Modeling and Evaluation of Reducing the Deer Creek Wastewater Treatment Plant Minimum Discharge Requirement

El Dorado Irrigation District (District) revisited the State Water Resources Control Board (State Water Board) water rights order WRO 95-9 which limited District to discharge a minimum 1.0 million gallons per day (mgd) of treated wastewater from the Deer Creek Treatment Plant (DCWWTP) when influent flows equal or exceed 2.5 mgd. The District would like to divert more of the treated wastewater to use as recycled water for landscape irrigation, thereby preserving upstream watershed diversions and potable water for other uses. RBI performed hydrologic and biological investigations of Deer Creek to determine whether reducing the minimum discharge requirement would adversely affect the riparian corridor, aquatic, or terrestrial life downstream.  Specifically, RBI:

• Conducted an initial feasibility study which included field flow-monitoring and stream gauging, biological and aquatic life surveys/assessment, assessment of flow loss in the creek during the summer period, and estimates of minimum discharge required to maintain surface flow throughout the riparian corridor.
• Developed a Hydrologic Engineering Centers River Analysis System (HEC-RAS) hydraulic model of the reach of interest, and performed calibration and validation studies to ensure the model would meet the project objectives.
• Performed field surveys of the creek to obtain geometric data for the hydraulic model.
• Applied the model under reasonable worst-case upstream flow conditions and typical summer flow losses to a range of potential DCWWTP discharge rates and compared the results to assess potential effects on aquatic, terrestrial, and riparian resources.

Based on the findings from this assessment, it was concluded that the DCWWTP minimum discharge requirement could be reduced to 0.5 MGD under all DCWWTP influent flow conditions without adversely affecting the aquatic, terrestrial, or riparian resources of Deer Creek.