Impact of Uranium Legacy Mining and Recent Mine Development

Over a century ago, rumors of mineral riches beyond imagining had drawn a small cartel of prospectors to northern Arizona. The deposits of copper and other ores found, however, proved to be too small to warrant development and the mines and prospects were abandoned. These abandoned mines are potential sources of contamination for water resources in the region. In the rush to locate uranium resources in the late 1950’s, uranium ore was discovered in association with many of these old mines that had been developed in mineralized breccia pipes. Today, some of the highest grade uranium ore in the country is believed to be located in the many potentially mineralized breccia pipes scattered across northern Arizona. Current mining practices have improved significantly from those used in the past owing to increased regulation since the 1970’s. Renewed interest in the exploration and mining of uranium ore in northern Arizona has again heightened public awareness of the environmental issues. This talk is a summary of recent USGS studies to determine the extent of naturally occurring dissolved uranium and other trace element concentrations in groundwater associated with ore deposits in the region and their relation to legacy and current mining practices. Deposition and dissolution of uranium occurs naturally in breccias pipes as groundwater infiltrates from the surface, through the ore bodies to perched water-bearing zones, and the underlying Redwall-Muav aquifer, in a process has been going on for millions of years. Dissolved uranium in water from wells and springs in northern Arizona range from less than 5.0 μg/L to 360 μg/L. The highest concentrations are associated with legacy mining activity. Recent well and spring samples collected in the Kanab Creek and the House Rock areas, where both unmined and recently mined deposits occur, range from less 1.0 μg/L to 20 μg/L dissolved uranium.