Thoughts From Engineers: Reusing Captured Stormwater

The residents of ancient Mediterranean cities presumed a state of water scarcity, which meant they valued water and collected it any way they could. The central hallways of homes of the wealthy in the ancient city of Pompeii, for example, often featured a large basin positioned directly below a circular opening in the roof—ready to collect vital rainwater. Just 15 or so miles from Pompeii, the city of Naples has enormous aqueducts below city streets—nearly as substantial as the built environment above—that stored a vast supply of freshwater during periods of drought. Given today’s shifting hydroclimatic regimes, few regions are completely water secure; we may have more in common with earlier civilizations now than perhaps any other time in history.

Regional drought and overextended aquifers as well as intense flooding coupled with growing urbanization and changing hydrologic trends are just a few pressing modern-day water issues. Widespread water uncertainty combined with resilience initiatives has led to innovative tools designed to diversify water supplies. From desalination plants to technologies capable of producing high-quality potable water from wastewater to domestic and commercial consumptive-use efficiencies and incentives, we’re clearly making important inroads.

But another significant water-diversification opportunity is still largely unrealized. This strategy aims to bolster municipal water supplies through the collection—and reuse—of stormwater. According to a report released in early 2024 by the Pacific Foundation in cooperation with 2NDNATURE (bit.ly/2ndNatureReport), urban areas throughout the country produce roughly 59.5 million acre-feet per year of stormwater runoff, which is roughly equal to an average of 53 billion gallons per day. The spatial model used in this report generated estimates based on annual stormwater loads using historical precipitation records—without factoring in potential shifts due to climate change—but the total volumes projected are nevertheless significant. These numbers underscore the need to continue to research and develop beneficial stormwater applications.

Leveraging Stormwater for the Common Good

During the last several decades, water-resource professionals and others have redeveloped and revitalized urban areas with a variety of stormwater technologies. Large tunnel systems divert stormwater to reduce combined sewer overflow events and polluted waterways. Curbside drywells trap stormwater, not only mitigating flood flows but introducing trees and green zones into otherwise gray spaces. We use cisterns, trenches and drains of various scales and design combined with suitable infiltration media to filter stormwater as it percolates toward groundwater. More-ambitious projects collect water over larger areas to integrate multiple objectives in the ultimate design—irrigated green spaces, recreational areas and floodwater detention all in one.

Many interesting, state-of-the-art projects have been developed to use reclaimed stormwater to meet irrigation, toilet-flushing and other needs. The National Park Service installed a stormwater capture and irrigation system with four 250,000-gallon cisterns for the National Mall in Washington, D.C. (bit.ly/USNationalMall). Collected runoff moves through underground chambers for treatment using ultraviolet disinfection before use for irrigation. The Capital Region Watershed District, which manages water resources across several municipalities including St. Paul, Minn., designed several interesting projects, including the Allianz Field project (bit.ly/AllianzStormwater), which uses stacked green infrastructure that collects rooftop runoff and stormwater from neighboring areas to repurpose for irrigation and non-
potable indoor stadium uses. These represent only a few of the many innovative stormwater reuse projects active around the country.

Recharging a Water Supply

Most systems so far consist of smaller-scale applications for non-potable use. A still unresolved issue—at least in most parts of the country—relates to whether large-scale stormwater infiltration systems can be safely designed to recharge a water supply. The Sun Valley Watershed-Sun Valley Park Drain and Infiltration System Project in L.A. County (bit.ly/LACountyWaterReuse) mitigates flooding and serves other beneficial uses as it collects stormwater from a 21-acre drainage area before channeling it through a treatment system to remove metals and suspended solids. Underground infiltration basins then further treat the water before it seeps down to groundwater.

Studies show that stormwater frequently contains metals, bacteria, salts, nutrients and petroleum hydrocarbons; more-recent research has identified contaminants such as flame retardants, common insecticides, perfluorinated compounds and plastic additives, among others. Those advocating for more caution (bit.ly/ACSUrbanStormwater) suggest that even with carefully designed projects, local water supplies are still vulnerable to contamination. This is an area that would benefit from extensive testing on field sites.

Ongoing monitoring should leverage new sensing technologies to track variability and subsurface movement of contaminants under differing hydrogeological-chemical scenarios and weather conditions, at the very least. Clearly, the complexity of factors potentially at play is significant—as is the risk.

It’s easy to see how an incomplete analysis or any number of other deficiencies in project design could quickly compromise a growing municipality’s last remaining source of clean water. For an interesting analysis of how one of the most populated cities in the world, Lagos, Nigeria, plans to meet a growing need for clean water using managed aquifer recharge, see “Water scarcity in the fast-growing megacity of Lagos, Nigeria, and opportunities for managed aquifer recharge” (bit.ly/LagosAquiferRecharge).

Expanding Uses

The considerable quantity of water that very literally moves through our communities is significant enough to make our efforts at collection and reuse worth the cost. We’ve made important advances in this regard, and our cities are the obvious beneficiaries of stormwater systems that capitalize on essentially “free” water to enhance our communities and bolster our supplies of a crucial resource. But as we expand on the many potential applications of stormwater—particularly to recharge groundwater supplies—exhaustive and long-term research and monitoring of test sites is key. Clearly, when you are dealing with a resource this important, maintaining the integrity of the receiving aquifers, in many cases our last pristine sources of drinking water, is paramount.