To bolster freshwater supplies, water managers are increasingly interested in recharging groundwater using storm water and recycled water. However, such multisupply groundwater recharge projects are hindered by the lack of planning tools to evaluate system design costs and trade‐offs. This study presents modeling advancements that provide enhanced insights into multisupply spreading basin systems (i.e., spreading basins that accommodate both advanced treated recycled water and dynamically available storm water), a form of managed aquifer recharge. The model identifies system designs that optimize infrastructure life cycle cost and water volumes infiltrated for groundwater recharge. To illustrate the model's application under realistic conditions, we present a case study of Los Angeles, California. In this case study, we find that competition between storm water and recycled water for spreading basin use is relatively minor. Moreover, compared to systems based on existing conservative assumptions, our methods identify optimal dynamic system designs that are 5%–20% more cost‐effective, primarily resulting from higher water recycling facility utilization. Overall, this approach, which considers the dynamic nature of storm water availability and variable recycled water production, can inform water planners of the cost, water volume, and energy trade‐offs associated with different multisupply spreading basin system designs, including varying levels of centralization.

To address water scarcity, cities are pursuing options for augmenting groundwater recharge with recycled water. Ozone-based treatment trains comprising ozone and biologically activated carbon potentially offer cost-effective alternatives to membrane-based treatment, the standard process for potable reuse in numerous countries. However, regulations in multiple states effectively limit the extent to which ozonebased treatment alone can produce recycled water for groundwater recharge. To investigate the trade-offs between treatment costs and regulatory constraints, this study presents methods for modeling and optimizing designs for (1) producing recycled water using membranebased treatment, ozone-based treatment, and hybrid treatment trains comprising ozone-based treatment with a membrane sidestream, and (2) delivering that water to stormwater spreading basins. We present a case study of Los Angeles, CA, to demonstrate the model's application under realistic conditions, including regulations that limit spreading recycled water based on its concentration of total organic carbon and the extent of dilution. While the membranebased treatment train exhibits economies of scale, we demonstrate how regulatory constraints create a diseconomies of scale effect for hybrid treatment systems because larger scales necessitate a higher proportion of recycled water undergo membrane treatment. Nevertheless, relative to membrane-based treatment, we identify opportunities for ozone-based or hybrid treatment trains to reduce treatment costs and energy use by up to 62% and 59%, respectively, for systems with up to 1 m 3 /s (23 million gallons per day) mean water recycling rate, potentially lowering the barrier for decentralized water recycling systems. This modeling approach could inform planning and policy regarding recycled water projects for groundwater recharge through spreading basins and, with additional modification, other potable reuse applications.

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