Naturally occurring contamination in the Mancos Shale

Area of Science:

• Environmental Science
• Geochemistry
• Hydrogeology

Background:

• Uranium mill tailings disposal cells are often sited on Mancos Shale.
• Mancos Shale contains contaminants similar to those found in mill tailings.
• Understanding background contaminant levels is crucial for environmental assessments.

Purpose of the Study:

• To determine the contribution of Mancos Shale to groundwater contamination.
• To assess naturally occurring contaminants in groundwater and seeps associated with Mancos Shale.
• To inform groundwater cleanup level evaluations.

Main Methods:

• Sampling of 51 groundwater locations across Colorado, New Mexico, and Utah.
• Chemical and isotopic analysis of groundwater and seep water.
• Geochemical modeling of major ion reactions and contaminant sources.

Main Results:

• Groundwater samples frequently exhibited high salinity and elevated concentrations of nitrate, selenium, and uranium.
• Higher contaminant levels were localized to groundwater associated with shale beds.
• Seep water showed distinctive coloration, high dissolved organic carbon, and preferential leaching of Uranium-234.
• Geochemical reactions involve calcite dissolution, pyrite oxidation, and ion exchange.

Conclusions:

• Naturally occurring contamination from Mancos Shale must be considered when setting groundwater cleanup levels.
• Weathering of organic matter and mineral surfaces are likely sources of contaminant release.
• Isotopic and chemical signatures indicate specific hydrogeochemical processes at play.