Uranium in Springs Sampled Near the Grand Canyon Likely from Natural Sources

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Scientists measured nine naturally occurring elements including uranium at 37 spring sites in the Grand Canyon area to establish baseline conditions and to understand the sources of uranium to local springs. Scientists found relatively greater concentrations of uranium at 6 of the 37 springs. A comprehensive geochemical analysis coupled with an understanding of the flow patterns in the area indicates that the presence of elevated uranium in the springs is likely from natural sources and not related to nearby mining activities.

Grand Canyon National Park in Arizona is a United Nations World Heritage Site, an international tourist destination, and the region is sacred place to many Native Americans. The Colorado River, which runs through Grand Canyon, is a primary source of drinking and irrigation water for mil¬lions of people in the United States and Mexico. The Grand Canyon region is also believed to host some of the highest-grade uranium ore in the United States and there is a limited amount of scientific data on the potential effects of uranium mining activities on cultural, biological, and water resources in the area.

Mountain Sheep Spring, Arizona

Mountain Sheep Spring in Sowats Canyon, Arizona, September 1, 2009. (Credit: Jamie P. Macy, US Geological Survey. Public domain.)

Therefore, U.S. Geological Survey scientists completed a comprehensive hydrologic and geochemical study on 37 springs discharging from rock formations north of the Grand Canyon to establish baseline concentrations for nine naturally occurring elements, some of which may be associated with uranium mining in the area (arsenic, boron, lithium, selenium, silica, strontium, thallium, uranium, and vanadium). Spring water was sampled owing to the lack of wells, which was due in part to the remoteness and depths to groundwater of more than 3,000 feet in some areas. Spring water geochemistry represents a snapshot of past conditions based on the age of the water and the contribution of water of different ages.

Although most of the spring sites had uranium values of less than 10 micrograms per liter, six springs had concentrations above that level, which is considered anomalous in the area. Scientists completed a comprehensive geochemical and hydrologic analyses to understand if these elevated concentrations were coincident with mining or representative of natural sources.

The geochemical characteristics of the six springs with elevated uranium include near-neutral pH, high specific conductance, and similar geochemical signatures, indicating they contain a mixture of modern and pre-modern waters. Similar geochemical compositions of spring waters with high levels of uranium are observed at sites located near and far from sites of uranium-mining activities in the present study.  Lines of evidence examined in this study suggest that the uranium in these six springs is likely from natural sources and therefore not likely related to nearby uranium mining.

Specialized teams of hydrologists, chemists, and geologists working together at sites in the Grand Canyon region are continuing to provide data to answer questions about the sources of uranium in air and water resources in the area. This information is important to understand exposure routes and potential health risks to humans and wildlife.

This study was funded by the U.S. Geological Survey Environmental Health Toxic Substances Hydrology and Contaminant Biology Programs and the Bureau of Land Management.

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