Not only is 97% of the nitrogen removed, but also trace pharmaceuticals. “You just have to focus on where the water is going,” says environmental engineer Aidan Cecchetti, referring to the UC Berkeley-Stanford-ReNUWIt team’s experimentation with three components of flow through the levee system—under the surface, over the surface, or into the air (through evapotranspiration). “In the wastewater pumped to the subsurface, you see full removal of every contaminant except phosphorous.” What’s most astonishing is how much of the work occurs in that subsurface drainage through the first 10% of levee slope. What happened further along the slope mattered less, the team found; plants only absorbed 8-12% of the nitrogen, with willows being the highest performing species. “Predicting performance is not nearly as complex as everyone imagines it to be, our three papers tie up everything nicely in bows,” says Cecchetti. One of papers is now out in Water Research X, while a second, describing an innovative isotope technique that helps calculate the amount of nitrogen taken up by the plants from wastewater versus the soil, is in peer review. Cecchetti, who developed the technique, says it’s a “shortcut to understanding exactly where nitrogen goes.” Future levees, the three papers are starting to conclude, may not need to be so careful about soil types throughout—imported dirt can be expensive to move and more locally sourced materials may prove adequate for some portions of the levee if the real work is going on in the first few meters. Meanwhile, how the levee system treats the concentrated salty “brine” produced by reverse osmosis in water recycling is the next testing frontier for the Oro Loma experimental levee. All these efforts are part of of regional study by Bay Area Clean Water Agencies, the San Francisco Estuary Institute and San Francisco Estuary Partnership of how best to build more, bigger, and longer horizontal levees around other wastewater treatment facilities vulnerable to sea level rise on the Bayshore. ARO