As rivers go, the Santa Cruz is a border-jumping nomad. Starting in Southern Arizona's lush San Rafael Valley, it forges a 35-mile loop through Mexico before flowing back across the line near Nogales.
Its continued existence is likewise an international affair. Nearly 30 miles of the river in the Santa Cruz Valley south of Tucson are almost entirely dependent on effluent that comes from Nogales, Sonora. That sewage is treated at a huge plant in Nogales, Ariz., before being released into the waterway.
Not long ago, underground pumping had sucked the Santa Cruz dry. Today, effluent from metro Tucson helps keep parts of the river alive on the northwest side of town. But it is the reclaimed water from Nogales that has sparked the most remarkable recovery, creating a riparian area that teems with life—from endangered minnows to native birds—beneath a whispering canopy of cottonwood trees.
Still, as ongoing drought puts a strain on water supplies, rivers such as the Santa Cruz are certain to be at the center of debate over the use of effluent. According to the National Research Council, some 32 billion gallons of municipal wastewater are dumped directly into the ocean or estuaries each year. In Arizona alone, effluent now helps sustain at least a dozen waterways. But its reach is shrinking, as treated sewage is put to other uses, such as agriculture or the cooling of industrial plants. A report by the Arizona Cooperative Extension shows that effluent sustained 351 miles of streams and rivers in 1987; by 2009, the number of miles had dropped to 91.
Impacts on the Santa Cruz in particular are tied to a new sewage treatment facility located 20 miles south of Nogales, Sonora. Just now coming online, the Los Alisos Wastewater Treatment Plant is expected to divert millions of gallons of sewage currently being treated under binational agreement at the International Wastewater Treatment Plant in Nogales, Ariz.
Instead of replenishing the Santa Cruz, that water will soon be pumped into a Mexican watershed.
An agreement dating from the 1980s authorizes the Arizona plant to treat up to 9.9 million gallons of Mexican sewage each day. But Mexico regularly exceeds that allotment by several million gallons, costing American taxpayers more to treat the excess. The Los Alisos plant is aimed—at least in part—at bringing imported Mexican wastewater closer to its allotted target.
Failure to do so is expensive. Figures provided by the U.S. government show that, from October 2010 to September 2011, Mexico paid $686,360 for treatment of its allotment, at a rate based on what it would cost to treat the waste to Mexican standards. Any excess beyond the 9.9 million is billed at actual treatment costs. As a result, during that same time period, Mexico sent an average of 11.84 million gallons to the plant each day; the total added cost for the extra treatment was $540,858.
When the new Mexican plant is fully operational, expectations are that those costs will be substantially lowered. But conservationists fear that Mexico's contribution to the Santa Cruz River could plummet as well.
According to U.S. officials, however, initial effects will be minimal. "We are estimating a reduction of 1.5 to 3 million gallons per day in what's coming across from Mexico," says Sally Spener, a foreign affairs officer with the International Boundary and Water Commission. As an organ of the U.S. State Department, the IBWC oversees water and boundary issues along the U.S.-Mexico border. Its tasks including operating the sewer plant in Nogales, Ariz.
Could demand for effluent ever reach the point that Mexico eventually keeps all of its Nogales wastewater? "Only in theory," Spener says. "From a legal standpoint, yes, they could. But we're downhill (from Mexico), and from a practical standpoint of keeping that water from flowing north across the border, that's just not reasonable. For them to hold all that in Mexico, they would have to create the infrastructure to pump it back up into Mexico. It's just not feasible."
In the meantime, the Santa Cruz River putters along. An upgrade to the Nogales plant in 2009 helped reduce the nitrogen levels in the water, and subsequently boosted oxygen levels, resulting in a surge of animal and plant life. "The native fish are starting to come back, and the riparian indicators have stayed pretty steady over those three years," says Claire Zugmeyer, an ecologist with the Sonoran Institute, which releases annual status reports on the Santa Cruz.
Among other things, the institute's monitoring also shows that the river's northward reach has diminished in recent years. "But that's not necessarily a bad thing, in that it means there's a greater infiltration of water" in the streambed, she says.
One reason may be a reduction in the so-called "clogging layer," created when water too high in nutrients and microorganisms clogs the river-bottom sediment, turning it into little more than an aqueduct. That could be the culprit behind a massive die-off of cottonwood trees near the treatment plant in 2005, Zugmeyer says. "Even though there was water in the river, trees couldn't get at it, because it wasn't infiltrating enough.
"While the Santa Cruz doesn't make it as far north, there is more water. It's full—aquatic life can now survive in the river. The water quality is still not perfect, because it's still treated wastewater. But it's much improved from before."
How long that remains the case is up to speculation. And despite government assurances, observers such as Zugmeyer say diversions to the new Mexican sewer plant will have a noticeable impact on the Santa Cruz. "There is a concern. (Sonora) is permitted to send 9.9 million gallons per day. So if they end up diverting all of that, and only 5.1 millions per day is coming from Nogales, Ariz., that's a huge difference."
Zugmeyer and others suggest that those changes point to the need for a larger discussion about the uses of effluent in an increasingly parched nation. "If we want to still have free-flowing rivers," she says, "especially in the Southwest, where so many of our rivers are effluent-dependent, we're going to have to make some tough choices in the future, as water becomes even more in demand."