Sewage sludge as a source of environmental selenium

Th~ Science of the Total Environment, 100 (1991) 177-205

177

Elsevier Science Publishers B.V., Amsterdam

SEWAGE SLUDGE AS A SOURCE OF ENVIRONMENTAI. SELENIUM ,

C H R I S J. C A P P O N

Environmental Health Sciences Center, University of Rochester Medical Center, 575 Elmwood Avenue, Box EHSC, Rochester, N Y 14642, USA

ABSTRACT Information is presented on the impact of land application of municipal sewage sludge on the selenium content and speciation in soil,groundwater and edible vegetation. Sources and typical concentrations of selenium in sludge are documented. A discussion of selenium uptake by agricultural crops from sludge-amended soil includes results from greenhouse and field studies. A comparison is made with crop selenium uptake from fly ash application. The effect of sludge treatment on animal and human dietary selenium intake is quantitativelyevaluated, and selenium guidelines for sludge application are summerized. The conc!ueion is made that future widespread use of sludge on agricultural land will result in increased selenium uptake by food crops and human dietary intake. While this may not present an increased human health risk,long-term risks are identifiedand recommendations are made to minimize them.

INTRODUCTION

M a n y municipalities in the United States and other industrialized ~ations are currently faced with the problem of disposing of increasing amounts of wastewater sewage sludge using economically and environmentally sound methods that minimize risk to human health. The proven fertilizerand soil conditioning value of sludge has made land applicatk n a more beneficial and feasible disposal method over other options (incine~ration,ocean dumping, landfilling,pyrolysis).The majority of sludge applied to land is used on general arable and grazing land; very littleis used in forestry and horticulture. In England during 1970, an estimated 80% ofeludge produced was disposed of on land; approximately 50% of this was applied to agricultural land (Sterrittand Lester, 1980).In South Africa, the widely-practiced land application of sludge amounts to 62% of annual sludge production (Vail and Devey, 1984). Sludge disposal on agricultural land is most likely to increase in the future because of economic and technological restrictionson alternative disposal methods. Despite the advantages, the presence of considerable amounts of toxic trace elements in sludge is the major environmental drawback to land application. This may lead to a long-term hazard to plants, animals, m a n and water resources, especially when sludge is used on cropland because of element

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178

uptake by food crops and subsequent entry into the human food chail~. The most common trace elements in sludge include aluminum, iron, manganese, antimony, arsenic, chromium, lead, mercury, selenium, cadmium, molybdenum, copper, nickel, and zinc (Diamont, 1979). The last five elements, especially cadmium, pose the greatest hazard. The potential problems relating to element uptake by vegetation grown on sludge-amended soil have been reviewed by Hinesley et al. (19'72), Chaney (1973), Page (1974), Dick (1977), and Sterritt and Lester (1980). In general, phytotoxicity in many food and forage crops appears at elemental concentrations higher than those toxic to animals and man, so that contaminated crops do not give toxicity warning symptoms. Cadmium is generally agreed to be the limiting factor in land application, and many countries have established sludge application guidelines and regulations based on sludge cadmium concentration. Unlike the considerable research data on Cd, Cu, Mo, Ni, and Zn, very little information exists concerning the nature of Se in sludge and sludge-amended soil, including uptake by crops grown on such soil. Possible reasons for this are: (i) the emphasis on the known hazards of Cd; (ii) the relatively low Se concentrations reported for sludge; (iii) the known nutritional essentiality of Se for animals (and possibly man); and (iv) the protective effect of Se against heavy metal toxicity (notably Cd and Hg) in several animals species (Wilber, 1980; Shamberger, 1981). However, Se is also highly toxic at elevated concentrations, and the relative difference between essential and toxic dietary conce~xations is small. A diet containing < 0.05 ppm Se may result in Se-deficiency diseases, while a diet containing > 4 ppm (dry weight) may result in toxicity to experimental animals (Levander, 1976; National Research Council, 1976). In addition, Se occurs naturally in several chemical forms which vary widely in their nutritional and toxicity impact (Allaway, 1975). These two facts themse}ves necessitate the need for additional detailed information on the environmental and human health impact of sludge-derived Se. In fact, the United States Environmental Protection Agency (1985) has recently concluded that Se should receive closer scrutiny as a potential contaminant of the food chain. Recently, there has been considerable documentation of enhanced Se uptake by vegetable crops (Furr et al., 1976c, 1977, 1978a, 1979; Gutenmann et al., 1981), sweet clover (Furr et al., 1978b), corn grain (Combs et al., 1980), woody plants (Scanlon and Duggan, 1979), Astragalus racemosus (Gutenmann and Lisk, 1979), forage crops (Gutenmann et al., 1979), and tobacco (Gutenmann et al., 1983) grown on fly ash-amended soil. This paper summarizes the information currently available on the impact on Se from sludgc and offers recommendations for future research. In addition, the effect of land application of sludge on human dietary Se intake will be quantitatively evaluated.

179 SELENIUM

IN SLUDGE

Sources The majority of Se present in municipal sludge is of industrial origin, the input arising mostly from the following activities: ore mining, smelting, and refining; and the production of pigments, rubber, glass, ceramics, alloys, electronic components, and specialty chemicals• Sludge Se content will vary with location, time of sampling, spectrum of nearby industries, their changes in production rates, and periodic relocation. There is very little direct information on the levels of Se discharged into receiving waters that eventually undergo various forms of wastewater treatment resulting in sludge production• Available estimates have been based on tenuous assumptions• For example, in 1970, total estimated worldwide industrial Se emissions were ~ 8000 tons,* of which ~ 3500 tons were released as solid and liquid waste (National Research Council, 1976). The amount that would finally appear in sludge was not determined. In the United States, a survey conducted by the state of Wisconsin gave the total annual amount of Se discharged into receiving waters as 3 tons (Konrad and Kleinert, 1974). Approximately 50% (1.5 tons) of the discharged Se passing through mumclpal wastewater ,.reatment facflltms was concentrated in sludge• No reliable data are available for estimating Se input to sludge from commercial, agricultural, and residential sources, although it would probably be relatively minor compared with that from industrial activity. •

•

•

~

•

•

•

Content Sludge concentration data for Se are not as well-documented when compared with trace elements such as As, Cd, Cr, Mn, Ni, Pb, Zn, etc. The range of reported sludge Se concentrations is < 1-9 ppm, the common value being < 5 ppm (both on a dry weight basis) (Sterritt and Lester, 1980). A summary of sludge and sludge ash Se levels is presented in Table 1. Corresponding data for soil and various recycled w~ste materials that also have been applied to cropland are included for comparison. Sludge is generally more enriched in Se than uncontaminated soil, compost, and commercial fertilizer materials• Residual ash from coal-fired power plants'and sludge iiminerators is higher in Se content (Furr et al., 1980)•

Chemical form The extent of environmental impact of Se from sludge applied to agricultural land may depend upon the specific Se chemical forms in the sludge, as this affects Se uptake by p!.ants. Plant uptake will be discussed in the following section• Only recently has information appeared concerning Se speciation in * 1 t o n = 1.016 m e t r i c t o n n e s ~,t).

180 TABLE 1 Selenium concentration in sludge, soil,fertilizerand related waste materials Material

Sludge Digested, secondary

A~h

Composted

Fly ash (coal) Soil Uncontaminated

Selenium (ppm dry weight)

Remarks

Reference

3.1 5.4 1.9 0.4-3.0 1.55 (rural) 2.80 (urban) 1.0 1.9 2.53 5.0 i.7-9.6 10, 13 0.2 3.9-26.0 0.6 1.8 1.1 0.8

Average, 16 U.S. cities Denver, C O Washington, D C Rochester, N Y England

Furr et al. (1976d) Kienholz et al. (1979) Furr et al. (1976b) Cappon (1981, 1984) Elliott et al. (1981)

Syracuse, NY Parkway, MD South Africa England Los Angeles, CA United States United States United States Milorganite Milorganite Pre-cured Cured

Elfving et al. (1981) Greenberg et al. (1981) Vail and Devey (1984) Vail and Devey (1984) Logan et al. (1987) Furr et al. (1980) Greenberg et al. (1981) Cappon (1984) Cappon (1984) Furr et al. (1976a) Cappon (1984)

Ithaca, NY Pennsylvania

Furr et al. (1977) Heaton et al. (1982)

World

Sterritt and Lester (1980)

United Kingdom

Bowen (1979)

Belgium (agricultural soils) Ontario, Canada (agricultural soils) Canada

Robberecht et al. (1982)

5.1-16 2.3-8.7 0.1-2.0 (mean 0.2) 0.01-12 (average 0.4) 0.04-0.27 0.13-1.67

Organic potting Cactus sand Compost

0.3-2 (mean 0.4) 0.7 0.01 0.19 0.32

Organic fertilizers Peat moss Cow manure Fish emulsion

0.2 0.29 0.12 0.45 0.70 (ng ml- ' )

Frank et al. (1979) Webber and Shsm~ss (1987~ Cappon (1984) Cappon (1984)

Residential (Pittsford, NY) Municipal

Composted Dehydrated

Cappon (1984) Cappon (1984) Furr et al. (1976a) Cappon (1984) Cappon (1984) Cappon (1984) Cappon (1984)

181

sludge. This is due largely to the availability of new analytical methodology that permits direct identification and measurement of ultratrace levels (ppb or lower) of specific Se species in various sample matrices. Elliot et al. (1981) adapted a soil speciation scheme to urban and rural sludge samples from England. The scheme employed chemical extraction techniques and spectrofluorimetric assay. The specific extraction reagents used and Se species extracted are summarized in Table 2. The predominant Se forms reflected the sludge origin and treatment. The urban sludge, largely of industrial origin, contained reduced Se [elemental Se, divalent or Se( - II)] with little organic or oxidized Se [tetravalent or Se(IV), hexavalent or Se(VI)]. However, the anaerobic digestion treatment may have reduced the oxidized forms and degraded any organic species to the elemental state. The rural sludge, mostly from domestic sources and receiving less advanced treatment, contained higher portions of organic Se (derived from human wastes) and reduced forms. It must be noted that chemical extraction procedures of this nature may represent only arbitrary divisions between different Se forms, and may not be completely selective for one particular form. The usefulness of these procedures is uncertain in cases when the objective is to establish trace element speciation. Recent investigations conducted in our laboratory have used electroncapture gas-liquid chromatography (GLC) for assessing Se speciation in sludge and related materials (Cappon, 1981, 1984). Secondary anaerobically digested sludge, including composted samples, was obtained from local municipal wastewater treatment plants servicing Rochester, New York, and i~s immediate suburbs. In addition, related soil amendment materials and soil samples were examined. Speciation data are presented in Table 3. A significant percentage (9.4-89.9%) of the total sludge Se was Se(VI), the remainder being S e ( - II) and Se(IV). The Se(VI) percentage was higher in sludge ash and other fertilizer materials, and lower in soil. For composted sludge, the post-cured TABLE 2 Selenium speciation in sludgesa Extractant

Species extracted

Percentage of sludge Se Urban

0.2 M K2SO 4 0.05 M NH3 50% HC1 9M HNO3 Residue

Soluble organic Se, selenates, selenites Organic Se associated with base-soluble humic material Heavy metal selenates and selenites, degraded organic Se Selenides, elemental Se Selenides, elemental Se

a Data from Elliott et al. (1981).

0

Rural

0

7.1

24.3

6.5 29.3 57.0

4.2 22.1 49.0

182 TABLE 3

Selenium species, as percentage of total selenium, in sludge, soil, and related fertilizer materials a Material

Sludge Secondary, anaerobically digested Composted Pre.cured Cured Milorganite A~h Compost Residential Municipal Fertilizers Peat moss Cow manut'e Composted Dehydrated Fish emulsion

Percent S e ( - II), (IV)

Percent Se(VI)

69.9-90.6

9.4-30.1

81.7 60.1 78.3 63.7--67.6

18.3 39.9 21.7 32.6-36.3

60.3 34.4

39.7 65.6

6!.2

42.8

60.9 55.6 76.4

39.1 44.4 23.6

Remarks

Range for four wastewater treatment plants

Cured 6 months

Range for three wastewater treatment plants

"Data from Cappon (1981, 1984).

sample was higher in percent Se(VI). Apparently, composting of sludge favors conversion of Se(IV), and possibly S e ( - II), to Se(VI). Hexavalent and tetravalent Se are likely to exist as the water-soluble ionic species selenate (SeO 2- ) and selenite (SeO~-), respectively. Divalent Se could exist in various chemical forms such as selenoproteins or selenoamino Acids (se]evocyste~ne, selenomethionine), and volatile inorganic (H2Se) and organic selenides. Reamer and Zoller (1980) reported the conversion of inorganic Se to volatile methylated species (CH3)2Se, (CH3)2Se~, (CH3)~SeO~ by microbial activity in sludge and soil. Similar soil Se speciation results were reported by Jiang et al. (1989) that provided additional evidence that natural biomethylation of Se is relatively widespread in certain soil environments and that anaerobic conditions favor this process. Dimethylselenide, (CH~)2Se, and dimethyldiselenide, (CH~)2Se2, comprised most of the volatilized soil Se. The GLC method used for Se analysis in our study does not differentiate between the possible forms of Se(-II) (organic and inorganic). Therefore, it was not possible to identify and measure actual S e ( - II) species in the samples studied. Nothing is known about the interaction of specific Se chemical forms with other elements and compounds also present in sludge. Selenium has a very strong affinity for sulfhydryl (-SH) groups, especially on proteins and peptides (cysteine and methionine residues). In sludge, Se is most likely to be bound to

183

the -SH sites of the organic matter. Specific interactions between Se and toxic heavy metals (especially Cd, Hg, and Pb) in sludge would be of great importance, since Se is known to bind directly to these metals to form various seleno-metal-protein complexes in animal tissues (Parizek et a}., 1974; Ohi et al., 1976). Since these corr~plexes are less toxic forms of the metal, their formation represents a key detoxification mechanism in animals. For sludges containing significant levels of Cd and other heavy metals, the presence of such Se complexes could have important implications, especially if they also render these metals less available for plant uptake. SELENIUM UPTAKE BY PLANTS GROWN ON SLUDGE-AMENDED SOIL

Impact of sludge addition to soil The normal concentrations of toxic trace elements in soils are generally much lower than those in pure sludge. In untreated soils, these elements may be effectively isolated from chemical and biological processes by their inclusion in water-insoluble mineral structures, making them less readily available for absorption by plant roots. Therefore, any addition of sludge to land would almost certainly increase the concentration of several toxic elements in the soil, as well as the level of plant-available element species. Gerritse et al. (1982) demonstrated this by evaluating the mobility of 26 trace elements, including Se, in two sludged soils (sandy and sandy loam). Using calculations based on absorption data, sludge treatment (compared with addition of aqueous salt solutions) enhanced soil mobility of Se. This was reflected in increased Se partitioning into the soil solution. Greenhouse studies by Furr et al. (1976a, 1980) demonstrated significant increases in Se concentration for soils amended with a commercially-processed sludge (Milorganite) and incinerated sludge ash (Table 4). In our field study (Cappon, 1981), sludge addition to a previously untreated garden soil increased the total Se content by a factor of 2.5 after the initial application and by a factor of 3 after the second application year (Table 5). The Se levels of the sludged soils in this study were similar to that of Furr et al. (1976a). The Se(VI) TABLE 4 Selenium content of soils treated with milorganite and sludge ash Se|cnit.~ ~T,?m dry weight)

Treatment Milorganite Sludge ash Indianapolis, IN Kalamazoo, MI

Application rate (dry tons acre- i ) 100 50 50

Amendment material 1.8 10 13

Treated soil 0.3 4.0 4.8

Reference Furr et al. (1976a) Furr et al. (1980)

184 TABLE 5

Effect of sludge treatment on soil selenium content a Selenium (ppm dry weight) Tre~tment yearb

Soil

Sludge

Soil

Percent Se(VI), soil

1

Pretreated c Treated d Pretreated c Treated d

1.6

0.13 0.30 0.27 0.39

1.8 3.7 1.5 2.9

2

1.9

' Data from Cappon (1981). b Sludge application rate was 10.Sdry tons acre- 1. c Soil samples taken from sludged garden plot immediately before sludge addition. d Soil samples taken immediately before planting, 3 weeks following sludge application.

percentages in the untreated and sludged soils were considerably lower than that for the sludges, averaging 2.2% of the total Se compared with 14.9% for sludge. A Canadian survey of 11 different trace elements in 228 untreated agricultural soilsof Ontario province (Frank et al.,1979) revealed that Se contents of 30 sludged soils were not significantly different from those of unsludged soils (Table 6). Selenium levels in clay soils appeared higher than those in sandy soils. Agricultural activities vegetable and cash crops, pasture, orchards m did not influence Se content. However, most of the treated soilssurveyed had received between one and five sludge applications of unknown rate and over unspecified time periods. A field study by EI-Bassan et al. (1977) examined the effect of long-term application of specific urban wastes (waste water, sludge, refuse compost) on

TABLE 6

Selenium content of agricultural soils of Ontario, Canada" Soil type

Number

Selenium (ppm dry weight) b

Untreated Sandy Loam Clay Organic All soils

,~

70 88 62 8 228

0.27 0.38 0.48 0.34 0.37

Sludged

~

30

" D a t a of Frank et al. (1979). bValues represent mean and range (in parentheses).

(0.10-1.32) (0.13-1.67) (0.16-1.43) (0.10-0.75) (0.10-1.67)

0.37 (0.21-0.59)

185

As an Se accumulation and movement in cultivated soils in Germany. Field soils had previously been treated with large quantities of waste (waste water since 1986, sludge at 3050 m3, refuse compost at 2480 ton ha-l). For Se, analysis revealed that the soil contained 0.47-1.40ppm Se in the upper surface layer. Sludge and refuse compost resulted in only a slight increase in surface soil Se content, while waste water application led to no significant accumulation. Little is known about the chemical behavior of Se in soil (even less for sludged soil), which may theoretically contain variable amounts of relatively stable forms [Se(- II), Se(IV), Se(VI), elemental Se]. These forms may be either unbound (neutral or ionic) or bound to soil minerals (e.g., Fe and Al oxides, orthophosphates), colloids, and organic matter. Geering et al. (1968) found the Se chemical form and content in soil to be governed by several factors: pH, dissociation constants and solubility products of specific forms, and soil redox potentials. The transformation rate o f S e ( - II) and Se(IV) to Se(VI) is relatively slow, the reduced form being favored under conditions of acid or neutral pH. This may explain the relatively low Se(VI) percentages we found for sludged soil. The increase in soil organic matter content due to sludge treatment may enhance microbial volatilizationof Se from the soil,i.e.ionic Se(IV) and Se(VI) being converted into volatile inorganic and organic selenides (Doran and Alexander, 1977;Reamer and Zoller,1980; Zieve and Peterson, 1981).Microbial Se volatilizationby organic matter may result in a gradual decrease in soilSe levels,especiallyin situations where long-term sludge application is sporadic.

Crop uptake from sludged soil This is one aspect of environmental Se behavior that has received very little attention. Only four investigations have appeared in the literature, two greenhouse (Furr et al., 1976a, 1980), a limited field study (Furr et al., 1976b), and a more extensive field study (Cappon, 1981). The results of these studies are described below in more detail.

Greenhouse studies One investigation (Furr et al., 1976a) used Milorganite, a commerciallyproduced sludge product. A small but significant increase in Se content was observed for seven different vegetable crops grown on the treated soil. The other investigation (Furr et al., 1980) employed municipal sludge ash from two midwestern U.S. cities. Compared with the Milorganite results, the Se concentrations increased more consistently in sludge ash-grown crops, especially when the ash and ash-soil mixture were of a more acidic pH. This may be partially due to the higher Se content of the sludge ash and untreated soils. However, selenium uptake was lower than that observed for the same series of crops grown on soil amended with soft-coal fly ash (Furr et al., 1978a). Pertinent data of these investigations are summarized in Table 7.

186 TABLE 7

Comparison of the selenium content of vegetable crops grown on sludge, sludge ash, and soft-coal fly ash. Greenhouse studies Crop

Selenium (ppm dry weight) Fly ash"

Beans Cabbage Carrots Onions Potatoes Tomatoes Millet Grain Straw Application rate (dry tons acre- ! ) ppm Se, material ppm Se, treated soil

Sludge ash b

Sludge c

Cd

Td

C

T

C

T

0.03 0.05 0.02 0.02 0.03 0.02

1.4 3.1 1.2 2.4 2.4 1.1

0.04 0.07 0.04 0.02 0.03 0.03

0.12 0.10 0.11 0.08 0.10 0.08

0.02 0.01 0.00 0.00 0.01 0.01

0.04 0.11 0.03 0.03 0.04 0.02

0.03 0.04

1.2 0.6

0.03 0.04

0.11 0.06

0.02

0.03

100 12 0.3

50 13 4.8

100 1.8 0.3

a Data from Furr et al. (1979a). bData from Furr et al. (1980). c Data from Furr et al. (1976a). d C, control soil; T, treated soil.

Field studies The limited field study by Furr et al. (1976b) involved Swiss chard grown on soil amended with 100 dry tons acre- lc,~of sludge from Washington, DC. A total of 41 elements was evaluated in the sludge, soil and plant material. Selenium was among the 32 elements whose plant availability was not significantly influenced by sludge treatment or soil pH (5.5 and 6.5). The Se content of the sludge-grown plants was less than that of the corresponding control plants (Table 8). This finding contrasts with the obse'rvation reported in the author's greenhouse study, which employed a similar sludge application rate, soil pH, and sludge Se content. This may be due to several factors influencing plant availability of Se (discussed later in this section) under actual field conditions as opposed to more controlled greenhouse conditions. In our field study, which also evaluated Hg content and speciation, crops were grown on residential garden plots under conditions typically employed by home gardeners. Total edible tissue Se content of 15 different sludge-grown crops averaged two times higher than that for control crops (Table 9). Sludgegrown crops also had higher percentages of Se(VI) (24.0 versus 15.5 for * l t o n a c r c - 1 = 0.251kgm -2

187

TABLE

8

Effect of sludge treatment on the selenium content of Swiss chard. Field study" Medium

Selenium (ppm dry. weight), Swiss chard

Control soil p H 5.5 p H 6.5

0.08 0.08

Sludged soil p H 5.5 p H 6.5

0.05 0.06

~Data from Furr et al. (1976b). Sludge Se content was 1.9ppm and the application rate was 100 dry tons acre- i.

TABLE

9

Effect of sludge treatment on selenium content and chemical form of vega.table crops. Field study"

'~

~'~ ~'~'~"

Crop Bean, green bush Beet Broccoli Cabbage, savoy Cauliflower Cucumber, slicing Cucumber, pickling Lettuce, head Lettuce, leaf Onion, northern Parsley Radish, red Pumpkin Tomato

ppb Total Se (ng g- 1dry wt)

Percent Se(VI)

Edible portion

Control soil (C)

Sludged soil (S)

S/C

Control soil

Sludged soil

Pod Seed Tuber Top Leaf Leaf Top Fruit Fruit Leaf Leaf Tuber Leaf Tuber 'Fruit Fruit

8.4 1.2 3.3 16.0 6.1 12.0 25.2 5.5 4.6 15.0 12.1 27.3 4.3 27.1 11.9 26.0

17.4 11.0 7.8 26.0 16.7 34.8 34.8 10.2 7.¢: 23.4 41.4 46.7 4.8 56.6 18.3 56.4 Averages:

2.1 9.2 2.4 1.6 2.7 2.0 1.4 1.9 1.7 1.6 3.4 1.7 I.I 2.1 I..~ 2.2 2.1

7.2 13.1 15.7 PI.9 25.6 20.8 25.0 17.1 21.7 18.8 23.0 23.8 18.6 7.0 15.I 15.2 15.5

26.4 22.7 21.8 44.2 38.2 32.2 31.7 27.6 40.2 34.6 27.3 10.1 16.7 15.5 23.9 32.8 24.0

,

Data from Cappon (1981).

controls). Twenty-one additional crops grown on sludged soil had comparable total Se levelsand Se(VI) percentages (Table 10).Results from greenhouse (Van Dorst and Peterson, 1984) and solution culture (Gissel-Nielsen, 1973) experiments also suggest that Se(VI) is more readily bound by plant tissue than is Se(IV). These findings are consistent with our data. Leafy and tuberous vegetables were highest in total Se content ( > 30 ppb), while cole crops, garlic,

188 T A B L E 10 Selenium content and chemical form in sludge-grown vegetable crops. Field study a Crop

Portion

ppb Total Se (ng g- l dry wt)

Percent Se(VI)

Bean, yellow bush

Stem Leaf Pod Seed Pod Seed Root Tuber Leaf Stem Leaf Leaf Tuber Stem Leaf Leaf Stalk Leaf Tuber Stem Leaf Tuber Stem Fruit Root Stem Leaf Leaf Leaf Fruit Fruit Fruit Fruit

41.0 18.7 15.0 10.4 28.3 9.7 15.4 7.8 23.9 30.8 42.1 73.1 26.5 6.7 4.7 7.4 30.7 19.8 235.5 48.6 41.4 60.5 92.3 16.1 22.2 27.0 34.3 35.1 18.2 8.6 11.5 2.6 8.!

5.6 10.2 19.2 11.5 8.8 13.4 27.3 21.8 49.8 35.7 39.7 12.1 19.0 4.5 4.3 12.2 16.3 11.7 48.1 23.6 27.3 17.8 21.7 5.0 19.8 20.4 31.5 4.3 1.1 9.3 13.0 15.4 2.3

36.9 46.5 41.1 56.4

22.0 3g.8 34.1 32.8

Bean, lima Beet

Cabbage, late Cabbage, red Carrot

Cauliflower Celery Collards Garlic Lettuce, leaf Onion, Spanish Pepper Spinach

Swiss chard, Fordhook Swiss chard, Ruby Red Squash, zucchini Squash, summer Squash, acorn Squash, vegetable spaghetti Tomato

Stem Leaf Immature fruit Mature fruit

Data from Cappon (1981).

spinach, tomatoes, and cucumbers were highest in Se(VI) percentages ( > 30% in the edible portion). The Se levels we found for the sludged soil and sludge. grown beans, cabbage, carrots, onions and tomato samples were in good agreement with the greenhouse data of Furr et al.(1976a).The Se content of the sludge and sludged soil was similar for both studies, although the sludged-soil

189

pH was higher in our study (6.8 versus 5.3). The extent of Se translocation from the roots to the aerial (leaf) portion of specific crops was also examined. This was especially vronounced for beets, lettuce, spinach, and onions. Translocation from stem to fruit in tomatoes and from the pod to seed in beans was also noted. A quantitative evaluation of relative plant uptake of specific Se species was performed by calculating appropriate plant/soil concentration factors (CF) for both control and sludged soils (Table 11). The Se CF values indicated that the element is more readily assimilated by sludged-grown plants. For both soil treatments, especially sludged soil, Se(VI) was more readily assimilated by plants, while Se( II) and Se(IV) behaved similarly. A preliminary plant-availability study conducted by Muntau et al. (1987) estimated uptake coefficients (Ccrop/Csoil) for Se and 18 other trace and nutrient elements for red cabbage (Brassica oleracea) grown on soil treated with sewage sludge for eight consecutive years. The calculated uptake coefficient for Se was 2.8, which was much greater (by a factor of 33) than the CF values reported by the present author. Only boron had a higher uptake coefficient. No data on sludge Se content and sludge application rate were presented. Other recent field studies involving crops grown on sludge-treated soil have revealed negligible enhancement of crop Sv accumulation. Logan et al. (1987) investigated long-term Se uptake by barley, Swiss chard and radish grown on soil treated for up to 10 years with composted sewage sludge. The annual sludge application rates ranged from 0 (control) up to 180 Mg ha ~~. At the end of the final application year, there was little or no measurable Se uptake by barley leaf and grain at the study's level of detection (0.05 ppm). There were also no significant increases in chard or radish Se concentrations with sludge addition. Setenium concentrations in chard (0.05-0.11 ppm) were several fold lower than those in radish (0.17-0.26 ppm). The authors also examined the soil Se levels at the, surface (Ot5 cm) and in subsoil (up to 150 cm) regions. Only 13-25% of the sludge-applied Se could be accounted for ~n the surface region and there was -

TABLE 11 Selenium plant/soil concentration factors (CF) ~ Treatment

Sludge

Sludge Sludge ash Fly ash Fly ash

CF (ng g ~plantb:ng g zsoil)

Total Se Se( - II), (IV) Se(VI) Total Se Total Se Total Se Total Se

Control soil (C)

Treated soil (T)

0.082 0.068 0.393 0.05 0.13 0.07 0.08

0.083 0.063 0.728 0.10 0.02 1.20 0.23

T/C

Reference

1.01 0.93 1.85 2.00 0.~5 17.1 2.88

Cappon (1981)

Furr et al. (1976a) Furr et al. (1980) Furr et al. (1976c) Gutenmann et al. (1981)

a Data represent the average of CF values calculated for all crop samples analyzed for Se. b Edible portion.

190

no measurable Se in the subsoil.The low Se recovery of sludge-Se in the surface -region was attributed to: (i)leaching to the groundwater, and (ii)formation of gaseous Se forms and subsequent volatilizationfrom the soilsurface. Under the conditions of this study, the organic carbon loading to the surface layer from sludge was high and these conditions are known to favor formation of volatile Se compounds (kabata-Pendias and Pendias, 1984). However, it was uncertain how important gaseous Se volatilization was in this study. Limited data for Se content of corn silage originating from sludge-amended soil were presented by Bray et al. (1985). The soil treatments involved 3-year cumulative additions of 0, 60, 120 and 180Mgha -~. At the highest soil treatment level, the Se content of silage was almost twice that of the control crop. A fieldstudy reported by Vail and Devey (1984) revealed no excessive Se uptake by pasture and turf grasses grown on sludge-treated South African coastal soils.A single 100 ton ha-~ application of heat-treated sludge with an average Se content of 2.5 ppm was employed. A comparison of the impact of crop Se uptake due to fly ash application with that of sludge is of interest because of the ever-increasing production of fly ash waste from coal-firedpower plants, and the need for its proper disposal. There is more research data, greenhouse and field,available on Se uptake by crops grown on fly-ash amended soil.Table 11 also includes corresponding CF data calculated for greenhouse studies by Furr et al. (1978a) and Gutenmann et al. (1981). These data indicate Se to be more available to plants from fly ash than from sludge. This is probably due to ~he higher Se content of fly ash and ashed soil. Calculations were also performed from the greenhouse data involving sludge ash applications (Furr et al.,1980). The results reveal that Se in sludge ash is the least available when compared with that in fly ash and pure sludge. Apparently, a significantportion of the Se is present as highly water-insoluble or refractory species formed during the high-temperature incineration process. It is also possible that sludge ash may inlmobilize a portion of native soil Se. Earlier solution culture and greenhouse studies on the comparative availability of different Se species to crops (wheat, barley, oats, corn) and Se indicator plants revealed the following increasing uptake order: selenide, selenite,selenate, organic Se (Rosenfeld and Beath, 1964). This would account for the observed higher uptake efficiency of Se(VI) in our study. Numerous factors may influence the nature and extent of Se root absorption and phytotoxicity in plants (Chaney, 1973; Epstein and Chaney, 1976). These include: (i) element factors (chemical reactivity, oxidation state, chelation, concentration, reversion to unavailable forms, toxicity); (ii)soil and sludge factors [redox potential, pH, presence of ions, organic matter and phosphate content, cation exchange capacity (CEC), aeration, moisture, temperature]; (iii) plant factors (species and varieties, organ uptake susceptibility, age, seasonal effects).The potential interrelationshipsbetween two or more of these factors render the prediction of Se availabilityto plants from sludged soilvery complex and difficult.

191

Foliar absorption Another important factor which may influence plant Se uptake from sludged soil is foliar absorption of volatilized Se from the soil {as opposed to Se deposition from other airborne sources such as mumcipal incinerators and coal-fired power plants). Doran and Alexander (1977) initially demonstrated Se volatilization from loam and clay soils treated with elemental, inorganic, and organic Se. The process resulted from aerobic and anaerobic microbial activity. However, maximum Se conversion was 0.5% for loam soil. This has also been shown for pure sludge and sludged soil (Reamer and Zoller, 1980; Zieve and Petersen, 1981). Recently, Zieve and Petersen (1983, 1984) demonstrated root and foliar absorption of a volatile Se species, dimethylselenide. The process was directly dependent on the soil level of water-soluble Se (selenite, selenate), soil temperature, season, and extent of microbial ac,~ivity. Thus, sludge addition may enhance Se volatilization from soil. The actual extent to which this process occurs and the potential decrease in plant-available Se has not been studied.

Toxicity implications Selenium soil content An immediate concern with sludge application to agricultural land is whether or not the resulting soil Se levels will produce phytotoxicity in crops. Earlier studies have demonstrated that the order cf diminishing Se toxicity in crop plants is: selenite, selenate, organic Se (Johnson and Whitehead, 1951). Selenite was shown to inhibit growth of mature wheat plants when soil concentrations exceeded 4 ppm. Levine (1925) observed the same order and found that seed germination and plant growth were adversely affected when the Se compound was supplied at concentrations of 0.01% (100ppm) or more. However, a recent greenhouse study (Singh and Singh~ 1979) revealed that dry matter yield of cowpea plants decreased with Se application to ~:~andysoil in the order selenate > selenous acid > selenite > elemental Se. Both selenate and selenous acid inhibited dry matter yield when applied at a level of 1.0 ppm, while the other forms produced inhibition at 2.5 ppm of added Se. For the three sludge-application studies cited in this report, the total Se content of the treated soil (even for a relatively high application rate of 100 dry tons acre -~) was < 0.5 ppm. These results indicate that sludge application on cropland will probably not, even for long-term application, produce phytotoxic soil Se levels. However, this may be a problem for areas where seleniferous soils predominate.

Selenium speciation As with other toxic elements (Hg, Pb, Sn, As, etc.), Se toxicity is highly dependent on chemical form. Selenium frora different sources produces different clinical and physiological symptoms. The toxicity order for most animal species is: seleniferous plant Se > selenite > selenate > organic Se

192

T A B L E 12 Selenium compounds in water extracts of cabbage leaves" Compound

Percentage of total Se

Se-methylselenocysteine Se.methylselenocysteine selenoxide Selenocystathionine Selenomethionine Selenohomocystine Se.methylselenomethionine Selenopeptides and proteins Selenocysteine-typecompound Selenocystine Unknown

26.70 21.51 19.96 14.62 9.22 2.90 1.11 0.61 0.41 2.96

"Data from Hamilton (1975).

(Rosenfeld and Beath, 1964). Hence, knowledge of Se speciation in edible vegetation grown on soil treated with sludge and other Se-enriched wastes is vital from a standpoint of livestock and human health. The proportion of organic and inorganic Se compounds accumulated in plants depends on the, plant species and the soil concentration of specific chemical forms. Inorganic Se in plants occurs almost entirely as selenate and only insignificant amounts of selenite or elemental Se are present under normal conditions (Beath and Eppson, 1947). Apart from the speciation results from our laboratory, little is known about specific Se forms (especially organic compounds) tha'~ may exist in the edible portion of sludge-grown crops. Selenium that is readily available from soil (selenite, selenate, organic Se) and absorbed by plants is known to be metabolized to several organic compounds and consequently incorporated into plant proteins. In water culture experiments employing added selenite (GisselNielsen and Bisbjerg, 1970), selenomethionine was the most dominant Secontaining compound observed in corn and barley. Hamilton (1975) identified several water-soluble Se species in seleniferous cabbage (Table 12). Plants were grown under greenhouse conditions and treated with Se as a dilute solution of selenite (as H2SeO~). HEALTH AND OTHER ENVIRONMENTAL

CONSIDERATIONS

Selenium guidelines for land application of sludge Guidelines limiting sludge application to agricultural land first appeared in the early 1970s. In many countries, these guidelines were based on Cd loading rates to the soil, but plant nitrogen demand, presence of other individual elements (Cu, Ni, Zn), soil CEC, and intended land use are also considered. The only currently existing Se guideline for sludge application is from England, where the recommended limit of total Se addition to soil is 5kgha -1, or

193 TABLE 13 Model for calculation of total sludge application duration based on sludge elemental content"

(TCms - ICms)4.2 × lC~ n

=

where n = TCms = ICm~ = Crow = W~ =

(Cmw)Ww number of years until tolerable plant levels have been achieved tolerable element soil concentration (ppm) initial element soil concentration (ppm) element concentration in waste material (e.g., sludge) (ppm dry matter) weight of waste material in kg dry matter h a ~ yearly

Application case

ppm Se, sludge

Application rate (dry tons acre l )

n (years)

Cappon (1981) Furr et al. (1976a)

1.6 1.8

10.8 100

92.9 977

"EI.Bassan and Tietjen (1977).

1.86 kg acre-1 (Sterritt and Lester, 1~80). Applying this guideline to the data of our field study (Cappon, 1981) and the greenhouse study of Furr et al. (1976a), where the respective sludge application rates were 10.8 and 100 dry tons acre- ~, respectively, the corresponding maximum s~udge Se levels would be 189.4 and 20.5 ppm (dry weight). These levels are way in excess of the actual levels (1.6 and 1.8 ppm, respectively) cited in these studies, as well as the reported range of sludge Se content, so that a considerable safety margin exists. Recently, the Province of Ontario (Canada) established a set of maximum permissable concentrations for 11 trace elements in sludge-t~°eated agricultural soil (OMAF/ MOE/MOH, 1986). The corresponding level for Se was set at 1.6ppm (dry weight). E1-Bassan and Tietjen (1977) developed a model for determining the total sludge load based on sludge elemental content and plant tolerances to soil levels of specific elements. The model assumed a medium soil of pH 6.5 and sludge incorporation to 30 cm depth. The formula is outlined in Table 13, along with the corresponding data for the two investigations mentioned in the previous paragraph. The tolerable soil Se level was specified as 10ppm, although this value appears excessive in light of documented soil phytotoxicity data discussed earlier. The relatively long calculated sludge application durations would not be very practical, especially when other toxic elements such as Cd and Pb are present. Impact on groundwater With sludge. 8~ with ~ny hazardous waste material disposed on land, there is always the real risk of toxic elements and compounds (including Se) leaching from the amended soil and eventually entering the groundwater. Enrichment

194

of Se in sludge from wastewater treatment, increased Se content of sludged soil, and several physical and chemical soil factors (Fuller and Alesii, 1979) (e.g., soluble salts, pH, organic matter content, ion-exchange capacity, subsoil texture, degth and permeability) which may mobilize specific Se species over extended time periods, may increase the potential for significant quantities of the element leaching into groundwater. Aquifer depth, flowrate, and volume are important factors in determining groundwater Se concentration, and may serve to reduce it. Selenium enters groundwater primarily as a result of leaching of rocks and soils high in Se (Craun, 1984). There is a wide variation of Se concentration depending on geological location. Table 14 summarizes selected reported

TABLE 14 A. Selenium content of groundwater Location

ppb Se (~g 1-1 )

Remarks

Reference

United States

< 10-330

Forty-four wells from South Dakota (seleniferous area) Oregon farm wells

Smith and Westfall (1937)

< 1-2 50-125 1.0-36.8 (ave. 3.$2)

Australia W. Germany

<1 1.6-5.3

Southeastern Colorado rural tap water supply Mean values from 3676 residences in 35 geographically dispersed areas Twenty-two village wells Tap and mineral waters from Stuttgard

Hadjimarkos and Bonhorst (1961) Tsongas and Ferguson (1977) Craun (1984)

Edmond (1967) Oelschlager and Menke (1969)

B. Drinking water quality and other criteria Source U.S, Public Health Services U.S. Environmental Protection Agency World Health Organization U.S. National Research Council, National Academy of Sciences Utah

ppb Se ~ g 1-1 ) 50 10 10

Remarks

Reference

1942 1962 1977 (interim study)

Rail and Hadley (1976) U.S. EPA (1980)

50 100-500

9000

Maximum no observed human effect level Onset of chronic human Se toxicity symptoms

Rail and Hadley (1976) NRC (1980)

Rosenfeld and Beath (1964)

195

values for groundwater Se content, taken mainly from areas where the water serves as a source of local (rural) drinking water. Cdrrent government Se standards for public drinking water and water quality criteria for aquatic life are included for comparison. Recent studies in the Central Valley of California indicate that naturally high levels of soil Se can be leached by irrigation and result in elevated concentrations in drainage water (Burau, 1985). However, there are no documented data on Se contamination of groundwater due to long-term land application of sludge. There are currently no leachate guidelines for Se pertaining to ground and surface water quality. The problem of potentially hazardous Se levels in groundwater has been restricted to seleniferous geographical areas where there is an excess of Se in rocks and soils (Rosenfeld and Beath, 1964). Currently, it appears that leaching of sludge-borne Se would have very little impact on groundwater quality, especially for nonseleniferous areas. This was observed for Cd, where leaching into groundwater from sludge disposal on land did not occur even when higher rates of industrial and municipal sludge were applied to moderately high permeable soils - - sandy loam and silt loam (Ritter and Eastburn, 1975). Similar results with other toxic elements (Co, Cu, Mn, Ni, and Pb) were obtained in leachate studies under greenhouse and field conditions incorporating liquid sludge on acid mine spoils (Urie et al., 1982). The elemental content of soil leachate decreased at 120 cm depth during 3 years following sludge application. This was probably due to element binding to sludge organic matter. Selenium may very well exhibit similar behavior, especially because of its known affinity f o r - S H sites on proteinaceous material. Migration experiments by E1-Bassan et al. (1977) revealed that downward. Se movement in sludged soils was less than that in untreated soils. Selenium reached a depth of 80 cm after irrigation with 700 mm of 1000 ppm Se solution for an 88-day period. Higher soil clay content reduced the Se downward transport rate. Potential impact on animal and human dietary intake Application of sludge to land used for edible crop and forage production is very likely to result in an elevated Se content of vegetation consumed by humans and domestic food animals. The major long-term concerns here are: (i) phytotoxicity, which would decrease crop yield, and consequent economic losses, and, more importantly, (ii) Se bioaccumulation in the food chain, which may result in elevated dietary intakes approaching and/or exceeding any acceptable Se intake guidelines for food. Animals Two recent studies have offered some insight into the extent of Se intake and tissue accumulation in animals fed diets of specific food crops grown on sludged soil (Furr et al., 1976b) or feed mixed with sludge (Kienholz et al., 1979). In the former study, Se was among 41 elements examined in selected tissues of guinea

196 TABLE 15 Selenium concentration in tissues of guinea pigs fed sludge-grown Swiss chard ~ Soil treatment

Control Sludge pH 5.5 pH 6.5

Selenium (ppm dry weight) Liver

Kidney

Muscle

Adrenal

Spleen

1.12

1.61

0.38

2.9

2.3

0.82 0.90

1.59 1.69

0.31 0.37

5.8 2.5

3.1 0.3

"Data from Furr et al. (1976b).

pigs fed for 28 days on diets containing 45% (by weight) of Swiss chard grown on sludged soils at two pH levels (5.5 and 6.5). The diet produced no observable toxicological effects. Selenium deposition, based on the observed tissue levels for three animal study groups (Table 15), was of the following order: adrenal > kidney > liver > spleen >> muscle. Except for spleen and liver, Se tissue levels in the animals fed sludge-grown Swiss chard were similar to, and even slightly lower than, those for the control animals. The results of Kienholz's study are summarized in Table 16. Selenium levels increased in liver, blood, and visceral fat of steers fed diets fortified with either 4 or 12% (dry weight) sludge for 94 days. There was no significant i~lcrease in Se content for edible muscle. Animal growth was lowered because the sludge component provided no dietary energy value. However, no toxicity due to sludge ingestion was observed. There is currently only one guideline for Se regarding the maximum tolerable dietary levels for agricultural livestock, established by the U.K. Department of the Environment (Department of the Environment, U.K., 1977). This was 2 ppm (dry dietary weight) for cattle and sheep. Humans Because of the lack of sufficient published data, it is quite difficult to assess the potential human health hazard from Se due to sludge application to agricultural land. At best, it is possible to obtain rough estimates of the expected increased Se dietary intake and relate it to suggested maximum dietary guidelines. The human daily dietary Se requirement is not known. However, it is believed that most people benefit from dietary intakes of 60-300 /~gday -1 (Weisz, 1983). Table 17 summarizes suggested human Se dietary guidelines for the United States. It has recently been suggested that the upper recommended United States dietary guideline may be too restrictive (Palmer et al., 1983). Definite toxic effects of Se of food origin in man have not been well documented. The only reported incidences were selenosis from a seleniferous areas of the United States (South Dakota) in the late 1930s (Lemly and Merryman, 1941) and the People's Republic of China in the early 1960s (Yang et al., 1983). Based on these observations and considering normal Se content of

4.2

12 0.72 630 Data from Kienholz et al. (1979). b Sludge Se c o n t e n t was 5.4 ppm. c No value reported. d No tissue analysis performed.

Liver 1.3 1.7

0.16 0.35

0 4

Total Se i n t a k e (mg)

5.4

5.4 c

Kidney

0.8

0.8 c

Muscle

Selenium (ppm dry weight)

140 320

ppm Se

P e r c e n t sludge b

Diet

Selenium c o n t e n t of tissues of steers fed a sludge-based diet for 94 days a

TABLE 16

0.1

0.1 c

Bone

0.9

0.9 d

Brain

1.7

1.3 d

Blood

1.2

1.2 d

Lung

1.3

1.4 d

Spleen

0.09

0.07 d

Fat

198 TABLE 17 Recommended human dietary guidelines for selenium: United States Guideline ~ g Se day- 1)

Remarks

Reference

50-200 60-120 50-100

1980 RDA 1976 RDA Supplements for persons living in low Se areas Infants (0-6 months) Infants (6 months-1 year) Children (1-3 years) Children (4-6 years) Children ( > 7 years) Maximum safe single dose (adult) Maximum safe multiple dose (adult)

NRC (1980) NRC (1976) NRC (1976)

10-40 20-60 20-80 30-120 50-200 50 (ug kg- l body wt) 5 (~g kg- 1body wt)

NRC (1980)

Olson (1986)

foods, 500/~g person - xday- ~was estimated as a maximum tolerable intake level (Sakurai and Tsuchiya, 1975; Lo and Sandi, 1980). Olson (1986) recently reviewed and evaluated the literature on acute and chronic Se toxicity in animals (farm and experimental) and man in order to establish a tentative maximum safe human Se intake. He suggested a maximum single oral dose of selenite, selenate, selenocysteine, or selenomethionine of 50/~g Se kg-x body weight (3.5 mg for a 70-kg adult). The suggested maximum safe multiple (long-term) oral dose was 5/~g Se kg-~ body weight (350/~g for a 70-kg adult). The latter dose is lower than the calculated intake for a hypothetical diet consisting of specific sludge-grown foodstuffs (see Table 21). It is important to note that these recommended maximum safe oral Se doses were based on very limited human toxicological data, and corresponding animal data was extensively used for the estimates. Surveys of Se content of foods (Morris and Levander, 1970; Levander, 1976; Lo and Sandi, 1980; Olson and Palmer, 1984), along with various composite dietary surveys (Mahaffey et al., 1975; Thompson et al., 1975; Thorn et al., 1978; Cross et al., 1978; Olson and Palmer, 1978) reveal that fruits, vegetables, cereals and grains, the foods that could originate from sludged soil, contribute up to 62% of the total daily adult Se intake. This is significant since Se from plant foods has a higher bioavailability than that of animal origin (Levander, 1976). Hallenbeck (1979) described a model that provided a quantitative estimate of the daily dietary increase of trace elements when foods are derived from sludged soil. Applying this model to Cd, it was shown that a diet, or part of a diet, derived completely from sludged soil is likely to be excessive in Cd and Jose a clear health hazard. This model was applied to the results of our field ~tudy. Tables 18-20 summarize the pertinent information and calculations. The diet evaluated is that employed by the United States Food and Drug Administration (FDA) for its Total Diet Studies and applies to a 15-20-year-old male

199 TABLE 18 Composition of FDA diet for total diet studies (1974) ° Food class

Dairy Meat, fish, poultry Grains, cereals Potatoes Leafy vegetables Legume vegetables Root vegetables Garden fruits Fruits Oils and fats Sugars and adjuncts Beverages Total

Average daily intake (g wet weight)

763 267 422 183 55 69 33 92 221 72 82 712

Median Se in food class ~ g g- l w e t wt) b

Median daily Se intake ~g)

0.069 0.378 0.387 0.007 0.016 0.008 0.019 0.007 0.006 0.017 0.006 0.020

52.6 100.9 163.3 1.3 0.9 0.6 0.6 0.6 1.3 1.2 0.5 14.2

2971

338.0

Diet composition based on FY 1974 U.S. FDA Total Diet Studies (Hallenbeck, 1~c79). bValues based on the data of Levander (1976).

TABLE

19

Selenium levels in crops raised on non-sludged soil and sludged soil~ Selenium Foodstuff

Control plot (/~gg- Idry wt)

Sludged plot (/~gg- ~dry wt)

Increase (l(gg- ~dry wt)

Wheat (grain) b Beet (tuber) Lettuce (leaf) Beans (legume) Carrot (root) Tomato (garden fruit)

0.22 0.008 0.012 0.010 0.011 0.026

0.44 0.022 0.041 0.028 0.027 0.056

0.22 0.014 0.029 0.018 0.016 0.030

a Crop data taken from Cappon (1981). b Control value taken from Mahaffey et al. (1975) and the sludged value assumed to be higher by a factor of 2.

(Table 18).The estimated quantity of additional dietary Se that may resultfrom sludge application (82.6/~g)is shown !LnTable 20, which is based on the data calculated in Table 19 for the increased Se levels in representative sludgegrown foodstuffs. Most of the increase in dietary Se results from grain and cereal intake. W h e n the 82.6/~gday-i increase is added to the present median intake of 338.0/~g day- 1, the total Se intake may be 420.6/~g day- 1, which is higher than most suggested recommended maximum intakes. A summary

200 TABLE 20 Estimation of the daily dietary selenium increase derived from sludge-grown crops Representativ ~ crop

Dry weight

Dry weight of food class a (g)

Wheat 88 371.4 Beet 20 36.6 Lettuce 5 2.8 Bean 10 6.9 Carrot 12 4.0 Tomato 6 5.5 Total qietary Se increase resulting from sludged soil:

Selenium increase in diet (ug day- ~) 81.7 0.5 0.08 0.1 0.06 0.2 82.6

a Calculated by multiplying the percent dry weight of the representative crop by the wet weight of the food class given in Table 18.

T A B L E 21 Human dietary intake of selenium Country

Intake ~g Se day- i)

Remarks

Reference

United States

150-169 23-200 331 186 159 159

F D A total diet study Hospital diets High animal protein diet Ovo-lacto-vegetarian diet Vegetarian diet Seafood-lacto.vegetarian diet South Dakota (high Se area) Glasgow

Mahaffey et al. (1975) Lo and Sandi (1980) Palmer et al. (1983)

216 Scotland England Canada Japan

234 60 98-224 50-150

Bangladesh Venezuela

63-122 326

Olson and Palmer (1978) Cross et al. (1978) Thorn et al. (1978) Thompson et al. (1975) Sakurai and Tsuchiya (1975) Levander (1976) Levander (1976)

comparison of this value with reported Se intakes from the United Ststes and other countries is presented in Table 21. This may be of long.term concern since the estimated increased Se intake was based on a relatively low sludge application rate (10.S dry tons acre -~) and sludge Se content (1.6ppm). Persons at greatest risk from increased Se intake due to agricultural land application of sludge would be strict vegetarians and neonates, especially from seleniferous reglons.

201 CONCLUSIONS

It appears that any future widespread use of municipal sewage sludge on cropland and subsequent Se uptake by food crops will lead to increased daily h u m a n d i e t a r y i n t a k e of Se. T h i s i n c r e a s e d i n t a k e m a y n o t pose a n i m m e d i a t e h u m a n h e a l t h h a z a r d , a t l e a s t for g e o g r a p h i c a l r e g i o n s w i t h n o r m a l or low soil Se c o n t e n t . H o w e v e r , t h e r e m a y be l o n g - t e r m localized r i s k s to s u s c e p t i b l e population subgroups (vegetarians and neonates), especially from seleniferous r e g i o n s . O n t h e o t h e r h a n d , Se p h y t o t o x i c i t y of e s s e n t i a l food a n d f o r a g e crops as well as g r o u n d w a t e r c o n t a m i n a t i o n will n o t likely be a p r o b l e m . To mL~.imize .-.ny p o t e n t i a l h e a l t h risks, t h e f o l l o w i n g a r e e s s e n t i a l : (i) C o n t i n u o u s a d e q u a t e e n v i r o n m e n t a l m o n i t o r i n g of Se c o n c e n t r a t i o n s in a p p r o p r i a t e m e d i a (sludge, soil, v e g e t a t i o n , l e c h a t e g r o u n d w a t e r ) i n v o l v e d in l a n d a p p l i c a t i o n p r o g r a m s . T h i s will e n s u r e t h e m a i n t e n a n c e of a n y e n v i r o n m e n t a l q u a l i t y c r i t e r i a ;

(ii) Conducting additional large-scale, long-term field experiments which include additional soil and crop types (especially grain and forage crops) in order to confirm the results of previous field studies; (iii) Establishing sludge application guidelines based on sludge Se content, although, in most cases, the more practical application rate will be based on the Cd content; (iv) Establishing dietary Se guidelines for susceptible population subgroups and reevaluating the recommended upper intake guidelines for the general population. REFERENCES Allaway, W.H., 1975. The effect of soils and fertilizers on human and animal nutrition. Agriculture Information Bulletin, No. 378. United States Department of Agriculture, Aglicultural Research Service, Washington, DC, iv + 32pp. Beath, O.A. and H.F. Eppson, 1947. The form of selenium in some v~getation. Wyo., Agric. Exp. Stn, Bull., 278:1 20. Bowen, H.J.M., 1979. Environmental Chemistry of the Elements. Academic Press. London, ix + 333pp. Bray, B.J., R.H. Dowdy, R.D. Goodrich and D.E. Pamp, 1985. Trace metal accumulations in ti~,sues of goats fed silage produced on sewage sludge-amended soil. J. Environ. Qual., 14:114 118. Burau, R.G., 1985 Environmental chemistry of selenium. Calif. Agric., 39: 16-18. Cappon, C.J., 1981. Mercury and selenium content and chemical form in vegetable crops grown on sludge-amended soil. Arch. Environ. Contain. Toxicol., 10: 673-689. Cappon, C.J., 1984. Content and chemical form of mercury and selenium in soil, sludge, and fertilizer materials. Water, Air, Soil Pollut., 22: 95-104. Chaney, R.L., 1973. Crop and food chain effects of toxic elements in sludge and effluents. In: Proc. Joint Conf. Recycling Municipal Sludges and Effluents on Land. National Association of State University and Land-Grant Colleges, Washington, DC, pp. 129142. Combs, Jr, G.F, S.A. Barrows and F.N. Swader, 1980. Biologic availability of selenium in corn grain produced on soil amended with fly ash. J. Agric. Food Chem., 28: 406-409. Craun, G.F., 1984. Health aspects of groundwater pollution. In: G. Britton and C.P. Gerba (Eds), Groundwater Pollution Microbiology. Wiley, New York, pp. 135 179.

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