The Turlock Irrigation District (TID) provides irrigation water to agricultural lands in areas near Turlock, California. The Harding Drain, which is part of the TID system, is a constructed agricultural drain that collects and conveys discharges from several types of sources, including
runoff from agricultural and urban areas and effluent discharges from the Turlock Regional Water Quality Control Facility (RWQCF). The Harding Drain was included on the State of California's impaired waters (Section 303[d]) list for unknown toxicity, ammonia, and organophosphorus
(OP) pesticides, specifically diazinon and chlorpyrifos. The drain discharges to the San Joaquin River, which is also on the 303(d) list for a number of pesticides and metals and unknown toxicity. In 2004, the California State Water Resources Control Board (SWRCB) established the Water
Quality Control Policy for Developing California's Clean Water Act Section 303(d) List (Listing/Delisting Policy) (SWRCB, 2004). The State of California is the first to have developed and implemented policy related to listing and delisting a waterbody under the 303(d) list. The
consequences of a waterbody being included on the 303(d) list include the development and implementation of a total maximum daily load (TMDL) for the listed constituent, which can be a labor intensive and costly effort. To remove a waterbody from the 303(d) list, compliance with the criteria
included in the Listing/Delisting Policy is necessary. This paper describes the process of characterizing current water quality conditions within the Harding Drain and applying the results to delist the drain for ammonia and diazinon. Also included are the lessons learned and advice for
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