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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
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. 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
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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
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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
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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. REFERENCES Boon, D. Y. 1989. Potential selenium problems in Great Plains soils. Pp. 107-121 in Selenium in Agriculture and the Environment. Special Publ. No. 23. L. W. Jacobs, ed. American Society of Agronomy, Inc., Madison, Wisconsin. Brady, N. C. 1974. The Nature and Properties of Soils. 8th Ed. MacMillan, New York. Burk, R. F. 1984. Selenium. Pp. 519-527 in Present Knowledge in Nutrition. 5th Ed. R. E. Olson, chairman. The Nutrition Foundation, Washington, D.C. Deason, J. P. 1989. Irrigation-induced contamination: How real a problem? Journal of Irrigation and Drainage Engineering 115, 9-20. Deverel, S. J., and S. K. Gallanthine. 1988. Relation of Salinity and Selenium in Shallow Groundwater to Hydrologic and Geochemical Processes, Western San Joaquin Valley, California. U.S. Geological Survey, Open File Report 88-336. Deverel, S. J., and S. P. Millard. 1988. Distribution and mobility of selenium and other trace elements in shallow groundwater of the Western San Joaquin Valley, California. Environ. Sci. Technol. 22~6~:697-702. Elrashidi, M. A., D. C. Adriano, and ~ L. Lindsay. 1989. Solubility, speciation, and transformation of selenium in soils. Pp. 51-63 in Selenium in Agriculture and the Environment. Special Publ. No. 23. L. W. Jacobs, ed. American Society of Agronomy, Inc., Madison, Wisconsin. Freeze, R. A., and J. A. Cherry. 1979. Groundwater. Prentice-Hall, Englewood Cliffs, New Jersey. Fujii, R., S. J. Deverel, and D. B. Hatfield. 1987. Distribution of Selenium in Soils of Agricultural Fields, Western San Joaquin Valley, California. U.S. Geological Survey, Open File Report 87-467. Jonez, A. R. 1983. Controlling salinity in the Colorado River basin, the arid West. Pp. 337-348 in Salinity in Water Courses and Reservoirs. R. H. French, ed. Butterworth, Boston. Klasing, S. A., and S. M. Pitch. 1988. Agricultural drainage water contamination in the San Joaquin Valley: A public health perspective for selenium, boron, and molybdenum. San Joaquin Valley Drainage Program, 2800 Cottage Way, Rm. W-2143, Sacramento, California. Klasing, S. A., and M. B. Schenker. 1988. Public health implications of elevated dietary selenium. In Selenium Contents in Animal and Human Food Crops Grown in California. R. Tanji, ed. Publication 3330. University of California SalinitylDrainage Task Force. Cooperative Extension, University of California, Division of Agriculture and Natural Resources. 102 pp.
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52 IRRIGATION-INDUCED WATER QUALITY PROBLEMS Lakin, H. hi! 1972. Selenium accumulation in soils and its absorption by plants and animals. In Geochemical Environment in Relation to Health and Disease. H. L. Cannon and H. C. Hopps, eds. Special Paper 140. Geological Society of America, Boulder, Colorado. Letey, J., C. Roberts, M. Penberth, and C. Vasek. 1986. An Agricultural Dilemma: Drainage Water and Toxics Disposal in the San Joaquin Valley. Special Publication 3319. Agricultural Experiment Station. University of California, Riverside. Division of Agriculture and Natural Resources. Levander, O. A. 1986. Selenium. Pp. 209-280 in Trace Elements in Human and Animal Nutrition. Vol. 2. 5th Ed. ~ Mertz, ed. Academic Press, New York. Longnecker, M. P. 1989. Selenium: The public health connection. Health and Environment Digest 3~3), 1-3. National Research Council, Assembly of Life Sciences, Committee on Medical and Biological Effects of Environmental Pollutants. 1976. Selenium. National Academy of Sciences, Washington, D.C. National Research Council, Food and Nutrition Board, Committee on Dietary Allowances. 1980. Recommended Dietary Allowances, 9th Ed. National Academy Press, Washing- ton, D.C. Olson, O. E. 1986. Selenium toxicity in animals with an emphasis on man. Journal of the American College of Toxicology 5, 45-70. Oster, J. D., J. E. Tracy, J. L. Meyer, and M. J. Snyder. 1988. Selenium in or near the southern Coastal Range: well waters and vegetable crops. Pp. 51-55 in Selenium Contents in Animal and Human Food Crops Grown in California. R. Tanji, ed. Uni- versity of California SalinitylDrainage Task Force. Cooperative Extension, University of California, Division of Agriculture and Natural Resources, Publication 3330. 102 PP Public Law 93-320. 1974. United States Statutes at Large, 93rd Congress, 2nd Session, Vol. 88 (Part D: 266-275. Rosenfeld, I., and O. A. Beath. 1964. Selenium: Geobotany, Biochemistry, and Nutrition. Academic Press, New York. San Joaquin Valley Drainage Program (SJVDP). 1987. Developing Options: An Overview of Efforts to Solve Agricultural Drainage and Drainage-Related Problems in the San Joaquin Valley. San Joaquin Valley Drainage Program, 2800 Cottage Way, Rm. W-2143, Sacramento, California. Shacklette, H. T., and J. G. Boerngen. 1984. Elemental concentrations in soils and other surface materials of the conterminous United States. Geological Survey Progress Paper 1272. Tracy, J. E., J. D. Oster, and R. J. Beaver. 1989. Selenium in a southern Coastal Range of California: Well waters, mapped geologic units and related elements. J. Environ. Qual., in press. U.S. Committee on Irrigation and Drainage (USCID). 1986. Toxic Substances in Agricultural Water Supply and Drainage: Defining the Problem. Proceedings of 1986 Regional Meetings. U.S. Committee on Irrigation and Drainage, Denver, Colorado. U.S. Fish and Wildlife Service. 1982a. Regional Resource Plan. Volume II. Central Valley of California Section. U.S. Fish and Wildlife Service, Portland, Oregon. 20 pp. U.S. Fish and Wildlife Service. 1982b. Regional Resource Plan. Volume IV. Central Valley of California Section. U.S. Fish and Wildlife Service, Portland, Oregon. 15 pp. U.S. Fish and Wildlife Service. May 1986. North American Waterfowl Management Plan: A Strategy for Cooperation. U.S. Department of the Interior, Washington, D.C.
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