Naturally Occurring Contamination in the Mancos Shale

Abstract

Some uranium mill tailings disposal cells were constructed on dark-gray shale of the Upper Cretaceous Mancos Shale. Shale of this formation contains contaminants similar to those in mill tailings. To establish the contributions derived from the Mancos, we sampled 51 locations in Colorado, New Mexico, and Utah. Many of the groundwater samples were saline with nitrate, selenium, and uranium concentrations commonly exceeding 250, 000, 1000, and 200 μg/L, respectively. Higher concentrations were limited to groundwater associated with shale beds, but were not correlated with geographic area, stratigraphic position, or source of water. The elevated concentrations suggest that naturally occurring contamination should be considered when evaluating groundwater cleanup levels. At several locations, seep water was yellow or red, caused in part by dissolved organic carbon concentrations up to 280 mg/L. Most seeps had (234)U to (238)U activity ratios greater than 2, indicating preferential leaching of (234)U. Seeps were slightly enriched in (18)O relative to the meteoric water line, indicating limited evaporation. Conceptually, major ion chemical reactions are dominated by calcite dissolution following proton release from pyrite oxidation and subsequent exchange by calcium for sodium residing on clay mineral exchange sites. Contaminants are likely released from organic matter and mineral surfaces during weathering.

Use of Chemical and Isotopic Signatures to Distinguish Between Uranium Mill‐Related and Naturally Occurring Groundwater Constituents

“…Similar values (AR > 2) were found in Mancos Shale groundwater seeps distributed over much of the Mancos depositional basin (Morrison et al. ). Thus, the uranium isotopic signatures suggest a Mancos Shale origin for the groundwater U at Eagle Nest Arroyo, Many Devils Wash, Salt Creek Wash, and the swale.…”

“…Morrison et al. () measured elevated concentrations of NO 3 , Se, SO 4 , and U in seeps issuing from the Mancos Shale throughout much of its depositional basin, including Salt Creek Wash and Eagle Nest Arroyo that are approximately 8 and 11 km, respectively, from the disposal cell and have no anthropogenic sources of contamination. To help determine the origins of the contamination, we collected chemical and isotopic groundwater data from six study areas: Shiprock disposal cell, the adjacent floodplain escarpment, a buried alluvial channel (swale), Many Devils Wash, Salt Creek Wash, and Eagle Nest Arroyo (Figure ).…”

Elevated aeolian sediment transport on the Colorado Plateau, USA: The role of grazing, vehicle disturbance, and increasing aridity

“…Areas that have been pushed into such degraded ecological states are often unable to recover biotic and hydrological functioning autogenically, necessitating active management intervention (Bestelmeyer et al, 2009;Miller et al, 2011;Webb et al, 2014;Fick et al, 2016) (Figure 5). Of further concern is the elevated concentrations of uranium, vanadium, selenium, and other associated metals and salts in Mancos and Morrison geologic formations which could be mobilized with dust emission events (Pliler and Adams, 1962;Morrison et al, 2012;Statwick and Sher, 2017) and have downwind consequences (Sankey et al, 2012a). However, at present these annualized-bare areas with large q values are relatively localized and generally fit the 'hot spot' concept posited by Gillette (1999).…”

Anoxia stimulates microbially catalyzed metal release from Animas River sediments

“…The site-specific nature of metal release may be linked to different reductive metabolisms, with Fe-reduction driving dissolution of grain coatings and alkalinity increases during SO 4 2− reduction offering another mechanism for metal desorption. 24 Given the Fe-and S-rich nature of the Colorado Basin, 3,13 these complex processes represent a challenge for the tracking of mining-impacted biogeochemistry and associated water quality issues, and emphasize the need monitoring efforts that account for the dynamic nature of fluvial systems and their ability to moderate strong spatial and temporal gradients in redox status. These results carry important implications for the role of microbes in the mobilization of sediment associated metal pools across a wide range of environments.…”

“…9,10 Microbes may directly reduce and dissolve Fe 3+ and Mn 4+ coatings, 11 while biogenic S 2− (produced via microbial SO 4 2− reduction) can abiotically catalyze the same process. 12 Groundwater in much of the Colorado Basin has naturally elevated levels of SO 4 2− due to the weathering and leaching of pyritic shales and gypsum-rich evaporites, respectively, 13 and oxidation of metal sulfide-rich ore deposits 14 within mineralized rock, with SO 4 2− entering river channels via groundwater discharge and hyporheic exchange. 15 Regardless of mechanism, the reductive dissolution of reactive Fe 3+ and Mn 4+ grain coatings can lead to the release of co-associated metals into the aqueous phase, 7,8 with subsequent implications for downstream water chemistry and long-term monitoring plans for watersheds such as the Animas River.…”