The Grand Canyon in northern Arizona is a United Nations World Heritage Site and an international tourist destination. The Grand Canyon region is a home or sacred place of origin for many Native Americans and its cultural significance goes back thousands of years.
The Colorado River, which carved the Grand Canyon, is a primary source of drinking and irrigation water for 35 million people in the United States and Mexico. Most communities in the immediate Grand Canyon area, however, do not have rights or access to water from the river and are dependent upon groundwater to meet all water needs. These groundwater-dependent communities include the Havasupai Nation, the Hualapai Nation, the towns of Tusayan, Williams, and Jacobs Lake, and even Grand Canyon National Park, which is visited by over 6 million people each year. Additionally, important ecosystems in the area are sustained by groundwater discharge at spring sites.
The Grand Canyon region also hosts some of the highest-grade uranium ore in the United States. In 2012, then-U.S. Secretary of the Interior Ken Salazar signed a Record of Decision (ROD) to withdraw over 1 million acres in three segregation areas of Federal land in the Grand Canyon region from new uranium mining activities for the next 20 years, subject to valid existing rights. A key factor in the decision for the withdrawal was the limited amount of scientific data and resulting uncertainty on potential effects of uranium mining activities on cultural, biological, and water resources in the area. Since 2014, the USGS has planned and conducted scientific investigations to address the uncertainties of potential uranium-mining effects noted in the ROD. USGS collects groundwater samples in the Grand Canyon region to understand the current state of groundwater chemistry in the region, to monitor for changes in groundwater chemistry that may be the result of mining activities, and to identify "hot spots" with elevated concentrations and investigate the causes.
The purpose of this interactive map is to present results for uranium concentrations in groundwater samples in the Grand Canyon region. The map allows users to navigate to different parts of the Grand Canyon region and see for themselves the uranium concentrations in wells and springs that have been sampled in the area. For this map, the Grand Canyon study area is defined by surface water drainages between Lee's Ferry to the east and Lake Mead to the west. Although uranium in groundwater may be of particular concern to residents and visitors of the area, other metals that are commonly associated with mineralized breccia pipes may be a greater risk to human health (figure 1).
For example, arsenic has an allowable drinking water concentration of 10 µg/L, which is one-third of the 30 µg/L allowable drinking water concentration for uranium (figure 1). The concentration of other important metals in groundwater in the Grand Canyon region will be added to this interactive map in the future.
A total of 560 groundwater sample results are currently available from 26 groundwater wells and 180 spring sites in the area (figure 2). Overall, Grand Canyon area groundwater samples to date indicate 95% (195) of all sites that have been sampled have uranium concentrations less than the 30 µg/L maximum contaminant level (MCL), with 79% of sites (163) having <10 µg/L uranium concentrations. Of the 11 sites with concentrations above the uranium MCL, 6 sites are in the Horn Creek drainage and 1 is in the adjacent Salt Creek drainage, which are both near the abandoned Orphan Mine uranium mine in Grand Canyon National Park. Uranium concentrations at sites in Horn Creek have been observed as high as 260 µg/L. USGS is conducting water-quality investigations in and near Horn Creek to explore the potential relation between elevated uranium concentrations in groundwater and the nearby abandoned mine. Other sites with uranium concentrations above the MCL include Pigeon Spring, which was determined to likely not be affected by the nearby Pigeon Mine uranium mine in a previous USGS study; Johnson Spring and Ide Valley Spring that both issue from geologic units that are above uranium ore zones in mined breccia pipes in the area; and an un-named spring west of the Hualapai Reservation, over 100 miles from the nearest breccia pipe uranium mine south of Grand Canyon. USGS continues to monitor water chemistry at the Horn Creek and Pigeon Spring sites, among other important sites in the region.