es203211z_si_001.pdf (3.84 MB)
Naturally Occurring Contamination in the Mancos Shale
journal contribution
posted on 2016-02-22, 06:43 authored by Stan J. Morrison, Craig S. Goodknight, Aaron
D. Tigar, Richard P. Bush, April GilSome 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 234U to 238U
activity ratios greater than 2, indicating preferential leaching of 234U. Seeps were slightly enriched in 18O 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.