WSP07583

• File Number: 8270.200
• Description: Selenium
• State: CO
• Basin: Gunnison
• Water Division: 4
• Date: 1/1/1996
• Author: USGS
• Title: Detailed Study of Selenium and Other Constituents in Water-Bottom Sediment-Soil-Alfalfa and Biota Associated with Irrigation Drainage - Uncompahgre Project Area and in the Grand Valley - 1991-93
• Water Supply Pro - Doc Type: Report/Study

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Detailed Study of Selenium and Other Constituents
in Water, Bottom Sediment, Soil, Alfalfa, and
Biota Associated with Irrigation Drainage in the
Uncompahgre Project Area and in the Grand Valley,
West-Central Colorado, 1991-93

byDavid L. Butler, Winfield G. Wright, Kathleen C. Stewart, Barbara Campbell Osmundson,
Richard P. Krueger, and Daniel W. Crabtree

Abstract

In 1985, the U.S. Department of the Interior
began a program to study the effects of irrigation
drainage in the Western United States. These
studies were done to determine whether irrigation
drainage was causing problems related to human
health, water quality, and fish and wildlife
resources. Results of a study in 1991-93 of
irrigation drainage associated with the
Uncompahgre Project area, located in the lower
Gunnison River Basin, and of the Grand Valley,
located along the Colorado River, are described in
this report. The focus of the report is on the
sources, distribution, movement, and fate of
selenium in the hydrologic and biological systems
and the effects on biota. Generally, other trace-
constituent concentrations in water and biota were
not elevated or were not at levels of concern.
Soils in the Uncompahgre Project area
that primarily were derived from Mancos
Shale contained the highest concentrations of
total and water-extractable selenium. Only 5 of
128 alfalfa samples had selenium concentrations
that exceeded a recommended dietary limit for
livestock. Selenium data for soil and alfalfa
indicate that irrigation might be mobilizing and
redistributing selenium in the Uncompahgre
Project area.
Distribution of dissolved selenium in
ground water is affected by the aqueous geochem-
ical environment of the shallow ground-water

system. Selenium concentrations were as high as
l,300 micrograms per liter in water from shallow
wells. The highest concentrations of dissolved
selenium were in water from wells completed in
alluvium overlying the Mancos Shale of
Cretaceous age; selenium concentrations were
lower in water from wells completed in Mancos
Shale residuum. Selenium in the study area could
be mobilized by oxidation of reduced selenium,
desorption from aquifer sediments, ion exchange,
and dissolution. Infiltration of irrigation water
and, perhaps nitrate, provide oxidizing conditions
for mobilization of selenium from alluvium and
shale residuum and for transport to streams and
irrigation drains that are tributary to the
Gunnison, Uncompahgre, and Colorado Rivers.
Selenium concentrations in about
64 percent of water samples collected from
the lower Gunnison River and about 50 percent
of samples from the Colorado River near the
Colorado-Utah State line exceeded the
U.S. Environmental Protection Agency criterion
of 5 micrograms per liter for protection of aquatic
life. Almost all selenium concentrations in
samples collected during the nonirrigation
season from Mancos Shale areas exceeded the
aquatic-life criterion. The maximum selenium
concentrations in surface-water samples were
600 micrograms per liter in the Uncompahgre
Project area and 380 micrograms per liter in the
Grand Valley.

Abstract 1