3. Potentially Toxic Elements in Sewage Sludges, Occurrence in Soils and Sludges and Assimilation by Food Crops
For an element to exert an effect on plant growth or to be assimilated by a plant, it must be present in solution. An. element can exist in solution as a simple ion, a complex ion or alternatively, in a chelated form. The composition of the soil solution is determined by physical, physicochemical and chemical factors which are interrelated. The distribution and concentration of the roots of a plant in the soil will determine to what extent any particular crop species or cultivar will assimilate elements from the soil solution.
3(a) Effect of Soil Properties
The principal soil properties which influence the elemental composition of the soil solution are:
(ii) Cation Exchange Capacity (C.E.C.), and
A general note on each of these soil properties is appropriate, prior to a detailed examination of the literature.
The pH of a soil is not a simple concept like the pH of an aqueous solution and as usually determined is an ‘apparent pH’. Soil pH is usually measured on a suspension obtained by shaking soil with distilled deionised water. The International Society of Soil Science recommend a ratio of one part of the soil to 2.5 parts of water. Most American workers use a 1:1 soil:water suspension. Some workers use a solution of 0.01 IA calcium chloride solution in place of water to simulate the activity of calcium ions in the soil solution. The measured pH will depend on the method used, which is not always reported. The pH of soil in the field will vary according to the state of oxidation or reduction of some of the soil constituents. An almost neutral waterlogged soil containing sulphides could become appreciably acid upon being drained and aerated, owing to the oxidation of sulphides to sulphates and sulphuric acid. Agronomic practices such as rolling, which increase the carbon dioxide concentration of the soil atmosphere will also lower soil pH. Similarly, soil pH will show considerable seasonal variation depending on the rate of leaching, root respiration, organic matter decomposition and nitrification. Russell (165) concluded that the pH of a soil could have no precise significance in agricultural practice. The implication of this for the disposal of sewage sludges to land is that if the soil pH is 6.5 at the time of sludge application it will almost inevitably fall below that level subsequently. A soil pH of 6.5 is considered desirable for most agricultural purposes.
The cation exchange capacity of a soil is a measure of the negative charge present on soil constituents and is an indication of the ability of a soil to adsorb cations. The adsorbed cations are by definition, exchangeable with the soil solution and hence, are not unavailable for uptake by plants. Cations adsorbed on exchange sites exist in equilibrium with the soil solution. The clay minerals and organic matter both contribute to the cation exchange capacity of a soil.
Clay minerals differ with respect to C.E.C. so that soil C.E.C. will vary both with the types and amount of clays present. For practical purposes in relation to sewage sludge disposal the type of clay present in the soil is of no particular importance in the U.K., but the total amount present is. The C.E.C. of clay minerals is virtually independent of soil pH. The C.E.C. of soil organic matter is however, pH dependent and increases linearly with increasing pH.
There are three main types of soil clays, , montmorillonitic and illitic having C.E.C.’s of approximately 3—15, 15—40 and 80—100 meq 100 /g. These clays however, seldom occur in the pure state in soil, so agricultural soils high in clay content do not have C.E.C.’s much in excess of 50 meq 100 gl of soil, while sandy soils low in clay content would have only a minimal cation exchange capacity.
Soil texture can be interpreted in either of two ways, Commonly, especially in laboratory determinations of texture, it is defined by the proportions of sand, silt and clay present as found by mechanical analysis. Field determinations of soil texture include allowance for the organic matter content of the soil. In the literature, it is not always clear whether texture has been determined by mechanical analysis in the laboratory, or by assessment in the field. Soil texture is of importance in determining cation exchange capacity, soil moisture holding properties and hydraulic conductivity all of which may influence the availability of trace elements.
3.1 Cadmium
3.1.1 The biochemical effects of cadmium include the uncoupling of oxidative phosphorylation, interference with enzymatic activity and the ability to interact with nucleic acids (199). In the body cadmium accumulates in liver and kidney tissue where histological and functional damage occurs. It can also induce gonadal necrosis and testicular atrophy (174). Ingestion of cadmium in food and water has been implicated in Itai-Itai disease in polluted areas of Japan (68). Of the elements presently known to exist in substantial quantities in sewage sludges cadmium gives the greatest cause for concern.
3.1.2 The average dietary daily intake of cadmium has been estimated at 15—30 ~ug for the U.K. (138) with an estimated maximum intake of 35 hg. A mean value of 92 ~ug has been reported in the U.S.A. (146). Since the biological half—life of cadmium in the human body is probably of the order of 10 to 30 years (68) and the acceptable body burden is 200 ~ug /g wet weight in the renal cortex (138) it is desirable to restrict and if possible reduce the dietary intake of cadmium.
3.1.3 Soils generally contain somewhere in the region of 0.01 - 0.7 /g cadmium (19) whilst Klein (107) reported an average concentration of 0.57 ug gl cadmium for agricultural soils. A more recent survey of the cadmium content of nearly 700 soil samples collected in connection with the “Survey of Practice” indicates that the mean cadmium content of British soils is less than 1.2 /g while most soils contained less than 1.0 ug cadmium g (8). This mean value is comparable with the levels reported for agricultural soils in Ontario (67), Manitoba (133) and Alberta (61).
Sewage from non—industrial areas generally contain less than 15 ug cadmium gl (128) although the mean cadmium content of 42 British sludges was found to be less than 100 ug /g (19). Lisk (117) reported that cadmium could often not be detected in sludges. The danger of applying sludges containing cadmium to agricultural land, would appear to be due to the high rates of application used. Cadmium occurs naturally, as an impurity in phosphatic fertilisers which have been widely used in agriculture for many years. In Wisconsin it has been estimated that more cadmium is applied to agricultural land together with phosphatic fertiliser, than is applied in sewage sludges (113). The distribution is however, inevitably different. It has been estimated by Lee and Keeney (113) that a sin~le application of sewage sludge, containing l8 ug cadmium g at a rate of 9 t hal would add an amount of cadmium equivalent to that during l86 years of phosphatic fertiliser application. Where sewage sludge has been applied for a long period of time or at heavy rates of application soil cadmium contents significantly greater than 1 ug /g are to be found. For example, at one sewage farm site, soil cadmium levels of 26 ug have been reported (Table 9), whilst where difficulties have been encountered in obtaining even distribution of sewage sludge, levels
in excess of 100 cadmium /g exist, usually over a comparatively small area and particularly adjacent to headlands where flood or furrow irrigation has been used (Appendix A4,7). Similarly, in another region, where sewage sludge has been applied at heavy rates for its soil conditioning properties on market—garden land, up to 76 ~ug cadmium /g have been reported (Appendix A2, 12). Levels of 16 ~ug cadmium /g were found to exist in soils on an allotment adjacent to a sewage treatment works (Appendix Al,5). Insufficient data exists to enable accurate estimates to be made concerning the area of land which contains cadmium above any particular concentration.
3.1.4 The cadmium content of most foodstuffs including cereals, meat, fish fats, fruits, root and green vegetable, milk and water in the U.K. is of the order of 0.01 ~ug g or lower (138) and the low levels probably reflect the generally low concentration of cadmium to be found in most soils. Scientific literature published in the course of the last six years indicates that the general tendency is, for an increase in soil cadmium content, in whatever form, to result in an increased cadmium content of any crop grown on that soil. In studies incorporating cadmium contaminated sewage sludges together with sewage sludges spiked with soluble inorganic salts of cadmium, uptake of cadmium by the crop has generally been greater from spiked sludges when compared to an unspiked sewage sludge containing a similar concentration of cadmium.
Vegetable Crops
3.1.4.1 The application of cadmium to the soil, either in the form of inorganic salts such as cadmium chloride (97) or cadmium sulphate (20) can increase the cadmium concentration of radishes. Generally, much lower concentrations of cadmium have been found in radish plants grown in sludge amended soils (59) than in experiments involving the use of inorganic salts. Radish tops accumulate cadmium to a greater extent than do roots (97, 109) indicating the cadmium is not immobilised in the root but is translocated to the above ground parts of the plant. Bingham et al., (20) found that a cadmium concentration of 21 ~ug /g in edible tissue was associated with a 25% reduction in radish yield in studies involving the use of sludges spiked with cadmium sulphate. However, Dowdy et al., (59) reported no detrimental effects on radish yields when sludge was applied at 450 t /Ha, while edible tissue contained only 0.31 ug cadmium /g compared to 0.13 ~ug /g for the control plants. Symeonides et al., (182) reported excessively high concentrations of cadmium in radish tops, some in excess of 500 ~ug /g for soils containing over 40 ~ug cadmium /g extractable with 1 N ammonium nitrate solution. For radish, Turner (186) found the zinc/cadmium ratio of plant tops approached unity as cadmium/zinc ratios in nutrient solution approached eight.
3.1.4.2 Lettuce is known to accumulate cadmium very effectively. Lettuce leaf cadmium concentrations of 2.7 ~ug /g have been reported —l (49) for plants grown on soil to which sludge was applied at 450 t ha when control plants contained 0.61 ~ug cadmium /g. However, John et al., (197) reported leaf cadmium concentrations~f 2.3 ~ug /g for control plants. Concentrations of 70 ~ug cadmium g in lettuce leaves have been associated with yield reductions of 25% in experiments with cadmium sulphate spiked sludge (20) while 13.6% yield reductions are reported (97) for leaf concentrations of 138 ~ug cadmium /g resulting from the use of a cadmium chloride spiked sludge. It has also been shown that the uptake of cadmium by lettuces from fertiliser is decreased when sludge is applied (97). For lettuces grown in nutrient solutions, uptake of cadmium is directly proportional to the concentration of cadmium in the solution (186, 151) a steady increase in leaf cadmium concentration being obtained up to a solution concentration of 1 ~±g cadmium mi?1. A leaf cadmium content of about 300—400 1~.ug /g can be expected when the solution cadmium content is 1 ~ug mi?1. Above this solution concentration of cadmium, yields decrease substantially. Soils containing up to 16 ~ug cadmium g~1 as a result of sewage sludge application are reported to have produced lettuces containing up to 28 ~ug cadmium /g (Table 10). There is evidence available that the uptake of cadmium varies with the species of cultivar grown. Lettuce has consistently been found to accumulate cadmium to a greater extent than any other crop, radish being an exception. Cadmium added at a rate of 5 ~ug /g to soil as the soluble chloride salt results in loss of yield (189).
3.1.4.3. It would appear that potato yields are significantly increased by the application of sewage sludges without any significant concommitant increase in tuber cadmium content. Applications of 450 t /Ha sludge containing 7~g cadmium /g have been reported (59) to produce only small quantitative increases in tuber cadmium content though these were of the order of 100% (0.23 ug cadmium /g as opposed to 1.12 ~.ug cadmium /g for control plants). Application of up to 80 t digested sewage sludge ha’ containing up to 37 ~ug cadmium /g resulted in significant increases in tuber yield without any detectable effect on tuber cadmium content the concentration being mostly below 0.05 ~g cadmium /g fresh tubers (11). Maclean (121) reported a background level of 0.18 ~ug cadmium /g in potato tubers. Recent evidence (162) shows that trace elements tend to accumulate mostly in the peel of potato tubers. Even on soils with high cadmium contents as a result of continued application of sewage sludge, potato tubers do not accumulate great amounts of cadmium. Evidence for this exists in analytical data obtained for potatoes grown at a sewage farm site where on a soil containing 27 ‘~ug cadmium /g potato tubers contained 0.5 ~ug cadmium /g (Table 9).
3.1.4.4 A mean cadmium concentration of 0.04 ug /g is reported for a sample of cabbages obtained from retail outlets throughout Britain (185). In studies involving the use of a silt loam soil amended with a dried sewage sludge and cadmium sulphate solution Bingham (20), described cabbage plants as being relatively tolerant to high soil cadmium levels, up to 170 ug cadmium /g being required to decrease yields by 25%. Cabbage leaves however, accumulated cadmium to a much lesser extent than either radishes or lettuces, at comparable soil cadmium levels. Cabbages grown in a high cadmium soil were found however to contain in general less than 1 ~ug cadmium /g (Table 10). This is a 20 fold increase over mean control values. An allotment soil containing l6 ug cadmium g’ as a result of heavy applications of sewage sludge produced cabbages containing 6 ug cadmium g~1 (Table 11).
3.1.4.5 Vegetables other than radish, lettuce and cabbage appear to have been studied to a lesser extent in relation to cadmium uptake. Tomato (Lycospersicon esculentum Mil), carrots (Daucus carota L), turnips (Brassica rapa L), spinach (Spinacia oleracea L), curley— cress (Tepidium sativum L), and field beans (Phaseolus vulgaris L), are among the other crops which have been investigated for uptake of cadmium. There is no distinct relationship between cadmium tolerance with respect to yield and accumulation of cadmium in the diagnostic tissue of these vegetables, though the cadmium concentration in the edible tissue of all these crops was increased by additions of cadmium sulphate to the soil. Spinach and curley cress contained 75 and 80 ug cadmium /g at levels of soil cadmium producing a 25% reduction in yield indicating that these two vegetables, together with lettuce can accumulate significant amounts of cadmium (20). Carrots have also been reported to accumulate cadmium to as great an extent as spinach (185). Carrots accumulate relatively large amounts of cadmium in experiments with spiked sludge (20). Peas (Pisum satiurum L), do not appear to accumulate cadmium though pea vines do (59). Tomato fruits accumulate significantly more cadmium when soil is amended with sludge (59) though at relatively low levels (0.20—0.32 ug /g) while fruit contained 7 ug cadmium g when yield was reduced 25% by applications of cadmium sulphate spiked sludge (20). Tomato leaves would appear to accumulate cadmium when irrigated with sludge extracts though the extent to which accumulation occurs is not related to the cadmium composition of the sludges used, reflecting differences in cadmium solubility between sludges (30). Soybean cadmium concentrations of up to 3 ug /g (seed) and 13.6 ug g’ (leaf and petiole) have been reported for cumulative additions of 59.4 kg cadmium /Ha in a two year period though actual concentrations in plant tissue can vary considerably from year to year (85).
Though other root crops such as beetroot (186, 151) have been examined for uptake of cadmium, often only leaf tissue concentrations are given. This may be useful as an indication of future yield prospects but is of little use in protecting the consumer. It would appear from the foregoing that leafy tissue is the greatest accumulator of cadmium, while tubers, seed and fruit tissue accumulate cadmium to a much lesser extent (20). On the same basis it might be expected to find that cereal grains accumulate cadmium to a lesser extent than do vegetative tissues.
Grass and Cereal Crops
3.1.4.6 Cadmium concentrations of 0.20 u/g in samples of wheat collected from a range of agricultural soils, isolated from apparent sources of pollution are reported in the literature by American workers (90). For well drained soils, Canadian workers have reported wheat and barley cadmium contents of 0.40 ug /g (61) with oats containing 0.60 ug cadmium /g. In Sweden, the cadmium content of wheat seeds has been found to be generally lower than the levels
reported by North American workers, the highest being 0.1 ug cadmium /g (155), with consistent differences between cultivars of both wheat and barley. The highest cadmium content reported for barley was 0.04 ug /g. Wheat grain is reported to have contained less than 0.20 ug cadmium /g when sewage sludge containing 300 ug cadmium /g was applied at 100 t /Ha (167). The same Canadian workers found straw cadmium concentrations to be less than 0.7 ug g for wheat and barley grown on brown earth soils. In general, on various soil types wheat straw contained least cadmium, followed by oats with barley containing most cadmium. Friberg (69) quotes the cadmium content of wheat flour determined by research workers in several countries, the maximum being 0.10 ug /g (203). Analysis of cereal grains consumed in the U.K. (138) indicate that the figures quoted by foreign workers as normal cadmium concentrations are 7—10 times those found in the U.K.
In experiments with barley (Hordeum vulgare L) plants grown for 69 days, cadmium could not be detected in grain, even at soil concentrations of 205 ug cadmium /g (105). Six week old rye (Secale cereale) grown in soil amended with sewage sludge at rates of application of 63 and 502 t /Ha grain contained between 1.1 and 9.3 ug cadmium /g (49).
Several studies on maize (Zea mays) substantiate the view that cadmium concentrations of grain are not greatly increased by the use of sewage sludge as . Application of 450t sludge /Ha was found to increase maize grain cadmium concentration from 0.02 to 0.05 ug /g (59) although applications of different sludges resulted in grain concentrations of as high as 1.0 ug cadmium /g forage concentrations being as high as 7.9 ug cadmium gl (75) and up to 30.4 ug cadmium /g (49). Irrigation of maize throughout the growing season with sewage effluent which was very low in cadmium did not produce plant tissue containing more than 0.27 ug cadmium g l (171). Garcia et al., (72) found the cadmium content of maize grain on sludge—treated—strip mine—soil to lie within the range of values obtained for control plants. Kirkham (106) in a study involving a site which had received sewage sludge for some 35 years at 28 t /Ha p.a. found that cadmium did not accumulate in maize grain, though maize roots contained large amounts of cadmium. Maclean (121) also found that cadmium tended to accumulate in the roots of maize plants. Cadmium in sewage sludge applied immediately prior to planting of maize is reported to be thirteen times more readily available to the crop than is cadmium applied as a constituent of sewage sludge during all previous years (85).
Although Bingham et al., (20). showed by experimentation with addition of soluble cadmium to soil that wheat (Tricticum aestivum L) grain contained 11.5 ug cadmium /g at soil cadmium levels resulting in a 25% decrease in yield, this figure is rather high when compared with analyses of wheat grain produced on United Kingdom farms with a history of sludge disposal which suffer from losses in yield of the same order of magnitude. The highest cadmium concentration found in winter wheat grain grown on- one authority’s farms was 0.71 ug /g and for barley grain 0.35 ug /g (Table 9). In another area one field, in winter wheat, which in 1976 yielded 4.2 t grain ha 1 produced grain containing less than 0.2 ug cadmium /g. Another field, which yielded 2.2 t grain /Ha produced grain containing up to 0.6 ug cadmium /g (Table 12). Although the former sewage farm soils contained relatively little cadmium (Table 9) farm soils in the latter area contained about 10 ug cadmium /g. It would appear therefore, that experiments involving addition of the soluble metal salts to soil are of very limited use in evaluating what might happen in actual practice. Some evidence for the view that cereal grains do not accumulate cadmium excessively also exists, in that soils which had received a press cake sludge in the Dunstable area produced wheat grain containing 0.2 ug cadmium /g when the soil contained 3.5—4.0 ug cadmium /g (Appendix A4, 2).
Little data exists for the uptake of cadmium from sludge amended soils by pasture grasses. Tall fescue (Festuca arundinacea ) cadmium concentration has been shown to be significantly increased by application of sewage sludge containing l65 ug cadmium g l at 16.8 t /Ha (3 kg cadmium /Ha) over a 14 month period (28) although no toxicity symptoms were developed by the fescue. Bingham~ et al., (21) reported tissue concentrations of 24 and 37 ug cadmium g lT0 be associated with 25% yield reductions in alfalfa (Medicago sativa L) and tall fescue (Festuca elatior L) respectively. As for other crops, uptake of cadmium by ryegrass is reported to be greater from soluble cadmium salt amended soils than from soil containing equivalent amounts of cadmium added in sewage sludge (56). Grass does however accumulate cadmium from the soil to a greater extent than cereal grains. Grass cadmium contents of approximately 5 ug g I have been found, originating from soil containing l ug cadmium g l (Appendix A4, 1).
Soil Properties Affecting the Availability of Cadmium to Plants
3.1.4.7 The factors influencing the uptake of cadmium by plants, from sludge amended soils must initially be influenced, to some degree by the form in which cadmium is present in the sludge. As the sludge organic matter is degraded an equilibrium will be achieved with the original components of the soil matrix. Once this equilibrium is attained, the factors controlling the uptake of cadmium by plants will essentially be those which affect control of the soil solution cadmium concentration. Leeper (114) remarking upon the lack of information on the reactions of cadmium in soil, hypothesised that these could be predicted to some extent, from what was known of the reactions of zinc in soil. However, in view of the overriding importance of cadmium from the public health viewpoint, this would be a somewhat unsatisfactory state of affairs. Stover et al., (178) attributed means of 14.8% and 48.8% of the cadmium in twelve sludges, to organic forms and carbonates respectively, the remainder probably being present as water insoluble sulphides. No exchangeable or absorbed cadmium was found to exist in the sludges examined. The data were based upon sequential extraction with 1 M potassium nitrate (16 h, solution: solid ratio 50:1); 0.5 IA potassium fluoride (pH 6.5, 16 h, 80:1); 0.1 IA sodium pyrophosphate (16 h~ 80:1); 0.1 IA ethylenediaminetetraacetic acid (pH 6.5, 8h double extraction, 80:1); and 1 IA nitric acid (16 h~ 50:1), to represent exchangeable, absorbed, organic, carbonate and sulphide forms respectively. However, the validity of such a procedure remains uncertain. It should be noted that a similar procedure developed by McLaren et al., (126) for soil copper, upon which Stover et al., based their method was in fact, validated by correlation studies of the amounts of copper extracted, with appropriate soil parameters e.g. %clay, %organic carbon, etc. No such correlation studies were performed by Stover et al., (178).
Studies on the simulated weathering of sludge (induced by the use of hydrogen peroxide) indicate that the extractability of cadmium by 0.~0l IA hydrochloric acid is increased by weathering (111). Thus although only a certain proportion of the cadmium present in sludge may be readily leached by water, the probability is, that all of the cadmium present will, eventually be solubilised and become available either for plant adsorption or leaching, if no further reaction occurs with soil particles. Once the retentive capacity of a soil is reached (i.e. all exchange and adsorption sites are occupied), the solution concentration of cadmium is likely to be increased and it appears that this is a possibility, if either a sludge containing large amounts of cadmium, or a soil low in retentive capacity is utilised(30). It has been suggested that the rate of movement of 0.01 -IA hydrochloric acid extractable cadmium downwards through the soil profile varies with the moisture content of the crust formed on the soil surface by application of anaerobically digested sludge (105). From a pedological viewpoint, it is extremely difficult to prove that an element has actually been translocated within a soil profile and not been solubilised at depth, by weathering of soil minerals in situ.
Giordano et al., (75) reported that some immobilisation of cadmium occurred with time of reaction of sludge with soil, having examined this effect on the topmost 20 cm of the soil profile. Anaerobic incubation of sludge increases the amount of cadmium extracted by water and correspondingly decreases acetic acid and ethylenediamine tetra—acetic acid soluble cadmium, while addition of soil or lime to sludge during aerobic incubation, generally increases the extractibility of cadmium in all three reagents C23). Kirkhan. (105) noted a decrease in the acid (0.05 N hydrochloric acid + 0.025 N sulphuric acid) extractable cadmium of sludge amended soil with increase in soil depth. When sludge was applied daily and the soil remained continuously wet, little extractable cadmium moved into the soil (105). This therefore, would appear to substantiate the results obtained by Bloomfield (23) for the effect of anaerobic incubation on cadmium solubility. Plant cadmium contents have been found to be better correlated with cadmium extracted by 1 N ammonium acetate (97), than with the total amount of cadmium present in the soil. The 1 N hydrochloric acid extractable cadmium closely correlates with total soil cadmium (97) which explains why plant cadmium content is not well correlated with soil cadmium extractable by 1 N hydrochloric acid (109). Williams and David (195) used 1 N hydrochloric acid to determine total soluble cadmium in soil and 1 N ammonium chloride to determine exchangeable cadmium. There was however, little difference in the amounts of cadmium extracted by 1 N ammonium chloride and 1 N hydrochloric acid. Giordano et al., (75) found plant uptake of cadmium to correlate well with 0.05 N hydrochloric acid extractable cadmium. Cadmium extracted by 0.1 N hydrochloric acid has also been used to determine its availability to plants (171). Close agreement has been obtained between levels of diethylenetriaminepentaacetic acid (DTPA) — extractable cadmium and the amounts of cadmium sulphate added to soil (20), between water saturation extractable cadmium uptake and crop yield data (21).
The random choice of an for soil cadmium, followed by regression analysis, in an attempt to relate plant uptake to soil factors has proved only partly successful. It does not provide information relevant to the long term availability of cadmium when successive transformations will have altered the relative importance of different parameters. Soil factors playing a direct role in controlling the availability of cadmium for plant. uptake, include soil type, as determined by cation exchange properties (195), organic matter (95) and pH, availability being less at higher ph.
John (95) examined thirty Canadian soils and found cadmium adsorption data to fit the Langmuir isotherm:
C/(x/m) = 1/kb + C/b
where C solution concentration of cadmium;
x/m amount of cadmium absorbed g /l00g
and b is the adsorption maximum and k is a constant proportional to the bonding energy. The bonding energy coefficient for cadmium was found to depend on soil texture, decreasing in the order:
organic> heavy clay>sandy and silt loam>sandy, in addition to being significantly correlated with soil aluminium and iron. None of the adsorption equation parameters was significantly correlated with cation exchange capacity and none of the parameters associated with surface area was significantly correlated with the Langmuir equation parameters, derived for individual soils. Levi—Minzi et al., (116) also found the adsorption of cadmium by soils to fit a Langmuir isotherm, adsorption being favoured by high cation exchange capacity, high organic matter content and high pH. Adsorption was not significantly affected by temperature, and was correlated with the Langmuir parameters.
The cation exchange capacity of soil would therefore appear to play a major role in the adsorption of cadmium from solution. Haghiri (78) reported that the retaining power of organic matter for cadmium was predominantly through its cation exchange property.
The cation exchange capacity of a soil is determined by the individual components namely, clay and organic matter. No work appears to have been done on the adsorption of cadmium by clay. Geothite (FeOOH), one of the hydrous oxide components of soil, has a low intrinsic affinity for cadmium (66) adsorption only occurring at pH above 5.9, while at pH 8 adsorption ~s only 61% complete.
It would appear probable that the cadmium adsorbed by soil organic matter remains essentially available for plant uptake since et al., (33) found the desorption of cadmium from peat to be comparatively rapid. This would help to explain why crops grown on spiked sludges accumulate more cadmium than when unspiked sludges are used. Cadmium added as salts to sludge, when applied to soil, would probably not exist in an organic form but would merely supplement a reservoir of available cadmium by being adsorbed on cation exchange sites of soil components. In this respect, it is interesting to note that in a comparative experiment of common soil extractant solutions Andersson (3) commented that cadmium was remarkably easily brought into solution.
Haghiri (78) found that the increase in cation exchange capacity of soil caused by the addition of organic matter resulted in increased growth of oats (Avena sativa L). The increased growth was stated as probably being due to a decrease in the uptake of cadmium by the oat plants. Haghiri also reported however for the same experiment, that cadmium concentration in oat shoots was not significantly affected by the addition of organic matter and neither was soil exchangeable cadmium affected by the addition of organic matter. This would appear to be a contradiction because one would expect changes in the cation exchange capacity of soil to be correlated with the amounts of organic matter added.
3.2 Lead
3.2.1 Unlike cadmium, lead has long been as a toxic element. Because of its multivarious uses lead is distributed in a ubiquitous manner. Lead concentrations increased from 0.005 ug/kg to more than 0.2 ug/kg in the Greenland ice sheet from 800 BC to 1965 AD (81). Like cadmium, lead interferes with the metabolism of a wide range of enzymes, can form complexes with nucleic acids and adversely affects their synthesis (199). Furthermore it has been implicated, though rather dubiously, in cancer causing mechanisms (70).
3.2.2 The estimated daily intake of lead in the U.K. is in the region of 220 ug while the 16th FAO/WHO Expert Committee on Food Additives proposed a provisional estimate for a tolerable weekly intake of 3 mg for adults (136). A recent Department of the Environment report (53) concluded that no evidence existed to show that this amount of lead in the diet would cause any harm to human health, the chief danger of high soil lead levels being the consumption of such soil by young children. The same report states that high soil lead levels have, in general, only small effects on the lead content of the edible portions of most plants used as foodstuffs by humans, an exception being root crops; that animal products contribute only about 18% of man’s total dietary intake of lead and that ruminant animals (cattle and sheep) excrete about 99% of ingested lead (22). Presently, the lead content of most foodstuffs is limited to a maximum of 2 ug /g by the Lead in Food Regulations 1961 (SI 1931) as amended in 1972 (SI 1843) and in 1973 (51 1053 and SI 1340). These regulations were reviewed by the Food Additives and Contaminants Committee in 1975, when it was recommended that the general limit for lead in food be halved to I ug /g (34). The mean lead content of food in the U.K. is 0.13 ug /g, green vegetables, cereals, meat and root vegetables contain in the region of 0.2 ug gl (136).
3.2.3 Soils, on average contain in the region of 30 ug lead (19). More recently a mean of 57 ug g~1 has been found for British soils, with a range of 5—1200 ug /g (8). Generally, levels lower than 57 ug /g have been found to occur in various Canadian soils (60, 67, 133). A mean lead content of approximately 13 ug g~- was found for a range of samples collected from general agricultural land in Ontario, including general arable and vegetable growing land (67). The highest level found in the soil A horizon in various Manitoba soils was 23 ug /g (133). In Alberta, 6—7 ug lead /g are to be found in the plough layer of well drained soils (60). In general, soil lead levels are higher in soils with high clay contents in Manitoba (133). Surveys of the lead content of soils derived from various parent materials in Britain (8) do not support this latter finding. Though sandy and chalky soils tended to have somewhat low soil lead contents of about 30 ug /g, clay soils contained 40 ug /g and soils derived from limestones averaged 80 ug /g. Soils derived from shales contained lead at levels equivalent to those derived from limestones whilst peaty soils contained least lead. Berrow and Webber found the mean lead content of 42 British sewage sludges to be 820 ug /g (19).
Vegetable Crops
3.2.4 A mean concentration of 0.05 ug lead /g for a wide range of vegetables obtained from retail outlets throughout England and Scotland was reported by Thomas et al., (185).
Lagerwerff (109) demonstrated that radish tops and roots accumulated lead as the soil lead content was increased but Dowdy et al., (59) found no significant increases in radish lead content when sludge (515 ug lead /g) was applied at 450 t /Ha. While Dowdy et al., (59) found sludge applications at 450 t /Ha increased the lead content of carrots from 0.4 to 0.9 ppm lead, Warren et al., (187) reported a naturally occurring range of 0.2—11 ug lead gl (mean 4 ug gl) in carrots obtained from a variety of locations in the United Kingdom and Canada. Similarly, Warren et al., (187) reported a mean lead content of 4 ug /g for various varieties of beans, with a maximum of 12 ug /g. Bradford et al., (30) found between 1.0 and 2.3 ug lead /g in beans grown under different sludge regimes. Concentrations of up to 27 ug lead /g were found in the leaves of the bean plants however, presenting the danger of translocation of lead from leaf to fruit. Applications of sludge containing 550 ug lead gl at 450 t /Ha failed to produce significant increases in the lead content of lettuces or potatoes (59). The concentrations reported are well within the normally occurring range of 0.3—5.6 ug lead gl for lettuces and 0.2—7.6 ug lead /g for potatoes, found by Warren et al., (187) and well below the means of 12 and 1.6 ug /g reported respectively for lettuces and potatoes. Although relatively high lead levels have been found in tomato leaves resulting from irrigation with water saturation extracts of sludge(30), Dowdy et al., (59) found tomato fruits did not accumulate lead from sludge amended soils.
Analyses of vegetable crops grown on sites with a history of sewage sludge application substantiate findings reported in the scientific literature that lead is not readily assimilated by these crops. Baerug et al., (11) found potatoes did not accumulate lead from sludged soils and that tuber lead contents bore little relation to sludge lead content, a point also made by Richardson (162) who found lead to accumulate mostly in the peel. Potatoes grown on one water authority’s farms contained no more than 1.25 ug lead /g at soil lead contents of 20 ug /g while at soil lead levels of 180— 350 ug /g, potatoes contained less than 1 ug /g (Table 9). Lettuces and cabbages grown in another area never contained more than 20 ug lead /g and usually less than 1 ug g’ despite soil lead levels of between 70—390 ug /g (Table 10).
Grass and Cereal Crops
3.2.5 Grass herbage contains 0.3—1.5 ug lead /g during periods of active growth but samples taken during winter or autumn usually contain considerably more, typically about 10 11 ug /g. Clovers contain slightly less lead than herbage grasses (141). Details of the elemental composition of grasses are rare in the scientific literature. In a recent survey of sites throughout England and Wales which had received sewage sludge, Richardson (162) found lead contents to generally be between 1 and 5 ug gl. One water authority, however, has performed some metal analyses on grass fields in its area. The area is divided into two parts, one of which received sludge by an irrigation technique and another where sludge was spread by tankers. Analyses of soil from the tanker spray area revealed lead contents of about 400 ug /g, while samples of washed grass from this area contained from 828 to 1345 ug lead /g. Unwashed grass samples from an adjacent part of the field which had not been sludged contained only 3.9 ug lead /g. The lead levels in this sludged grass are far in excess of any reported in the literature (Appendix A4, 1).
Barley irrigated with sludge extracts tended to show increased lead concentrations in the leaf tissue (30). At concentrations below 8 ug /g Dowdy et al., (58) reported that more lead was taken up by barley seedlings from acid soils than from soils of pH 7.9 containing free carbonates, while lead uptake was increased by as much as 50% by additions of 30 t sludgeto acid soil (pH 5.9). Sabey et al., (167) conducted field trials on wheat and found that the application of 100 t /Ha of sludge did not increase wheat grain lead content above the control level of 0.15 ug gl.
Lead appears not to be accumulated by maize grain from sludge amended soils. Dowdy et al., (59) reported no significant change in maize grain lead content resulting from application of 450 t sludge /Ha. Sidle et al., (171), Garcia et al., (72) and et al., (106) have all reported that maize grain does not accumulate lead from sludge amended soils. Kirkham et al., (106) determined the heavy metal contents of maize grown on a site which had received sludge for 35 years. These show that lead is excluded from stems, leaves, husks and grain but accumulates in maize roots. Those roots grown on sludge treated soil contained approximately five times as much lead as did the roots of plants grown on control sites (114 ug /g for control). Giordano et al., (75) reported lead contents of maize forage and grain grown on soil amended with a zinc sulphate spiked sludge. Apart from seasonal variation in lead uptake, sludge amendment did not greatly increase the lead content of the plants where increases occurred and in some cases, lead uptake appeared to decrease as sludge zinc content increased.
Winter wheat and winter barley grain grown on farms of a water authority with a history of sewage sludge application contained no more than 1.8 ug lead /g and generally much less, while soil lead levels varied from 24 to 188 ug g (Table 9). Similarly, analyses of soil treated with a press cake sewage sludge in another area and of wheat and barley grown on the site revealed that, for wheat, soil lead levels of 454 ug /g produced grain containing no more than 6 ug lead g . Soil containing 36 ug lead /g produced barley grain containing 2—5 ug lead /g (Appendix A4, 2). These results agree closely, in so far as grain lead content is concerned with the results of a wider survey reported by Richardson (162). It appears very probable that cereal grain lead contents could be increased significantly by sewage sludge application to soil.
Soil Properties Affecting the Availability of Lead to Plants
3.2.6 The uptake of lead by plants appears to be a purely passive process which is not reduced by either metabolic inhibitions or low temperatures (9). Soil factors which control the solution concentration of lead are therefore, important in determining the uptake of lead by plants, the plant itself not having much impact upon the situation.
While the addition of phosphatic may provide a sink for lead added to soil (168) through the precipitation of insoluble lead phosphates, this is not an effective means of controlling lead stability in soils (202). The addition of lime to soil decreases the solubility of lead (120). The effect of liming on the uptake of lead by maize from soil amended with lead nitrate is however, erratic and appears to exert a reduction in the ability of maize plants to translocate lead rather than reducing actual uptake (202). Table 13 indicates that liming has some effect in reducing shoot lead concentrations. When lead was applied at 920 ug /g, the effect of lime on root uptake was shown to be erratic. Lead uptake by rape plants from sludge amended soils is less at ~H 7.2 than at pH 6.0 for sludge applications of up to 58 t ha (7). The lead contents of spring wheat grown in the same trials were similar when grown in soil of pH 7.2 and pH 6.0. At high rates of sludge application, lead uptake by both rape and spring wheat was lower than at low rates of application indicating that organic matter might have restricted uptake. It has been suggested (202) that the effects of organic matter in immobilising lead are temporary and would lessen as decomposition took place.
In-leaching experiments using columns filled with a prepared soil (40% pore space) Giordano et al., (75) found 0.05 N hydrochloric acid extractable lead to be increased by sludge application, although the mobility of lead in these soil columns was low. Boswell (28) found the 0.1 N hydrochloric acid extractable lead content of soil was increased significantly by sludge application in the surface (0—8 cm) horizon, and increased to a level which was not significantly higher than control levels, at lc~er depths. Anaerobic incubation of sludge increases the solubility of lead in water, 0.05 IA ethylenediamine tetraacetic acid and 0.5 IA acetic acid, the increase in solubility being greatest for ethylenediamine tetraacetic acid and acetic acid. Subsequent aerobic incubation results in further increases in solubility of sludge lead in acetic acid and ethylenediamine tetraacetic acid. Addition of soil or lime to sludge decreases the solubility of lead in acetic acid immediately, and in water over a 4 month period, but solubility in ethylenediamine tetra— acetic acid is considerably enhanced by such additions (23).
Stover et al., (78) attributed 64.5% of sludge lead to carbonate forms (determined by extraction with ethylenediamine tetraacetic acid), 38.5% to organic, 1.8% to adsorbed and 9.3% to sulphide forms respectively. Largerwerff et al., (111) found that only a very small proportion of sludge lead could be removed either by leaching with water or 0.06 N calcium chloride solution (0.3% and 0.5% respectively). Santillan—Medrano et al., (128) studied the solid phase formation characteristics of lead and cadmium in soils and concluded that at any given pH, the activity of lead was considerably less than that of cadmium. Lead was found to be very readily adsorbed by geothite (66) even at relatively low pH. Bunzl et al., (33) reported lead to be very rapidly- adsorbed by peat and the rate of desorption was moreover, comparatively slow in contrast to the behaviour of cadmium.
3.3 Zinc
3.3.1 Zinc is an essential element for both plants and animals (1) though high concentrations are potentially phytotoxic. The larger the amounts of zinc added to soil the less likely it is to be inactivated (114). The phytotoxicity of zinc is used as the basis for the zinc equivalent concept (41) which is discussed elsewhere.
3.3.2 The zinc content of institutional diets in the U.S.A. has been estimated at between 2.7 to 6.4 ug /g (146). In the U.K. a voluntary limit of 50 ug /g for foodstuffs was proposed in 1953 though current thinking is in favour of an increase in dietary zinc as much zinc is lost during food processing.
3.3.3 Sewage commonly contain high concentrations of zinc, up to 49,000 ug /g having been reported (19). Soils contain much less; a range of 10—300 ug /g was found by Allaway (1) while the mean of a large number of British soil samples was found to be 82 ug /g (8) the highest value found being 816 ug /g. Much of the zinc present in sewage sludges is in readily soluble forms, up to 44% being soluble in 2.5% acetic acid (19). Application of sewage sludges can therefore significantly increase both total soil zinc levels and also the amount of zinc readily available for uptake by plants.
Vegetable Crops
3.3.4 Zinc concentrations in lettuce tissue are significantly increased by applications of sludge, up to 225 ug /g having been obtained for applications of 450 t sewage sludge /Ha (59). The tissue concentration of zinc which is regarded as phytotoxic is approximately 200 ug /g (188). Experiments with solutions of zinc sulphate added at rates up to 250 ug zinc mi?1 to a sandy loam and a peaty soil had no detrimental effect on mean yield of lettuces below 50 ug added zinc mi?1 but yields decreased when more than 50 ug zinc mi?1 was added, with corresponding lettuce leaf concentrations of 113 ug zinc /g (189). Municipal compost applications can also significantly increase lettuce zinc content (158) without affecting growth, while the higher zinc contents produced by sludge application can decrease yield (73). Warren and Delavault (187) reported a mean tissue zinc concentration of 86 ug gl for lettuces, with a range of 30—250 ug zinc /g, so sludge applications may not be the only agent responsible for producing abnormally high concentrations of zinc in lettuce tissue. Lettuce, with up to 140 ug zinc /g have been grown at one site without any reports of yield losses (Table 10).
Radish zinc concentrations are also increased by applications of sludge (59) though not to the same extent as lettuce. Uptake by radishes has been sh~n to increase with soil zinc content, up to 121 ug /g being accumulated by roots at a pH of 5.9. Potatoes have been found to have a mean content of 18 ug zinc /g (187). The tuber zinc concentration has been increased to 53 ug zinc g by 450 t sludge /Ha (187) and to 42.6 ug zinc /g by 100 t municipal compost /Ha (158). However, Le Riche (112) found no evidence to suggest accumulation of zinc in potato tubers as a result of sludge treatment. Baerug et al., (11) reported that addition of up to 30 kg zinc /Ha in sewage sludge had little effect on potato tuber zinc content but that at rates in excess of this, a considerable increase in tuber zinc content took place, even though only up to 7% of applied zinc was accumulated by the tubers, the highest concentration being l0.3 ug /g. The highest concentration of zinc found in potato tubers grown in one water authority’s farms was 45 ug /g, from a soil containing 525 ug zinc /g (Table 9). As is the case for cadmium and lead, the concentration of zinc in potato peel has been shown to be greater than that found in whole potatoes (112).
Carrots may contain up to 59 ug zinc /g when grown in soils naturally high in zinc, although the mean zinc content of a sample collected from British and Canadian sites was 29 ug /g (187). Le Riche (112) found only up to 44 ug zinc /g in carrots grown on land which had received a total of 1430 t sludge hal, but 103 ug zinc /g have been found in carrot tubers grown on soil which had received 450 t /Ha (59). Similar levels of zinc to those found by Dowdy et al., (59) have been reported in carrots gr~n on sandy loam soil to which digested sewage sludge had been applied for approximately 15 years (162). Most of the zinc taken up by carrots appears to be translocated to the tops as Richardson found higher levels in tops than in roots (62).
Tomatoes grown on sandy soils amended with sludge contain significantly more zinc than tomatoes grown on clay soil (73). Tomatoes grown on sludge alone, may contain as much as 87 ug zinc /g while applications of 450 t /Ha have resulted in concentrations of 31 ug zinc g~1 (59). Concentrations as high as 295 ug zinc /g in tomatoes, resulting from the use of commercial sewage sludge products have been obtained. Extracts of municipal sludges containing greater amounts of zinc, when used for irrigation, produced tomatoes with lower zinc concentrations, reflecting differences in the solubility of zinc in different sludges.
It has been noted that Swiss chard and spinach have a tendency to accumulate zinc (24). Other vegetables investigated for zinc uptake from sludge amended soils include leeks, which have been reported to contain up to 160 ug zinc /g and globe beet roots which can accumulate up to 290 ug zinc /g when grown on soil amended with 1410 t hal (112). Concentrations of zinc in Soybean seed of up to 94 ug g—l have been reported following cumulative additions of 1306 kg zinc /Ha in sludge (85).
Grass and Cereal Crops
3.3.5 Barley has the ability to absorb large- quantities of zinc from alkaline soils (25). Grain zinc concentrations of up to 67 ug /g following application of 183 t /Ha sludge to soil pH 7.2 containing 4,350 ug zinc 3l have been found in 69 day old barley (105). 75 ug zinc /g is regarded as the upper limit of the normal concentration range in grain (105).
Sludge applied at 100 t /Ha increased wheat grain zinc concentrations from 34.8 to 54.2 ug /g (167). Addition of zinc salts to sludge can produce very high zinc concentrations in rye plants, up to 1366 ug /g having been reported (50). Control rye plants contained 45 ug zinc /g, sludge application at varying rates having produced average concentrations of 212 ug /g (49). On sewage works farms with a history of sewage sludge application levels of zinc found in winter barley and winter wheat grain ranged up to 85 ug /g (Table 9).
Zinc uptake by maize increases with organic waste application (59, 75, 144, 171). Between 84% and 92.1% of the zinc applied to growing maize by irrigation with wastewater can be accumulated by the crop (171). Maize grown normally contains approximately 40 ug zinc /g (58, 75), while forage contains about as much zinc as does the grain (75). When large amounts of zinc are added to the soil in sewage sludge, then forage appears to accumulate zinc to a greater extent than does the grain. Application of 450 t sludge (1070 ug zinc gl) /Ha increased grain zinc content from 41 to 65 ug g l(58). In experiments with zinc sulphate spiked sludge applied at rates varying up to 360 kg zinc /Ha grain zinc concentration increased from 37 ug /g for control plants to 44 ug /g and forage zinc was increased from 94 to 97 ug /g (75). In the second year of experiments with zinc sulphate spiked sludges Giordano et al., (75) found zinc contents of grain to be similar to that in the year immediately following sludge application, but forage zinc was increased by a factor of 2 at the highest rate of application. This was despite the fact that 0.5 N hydrochloric acid extractable zinc decreased during the interval between the two crops, indicating that no correlation existed between plant uptake and 0.5 N hydrochloric acid extractable zinc.
Mortvedt and Giordano (144) in contrast, found that the zinc concentrations iii the tissue of three maize crops each grown for 7 weeks, treated with incubated sludge (for 0, 21 and 36 weeks) decreased with sludge incubation time. This indicated that the zinc was becoming less available with time. In these experiments less than 5% of the zinc applied was removed by the crop. Mortvedt et al., (144) found zinc uptake to be greater with zinc sulphate applications than with comparable rates of sludge application. In addition, although 0.5 N hydrochloric acid extractable zinc increased with application rate of zinc in whatever form, lower levels of extractable zinc were found after cropping; 240 ug zinc /g as zinc sulphate decreased forage yields whereas 240 ug zinc /g as sludge did not (144). Cunningham et al., (49) found zinc uptake by maize to differ according to both source and rate of application in almost all cases. Zinc concentrations of 381 ug /g in maize leaves have been found on land which over a period of 7 years had received over 2300 kg zinc /Ha in sewage sludge (85). This level was considered to be well above that thought to be potentially toxic and yet no decrease in corn yields was observed. Heinously et al., (85) considered it unlikely that 2000 kg zinc /Ha supplied in sludge over a period of several years would cause zinc concentrations in maize grain to be increased by more than 20 ug /g if applications of sludge were terminated.
Soil Properties Affecting the Availability of Zinc to Plants
3.3.6 A high proportion of sludge total zinc is water soluble. Lagerwerff (111) reported that 31% of sludge total zinc could initially be leached with water, while a total of eight successive leachings removed up to a further 36% of total zinc. However, Jenkins and Cooper (92) found only 1/60 of sludge total zinc to be extracted by 16 successive extractions with distilled water. It would appear that the presence of bases enhances the leachability of zinc•
When a sludge was leached with 0.06 N calcium chloride solution, 81% of the zinc was removed, contrasted with 36% when distilled water was used (92). The extractability of zinc however differs between sludges, extraction of a Washington D.C. sludge with 0.06 N calcium chloride solution removed only 0.37% of total zinc (111)
Nevertheless, zinc present in sludges is much less mobile in soil, than zinc derived from inorganic salts. In a leaching experiment to compare the mobility of zinc from sludge with that derived from inorganic salts Giordano et al., (75) found most of the zinc present in sludge to be retained in the topmost 0—0.4 cm of a sandy loam soil whereas significant amounts of zinc were translocated well into the soil column when inorganic salts were used to supply the zinc. Experiments on field plots indicate that levels of 0.1 N hydrochloric acid extractable zinc in soil are increased by applications of sludge, while very little translocation of zinc occurs in the soil profile over a one year period (89). In addition, Brown (31) found that most of the zinc applied over a 3 year period of sludge irrigation, to a silt loam soil, remained in a form extractable by 0.1 N hydrochloric acid, a minimum of 74% of the total zinc applied being found to occur in this form. Such applications of sludge greatly increased the amounts of 0.1 N hydrochloric acid extractable zinc in comparison to the average extractable zinc for Midwestern soils of the U.S.A. If 2% citric acid is used as an extractant for sludge zinc, as much as 58% of total zinc is extracted, so that if metals are present in sludge in association with organic matter, these compounds must be readily hydrolysed (92). Anaerobic incubation of sewage sludge has been found to decrease the solubility of zinc in 0.5 M acetic acid and 0.05M ethylenediamine tetraacetic acid, whereas subsequent aerobic incubation increased solubility of sludge zinc in acetic acid, ethylenediamine tetraacetic acid and water. The addition of soil to sludge decreased the solubility of zinc in water and acetic acid but increased solubility in ethylenediamine tetraacetic acid (23). Stover et al., (178) attributed 56% of sludge total zinc to organic forms and 12% to carbonates. This would appear to agree with the supposition of Jenkins et al., (92) that 2% citric acid readily hydrolyses organic zinc compounds to extract up to 58% of sludge total zinc, though of course individual sludges must inevitably differ (178).
Experimentation on a Peterborough farm where sludge has been used for sewage irrigation since 1880 and for arable farming since 1955 (with continued applications of sludge) reveals a correlation between the soil organic matter and both total and available zinc. Available zinc determined by extraction with. 0.5M acetic acid was largely related. to soil total zinc and was not correlated with soil pH (175). Rohole (164) associated the exhaustion of sewage irrigated land with the increase in root soluble zinc brought about by the fixation of zinc by soil organic matter.
3.4 Copper
3.4.1 Copper is similar to zinc in that it is an essential element for both animals and plants though excess can give rise to symptoms of toxicity. In the plant, copper is a constituent of certain oxidising—reducing enzymes such as tyrosinase and ascorbic acid oxidase, but apart from this catalytic role, no definite relationship has been identified for copper in any other physiological process. In animals, copper occurs mainly in the liver and plays an intimate part in haemoglobin synthesis. Deficiency of copper can therefore cause nutritional anaemia, and ‘swayback’ in lambs. Copper metabolism can be severely affected by an excess of other elements, particularly molybdenum and zinc which may occur when sheep ingest soils rich in these elements (180). Similarly, sheep are more susceptible than other animals, to copper toxicity (55).
3.4.2 Murthy et al., (147) reported that institutional diets provided to U.S. children aged 9 to 12 contained between 0.82—1.48 mg of copper with an average of 1.1 mg per day, of which 94.4% originated from foods other than milk.
3.4.3 Sludges contain relatively large amounts of copper, a range of 200—8000 ug /g with a mean of 970 ug /g having been reported by Berrow and Webber. Soils generally contain much lower amounts of copper, typically 20 ug /g with a range of 2—100 ug /g (19) being reported in the literature. More recent data for British soils gives a range of 2—200 ug /g with a mean of 20 ug g~1 (8).
Vegetable Crops
3.4.4 Lettuces have been found to contain an average of 10 ug copper /g in a study on plants collected from several different localities in both Britain and Canada (187). Applications of 100 t /Ha of municipal compost (158) and 450 t /Ha sludge (59) containing l1.6 and 11.9 ug copper produced lettuces respectively; below the maximum of 15 ug copper g l occurring naturally which was reported by Warren et al., (187). In experiments with sludge applied to sandy loam and clay soils, uptake of copper was greatest from silty loam soil while uptake of copper from clay soil was only just greater than a normally fertilised soil (73).
In laboratory experiments, copper has been found to inhibit lettuce root growth at concentrations of 5 x 10-2 M while germination was stopped completely at a concentration of 0.1M copper. Transferring lettuce seeds germinated in the presence of soluble copper to tetraacetic acid, indoleacetic acid or gibberellic acid partially reversed the growth inhibition induced by copper (145).
Richardson (162) found only slightly higher concentrations of copper in lettuce plants grown on soil which had a history of sludge applications than those to be found on similar soil which had not. Despite soil copper levels approaching the upper end of the range found naturally i.e. 170—200 ug /g, lettuces grown in such soils rarely contain more than 15 ug copper g l (Table 10).
Copper concentrations in potato tubers were not increased where sludge had been applied in the Woburn Market Garden experiment, though potato tops contained more copper where sludge had been applied (112). Similarly, more recent literature supports the finding that potato tubers do not accumulate copper from sewage sludge amended soils (11, 162, Table 9). Applications of 100 t hal municipal compost (158) and 450 t sludge /Ha (59) produced potatoes containing significantly greater concentrations of copper (7.4 and 19.0 ug copper /g respectively) than control plants (6.6 and 8.6 ug copper /g respectively). A naturally occurring range of 0.5—6 ug copper /g with a mean of 3 ug /g for potatoes was reported by Warren et al., for tubers grown in various parts of Britain and Canada (187).
Sludge applications do not unduly increase copper uptake by carrots, for which a mean concentration of 3 ug copper gl was reported by Warren et al. In field experiments, Dowdy et al., (59) found copper concentrations in carrots increased significantly from 0.3 ug /g to 1.5 ug when sludge application was increased from 225 to 450 t hal. Carrots grown in the Woburn Market Garden experiment (112) contained relatively high concentrations of copper, some of the treated plants containing l2 ug copper /g in the tops but root concentrations were within the upper part of the naturally occurring range of 0.5—6 ug /g reported by Warren et al., (187). A wider sample of soils which had received sludge (102) shows that carrots grown on treated soils contain only slightly greater concentrations of copper than those grown on untreated soils.
Irrigation with saturation extracts of sludges (30) or incorporation of sludge with soil can produce tomato plants containing more copper than control plants, uptake being greater from sandy loam soils than from clay soils (73). Applications of up to 450 t sludge hal containing 245 ug copper /g did not increase tomato fruit copper concentrations beyond that of the control concentration of 0.3 ug copper g l ~ By incorporating sludge with soil (73) tomato fruits containing up to 9.4 ug copper /g were produced.
Applications of 450 t /Ha sludge and 100 t municipal compost ha -l failed to significantly increase uptake of copper by peas, though vine copper concentrations were significantly increased (59). Similarly, although irrigation of beans with sludge extracts elevated leaf copper concentrations (30) applications of 100 t municipal compost /Ha failed to significantly increase pod copper concentrations (59). Indeed the copper concentrations in beans, resulting from applications of municipal compost (158) were found to be within the naturally occurring range reported by Warren et al., (187). Other vegetables investigated for copper uptake from sludge amended soils include globe beet and leeks (112), in addition to radishes (59). Copper concentrations in leeks (112) were found to be elevated in the Woburn Market Garden experiment when compared to control plants, but not in the more recent survey by Richardson (162).
Grass and Cereal Crops
3.4.5 Data on uptake of copper by cereals is sparse. Bradford et al., (30) reported leaf copper concentrations as high as 17 ug gl in barley irrigated with saturation extracts of commercial sludge products. Dowdy et al., (58) found that sludge—applied copper was not adsorbed by barley from either acid or calcareous soil, even when applied at a rate of 83 ug copper /g. Patterson (153) observed that sludge applied at 134 t hal had no effect on copper uptake by oat plants (Avena sativa L) at pH 5.3 or 6.8.
Significant increases in what grain copper concentrations were reported by Sabey et al., (167) to have resulted from applications of 100 t sludge ha containing in the region of 74—101 ug copper /g.
Leaf copper concentrations of between 5.8—39.4 ug for maize (6 weeks old) irrigated at various rates with sludge were reported by Cunningham et al., (49). However, applications of 450 t sludge (245 ug copper gl) hal failed to produce any significant increase in the copper concentrations found in mature maize grain (58). Seasonal difference in copper concentrations of maize tissue irrigated with sludge were reported by Sidle et al., (171) the control plants in one year, containing more copper than treated plants.
In practice, copper contents of winter wheat and winter barley grain grown on sites with a history of sludge application and a wide range of soil copper levels (8 to 125 ug gl) have not been found to vary greatly (Table 9).
Soil Properties Affecting the Availability of Copper to Plants
3.4.6 Sludge copper is not readily soluble in water. In leaching experiments Jenkins et al., (92) found that 16 successive percolations of sludge with distilled water removed no more than 1/350th of total sludge copper. Similarly, Lagerwerff et al., (111) reporting on leaching studies conducted upon two sludges, found only a maximum of 1.8% of the total sludge copper to be removed after 8 successive leachings with distilled water. Leaching with a solution of 0.06 N calcium chloride solution similarly removed only a maximum of 2% of total sludge copper (111). A much larger proportion however, of sludge copper is soluble in 2.5% acetic acid (19) and 2% acetic acid (92). Berrow and Webber (19) reported that between 0.5 and 31% of sludge copper was soluble in 2.5% acetic acid with a mean of 6.9% from a study of 42 sludges collected from England and Wales. Jenkins and Cooper (92) found 57.8% of copper from one particular sludge to be soluble in 2% acetic acid. Boswell (28) reported results of field trials indicating that very little movement of sludge applied copper occurred in the soil profile, though-sludge could appreciably increase surface soil (0—8 cm) copper concentrations. Bloomfield et al., showed that anaerobic incubation of sludge decreased the amount of copper dissolved by water, 0.05 IA ethylenediamine tetra— acetic acid and 0.5 IA acetic acid whilst solubility was increased following subsequent aeration. Addition of soil decreased the solubility of copper in all three reagents (23).
Stover et al., (178) estimated that 6.4%, 10.4% and 22.5% of sludge copper was in exchangeable, adsorbed, organic and carbonate forms respectively, with possibly 35.1% accounted for as sulphides. Sewage irrigated soils in Berlin and Paris contained 2.3 and 1.3 times as much root soluble copper as ‘healthy’ soils (164). It has been indicated that accumulation of copper by organic matter in sewage irrigated soils might cause soil exhaustion. Brown (31) found that most of the sludge copper added to a silt loam soil remained in a form extractable by 0.1 N hydrochloric acid.
3.5 Nickel
3.5.1 The zinc equivalent concept developed by Chumbley (41) as a guideline for land disposal of sewage sludge assumes nickel to be 8 times more toxic to plants than zinc. Indeed, nickel is toxic to a wide range of crops at very low concentrations causing induced iron deficiency (14).
3.5.2 The effects of nickel on humans and animals are not well documented. Some organic compounds of nickel are carcinogenic, the metal normally accumulating in muscle tissue (JO). It was not until 1974 that nickel was found to be an essential trace element in animals (148) though its metabolic role is not well understood at present. Little data appears to be available on the amounts of nickel in the daily diet. Murthy et al., (147) reported amounts of 0.29—0.70 mg per day for institutionalised children in the U.S.A.
3.5.3 Soils contain between 4 and 230 ~.ug nickel /g, the mean content being about 30 ug /g (8). Sewage sludges contain in the region of 20—5300 ~ug nickel /g (19).
3.5.4 Few studies on the uptake of heavy metals from sludge amended soils refer specifically to nickel. Vegetables grown on sludge amended soils in the Woburn Market Garden experiment (112) were found to contain levels of nickel above that found in control plants. Leeks contained up to 7.20 ~ug nickel /g, globe beet roots 15.0 ug /g and potatoes 0.9 ug /g on amended soils as compared to 3.0, 2.5 and 0.25 ~ug nickel gl in respective control plants. Carrots grown on sludge treated soils have been found to contain no more nickel than control plants (112). Bradford et al., (30) conducted experiments whereby bean, tomato and barley crops were irrigated with saturation extracts of both sewage sludges and commercial sludge preparations and found that generally plant nickel contents were increased above control levels. Nickel contents of maize and rye vegetative tissues have been found to range up to 25.7 and 50.3 hg nickel /g respectively when grown on sludge amended soils (49).
Jenkins et al., (92) found that sludge nickel was less soluble in water than sludge zinc but more so than sludge copper. However, when 2% citric acid was used as a criteria for solubility, up to 73% of sludge nickel was soluble (75% for zinc).
Solubility of sludge nickel in water and 0.5 M acetic acid decreases upon anaerobic and increases upon aerobic incubation, whereas solubility in 0.05 M ethylenediamine tetraacetic acid is not greatly altered during incubation.(23). Stover et al., (178) attributed as much as 27% of sludge nickel to exchangeable forms and 15.5% to carbonate forms, while only 9.8%, 11.2% and 5.5% respectively were attributed to adsorbed, organic and sulphide forms. Brown (31) found that most sludge nickel applied to soil remained in a 0.1 N hydrochloric acid extractable form, though as little as 18% remained extractable with low application rates.
Concentrations of nickel in excess of 25 ~ gl in plant tissue are regarded as being phytotoxic (188). Such levels are not readily to be found in either vegetables (Table 10) or in cereal grains (Table 9) grown on sites with a history of sewage sludge application. Generally, the concentration of nickel in soils on these sites does not exceed those found to occur naturally, i.e. 4—230 ppm.
3.5.4 Despite the assumption that nickel is eight times as phytotoxic as zinc and that some sewage sludges contain significant concentrations of nickel, no crop failures are known to have been attributed solely to the application of nickel in sewage sludge. High soil organic matter contents and high pH can reduce the availability of nickel to plants (80).
3.6 Metal Availability
3.6.1 Commenting upon the experience of agricultural advisors, Patterson (153) suggested that crop damage could rarely be attributed to the effects of a single element. In developing the zinc equivalent concept for the use of sewage sludge on agricultural land, Chumbley (41) took account of zinc, copper and nickel toxicities. The zinc equivalent concept supposes copper to be twice and nickel eight times as toxic to crop plants as zinc, weight for weight. In addition, it assumes the toxicities of these three elements to be additive. The distinct advantage of the zinc equivalent concept is its simplicity and therefore ease of use.
Chumbley proposed that a top soil content of 250 ~.ug zinc gl equivalent was acceptable. This corresponds to 560 kg zinc /Ha and was meant to be applicable to a soil maintained close to pH 6.5. Toxic limits for zinc, copper and nickel of 500, 30 and 25 ~ug /g in dry plant matter respectively, were proposed by Leeper (114). The Agricultural Development and Advisory Service consider 200 ~ug zinc gl, 25 ~g copper /g and 50 ~ug nickel /g in dry plant material, to represent the toxic level of each of these elements C188). A major disadvantage of the zinc equivalent concept is that, except for pH, soil factors which affect trace element availability, are not taken into account. Similarly, Leeper (114) and Chaney (37) suggested, without considering soil factors other than pH, that toxic metal additions to soil should not exceed zinc equivalent levels equal to 5% of the unamended soil cation exchange capacity.
Soil factors other than pH, which have been implicated in controlling the phytotoxicity of metals added to soil include cation exchange capacity and soil organic matter. In addition to these factors, hydrous manganese and iron oxides and soil redox potential also play a role in controlling soil solution concentrations of trace elements (91).
The pH of a soil has no precise meaning (165) in that it is dependent upon the redox potential of the various soil constituents, the carbon dioxide concentration of the soil air and the activity of calcium ions in soil solution. Soil pH does, however, influence to a large extent, the cation exchange capacity (c.e.c.) of soil organic matter. The relationship between the c.e.c. of organic matter and pH is almost linear, being dependent upon the of carboxylic, hydroxylic and phenolic hydroxyl groups. As soil pH increases therefore, the proportion of the soil c.e.c. contributed by organic matter increases (83).
It is probable the pH controls the solubility and hence availability of trace elements by exerting indirect effects. Numerous studies indicate that the availability of trace elements for plant uptake is greater at low pH than at high pH. In reviewing the literature, Jenne (91) indicated that pH effects could be explained by the combined effect of the hydrogen ion on:
(1) The direct precipitation of the metal as the oxide
(manganese) of hydroxide (nickel, copper, zinc).
(2) The concentration of carbonate, phosphate and possibly
silicate ions in the associated aqueous phase.
(3) The precipitation — dissolution of manganese and iron
oxides. The rate of sorption and desorption of heavy
metals by the hydrous oxides.
The usual means of increasing soil pH is to apply lime, the optimum for most crops being a soil of pH 6.5, though some crops, notably spinach, grow best at more alkaline pH values. Brassicas are exceptional and are often grown in soils of pH 7.0 as a means of controlling club root (Plasmidiophora brassicae). Overliming soils can cause trace element deficiencies, so that for practical purposes, a soil pH much higher than 6.5 is un— desirable for agricultural soils. Conversely, the lowering of soil pH increases the concentration of cations in soil solution according to the equation:
(M1)1/n / (M2)1/m = k
known as the Ratio Law, developed by (165) where (M1) and (M2) represent the activities of ions M1 of valency n and IA2 of valency m. This law has been found to generally be applicable for soil systems comprising potassium, calcium and magnesium and an attempt has been made to apply it to aluminium, cadmium and calcium—cadmium systems (110). Generally the law predicts that an increase in acidity, and hence concentration of hydrogen ions would increase the concentration of other elements present in solution, proportional to the power of the valency of the elements concerned. The Ratio Law has not been tested with zinc, copper or nickel systems. Addition of lime to sludge increases the solubility of sludge cadmium in water, 0.5 IA acetic acid and 0.05 M ethylenediamine tetraacetic acid even in the presence of soil (23).
In a comparison of different soil , 0.5 IA ammonium lactate + acetic acid solution (pH 3.75) was found to extract only slightly greater amounts of cadmium than 0.25 IA sodium ethylenediamine tetraacetic acid (pH 7.0) solution, while ammonium oxalate (pH 3.2) extracted only about a fifth of the amount extracted by the ammonium—lactate extractant (3). The adsorption of cadmium added as cadmium chloride to soils has been found not to be correlated with pH (116). Recent data obtained from experiments involving the addition of cadmium chloride solution of 5 ~ug /g of various soils substantiate this finding (121, Table 14).
This data illustrates three important points:
(1) The diethylenetriamine pentacetic acid (DTPA) extractable cadmium is not dependent upon soil pH.
(2) Uptake of cadmium by lettuces would appear to be less at soil pH 6.8 than at pH 5.9 and not dependent upon the amount of DTPA extractable Cd in the soil.
(3) Uptake of cadmium by lettuces is less when the soil contains a significant amount of organic matter.
Adsorption of cadmium has been found to be correlated with both soil organic matter content and soil cation exchange capacity (c.e.c.) (116) suggesting that organic matter controls cadmium adsorption more through its exchange capacity than through its chelating ability. This is contrary to the widely held belief that the chelating ability of soil organic matter is of greater importance than its c.e.c. (37, 191). Soil exchangeable cadmium is however, not significantly affected by the addition of organic matter (78).
Uptake of cadmium by lettuces from fertilised soils is generally less when the organic carbon content of the soil is increased by sludge application (73). The evidence for the immobilisation of cadmium by organic matter originating from either soil or sludge is somewhat circumstantial and also conflicting. All that can be said with certainty from the results published in the literature, is that cadmium in spiked sludges is more readily assimilated by plants than is cadmium present in sludge itself. There is, however, no evidence for the long term immobilisation of cadmium by soil or sludge organic matter.
The rate of desorption of cadmium from soil organic matter has been found to be comparatively rapid (33) and furthermore, cadmium is not tightly bound to soil organic matter. Cadmium is also less readily adsorbed by goethite than is copper, lead or zinc, adsorption only occurring to any significant extent at greater than pH 6.4 (66).
The effect of pH and organic matter on the availability of copper, zinc, nickel and lead in sludge amended soils has been studied even less than that of cadmium. Zinc has been studied more so, because of interest in reducing the uptake of cadmium by plants by the maintenance of high zinc/cadmium ratios. (36) suggested that sludges applied to agricultural land should not contain cadmium in amounts exceeding l% of the sludge zinc content. Although it has been reported that at high pH, organic matter appears to increase the plant availability of zinc (36) the evidence for this is not substantial. Bloomfield et al., found the extractability of sludge zinc to increase, during aerobic incubation and this coincided with a decrease in ~H (23). Data obtained by Maclean (Table 15) for the uptake of zinc by lettuces from different soils suggest that while organic matter may maintain zinc in an extractable form, actual uptake of zinc is not necessarily enhanced by high levels of soil organic matter.
Uptake of zinc by lettuces has been found to be greater from sludge amended soils of pH greater than 6.5 than at lower pH values (73). The application of lime to sludge amended soils reduces the zinc/cadmium ratio in lettuces whilst high soil organic matters tend to elevate zinc/cadmium ratios in plant tissue (121). It would appear possible that liming increases the adsorption of zinc by inorganic constituents of soil more so than that of cadmium. Zinc has been found to be adsorbed more completely than cadmium by goethite for any given pH (66).
The amount of 0.5M acetic acid extractable zinc, nickel and copper in soil is not affected by the addition of lime (27). The toxicity however of these elements to perennial ryegrass is markedly dependent upon soil pH. The pH of the extractant used for soil zinc and copper is not the only factor which determines how much of the element is extracted. This is quite clearly illustrated by the data of Anderson (3) which show that for organic extractants, the nature of the ligand is as important as the pH of the extractant solution.
There is a definite lack of conclusive evidence as to the chemical nature of the complexing materials with which cadmium, lead, copper, nickel and zinc are associated in either soils or sludges. Most of the work on soil organic matter—metal complexes has been done at acid pH values well below the pH of agricultural soils. It has been demonstrated that soluble fulvic acid—metal complexes can be formed at alkaline pH and that these are negatively charged. Only when the organic matter/metal ratio reaches a critical value will precipitation of organo—metallic complexes occur (119).
Organo-copper complexes in soil having a molecular weight less than 1000 are readily available to growing crops, whereas complexes of molecular weight greater than 5000 are not (129, 130). Four fractions of soil copper were proposed by McLaren et al., (126, 127):
(a) soil solution and exchangeable copper;
(b) copper weakly bound to specific sites;
(c) copper occluded by oxide material;
(d) residual copper in clay lattice structures.
In an examination of 11 agricultural soils of widely different textures, Mercer et al.~ (129) found that the proportion of the total soil copper in solution did not differ greatly between soils. Presumably the equilibrium between the different soil copper fractions will shift in order to maintain a given concentration of copper in solution. This view is substantiated by data of Mercer et al., (126) obtained for 24 different soils. Copper is mainly adsorbed by soils at sites, or by processes, other than ‘normal’ ones. Adsorption is primarily dependent upon the organic matter and free manganese oxide content of the soil (127). The finding that specific adsorption of copper by soils increases with increasing pH (126) would therefore appear to substantiate the view that pH controls trace element availability by indirect effects (91). It is not surprising therefore, to find that in the soil pH range of agricultural interest, pH is not a completely reliable guide to the availability of toxic elements. The inorganic components of the soil matrix will determine, to a large extent, what the indirect effects of changes in soil pH will be.
Digested sludge organics, it has been previously concluded, are unreliable metal complexing agents (175). Jenne (91) proposed that the accumulation of metals in surface soil horizons could be explained by:
(1) under vegetative cover, heavy metals are extracted from the soil profile by plant roots and the litter decomposes on the soil surface;
(2) the greater portion of these metals, released from decaying vegetation, will be retained in the upper few inches of the soil by the hydrous oxides except where the hydrous oxides have largely been leached to lower horizons.
The argument that soil organic matter might immobilise toxic elements permanently, runs counter to present knowledge of soil forming processes. Knowledge of trace element mobilisation has largely been derived from studies of podsols (highly leached soils) and although this is a somewhat unsatisfactory state of affairs, these indicate that translocation of organo—metal complexes is a normal feature of soil development. Translocation of such complexes is normally, however,a very slow process.
Chernozems contain much larger amounts of fulvic acids than Podzolic soils without displaying any marked organic matter migration within the profile (71). This would appear to indicate that marked migration of organic matter (and hence complexed elements) will not occur until the bases (calcium-and magnesium)
flocculating the organic matter are leached out of the surface. The rate at which this will occur will depend principally upon seasonal factors, especially rainfall (179). The association of organic matter with (clay skins) isolated from Luvisols (18) indicate that the migration$ of clay and organic matter are linked. The possibility exists therefore, that metallo-organic complexes can be translocated downwards in the soil profile either as soluble compounds or in association with clay during eluviation.
Whilst traditionally, studies on podsolisation have examined the translocation of iron and aluminium (122, 123), more recent studies (76) have been concerned with the mobilisation of heavy metals. Chromium, copper, gallium, manganese, nickel and lead are among the elements which have been found to be trans— located downwards in the soil profile by polyphenols derived from aqueous plant extracts (76). The ligands involved in complexation were primarily hydroxyl groups in plant extracts and amino groups in soil extracts. In podsols, organic matter which has been trans— located is precipitated further down the soil profile by mechanisms which are not fully understood. However, the possibility exists that localised environmental changes may result in further trans location of organic matter and associated metals, eventually posing a threat to water quality.
3.6.2 There has been much criticism of the retention of the zinc equivalent concept in its original form in the recently published Report of the Working Party on the Disposal of Sewage Sludge to Land (55). Much of this criticism has been based on results of field experiments with metal contaminated sewage sludges, presented by the Agricultural Development and Advisory Service which showed that, for red celery and lettuce, copper was generally less than twice as toxic as zinc, while nickel was less than three times as toxic as zinc when total soil metals were considered.
When extractable metals were considered (0.5 M acetic acid extractable zinc and nickel, and 0.05 H ammonium EDTA at pH 7.0 for copper), the relative toxicities for the same crops were found to be:
zinc : copper : nickel
1 : 0.44 : 5.91
In this particular series of experiments, approximately 60% of the variation in crop yield was accounted for by regression equations incorporating extractable zinc, copper and nickel concentrations.
3.6.3 Zinc, copper and nickel are the most common phytotoxic contaminants of sewage sludge since a large part of the variation in crop yield can be explained by a regression equation which combines the concentration of zinc, copper and nickel. It appears that the zinc equivalent concept is of use in determining acceptable additions of metals to soil. There is a case for revising the actual relative toxicity ratios of the three elements, so that the revised ratios would provide protection of productive capacity for all crops which might be grown on a particular site.
The zinc equivalent concept provides a simple and readily obtainable indication of permissible applications of contaminated sewage sludges to land. It is therefore, more likely to be used in practice than more complicated guidelines.
3.6.4 Where metal additions to soils have not exceeded levels specified in the Working Party Report (55), no deleterious effects have been reported.
Though applicable to only a particular site (Great Billing), the Anglian Water Authority has observed clinical reduction in crop yields when soil zinc equivalents based on total metals exceeded 24Oppm (Appendix A4, 7). Since this relates to a site with a history of sludge disposal, and in particular to what is now a soil formed from the sludge itself representing the climax of a sludge disposal operation, the maximum zinc equivalent additions suggested by the Working Party would appear to be of the right order of magnitude.
3.6.5 Further information on the relationship between total metal additions to soil and increases in soil available metals could be obtained through the statistical analysis of data supplied by Chiltern Division of Thames Water Authority. Soil analyses between successive sludge applications would enable correlation coefficients to be obtained between the amounts of metals applied in sludge and increases in:
—
(1) (a) soil total metals;
(b) soil ‘available’ metals;
(2) The analysis could be performed on subsets of data
having divided the soils on the basis of their
original pH values, since these could well affect
the ‘available’ metal levels in the soil.
Such analysis will be made possible by the transfer of original records to a form suitable for computer analysis. It is understood that this may be performed by the Water Data Unit of the Department of the Environment.
3.6.6 At this point in time, scientifically valid evidence which would support a major revision of the relative toxicity ratios used in the •calculation of zinc equivalents~ does not exist. In practice however, few incidents of loss in crop yield as a result of the use of sewage sludge on agricultural land have been reported or documented. The recommendation of the Working Party (55) that permissible applications of zinc equivalents to land be increased by a factor of up to 2 on permanent pasture land, or on calcareous soils where the pH is likely to remain above 7.0,appears reasonable in the absence of detrimental effects having been reported in these situations. Further research is needed to establish whether these recommendations could be amended to allow higher application rates of metals in sewage sludges.
3.7 - Arsenic
Sewage sludges can contain noticeably greater concentrations of arsenic than most soils, up to 1,000 ug arsenic /g being reported for sludge (47). Soils were reported by Bowen (29) to contain 0.1—40 ug arsenic /g. Addition of sewage sludges to soils can therefore increase the arsenic content of soil. There appears to be little data available on the uptake of arsenic from sludge—amended soils by crop plants. Arsenic has been reported to be excluded by plants (1), only occurring to the extent of 0.1— 5.0 ug /g (39) in tissue. Data for the arsenical content of various classes of foodstuffs obtained by Schroeder and Balssa (see 117) indicate that cereals and grains may contain up to 2.4 ug arsenic /g.
A recent survey of agricultural soils in Ontario shows that the application of lead arsenate as pesticide to orchards in the past, is reflected in relatively high soil arsenic levels (Table 16). Although vegetable and meat arsenic contents in excess of 1 ug have been reported (117),there is little data which can be used to relate arsenic content of soil to crop uptake.
—
- The chemistry of arsenic compounds in soil has been assumed to be similar to that of phosphorus (198). To what extent arsenate is adsorbed by either clay minerals, iron and aluminium hydrous oxides or organic matter is not reported in the literature. The view that arsenate is strongly adsorbed by soil clay minerals has, however, been put forward (47). Adsorption of anions by clays and hydrous oxides does, in fact, decrease as soil pH increases.
Data obtained by Frank et al., (67) lend support to the view that clays do adsorb arsenic in some form, while sandy soils and organic soils do so to a lesser extent (Table 17).
Peas and beans appear to tolerate up to 9 /g of soluble arsenic in soil (198). Barley is injured by 2 ug arsenic /g when present at this level in soil solution. Barley grown on zinc—contaminated soils in Norway is reported to contain in the region of 0.1 ug arsenic /g in grain (173). Bohn (26) presented an analysis of the possible forms of arsenic in soil, taking into consideration soil pH and redox potential. From this it would appear that H3AsO4 , HAsO2 and As2S33-