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OCR for page 37
Understanding the Scientific Dimensions
of an Environmental Problem
Decisionmakers must have a basic understanding of the general pro-
cesses by which irrigation degrades water quality before they can resolve
irrigation-induced problems, and this understanding needs to encompass
both scientific and institutional dimensions. This chapter briefly reviews
some of the hydrological, chemical, geological, ecological, and other phys-
ical factors that affect and are affected by irrigation. Understanding that
these factors set the stage for the development of problems is critical to any
attempt to select potential solutions, because no solution can be successful
unless it reflects some knowledge of the underlying natural processes at
work. The issues highlighted here are discussed extensively in other pub-
lications (e.g., Letey et al., 1986; SJVDP, 1987; USCID, 1986~. Chapter 3
examines the relevant institutional issues.
HOW IRRIGATION DRAINAGE ALTERS WATER QUALITY
Irrigation, simply defined, is the act of supplying land with water by
artificial means. Like other uses of water, irrigation degrades water quality
for some later users, particularly in arid climates. Irrigation also can lead to
an increase in soil salinity. The processes by which these changes occur are
natural, but they can be significantly accelerated under irrigation because
of the increased quantities of water involved (Brady, 1974~. Substantially
less natural leaching occurs in arid and semiarid regions than in humid
areas because less water is available; thus the changes caused by irrigation
can be more pronounced.
37
OCR for page 38
.
38
IRRIGATION-INDUCED WATER QUALITY PROBLEMS
Like other uses of water, irrigation can degrade water quality. This happens because
all irrigation water contains dissolved salts, and these salts are left behind as the water
evaporates from the soil surface or is taken up by plants and returned to the atmosphere.
If irrigation is to be maintained, adequate water must move down through the soil profile
to reduce the concentration of dissolved material in the root zone. If irrigation is a desired
use of water, then its waste waters must be treated and/or disposal provided for.
CREDIT: U.S. Bureau of Reclamation, J. C. Dahilig.
The potential for water quality degradation and salinity problems
arising from irrigation exists because all water contains dissolved salts. The
concentration of these salts, however, varies considerably depending on the
origin of the water. Once irrigation water has bean annlied to ~ field it
moves away from the point of application by various routes. Some water
evaporates from the soil surface, but much more is taken up by plants
and returned to the atmosphere through plant leaves. As this transpiration
continues, the salts originally dissolved in the irrigation water are left
behind. These mineral salts remain in the soil unless sufficient quantities
of water are applied to leach out the salts and carry them below the root
zone. Any water remaining in the root zone has a higher concentration of
dissolved salts (salinity) than the originally applied irrigation water had. If
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THE SCIENTIFIC DIMENSIONS
39
the salinity in the root zone increases too much, plants grow more slowly,
salt-sensitive plants die, and agriculture in the area suffers.
Adequate drainage whether natural or provided through installation
of drainage systems is a necessity to maintain irrigated agriculture over
time. Without leaching, the concentration of salts dispersed in soil solutions
continues to increase and can become sufficiently high that it prevents
crops from absorbing water. Without drainage to remove the leaching
water, the water table will rise. The end result is a waterlogged, saline soil.
Other factors can complicate the picture by either slowing or hastening the
fundamental trend, but they do not stop it.
Irrigated agriculture will always be a short-lived enterprise unless the
salts accumulating in the root zone are leached out. In most unaltered
(by humans) ecosystems, the most common path for soluble salt removal is
through the natural drainage system (e.g., rivers and creeks) to the ocean.
Over geologic time, the ocean is the ultimate sink for all dissolved salts
in the surface drainage system. Not all areas drain to the sea in a human
time frame, however. Drainage water can and does collect in closed basins.
Examples include the Dead Sea on the Jordan-Israel border, the Salton
Sea in southern California, and the Great Salt Lake in Utah, as well as the
reservoirs at both the Stillwater Wildlife Management area in Nevada and
Kesterson National Wildlife Refuge (NWR) in California.
As the names of some of these areas suggest, these natural or human-
made low points accumulate both water and salts. The water also leads
to the growth of riparian vegetation, and this attracts waterfowl and other
wildlife. When such enclosed water bodies are used to dispose of irrigation
drainage water, they may, through evaporation and other processes, be-
come saline quite quickly and can ultimately lose their capacity to support
biological productivity and diversity. The accumulation of trace elements
(some of which are toxic in low concentrations) and of agricultural pol-
lutants, such as pesticides or nitrates and phosphates from fertilizers, can
accelerate the deterioration of water quality, sometimes with disastrous
results.
As irrigation waters are concentrated in the soil, some of the dissolved
salts precipitate and form solid-phase minerals; thus the minerals gypsum
(CaSO4-2H2O) and calcium carbonate (CaC03) often accumulate in the
solid phase, becoming part of the soil. In other circumstances, salts may be
dissolved from the soil, a process that can lead to an increased concentration
of dissolved salts in the soil water. Highly saline drainage waters may also
displace good-quality ground water.
The leaching of soluble minerals from the soil and the displacement of
ground water are natural processes. Irrigation accelerates both processes.
Any water (whether from rainfall or irrigation) applied to the land in
excess of the evaporative demand passes down through the root zone and
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40
IRRIGATION-INDUCED WATER QUALITY PROBLEMS
becomes part of the local ground water. A rising ground water table,
in turn, increases local drainage how. (A "mound" of water develops
under the irrigation project.) On its subsurface path toward an outlet and
ultimate disposal or dispersal, the drainage water displaces older ground
water; frequently, this ground water contains dissolved salts of geologic
origin. Both drainage and displaced water ultimately flow via the surface
or subsurface drainage system and eventually end up in the ocean. When
the drainage process is interrupted by a closed basin (such as in Kesterson
NWR), the waters become trapped and can form "salt" lakes.
As the amount of land irrigated in the arid West increases, stream
salt loadings in areas of irrigation will also increase. Thus, the quality of
water is degraded as it mores downstream through a watershed in an arid
climate. An illustration may help to clarify this point. The total dissolved
solids concentration (TDS) in the upper reaches of the Colorado River is
generally less than 200 milligrams per liter (mg/1~; the TDS of the lower
reaches, where the river enters Mexico, typically is around 800 to 900 mg/1.
Over one-third of this increased salt load is contributed by the irrigated
areas in the Colorado River basin (Jonez, 1983~. Major salinity control
programs have been undertaken (cf. P.L. 93-320) to obtain a salinity level
agreed upon by treaty with Mexico, the last user on the river.
In the Grand River valley of Colorado, water is diverted from the
Colorado River for irrigation. Some of it is used consumptively, and some
returns to the river, but a substantial part (20 percent) infiltrates into the
soil and displaces ground water from the underlying salt-rich substrates.
This displaced saline ground water then flows into the river, increasing the
salt load. Water from the same river is again diverted for irrigation in
the Imperial and Coachella Valleys in California. Irrigation drainage water
from these areas then carries the salts leached out of the soils to the Salton
Sea.
In many areas, natural drainage rates are adequate to meet the needs
of irrigated agriculture. In other locales, the rates are too slow, and human-
made drainage systems such as underground collector tubes or tile drains,
open ditches, or pumped wells are added. Whatever their engineering
configuration, their purpose is to collect drainage water, sometimes of high
salinity, for disposal. The distinction between drainage water from a human-
made collector system and drainage water from natural processes can raise
important institutional issues and can affect capital costs. Conceptually,
however, there is no difference between them: the drainage water must be
removed to avoid waterlogging and salinization.
The dominant dissolved salt species involved in these processes include
the carbonates, bicarbonates, sulfates, and chlorides of sodium, calcium, and
magnesium. The adverse effects of these salts have long been recognized,
and considerable efforts have been expended in learning how to minimize
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THE SCIENTIFIC DIMENSIONS
41
them. Past experience with elements present in minor or trace amounts
indicated that, in most instances, their concentrations were low enough
that adverse effects were of little concern. However, recent investigations
have shown potential, serious impacts, especially for the trace elements
selenium, molybdenum, and arsenic (Deason, 1989~.
These elements were not carried in by irrigation water in most cases
but instead originated from in situ dissolving of geological materials. This
situation has added a new dimension to the problem of irrigation water
management. Drainage must now be managed not only to reduce salt
accumulation in the root zone and salt disposal in streams, but also to limit
the toxic effects of selected trace elements contributed by the local geology.
Furthermore, as noted earlier, agricultural drainage waters often contain
other contaminants (e.g., nitrates, pesticides, and soluble constituents).
Nitrate, for example, is a particularly mobile component, is easily leached,
and moves readily with water. It has been identified as a common cause of
ground water contamination beneath agricultural lands in California and
elsewhere (Freeze and Cherry, 1979~.
If irrigated agriculture is to be maintained, adequate water must move
down through the soil profile to reduce the concentration of solutes (dis-
solved material) in the root zone so that they do not exceed the level that
can be tolerated by the crops. Although the quantity of this flux can be
managed, the removal of excess salts is mandatory. Thus irrigated agr~cul-
ture over time cannot avoid causing an adverse oKsite effect. This effect
must be acknowledged: it can be minimized, internalized, or rejected, but
it cannot be ignored. If irrigation is a desired use of water, then its waste
waters must be treated and/or disposal provided for.
HYDROLOGY AND SOILS
The problems in the San Joaquin Valley and the selenium contami-
nation at Kesterson NWR vividly illustrate the relationship between the
physical environment and irrigation-induced water quality problems. Two
critical preconditions that set the stage in this case, and that play a similarly
fundamental role in these kinds of problems elsewhere, are hydrology and
soil composition. The soils on the west side of the San Joaquin Valley
are primarily derived from marine sedimentary rocks in the Coast Range.
These soils contain materials commonly found in areas of salt water depo-
sition. The sediments are fine-textured, and they contain impermeable clay
layers and elevated levels of trace elements (e.g., chromium, arsenic, and
boron) that are toxic at low concentrations.
Soils on the east side of the San Joaquin Valley are derived from
granitic parent material of the Sierra Nevada mountains to the east. These
soils are coarser-textured, contain little salt, and have fewer water-restricting
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42
IRRIGATION-INDUCED WATER QUALITY PROBLEkIS
clay layers. Consequently, waters on the east side contain much lower
concentrations of dissolved salts (Letey et al., 1986~.
The confining clay layers underlying the valley's west side inhibit deep
infiltration of the irrigation waters. Thus, as irrigation water was introduced
from outside the valley, the level of the prevailing water table rose. A high,
saline water table extending up into the root zone developed. High water
tables reduce crop productivity and increase soil management problems.
Many farmers have installed subsurface drain tubes buried 6 to 10 ft
deep, to supplement the natural drainage by collecting and conducting the
leachates out of waterlogged fields. The waters are then collected in sumps
and either pumped into discharge channels or conveyed to them by gravity.
These waters eventually work their way into the regional drainage system.
GEOLOGY AND GEOCHEMISTRY
Selenium is found in a variety of geologic formations. The marine
shales in California, South Dakota, and other western states, coal from
West Virginia and Kentucky, and volcanic formations in Hawaii all provide
high selenium concentrations. Selenium normally enters the biosphere by
natural weathering from the rocks that contain it. Areas of low and high
endemic selenium intake in humans and livestock have been identified
around the world. Western Oregon, parts of the midwestern United States,
most of New Zealand, and several areas of China generally have low or
very low soil levels of selenium, which may lead to low intakes in humans
and animals. Other areas of China, parts of Venezuela, and some localities
in the Great Plains region of the United States have high or very high soil
selenium levels with the potential for excess intakes (Burk, 1984~.
The selenium problems at Kesterson NWR resulted from a combina-
tion of natural geologic factors and human influences. The San Joaquin
Valley is a structural trough or valley lying between the Sierra Nevada
mountains on the east and the Coast Range on the west. As noted, the
soils in the basin trough were developed from a mix of geologic materials
derived from both ranges, but predominantly from the Sierra Nevada. The
deposits on the west side of the trough were formed by ephemeral and
intermittent streams coming out of the Coast Range.
The Coast Range shale deposits are of marine origin and contain a high
level of soluble salts and pyritic material. Selenium and seleniferous salts
are commonly associated with pyritic materials. In contrast, the igneous
rocks of the Sierra Nevada tend to form soils low in soluble salts and low
in pyrite and selenium-bearing minerals. Both geologic sources, however,
contribute significant amounts of other trace elements, such as arsenic,
boron, and molybdenum. (Deverel and Millard, 1988~.
Ground water moving downgradient toward the valley carries with it
OCR for page 43
THE SCIENTIFIC DIMENSIONS
43
soluble salts and, specifically, selenium. In time, transpiration and evapo-
ration cause salts to accumulate in the trough. Much of the variation in
salinity and selenium found in the shallow ground water or in the local
soils today is the result of natural processes and the impact of irrigation.
For example, soils that have been irrigated longer tend to have lost most
of their soluble selenium and other soluble salts, whereas in more recently
irrigated soils, present-day ground water salinity is closely correlated with
soil salinity (Deverel and Gallanthine, 1988~.
Very likely, the San Joaquin Valley's problems with selenium would
never have surfaced had it not been for human intervention. Natural
drainage from the valley was provided by the San Joaquin River system,
and dissolved salts, including selenium, were transported by this system
through to the delta and thence to the ocean. Introduction of irrigation,
with water imported from outside the valley, led to the need for additional
drainage capacity. It also mobilized the salts and selenium stored in the
soil profile.
Although the U.S. Bureau of Reclamation (USBR) anticipated prob-
lems associated with the management of saline soils and drainage water
in the San Joaquin Valley and made plans to mitigate them, it did not
anticipate the selenium problem; neither the USER nor any other group
anticipated that selenium could or would be a problem until it was actually
encountered in the drainage water. Selenium was not recognized as a
problem associated with the management of saline soils and drain water
until recently (Fuji) et al., 1987~. The problem stems from the cycling of
selenium induced largely by irrigation, collection of drainage water in a
master drain, delivery and storage in a closed basin~(Kesterson NWR),
concentration by evaporation, accumulation by biota, and transfer up the
trophic chain. Figure 2.1 shows in a general way the biogeochemical cycling
of selenium from its primary source (igneous extrusions and volcanic basest
through pathways to aquatic life, man, and animals.
The source of the selenium problem in the Kesterson NWR begins
with the chemical form of the selenium that occurs in soils and in the
parent materials. The parent materials for most of the seleniferous soils
in the western United States, including those in the Kesterson region,
are Cretaceous shales (Boon, 1989~. Compared to igneous and other
sedimentary rocks, the shales of Cretaceous age are elevated in selenium.
For example, whereas the average concentration of selenium (Se) in the
earth's crust is approximately 0.09 mg Se · kg-i, Cretaceous Pierre and
related shales average 2 mg Se . kg-i and may contain as much as a few
hundred mg Se . kg-i (Lakin, 1972~. Soils developed from these parent
materials commonly contain from 1 to 10 mg Se · kg-i, compared to a
mean for the entire western United States of 0.23 mg Se · kg-i (Shacklette
and Boerngen, 1984~. Where soils are alkaline, selenium occurs mainly in
cat ~
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44
IRRIGATION-INDUCED WATER QUALIW PROBLEMS
/K ~
~ PLANTS ~ SOILS
/~\ ANIMALS
-
~,
J SEDIMENTS AN:
T SEDIMENTARY
ROCKS
T
/ /AQUATIC \
// LIFE \
OCEANS,
SEAS,
AND
\ \ I ~ gES
~ \\ \!
ATMOSPHERE
\
VOLCANISM ~
FIGURE 2.1 Cycling of selenium in nature. EARTH'S
SOURCE: National Research Council, 1976. CORE
MOLTEN
-ROCK
RUNNIN;
AND
GROUND
WATERS
/
IGNEOUS ROCKS
the selenate (+6) form. Owing to its stability at alkaline pH values, its
high solubility, and its ready availability to plants, selenate is considered
to be the most dangerous chemical form of selenium as far as potential
environmental problems are concerned.
The chemistry of selenium resembles that of sulfur (S). Like sulfur,
selenium has four oxidation states: - 2, 0, +4, and +6. The solubility
and chemical form of selenium in soil solutions and surface waters depend
mainly on the pH and the redox condition of the system. In reducing
environments (waterlogged and/or flooded conditions), selenium exists in
the-2 (selenide) or 0 (elemental) oxidation state. Selenides and elemental
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THE SCIENTIFIC DIMENSIONS
45
selenium are very insoluble in water and as such quite inert and essentially
unavailable to biota (Elrashidi et al., 1989~.
In aerated systems, selenium occurs in either the +4 or +6 oxidation
state, depending mainly on the pH of the system. At high redox potentials
(~400 millivolts) and under alkaline conditions, the +6 form of selenium
as selenate ion (SeO4-2) is the dominant dissolved species. At moderate
oxidative potentials (100 to 400 millivolts) and near neutral to slightly
alkaline conditions, the selenite species (SeO3-2) is dominant, whereas
under acid conditions (pH ~3 to 7), the biselenite (HSeO3-) species
dominates.
Selenium is an element essential for animal nutrition, but the range
between dietary requirements and toxic levels is quite narrow. In general,
dietary requirements for most animals range from 0.05 to 0.3 mg Se. kg-i,
while a dietary concentration of 2 mg Se · kg-i on a continuing basis is
suggested as a maximum tolerable level for all species (National Research
Council, 1980~. The availability of selenium to biota depends largely on
its chemical form and competitive interactions among similar constituents
(i.e., sulfate, arsenate, and so on). In humid areas where parent materials
are high in selenium, slightly soluble selenite and biselenite oxyhydroxides
of iron and manganese are likely to form (Elrashidi et al., 1989~. Because
of the sparingly soluble nature of the selenite forms of selenium, plants
grown on soils in these regions will not contain levels of selenium that will
produce forage with levels potentially harmful to animals. However, they
should produce vegetation containing adequate selenium to protect wildlife
and domestic animals from selenium deficiency. In humid regions where
parent materials are low in selenium, plants produced are likely to contain
insufficient selenium, and deficiencies can result.
In well-drained, arid and semiarid regions where parent materials are
high in selenium, selenate and organic forms commonly dominate. The
selenate salts are highly soluble in water and readily available to biota.
Consequently, these areas are most likely to produce terrestrial vegetation
containing potentially toxic levels of selenium. Likewise in poorly drained,
periodically hooded areas where parent materials are high in selenium (e.g.,
Kesterson NWR) under alkaline conditions, both terrestrial and aquatic
vegetation may accumulate sufficient selenium to harm aquatic organisms,
waterfowl, and wildlife.
The selenium distribution in soils on the west side of the San Joaquin
Valley is influenced by landscape, topography, evaporation, and leaching
characteristics. Not surprisingly, soils located near selenium-containing
geologic materials have higher concentrations of selenium in the upper soil
horizons. Where the parent material source is more distant, weathering
and leaching result in the selenium being found in the subsoil. Selenium
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46
IRRIGATION-INDUCED WATER QUALITY PROBLEMS
The value of agriculture is no less important to the nation today than in the past; however,
the value of other natural resources has increased in the public's perception. Efforts
to respond to imgation-induced water qualifier problems need to recognize the increased
importance of these other environmental values. The availability of adequate fish and
wildlife habitat, as illustrated here along the Colorado River, is a critical consideration.
CREDIT: U.S. Bureau of Reclamation, E. E. Hertzog.
thus exists in shallow ground water at concentrations ranging from levels
too low for detection to levels of several hundred micrograms per liter.
FISH AND WILDLIFE CONSIDERATIONS
Another important scientific consideration for decisionmakers studying
irrigation-related contamination is fish and wildlife habitat. Public concern
for the quality of the habitat provided at Kesterson NWR was the major
force motivating cleanup efforts at that problem site and is but one example
of society's increased attention to non-economic environmental values.
Once again, the California example is illustrative. The Central Valley
of California once contained some of the finest bird and anadromous fish
habitats in the world. As the valley was developed and lands were converted
to agricultural use, fish and wildlife resources declined (Figure 2.2~. Today
OCR for page 47
THE SCIENTIFIC DIMENSIONS
in 4
o
._
. _
c
oh
~ 1
3
2
o
47
1850 - 4.1 million to 5.0 million acres of wetlands
1906 - 3.7 million acres of wetlands
1922 - 1.2 million acres of wetlands
1954 - 482,000 acres of wetlands
Present - 425,000 acres of wetlands
1906 \
Survey \
1 922
Surveys
-
1954
Survey
1 1 1 1 1 1 1 1 1 1 , , 1 -- °
1850 1860 1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1977
YEAR
FIGURE 2.2 Wetland losses in California, 1850 to 1977. Note: Estimates prior to 1900
range from 4.1 million to 5 million acres.
SOURCE: SJVDP, 1987.
only 300,000 to 425,000 acres of wetlands remain out of an estimated 4
million. In addition, an historic 6000 miles of productive stream and river
habitat have been reduced by about 85 percent, to only 950 miles, as a
result of the construction of dams and other major water developments
(U.S. Fish and Wildlife Service, 1982a).
Reduction of productive habitats has resulted in corresponding re-
ductions of fish and wildlife populations. A large number of species of
migratory birds, including waterfowl, shore birds, waders, raptors, and
passerines, winter in or pass through the Central Valley. About 60 percent
of the waterfowl population of the Pacific Flyway, including the entire
population of the endangered Aleutian Canada Goose, use the remaining
valley wetland habitat (U.S. Fish and Wildlife Service, 1982b). The North-
ern American Waterfowl Management Plan, adopted in 1986 by Canada
and the United States, recognized the importance of wetland habitat for
wintering flyway populations and established a goal of improving the quality
of all publicly managed habitat areas (U.S. Fish and Wildlife Service, 1986~.
Agriculture has long been considered the primary water user in the
West and has rights to the best-quality water. Many wildlife refuges in the
arid West are, in essence, terminal points for irrigation drainage schemes.
These areas often are located in closed basins with no outlet to the sea, and
so water quality problems can become severe. In all cases, the reduction in
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48
IRRIGATION-INDUCED WATER QUALITY PROBLEMS
stream How and the deteriorating quality of water have been stressful for
fish and wildlife.
The discovery of selenium in these terminal points Kesterson NWR
is but one example, albeit the first-is of particular concern because of (1)
selenium's effect on reproductive capacity, (2) its developmental toxicity,
and (3) its ability to bring about mortality for selected waterfowl. These
harmful effects indicate the hazards of relying on irrigation drainage water
as a source of water for wildlife refuges. Other elements such as arsenic,
cadmium, lead, chromium, boron, mercury, and molybdenum also are
found in selected drainage waters. Significantly high concentrations of any
of these elements in western wildlife refuges can be deleterious to living
things. Plant life, for example, is particularly sensitive to boron.
For the San Joaquin Valley, the decision to build the upper part of
the San Luis Drain before building the lower part (the outlet) set an
unexpected series of events in motion. As drainage waters were conveyed
to the ponds in Kesterson NWR and evaporated, salts accumulated as
expected. What was unanticipated was the buildup of high concentrations
of selenium and its consequent bioconcentration, which has had severe
impacts on waterfowl and fish. These effects, in turn, called attention to
other problems, such as the high levels of boron in the drainage waters,
and also sparked investigations looking for similar problems elsewhere in
the West.
PUBLIC HEALTH CONSIDERATIONS
Irrigation drainage waters often contain elevated concentrations of
many elements of geologic origin, as well as agricultural chemicals. When
the presence of elements of concern results in potential direct or indirect
exposure of humans, a public health concern may arise. ~ date, the most
frequently encountered public health concern from irrigation in the arid
West has been caused by elevated selenium concentrations, and therefore
its implications are discussed here as an example. Selenium is an essential
element necessary to human and animal health, but it has the potential
to cause toxicity at elevated levels. The margin of safety between levels
considered essential and levels associated with toxicity is small.
Natural sources of selenium have been known to cause toxicity in
free-living animal and human populations in regions around the world. In
areas of China and South Dakota, for example, "alkali disease" and "blind
staggers" (chronic and acute forms of animal selenium toxicity, respectively)
have been seen in animals grazing on seleniferous forage (Klasing and
Schenker, 1988~. Blind staggers occurs in animals that consume selenium-
accumulator plants over a period of weeks or months. The affected animals
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THE SCIENTIFIC DIMENSIONS
49
have impaired vision, and they wander, stagger, and finally succumb to
respiratory failure (Rosenfeld and Beath, 1964~.
Selenosis in humans is characterized by hair and nail changes, gastroin-
testinal discomfort and diarrhea, skin abnormalities, garlic breath, nausea,
fatigue, and irritability. No human deaths clearly attributable to selenium
toxicity from chronic exposure have ever been reported, although there
have been cases of acute selenium poisoning (Longnecker, 1989~. Because
deficiency and toxicity syndromes are relatively rare in humans, the effects
of chronic low or high selenium intakes in humans have not been clearly
defined (Klasing and Schenker, 19884. An episode of human selenium
toxicosis was observed in a region of China where environmental selenium
was unusually high and where human exposures were increased because of
drought conditions (Levander, 1986~.
As the San Joaquin Valley experience illustrates, human activities can
hasten the entry of selenium into the biosphere. Irrigation is not the only
mechanism: for instance, some coals produce a seleniferous fly ash that,
if improperly disposed of, can release selenium to enter the food chain.
In some South Dakota rivers, erosion and damming have contributed to
elevated selenium levels in the water. Similarly, irrigation-induced selenium
in water also has contributed to abnormally high selenium levels in wildlife
in Utah. Concern for public health surfaces because hunters and fishermen
may ingest fish and wildlife containing elevated concentrations of selenium.
The average American diet contains a safe and adequate selenium
intake, 50 to 200 micrograms per day (pa/day) (National Research Council,
1980~. Meats, especially liver and kidney, dairy products, eggs, certain
seafoods, and wheat products contribute most of the selenium. Selenium
intake depends on the amount of these foods consumed and their selenium
concentration, which varies by region. In general, it appears that healthy
adults are unlikely to suffer from selenium deficiency if their daily intake
is greater than 50 fig, and they are unlikely to suffer from selenium toxicity
if their daily intake is less than 5 ,ug per kilogram of body weight (or 350
,ug/day for a person weighing 70 kg) (Olson, 1986~.
Studies initiated on behalf of the San Joaquin Valley Drainage Program
on human health concerns associated with selenium in or near the Kesterson
Reservoir indicated that there was no basis for serious alarm in that region
(Klasing and Pitch, 1988~. That does not mean, however, that there is no
reason for further study of selenium uptake into food plants from irrigation
water in other areas. For example, in a survey of 107 irrigation and 44
livestock wells in the southern California Coast Range, selenium levels
were found to be above the drinking water standard (10 ~g/1) in 26 wells;
the irrigation water standard (20 ~ug/1) was exceeded in 11 wells (Oster et
al., 1988~. Other preliminary studies also have shown elevated levels of
selenium in some food products, but not to levels that warrant ameliorative
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50
IRRIGATION-INDUCED WATER QUALITY PROBLEMS
action. Thus the evidence to date indicates somewhat elevated levels of
selenium in some irrigation waters, with consequent elevation in some food
crops or animal products; however, to date the probability of an adverse
effect on the general population is remote. The relationship between
selenium in well waters and geological mapping units is clear enough that
reasonable predictions are feasible without excessive monitoring (Tracy et
al., 1989~.
In more general terms, any actions taken to address the agricultural
drainage problem in the San Joaquin Valley (or elsewhere) will result
in changes in the distribution, concentration, and possibly the types of
potentially hazardous contaminants to which people are exposed. Thus any
program to evaluate the feasibility and desirability of various actions to
resolve the problems of agricultural drainage should explicitly address the
public health concerns that might be raised by such actions. The public
health component of these evaluations should include the following steps:
1. An analysis of potential changes in the physical, chemical, and
biological transport and fate of contaminants resulting from a proposed
action.
2. An analysis of the potential intensity and extent of human exposure
resulting from those actions. This exposure assessment should consider total
exposure, including exposure through drinking water, air, foodstuffs, and
other possible routes.
3. An analysis of possible health effects that might result from the
exposures identified in step 2 above. To the extent feasible, these as-
sessments should consider synergistic and antagonistic effects among the
contaminants, and other possible health risks that exposed populations
might face.
CONCLUSIONS
Irrigated agriculture remains the most significant water user in the
West. Throughout the West, however, there is increasing pressure on a
diminishing and deteriorating water resource from numerous competing
interests (e.g., urbanization). With this competition comes an increasing
need, real and perceived, to find solutions to water quality problems ac-
ceptable not only to the irrigation interests but also to other parties. The
historic conversion of wetlands and wildlife habitats to agricultural and
other uses compounds the need to protect the remaining natural areas and
to ensure an uncontaminated water supply for state and federal refuges
and other wetlands.
The discovery of selenium poisoning in Kesterson NWR raises the
question of whether similar problems are occurring elsewhere. The answer
is clearly affirmative. The geohydrology of the West is such that the
OCR for page 51
THE SCIENTIFIC DIMENSIONS
51
processes that caused the accumulation of selenium in the San Joaquin
Valley are likely to play a similar role at other locations. The events that
have happened there over the past few years not only have heightened
the nation's awareness of such problems, but also have added a sense of
urgency to the search for solutions.
The primary problem associated with irrigation traditionally has been
salinity and how to dispose of drainage water at minimal cost to the
irrigator. Now, however, there Is an added dimension: how to protect
downstream and offsite users from the adverse effects caused by selenium
and other trace element contaminants. The events in California's San
Joaquin Valley and Kesterson NWR have caused some people to challenge
past assumptions that the benefits of irrigation always outweigh the costs.
As the demand for water increases, these problems will become more acute.
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Representative terms from entire chapter:
joaquin valley
