Colorado State University Study Shows Chemicals in Recycled Water Should be Considered When Used in Landscapes

More urban area landscape managers are turning to recycled wastewater for irrigation in the wake of Colorado’s recent dry years, but they’ll need to consider possible higher levels of certain chemicals present in recycled water, and the potential effect on plants. A Colorado State University preliminary study shows that management practices will be important in considering higher levels of chemicals, such as those in soaps and detergents, left in the wastewater after it is recycled from human use.

Treated wastewater, used increasingly by landscape managers on golf courses, large parks, open spaces and greenbelts, contains different chemical properties than other water sources. Recycled, or treated, wastewater is urban sewage that is treated to state and federal government regulations, but is not treated to drinking water standards. Treatments remove solids and reduce levels of pathogens such as E. Coli, as well as nutrients such as nitrogen and phosphates.

As other water sources along the Front Range dwindle, such as raw water —  or untreated well, lake or stream water – and potable water, which is treated water that is safe to drink, treated wastewater sources increase. In fact, wastewater treatment plants discharge 72 billion gallons of effluent water directly into streams in the South Platte River basin each year.

"New water supplies along the Front Range are not currently available and are costly to develop, so using recycled wastewater is an environmentally-sound practice. It can improve water-use efficiency, save a substantial amount of water and help provide nutrients to plants that they need to grow and maintain their health," said Yaling Qian, Colorado State University associate professor who led the study within the Department of Horticulture and Landscape Architecture. The study was funded by the Colorado Water Resources Research Institute, a branch of Colorado State University.

"However, it’s not a picture-prefect option because of different chemical properties that recycle water possesses as opposed to raw water. Some chemicals in detergents and water softening agents are not removed in treatment plants, and managers should make allowances for the levels of salt, sodium and boron in recycled water."

Over the long-term, about 5 to 15 years, these chemicals can impact the soil and plants in a landscape, particularly by making the soil salty, and recycled wastewater contains high levels of ions – sodium, chloride and boron – that are toxic to some trees and shrubs. However, some chemicals in the wastewater, such as nitrogen, can benefit plants.

The study also indicated that landscapes watered with recycled wastewater may be more prone to drought, and some conifer trees’ health within the landscape appeared to decline over a period of time if they were in an area with poor drainage or an area with a shallow water table.

For the study, Qian took samples of recycled water from sites in Englewood, Aurora and Westminster where recycled water was used to irrigate landscapes and then tested the water for chemicals. She also collected soil samples from sites watered with recycled wastewater over varying lengths of time and compared them to soil samples from sites where recycled water was not used.

Study results indicate that potential problems exist when supporting landscape needs with recycled water; high levels of sodium and bicarbonates in the water often change the soil’s chemical and physical properties. Some of those changes can be countered with landscape care strategies, such as amending the soil with gypsum to reduce higher levels of sodium, and selecting salt-tolerant grass and plants.

In addition, the study indicates that landscapes irrigated with recycled wastewater may also need more water than average because the soil will need to be "flushed" to rinse out salt left behind.  

"We need to continue to study the effects of recycled wastewater on landscapes," said Qian. "And, landscape managers should be sure to be up-to-date on information about managing landscapes watered with recycled wastewater before they begin and while they’re using this type of treated water. The community can help reduce the sale problem by minimizing the use of water softeners and detergents with high sodium contents."  

This year, Qian will continue studies of the impact of recycled wastewater, including looking at how using recycled wastewater can impact the chemical composition in plants as well as options to mitigate excessive chemicals.


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