ARCHIVED REPORTS_XR0004413 - Laserfiche WebLink

Home Browse Search

in VM-4 In December 1997, the water level was 4 to 5 feet over screen in VM-2 and VM-3, and 6 to 7 feet over screen in MW-1 4.2 Groundwater Gradient Figure 3 illustrates the general direction of groundwater flow Presently, flow is to the east- northeast The gradient decreases from 0 0055 ft/ft in the northern part of the site to 0 0022 ft/ft in the southern part of the site The steeper gradient near VM-4 is probably due to the low- permeability clay that is present in the northern part of the site This clay may confine groundwater to a slightly deeper depth by restricting flow from the west 4.3 Analytical Results Despite the fact that the static water level is considerably above the screened interval in three of the four wells, there has been no decrease in hydrocarbon concentrations in any of the wells In fact, concentrations in VM-1 have risen slightly, concentrations in VM-2 have risen significantly, and concentrations in VM-3 have risen above detection limits for the first time (Table 2) These increases are almost certainly caused by groundwater rising into the contaminated vadose zone and dissolving residual gasoline Gasoline and benzene concentrations are contoured in Figures 4 and 5 The maps confirm previous speculation that the dispenser was the source of the unauthorized release They also indicate that contamination has migrated to the cast and southeast since water samples were last collected. in December 1997, implying that the plume has enlarged Normally, as contaminants spread out, they become diluted and concentrations decrease In this case, however, concentrations have increased, which indicates that soil in the vicinity of the former dispenser island is still a source for further groundwater contamination 5.0 CONCLUSIONS AND RECOMMENDATIONS As anticipated, the depth to groundwater has continued to decrease from its levels in 1997, but the water samples collected in August yielded unexpected results Despite the fact that the static water level is more than 10 feet above the screen in most wells, hydrocarbons continue to be detected at increasing concentrations in all wells These results imply that not only is the plume spreading, the overall mass of contaminants in groundwater is continuing to increase For this to occur, there must be a continuing source of gasoline to provide hydrocarbons to groundwater Presumably, this source is the contaminated soil that exists in the depth interval 10-35 feet in the northwest corner of the property, in the area around VM-2 and HP-4 (Figure 2) Additional information on the extent of soil contamination can be found in the April 1998 Soil Assessment Report prepared by Upgradient At this time, most of the information that is required for the preparation of a Problem Assessment Report as defined in Appendix A of the Tri-Regional Board Recommendations for Preliminary 4