ARCHIVED REPORTS_DRAFT ENVIRONMENTAL IMPACT REPORT

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discharges from wetlands and publicly owned wastewater treatment facilities. Diversions can remove a significant amount of San Joaquin River flow, especially during periods of below normal rainfall. The Lower San Joaquin River is not typically affected by tidal flows,being sufficiently upstream of the Pacific Ocean's tidal influence. The second main section of the Lower San Joaquin River is the tidally influenced reach from Vernalis to its confluence with the Sacramento River near Collinsville. Major tributary inputs to this section are provided by the Cosumnes and Mokelumne rivers where their commingled flows enter the central Delta near Webb Tract. Significant nontributary inflows are provided from irrigation return flows that are pumped from adjacent agricultural lands into the San Joaquin River. The major diversion of San Joaquin River water occurs at the junction of Old River,where,depending on Delta hydraulics,up to 50%of San Joaquin flows may be diverted to the south Delta(Quinn and Tulloch 2002). An understanding of the hydrology and hydrochemistry of the San Joaquin River can be gained through a review of the relative flow contributions made by eastside tributaries,westside tributaries, and other inflows. The major eastside tributaries include the Merced,Tuolomne, and Stanislaus rivers which join the Lower San Joaquin River upstream of Vernalis. There are also three minor eastside tributaries in the Basin, including Bear Creek,the eastside Bypass, and the Mariposa Bypass. There are nine significant streams and conveyances that drain the westside of the San Joaquin Basin and are tributary to the San Joaquin River. These tributaries include Panoche- Silver Creek, San Luis Drain, Salt Slough,Mud Slough, Spanish Grant Drain, Orestimba Creek,Hospital Creek, Ingram Creek, and Del Puerto Creek. Many of these streams are ephemeral, conveying rainfall runoff during the winter season and agricultural return flows during the summer months. The San Luis Drain is a concrete-lined conveyance that once formed part of a Valley Master Drain system providing drainage for the entire westside of the Basin. The Drain presently serves five agricultural water districts and conveys subsurface drainage water into Mud Slough, six miles upstream of the confluence with the San Joaquin River. Due to the Drain's importance to the hydrology of the San Joaquin River, it is considered a westside tributary(Quinn and Tulloch 2002). A comparison of flow and TDS (salts)inputs into the San Joaquin River by a variety of sources is provided in Table 4.9-1. During the period 1985-1994, annual average flows from all eastside tributaries collectively accounted for 70%of the flow measured in the San Joaquin River at Vernalis. In contrast,westside tributaries contributed 4%of the flow measured at Vernalis. While eastside tributaries contributed approximately 17.5 times the flow of westside tributaries,the estimated TDS loading of the eastside tributaries was about three quarters of that provided by westside tributaries. This difference in TDS loading can be attributed to the fact that snow-melt comprises a large proportion of eastside flow volume,while return flows from agriculture and wetlands dominate westside hydrology. Tributary contributions not withstanding,all other sources collectively contributed 26% of total flow and 63%of estimated TDS loading in the San Joaquin River at Vernalis. The largest nontributary TDS loadings were made by groundwater(191,000 tons) and surface water return flows(150,000 tons). Municipal and industrial sources contributed minor discharge volumes (1%) and TDS loads (2-3%), similar in magnitude to contributions from subsurface return flows (Quinn and Tulloch 2002). Surface Hydrology in the Project Area The WQCF site and proposed pipeline alignments are located in the Central Valley within the central portion of San Joaquin County in the westernmost portion of City. Existing ground slopes in the area are relatively flat,with an elevation of approximately 25 feet above mean sea level. Average annual rainfall within the City is approximately 14 inches,with most of this rain occurring between November and March(Climate Zone 2006). Major drainages in the project vicinity include Lone Tree Creek approximately 6 miles to the north,the Stanislaus River approximately 7 miles to the south,and the San Joaquin River approximately 2 miles west of the site. The nearest significant body of water is the San Joaquin River. Two agricultural ditches traverse the WQCF site in the western and southern portions of the project site and both are nonjurisdictional features under both state and federal regulations. The southwestern part of the proposed project area,including the WQCF site, is located in the 500-year floodplain. . Portions of the wastewater collection system,recycled water disposal system, and effluent DER EDAW City of Manteca 4.9-3 Hydrology and Water Quality