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D R A F T January 4, 2012 agricultural well pumping, but that effect tends to be localized due to the continual groundwater recharge by the Delta's surface water channels. The land surface elevations on the City's property range from about 0 to 5 feet above mean sea level near the western edge of the property to about 10 feet above mean sea level near the eastern edge. Groundwater underlying the Facility is very shallow, ranging from the ground surface near the western edge of the property to more than 20 feet below mean sea level near the eastern edge of the property. Groundwater elevations in the immediate vicinity of the treatment plant change little throughout the year, ranging from 1 to 2 feet below mean sea level in the spring to about 2 to 4 feet below mean sea level in the fall. Evidence in the record indicates that there is a persistent, slight groundwater mound underlying the Facility, which influences the groundwater gradient and flow direction within the City's property.31 While groundwater from the mound does flow to the east, in response to the regional gradient, the mound also appears to induce flow away from the facility to the south and west. City property located immediately north of the Facility exhibits a northeasterly flow; while property located immediately southeast exhibits a seasonal shift from southeasterly in the winter months to northeasterly/easterly in the summer months.32 Property located further to the east from the mound shows a dominant easterly flow throughout the year. At the southwest corner of the City's property, the mound apparently induces flow away from the Facility to the south and west. The most likely cause of the groundwater mound is a continued discharge to groundwater in the area of the Facility. The unlined storage ponds at the Facility are the most likely source for the flow causing the groundwater mound. The onsite storage ponds are extensive, and they appear to provide the only large quantity, constant source of water available for creating and maintaining the mound. The four storage ponds have depths ranging from 7 to 9 feet, and they hold approximately 97 million gallons when full. Evidence in the record indicates that there is significant leakage from the ponds. The average annual percolation rate from the 49-acre pond area is estimated to be approximately 0.3 inches per day, totaling about 109 million gallons of wastewater per year lost to percolation.33 This equates to nearly 10 percent of the average annual flow to the agricultural 31 See, e.g., City of Lodi White Slough Water Pollution Control Facility Groundwater Monitoring Status Report(June 2003),AR, vol. 12, item 412. 32 Id. at 5. 33 City of Lodi White Slough WPCF Soil and Groundwater Investigation Existing Conditions Report, Final Report (Sept. 2006) (Final Report) at pp. 2-18.AR,vol. 13, item 415. 13.