Salt of the earth: How evaporite problems evolved
While the nearest seaside property is far from Colorado now, that wasn’t always the case.
The state continues to be reminded of that fact today, with problems ranging from sinkholes to salinity in water, and also a benefit in the form of gypsum mining.
According to a new Colorado Geological Survey report principally authored by Jonathan White, from 300 million to 150 million years ago, much of what is now Colorado was near sea level. As shallow seas, near-shore desert estuaries, and saline lakes variously came and went, water evaporated, leaving behind solutions concentrated with salts and sulfates. These minerals then precipitated out and slowly accumulated into thick deposits that turned to bedrock.
That bedrock since was buried by younger sediment deposits. But after the rise of the Rocky Mountains, millions of years of erosion and downcutting of rivers has exposed some of these evaporite rocks at the surface.
From a geological hazard standpoint, the problem with that bedrock is that it is dissolved by fresh water, the report says.
“The dissolution of evaporite rock alters ground and surface water flows, and creates subsurface voids such as caverns, open fissures, and solution pipes. Collapse of these subsurface voids manifests itself at the surface as subsidence, which can be a geologic hazard and risk for structures located thereon,” it says.
Evaporite rock also can deform and “flow” underground, in a process called salt tectonism, and can pierce through overlying rock layers.
“The evaporite rocks are eventually exposed at the surface or at shallow depths where they are more influenced by surface weathering, exposure to fresh water, and increased solubility so that regional subsidence occurs,” according to the report.
That is, in addition to creating features such as sinkholes and ground depressions, evaporite dissolution and salt tectonics have created what geologists say are large-scale, regional collapse centers marked by a general lowering of the ground surface. Such centers typically correspond with areas with sinkholes and similar hazards, and have been identified in the Carbondale and Eagle areas, in the Buford area east of Meeker, and in the Paradox area in Montrose County. In the Paradox Valley, the rise of an underground salt dome and its subsequent dissolution and collapse led to the creation of the oddity of a valley cut perpendicularly rather than lengthwise by a pre-existing river now known as the Dolores. Hence the valley’s name.
Regional collapse of some 4,000 feet has been identified in the Carbondale area. A possible collapse center also has been identified in southeastern Colorado.
How fast collapse areas are subsiding isn’t known, White’s report says.
The risk of damage from regional collapse, as opposed to things such as sinkholes, is also unknown, “but likely very low for current or planned developments for the design life of normal residential structures,” the report says. “Movement over geologic time, ranging from hundreds to thousands of years, could still be significant. For the planning and construction of long-term or critical facilities, more in-depth study of the collapse regions should be considered.”
With such massive dissolution of evaporite, all that salt has to go somewhere. Unfortunately, it ends up in rivers and water wells, according to the report, which notes how prolific names such as Salt River and Alkali Creek are in areas of evaporite rock.
The hot springs in Glenwood Springs, the biggest single source of salt loading in the Upper Colorado River Basin, have been estimated to contribute 141 cubic yards of dissolved salts a day into the river.
“At current concentration rates, this spring alone could account for a cubic mile of evaporite dissolved and washed down the river in 100,000 years,” it says.
The U.S. Bureau of Reclamation’s Colorado River Basin Salinity Control Program is addressing evaporite issues in the region. One of its projects intercepts saline groundwater from evaporite rocks before it reaches the Dolores River in the Paradox Valley. Another, along the White River outside Meeker, involved plugging abandoned oil and gas wells that were leaking saline water from evaporite, the report says.
But one common form of evaporite has proven to have significant economic value. Gypsum has been mined in many areas in the state. The substance is mined near its eponymously named town, Gypsum, in Eagle County, by the American Gypsum Co., which uses it to make wallboard in a plant in the town.