Denver Water and engineering partners resolve major water quality challenge in crucial South Platte River exchange reservoirs.
Two years ago, Denver Water faced a big challenge at four of its downstream exchange reservoirs along the South Platte River in Commerce City.
Dissolved oxygen levels in the water were plummeting, leading to the generation of a gas called hydrogen sulfide that can damage equipment and affect water chemistry.
The root of the problem was tied to water quality.
The water routed into the reservoirs comes from the South Platte River downstream of Denver and carries discharge from upstream water treatment plants, as well as runoff from urban areas.
The water is rich in nutrients, and when contained in a relatively small reservoir like those along the river, the “bacteria and algae go nuts,” said Daniel Babyak, construction project manager for Denver Water.
Those also are conditions that consume the dissolved oxygen in the reservoir bottom and lead to hydrogen sulfide production.
The conditions had Denver Water and state health officials concerned. They also kept the utility from releasing water downstream due to the poor water quality. Such releases are an important part of how water supplies are exchanged in Colorado and help Denver Water gain access to water higher upstream for its customers.
Dunes Reservoir, one of Denver Water’s North Complex reservoirs near the South Platte River, benefits from an aeration system that keeps water oxygenated. Patches visible on the surface result from underwater equipment that injects air and keeps the water churning. Photo credit: Denver Water.
Denver Water gets half of its water supply from the Colorado River. Because that water comes from the West Slope and is not native to the Front Range, the utility can use the water multiple times, including storing it in its downstream reservoirs. Denver Water can then release the water stored in these reservoirs when it’s needed — instead of releasing water from mountain storage. That helps keep water in the mountains while making the most of the water available.
But the water quality issue was limiting Denver Water’s flexibility in how it used its downstream reservoirs.
“We can only release water back to the river that has an equal or better quality than when it was first diverted from the river,” recalled senior engineer for Denver Water Brett Balley.
The four reservoirs — Welby, Bambei-Walker, Dunes and Tanabe — are part of Denver Water’s North and South reservoir complexes in the Commerce City area.
A pipe carries air from a nearby compressor station into Dunes Reservoir northeast of Denver. The injected air keeps the water in the reservoir churning and oxygenated. Photo credit: Denver Water.
Denver Water brought in a team of lake scientists who studied the problem and suggested solutions. Then the utility partnered with Brown and Caldwell and Filanc on an unusual “design-build” contract that reduced the time to design, construct and install a system from 24 to 14 months.
The solution involved installing porous hoses running thousands of feet in length near the bottom of the four reservoirs. Compressed air is pumped into the hoses and dispersed through small openings.
That adds oxygen to the water and creates a mixing effect in the reservoir that moves water around, preventing the lake from becoming stratified by oxygen levels and temperature, conditions that lead to problems such as hydrogen sulfide development.
“Pumping compressed air into the water and keeping the reservoir mixed is good for water quality and allows us to release this water when we need to,” Balley said.
At times, the four reservoirs — each 30 to 50 feet deep and collectively holding nearly 6,000 acre-feet of water — are turning over their water supplies every year, as water is released to downstream users and the reservoir is refilled over the course of the winter, spring and summer.
“These reservoirs don’t always get as much respect, but they’re valuable,” Babyak explained. “When these things are up and going, they’re real workhorses.”
The project was unusual for Denver Water in a couple of ways.
The utility is far more accustomed to managing water from its supplies higher up in the watershed, collected in big mountain reservoirs like Dillon and Gross. That’s a far different process than collecting nutrient-rich water in the smaller downstream reservoirs for exchange purposes.
In part because of that inexperience, the utility sought an agreement that put greater responsibility in the hands of the contractor designing and building the project.
These design-build contracts differ from the more common design-bid-build contracts. In a design-bid-build contract Denver Water is more deeply involved in both the design and the construction of the remedy.
The Fulton Ditch brings water from the South Platte River to North Complex reservoirs near Commerce City. Photo credit: Denver Water.
A design-build contract “helps with the timeline and puts accountability on just one firm,” Balley explained.
Added Babyak: “It removes the start-stop nature of these larger projects where you have the intricacies of multiple contractors and contracts in play.”
Denver Water is installing similar systems in three additional North Complex exchange reservoirs still in development as well as two similar reservoirs planned for the Lupton Lakes complex near Fort Lupton in the future.
Once completely built out, the reservoirs will hold roughly 32,000 acre-feet of water. That’s more than four times the capacity of Denver Water’s Strontia Springs Reservoir southwest of Denver and 1 1/2 times the storage in Antero Reservoir in Park County.