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National Water-Quality Assessment (NAWQA) Program

U.S. Geological Survey
Circular 1180

Areas Susceptible to Irrigation-Induced Selenium Contamination of Water and Biota in the Western United States

By R.L. Seiler, J.P. Skorupa, and L.A. Peltz

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Abstract

The U.S. Department of the Interior (DOI) studied contamination induced by irrigation drainage in 26 areas of the Western United States during 1986-95. Comprehensive compilation, synthesis, and evaluation of the data resulting from these studies were initiated by DOI in 1992. Soils and ground water in irrigated areas of the West can contain high concentrations of selenium because of (1) residual selenium from the soil's parent rock beneath irrigated land; (2) selenium derived from rocks in mountains upland from irrigated land by erosion and transport along local drainages, and (3) selenium brought into the area in surface water imported for irrigation. Application of irrigation water to seleniferous soils can dissolve and mobilize selenium and create hydraulic gradients that cause the discharge of seleniferous ground water into irrigation drains. Given a source of selenium, the magnitude of selenium contamination in drainage-affected aquatic ecosystems is strongly related to the aridity of the area and the presence of terminal lakes and ponds. Marine sedimentary rocks and deposits of Late Cretaceous or Tertiary age are generally seleniferous in the Western United States. Depending on the origin and history, some Tertiary continental sedimentary deposits also are seleniferous. Irrigation of areas associated with these rocks and deposits can result in concentrations of selenium in water that exceed criteria for the protection of freshwater aquatic life.

Geologic and climatic data for the Western United States were evaluated and incorporated into a geographic information system (GIS) to produce a map identifying areas susceptible to irrigation-induced selenium contamination. Land is considered susceptible where a geologic source of selenium is in or near the area and where the evaporation rate is more than 2.5 times the precipitation rate. In the Western United States, about 160,000 square miles of land, which includes about 4,100 square miles (2.6 million acres) of land irrigated for agriculture, has been identified as being susceptible. Biological data were used to evaluate the reliability of the map. In 12 of DOI's 26 study areas, concentrations of selenium measured in bird eggs were elevated sufficiently to significantly reduce hatchability of the eggs. The GIS map identifies 9 of those 12 areas. Deformed bird embryos having classic symptoms of selenium toxicosis were found in four of the study areas, and the map identifies all four as susceptible to irrigation-induced selenium contamination.

Table of Contents

Abstract
Introduction
Background
Purpose and scope
Acknowledgments
Sources of selenium
Study methods
Physical classification of areas
    Geology
    Climate
    Hydrology
Classification of study areas by selenium concentrations
    Water
    Biota
Use of geographic information system to create maps
    Geologic data layers
    Climatologic data layers
    Land-use data layers
Factors affecting selenium concentrations in water
Geology
Climate
Hydrology
Deformities in aquatic birds and selenium concentrations in bird eggs
Map identifying areas susceptible to selenium contamination
Assessing map reliability
    Test areas
    Correctness of analysis
    Accuracy and precision of maps
Land use within susceptible areas
Discussion and summary
References cited

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