Non-potable water saves money, but some say it’s contributing to killing off trees earlier than normal

About 90 mature evergreen trees — some up to 100 years old — have died in a patch of mountain forest in the middle of Washington Park, and neighbors are blaming the recycled irrigation water.

The park has been irrigated with recycled water — storm and wastewater cleaned up not quite to drinking standards but OK for lawns and other landscaping — since 2004. But in the last three years the canopy shading what is known as Evergreen Hill has begun to disappear.

Last month alone, 21 dying conifers — mostly blue spruce and Douglas firs — were removed from the hill along Virginia Avenue near Lafayette Street at the north end of the park.

“This last one was really shocking to people,” said Sonia John, a board member of the Friends and Neighbors of Washington Park organization.

Denver Parks and Recreation began using recycled water because it’s less expensive and conserves drinking water, but experts say the water contains more salt, which makes it harder for trees to absorb water. Neighbors want the city to take action before more of the urban canopy is harmed.

“I personally have seen damage on (trees) that I’m pretty sure we can pinpoint to reclaimed water,” said Colorado State University professor of landscape horticulture James Klett, adding that he’s taken samples and seen decline at places such as the Denver Country Club and the Park Hill Golf Course, which also use recycled water.

Conifers are affected more because they keep their needles throughout all four seasons, absorbing more of the salt, Klett said.

Denver Parks and Recreation deputy director Scott Gilmore said he believes the recycled water has contributed to the demise of some of the trees, but he said other factors are also at play.

Horticulturists point to climate conditions, such as last year’s November temperature plunge that saw a 77 degree drop in 48 hours. Soil quality can vary, disease and pests can attack, and even heavy construction can stress trees. But the salt is definitely a factor.

“It does impact the trees, I’m not going to sit here and lie. There are other things, though,” Gilmore said, adding that weaker trees are more susceptible to other agents and sometimes cannot recover.

Evergreens in Washington Park in Denver have been dying. They have replanted new evergreens on Evergreen Hill which is located on the north side of the park. A walker strolls past a new evergreen on the trail along E. Virginia Ave. on Monday, November 16, 2015. (Cyrus McCrimmon, The Denver Post)

Washington Park has lost 103 conifer trees during the past six years, including 39 this year, with 36 coming from Evergreen Hill, according to Parks and Recreation. More than 400 conifer trees have been planted at Washington Park since 2007, including 125 on Evergreen Hill. Those trees are not yet mature and many still are tied to wooden poles. Conifers can live past 150 years and can take more than 30 years to reach maturity.

Now, the Friends group and the Inter-neighborhood Cooperation (INC) — Denver’s association of registered neighborhood organizations — want the city to take action to try and mitigate the damage to trees in parks using recycled water.

“The ultimate goal is we preserve all the park trees and don’t do damage to them,” said Cindy Johnstone, president of the Friends and Neighbors of Washington Park and chairwoman of the board of the INC Park and Recreation Committee. “You can save the money, but if they have to keep trying to figure out how to replace these trees, that costs something, too.”

INC has also written a letter to Mayor Michael Hancock asking that the parks department supplement the irrigation with potable water to flush out the salt content throughout the year and continue to study the issue to find a solution to reduce the amount of sodium.

“Our feeling is, it’s well enough demonstrated that the recycled water is causing the problems. Let’s at least do the easiest mediation measure now as a stop gap, which is watering with pure water to buy some time,” John said.

Denver Water opened its recycled water plant in 2004 in Commerce City, and Denver Parks and Recreation began using it at certain parks throughout the system. Today, about 20 percent of Parks and Rec irrigated land uses recycled water, saving almost $1 million per year, according to Gilmore.

Denver Water contracted with CSU to test the soil of those areas. A study released in 2010 showed sodium increased 2.7 percent on average at the sites that were tested, including Washington Park.

Gilmore said his department and Denver Water believe a new study from Colorado State out as soon as next month will show reduced amounts of sodium. The two organizations plan to form a working group made up of representatives from several organizations and members of the community to discuss solutions to the problem.

But there are no plans to stop using recycled water.

“As communities look to expand their water sources, having multiple sources keeps us able to have supply,” said Russ Plakke, water treatment plant supervisor for Denver Water. “We have to be more mindful of the resources available to us and minimize how the population meets the environment.”

In Cheesman Park, the city started using recycled water in 2013 and the park has not seen the amount of turnover in conifers that Washington Park has, with only 16 conifers taken down since 2009. At present, Cheesman has 541 conifers to Washington Park’s 456.

Klett said he doesn’t know if you can totally lay the blame on recycled water, but he said it’s certainly a contributing factor.

“Some of them I’m sure would still be alive” had recycled water not been used, Klett said of the trees at Washington Park. “It’s a dilemma. You hate to lose these trees they’ve been maintaining all these years.”

Joe Vaccarelli: 303-954-2396, jvaccarelli@denverpost.com or @joe_vacc