330 Hydrogeology III - Uranium, Metals, and Other Contaminants in Groundwater

Thursday, 9 October 2008
8:00 AM - 12:00 PM

In-Situ Leach Uranium Mining In the United States- Overview of Production and Remediation Issues

Paper number: 330-1
Program: Technical Sessions
Time: 8:00 AM – 8:15 AM

Authors

Abstract

In 2007, 4.53 million pounds of U3O8 were produced in the U.S., virtually all of it from in- situ leach (ISL) deposits in Wyoming, Nebraska, and Texas. Uranium production in the U.S. will likely be dominated by such mining during the next 5-7 yr, until existing conventional mills come into full production and new conventional mills are built. From its beginning in the 1960s, ISL mining has been considered as a lower-cost, more environmentally friendly alternative to conventional underground or open-pit mining and surface milling. The character of the leach solution has changed since the early use of sulfuric acid, which was abandoned early on in the U.S. because of plugging problems and the difficulty in restoring acid-treated aquifers. Subsequent leachates have included sodium, calcium, and ammonium bicarbonate, hydrogen peroxide, oxygen, and carbon dioxide. In each case, an oxidant and a complexing agent are needed to solubilize the uranium and form complexes to keep it in solution.

Leaching solutions not only dissolve uranium but many other unwanted components from minerals in the host formation, many of which typically remain dissolved in the aquifer being mined at levels above baseline after the leachate no longer carries enough uranium to be economically recovered. Moreover, many of the dissolved major elements are at concentrations sufficiently high to exceed the solubilities of various minerals which then precipitate, plugging the formation and operational plumbing, and reducing the effectiveness of extraction and subsequent remediation. Remediation techniques include flushing, reverse osmosis to clean waters used for flushing, introduction of reductants, and introduction of agents to reduce undesirable solute species or to control plugging. Bioremediation has been studied experimentally.