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i1 r i ' Site Conceptual Model LATHROP GAS AND FOOD MART ' 14800 South Highway 99,Manteca,California 1.0. INTRODUCTION Advanced GeoEnvironmental, Inc (AGE) has prepared this Site Conceptual Model (SCM) for ' Lathrop Gas and Food Mart,located at 14800 South Highway 99,Manteca,California(the site) The purposes of the SCM are to 1) identify the source(s)of contamination, 2)determine the nature and the extent of the contamination,3)identify the dominant fate and transport characteristics of the site, ' 4)specify potential exposure pathways, and 5) identify potential receptors that may be impacted by the contamination The site setting is illustrated on Figure 1 Site structures, soil borings, and monitoring well locations are depicted on Figure 2 This SCM has been prepared utilizing existing data collected during previous site subsurface , investigations, and includes a discussion of site background and a summary of previous subsurface ' investigations This model has been prepared in accordance with guidelines issued by the California Regional Water Quality Control Board- Central Valley Region(CVRWQCB)for the investigation of underground storage tank (UST) sites • 2.0, BACKGROUND The site is located at 14800 South Highway 99, Manteca, California, and is situated in an area of mixed commercial and agricultural use The site is bordered on the east by Highway 99,on the west by Highway 99 Frontage Road, on the north by the Lathrop Road off-ramp, and on the south by Lathrop Road Prominent features on the subject property are depicted on Figure 2 1 21 REGIONAL GEOLOGIC/HYDROGEOLOGIC SETTING According to Geologic Map of California, published in 1966 by the California Department of Conservation Division of Mines and Geology, the site area is shown to be within the Great Valley Geomorphic Province(GVGP)of California,a large,elongate,northwest-trending structural trough The GVGP is subdivided into two major divisions, designated the Sacramento and San Joaquin Valleys, which have been filled to the present elevation with thick sequences of sediment derived ' from both marine and continental sources Thickness of the sedimentary fill ranges from thin veneers along the valley edges to more than 20,000 feet in the south central portion of the valley The sedimentary formations vary in age from Jurassic to Recent,with the older deposits being primarily ' marine in origin and the younger deposits continental Continental-derived sediments were typically ` deposited in lacustrine,fluvial,and alluvial environments with sediment sources being the mountain ' ranges surrounding the valley(Olmsted and Davis, 196 1) The site is located on unconsolidated and semi-consolidated alluvium,lake,playa,and terrace deposits of Quaternary age(California Division of Mines and Geology, 1977) Rocks composing the basement complex of the province have not 01' Advanced GeoEnvironmental,Inc 1