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Evaluation of Natural Attenuation_ 7500 West Eleventh Street, Tracy, CA. Page 44 8.2.2.1 MAROS Input Data The MAROS analysis requires the locations of all wells in the monitoring well field to be defined by coordinates. Because it is situated adjacent to the northern boundary of the pump island of the former fueling station and on the axis of the principal plume of contaminated groundwater, SJC set the origins of the coordinates at Monitoring Well 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 F" Well MW-3 and are negative to the south of that point. The coordinates of the wells as set by that system are presented in the first table in Appendix A. The well locations in relation to the coordinate system are diagrammed on the Well Locations diagram that is r also included in that Appendix. `a The MAROS data input distinguishes between groundwater-monitoring wells that are at the "source" (5) 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 form the source area). SJC assigned Monitoring Wells MW-I, j MW-2, MW-3 and MW-4 to be in the source area and the rest of the wells in the array to G be in the tail area uired for the MAROS evaluation is closely similar to the The geochemical data input req historical groundwater chemistry data presented in Table 2. However, the MAROS software also permits analytical results taken from samples recovered at different times but that were separated by only a few days to be treated as though they were obtained from samples recovered on a single date. This permits data from a sampling round that required more than one day to complete to be treated analytically as a single event. In I; addition, for the purpose of the MAROS analysis, SJC opted for the concentrations of analytes recorded by the laboratory as "not detected" to be set at 50% of the method detection limit (NML). The "Reduced Data Table"presented in Appendix A presents the geochemical data as it was adapted for the MAROS analysis. The MAROS analysis also requires definition of the porosity of the aquifer, the saturated thickness of the aquifer, the direction of groundwater flow and the rate of groundwater flow. 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). As can be derived from Figure 3, the direction of groundwater flow, as defined by the long axis of the principal contaminant plume, is k--- North 25° East. As is shown on Figures 4 and 5, the mean saturated thickness of the contaminated aquifer is approximately 10 ft. 8.2.2.2 Plume Stability To evaluate plume stability, the MAROS protocol applies regression techniques and the more sophisticated Mann-Kendall analyses to the time/analyte concentrations data for each monitoring well in the well field. The MAROS software also evaluates plume �J SJC