Unforeseen Consequences of Irrigation 

Drainage

 	

 Before 1980, regulation of irrigation return flow from agricultural projects focused mainly on management of salts, nutrients, and pesticide 

residues. In the early 1980's, selenium mobilized by irrigation water was 

discovered to be the cause of congenital deformities and mortality of birds 

at Kesterson Reservoir, a National Wildlife Refuge in central California. 

This unforeseen result of using irrigation drain water to sustain a wetland 

prompted the U.S. Department of the Interior (DOI) to create the National 

Irrigation Water Quality Program (NIWQP) to determine whether events 


at Kesterson could happen elsewhere in the United States.


 	

 Satellite imagery indicates that more than 66,000 mi2 of land in the 17 

conterminous Western States is irrigated for agriculture. Because complete 

investigation of every irrigated area in the Western United States is impractical, managers need to be able to predict where selenium contamination is 

likely.


Investigations in U.S. Department of the 

Interior Irrigation Project Areas

 	

 The DOI has constructed or manages more than 600 irrigation or 

irrigation-drainage projects and National Wildlife Refuges in 17 Western 

States. NIWQP grouped these projects and refuges into 192 areas. Scientists from the U.S. Geological Survey (USGS), U.S. Fish and Wildlife 

Service (USFWS), and Bureau of Reclamation investigated 26 of the 192 

NIWQP areas (fig. 1) to determine if irrigation drainage had caused or had 

the potential to cause harmful effects on human health or on fish and wildlife. Detailed investigations were made in 9 of the 26 areas after reconnaissance investigations confirmed that irrigation drainage had caused significant harmful effects. Summaries of these investigations and identification 

of the principal factors involved in irrigation-related selenium contamination of water and biota are contained in reports by Engberg and Sylvester 

(1993), Presser and others (1994), and Seiler (1995).


 	

 The 26 NIWQP areas were classified as selenium contaminated or 

uncontaminated on the basis of selenium concentrations measured in 

surface water (Seiler, 1995). An area was considered contaminated if at 

least 25 percent of the surface-water samples from it contained levels of 

selenium that equal or exceed the U.S. Environmental Protection Agency 

chronic criterion for the protection of freshwater aquatic life (5μg/L). By 

using this criterion, 12 of the 26 NIWQP areas were classified as contaminated (fig. 1).


Factors Affecting Severity of Selenium 

Contamination

 	

 During the 1930's, irrigation was considered a remedial measure for 

seleniferous soils because the drain water removed soluble selenium from 

the soils. Whether selenium is mobilized by irrigation and reaches levels 

toxic to fish and wildlife depends on the geology, climate, and hydrology 

of an area.

Geology

 	

 Knowledge of bedrock lithology is very important because the bedrock 

commonly is the parent material for the irrigated soils. The sources of 

selenium in soils and the distribution of seleniferous rocks in the United 

States were studied intensively during the 1930's and 1940's after selenium 

in pasturage was shown to cause a fatal disease in cattle and horses in 

South Dakota. One of the conclusions of that research was that any area 

where the soil is derived from material of Cretaceous age is likely to contain harmful quantities of selenium (Lakin and Byers, 1941). In 12 of the 

26 NIWQP areas, Upper Cretaceous marine sedimentary rocks form the 

bedrock, and 9 of these 12 areas have selenium-contaminated water. In the 

17 Western States, areas of Upper Cretaceous marine sedimentary rocks 

total about 311,000 mi2 (fig. 2).
 	

 Upper Cretaceous and Tertiary marine sedimentary deposits in nearby 

mountains are the principal sources of selenium for contaminated areas in 

the San Joaquin Valley of California (Presser, 1994). Erosion and local 

drainage carry selenium from seleniferous rocks in the mountains to irrigated land in the valley. Upper Cretaceous and Tertiary marine sedimentary rocks form the bedrock beneath 396,000 mi2 in the Western United 

States (fig. 2).


 	

 The occurrence of elevated selenium concentrations in other sedimentary rocks in the Western United States is localized. Trelease and Beath 

(1949) noted that Tertiary sedimentary deposits differ greatly in selenium 

content. Continental deposits of Tertiary age may be seleniferous if they 

were derived from Upper Cretaceous marine sedimentary rocks. 


 


Climate

 	

 Selenium contamination caused by irrigation has been observed only in 

arid or semiarid environments. To compare the aridity of different parts of 

the United States, the precipitation and evaporation rates were combined 

into a single number called the evaporation index (EI). The EI is calculated 

from the expression


 EI = FWSE ÷ PRECIP
 


where FWSE is annual free-water-surface evaporation and PRECIP is the 

annual precipitation. In the Eastern United States, the EI is less than 2.0, 

but in the Western United States, EI values greater than 2.5 characterize 

about 1,020,000 mi2, or more than half the land area (fig. 3). 

U.S. Department of the Interior—U.S. Geological Survey

Methods to Identify Areas Susceptible to 


Irrigation-Induced Selenium Contamination 


in the Western United States

120°

45°

40°

35°

30°

115° 110° 105° 100°

95°

0 250 500 MILES

0 250 500 KILOMETERS

Figure 1. Location of National Irrigation Water Quality Program areas 


investigated for potential irrigation-induced selenium contamination. Red 


signifies areas that were classified as selenium contaminated because 


25 percent of the surface-water samples contained levels of selenium 


that equal or exceed 5μg/L.

State boundaries from TIGER/Line


digital data, 1:100,000, 1992; coastlines


and hydrology from U.S. Geological Survey


digital data, 1:2,000,000, 1970


Albers Equal-Area Conic projection


Standard parallels 29°30' and 45°30',


central meridian 96°00'

MONTANA

WYOMING IDAHO

OREGON

NEVADA

WASHINGTON

CALIFORNIA

ARIZONA

UTAH

COLORADO

NEW


MEXICO

TEXAS

OKLAHOMA

NEBRASKA

KANSAS

SOUTH


DAKOTA

NORTH
 MONTANA

DAKOTA

WYOMING IDAHO

OREGON

NEVADA

WASHINGTON

CALIFORNIA

ARIZONA

UTAH

COLORADO

NEW


MEXICO

TEXAS

OKLAHOMA

NEBRASKA

KANSAS

SOUTH


DAKOTA

NORTH


DAKOTA
 Analysis of data from the 26 areas investigated by the NIWQP showed 

that an EI value of about 2.5 separated contaminated from uncontaminated 

areas (Seiler, 1995). Contamination was not observed in any NIWQP study 

area where the EI value was less than about 2.5, even where a local source 

of selenium has been identified. Given local sources, areas were found to 

be contaminated if EI values equaled or exceeded about 3.5, and most areas 

were contaminated at EI values of about 3.0. Where seleniferous rocks are 

upland from irrigated lands or where selenium is imported in water used for 

irrigation, EI values of about 4.5 or 8.0, depending on geology (fig. 4), 

separate contaminated from uncontaminated areas.


Hydrology

 Because drain water frequently is reused for irrigation downstream, it 

can be a source of selenium to other areas. An example of an area contaminated by imported selenium is the Imperial Valley in California, which 

receives selenium from drain water discharged to the Colorado River from 

irrigated areas in Utah, Colorado, and New Mexico. Because Imperial 

Valley has no local geologic source of selenium, it is contaminated because 

irrigation water supplies the selenium (Seiler, 1995).


 	

 Whether lakes or ponds in an area are terminal (that is, have no outlet) 

during nonflood years is also important. In terminal water bodies, selenium 

accumulates but is not flushed out during normal spring runoff.


Methods to Identify Areas Susceptible to 

Irrigation-Induced Selenium Contamination


 	

 Two methods were devised to allow managers to identify lands susceptible to selenium problems caused by irrigation. The first uses a decision 

tree (fig. 4) that poses questions about regional geology, climate, and 

hydrology to predict whether a specific irrigation project area is likely to 

have selenium problems (Seiler, 1995). The second uses a map (fig. 5) that 

combines geologic and climatic data to identify geographic regions where 

selenium problems are likely. 


 


 Because typically only limited site-specific information is available 

during an initial screening, many factors and processes that influence the 

actual amount or extent of contamination in a given area are not considered 

when using these methods. For example, the amount of land that overlies 

Upper Cretaceous or Tertiary rocks and the actual selenium content of the 

rocks are not considered. 

Individual Areas

 The decision tree (fig. 4) ranks an irrigation area according to whether a 

selenium problem is unlikely, possible, likely, or very probable. On the 

basis of this classification, a resources manager or planner can assess the 

need to collect additional information to quantify potential selenium 

contamination from irrigation drainage.


 	

 To use the decision tree, several yes/no questions are answered sequentially. Whether a particular area is characterized by Upper Cretaceous or 

Tertiary marine sedimentary deposits may be determined from the geologic map of the United States (King and Beikman, 1974) or from a 

larger-scale map such as a State geologic map. Approximate values for EI 

can be obtained from figure 3. If more exact values are needed, the FWSE 

rate from evaporation maps of the United States (Farnsworth and others, 

1982) can be divided by the annual precipitation. Answers to hydrogeologic questions can be obtained by examining topographic and geologic 

maps of the drainage basin supplying water to the area of interest.


Broad Geographic Areas

 Areas susceptible to selenium problems caused by irrigation were identified by creating a map (fig. 5) showing areas where Upper Cretaceous or 

Tertiary marine sedimentary rocks form the bedrock (fig. 2) and where EI 

values exceed 2.5 (fig. 3). About 160,000 mi2 of land in the Western 

United States thereby was identified as susceptible. The greater the EI, the 

more likely is the occurrence of selenium problems. The areas that are 

most susceptible to selenium problems have EI values greater than 3.5 

(fig. 5)—about 52,600 mi2. 


 Land adjacent to areas mapped as susceptible should be treated provisionally as susceptible because rocks upland from irrigated areas can yield 

selenium to downgradient areas. In California, the areas mapped as susceptible in figure 5 are all in the mountains, whereas the actual susceptible 

areas are in the valleys below.


120°

45°

40°

35°

30°

115° 110° 105° 100°

95°

0 250 500 MILES

0 250 500 KILOMETERS

Figure 2. Areas of Upper Cretaceous (green) or Tertiary (yellow) 


marine sedimentary deposits. Geology from King and Beikman (1974).

120°

45°

40°

35°

30°

115° 110° 105° 100°

95°

0 250 500 MILES

0 250 500 KILOMETERS

Evaporation index:

2.5 3.0 3.5 5.0

Figure 3. Areas where evaporation index is greater than 2.5. Climatic 


data from Farnsworth and others (1982) and G.H. Taylor (Oregon State 


Climatologist, written commun., 1994). 
 The susceptibility map (fig. 5) cannot identify every susceptible area 

because important hydrologic factors, such as whether an area downstream 

from irrigated land has terminal ponds and whether selenium is imported 

into an area, are not readily mappable. However, most of the areas where 

the NIWQP found irrigation-induced selenium contamination of surface 

water and all the areas where deformed birds were found can be identified 

by the map. 


 	

 A great advantage of maps as a means of identifying susceptible areas is 

that maps of land-use data can be superimposed. Then managers quickly 

can identify areas of interest where selenium problems may occur. Maps 

showing the susceptible areas and the locations of National Wildlife 

Refuges and Bureau of Reclamation project areas are being prepared 

jointly by the USGS and the USFWS for the DOI. However, managers of 

all Federal, State, local, and private entities that provide irrigation water or 

that receive irrigation drain water need to know whether and where selenium problems are likely. Figure 6 shows the irrigated areas in the Western 

United States and, by inference, which of those areas are susceptible to 

irrigation-induced selenium contamination. About 4,100 mi2 of irrigated 

land is within areas mapped as susceptible.


Limitations of the Methods

 Identifying circumstances when the methods presented here will not 

work is important. Both the decision tree and the map were constructed as 

if only those soils derived from Upper Cretaceous or Tertiary marine sedimentary deposits are seleniferous. Thus, the methods fail to identify areas 

where soils are derived from local seleniferous deposits that are not of Late 

Cretaceous or Tertiary age. 


 	

 Although no specific instances have been identified, another circumstance under which the methods might fail is where soils in irrigated areas 

are much more seleniferous than in the areas investigated by the NIWQP. 

In these areas, toxic concentrations could be reached even though EI values 

are less than 2.5. In extremely seleniferous areas, irrigation water could 

mobilize so much selenium that toxic concentrations could occur without 

much evaporative enrichment.

Does area


have terminal


lakes or ponds,


and is evaporation


index ≥ 8?

Selenium


problem


very


probable

Selenium


problem


unlikely

Does area


have major


upstream


selenium


source?

Selenium


problem


possible

Selenium


problem


likely

Is evaporation


index ≥ 4.5?

Does area


have terminal


lakes or ponds?

Does area


have terminal


lakes or ponds?

Does area


have terminal


lakes or ponds?

Is evaporation


index ≥ 2.5?

Is evaporation


index ≥ 3.5?

Is evaporation


index ≥ 3.5?

Selenium


problem


unlikely

Selenium


problem


possible

Selenium


problem


likely

Selenium


problem


unlikely

Selenium


problem


very


probable

Selenium


problem


likely

Selenium


problem


likely

Selenium


problem


possible

No Yes

No

No

Yes Yes

Yes

Yes Yes

Yes Yes Yes

Yes Yes

No

No

No No

No

No No

No

Figure 4. Decision tree for predicting likelihood of selenium problems. ≥, greater than or equal to. Modified from Seiler (1995) and used with permission 


of American Society of Agronomy.

Are


areas of


Upper Cretaceous


or Tertiary marine


sedimentary deposits


upland from


irrigated


lands?

Are


irrigated


lands on either


Upper Cretaceous or


Tertiary marine


sedimentary


deposits?

120°

45°

40°

35°

30°

115° 110° 105° 100°

95°

0 250 500 MILES

0 250 500 KILOMETERS

Evaporation index in areas where geologic units


 are mainly Upper Cretaceous or Tertiary marine 


 sedimentary deposits:

Figure 5. Areas predicted to be susceptible to irrigation-induced selenium 


contamination on the basis of geologic (fig. 2) and climatic (fig. 3) data. 

2.5 3.0 3.5 5.0
Which Method to Use?

 The advantages and disadvantages of each method are presented here to 

help decide which method is the most appropriate. The principal advantages of using the decision tree (fig. 4) are that it considers more of the 

factors that determine whether irrigation causes selenium contamination 

and that it actually ranks the likelihood that an area has (or may have) 

selenium problems. By slightly modifying the questions, the decision tree 

could be used for areas where seleniferous rocks of other than Late Cretaceous or Tertiary age are known to be the parent material for the soils. A 

disadvantage of using the decision tree is that it requires more information 

and is more time-consuming than using the maps. 	

 


 The principal advantage of using the maps (figs. 5 and 6) is that the risk 

of selenium problems in broad geographic regions can quickly be evaluated 

without the need to gather new information. Land-use overlays can be 

created to aid Federal, State, and local water managers to evaluate whether 

areas for which they are responsible are susceptible to selenium contamination. A disadvantage of the maps is that they do not take into account 

some important hydrologic factors in evaluating the risk of irrigationinduced selenium problems. Also, the small scale at which they are produced herein may reduce their usefulness, but the principles identified here 

can be used to create larger-scale State maps.

Sources of Additional Information

 The publications cited in this fact sheet contain additional information on 

selenium in irrigation drain water and on the methods used to predict where 

selenium problems may occur:


Earth Resources Observation System Data Center, 1990, Prototype 1990 


 conterminous U.S. land-cover characteristics data set [CD–ROM]: 


 U.S. Geological Survey, Earth Resources Observation System Data 


 Center.


 


Engberg, R.A., and Sylvester, M.A., 1993, Concentrations, distribution, 


 and sources of selenium from irrigated lands in Western United States: 


 Journal of Irrigation and Drainage Engineering, v. 119, no. 3, 


 p. 522–536.


 


Farnsworth, R.K., Thompson, E.S., and Peck, E.L., 1982, Free water 


 surface exchange, 1956–70, map 3 of Evaporation atlas for the con- 


 tiguous 48 United States: National Oceanic and Atmospheric Admin-


 istration Technical Report NWS 33, scale 1:5,000,000.


 


King, P.B., and Beikman, H.M., comps., 1974, Geologic map of the United 


 States (exclusive of Alaska and Hawaii): U.S. Geological Survey, three 


 sheets, scale 1:2,500,000.


 


Lakin, H.W., and Byers, H.G., 1941, Selenium occurrence in certain soils 


 in the United States, with a discussion of related topics—Sixth Report: 


 U.S. Department of Agriculture Technical Bulletin 783, 27 p.


Presser, T.S., 1994, "The Kesterson effect": Environmental Management, 


 v. 18, no. 3, p. 437–454. 


Presser, T.S., Sylvester, M.A., and Low, W.H., 1994, Bioaccumulation of 


 selenium from natural geologic sources in the Western United States and 


 its potential consequences: Environmental Management, v. 18, no. 3, 


 p. 423–426. 


Seiler, R.L., 1995, Prediction of areas where irrigation drainage may 


 induce selenium contamination of water: Journal of Environmental 


 Quality, v. 24, no. 5, p. 973–979.


Trelease, S.F., and Beath, O.A., 1949, Selenium—Its geological occurrence 


 and its biological effects in relation to botany, chemistry, agriculture, 


 nutrition, and medicine: Burlington, Vt., Champlain Printers, 292 p. 


 Information on geochemical and statistical tools to identify irrigated 

areas where surface water might contain elevated concentrations of selenium are available in U.S. Geological Survey Fact Sheet FS–077–96. Fact 

sheets and information on other technical reports and hydrologic data 

related to the National Irrigation Water Quality Program can be obtained 

from


Richard A. Engberg, Manager


National Irrigation Water Quality Program


Department of the Interior (6640–MIB)


1849 C Street, NW


Washington, DC 20240


e-mail: rengberg@ios.doi.gov


A product of the National Irrigation Water Quality Program


—Ralph L. Seiler (rseiler@usgs.gov)

March 1997 Fact Sheet FS–038–97

120°

45°

40°

35°

30°

115° 110° 105° 100°

95°

0 250 500 MILES

0 250 500 KILOMETERS

EXPLANATION

Area susceptible to contamination—Where evaporation 


 index is equal to or greater than 2.5 and where geologic 


 units are mainly Upper Cretaceous or Tertiary marine 


 sedimentary deposits

Irrigated agricultural land

Figure 6. Spatial relation between irrigated areas and areas susceptible 


to irrigation-induced selenium contamination. Irrigated-areas data derived


from Advanced Very High Resolution Radiometer data (resolution, 1.0 


km2 or 0.39 mi2) from the Earth Resources Observation System Data 


Center (1990).