Will Denver’s future water reservoirs lie underfoot and not behind dams?
by Allen Best
We think of reservoirs as bodies of water, places created by dams where you can go sailing or fishing. Denver Water is investigating whether Denver’s future reservoirs will lie several hundred feet below the feet of its customers in aquifers called the Arapahoe and Laramie-Fox Hills.
Aquifer recharge has been used in many places as a way to store water. Arizona, for example, stores water for Las Vegas in an innovative partnership as well as water for its own use. In metropolitan Denver, the Centennial Water and Sanitation District, which serves Highlands Ranch, has also been pumping water into an aquifer, for withdrawal when needed. Others in the Denver area have also used it, with various degrees of success.
Denver has 17 reservoirs already able to store a combined maximum of 690,000 acre-feet. The adequacy of that storage is challenged by the uncertainties posed by the changing climate and continued population growth, said Bob Peters, a water resource engineer with Denver Water, speaking at a National Groundwater Association conference in Denver on April 25. Among the options now being studied is whether the aquifers underlying the city could also provide storage.
The city is bisected by the South Platte River. For most of the year, the river is over-appropriated, meaning there is no new water to be claimed. Furthermore, many of Denver’s existing rights from the South Platte are junior, meaning Denver might be left short in years of little snow or rain.
In a PowerPoint presentation, Peters also showed a variety of scenarios, all depicting gaps between needs and supplies. Denver is pursuing stepped-up conservation and greater reuse.
Denver also wants to divert more water from the Colorado River Basin through its Moffat Tunnel delivery system near Winter Park. That Moffat system expansion would include raising the height of Gross Dam, located southwest of Boulder, by 125 feet, nearly tripling the capacity of the reservoir. Denver has not received final authorization from the U.S. Army Corps of Engineers.
Aquifer storage might also play a role in Denver’s future. Pumping water underground results in no evaporation, Peters said, requires fewer permits, and has less of an environmental footprint. Plus, it’s less costly than above-ground storage and can be done in small increments, unlike dams.
Challenges include figuring out where to put wells in urban areas, questions about the quality of water to be injected, and uncertainty about how much the water can later be recovered.
“We know it’s feasible. The question is whether it will work for Denver Water,” said Cortney Brand, of Leonard Rice Engineers, a consulting group.
Brand outlined Denver’s aquifers. The Arapahoe Basin is 500 to 2,100 feet thick, but the water-bearing sands of that formation are only 150 to 250 feet thick and not necessarily in one seam. The water-bearing sands of the Fox Hills has average thickness of 382 feet. These are averages for wells logged within Denver, but the city is only 2.5 percent of the much broader Denver Basin.
But the understanding of what lies underneath is not as sharp as those figures might suggest. To get a clearly image of the ability of the aquifers in specific areas to store water, three or more wells are being drilled this year.
With those additional wells, he said, engineers expect that they can deliver designs and cost estimates of a pilot project for an aquifer storage and recovery project by the end of 2016.
How much storage might these wells provide? The study intends to answer that question, but Denver Water’s website suggests that nobody should expect a quick Dillion Reservoir. One recharge sit could store an estimated 20 to 150 acre-feet of water per year. That compares with the 7,863 acre-feet stored by Denver’s smallest surface reservoir, Strontia Springs.
For more information about Denver’s project, see the agency’s website explanation.
For more about the conference, see: https://ngwa.confex.com/ngwa/2016gws/webprogram/Paper10883.html