In-situ leach mining (ISL) uses water wells to inject water and chemicals into a sandstone aquifer to release uranium deposits and then pump out the resulting uranium solution.  The solution is then processed to remove the uranium. 

Injection and extraction wells are drilled 50-100 feet apart in a grid pattern above the ore bodies.  Fresh water is pumped out of the aquifer, mixed with sodium bicarbonate and oxygen gas, and pumped back into the aquifer by high pressure injection wells.  Extraction wells “pull” the chemical solution across ore deposits, releasing the uranium. Other radioactive elements are mobilized during this process, including thorium, radium, and radon, as well as heavy metals such as arsenic, selenium, cadmium, vanadium, molybdenum, and lead.  

This solution of radioactive and toxic metals is pumped out of the ground and run through ion-exchange columns where resin beads remove the uranium for further processing into “yellowcake”.  The process water, contaminated with radioactive elements and heavy metals, is recharged with oxygen and sodium bicarbonate and re-injected into the aquifer.  This process continues until 60-80% of the uranium is extracted. 

In-situ mining is not a closed process.  There is no physical barrier between the mining zone and the surrounding aquifer which supplies drinking water to residents with wells.  More water is extracted than injected in an attempt to use water pressure to control the leaching solution and keep it from spreading outside the mining zone.  Pump failure, electric power loss, or computer control malfunction can result in an escape of leaching solution (containing uranium, radium, and heavy metals) from the mining zone into adjacent areas of the aquifer used for drinking water.  This is euphemistically referred to by the mining industry as an “excursion”.

Monitoring wells are intended to detect excursions.  Upon detection of an excursion, injection and extraction well pump rates are adjusted to try to draw the escaping solution back towards the extraction well.  However, poor engineering of monitoring well locations as well as variability and complexity of geologic structures that form the ore deposit can result in failures to detect excursions. 

The excess water extracted from the mining zone, called “bleed”, ranges from 1-5% of the total quantity of water pumped from the wellfield.  Since this bleed water is contaminated with radioactive elements and toxic heavy metals, it is disposed of by pumping into open-air evaporation ponds.  The resulting toxic sludge is packaged and transported to hazardous waste facilities.  Alternatively, the contaminated bleed water can be injected into deep disposal wells.  An ISL mining operation that operates at 2,000 gallons per minute with a 2% bleed rate will consume over 21 million gallons of water per year. 

When most of the economically-recoverable uranium is extracted, restoration of the aquifer mining zone is attempted.  Injection ceases, but extraction continues.  This “sweeping” draws clean water into the leaching zone from the surrounding aquifer.  The contaminated water is pumped into deep disposal wells or into evaporation ponds.  Sweeping cannot remove all contaminants, so a second phase involves treatment by reverse osmosis and re-circulation through the mining zone.  Even this process fails to remove all contaminants.  Hydrogen sulfide and other chemicals may be injected into the aquifer to try to stabilize any remaining uranium and heavy metals, but contaminant levels can actually rise over time after restoration efforts are ended.  

Aquifer restoration takes several years and invariably fails to bring all contaminants to pre-mining levels, leaving elevated levels of radioactive elements and heavy metals.  When the pumps are turned off and the mining company leaves, there is no way to prevent these groundwater contaminants from migrating outside of the mining zone.  According to a January 2007 report by the U.S. Nuclear Regulatory Commission on restoration of uranium ISL mines, “Industry experience shows that elevated concentrations (above baseline) of arsenic, selenium, radium, uranium (Table 3), molybdenum, radium, uranium, and vanadium (Tables 4 and 5) still existed after extensive groundwater restoration activities.”