A parched compound within earshot of the whoosh of I-10 traffic in Marana, about 20 miles northwest of the University of Arizona campus, seemed an unlikely place for fish. But there were tilapia, darting to the surface and then back into the murky depths of a tank of brine. Hedgerows of saltbush fit more naturally into the desert setting. A putting-green-sized carpet of salt-tolerant turf grass lay at the end of a row of saltbush.
Through a chain-link fence was another part of the puzzle: a trailer containing water desalination equipment and a shed housing an even more sophisticated water-treatment unit.
Here also was where a finger of the Central Arizona Project canal diverted for irrigation some of its Colorado River water bound for Tucson. It was an ideal setting for a pair of complimentary research projects involving water sustainability.
Salt concentrates in the CAP water as it evaporates while flowing through the canal. One of the challenges in managing the water supply is determining the optimal way to extract salts from the CAP water, which, together with groundwater, provides about half of Tucson's water.
Desalination processes like reverse osmosis, in which water is forced at pressure through membranes, is one way to go, but 20 percent of the water is lost as concentrated brine.
A secondary treatment called Vibratory Shear Enhanced Processing, or VSEP, concentrates the brine in about four percent of the water. But the tradeoff for having more water to drink is that both processes are energy intensive – costly –- and produce brine that must be disposed of somehow.
"The chemical engineering approach is to squeeze as much water out and then find the minimum amount of brine to discharge," said Martin Yoklic, associate research scientist at the UA's Environmental Research Laboratory. "Our approach is to find ways to use the brine, but if it gets too salty, the use is limited."
Yoklic and his research partner, Fiona L. Jordan, assistant research scientist, began to experiment with ways to put the brine to use – irrigating salt-tolerant plants, or halophytes – that can be used as forage additive for animals, and raising tilapia.
Salinity is just one of the many questions water researchers across dozens of disciplines are tackling at the UA.
"Stemming from our land grant heritage, the UA has had a longstanding interest in helping to ensure a reliable, clean water supply for Arizona's citizens," said Leslie P. Tolbert, UA vice president for research, graduate studies and economic development. "We have a number of very highly respected water programs focusing in the science of water and water flow, engineering methods that minimize water use, water conservation and development of sustainability efforts that will apply in semi-arid regions around the globe.
Global warming models make clear that Arizona needs to be resourceful and creative in water use in order to be able to maintain a decent quality of life in coming decades."
SAHRA, which stands for Sustainability of semi-Arid Hydrology and Riparian Areas, is one of the lead interdisciplinary groups studying water issues.
"Hydrology was kind of invented here," said Gary Woodard, SAHRA's associate director for knowledge transfer and international activities. "We had the first hydrology department," organized in 1966. "As near as I can tell, there are more faculty and professional staff at this university involved in water-related stuff than anywhere on the planet."
The desalination side of the fence at the Marana compound is what Robert G. Arnold, professor of chemical and environmental engineering, is focused on. He's studying ways to make the polymer membranes in the reverse osmosis units last longer.
The CAP water is pretreated in a sand-filtration tank to strain large particles that can foul the RO membranes. A microfiltration system does a better job of pretreatment, but uses more energy.
"The Central Arizona Project brings us 200,000 metric tons of salt every year, and none leaves," Arnold said. "The water is gone; it's used, but the salt stays behind, and it has to go somewhere so it's going to end up in our soils or in our groundwater."
Eventually, this will drive up the salt content of groundwater – in 50 years it would double at the current rate. "There's a sustainability issue there that people eventually will have to face – it doesn't have to be today, but pretty soon. They have to decide what to do about salt. And all inland metropolis areas face that problem in the Southwest."
Yoklic thinks he may have an answer. "The ultimate goal is to put a system together where the objective of this industrial ecosystem approach is to take the waste brine and make product out of it, through some high-value system like protein production, and then eventually discharge it into the halophytes that we can harvest for high-value plant additive for animal feed."
On a hot August day, Desirie Soliz, a doctoral student in environmental science, spent her first day at the Marana site, which is a collaboration with the federal Bureau of Reclamation and several local water districts.
"We're studying to see how the plants are reacting with the brine and is it going to get lower in the ground or is it going to stay within the plants," she said. "You don't want it into the water table.
"We're going to do a core sample. We're going to do tree samples. We're going to test how much salt is in the soil. And we'll test how much salt is in the drainage content."
Yoklic said they would also be measuring the root zone of the plants and their biomass production.
"The plants are three years old," he said. "We're going to do a major harvest this fall and do a major data collection and then restart it – probably redo the irrigation lines and then keep it going as long as we can keep sponsorship going on it, because the longer the term of the experiment the better data we'll have."