USGS Publications Warehouse

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 watrer-extractable selenium. Only 5 of 128\x11alfalfa 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\x11micrograms 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\x11percent 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.\x11Environmental Protection Agency criterion of 5\x11micrograms 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\x11micrograms per liter in the Uncompahgre Project area and 380\x11micrograms 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 from 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 the 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\x11micrograms 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\x11bird 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.