Once a Smelly Nuisance, Mexicali’s Wastewater Now Brings Life to the Colorado Delta

This post is part of a series on the Colorado River Delta.

If there is one place that transforms wastewater from trouble-maker to life-saver it’s the site of Las Arenitas sewage treatment plant in the Mexican state of Baja California.

There, nasty urban wastewater that once made a smelly health hazard of the New River near the U.S.-Mexico border is now sustaining a wondrous wetland and bird-watchers’ paradise in the northwest corner of the Colorado River Delta.

In this way, Las Arenitas joins La Ciénega de Santa Clara – the “accidental wetland” – in showcasing the revival possible by adding water to the once-verdant, but now desiccated delta.Colorado River delta series

Located about 16 miles (26 kilometers) southeast of the border city of Mexicali, Las Arenitas at first glance looks like any municipal wastewater treatment plant.  A big underground pipeline daily delivers nearly 20 million gallons of Mexicali’s sewage, which then undergoes conventional physical, biological, and chlorination processes to remove bacteria and other harmful pollutants.

The plant was designed to discharge the treated effluent to an agricultural drain that empties into the Hardy River, which joins the Colorado River before reaching the sea.  After its start-up in 2007, however, the plant not only failed to meet the required water quality standards, it was sending raw sewage down the Hardy.

A year later, even as the public services commission in Mexicali took steps to improve the plant’s operation, two conservation groups – Pronatura Noroeste, based in Ensenada, Baja California, and the Sonoran Institute, based in Tucson, Arizona – seized the opportunity to help solve the pollution problem while restoring vital delta habitat:  they partnered with the commission to construct a 250-acre (100-hectare) wetland.

Just as wetlands do in nature, the constructed cattail marsh at Las Arenitas removes pollutants from the wastewater, giving it an additional cleansing before its release to the river or to farmers.

At the same time, the wetland creates a home for resident and migratory birds that have seen their delta habitat shrink nearly to oblivion from the damming and diverting of the Colorado River upstream.

And treated wastewater from the marsh has roughly doubled the flow of the Rio Hardy, enhancing fishing and economic opportunities.

During a visit to Las Arenitas in February, the marsh was a riot of birds.  A yellow-rumped warbler sang from atop a mesquite tree.  Flotillas of American coots plied the lagoons.  And hiding in the cattails, a secretive sora delivered its telltale descending whinny call.

For Javier Orduño Valdez, director of the state public services commission in Mexicali, Las Arenitas anchors a grand vision for recreation, tourism, and economic development in the region.  He described plans to build a nature park with trails around the marshes, along with exhibits to explain the importance of the wetlands to the Rio Hardy and the delta.

“With this water we give life to all this area,” said Orduño Valdez.  “The important thing right now is not us, but the future generations.”

There are plans to double the treatment capacity of Las Arenitas over the next several years.  And in exchange for constructing additional wetlands at the site, the conservation groups have the opportunity to acquire more flows for the Hardy River.

Las Arenitas has benefited residents north of the border, too.  For decades, sewage from Mexicali flowed untreated into the New River, which begins 15 miles (24 kilometers) south of the city, flows north across the border, and then traverses sixty miles (97 kilometers) of California’s Imperial Valley before emptying into the Salton Sea.

At times, the stench near the border was overpowering.  Signs placed along the river’s course warned visitors to avoid contact with the river’s hazardous brew.

But today, thanks to Las Arenitas, the New River is much cleaner, especially near the border.

And the 20 million gallons a day of raw sewage that used to pollute the New now sustains wetlands, birds, fish, wildlife, and river flows in the delta.

It’s a great example of taking the “waste” out of wastewater – and of the promise of restoring the Colorado Delta.

Help restore water to the Colorado River by joining Change the Course. Sign up online or text ‘River’ to 77177.

Special thanks to Silk and Coca-Cola, Charter Sponsors for Change the Course. Additional funding generously provided by the Walton Family Foundation.


Sandra Postel is director of the Global Water Policy Project and Freshwater Fellow of the National Geographic Society. She is the author of several acclaimed books, including the award-winning Last Oasis, a Pew Scholar in Conservation and the Environment, and one of the “Scientific American 50.” She is co-creator of Change the Course, the national freshwater conservation and restoration campaign being piloted in the Colorado River Basin.

Sandra Postel is director of the Global Water Policy Project and author of Replenish: The Virtuous Cycle of Water and Prosperity. From 2009-2015, she served as Freshwater Fellow of the National Geographic Society. Sandra is also co-creator of Change the Course, the national water stewardship initiative awarded the 2017 US Water Prize for restoring billions of gallons of water to depleted rivers and wetlands. The recipient of several honorary degrees, she works to bridge science, policy, and practice to promote innovative ways of securing water to meet both human and ecosystem needs.
  • Mike Ester

    We have been on the Rio Hardy at Campo Mosqueda since 1973. The river back then was way cleaner then in recent years. The wildlife and fish were more plentiful. There was birds galore, big mouth bass, huge cat fish and tons of frogs, which is no longer. It would be awesome to see that come back without the local fisherman taking out everything that they can trap in their gill nets.I don’t have a problem with making a living at fishing, but don’t rape it to the point that there is nothing left. I have stood on my dock and watched the local fisherman drag in gill nets and not throw one thing back in. No matter what it is or the size, and that is wrong.

  • Paul Fairall

    Brilliant what 4 or 5 Hydrophytes dominate the system.

  • Krystal Reardon

    Great to see the Rio Hardy (and the Colorado in general) getting some remedial attention 🙂 See you in Baja!

  • Doug West

    I miss the frogs too! Hopefully they come back strong….I agree the fisher men are raping the fish @ the rio.

  • fran

    What an amazing story and a great start for the Colorado River Delta. The conservationists and governments who worked together to make this happen should be so proud of their achievements. I look forward to seeing how this wetland grows and changes. Nice work Sandra and can’t wait for the next post in this series.

  • Kristiana Teige

    We need more stories like this! There is so much for us to learn from nature about managing and improving our built environment. Thanks, Sandra!

  • Mozart J Fialho

    If we all could give a hand to the Nature, we’d be returning everything what we remove of it. Here in Brazil, we try not to degrade our sources, but this is not easy, especially in very populous areas. Anyway, I’m afraid ’bout the future and what “progress” can bring to our natural resources.
    Posts like yours, Sandra, are very important to guide us in respect to what to make to protect such resources.
    Thanks a lot and congratulations for helping the Earth.

  • Lise Bolton

    It is really great to see this being done on such a large scale. Wetlands have been degraded worldwide and wastewater treatment is becoming a big problem. By using wetlands to treat wastewater habitats can be restored and pollution can be reduced. I work for a wetland treatment company in Australia http://www.ecoteam.com.au we are also working on similar project at slightly smaller scale. A great project in Northern NSW which has also used wastewater to turn a degraded wetland into a tourist facility for bird watching http://www.wastewaterwetlands.com.au

  • tony kelly

    Wonderful to see & read how mankind is directing it’s energies at restoration rather then destruction


    A pleasure to read intelligent comments,.

  • John Mackey

    Thank you for a truly inspiring piece. We, who love The Salton Sea, will not only learn from this Rio Hardy enterprise, but also, the collaboration between Environmentalists & Baja Government is an Inspiration to our group, based @ ecomedia in Salton City.

  • Lautaro Arauca


About the Blog

Researchers, conservationists, and others share stories, insights and ideas about Our Changing Planet, Wildlife & Wild Spaces, and The Human Journey. More than 50,000 comments have been added to 10,000 posts. Explore the list alongside to dive deeper into some of the most popular categories of the National Geographic Society’s conversation platform Voices.

Opinions are those of the blogger and/or the blogger’s organization, and not necessarily those of the National Geographic Society. Posters of blogs and comments are required to observe National Geographic’s community rules and other terms of service.

Voices director: David Braun (dbraun@ngs.org)

Social Media