Geochemistry and hydrology of perched groundwater springs: assessing elevated uranium concentrations at Pigeon Spring relative to nearby Pigeon Mine, Arizona (USA)

Abstract

The processes that affect water chemistry as the water flows from recharge areas through breccia-pipe uranium deposits in the Grand Canyon region of the southwestern United States are not well understood. Pigeon Spring had elevated uranium in 1982 (44 μg/L), compared to other perched springs (2.7-18 μg/L), prior to mining operations at the nearby Pigeon Mine. Perched groundwater springs in an area around the Pigeon Mine were sampled between 2009 and 2015 and compared with material from the Pigeon Mine to better understand the geochemistry and hydrology of the area. Two general groups of perched groundwater springs were identified from this study; one group is characterized by calcium sulfate type water, low uranium activity ratio 234 U/ 238 U (UAR) values, and a mixture of water with some component of modern water, and the other group by calcium-magnesium sulfate type water, higher UAR values, and radiocarbon ages indicating recharge on the order of several thousand years ago. Multivariate statistical principal components analysis of Pigeon Mine and spring samples indicate Cu, Pb, As, Mn, and Cd concentrations distinguished mining-related leachates from perched groundwater springs. The groundwater potentiometric surface indicates that perched groundwater at Pigeon Mine would likely flow toward the northwest away from Pigeon Spring. The geochemical analysis of the water, sediment and rock samples collected from the Snake Gulch area indicate that the elevated uranium at Pigeon Spring is likely related to a natural source of uranium upgradient from the spring and not likely related to the Pigeon Mine.

An assessment of uranium in groundwater in the Grand Canyon region

“…Pigeon Spring discharges from perched groundwater within the Toroweap Formation (Fig. 2 ), most likely after having moved down through the Kaibab and Toroweap Formations 62 . USGS has been monitoring water chemistry at Pigeon Spring since 2012, and uranium concentrations at the site are the highest observed in the study area north of Grand Canyon—as high as 104 µg/L on March 15, 2012 (Fig.…”

“…Temporal changes in uranium concentrations, which may be due to fluctuations in recharge amount and seasonality, are apparent in the dataset, with concentrations averaging 73 µg/L and varying from 56 to 104 µg/L between 2012 and 2020 63 . Beisner et al 62 published a detailed description of water chemistry at Pigeon Spring and nearby springs and investigated the likelihood that the source of elevated uranium concentrations at the site was Pigeon Mine. Based on estimated direction of groundwater flow inferred from spring elevations in the area and comparison of uranium concentrations at Pigeon Spring to leaching experiments involving Pigeon Mine waste rock, ore, and nearby soil, Beisner et al 62 concluded that current evidence indicate it is unlikely that elevated uranium at Pigeon Spring is coming from the former Pigeon Mine.…”

“…Beisner et al 62 published a detailed description of water chemistry at Pigeon Spring and nearby springs and investigated the likelihood that the source of elevated uranium concentrations at the site was Pigeon Mine. Based on estimated direction of groundwater flow inferred from spring elevations in the area and comparison of uranium concentrations at Pigeon Spring to leaching experiments involving Pigeon Mine waste rock, ore, and nearby soil, Beisner et al 62 concluded that current evidence indicate it is unlikely that elevated uranium at Pigeon Spring is coming from the former Pigeon Mine. This conclusion is important for establishing expectations of naturally occurring concentrations of uranium (and other associated trace elements) in groundwater in the region.…”

“…Ongoing monitoring at the site and samples from a planned nearby well will provide additional data on temporal changes in geochemistry in groundwater at the site and allow for further evaluation of the relationship between the mine and spring.

Figure 5 Map of Pigeon Spring and the former Pigeon Mine with uranium concentrations through time at the spring and inferred direction of groundwater flow from Beisner et al 62 . Created in Adobe Illustrator 25.4.1 ( https://www.adobe.com/products/illustrator.html ).

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Assessing uranium and select trace elements associated with breccia pipe uranium deposits in the Colorado River and main tributaries in Grand Canyon, USA

“…While several studies have been published on water quality of groundwater discharging from springs in the Grand Canyon region [e.g., 9 , 10 , 17 – 19 ], few published studies have focused on the concentration of metals in the Colorado River and major tributaries that are related to uranium mining in the area. Hopkins et al [ 20 ] collected water and stream sediment samples in March of 1982 in the Snake Gulch drainage of Kanab Creek as part of determining mineral resource potential in the area.…”