Oro Loma Sanitary District’s unique horizontal levee offers innovative solutions to two critical environmental problems. Photo by Kristopher Decker.

This small rectangle of wetland near the San Francisco Bay in San Lorenzo doesn’t look particularly visionary. Above ground, it’s an appealing — if unusually orderly — array of meadows, cattails, and willows. But there’s far more here than meets the eye. This modest strip of land, just 38 by 150 feet, in the Oro Loma Sanitary District promises to help solve two of the Bay Area’s most pressing concerns: sea level rise and nutrient pollution.

“In many ways, the project has been a wild success,” said Jason Warner, the sanitary district’s general manager.

Below ground, effluent from Oro Loma’s wastewater treatment plant percolates through layers of sand and gravel. The goal is to test whether microbes living down there can remove nitrogen and other contaminants that currently pass right through the treatment plant and into the Bay. Nitrates can harm aquatic environments by fueling algae blooms, which in turn can deprive other aquatic life of oxygen.

Another key feature of the project that’s not immediately obvious is its shape. The land is higher at one end than the other, forming a subtle wedge. The hope is to protect tidal marshes, which help buffer the Bay shoreline against crashing waves, from rising seas. When bounded by conventional steep levees, marshes have nowhere to go and are projected to drown. In contrast, the gentle slope of the Oro Loma project — called a horizontal levee — mimics the natural transition between upland and marshes, giving these vital wetlands room to move inland as the sea rises.

The horizontal levee was inspired by seeps that historically flowed from uplands to marshes around the Bay, before conventional levees broke that connection. “There was a native flora that depended on freshwater seeps into salt marshes,” said coastal ecologist Peter Baye, who helped design the Oro Loma project. “The horizontal levee is like a freshwater seep, except the water source is treatment plant effluent.”

A few remnants of the natural upland-to-marsh transition are left today, including one near the horizontal levee site in Coyote Hills Regional Park. Baye chose more than 20 plant species growing there for the Oro Loma project, including sedges, rushes, and goldenrod. The next step, spearheaded by the Oakland-based nonprofit Save the Bay, was establishing the native plants in the horizontal levee.

This was a huge undertaking that took about 70,000 plants. Collection permits in hand, Save the Bay staff gathered plant materials from parks and reserves in the fall of 2014. At Baye’s suggestion, they focused on rhizomes — underground stems that readily grow roots and shoots — and planted them in raised beds at Oro Loma.

The beds were alive with new shoots by spring and in the fall of 2015, Save the Bay began the task of transplanting them in the horizontal levee. All told, the effort took about two months and 5,000 volunteers. And once the transplants took off in their new home, the project was ready for testing.

“The water quality improvements have been stellar,” Oro Loma’s Warner said.

Preliminary results show that the horizontal levee removes essentially all of the nitrate from the wastewater treatment plant effluent. “Nitrogen disappears almost as soon as it comes into the system,” said David Sedlak, an environmental engineer at UC Berkeley and co-director of the Berkeley Water Center, who led the water quality testing.

The horizontal levee also removes pharmaceuticals such as carbamazepine, an anticonvulsant used to prevent seizures. “It was quite a pleasant surprise,” Sedlak said. “It’s disappearing in this wetland — the microbes growing there break it down much more effectively than microbes in the sewage treatment plant.” Bacteria in the horizontal levee come from the landscape, while those in treatment plants come from sewer pipes and people.

Currently, the microbes growing in the horizontal levee’s underground sand and gravel get their carbon from wood chips that were added when the project was built. When those wood chips are gone, roots from the native plants growing on top will provide a new carbon source.

Another benefit of the project is that it gives wildlife a place to live. “Critters are moving in,” Warner said, adding that he sees more birds and foxes than before the horizontal levee was built.

But for all the promise of the horizontal levee, getting it approved was quite a challenge. “We did this to advance the science of response to sea level rise, but existing regulations hinder our ability to respond in an environmentally-friendly manner,” Warner said. Notably, the horizontal levee was classified as artificial treatment rather than wetland restoration. “The laws are set up to protect endangered species, and don’t differentiate between a horizontal levee and a big box store,” he explained.

This meant that the approval process took a lot of money — about a quarter of the project’s total $2 million cost — and time. “It took four years to get the permits and only six months to build,” Warner said.

Now, the horizontal levee concept is being explored for sites all around the Bay, including Novato in the North Bay, the salt pond restoration in the South Bay, and the Palo Alto Baylands Nature Preserve on the peninsula. The latter has a spot that seems ideal for a horizontal levee. “It’s near a wastewater treatment plant and there’s a pre-existing slope,” Baye said.

While expansion is important for optimizing horizontal levees, so is making sure that they don’t get built where they don’t belong. “They need wide marshes and mudflats, not cliffs or bluffs,” Baye said. “The biggest danger is misapplication, which breeds poor results.” Just as in other parts of life, ecological fixes can become too trendy for their own good. For example, Facebook and Google were intrigued by horizontal levees, but the sites they had in mind weren’t a good fit.

Collaborating with Baye, Sedlak, and the rest of the team is one of Warner’s favorite parts of the project. “We don’t always solve problems this way — sometimes people stay in their boxes,” he said, adding, “When you do something special for the environment, it brings out the best in everyone.”

Robin Meadows covers water for the Monitor.