Beech foliage as a bioindicator of pollution near a waste incinerator

Environmental Pollution 85 (1994) 185-189

BEECH FOLIAGE AS A BIOINDICATOR OF POLLUTION N E A R A WASTE INCINERATOR T h . K e l l e r , R . M a t y s s e k & M . S. G 0 n t h a r d t - G o e r g Swiss Federal Institute for Forest, Snow and Landscape Research, Birmensdorf ZH, CH 8903, Switzerland

(Received 24 November 1992; accepted 4 May 1993)

beech forest on the slope above. An attempt was therefore made to check for contamination by foliar analyses of beech ('passive monitoring' according to Steubing, 1982). The observation, however, that 'acid precipitation' (cf. Morrison, 1984), particularly in the presence of NH~ (Roelofs et al., 1985), might increase the leaching of foliage, led to the hypothesis that such leaching, together with wash-off, might counteract any accumulation of pollutants. Even if not a key factor in the development of mineral deficiencies (Ashmore et al., 1990), this effect may restrict the use of foliar analysis for detecting slight pollution and may deserve attention in the context of the biomonitoring of watersoluble pollutants. This paper aims to answer the following questions:

Abstract Since 1971 unshaded leaves from the top o f marked beech trees (Fagus sylvatica L.) in the vicinity o f a regional waste incinerator have been sampled every year in early September. The unwashed leaf samples were analyzed for the concentration o f Cl and, in some years, for 16 other elements. The operation of the waste incinerator distinctly increased the Cl concentration in the foliage. When the flue gas filtration did not work properly, several other elements also accumulated (without any obvious dust accumulation). There were no significant correlations between precipitation and concentration o f watersoluble elements in foliage samples. This suggested that precipitation was not accelerating foliar leaching so that the bioindication of pollutant accumulation is not restricted in foliage with a well developed cuticula.

- - I s beech foliage a suitable accumulating bioindicator for the presence of air pollutants like CI or cations? --Does foliar accumulation of elements reveal the operational practice of the polluting source? --Does precipitation affect foliar concentrations of water-soluble elements, such that the bioindicative value of foliar accumulation is restricted?

INTRODUCTION

A by-product of modern civilisation is a huge volume of waste which is treated by incineration. Waste incinerators pollute vast volumes of air if flue gases are not adequately purified. In particular, waste contains PVC and other compounds which release HC1 upon incineration. For example Stficheli (1989) estimated for Switzerland an emission of up to 5.6 g HC1 per kg waste by 1983, predicted to decrease to 1.4 g kg 1 by 1990 (different waste composition, better gas cleaning). In addition to flue gases, waste incinerators may also emit dust containing heavy metals. The air-cleaning effect of vegetation has been known for some time (cf. Smith, 1990), and this filtering accumulates pollutants. Consequently, foliar analysis has often been used to detect elements which may be present in the air in tiny concentrations or only temporarily. Plant organs with a large surface area per unit weight (such as lichens or mosses) or with a long life span (such as conifer needles) are considered to be good accumulators, whereas there is little information regarding foliage of deciduous trees. In the early 1970s, a regional waste incinerator was built which had the potential to pollute a protective

MATERIALS AND METHODS

The topographical situation of the waste incinerator is shown in Fig. 1. Construction began in 1971 and operation with an electrofilter started in 1974. An additional dry scrubber for gases became effective in 1986, but for a period in 1987 it worked only for the control of particulates, whereas since 1988 the dry scrubber and filter have seemed to work well. The main wind direction is perpendicular to the slope and wind speed was considered to be high enough to dilute the pollutants to very low concentrations. It was suspected, however, that the emissions of the waste incinerator would also be incorporated in the wind circulation of the slope and might endanger the forest there. Therefore, at different altitudes on the slope, 16 sample plots of two adult beech trees each (Fagus sylvatica L.) were marked in 1971. The foliage of two trees was mixed upon harvest to give one sample/plot. These 16 sample plots were grouped to yield the six groups at the different altitudes shown in Figs 1 and 3. Four additional plots

Environ. Pollut. 0269-7491/94/$07.00 © 1994 Elsevier Science Limited, England. Printed in Great Britain

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Th. Keller, R. Matyssek, M. S. Gfinthardt-Goerg

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chloride concentration (ppm) Fig. 3. CI concentration for several years at different altitudes on the slope (each point is the average of three values). 1967) was used; after 1983 H P L C ( L a n d o r et al., 1989) was applied which yielded results comparable to colorimetry. With the installation of I C P - A A at our institute in 1985, additional elements (mainly metals) were determined annually. In addition, the samples from the earlier years 1971, 1973, 1977, 1981 and 1984 were analyzed in this way. Data for monthly precipitation were obtained from a meteorological station close to the waste incinerator. The Spearman rank correlation test was used to test for correlations between precipitation and foliar concentrations of some elements. Otherwise significance was tested by the Wilcoxon u-test. R E S U L T S AND D I S C U S S I O N Accumulation of chlorides Figure 2 shows the fluctuations in the average C1 concentrations (mean + standard deviation) over 20 years and, in addition, the approximate amount of waste burnt annually between 1 May and 31 August. The rise of chlorides began in 1974, immediately after the waste incinerator started its operation. All values between 1974 and 1990 are significantly higher than before. A peak was reached in 1977. most likely due lo a filter defect. This value was slgnificant!y higher than in all other years except 1985 and 1987. When a dry scrubber was installed iv. 1986, which removed gases to a large extenl, the CI content declined again. In the following )ear, however, the dry scrubber was out of operation l~,r s,'me time /overhaul) which prompted an abrupt

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increase. We conclude from this that C1 is emitted mainly in a gaseous form. In the years 1988 and 1989 a further drop of CI content became noticeable, but these concentrations are still elevated if compared to the original values (before 1974). Recent values (after 1990) fortunately reveal a further decline, in spite of the fact that the amount of incinerated waste has triplicated. With increasing height above the chimney (emission source), CI concentrations in foliage show a decreasing trend except for 1971/73 (before operation of incinerator; Fig. 3). This indicates that C1 concentration in the

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foliage is enhanced with increasing proximity to the emission source. The curves shown in Fig. 3 were chosen from Fig. 2 for several years with different operational practice (see above). In many years (1981/84/85, 1986, 1987) C1 contents reached their maximum values, not at the height of the chimney top but about 50 m above it, where the plume probably reaches the forest most often. In addition, a cloud layer is often observed at this height and we assume that gaseous pollutants are trapped there and so increase the foliar concentrations. The distinct rise of C1 concentrations in beech foliage with the beginning of waste incineration (Fig. 2) confirms the suitability of such foliage to act as an accumulator and biomonitor of the environmental pollutants released by this installation. Annual fluctuations have been largely explained by the operational practice of the incinerator, but of course, waste composition and wind regime also play a role in the fluctuations from year to year. It should be pointed out that C1 concentrations in foliage grown relatively close to the ocean may normally be higher than those observed in this study (Garber, 1967). Accumulation of other polluting elements Waste incinerators are known to emit a broad array of elements. Accordingly, the analytical data reveal a peak of many elements in 1977; a few elements also showed elevated concentrations in 1987. Table 1 gives in Table l. Absolute (A) and relative (B-F as % of A) element concentrations in beech foliage for six periods with differing operational practice (A-F) of the waste incinerator

A B C D E F Element 1971/73 1977 1981 1986 1987 1988-90 (/xg g ~) (%) (%) (%) (%) (%) C1 Al Ca" Cd Cr Cu Fe Ka Mg~ Mn Na Ni P Pb S V Zn

CV (%)

170 1418 765 829 1324 664 -62 119 87 95 118 94 32.4±11-0 12-26 128 104 100 99 94 16.9±9.2 <0.15 >400 >120 >150 >150 >133 1.51 119 115 94 b h 7.31 107 110 102 95 87 -121 111 91 92 105 89 17.7±3.0 7.02 90 100 105 96 78 11-1±6.5 1-30 116 78 90 72 86 25.7±10-1 420 113 123 115 96 83 32.9±28-2 25 180 136 168 128 128 27.5±13.3 2.25 128 115 b 162 107 974 99 101 98 100 95 10.4±6-2 7.5 260 153 77 86 60 -858 118 104 100 102 86 10.6±3.5 0-52 188 125 b b b __ 27.2 169 133 105 122 97 16.3±4.2

a mg g ~ instead of/~g g 1. b not determined or too many values below detection limit. A: before operation of the waste incinerator. B: filtration malfunctioning. C: emission control by dust filtration as in 1974-76, 1978-85. D: dry scrubber plus filtration. E: (temporarily) faulty emission control. F: normal operation of dry scrubber + filter. CV: coefficient of variation for 10 sites and 5 years.

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Th. Keller, R. Matyssek, M. S. Gfinthardt-Goerg

column A average absolute element concentrations for 1971/73, before emissions started. Columns B - F give relative values for five periods with differing operational practice of the incinerator. Since the values in column A (1971/73) equal 100%, the relative values of B - F reveal the accumulation of pollutants in different years. Because element concentrations fluctuate with year and site, the table contains the coefficient of variation (cv = standard deviation in % of the mean) for those 10 elements which were considered to be important for nutrition or toxicity. When this cv was calculated only those 10 sites were selected which seemed to be most exposed to the emissions from the incinerator. For these sites only the five years 1971, 1973, 1981, 1984, 1985 were included which were not extreme with regard to the emissions. From Table 1 it appears that in 1977 (aside of CI ) especially Cd, Pb, V, Na, Zn, Ni and Ca were emitted. The percentage of sites in which the different elements reached maximum concentrations in 1977 was determined to be: 75-100% of sites: 50-74-9% of sites: 2 5 4 9 . 9 % of sites: >25% of sites:

Cd, Pb, V, Sb, Zn, Na, Ca Co, Ni, Mn, S Mg, Fe, B, Cr A1, K, Cu, P.

(Analyses for Sb and Co were above the detection limit only for 1977 and were stopped in 1985). We think that the above elements were those particularly emitted when flue gas filtration was defective. (Some of these elements were elevated again in 1987 when C1 showed a second peak.) Whilst concentrations of Cd, Pb, V, Na, Zn, and Ca were strongly increased in 1977, in later years mainly Cd and Na distinctly exceeded their initial concentrations. The discussion is restricted to these six elements. Ca: this macronutrient exceeded 15 mg g l only in 1977, apparently due to defective dust filtration, but even this increased concentration caused neither visual foliage injury nor observable dust accumulation. Cd: like C1 this element rose distinctly with the start of waste incineration. Because the initial values were very low (often below the detection limit of the method) the relative value in Table 1 is elevated. After 1977 the concentration, however, dropped below the detection limit at several sites. According to Stoeppler (1991) Cd levels in plants are generally below 0-5 /xg g l; this limit was never exceeded in any of our sample plots during the last decade. Na: with the exception of 1988, the relative concentration of this element was distinctly increased. Its absolute amounts, however, were less than 1% of the K concentrations and are considered to be too low to deserve attention. Pb: waste incinerators are well known sources of Pb and, in accordance, highest values were found in 1977 when the filter apparently did not work.

However, the major part of Pb in the atmosphere is considered to result nowadays from the combustion of leaded gasoline, as Pb may be transported over hundreds of kilometers (Ewers & SchlipkSter, 1991). The contributory effect of vehicular lead emissions becomes evident, as recent values are lower than before the start of the incineration. This fact may reflect the modem use of unleaded gasoline as well as the diversion of traffic to a more distant highway. V: the determination o f this element (prominent in 1977) was discontinued, because many values dropped below the detection limit after 1981. Zn: this element also showed maximum values in 1977, but even then it remained well below 400 /xg gl, the limit considered as 'toxic' for plants (Ohnesorge & Wilhelm, 1991). Thus the concentrations of all elements remained below their assumed levels of toxicity and the foliage never showed any visible symptoms of injury. The influence o f precipitation

In Fig. 4 average foliar concentrations are plotted against precipitation (either during the whole growing season or during August only, the month immediately before foliage harvest). Whereas K and Mg are reported to be subject to leaching (Larcher, 1980), CI and Zn may be leached with difficulty (Fliickiger et al., 1988). It is not clear, however, whether these authors mean by 'leaching', a general depletion o f elements by precipitation (including wash-off), or specifically an extraction from the mesophyll. No relation between element concentrations and precipitation was evident in Fig. 4 for any of these elements. Nevertheless, data for individual trees (instead of annual averages as in Fig. 4) were subjected to Spearman's rank test. Results are given in Table 2. They reveal statistically significant increases in concentration for CI (both precipitation periods) and Zn (growing season only) with increased rainfall, whereas relationships for K and Mg remained without significance. In contrast, washing and leaching is expected to decrease concentration. It has been shown in Table 1 that the waste incinerator is not a source o f K and Mg, as compared with C1- and Zn. The parallel increases of the latter two elements with increasing precipitation therefore might be an indication for their increased trapping by rain or mist and subsequent deposition. Table 2. Spearman rank correlations for some foliar concentrations and precipitation either in August or during the whole growing season

n August Growing season,

r p r p

C1

K

Mg

Zn

342 0.24 0.000 0.17 0.001

209 -0.13 0.054 0.11 0-10

190 0 0.31 0 0.75

190 0 0.08 0.19 0.01

r -- rank correlation, p = probability.

Beech foliage as a bioindicator o f pollution

Foliar element concentrations may be caused by both leaf-internal processes and external deposition. --Leaf-internal process (a) root uptake from soil solution and subsequent upwards transport by transpirational flow. These two processes depend on moisture conditions in the root horizons. With increasing precipitation increasing amounts of pollutants are washed into the soil by wet deposition and enter the soil solution. Especially in forest soils with little surface run-off, an increase in precipitation raises the amount of soil solution which is available to the roots. In addition, deposits already present in the soil may become dissolved and available. (b) uptake of gases from the atmosphere through the stomata into the mesophyll. (c) leaching of the mesophyll. According to Morrison (1984) 'there is some evidence ... that foliage leaching in the sense of removing minerals originally taken up through roots does occur ... and is increased by acid precipitation'. But Ashmore et al. (1990) conclude in their literature review that the role of leaching is unclear and controversial. --External deposition (d) epidermal contamination by dry deposition of airborne particles and gases; (e) (partial) removal of surface deposits by precipitation (wash-off). While increased precipitation may increase foliar concentrations through (a), it may decrease it through (b) and (d) by cleaning the atmosphere as well as through (c) and (e). We assume that mainly (a) affected the tendency of slight simultaneous increases of element concentrations and precipitation during the growing season (Table 2, Fig. 4). At best the slight negative correlation for K in August (not significant) may support the suggestion that increased precipitation just before harvest might have washed off some surface deposits (e.g. dust). CONCLUSIONS The present investigation leads to the following conclusions: --beech foliage is a suitable accumulating bioindicator for mineral pollutants; --foliar concentrations of several elements, including C1, may monitor the operational practice of polluting sources;

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--since no significant negative correlations were detected between element concentrations in beech foliage and precipitation, we conclude that precipitation did not restrict the bioindicative value of foliar pollutant accumulation. ACKNOWLEDGEMENTS We thank the local Forest Service for careful sampling, the many laboratory technicians who performed analyses during the long span of time and Dr C. Hoffmann for statistical calculations. Financial support by the management of the incinerator (for analyses) and stimulating discussions with Drs J. B. Bucher and J. Innes are gratefully appreciated. REFERENCES Ashmore, M. R., Bell, J. N. B. & Brown, I. J. (1990). Air pollution and forest ecosystems in the European Community, CEC Air Pollut. Res. Rep 29 (Brussels). Ewers, U. & Schlipk6ter, H. W. (1991). Lead. In Metals and Their Compounds in the Environment, ed. E. Merian. VCH, Weinheim, pp. 971-1014. Fltickiger, W., Leonardi, S. & Braun, S. (1988). Air pollutant effects on foliar leaching. In Scientific basis of forest decline symptomatology, ed. J. N. Cape & P. Mathy. CEC Air Pollut. Rep. 15, pp. 160-69 (Brussels). Garber, K. (1967). Luftverunreinigung und ihre Wirkungen. Borntraeger, Berlin-Nikolassee. Landolt, W., Guecheva, M. & Bucher, J. B. (1989). The spatial distribution of different elements in and on the foliage of Norway spruce growing in Switzerland. Environ. Pollut., 56, 155 67. Larcher, W. (1980). Physiological Plant Ecology, 2nd edn. Springer, Berlin, Heidelberg, New York. Morrison, I. K. (1984). Acid rain. A review of literature on acid deposition effects in forest ecosystems. For. Abstr., 45, 483 506. Ohnesorge, F. K. & Wilhelm, M. (1991). Zinc. In Metals and Their Compounds in the Environment, ed. E. Merian. VCH, Weinheim, pp. 1309 42. Roelofs, J. G. M., Kempers, A. J., Hondrijk, A. L. F. M. & Jansen, J. (1985). The effect of airborne ammonium sulphate on Pinus nigra var. maritima in the Netherlands. Plant and Soil, 84, 45-56. Smith, W. H. (1990). Air Pollution and Forests, 2nd edn. Springer, New York, Heidelberg, Berlin. Steubing, L. (1982). Problems of bioindication and the necessity of standardization. In Monitoring of Air Pollutants by Plants, ed. L. Steubing & H. J. Jaeger. Junk, The Hague, Boston, London, pp. 19-24. Stoeppler, M. (1991). Cadmium. in Metals and Their Compounds in the Environment, ed. E. Merian VCH, Weinheim, pp. 803 51. Stticheli, A. (1989). Siedlungsabfallaufkommen, Kehrichtverbrennung und Luftschadstoffbelastung. Umwelttechnik, 23, 2-7.