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Foothill Sanitary Landfill Page 9 of 53 San Joaquin County DPW - Solid Waste Joint Technical Document Revised 6/09/2020 The mean annual Pan A evaporation at the site is about 52.2 inches per year based on monthly average historical data from DWR’s CIMIS Weather Station 70 (Manteca) about 22 miles southwest of the site. Mean monthly evaporation is estimated to exceed mean monthly precipitation in all months of the year, except January, February, and December. Net average annual evaporation at the site is estimated to be about 33.4 inches (2015 WDR). 4.6.5. Run-on and Runoff Volume and Pattern The natural topography of the Foothill Landfill area controls run-on onto the site, with the refuse fill generally at a higher elevation than the surrounding soil. In addition, the active fill area is a significant distance from the property boundary, providing area for drainage features to channel water away from the landfill. To control runoff, a perimeter ditch surrounding the landfill directs storm water into various storm water basins, the largest which is on the east side of LF-1 of the site (Figure 2). Overflow of this basin has not occurred in recent history, to the knowledge of the County. Run-on near the active area is diverted to a sedimentation area located southeast of Module 1 (Figure 2). If this sedimentation area overflows, the water flows overland through grassland to the water pond on landfill property. Water leaving the landfill property along this route is sampled for National Pollution Discharge Elimination System requirements. 4.6.6. Wind Rose The mean annual wind speed is 8.3 mph in a Westerly direction. Monthly predominant wind direction varied between southeast in January, February, and December and west during the remaining months (Figure 10). The Western Regional Climate Center was contacted for historical information. The closest station to the landfill is the Stockton Metropolitan Airport. 4.7. Geology 4.7.1. Regional Geology