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SEC ® R SIEC©R o. 6.0 CONCLUSIONS AND RECOMMENDATIONS This Site Conceptual Model was meant to summarize site activities and provide a guide for future assessment and remediation. The following conclusions result from the above collection of data. • Petroleum hydrocarbons detected in soil and groundwater during the removal of USTs, product lines, and dispensers in 1997 and subsequent site investigation resulted from operations of gasoline USTs and associated piping and dispenser at the site. The majority of hydrocarbons in soil have been found in the area of the former underground storage tanks to depths of approximately 25 feet bgs. • Receptor surveys for the site indicated that the nearest wells are approximately � . 700 feet to the east (crossgradient) and 792 feet to the south (upgradient). The nearest downgradient (1,584 ft northwest) well is an irrigation well at Lathrop Grammer School. A well influence study conducted in 2003 indicates that the ' Lathrop Grammar School well did not influence groundwater flow direction or gradient beneath the Circle K site. ' • Monitoring and sampling performed at the site since 1998 indicated the presence of TPHg, benzene, and MtBE dissolved in groundwater beneath the site, with a majority of the dissolved hydrocarbons in the shallow portion of the water bearing wzones. The extent of TPHg, benzene, and MtBE in groundwater are delineated to low or non-detectable levels. • Based on deep soil borings completed in February 2005, vertical delineation of the plume has not been completed. Potential exposure medium include the following. groundwater, soil, and air. Groundwater is typically only encountered during monitoring well gauging and sampling ti. . and well installation. Soil is typically only encountered during well installation or onsite excavation. Despite the presence of asphalt at the site, air (ambient) may be t t encountered via cracks in the asphalt surfaces should the site be redeveloped. Figure ` 11 presents an exposure pathway model, depicting sources responsible for potential t5 exposure and potential pathways of exposure. r� Based on the above information SECOR recommends the following: • The site should continue to be monitored and sampled on a quarterly basis to confirm groundwater concentrations and to evaluate plume stability in the shallow zones. The new wells (MW-17 through MW-19) should be added to the quarterly r monitoring and sampling schedule. Shallow zone isoconcentration maps for 1 TPHg, benzene, and MtBE should be included in future quarterly monitoring and sampling reports. • Deep groundwater monitoring wells should be installed to a depth of £E` approximately 80 feet to delineate the lateral extent of hydrocarbons beneath the Circle K Station. 1205 5CM,doc ) ` AyRa" t�