ARCHIVED REPORTS_XR0013078 - Laserfiche WebLink

Home Browse Search

15 June 2009 �..r AGE-NC Project No.05-1306 Page 9 of 33 4.2. GROUND WATER FLOW DIRECTION AND GRADIENT The regional ground water flow is generally southwest, however, the local ground water flow direction appears to be more variable. Between May 2005 and March 2009, ground water flow direction was typically inferred to be towards the northeast and southwest.During several monitoring events divergent gradients are observed on site. The observed variability is likely caused by tidal influence and/or surface water elevation fluctuations in the Port of Stockton's Deep Water Channel, located approximately 650 feet north of the site. Additionally, the divergent gradient is likely attributed to large-grained backfill material used in the September 2005 excavation. It is believed the backfill material, which is more permeable than the surrounding native soil, created an area of low pressure observable as a slight mounding in the ground water table. Ground water monitoring data is summarized in Table 5. Ground water flow direction and gradient are included in Figure 12. Historic ground water flow directions and gradients are illustrated in Appendix H. ti 4.3. DISSOLVED CONCENTRATIONS 4 Between August 1998 and March 2009, thirty six ground water monitoring events have been performed at the site to monitor petroleum hydrocarbon concentrations in ground water; based on 'r the results of the August 1998 to March 2009 quarterly ground water samples, TPH-d has been established as the contaminant of concern (Table 5). `' Ground water samples collected from monitoring wells installed within the affected soil area(MW-2 through MW-4 and MW-7)have consistently been reported by analytical laboratories to be impacted by TPH-d. The greatest concentrations of dissolved TPH-d were reported in the area of wells MW-2/MW-2R,between August 2004 and just prior to the removal and abandonment ofwell MW-2 in August 2005. Thereafter,TPH-d was reported only once in ground water samples collected from &W replacement well MW-2R during November 2005. Additionally, TPH-d has been reported in wells MW-1, MW-5, MW-6, and MW-11. However, between 1998 and 2009, TPH-d concentrations reported in wells MW-3, MW-4 and MW-7 have shown an average decline of approximately 43 percent, 56 percent, and 31 percent, respectively. Furthermore, well MW-1, located approximately 63 feet northeast of the former tank basin, had a LW maximum reported TPH-d concentration of 79µg/l.Wells MW-5 and MW-6,located approximately 81 feet southwest and 108 feet southeast of the former tank basin,had a maximum reported TPH-d concentration of 170 µg11.Well MW-11 located approximately 72 feet northeast of the former tank +- basin, had one reported TPH-d concentration on 86 µg11. Advanced GcoEnvironmental,Inc. i.r