An assessment of uranium in groundwater in the Grand Canyon region

Abstract

The Grand Canyon region in northern Arizona is a home or sacred place of origin for many Native Americans and is visited by over 6 million tourists each year. Most communities in the area depend upon groundwater for all water uses. Some of the highest-grade uranium ore in the United States also is found in the Grand Canyon region. A withdrawal of over 4000 km2 of Federal land in the Grand Canyon region from new uranium mining activities for 20 years was instituted in 2012, owing in part to a lack of scientific data on potential effects from uranium mining on water resources in the area. The U.S. Geological Survey has collected groundwater chemistry samples since 1981 in the Grand Canyon region to better understand the current state of groundwater quality, to monitor for changes in groundwater quality that may be the result of mining activities, and to identify "hot spots" with elevated metal concentrations and investigate the causes. This manuscript presents results for the assessment of uranium in groundwater in the Grand Canyon region. Analytical results for uranium in groundwater in the Grand Canyon region were available for 573 samples collected from 180 spring sites and 26 wells from September 1, 1981 to October 7, 2020. Samples were collected from springs issuing from stratigraphic units above, within, and below the Permian strata that host uranium ore in breccia pipes in the area. Maximum uranium concentrations at groundwater sites in the region ranged from less than 1 µg/L at 23 sites (11%) to 100 µg/L or more at 4 sites (2%). Of the 206 groundwater sites sampled, 195 sites (95%) had maximum observed uranium concentrations less than the U.S. Environmental Protection Agency’s Maximum Contaminant Level of 30 µg/L for drinking water and 177 sites (86%) had uranium concentrations less than the 15 µg/L Canadian benchmark for protection of aquatic life in freshwater. The establishment of baseline groundwater quality is an important first step in monitoring for change in water chemistry throughout mining lifecycles and beyond to ensure the health of these critical groundwater resources.

Arsenic in groundwater in the Grand Canyon region and an evaluation of potential pathways for arsenic contamination of groundwater from breccia pipe uranium mining

“…Maximum observed arsenic concentrations at groundwater sites in the Grand Canyon region ranged from <1 μg/L at 52 sites to 875 μg/L at Pumpkin Spring (USGS Site ID 355459113195900, S1 in Fig 4). About 88% (202) of the 230 groundwater sites had a maximum arsenic concentration at or below the USEPA MCL of 10 μg/L (Figs 4 and 5), substantially less than the 95% of groundwater sites with uranium concentrations below the uranium MCL reported by Tillman et al [8]. Of the remaining 12% of sites, all but three had maximum concentrations <40 μg/L.…”

“…Available arsenic concentration data in the Pinenut Well have varied somewhat unpredictably over time, with concentrations before the most recent mining period observed at 6.2 μg/L (2009) and 13.5 μg/L (2012), during mining at 4.1 μg/L (2014), and after mine closure at 11 μg/L (2018). The closest spring to the Pinenut Mine, Willow 1 Spring (S11 in Fig 4), is located 7 km north of the mine and has been monitored by USGS since 2009, owing to its elevated uranium concentrations [8]. Arsenic concentrations at Willow 1 Spring have ranged from 0.6 μg/L to 3.1 μg/L in 26 samples from the site (Table C in S1 Table ).…”

“…In 2017, an additional groundwater well was installed adjacent to the mine to monitor the perched groundwater of the Coconino Sandstone. Tillman et al [8] hypothesized that potential effects from mining activities on groundwater would likely first be observed in the shallow system closest to the mine shaft and ore deposits. Arsenic concentrations in the perched well have decreased in the seven samples from this site, from 4.6 μg/L in 2017 to 0.51 μg/L in 2022 (Table C in S1 Table ).…”

“…In 2012, about 400,000 hectares of federal land near Grand Canyon was withdrawn from future uranium mining for 20 years, subject to valid existing rights, owing to, among other issues, uncertainties of mining effects on water resources in the region (Fig 1) [3]. Since that time, the U.S. Geological Survey (USGS) has collected environmental data (e.g., [4]) and published investigations (e.g., [5][6][7][8]) to address these uncertainties and to provide information to decision makers who will determine if the mining withdrawal should be eliminated, extended, or made permanent.…”

Synthesis, characterization and application of a zirconium-based MOF-808 functionalized with isonicotinic acid for fast and efficient solid phase extraction of uranium(VI) from wastewater prior to its spectrophotometric determination

“…It can be entered to water because of mining, test of nuclear weapons and leaking from nuclear facilities. Groundwater contamination with uranyl ions in different parts of the world is extensively reported [ 2 – 5 ]. Entrance of this element to human body is dangerous because of both its chemical toxicity and its radioactivity.…”