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• 2.1 PHYSIOGRAPHIC AND CLIMATIC SETTING 2.1.1 Landfill Setting The CHSL is situated southeast of the intersection of Corral Hollow Road and State Route 580, and occupies a portion of Section 18 in Township 3 South, Range 5 East of the Mount Diablo Baseline and Meridian system. Physiographically, the site is located on the northeastern flank of the Diablo Range at the transition to the San Joaquin Valley. The site is bounded by Corral Hollow Creek on its eastern side and Corral Hollow Road on its western side. The ground surface generally slopes to the northeast, with ground surface elevations ranging from approximately 240 feet above mean sea level (amsl) at the northeastern corner of the site to approximately 330 feet amsl at the southwestern corner of the site. The landfill waste prism forms a roughly rectangular pyramid that rises from the adjacent terrain to a maximum top deck elevation of approximately 352 feet amsl. 2.1.2 Climate The climate for the region surrounding the CHSL is Mediterranean, characterized by mild winters and moderate summers. According to the California Department of Resources, California Irrigation Management Information System (CIMIS), during the last 10 years, temperatures range from a minimum average of 45.6 degrees Fahrenheit to a maximum average of 71.6 degrees in the nearby city of Tracy. The area surrounding the CHSL receives an average of about 10 inches of rain per year (CIMIS, 2011), with approximately 70 percent of rainfall occurring between November and April of each year. 2.1.3 Landfill Configuration and Improvements The CHSL is a closed, unlined Class III landfill. The landfill property is approximately 58 acres, with a refuse footprint that is approximately 44 acres (including recently discovered refuse outside the closure cap). In 1995 the refuse known at that time was closed with a prescriptive final cover: a foundation layer, a low-permeability clay layer, and an overlying vegetative soil layer. In 2001, a landfill gas extraction system was installed to mitigate landfill gas migration. The original landfill gas extraction system included 39 gas extraction wells connected by header lines to a flare at the southern end of the site. In 2008, five Engineering Feasibility Study Geo-Logic Page ■ 3 Corral Hollow Sanitary Landfill ASSOCIATES January 25,2013