Abstract
Abandoned uranium mines in the Monument Valley and Cameron mining districts that have partially filled with water were studied to define hydrologic and chemical characteristics of mine water and shallow ground water and to evaluate possible chemical interactions of shallow ground water and the mine-spoil material that will be used in mine reclamation. Uranium mines in the Monument Valley area were established predominantly in channel-fill deposits within the Shinarump Member of the Chinle Formation. The Shinarump Member yields ground water to wells and may yield water to the Moonlight and Radium Hill mines. Depth-to-water measurements in the area of the Moonlight and Radium Hill mines indicate that local ground-water flow is from the southeast to the northwest along the trend of Oljeto Wash. In the study area near Cameron, uranium was mined from channel-fill deposits within the Petrified Forest Member of the Chinle Formation. Units of the Petrified Forest Member do not yield ground water to wells in the area, but fractures in the lower part of the Petrified Forest Member are probable pathways for upward flow of ground water from the Shinarump Member. Depth-to-water measurements were not sufficient to determine local ground-water flow directions, although previous investigations determined that regional flow in the area is toward the Little Colorado River. In the Cameron area, water in mines can originate from several sources. Most of the mines receive water from surface inflow of rainfall runoff, but ground water also may be transmitted to open pits and drill holes in the subsurface through fractures or along faults in the Petrified Forest Member. Uranium-238 activities in shallow ground water from mines ranged from 150 to 14,000 picocuries per liter and radium-226 activities ranged from 0.10 to 110 picocuries per liter. Uranium-238 activities in pit water from mines ranged from II to 22 picocuries per liter. Radon-222 activities from three groundwater samples ranged from 590 to 250,000 picocuries per liter. Radionuclide activities in well and spring water generally were less than in shallow ground water and pit water. Water from Clay Well spring, which is about 1.9 miles from the nearest mine, contained a uranium-238 activity of 27 picocuries per liter. Radionuclide activities in well and spring water may result from naturally occurring mineralization in waterbearing rock units. The effects of mining activity could not be determined from chemical analyses of well and spring water. Laboratory-batch tests indicate that radionuclide activities varied in leachate and generally correlated with field gamma measurements. Uranium concentrations in leachate samples ranged from 20 to 7,700 micrograms per liter and radium-226 activities ranged from 0.95 to 34 picocuries per liter. Batch tests were done with material that was 2.00 millimeters and smaller. Particle-size data indicate that spoil material near sampling locations is predominantly gravel and coarser sediments at three of the mines and sand-size sediments at the fourth. The radiochemistry of leachate from coarser sediments was not determined, and the specific rate and magnitude of radionuclide leaching are dependent on site-specific conditions that include the amounts of oxygen and organic material present, temperature, spoil mineralogy, and local ground-water composition.
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