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Environment

Denver’s Urban Waterways Are Flowing With Tap Water From Your Sprinkler

According to a new study from Colorado State University, an influx of water from front yards keeps city creeks active during the summer.

If you’ve biked along Little Dry Creek in Westminster this summer, or walked your dog near Dry Gulch downtown, you may have noticed something unusual: water. As their names suggest, these channels aren’t known for their flow. So where’s the water coming from? A new study points the finger at your front yard.

Using water samples from the summer of 2019, researchers at Colorado State University determined more than 65 percent of water flowing through Denver’s urban streams is tap water. That’s an eyebrow-raising figure for a region constantly contending with water issues.

Samples for the study came from 13 urban streams, including Lakewood Gulch and several points along Little Dry Creek . For reference, the researchers also collected samples from two natural grassland streams on Rocky Flats National Wildlife Refuge, and from taps at various gas station and restaurant restrooms across the Denver metro area.

Back in Fort Collins, the researchers analyzed isotopes from the samples. A small percentage of molecules in water are naturally “heavy,” containing an extra neutron or two. The ratio of heavy to light isotopes differs with altitude. That makes it a useful analysis method in Denver, where tap water comes from the mountains, while urban creeks theoretically drain water from the prairie.

During the 2019 research season, the reference streams in Rocky Flats dried up, but the urban creeks kept flowing. Analyzing the isotopes showed that water in urban streams didn’t match the grassland streams, but instead showed a similar source to the gas station and restaurant taps.

“For many days of the year, the water looks more similar to water in the mountains,” says Aditi Bhaskar, an assistant professor of environmental engineering at Colorado State and a co-author on the paper. That means precipitation isn’t a key contributor to urban streams, she says. “Tap water is playing a bigger role.”

Tap water can enter streams in a few ways. Wastewater is one path, but none of the 13 Denver streams had sewer inputs. Water utilities inevitably lose some water through leaky pipes, overflowing tanks, and firefighter efforts, but Denver’s reported water loss of four percent is similar to other semi-arid cities (Los Angeles has a rate of six to eight percent).

Lawn irrigation is the last major option. That includes water running down the sidewalk to neighborhood drains, along with excess water seeping through saturated yards. Watering lawns can account for 50 percent of a city’s water budget in Western cities, and the Mile High City seems particularly built to overwater. A 2016 survey by Denver-based Water Research Foundation found 74 percent of Denver homes had in-ground sprinkler systems installed.

Overall, though, urban water use is decreasing, according to Travis Thompson, Denver Water’s communications manager. The ten-year-old “Use Only What You Need” campaign—which plastered the metro area in orange billboards and bus ads discouraging overwatering—helped cut per-person use by 22 percent, he says. The utility sends its customers an annual mailer on summer watering rules and uses recycled water to irrigate parks. “Our customers have done a great job conserving water and increasing their efficiency, especially in the years since the 2002 drought,” he says.

Bhaskar agrees. “Even though Denver and other cities in the Front Range are growing in population, their total water use is actually going down,” she says. That’s thanks in part to indoor innovations like more efficient appliances, but “there’s room for further reductions in outdoor water use,” she says. That can be as simple as ensuring sprinklers are aimed at plants and not the pavement. Every drop that hits the driveway contributes to water loss.

(Read more: Could Greywater Recycling at Home Be an Answer to Denver’s Water Woes?)

Without historical streamflow data, it’s hard to say whether urban waterways like Dry Gulch should actually be dry. But the reference streams in Rocky Flats were dry for 40 percent of the summer in 2019. It’s reasonable to assume urbanized streams would behave similarly in a natural state.

“Dry streams might be closer to what we had before, before they were supported by lawn irrigation,” Bhaskar says.

So, if your neighborhood gulch is suddenly dry, that’s not necessarily evidence of a water crisis. It could be a good thing; a sign that upstream neighbors are watering more efficiently and returning the Denver area’s urban streams to a natural, drier summer state.

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