Birds flying above Las Arenitas wetland.
Las Arenitas Birds flying above Las Arenitas wetland. © Bill Hatcher/Sonoran Institute, 2019

Colorado River Basin

Turning Waste into Water

How improving wastewater treatment in a Mexican city will help restore water to the Colorado River Delta.

The Nature Conservancy, along with a coalition of environmental partners, signed a deal July 1st with Mexican water authorities that secures increased freshwater flows to the Colorado River estuary. The Las Arenitas Wetland Project funds infrastructure improvements for one of Mexicali’s major wastewater treatment plants, including the construction of a wetland that will help filter wastewater and serve as habitat for migratory birds as they travel along the Pacific Flyway.

Most importantly, the agreement secures about 11,000 acre-feet (3.586 billion gallons) of treated water annually for the parched Colorado River Estuary.

Las Arenitas: An Unlikely Oasis

Highway 5 exits Mexicali, the capital of Baja California, and cuts south, bisecting the Sonoran Desert and the Colorado River Delta. To the east, the impact of the river is easy to spot: green fields of alfalfa and cotton. Father south the farms disappear, as does any sign of freshwater, and the landscape becomes flat and salty for several miles before reaching the Gulf of California.

Somewhere between the farms and the barren land sits the Las Arenitas wastewater treatment plant: 16 oxidation pools and a 250-acre wetland that treat about 50% of Mexicali’s sewage. The stench from the blackwater would drive most people running back to the desert, but for environmental groups working to protect the Colorado River Delta, Las Arenitas is a beacon of hope.

“For me, this place is very special,” explains Edgar Carrera, The Nature Conservancy’s (TNC) Colorado River Delta Project Coordinator. A native of Mexicali, Carrera spent hours wading waist-deep in Las Arenitas’ treatment pools when he was doing research for his master’s thesis at the University of Baja California. “Not only is it fascinating from the environmental perspective, showing us how we can use green infrastructure to more efficiently reuse our water and return it to the river, I also believe that Las Arenitas showcases how the U.S. and Mexico can work together to manage the Colorado River.

Las Arenitas has been a joint U.S.-Mexican venture since its conception in the mid 2000s. Prior to the construction of the plant, Mexicali was releasing untreated sewage into the New River, which runs north into California and terminates in the Salton Sea. In an effort to fix this pollution problem, the North American Development Bank helped finance the construction of the treatment plant. Although the New River remains an extremely polluted waterway, the construction of Las Arenitas greatly reduced raw sewage inflows into the river.

Shortly after the plant became operational, two conservation groups (the Sonoran Institute and Pronatura Noroeste) approached the State Commission of Public Services for Mexicali (CESPM), which manages Mexicali’s water, with a proposal to construct a wetland next to the treatment plant. This provided additional treatment capacity to the plant, while also creating habitat for the hundreds of bird species that rely on this area. Seen from the sky, the wetland is a wild oasis in a sea of desert and farmland. 

The Las Arenitas water treatment plant, with its oxidation pools and wetlands, lies in stark contrast to its desert surroundings.
Desert oasis The Las Arenitas water treatment plant, with its oxidation pools and wetlands, lies in stark contrast to its desert surroundings. © Bill Hatcher 2015

Polluted Water Threatens People and Nature

As the city of Mexicali grew to a population of 750,000, so did the demand for sewage treatment. At the same time, Las Arenitas’ treatment capabilities declined due to a maintenance backlog. The plant now services half of Mexicali’s wastewater needs and receives about 20% more inflows than it can fully treat. The polluted water is discharged into the Rio Hardy, threatening both humans and wildlife downstream. The public services commission was aware of the problem, but struggled to obtain the funds for the necessary fix.

TNC, along with its partners, saw an opportunity: fund the much-needed improvements, and increase both the quantity and quality of water flowing into the Colorado River Estuary. But first, local residents, the public services commission, and Mexico’s federal water authority would need to be convinced that a project focused on acquiring water for the environment would not mean less water for farmers and cities, a hard sell in this parched region. Making matters more difficult, Mexicali Valley farmers and residents have often had to fight for their water from the Colorado River, causing them to bristle at the idea of deals involving water allocations. 

Much of the landscape near Las Arenitas is desperately dry.
Parched earth Much of the landscape near Las Arenitas is desperately dry. © TNC

Collaboration and Community

One skeptical group was the Ecological Association of Users of the Hardy and Colorado Rivers (AEURHYC), which represents the downstream water users of the Rio Hardy. Francisco Javier Mosqueda, the president of AEURHYC and the co-owner of a large tourism camp along the river, explains that agreements in the past had done little to clean up the river. “Previous deals were signed and water quality was not improved much,” says Mosqueda. “Fish are still dying in the Rio Hardy.”

Still, he is hopeful that with advancements to the treatment plant and monitoring by TNC and its partners, the new agreement will bring improvements to the river. “A cleaner Rio Hardy would bring huge economic benefits to the region,” says Mosqueda, going on to detail the fishing and tourism that the upper river has historically supported. “This is the best effort for water quality and quantity improvements in the last 10 years.” 

The Las Arenitas Wetland Project secures about 11,000 acre-feet (3.586 billion gallons) of treated water annually for the parched Colorado River estuary.
Estuary restoration The Las Arenitas Wetland Project secures about 11,000 acre-feet (3.586 billion gallons) of treated water annually for the parched Colorado River estuary. © TNC

“It took years of collaboration and trust building between the environmental partners, water authorities and local groups,” says Francisco Zamora, director of the Sonoran Institute’s Colorado River Delta Program. Zamora was one of the central figures in building the first wetland area next to Las Arenitas, so he understood from the beginning that success was possible.

As talks progressed, the Mexicali public services commission felt comfortable sharing sensitive water quality and flow data with the environmental groups. Armed with this information, TNC and its partners helped develop plans for an improved treatment plant, incorporating a new wetland, that would target the specific contamination issues Las Arenitas was experiencing. Negotiations, spearheaded by Carrera and Zamora, led to a plan with which the water managers, environmental groups and local organizations were happy. 

Hundreds of species of birds, including these American avocets, have been found in the Las Arenitas wetland.
Habitat for birds Hundreds of species of birds, including these American avocets, have been found in the Las Arenitas wetland. © Bill Hatcher/Sonoran Institute

The Las Arenitas Agreement

The agreement signed July 1st provides funding for maintenance and improvements to the existing treatment infrastructure and the construction of a new wetland area, and pledges that a volume of water equal to 50% of the inflow to Las Arenitas will be allocated to the environment. Importantly, these increased environmental flows will come at minimal costs to downstream farmers and residents, who maintain their current water allocations and will have the added benefit of a cleaner Rio Hardy. 

“The most important lesson that we have taken away from this process is that we can all win: the people, the institutions and the environment” says Isaac Vizzuett Herrera, the public services commission’s subdirector of sanitation and water. Herrera is hoping to take the lessons learned in Las Arenitas and apply them to other communities in Baja California that are struggling with wastewater treatment. 

For Carrera, the agreement is a sign of hope for the entire Colorado River basin. “What is happening in Las Arenitas and in the delta allows us to see the connection between the upstream and the downstream in the Colorado River,” he said. “This is one system, one basin, binational, working to restore the delta ecosystem.”

This piece was written by Lucas Isakowitz an environmental writer, currently pursuing a master's degree at the Yale School of Forestry and Environmental Studies.