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City of Tracy Wastewater Treatment Plant Holding Ponds Investigation Introduction The City of Tracy, California (the City) operates a network of holding ponds at the City's wastewater treatment plant that provide treatment for peak industrial wastewater flows during the summer canning season. The location of the ponds is shown on Figure 1. Without the ponds, the capacity of the plant's secondary treatment processes would be overloaded during the canning season, resulting in potential violations of the plant's waste discharge requirements. In 1989, the California Regional Water Quality Control Board (RWQCB) requested that the City install monitoring wells at the holding ponds to evaluate the potential impact of the ponds on total dissolved solids (TDS) concentrations in shallow groundwater. The City initiated a groundwater investigation in 1990 that included the following tasks: • Construct four monitoring wells for sampling. • Gather data on regional hydrogeology. • Canvass wells in the vicinity of the ponds and collect samples from five of these wells. • Collect samples from the ponds themselves. The location of the four monitoring wells is shown on Figure 2. The investigation report, released in June 1991, concluded that the holding ponds did not appear to be affecting TDS concentrations in the local shallow groundwater (CH2M HILL, June 1991). The RWQCB then ordered the City to conduct quarterly monitoring in the ponds and monitoring wells for 1 year. This monitoring included the collection of samples from the ponds and monitoring wells, with analyses being performed for TDS, pH, electrical conductivity(EC), and chlorides. In addition,the monitoring included the collection of water level data in the monitoring wells and in nearby Sugar Cut Slough. The monitoring began in March 1992. Altogether, six quarterly sets of data were collected by the City. The monitoring data appear to confirm that the holding ponds are not negatively affecting the water quality of the shallow groundwater or Sugar Cut Slough. However, some of the data are contradictory. In addition, the RWQCB has expressed continued concern about possible releases from the ponds. Therefore, the City has decided to perform additional investigation to see if pond water is infiltrating and affecting TDS concentrations in the shallow groundwater or Sugar Cut Slough. SAC/SAC36357/041.WP5 1 8/94