Assimilation Wetlands

Scientists are fertilizing wetlands with treated sewage

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From Breaux Bridge to Luling, Thibodaux to Mandeville, treated wastewater is flowing into wetlands. Some scientists and coastal advocates are excited about it, claiming that the valuable nutrients derived from properly treated sewage, which is typically released into bodies of water such as the Mississippi or Amite rivers, could give ailing wetlands a fertile boost as coastal restoration projects move forward in Louisiana. Others think that some of these landscapes are too delicate to handle wastewater, and they say that one past project has done more harm than good.  

One of the newest wastewater-to-wetlands, or “assimilation wetlands,” projects is in the Central Wetlands Unit: thirty thousand marshy acres stretching from the 9th Ward in New Orleans into St. Bernard Parish. At the southern end of their reach sits the Riverbend Oxidation Pond, which treats and releases half a million gallons of municipal wastewater from the communities of Violet and Poydras into the Central Wetlands each day. “It’s quite beautiful,” Jake Groby III told me. Groby manages the St. Bernard Parish Wetlands Reclamation Project, and he helped bring the Riverbend project online back in 2014. 

Except for the odors (which ranged from port-o-let to chemical floor cleaner, depending on the wind), the scene was quite beautiful. Mats of brown hyacinth covered the three-acre oxidation pond. A heron fished, and an osprey hovered and dove into the remarkably clear water. Below the surface, bacteria and algae were busy feasting on the leftovers of twelve thousand people. At the far end of the pond, the water was shooting through a series of ultraviolet filters, whose bright blue jets sanitized the water before finally carrying it by pipe into the wetlands beyond. “The water coming out of that pond is incredible,” Groby said. “It’s nutrient rich, high in nitrogen and phosphorous, going right into the wetlands.” He called the marriage of treated water and wetlands “a match made in heaven.”

Over the last century, the Central Wetlands have seen dramatic changes. Charlie Canzoneri, who works at a stormwater pumping station next to the Riverbend site, has watched the transformation over the course of his life. As a boy, he and his friends hunted wood ducks in what used to be dense cypress and tupelo forests along the levee. “Thick,” he said of the cypress. “You’d go in there, no matter how bright it was outside, and it would be dark in there because of the canopy.”  

According to the USGS, the Central Wetlands Unit has lost about six thousand acres of land in the last eighty-four years. The reason for this collapse, according to the Lake Pontchartrain Basin Foundation (LPBF), was a combination of levee-building, logging, oil and gas exploration, and saltwater intrusion. As Canzoneri and his friends prowled the swamp in the 1960s, crews to the east were completing a seventy-six-mile canal that connected the Gulf of Mexico to the Port of New Orleans. The now-infamous MRGO, as the canal is called, was a straight-shot conduit for massive amounts of saltwater to creep in and change the wetlands’ entire ecosystem.   

Over the levee, Canzoneri pointed out a single patch of cypress trees, which he said only manage to survive because stormwater from the pumping station has kept the saltwater in check. It’s one of the only cypress swamps remaining in the Central Wetlands. Beyond that, the bottomland hardwood forests, freshwater marshes, cypress, and tupelo swamps have turned largely to saline marsh and open water with the occasional mangled, gray skeleton of a cypress sticking up in the sun.

Dr. John Day, distinguished professor of environmental science at LSU’s College of the Coast and Environment, has studied assimilation wetlands for thirty years and says that the loss of this swamp costs more than just habitat. In a report he co-authored in the Journal of Coastal Research, Dr. Day and his colleagues found that MRGO and its impact on the Central Wetlands played a major role in the flooding in New Orleans and St. Bernard Parish caused by Hurricane Katrina. “We estimate that had the cypress swamps been in place in the Central Wetlands during Katrina, it would have reduced flood damages by about eighty percent,” he told me.

Now that the channel has been closed, he and others are hoping that treated effluent can help regrow surge-buffering wetlands quickly, especially as rising sea levels pose an increasing threat. Dr. Gary Shaffer, professor of biology at Southeastern Louisiana University, has found that cypress trees fertilized with treated sewage can grow up to five times faster than traditionally grown cypress.

But others are wary of the wastewater. Scientists with the LPBF agree that wetlands are well-known for their ability to treat wastewater effectively, but they say that wetland assimilation projects are risky because it’s difficult to balance the flow of municipal wastewater with ideal water levels in the natural world. “You don’t have to raise water levels that much to have an effect on wetlands,” said Dr. Theryn Henkel of LPBF. She pointed to a much larger wastewater wetlands project in Hammond that is widely believed to have accelerated land loss in the area. Some three hundred acres of freshwater wetlands were converted to bare mudflats and open water after the city of Hammond began releasing secondarily treated wastewater into the area.  

Some suggest that the boost in nutrient levels caused a population explosion of nutria, which, in turn, consumed the wetlands in question. Others have hypothesized that the accumulation of wastewater in this poorly draining area effectively drowned the vegetation (which has since begun to recover). Hammond’s sewage treatment facility reportedly exceeded the amount of wastewater it was permitted to release on several occasions; between 2009 and 2010, the site violated permitted limits nineteen out of twenty-four months, according to a 2010 presentation by Matt Rota of the Gulf Restoration Network. Local scientists haven’t reached an agreement on the precise cause.

Across Louisiana, there are around twenty wetland assimilation projects in operation. Groby said that the Riverbend site in St. Bernard doesn’t release enough water to have an effect on water levels, but the site has yet to create a plan to control the nutria population beyond supplying plastic collars that protect young cypress from the rodents. He regularly submits a slate of water quality tests to the Louisiana Department of Environmental Quality, including phosphorous and nitrogen levels, which he said have been ideal; and he’s confident that the cypress and tupelo will enjoy the nutrients. “We’ve got so much going for us,” Groby said, “but it’s the world’s best kept secret.”