ARCHIVED REPORTS_XR0005243 - Laserfiche WebLink

Home Browse Search

i ` Former Beacon Station No. 12474 March 18, 2002 Stockton California Problem Assessment Report/Corrective Action Plan L u The highest gasoline constituent concentrations in soil samples were collected from the vapor well VW-11213 and MW-12 borings located near the north end of the eastern dispenser and occur predominantly at depths of 85 feet or less. Relatively low gasoline constituent concentrations in soil samples from monitoring well MW-6, vapor wells VW-4/516, and air sparge well AS-1 borings indicate proximity to the margin of gasoline constituent soil impacts. The lateral extent of gasoline constituent concentrations in soil is delineated by monitoring wells MW-8, MW-10 and MW-11. The vertical extent of gasoline constituents in soil in the dispenser island area is defined by samples collected from the vapor well VW-7 and air sparge well AS-1 borings. Proximity to the vertical extent of gasoline constituents in the monitoring well MW-12 boring is suggested by the decreasing and near-reporting limit constituent concentrations with depth from approximately 92 to 112 feet bsg. 5.2 Distribution of Petroleum-Impacted Groundwater The quarterly monitoring data (Appendix.A) through first quarter 2002 indicate the highest concentrations of-gasoline constituents in groundwater have been regularly detected in monitoring wells MW-2, MW6 and MW-12, and vapor wells VW-1 and VW-4. Quarterly groundwater monitoring analytical data from first quarter 2002 indicate concentrations of TPHg ranging up to 230,000 micrograms per liter (parts per billion or ppb), and benzene ranging up to 29,000 ppb. The lateral extent of TPHg in groundwater, as shown on Figure 7, is delineated to the north and east by monitoring wells MW-3, MW-4, MW-5 and MW-7 and extends west and south of the dispenser island area to monitoring wells MW-1, MW-9 and MW-10. Concentrations of TPHg in groundwater decrease by greater than two orders of magnitude from the dispenser island area to monitoring wells MW-1, MW-9 and MW-10 suggesting reportable TPHg concentrations in groundwater extend only a limited distance westward and southward as shown on Figure 7. The lateral extent of groundwater benzene concentrations, as shown 474PARCAP3-02.doc 7 Project No. 1474.23 HORIZON ENVIRONMENTAL INC.