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Evaluation of Natural Attenuation: 7500 West Eleventh Street, Tracy, C.A. Page 42 When the source of the contamination has been eliminated (e.g., by the removal of an underground tank), these fluctuations in contaminant concentrations occur relative to a I' f trend line of declining mean concentration with time as the reservoir of fuel hydrocarbons in the smear zone is slowly depleted by desorption, the dissolved phases of the 3 contaminants are moved away from the source area by groundwater flow in the down- gradient direction, and the contaminant mass is reduced by other natural attenuation processes. I As has been discussed in all of SJC's engineering reports for this project, groundwater elevations at the Navarra Site and in the Tracy area generally do not follow a Spring, Summer, Fall, Winter cycle such as prevails in the eastern United States where the tradition of quarterly groundwater-quality monitoring schedules originated. In the Central Valley of California, there are only two significant climatic periods: a wet season that typically occurs in November through March and a dry season. Given these conditions, on first inspection these facts would suggest that groundwater-quality monitoring in the area should be scheduled for the end of the wet season and, if necessary, at the end of the n dry season However, at the 7500 West Eleventh Street site and in the Tracy area in general, as is typical of many regions of the Central Valley, the elevation of groundwater in the shallow aquifer is relatively little influenced by precipitation, but is dominated by agricultural irrigation that uses water imported from the Delta of the Sacramento and San Joaquin Rivers via the California Aqueduct and the Delta-Mendota Canal, which supply is also supplemented by pumping from deep wells. Due to the typical schedules of agricultural cropping in the area, this results in groundwater elevations being at their highest in the middle of the climatic dry season and at their lowest during the wet season. The periodic variations in the concentrations of analytes of concern in the groundwater at -� the Navarra Site can be readily observed in the data gathered by the groundwater- monitoring program over the last five years. The data show that the highest concentrations of analytes in groundwater typically occur in late July of each year, while the lowest concentrations have typically occurred in the wet season. This is consistent with the agricultural irrigation practices that control elevations in the shallow aquifers in 1. the Tracy area. ---� The relationship between groundwater elevations and contaminant concentrations are illustrated on Figure 58, which plots the concentrations of diesel with time in samples recovered from Monitoring Well MW-3 and compares that plot with a plot of the variations in groundwater elevation in that well on the days samples were recovered. The fluctuations in the concentrations of that contaminant in response to the rise and fall of the groundwater table is clearly seen and demonstrates that, at the Navarra Site, the occurrences of the high concentrations of diesel occur at the opposite season to those .� which would be assumed based on weather patterns alone. { Sic