From The Aspen Daily News (Curtis Wackerle):
Building concrete bulkheads in 130-year-old mine tunnels and stopes that connect Aspen and Smuggler mountains underneath town could form storage vaults capable of holding between 1,000 and 2,000 acre feet of water, according to a report from Deere and Ault Consultants of Longmont, presented in a work session to Aspen City Council last week.
The report listed pros and cons of using underground mines, which are an alternative approach to water storage the city is investigating as it looks for options other than 150-plus-foot tall dams creating reservoirs on upper Castle and Maroon creeks. Council also heard a presentation on Monday from Deere and Ault about a preliminary review of “in situ” storage — meaning in lined storage vaults underground where water mixes with gravel and is then covered. The golf course was identified as a site that could lend itself to such a reservoir, the consultants told council.
Deere and Ault’s review of underground mine storage, resulting in a 20-page report, found that the pros of using Aspen’s underground mines for storage — such as proximity to water rights and infrastructure, usefulness as drought hedging and good baseline water quality — to be outweighed by the potential cons.
“Maintaining dominion and control” of stored underground water is first among those concerns, according to consultants. Even with a complex system of bulkheads at virtually every level in the mines, “water could still potentially leak out through the natural faults, shear zones and fracture zones,” the report says.
Constructing the vaults would be hazardous, since water that collects naturally in the tunnels would have to be pumped out. When water is pumped out of old mines, “that’s when collapses occur,” Victor deWolfe, with Deere and Ault, told the council.
Raising the water table — hard to avoid with underground storage — also carries risk. It could create more surface landslides and exacerbates flooding issues. And while water occurring in the limestone layers has tested for good quality overall, raising it and lowering it above the water table could “exacerbate metal leeching and acidification.” That contaminated water could then leak into neighboring freshwater streams.
The city would also have to build an expensive pumping and pipeline system to connect the vaults to the water treatment plant near the hospital. New treatment techniques and infrastructure may also be needed to deal with alternative water storage method.
“We noted that there is no real precedence in Colorado for storing raw water in underground hard rock mines for municipal use,” the report says.
The consultants did envision a possible system, however, where water from the Salvation Ditch is used to fill the Molly Gibson Shaft. Or, bulkheads could be installed to raise water levels beneath Aspen Mountain, and the city could tap water flows coming out of the Durant Tunnel at the base of the mountain, where it owns a 3 cubic feet per second water right.
Aspen’s complex geology is well understood, the consultants note, and their study drew on maps showing the many rock layers and mine tunnels. The pond at the current Glory Hole Park, off Ute Avenue, formed in the mining era when alluvial soils were swallowed by a sinkhole, taking a locomotive and two boxcars with it.
Council members thanked the consultants for the report — the study, which included site visits, cost the city $15,000 — but showed little appetitive for pursuing the issue further. Councilman Bert Myrin said it’s crucial for the city to determine how much water it needs to store before it can make sense of options.
Deere and Ault will continue looking into the “in-situ” reservoir concept, which also could yield 1,000 or more acre feet of storage.