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Former Beacon Station No 12474 June 7,2005 <br /> Stockton California Results of Additional Groundwater Assessment <br /> . induce a subordinate lateral component to constituent migration Certain natural attenuation <br /> processes in the soil and groundwater environments, including bio-degradation and other <br /> processes, tend to decrease the concentrations of hydrocarbons at the margins of the plume <br /> Because of the gentle groundwater gradients detected at the site and the tendency for <br /> dissolved hydrocarbons to lag behind the migration of groundwater, the majority of the <br /> TPHg and benzene groundwater impact remains limited to the site as shown on Figures 7 <br /> and 8 Interpretation of groundwater monitoring data indicates the down-gradient extent of <br /> dissolved TPHg may extend to the southern property boundaries as shown on Figure 7 The <br /> geometry of the groundwater TPHg plume indicates a predominant tendency for migration <br /> toward monitoring well MW-7 Coarse-grained sediments between 50 and 80 feet bsg, <br /> extending from the source area toward monitoring well MW-7, may have provided the <br /> preferential migration pathways for impacted groundwater <br /> Transport Mechanisms and Exposure Pathways to Potential Receptors <br /> The subsurface soil secondary source is isolated under paved surfaces and is limited laterally <br /> to the dispenser area Underground utility trenches do not traverse the impacted soil Wind <br /> erosion of surface soils, storm water, surface water and atmospheric dispersion are not <br /> significant transport mechanisms at this site Volatilization and enclosed space <br /> accumulation from subsurface soil and groundwater through paved surfaces, leaching and <br /> groundwater transport are potential transport mechanisms at this site No non-aqueous phase <br /> liquids (free product) are currently present in the monitoring wells <br /> In general, there is no potential of impacted soils at the site coming into direct contact with <br /> humans except in the case of construction workers excavating in the vicinity of the eastern <br /> dispenser island In this case, the dermal contact or ingestion exposure pathways have the <br /> potential to be completed During excavation work in the vicinity of the eastern dispenser <br /> island, these exposure pathways can be mitigated by the use of personal protective <br /> equipment and proper industrial hygiene practices <br /> Addtn1GWassmnt <br /> Project No 1474 15 10 HORIZON ENVIRONMENTAL INC <br />