Science for Decision Making on Uranium Mining in Arizona

The Colorado River, which carved Grand Canyon over the last 6 million years, is one of the most important water sources in the western U.S., serving the needs of more than 38 million people in the U.S. and Mexico. The Grand Canyon region also hosts some of the highest-grade uranium deposits in the U.S. and has subsequently experienced varying levels of uranium mining activity since the 1950s.

Since late 2015, the USGS has monitored major surface waters in the Grand Canyon area for elements associated with mineralized uranium deposits. Dissolved constituents in the Colorado River are monitored upstream (Lees Ferry), near the middle (Phantom Ranch), and downstream (above Diamond Creek) of Grand Canyon National Park. Additionally, dissolved and sediment bound constituents are monitored at the mouths of the Little Colorado River, Kanab Creek, and Havasu Creek tributaries.

Difficult sampling conditions exist at these remote tributary monitoring sites, including limited access and extremely variable flow from baseflow to flood runoff conditions. Water chemistry results from the mainstem Colorado River sites are available in the USGS National Water Information System database (see links below). Water and sediment chemistry results from the Little Colorado River, Kanab Creek, and Havasu Creek are available on the USGS ScienceBase web site.

Continued monitoring at these and additional tributary sites in Grand Canyon will provide a robust dataset with which to compare potential future changes that may result from uranium mining in the area.

Findings from this investigation¹ to date include:

• From 2015–2018 major surface waters in Grand Canyon were monitored for select elements associated with breccia-pipe uranium deposits in the area, including uranium, arsenic, cadmium, and lead.
• Dissolved constituents in the Colorado River were monitored upstream (Lees Ferry), in the middle (Phantom Ranch), and downstream (Diamond Creek) of uranium mining areas.
• Additionally, dissolved and sediment-bound constituents were monitored during a wide range of streamflow conditions at Little Colorado River, Kanab Creek, and Havasu Creek
• tributaries, whose watersheds have experienced different levels of uranium mining activities over time.
• Concentrations of uranium, arsenic, cadmium, and lead at the main-stem Colorado River sites varied little during the study period and were all well below human health and aquatic life benchmark criteria.
• Samples from the tributary sites contained ≤3.8 μg/L of dissolved cadmium and lead, and ≤17 μg/L of dissolved uranium.
• Dissolved arsenic also was mostly below human and aquatic life criteria at Little Colorado River and Kanab Creek; however, 63% of water samples from Havasu Creek were above the maximum contaminant level for arsenic.
• Arsenic in suspended sediment was greater than sediment quality guidelines in 9%, 35%, and 35% of samples from Little Colorado River, Kanab Creek, and Havasu Creek, respectively.
• Irregular loading from the tributary sites is controlled by suspended sediment discharge occurring during flood events at the sites, resulting in loads that can range over 3+ orders of magnitude between baseflow and high flows for some elements.

At the concentrations observed during this study, tributaries contributed on average only about 0.12 μg/L of arsenic and 0.03 μg/L of uranium to the main-stem river.

Water chemistry results from the mainstem Colorado River sites are available on the USGS National Water Information System

COLORADO RIVER AT LEES FERRY, AZ

COLORADO RIVER NEAR GRAND CANYON, AZ

COLORADO RVR ABV DIAMOND CREEK NR PEACH SPRINGS AZ

Questions this study could help answer

What is the flux (mass per time) of uranium and associated trace elements in the Colorado River upstream and downstream from the withdrawal areas?

What is the flux of uranium and associated trace elements in the major tributaries to the Colorado River in the Grand Canyon area?

Is there a difference in flux from drainages with unmined breccia pipes, currently mined breccia pipes, and formerly mined breccia pipes?

Is there a seasonal or runoff-driven change in flux over time?

Objectives

Monitor dissolved constituents in the Colorado River upstream (Lees Ferry), near the middle (Phantom Ranch), and downstream (above Diamond Creek) of Grand Canyon National Park.

Monitor dissolved and sediment bound constituents at the mouths of the Little Colorado River, Kanab Creek, and Havasu Creek tributaries.

Status

Ongoing

Specific Tasks

Task 5b: Collect/analyze water and sediment samples from streams and springs
Task 12: Surface water monitoring of chemical constituents in drainages with historical and current mining

Sources

Principal Investigators

Fred Tillman
Hydrologist
Arizona Water Science Center
520.670.3312
ftillman@usgs.gov

Joel Unema
Hydrologist
Arizona Water Science Center
928.556.7363
junema@usgs.gov

Jessica Anderson
Hydrologic Technician
Arizona Water Science Center
928.556.7357
jranderson@usgs.gov

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