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21 June 2019 AdvancedGeo AGE Project No. 19-4745 Environmental Page 5 of 14 `� X ADDRESS: 1155 Gertrude Avenue LOCATION: 0.20 miles southwest of property SEPTIC SYSTEM: Permit date 1978; septic system and leach line installation. 3.6. PROPOSED MAXIMUM SEPTIC TANK USAGE ON SITE The current septic system installed on the property is connected to the small modular office building at the site; there are no other buildings/structures at the property. The septic system is located to the west-southwest of the modular office. The facility is open for 24-hours a day and seven days a week. The modular office will be used by a maximum of six people during a 24-hour period (workers and visitors); three people per 12-hour shift. The office has one sink and one toilet, the average and reasonable usage of three to five gallons per day per person is typical for a worker or visitor to the site setting. 4.0. GROUNDWATER INFORMATION - REGIONAL HYDROGEOLOGY The Geologic Map of California, published in 1966 by the California Department of Conservation Division of Mines and Geology, shows the site area within the Great Valley Syncline (GVS), a large, elongate, northwest trending structural trough. The GVS 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 ranging in age from Jurassic to present day, creating a nearly flat-lying alluvial plain extending from the Tehachapi Mountains in the south to the Klamath Mountains in the north. The western and eastern boundaries of this province are comprised of the California Coast Range and the Sierra Nevada, respectively. Geologically, the area around the property generally consists of Holocene-aged flood-basin deposits of clay, silt, and sand. Miocene to Holocene age-units are located in the surrounding area. These deposits are heterogeneous mix of generally poorly sorted clay, silt, sand, and gravel with some beds of claystone, siltstone, sandstone, and conglomerate (Page, 1986). Most of the fresh groundwater in the Central Valley is contained in the post- Eocene-aged continental rocks and deposits and in the Holocene-aged river deposits consisting of gravel, sand, silt, and minor amounts of clay. In general, these geologic materials comprise a major widespread aquifer extending hundreds of feet deep and occasionally containing individual confining layers. The Modesto, Riverbank, Turlock Lake Formations and overlying recent alluvium are the principal source of domestic groundwater in the 13,500-square mile San Joaquin Valley Groundwater Basin (Basin 5-22). This basin is drained primarily by the San Joaquin River, which is approximately 3 miles west of the property. The nearest surface water features near the property is McLeod Lake and the Mormon Slough, which are located approximately 1.5 miles west of the property.