ABSTRACT

Between 1955 and 1967, approximately 590,000 pounds of uranium oxide was recovered from 85,000 tons of lignite mined from the Fort Union Formation in western North Dakota. The lignite, which has a U3O8 content ranging from 0.001 to more than 1.8 percent, was strip mined in 12 mine pits. The lignite was burned in mine- pit bottoms or nearby ashing kilns to concentrate the uranium by approximately a factor of 10. The resultant ash was transported to uranium processing plants in other States. Stockpiles of lignite and ash remained in mine- pit bottoms when the mines were abandoned, and reclamation has been initiated at only one site. Most mine pits have become flooded, because the lignite beds commonly are local aquifers. Contact between mine wastes and water in the mine pits facilitates transport of radioactive elements from the mine sites to regions where human exposure is more likely.

Uranium concentrations in mine-pit waters range from 12 to 19,000 micrograms per liter. Accompanying radium- 226 concentrations range from 1 to 73 picocuries per liter, commonly much greater than the primary drinking water standard of 5 picocuries per liter radium-226. Base flows in streams as distant as 4 miles down the hydrologic gradient from the nearest mine have uranium concentrations in excess of 200 micrograms per liter, approximately 10 times regional background concentrations.

Aerial measurements of gamma-ray surface exposure levels indicate that radiation from natural outcrops of the uraniferous lignite ranges from 10 to 30 microrems per hour, radiation from ash piles ranges from 14 to 50 microrems per hour, and radiation from spoils piles ranges from 10 to more than 200 microrems per hour. Investigation of the lignite in outcrops and interpretation of geophysical logs indicates most of the uranium occurs as a thin layer of hydrated uranium sulfates immediately above the lignite and is not adsorbed on organic materials. Apparently, most of this uranium was stripped and remained in the spoils during mining at some sites.

Accordingly, introduction of radioactive species to the ground water is controlled principally by infiltration through the spoils and not by reaction between water and ash in mine-pit bottoms. Preliminary analysis of saturated extracts and solutions from accelerated- weathering experiments of spoils indicates uranium concentrations in leachate are governed by the availability of dissolved organic compounds. Complexation of uranium on soluble hydroxybenzenes and other organic compounds derived from oxidation of lignite appears to be a contributing mobilizing process. Solutions generated from ash contain uranium concentrations approximately 10 to 50 times less than those produced from spoils of identical initial uranium content.