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