How much water is under #Moab? Scientists say it’s less than once thought — KSL.com #ColoradoRiver #COriver #aridification

Colorado River near Moab, Utah.

From KSL.com (Graham Dudley):

Moab City and Grand County are reckoning with recent studies that suggest their underground water supply might not be as abundant as originally thought.

Now the city is working to solidify an estimate and determine what restrictions or changes might be necessary to keep the growing area and thriving tourist destination hydrated.

In the early 1970s, a study from U.S. Geological Survey estimated there was 22,000 acre-feet of water entering and leaving the Spanish Valley aquifer system each year…

But a 2019 study from the Geological Survey and a 2020 article in the Journal of Hydrology suggest that there’s actually more like 13,000 to 15,000 acre-feet of water recharging the aquifers.

There are two main aquifers supplying water to the area: the valley-fill aquifer and the Glen Canyon Group aquifers. The city’s culinary water comes entirely from the Glen Canyon Group aquifer, particularly its deeper sections. Douglas Kip Solomon, a University of Utah geologist who helped author both recent reports, told KSL.com that “essentially all” the water recharging the aquifer each year is already being withdrawn for use, about 3,600 acre-feet per year between all entities.

In other words, withdrawing more water would require “mining” the aquifer, or taking out more than is going back in. “There just isn’t any unaccounted-for water,” Solomon said, “that was somewhat, I think, previously assumed.”

Why not just use another source, like the valley-fill? Solomon said the water rights from the valley-fill aquifer and the shallow Glen Canyon waters are already claimed and are used primarily for irrigation and agriculture. They are treatable, he said, but not as high-quality as the Glen Canyon Group waters.

“Water from the Glen Canyon Group aquifer, especially the deep aquifer that the city of Moab uses, is outstanding quality water,” Solomon said. “Just the right amount of salt to be really tasty. It’s thousands of years old, it’s free of contamination — it’s just an excellent source of water supply.”

Solomon said the City of Moab will “have to really think about other sources of water” other than drilling wells into the Glen Canyon Group aquifer. “They may have to think about using water from the Colorado River,” he said, but that’s an “expensive proposition.”

[…]

[Mike] Duncan wants the city to start carefully measuring how much water it’s using, tracking its future commitments and, if necessary, considering a quota system for future allocations. “The city has plenty of water rights,” he said, “but that’s not the issue anymore. How much real water do we have to use?”

Other potential sources include Mill Creek, surface water supplied from the Glen Canyon Group aquifers, which is currently used agriculturally by the Moab Irrigation Company. There’s also the valley-fill aquifer, but its waters would be expensive to treat, and drawing it down could have environmental impacts. Using Colorado River waters would also be expensive.

Every option has its tradeoffs, Duncan and Solomon agree, but it’s important to start this conversation now.

Graphic credit: The Journal of Hydrology

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