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An Environmental Critique of In Situ Leach Mining : The Case Against Uranium Solution Mining - By Gavin Mudd, Victoria University of Technology - July 1998


(Full report - pdf)


In Situ Leach (ISL) or Solution Mining is a third alternative mining technique, quite
different to conventional open cut or underground mining and milling operations. The
ISL process can be applied to the extraction of uranium, copper, silver, gold, nickel and
many other metals. For an ore deposit to be amenable to ISL it must occur within
permeable materials, such as sands or sandstones, be saturated (ie - within an “aquifer”
or groundwater system), and be confined above and below by low permeability
materials (such as clays or shales). Given these general criteria, chemical leaching
solutions are pumped into the ore deposit “In Situ” and dissolves the metal of interest.
These metal-laden solutions are then pumped back to the surface and extracted in a
processing plant. In this way, instead of the ore being excavated, crushed and chemically
treated in a large surface processing plant, the process chemicals are injected
underground into the ore directly. As such, the ISL technique generally has lower
production costs than conventional mines. The ISL mining method has many advantages
as well as inherent problems, and the success of this technique applied to the extraction
of uranium is contained within this report and briefly summarised here.

The ISL Technique

The ISL technique, as described above, involves the drilling and operation of an
extensive series of groundwater bores, injecting and extracting the chemical solutions
continuously. There are two main options for the leaching chemistry - acid or alkaline
solutions. In general, acid solutions (such as sulphuric acid) will extract a higher
proportion of uranium and at faster rates than alkaline solutions (such as sodium or
ammonia bicarbonate). However, acidic solutions will also mobilise high levels of
environmentally sensitive and toxic heavy metals (such as cadmium, selenium,
vanadium, lead, and others). The level of radionuclides, such as radium, mobilised from
the ore into processing solutions tends to be higher in alkaline solutions.

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.


After the ore has been depleted and the maximum degree of uranium extraction
achieved, it is good policy and (generally) a legal requirement to restore the water
quality of the groundwater system that was mined to it’s pre-mining level. That is,
remove all of the remaining chemicals from the aquifer and return the uranium and
heavy metals to their original concentrations. In practice, this is extremely hard to
achieve, especially with acidic solutions. Despite nearly 25 years of commercial ISL
uranium mines in the United States (all using alkaline leaching solutions), regulators are
yet to review or approve a report on the full scale restoration of groundwater at these
sites, although they describe the restoration at earlier pilot sites as “satisfactory”.

With the current proposals at Beverley and Honeymoon in South Australia, and
presumably for the new proposal at Manyingee in Western Australia, the restoration of
groundwater is not planned nor is it being required by the regulators.

United States Experience

Texas has been the most prolific state for development of ISL uranium mines, with
Wyoming also being popular for ISL sites. The first trial of ISL uranium mining was
conducted at the Shirley Basin deposit by then Utah Construction & Mining Company2
in the early 1960’s with operation of an experimental full scale mine in the late 1960’s.
It was shut down and converted to a traditional open cut mine. The uranium industry,
keen to cut costs to compete with developing overseas uranium mines, nevertheless
enthusiastically developed new ISL trials and by 1975, the first large scale commercial
ISL uranium mine opened in Texas at Clay West. By 1992, there had a total of 62 sites
where ISL had been applied, only 24 of which reached commercial production of
uranium. Currently, due to the depressed nature of the world uranium market and higher
costs at conventional mines, only 5 ISL uranium mines are operating in the USA,
despite 12 being licensed.

The early ISL mines, such as Irigary Wyoming and Clay West/Burns in Texas, had
many technical problems which led to poor operational and environmental performance.
These included mineral precipitation of gypsum (CaSO4) and calcite (CaCO3) plugging
the aquifer, restricting groundwater flow and exacerbating excursions; complex
reactions of chemicals with clays in the aquifer soils leading to permeability loss;
excursions through old exploration boreholes; and excursions outside the mining zone.
Problems were also noted with radiation levels, especially at some Texan ISL mines.
The restoration of groundwater at many sites was not successful, and companies lobbied
regulators to relax cleanup standards, and some sites still had significant problems even
meeting these standards. The design of processing plants and infrastructure was also
inadequate at some sites.....