Res 2016-137/approving a Water Master Plan to guide the future maintenance and extension of the City’s Water Infrastructure

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Alan Plummer Associates, Inc. Water Master Plan Update 2016 7 A water age evaluation was completed for the 2013, 2020, 2025, and 2035 average day scenarios. The hydraulic model can estimate water age for any node in the system by running an extended period simulation and determining the age of water based on travel time and model demands. This analysis is highly dependent upon system demand and the operational controls, of which there are many combinations. For each year, the average day model was run for a simulation period of 300 hours. This is a long enough period to determine water age at all points in the system. In general, water age increases with distance from the fresh water source (the SWTP in this case). However, input from the active wells in the system will reduce water age by adding other fresh water inputs within local areas. The addition of major developments further from the source of most of the water in the distribution system (SWTP) will generate higher water ages than previously seen within the system. Since the City has already experienced high total trihalomethane (TTHM) and haloacetic acid (HAA) concentrations on occasion, service to these new areas comes with a concern for disinfection by-product related water quality related issues. The following sections describe the results of the water age analysis for each year modeled and identify any water quality concerns. Appendix E contains figures depicting water age for each scenario. Dead end lines with zero demand nodes are not included in these figures because they will show a high water age regardless of the surrounding area. These are typically locations where flushing is needed to maintain chlorine residuals in the system. 7.1 EXISTING SYSTEM Water age in the existing system varies from low water age (< 72 hours) in the central portion of the main SWTP and Upper Pressure Planes to medium water age (72 - 120 hours) as the system continues further out (Figure 7-1). Facilities at the end of long dead-end lines show the highest water age in the system. These areas of concern include the Ranch Road 12 EST and the dead-end water main serving the airport area (water age > 250 hours for both). Also, areas with a higher than average water age include the Cottonwood EST (about 150 - 200 hours) and the Blanco Vista Development (120 200 hours) in far northeast San Marcos. 7-1 m:\projects\0600\022-01\doc\report\draft\draftmasterplan-v15_hef.docx