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Home arrow Uranium Mining arrow In-Situ Leaching
In-Situ Leaching Print

Regardless of what they call it...

  • In-Situ Leaching
  • In-Situ Recovery
  • Solution Mining

It all means the same thing: changing water quality to leach uranium out of an aquifer.

In-situ leaching is planned in the northern area of the Centennial Project where deposits of uranium are 250 to 600 feet deep.

Location of Centennial Uranium Mining Project
Location of Centennial Uranium Mining Project

Detailed View of North and South Ore Bodies in the Centennial Project Area
Detailed View of North and South Ore Bodies in the Centennial Project Area

In situ leaching (ISL) for uranium is not a new technology. The first commercial ISL mine in the United States was licensed in 1975. ISL is the cheapest, and therefore the preferred, method of uranium mining companies. Three criteria must exist in the ground formations for in-situ leaching to work:

  • The uranium deposit is within a porous layer of rock such as sand or sandstone.
  • The layer is saturated, usually within an aquifer.
  • The layer is confined between rock layers that are not porous such as clay or shale.

The northern area of the Centennial Project meets the criteria for in-situ leaching within the Laramie-Fox Hills aquifer.

The Laramie-Fox Hills, the Arapahoe, the Denver and the Dawson are the four aquifers that make up the Denver Basin system. These four aquifers are layered within five geological layers.

The Laramie-Fox Hills aquifer covers most of the Denver Basin area, approximately 7,000 square miles along the Front Range from Wyoming to Colorado Springs and east to Limon.  The Laramie-Fox hills aquifer is below the Arapahoe aquifer, the source of Colorado’s artesian Deep Rock Water. 

In most areas the Laramie-Fox Hills aquifer and the Arapahoe aquifer are separated by 400 feet of shale, coal, siltstone and sandstone.  However, this layering has been altered by uplifts and faults.  The Laramie-Fox Hills is above the greater Dakota-Cheyenne aquifer.  These two aquifers are typically separated by as much as 7,000 feet of Pierre Shale. The depth to the top of the Dakota-Cheyenne aquifer “ranges from zero where it crops out along the Front Range on the western edge of the Denver Basin, to greater than 9,000 feet below ground surface near the center of the Denver Basin.” (Ground Water Atlas of Colorado)

A complex interfingering of sandstones and shales typically restricts movement of water within the Dakota-Cheyenne aquifer.  Faults and fractures found associated with geological uplifts throughout the Laramie-Fox Hills and the Dakota-Cheyenne aquifers also limit large-scale ground-water flow.  However, these same faults and fractures provide paths for water movement when an aquifer is subjected to additional pressures such as those introduced by ISL mining (see ISL impacts).

Commercial, municipal, agricultural and residential wells use the Laramie-Fox Hills aquifer extensively. In February of 2001 there were 33,700-recorded wells.  With the population and commercial growth in northern Colorado since 2001, it can be safe to assume the number of wells using this aquifer has increased significantly.

Laramie Fox Hills Aquifer
Laramie Fox Hills Aquifer

There is concern the aquifer’s days are limited. Municipal use of the Laramie Fox-Hills aquifer in Denver’s southeast metropolitan area has resulted in significant water level declines.  Some recharge does occur through percolation of lawn irrigation where formations lie at the surface. Rain and snowmelt in Colorado typically runs off into streams or is evaporated while only a small portion reaches the aquifer. The recharge rate is not enough to replenish the discharges from this aquifer. "With a growing population and the current discharge for the system, there is only a matter of time before the well runs dry." (Hydrogeology of the Denver Basin Aquifer)

Typically under artesian pressure, the Laramie-Fox Hills aquifer’s ground water flows at over 1000 square feet per day. In the northern portion of the aquifer where Powertech’s in-situ leaching has been proposed, movement of the aquifer water is from Wyoming southward toward Greeley and the South Platte River.  The proposed Centennial ILS project is situated between Wyoming and Greeley.  In addition to water movement within the aquifer there are discharges from the aquifer.  While human use is a significant source of discharge within the Denver Basin, the largest discharge is movement between the four Denver Basin aquifers.

To leach the uranium out of the geological formation, fresh water is treated with a caustic solution that will dissolve uranium as well as other metals. This leaching solution (lixiviant) is pumped into the uranium bearing aquifer. “Frequently, the ground water in the mined aquifer is a domestic or livestock water supply. As the lixiviant migrates through the ore body, uranium and various associated elements such as arsenic, selenium, molybdenum, vanadium and radium-226 are mobilized in the ground water.” (Source: SpringerLink) The metal laden water is pumped back to the surface where the uranium is removed and the remaining toxic water is pooled in a holding pond. In addition to the well-documented problems ISL creates with heavy metals disturbed in the leaching process. Selenium will create a unique and significant disposal problem in northern Colorado. (See Selenium Impacts.)

ILS uranium mining requires the installation of monitoring wells in an attempt to detect when leaks or excursions occur.  “Groundwater monitoring bores are drilled into the same aquifer outside the mining zone, and also into aquifers above and below the mining zone (if aquifers are present). It is intended that the wells are closely spaced so that any excursion of lixiviant will be intercepted by a monitoring well. . .” (Report from The Sustainable Energy and Anti-Uranium Service Inc. PDF).  

The best information available today to use as an estimate for determining how many drill holes would be within the Centennial project would come from existing Wyoming  ISL uranium mines.  In Wyoming an ISL well is drilled with monitoring wells in a five spot pattern, 100 feet apart.  There are ten wells per acre equaling 60 drill holes x 5,000 – 6,000 acres of the Centennial project.

Wellfield with Drilling Rig (Photograph courtesy of Power Resources, Inc.)
Wellfield with Drilling Rig

Uranium companies market in-situ leaching as a benign and safe method of mining. The facts show otherwise. Since in-situ leaching problems are not as visible as in an open-pit mining, problems can go undetected for years.

Spills, leaks, and mechanical failures plague in-situ mining. Restoration of groundwater is a continual problem. Gavin Mudd writes in his research report An Environmental Critique of In Situ Leach Mining: The Case Against Uranium Solution Mining (PDF): “The most critical part of the ISL process is to control the movement of the chemical solutions within the aquifer. Any escape of these solutions outside the ore zone is considered an excursion, and can lead to contamination of surrounding groundwater systems. Some of the most common causes of excursions, identified by international operations in the United States and across Europe, can be through old exploration holes that were not plugged adequately, plugging or blocking of the aquifer causing excess water pressure buildup and breaks in bores, and failures of injection/extraction pumps.”   All of these issues will exist within the Centennial project (See ISL impacts).

Diagram of the In-situ Leaching Process (
Diagram of the In-situ Leaching Process

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