by Jean E. McLain, Associate Director, WRRC
Agricultural production is vital to Arizona, contributing over twelve billion dollars to the State’s economy each year. It is also by far the largest water-consuming sector, accounting for about 70 percent of water demand in Arizona. With water shortages looming, water recycling could give some agricultural operations a much-needed additional water supply.
While no federal regulations exist pertaining to recycled water use for agricultural irrigation, there are recommendations set forth by the U.S. government. Originally drafted in 1980, the United States Environmental Protection Agency (EPA) Guidelines for Water Reuse established minimum standards and criteria recommended for the use of recycled water for agricultural irrigation. The EPA guidelines, which were updated in 2004 and again in 2012 to conform to advances in water treatment technologies, provide states with criteria to establish their own regulations on the use of recycled water.
Using the EPA guidelines as a model, the State of Arizona has established regulations on the use of recycled water for agricultural irrigation. These regulations are being revisited (see Guest View this issue), but the rules governing agricultural irrigation are not likely to change substantially.
Arizona’s recycled water standards establish five classes of recycled water, each requiring a specific minimum treatment in order to meet a limited set of numeric water quality criteria (TABLE 1). Class A is the required minimum quality of recycled water for irrigation of food crops, including vegetables that are eaten raw, while Class B suffices for surface irrigation of an orchard or vineyard. Forage, fib r, and seed crops require even less stringent (Class C) water quality. Though Arizona’s agriculture industry is extremely diversified, producing cotton, alfalfa, wheat, citrus, and fresh vegetables, all of which can be legally irrigated with recycled wastewater, the majority of crops currently being irrigated with recycled water are non-edible crops such as alfalfa, feed corn, cotton, and grasses.
Most agricultural use occurs near metropolitan areas, where high demand for freshwater sources exists and where water does not have to be moved great distances from wastewater treatment plants (WWTPs) to agricultural fields.
It is difficult to gauge the full extent of recycled water use for agriculture in Arizona, but a study by a University of Arizona graduate student, Jeremy Cusimano, found that in 2012, permitted reuse amounted to 140,000 acre-feet (AF) in Central and Eastern portions of the state. In addition, the Gila River Indian Community receives 40,000 AF in treated water from the cities of Chandler and Gilbert. In the Yuma area an abundant and extremely low-cost supply of Colorado River water has precluded widespread use of recycled municipal or industrial wastewater for agricultural irrigation.
In addition to permitted use, incidental reuse for agricultural irrigation from rivers that contain treated effluent is undoubtedly common, as treated wastewater is often discharged from WWTPs into dry riverbeds. For example, approximately 50 miles southwest of Phoenix, downstream irrigators in the Gila Bend area use the treated wastewater that dominates the flow of the Gila River.
In many cases, the use of recycled water for agriculture is a money-saving measure. While recycling water is energy-intensive, performing wastewater recycling on site or close to an agricultural field reduces the energy needed to move water longer distances or pump water from deep within an aquifer. On the other hand, growers can face several challenges with recycled water use. Often, recycled water is only readily available when irrigation demand for crops is low, such as in the winter months. In summer, increased demand for water to irrigate parks and other municipal green spaces decreases the availability of recycled water for agricultural irrigation.
Growers also may be reluctant to increase dependence on recycled water due to instability in long-term supply. In many cases, newly constructed WWTPs provide growers with recycled water only temporarily until supplies are diverted to other uses. For example, if urban development occurs near a WWTP, recycled water can be transferred away from agricultural use to irrigate urban landscapes.
Even when recycled water is readily available, growers may be reluctant to utilize it for crop irrigation. One issue, perhaps the most difficult to overcome, is the negative public perception that may remain regarding the use to recycled water for food crops. A 2008 survey of Arizona residents by Channah Rock, a University of Arizona researcher, and colleagues at Northern Arizona University found that while 58 percent of Arizona residents supported the use of recycled water for irrigation of non-edible crops, far fewer, - 28 percent, - found this policy acceptable for edible crops. Thus, it would be likely that growers with a range of water supply options may choose not to irrigate with recycled water.
Another issue associated with recycled water use is salinity. Recycled water typically contains a concentration of total dissolved solids that ranges from 800 to 1400 parts per million (PPM), and can be higher in some cases. In comparison, water delivered through Central Arizona Project canals in 2013 averaged about 600 PPM.
The high salt concentration of recycled water can be managed in agriculture by periodically applying a sufficient amount of excess irrigation water to flush salts below the root zone. Put simply, a grower can maintain the root zone salt balance by applying enough excess water to carry the same amount of salt out of the soil as the water itself brings in. If recycled water is readily available in adequate supplies, the cost savings of using recycled water instead of CAP or groundwater may make applying excess irrigation water to leach salts worth the additional cost. It is also important to note that many crops irrigated in Arizona with recycled water have a fairly high tolerance to salinity; these include barley, wheat, Sudan grasses, date palms, and cotton.
Finally, grower concerns may be related to the presence of contaminants of emerging concern in recycled wastewater. Though wastewater technology has made rapid advances in the production of a clean, safe water, very low (parts per trillion) levels of contaminants, including pharmaceuticals and cleaning agents, may remain. Some recent research has suggested that plants have the ability to take up such contaminants and thus, bring them into the human food supply. Many of these studies, however, involved hydroponic systems and excessively high concentrations of contaminants, while well-controlled field studies do not show uptake for the most part. Clearly, this is an area requiring more attention.
As Arizona’s population increases and demand for fresh water supplies escalate, the agriculture industry will probably become more dependent on recycled water. As Arizona grows, recycled water supplies will grow and will be available to meet more of the State’s water demand. With the growing availability of recycled water, it is important both to monitor its use in agricultural irrigation and to provide growers with a full accounting of its benefits and concerns regarding its use.