THE GRAND CANYON BRECCIA-PIPE URANIUM DISTRICT, NORTHWESTERN ARIZONA, AND POTENTIAL MINING-RELATED IMPACTS TO COLORADO RIVER WATER QUALITY

The Grand Canyon region contains over 1300 known or suspected breccia pipes, which are vertical, pipe-shaped bodies of highly fractured rock that collapsed into voids created by dissolution of underlying rock. Breccia pipes extend hundreds of meters upward from laterally extensive dissolution breccia in the Mississippian Redwall Limestone to as high as the Triassic Chinle Formation. Some breccia pipes were mineralized with uranium oxides as well as sulfides of copper, lead, zinc, and silver. The breccia-pipe district is estimated to contain ~40% of U.S. uranium resources. Uranium ore is typically many hundreds of meters underground, and deposits are mined using a pair of mine shafts. Mine closure and site remediation are readily accomplished by standard techniques. Total breccia-pipe uranium production as of Dec. 31, 2010 has been more than 10,700 metric tons (23.5 million lbs) from ore with average grade of 0.65% U₃O₈ derived from nine underground mines, eight of which are north of Grand Canyon near Kanab Creek.

Renewed exploration due to a steep rise in uranium prices during 2004-2007 led some to concerns about contamination of the Colorado River related to uranium mining. Colorado River water in Grand Canyon region contains about 4 µg/l (micrograms per liter) of uranium (equivalent to 4 parts per billion by mass), with approximately 15 cubic km annual discharge. Thus, approximately 60 metric tons of dissolved uranium, derived by natural weathering of rock over the Colorado River drainage basin, are carried annually by the Colorado River through the Grand Canyon. We consider a hypothetical, worst-case accident in which a truck hauling thirty metric tons (66,000 lbs) of one-percent-uranium ore is overturned by a flash flood in Kanab Creek and its entire load is washed 60 km down Kanab Creek into the Colorado River where it is pulverized and dissolved over one-year to become part of the dissolved uranium content of the river (such a scenario is extremely unlikely if not impossible). This addition of 300 kilograms (660 lbs) of uranium over one year would increase uranium in river water from 4.00 ppb to 4.02 ppb. Given that the EPA maximum contaminant level for uranium in drinking water is 30 ppb, this increase would be trivial. Furthermore, it would be undetectable against much larger natural variation in river-water uranium content.