet to the south of Monitoring Well MW- <br /> 3. As can be seen on Figure 2, this location is on the northern edge of the former pump <br /> - island, where the preponderance of leakage of fuel in the subsurface occurred at the <br /> fueling station that was located on the Navarra site. This selection of a point of origin for <br /> a coordinate system makes the location of Monitoring Well MW-3 to be 5 feet in the <br /> - northerly, down-gradient direction, which direction will be nominally designated the "y" <br /> direction, and at 0 feet in the approximately east-west or "x" direction, i.e., parallel to <br /> West Eleventh Street. Table 6 lists the "x" and "y" coordinates established according to <br /> that system for each of the monitoring wells that are involved in the evaluation of the <br /> distribution of concentrations of analytes of concern with distance from the source. <br /> Figure 10 is a plot of the concentrations of TPHg recovered in March 2005 from wells <br /> MW-1, MW-3, MW-13, MW-7, MWFP-1, MWFP-4, MW-14, MW-16, MW-18, MW- <br /> 11, and MW-19 with distance down-gradient. Those wells are all located on or close to <br /> the axis of the primary plume, so that the graph is essentially a plot of TPHg <br /> concentrations with distance down-gradient along that axis. Figures 11, 12, and 13 show <br /> f ' the same distribution of concentrations with distance for TPHd, BTEX (i.e., the sum of <br /> the concentrations of benzene, toluene, ethyl benzene, and total xylene isomers), and <br /> MTBE,respectively. <br /> i <br /> i <br /> ' <br /> sic <br />