**National Irrigation Water Quality Program**\
U.S. Department of the Interior

**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**

David L. Butler, U.S. Geological Survey\
Winfield G. Wright, U.S. Geological Survey\
Kathleen C. Stewart, U.S. Geological Survey\
Barbara Campbell Osmundson, U.S. Fish and Wildlife Service\
Richard P. Krueger, U.S. Fish and Wildlife Service\
Daniel W. Crabtree, Bureau of Reclamation

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 geochemical environment of the shallow ground-water system.
Selenium concentrations were as high as 1,300 micrograms per liter in
water from shallow wells. The highest concentrations of dissolved
selenium were in water form 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.

Irrigation drainage from the Uncompahgre Project and the Grand Valley
might account for as much as 75 percent of the selenium load in the
Colorado River near the Colorado-Utah State line. The primary source
areas of selenium were the eastern side of the Uncompahgre Project and
the western one-half of the Grand Valley, where there is extensive
irrigation on soils derived form Mancos Shale. The largest mean selenium
loads from tributary drainages were 14.0 pounds per day from
Loutsenhizer Arroyo in the Uncompahgre Project and 12.8 pounds per day
from Reed Wash in the Grand Valley. Positive correlations between
selenium loads and dissolved-solids loads could indicate that
salinity-control projects designed to decrease dissolved-solids loads
also could decrease selenium loads from the irrigated areas. Selenium
concentrations in irrigation drainage in Grand Valley were much higher
than concentrations predicted by simple evaporative concentration of
irrigation source water. Selenium probably is removed from pond water by
chemical and biological processes and incorporated into bottom sediment.
The maximum selenium concentration in bottom sediment was 47 micrograms
per gram from a pond on the eastern side of the Uncompahgre Project.

Selenium concentrations in some aquatic plants and aquatic invertebrates
exceeded a dietary guideline of 3 micrograms per gram dry weight for
protection of fish and wildlife resources. More than 75 percent of
whole-body fish samples collected from rivers and tributaries throughout
the study area had selenium concentrations that exceeded the 1984
National Contaminant Biomonitoring Program 85th-percentile concentration
of 0.73 microgram per gram wet weight. Whole-body fish samples from the
Gunnison River had significantly higher geometric mean selenium
concentrations than fish samples from the Uncompahgre and Colorado
Rivers.

Based on a risk assessment using selenium data for five Grand Valley
streams, the selenate form of selenium could be a moderate risk to the
swim-up life stage (less than 99 days old) of the endangered Colorado
squawfish and to three tested life stages of the endangered razorback
sucker. Zinc concentrations in Adobe Creek, Leach Creek, and Indian Wash
were in the high-risk category for the swim-up life stage of Colorado
squawfish.

Selenium concentrations in food items of migratory birds collected at
wetlands in the Uncompahgre Project area exceeded the dietary guideline
for protection of consumer wildlife. Fathead minnows from ponds had high
selenium concentrations, and the maximum concentration was 110
micrograms per gram dry weight. Selenium concentrations in bird eggs and
livers generally were in the range of uncertainty regarding biological
risk; however, some samples from the Uncompahgre Project area had
selenium concentrations in the high-risk category. Of 65 bird eggs
incubated, only one had any visual deformity. Five recently hatched
birds and two embryos that were found dead at two ponds in the
Uncompahgre Project area had selenium concentrations ranging from 19 to
43 micrograms per gram dry weight, but no abnormalities or deformities
were observed.

Toxicity tests on irrigation-drainage water from five Grand Valley
Streams indicated no significant toxicity differences between control
and test samples, except for the test on fathead minnows exposed to
water from leach Creek. Bioaccumulation of selenium from water to biota
was significant in the study area, and the highest bioaccumulation
factor was about 20,900 from water to bird livers. Selenium accumulates
in fish and birds in the study area to levels that would limit human
consumption if eaten on a daily basis. It is unlikely that fish and
birds taken in the study area are consumed by humans on a daily basis.