Horizontal levees, a relatively new concept, have been generating a lot of interest in the San Francisco Bay as nature-based shoreline projects with the potential to offer many benefits, from flood and sea level rise protection to wildlife habitat, water quality improvement, and more. But only one horizontal levee is currently in operation in the Bay Area, so these benefits await substantiation through implementation and research. Fortunately, a recently published study from UC Berkeley’s ReNUWIt (US National Science Foundation Engineering Research Center for Re-Inventing the Nation’s Urban Water Infrastructure) has now provided the initial results to affirm the promise that horizontal levees hold.
The two-year study, published in April in Water Research X, monitored for trace organic contaminants, nutrients such as nitrogen and phosphorus, and enteric pathogen indicators. The study authors found that subsurface flow through the Oro Loma horizontal levee removed nitrogen, trace organic compounds such as pharmaceuticals, and fecal indicator virus (F+ coliphage) from treated secondary effluent almost completely, and also removed a significant amount of phosphate. Subsurface flow capacity appears to have the most significant effect on contaminant mass removal, indicating that choice of substrate materials and planting regime will impact the effectiveness of the horizontal levee. In addition, native plants were found to rapidly out-compete non-native plants and attracted diverse fauna, offering another benefit of horizontal levees.
Additional research is needed to learn more about the mechanisms of contaminant and nutrient removal, but these findings offer encouragement to wastewater treatment plant operators and others who are seeking to meet new or anticipated regulatory requirements, adapt to a changing climate, and more. For more on the Oro Loma project and the Estuary Partnership’s work to advance horizontal levees, visit the Transforming Urban Water Initiative.
This research advances Actions 14 and 28 by increasing understanding of the potential for nutrient management in the Estuary, and by demonstrating how nature-based shoreline infrastructure can provide increased resiliency to changes in the Estuary environment.