A New Look at Stormwater – Utilizing Existing Infrastructure to Harvest Flows for Drinking Water Use
The City of Oxnard, CA (City) has completed a Public Works Integrated Master Planning (PWIMP), which addresses future planning needs for all major water resource utilities within the City’s jurisdiction including potable water, wastewater, recycled water, and stormwater. As part of the PWIMP, a previously unexplored linkage between stormwater and water supply was investigated by applying an approach to capture (or “harvest”) stormwater, both during dry and wet weather periods, and treat it at Oxnard’s wastewater treatment plant (OWTP). This innovative practice accomplishes multiple objectives: 1) it will provide an additional water source for the advanced water purification facility (AWPF) and subsequent reuse, 2) it will help equalize diurnal flows to the OWTP which reduces turndown requirements for water reuse production, 3) it significantly reduces dry weather stormwater flows as a source of pathogens and pollutant loadings reducing potential beach closures and recreational impacts, and 4) it helps to meet effluent NPDES requirements by reducing concentration effects through the AWPF.
The City of Oxnard is located on a relatively flat coastal plain with both a shallow perched aquifer of poor water quality, and deeper aquifers used for water supply. The existing storm drain infrastructure cuts through this shallow perched aquifer, and due to infiltration, captures a significant amount of flow during dry weather periods. One major stormwater drain that collects water from a third of the city conveys an average dry weather flow of almost two million gallons per day. Currently, these dry weather flows drain to the ocean. As Oxnard plans for future growth in the increasingly variable southern California climate, they have a need for additional water sources, and mining of dry weather stormwater flows could be one such source.
Samples of the perched aquifer stormwater show that water quality is similar to the quality of dry weather infiltration entering the OWTP on a daily basis through the sanitary sewer system. Adding perched aquifer stormwater to the OWTP not only serves as a new water source when coupled with the City’s water reuse system, but also helps with typical problems of reuse such as equalizing diurnal variability, and managing secondary effluent concentrations flowing to the AWPF. This treatment of dry weather stormwater could also benefit overall water quality in the basin by reducing dry weather pathogens that cause beach closures. Further water quality analysis, flow monitoring, and regulatory discussions are now underway to assess final feasibility.
There is also the ability to augment dry weather stormwater flows by harvesting rainwater from wet weather events. Since existing stormwater during wet weather events contain many pollutants during “first flush” of the system, capturing rainfall by storing flows up in the system and releasing these flow after the first flush passes could additionally augment the dry weather stormwater supply. This approach allows for the use of the existing stormwater collection system as a conveyance mechanism for transferred harvested rainwater in the upper reaches of the system where it would have limited pollutants, downstream by gravity to treatment facilities without any need for new infrastructure or pumping. Large commercial sites could be retrofitted to capture significant quantities of rainwater and provide for an additional water source to augment the dry weather stormwater during the rainy season. This source of water would also help dilute the brackish water from the near surface aquifers infiltration, thus reducing concentration effects at the AWPF.
This presentation will inform other agencies that may not have thought of using stormwater as a new water source, during both dry and wet weather periods, and will outline the application of this unique approach that Oxnard.