Mobilization of selenium from the Mancos Shale and associated soils in the lower Uncompahgre River Basin, Colorado

Applied Geochemistry
By: , and 



This study investigates processes controlling mobilization of selenium in the lower part of the Uncompahgre River Basin in western Colorado. Selenium occurs naturally in the underlying Mancos Shale and is leached to groundwater and surface water by limited natural runoff, agricultural and domestic irrigation, and leakage from irrigation canals. Soil and sediment samples from the study area were tested using sequential extractions to identify the forms of selenium present in solid phases. Selenium speciation was characterized for nonirrigated and irrigated soils from an agricultural site and sediments from a wetland formed by a leaking canal. In nonirrigated areas, selenium was present in highly soluble sodium salts and gypsum. In irrigated soils, soluble forms of selenium were depleted and most selenium was associated with organic matter that was stable under near-surface weathering conditions. Laboratory leaching experiments and geochemical modeling confirm that selenium primarily is released to groundwater and surface water by dissolution of highly soluble selenium-bearing salts and gypsum present in soils and bedrock. Rates of selenium dissolution determined from column leachate experiments indicate that selenium is released most rapidly when water is applied to previously nonirrigated soils and sediment. High concentrations of extractable nitrate also were found in nonirrigated soils and bedrock that appear to be partially derived from weathered organic matter from the shale rather than from agricultural sources. Once selenium is mobilized, dissolved nitrate derived from natural sources appears to inhibit the reduction of dissolved selenium leading to elevated concentrations of selenium in groundwater. A conceptual model of selenium weathering is presented and used to explain seasonal variations in the surface-water chemistry of Loutzenhizer Arroyo, a major tributary contributor of selenium to the lower Uncompahgre River.

Study Area

Publication type Article
Publication Subtype Journal Article
Title Mobilization of selenium from the Mancos Shale and associated soils in the lower Uncompahgre River Basin, Colorado
Series title Applied Geochemistry
DOI 10.1016/j.apgeochem.2014.06.024
Volume 48
Year Published 2014
Language English
Publisher Elsevier
Contributing office(s) Colorado Water Science Center
Description 12 p.
Larger Work Type Article
Larger Work Subtype Journal Article
Larger Work Title Applied Geochemistry
First page 16
Last page 27
Country United States
State Colorado
Other Geospatial Uncompahgre River Basin
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