September 2009, Vol. 21, No.9

The Verdict Is In

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Florida citrus trees prosper when irrigated by reclaimed wastewater.
 
After more than two decades of being irrigated with treated municipal wastewater, the citrus groves of central Florida have reached a verdict. They like it. They really like it.

That’s the finding of a long-term study conducted on behalf of Water Conserv II, a municipal reclaimed-water project operated by the city of Orlando and surrounding Orange County since 1986.

That’s the finding of a long-term study conducted on behalf of Water Conserv II, a municipal reclaimed-water project operated by the city of Orlando and surrounding Orange County since 1986. After more than two decades of being irrigated with treated municipal wastewater, the citrus groves of central Florida have reached a verdict. They like it. They really like it.

That’s the finding of a long-term study conducted on behalf of Water Conserv II, a municipal reclaimed-water project operated by the city of Orlando and surrounding Orange County since 1986.

Water Conserv II is not only the largest of Florida’s 440 reclaimed-water reuse systems, it also is the first project permitted by the state Department of Environmental Protection to use reclaimed water to irrigate not only golf courses and landscape but also crops destined for human consumption. And that, in the beginning, made many Florida citrus growers nervous, according to Kelly T. Morgan, who led the research.

“Initially, the growers had reservations about accepting reclaimed water, especially in the high volumes they were asked to take,” recalled Morgan, who is assistant professor of soil fertility and water management at the University of Florida’s Southwest Florida Research and Education Center (Immokalee).

“They didn’t know what heavy metals or salt might be found in treated wastewater and what impact they might have on their trees and the fruit they produced,” Morgan said.

The growers also were concerned about public opinion. Would consumers balk at buying oranges grown in groves that had been irrigated with treated wastewater?

Morgan’s research couldn’t definitely answer that last question. But his research team could monitor the orchards in Florida’s western Orange and eastern Lake counties to see if reclaimed irrigation water reduced citrus tree health, decreased fruit yields, affected fruit quality, or increased soil contaminants. On all counts, the answer was No.

Differences Are Negligible
The researchers’ findings, published in the April 2008 issue of HortScience, concluded that using reclaimed water for irrigation did little harm — and perhaps even offered a few benefits — to Florida citrus trees.

“By most measures, it was a wash,” Morgan said. The condition, size, and yield of trees irrigated with reclaimed water in the study were typically as good as, if not better than, groves irrigated with well water. The fruit quality was also similar, Morgan said.

The place that reclaimed water made its greatest mark was on the soil, which benefited slightly from the low levels of nutrients present in the water.

“The state of Florida has stringent regulations for water that is applied to agriculture,” Morgan explained. “And those requirements render the water unusable as a fertilizer source.”

Still, given their presence in reclaimed water, both boron and phosphorus could be eliminated from growers’ fertilizer programs. Because reclaimed water maintains a soil pH within the recommended range for citrus, growers in the region no longer need to apply lime to their soil either, Morgan said. They also could reduce but not eliminate the amount of zinc and magnesium applied to their soil due to its moderate presence in reclaimed water.

More Wastewater To Recycle
There was one thing, however, that growers noticed when they began to irrigate with reclaimed water: more weeds. Researchers attributed the increase to the higher soil water content in the fields irrigated with reclaimed water.

What made the soil water content so high?

Under the original 20-year contract with Water Conserv II, growers agreed to accept an acre-inch of reclaimed water a week, an amount that far exceeded their needs during most of the year, Morgan explained.

Growers were encouraged to use higher-than-recommended amounts of irrigation water because, for Orlando and Orange County, the water reclamation program served a second important purpose: It was their way of disposing of the region’s growing volume of treated wastewater.

With a more than fivefold population increase in the last 50 years, Florida has experienced similar growth in municipal wastewater. One reason the Water Conserv program was initiated, according to Morgan, was to meet state requirements for eliminating the discharge of treated effluent to surface waters.

“Increased use of reclaimed water for agricultural irrigation not only reduces the wastewater disposal problem, it also reduces the amount of water that would otherwise have to be withdrawn from Florida’s aquifers for irrigation,” Morgan said. “And that’s important, especially when the state faces local shortages during low rainfall years.”

And consumers don’t seem to mind. “From what I can tell, it’s been a nonissue,” Morgan said.

What happens in Florida, Stays in Florida?
University of Florida researchers concluded that long-term use of reclaimed water on the state’s citrus groves has required relatively little adjustment in crop production practices.

While good news for Florida citrus growers, these findings cannot be applied universally, according to Christopher Amrhein, a professor of soil chemistry in the Department of Environmental Sciences at the University of California (UC)–Riverside.

Amrhein said citrus growers in California and elsewhere have had a somewhat different experience with reclaimed irrigation water. Earlier this summer, in fact, Amrhein voiced opposition to a plan to replace the water in some Southern Californian irrigation canals with treated municipal wastewater. These canals serve, among other places, the 210-ha (510-ac) Citrus Research Center and Agricultural Experiment Station on the UC–Riverside campus.

Amrhein said the proposed switch could damage leaves and cause problems with fruit quality, yield, and soil permeability.

The question is, why might high-quality reclaimed water be appropriate in one geographic region but not another?

Average rainfall and soil conditions are two major reasons, according to Amrhein.

Riverside County receives only about 250 mm (10 in.) of rainfall a year, compared to about 1200 mm (47 in.) in the citrus-growing regions of Florida, Amrhein said.

So, while irrigation is considered supplemental in Florida, it is required for citrus production in California and constitutes a greater percentage of the water citrus trees receive, increasing its potential impact.

Florida’s coarse-textured, sandy soils also absorb substances found in reclaimed water, such as boron, more easily than the clay- and silica-rich subsoils of Southern California, Amrhein said.

“Citrus plants are very sensitive to boron,” Amrhein said, “and the concentration in the reclaimed wastewater is likely to cause leaf damage and yield reductions. In addition, the higher sodium content in reclaimed water will reduce soil permeability and make leaching of accumulated salt a problem.”

As proof, Amrhein cites a 6-year field study conducted near Escondido, Calif., where avocado trees irrigated with reclaimed water had a 42% yield decrease, compared with trees irrigated with municipal water. Trees irrigated with a 50–50 blend of reclaimed water and municipal water experienced a 27% reduction in yield.

To address such concerns, some countries have begun making changes to make reclaimed water more acceptable for irrigation, Amrhein said. For example, boron has been banned from laundry detergent in Israel, where reclaimed water is used extensively for irrigation.

For the use of reclaimed water in agricultural irrigation to grow, other shifts in practices — and in attitudes — are also needed, said Jay Prager, deputy program manager of the Wastewater Permits Program in the Maryland Department of the Environment.

“In many places, people still don’t think of reclaimed water as water reuse but as a waste disposal method,” Prager said.

“At some point, it’s going to seem odd that we’re flushing toilets with drinking water and treating our wastewater to drinking standards and then disposing of it,” Prager said. “Whatever happens, the future needs to be different than the present.”

— Mary Bufe, WE&T