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Report:Groundwater-quality Monitoring—March 29-31,2006, 7500 West Eleventh Street, Tracy,CA Page 17 i MW-3. X coordinates are positive eastward from that location and negative westward from that well. Y coordinates increase positively with distance north from Monitoring Well MW-3 and are negative to the south of that point. The MAROS data input distinguishes between groundwater-monitoring wells that are at the "source" (S) of the fuel hydrocarbon release (i.e., at, up-gradient from, or co-gradient to the point at which the fuel was released) and those that are in the "tail" (T) of the plume (i.e., down-gradient from the source area). SJC assigned Monitoring Wells MW-1, MW-2, MW-3 and MW-4 to be in the source area and the rest of the wells in the array to be in the tail area + The MAROS analysis also requires definition of the porosity of the aquifer, the saturated thickness of the aquifer, the rate and direction of groundwater flow, the current and maximum width and length of the plume of affected groundwater and the current and closest approach of the down-gradient edge of the plume to any receptor of concern. ` Based on work previously completed for the Navarra site, the porosity of the aquifer was set at 0.3, the down-gradient velocity of groundwater flow at 77 ft per year (The San Joaquin Company Inc. 2002c). The current length of the primary plume of affected groundwater as measured down-gradient from Monitoring Well MW-3 is approximately 750 ft. and the current width is approximately 360 ft. Although none is actually present t— within the neighborhood of the down-gradient limit of the plume, it was assumed, solely for the purpose of the MAROS analysis, that such a receptor is present at a distance of -� 1,000 ft. down-gradient from Monitoring Well MW-3. As can be derived from Figure 3, L the direction of groundwater flow, as defined by the long axis of the principal contaminant plume, is North, 25° East. As is shown on Figures 4 and 5, the mean saturated thickness of the contaminated aquifer is approximately 10 ft. 5.2 Plume Stability �. To evaluate plume stability, the MAROS protocol includes regression techniques and the more sophisticated Mann-Kendall analyses to the time/analyte concentrations data for each monitoring well in the well field (Gilbert 1987). The Mann-Kendall test is a statistical procedure that is well-suited for analyzing trends in data over time. It is a non-parametric test for the zero slope of the first-order regression of *- time-ordered concentration data vs. time. It does not require any assumptions as to the _ statistical distribution of the data(e.g., normal, lognormal, etc.) and can be used with data sets that include irregular sampling intervals and missing data. The MAROS software applies a very conservative decision-maker for characterizing the results of the Mann- Kendall test. For example, for the trend of an analyte in a given well to be classified as "increasing" or"decreasing," the slope of the trend must be greater than zero or less than zero, with a confidence of greater than 95%. Trends are classified as "probably increasing" or"probably decreasing" if the slope of a trend line can be determined with a confidence of 90-95%. In cases where the trend line cannot be computed with a confidence of 90% or more, the data is classified as having"no trend." sic