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<br /> <br />Foothill Sanitary Landfill Page 9 of 52 San Joaquin County DPW - Solid Waste <br />Joint Technical Document Revised 01/05/2026 <br /> <br />The average 24-hour precipitation event is about 1.6 inches, and the 100-year, 24-hour <br />precipitation event for this station is about 3.3 inches (2015 WDR). <br /> <br />4.6.4. Evaporation <br />The mean annual Pan A evaporation at the site is about 52.2 inches per year based on <br />monthly average historical data from DWR’s CIMIS Weather Station 70 (Manteca) about 22 <br />miles southwest of the site. Mean monthly evaporation is estimated to exceed mean monthly <br />precipitation in all months of the year, except January, February, and December. Net average <br />annual evaporation at the site is estimated to be about 33.4 inches (2015 WDR). <br /> <br />4.6.5. Run-on and Runoff Volume and Pattern <br />The natural topography of the Foothill Landfill area controls run-on onto the site, with the <br />refuse fill generally at a higher elevation than the surrounding soil. In addition, the active fill <br />area is a significant distance from the property boundary, providing area for drainage features to <br />channel water away from the landfill. <br /> <br />To control runoff, a perimeter ditch surrounding the landfill directs storm water into <br />various storm water basins, the largest which is on the east side of LF-1 of the site (Figure 2). <br />Overflow of this basin has not occurred in recent history, to the knowledge of the County. <br /> <br />Run-on near the active area is diverted to a sedimentation area located southeast of <br />Module 1 (Figure 2). If this sedimentation area overflows, the water flows overland through <br />grassland to the water pond on landfill property. Water leaving the landfill property along this <br />route is sampled for National Pollution Discharge Elimination System requirements. <br /> <br />4.6.6. Wind Rose <br />The mean annual wind speed is 8.3 mph in a Westerly direction. Monthly predominant <br />wind direction varied between southeast in January, February, and December and west during <br />the remaining months (Figure 10). The Western Regional Climate Center was contacted for <br />historical information. The closest station to the landfill is the Stockton Metropolitan Airport. <br /> <br />4.7. Geology