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TRACY WW7P-TASK 14 MS MONITORING WELL INSTALlAT10N WORK PIAN There are five existing monitoring wells at the W WTP(Figure 1)ranging in depth between 25 and 40 feet. These wells were installed between 1990 and 1994 to monitor groundwater quality conditions in the vicinity and upgradient of the holding ponds. In addition to the City-owned monitoring wells, the Spreckels Sugar facility(formerly Holly Sugar)located immediately north of the treatment facility and west of the holding ponds has numerous monitoring wells it regularly samples(see Figure 1). Table 1 summarizes the well construction details for the existing City monitoring wells and the Spreckels Sugar monitoring well located along Arbor Avenue. Local Geologic and Hydrologic Conditions The W WTP is located within the Tracy Subbasin of the San Joaquin Groundwater Basin. The subbasin is comprised of continental sand,gravel,and silt/clay deposits of the Tulare Formation,as well as alluvium and flood basin deposits(GE12007). The Corcoran Clay extends into the subbasin to the south and west of the W WTP and so it can be inferred that it is present beneath the plant itself. The Corcoran Clay divides the groundwater system into confined and unconfined aquifers(Figure 2). The focus of this effort is the unconfined aquifer. In the Tracy area,groundwater within the unconfined aquifer Bows to the north(Figure 3). Several surface water features occur near the W WTP,including Tom Paine Slough,Old River,and Sugar Cut. Only Sugar Cut occurs immediately adjacent to the W WTP-the upper reach is near the northern boundary of the holding ponds. Previous water quality and water level measurement studies have not shown a direct connection between the holding ponds and local surface water(CH2M HILL 1995). Facilities Identified for Monitoring by the CVRWQCB The CVRWQCB order identified three treatment,storage,and disposal(TSD) facilities at the W WTP requiring groundwater monitoring. These facilities,shown in Figure 4,are: • Sludge Drying Beds • Biosolids Storage Area • Lined Pretreatment Lagoons Facility summaries are based on information in the Evaluation of the Tracy W WTP in Response to RWQCB Letter Dated June 15,2004(CH2M HILL 2005). Sludge Drying Beds The recent W WTP improvements modified the configuration of the sludge drying beds. The 12 beds(before the plant expansion, there were 16 sludge drying beds)cover about 362,500 square feet and can hold approximately 3.4 million gallons of sludge(solids and water)at depths ranging from 1 to 3 feet. Ten of the 12 sludge drying beds are sand-lined. In the drying beds,water is primarily removed from solids by decanting supernatant and by evaporation. Decanting systems remove water from the surface of the beds and underdrains remove water that percolates into the sand. After decanting is complete,solids are periodically tilled to exposed moist solids and accelerate drying. OCMW 31,2007 2