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I r:Groundwater-quality Monitoring—March 29-31,2006, 7500 West Eleventh Street, Tracy, C4 Page 21 Repot q y g . to from the newly-expanded monitorin H statistical confidence m the data g well array immediately rose to values sufficient for the Navarra Site to be characterized reliably. Following the gathering of data by the sixth groundwater monitoring round, confidence in the sufficiency of the data to support conclusions regarding the characteristics, geometry of the plume and to predict future trends in analyte concentrations increased rapidly because data was then available from an adequate array of groundwater-quality p Y monitoring wells that extended the whole length of the plume of affected groundwater. By the time of the sampling round conducted on July 26, 2004, the coefficient of confidence had risen to well over 90%. Such high coefficient of confidence indicates that, by that time, the groundwater-quality database was sufficient to support robust conclusions regarding the characteristics of the plume of affected groundwater and subsequent rounds of monitoring were not cost-effective with respect to improving the quality of the site characterization program conducted at the site. 5.5 Optimal Monitoring Dates It has previously been established that, at the 7500 West Eleventh Street Site, fluctuations j in the depth to groundwater in the shallow aquifer are controlled by seasonal variations in agricultural irrigation in the region (The San Joaquin Company 2006d). This results in the highest elevations of the water table occurring during the climatic dry season, which typically extends from March through October in the area, while the water table is at its lowest elevation during the wet season, which usually extends from October through March. S As the water table rises and falls, groundwater is exposed to increasing and decreasing thicknesses of soil affected by components of fuel hydrocarbons beneath the Navarra Site. Contaminants adhered to soil particles and present in the interstices between the particles in this "smear zone" are slowly desorbed from the soil and pass into solution in the groundwater. When groundwater is high, the total volume of affected soil in contact with groundwater is large, so that concentrations of dissolved components of hydrocarbons reach their high periodic levels. Conversely, when groundwater levels are low, the volume of soil that serves as a source of contamination to groundwater is at a minimum and concentrations fall. i he concentrations of analytes of concern in the groundwater at The periodic variations n t c yt the Navarra Site and their relationship to groundwater table elevation can be readily observed in the data gathered over the past six years that is documented in Tables 1 and 2. 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 the Tracy area. The relationship between groundwater elevations and contaminant concentrations are illustrated on Figure 14, 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 sic