ELSEVIER

Environmental Pollution 86 (1994) 243 249 © 1994 Elsevier Science Limited

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THE USE OF PEAT IN THE HISTORICAL MONITORING OF TRACE METALS IN THE ATMOSPHERE

Carol Stewart & Jack E. Fergusson Department of Chemistry, University of Canterbury, Christchurch, New Zealand

(Received 18 January 1993; accepted 30 September 1993)

Abstract The concentrations of lead, zinc, copper, manganese, cadmium and calcium have been measured in three peat bogs. The distribution of Pb, Zn, Mn and Ca are depen- dent on the position and fluctuations of the water table. They all become depleted in the waterlogged, anaerobic zone. Copper and cadmium are uniformly distributed and appear to be immobilised, probably by the formation of metal/organic complexes, and independent of the acid and redox conditions existing in the bogs. The use of the concentration profiles of the metal ions as a means of historical monitoring of trace metal contamination is complex for Pb, Zn, Mn and Ca but may be more straightforward for Cu and Cd.

Keywords." peat, trace metals, historical monitoring

INTRODUCTION

Goodman and Roberts (1971) found that mosses, which are non-vascular and hence receive all nutrients from the atmosphere, could be used as indicators and integrators of aerial metal levels. The efficiency of mosses at collecting airborne particulates is thought to be due to their porous structure and high cation ex- change capacity, even at low pH (Clymo, 1983; Swaine et al., 1984). Thus, it has been suggested that peat bogs, which are accumulations of mosses and other higher species, may retain a historical record of atmospheric metal deposition. Although some authors have reported success (Ruhling & Tyler, 1973; Livett et al., 1979; Madsen, 1981), the complex physicochemical processes in peat bogs are not well enough under- stood to make results unequivocal (Coleman, 1985; Glooschenko, 1986; Livett, 1988). In particular, the position and variation of the water table can alter trace element distributions in peat bogs. Damman (1978) found that whereas lead, zinc and manganese are retained in well drained peat, they are mobilised and removed in the permanently anaerobic zone below the water table. Therefore, in the zone of water table variations, some elements accumulate, and some are lost.

The establishment of accurate dating is also prob- lematic within a peat profile. Radiocarbon dating is in- accurate for samples younger than 200 years before present. Contamination of older peat by the downward

243

leaching of younger material is a problem. Dating by pollen analysis requires historical information on vege- tation changes (Livett et al., 1979). Therefore, dating by two independent methods is preferable.

Cores were collected from three peat bogs located in the inland alpine area of Canterbury, South Island, New Zealand. One bog was located on the summit of Arthurs Pass, one about 2 km south of the pass, and one in Broken River basin, a much drier area 40 km to the southeast. The water table varied between the sur- face of the bogs and 10 cm below the surface. The cores had been dated using pollen analysis, with some cross-checking by 14C dating (Randall, 1990). The con- centrations of lead, zinc, copper, manganese, cadmium and calcium were measured.

The two Arthurs Pass bogs are comprised of cushion bog species, rather than moss. This is a complication as Glooschenko (1986) has suggested that the use of lower plants (mosses and lichens) is preferable because the root systems of higher plants may penetrate into under- lying substrates.

The three bogs can be considered as remote locations with respect to major urban areas. However, the region is traversed by a highway which carries an average of 780 cars day ~ (National Roads Board, 1988, pers. comm.). Elevated levels of lead in vegetation have been found to be detectable up to 100 m either side of a road (1200 cars day 1), with a roughly exponential de- cline away from the road (Ward et al., 1975). Two of the bogs are located within 100 m of the highway which must be considered as a potential source of aerosol. The geology of the area is strongly indurated mostly graded-bedded greywacke and argillite (NZ Geol. Surv., 1964).

EXPERIMENTAL

Sampling sites

Main divide (map reference N Z M S 260 K33 924102) The sampling site was located in a wet bog formed behind a moraine on the summit of Arthurs Pass, Canterbury, 100 m west of the highway. The bog consisted of fibrous, well humidified, dark brown peat of around 2 m depth. The main peat-forming species were the bog cushions Donatia novae-zelandia and Oreobolus pectinatus.

244 C. Stewart, J. E. Fergusson

Whites bridge (map reference N Z M S 260 K33 922080) The site was located on a fluvioglacial terrace 300 m south of Whites Bridge, Arthurs Pass, and 50 m south- west of the highway. The peat was approximately 1.5 m deep and was of similar composition to the Main Divide bog.

Craigieburn (map reference N Z M S 260 K34 069826) The site was located on a terrace of Cave Stream in Broken River basin, Canterbury, and approximately 400 m east of the highway. The bog consisted of light brown, uncompressed, fibrous peat overlying alluvial gravels at 102 cm depth. The main peat-forming species was the moss Sphagnum falcatulum, with some sedge remains also present. Scattered charcoal fragments were found between the depths of 24 and 38 cm.

Sampling Monolithic cores were sampled from the peat bogs by inserting PVC piping (15 cm internal diameter x 45 cm depth) into the bogs and slicing across the bottom edge with a stainless steel knife. In order to avoid compres- sion of the peat, teeth were cut into the corer so it could slice through the peat. The core ends were imme- diately sealed with plastic film and the cores trans- ported back to the laboratory in the pipes.

The cores were extruded from the pipes and cut into 2 cm sections with a stainless steel knife. Roots were removed with stainless steel tweezers to minimise con- tamination from younger material. For heavy metal analysis, subsamples were removed from the centre of each section (to avoid surface contamination from the corer) with an acid-washed glass spatula. Analytical samples were dried to constant weight in a clean oven at 50°C.

Field measurements

Water table The positions of the water tables were recorded at all sites, at the time of coring and at a later visit. They varied between the surface and 10 cm below the sur- face.

pH Because of a shortage of material, a complete pH depth profile was obtained only for the Whites Bridge core. Five grams of field-moist peat were mechanically stirred with 12.5 ml 0-01M CaCI2 and left to settle overnight. The pH of the supernatant solution was measured with a glass/calomel electrode couple. The pH of the groundwater was also determined for each bog in the laboratory for a sample collected in a polypropylene bottle.

E~ The Eh was measured in situ by inserting a platinum probe to the required depth; a calomel electrode was placed in a KCl-soaked filter paper in contact with the surface of the peat. Each reading was taken after 4-5

min, the time necessary for a constant reading to be obtained. Care was taken to avoid contact with the air (Hermann & Neumann-Mahlkau, 1985).

Analytical procedures Following the technique recommended by Livett et al. (1979) and Damman (1978), 5 ml of 4 : 1 concentrated HNO3 : HC104 (Aristar grades) was added to each dried subsample (0.7-1.0 g). This mixture was boiled gently in covered Pyrex beakers for 30 min and then filtered through acid-washed Whatman No. 540 filter paper into 25 ml volumetric flasks.

Copper, lead and cadmium were analysed using graphite furnace AAS. Matrix modification with H3PO4 was used to improve peak shape (Czobik & Matousek, 1977). Zinc and manganese were analysed by flame AAS. The solutions were accurately diluted 10 times to avoid the problem of the burner blocking with salts.

Calcium measurements were made on a separate 5 ml subsample removed from each solution. To avoid interference by phosphates (due to the formation of r e f r a c t o r y Ca3(PO4)2) , the hotter C2H#N20 flame was used. Potassium (2000/zg g-l) was used as an ionisation suppressant.

The organic content of the peat was estimated by the per cent weight loss after ignition. Accurately weighed 0.2 g samples were ashed to constant weight at 430°C.

Reference materials IAEA Cotton Cellulose V9 and IAEA Hay Powder V-10 were analysed satisfactorily for Cd, Cu, Mn, Pb and Zn as described and presented previously (Stewart et al., 1991).

RESULTS AND DISCUSSION

Redox potential, pH and organic content of peat profiles The E h depth profiles at each of the three sites are shown in Fig. 1. The rainfall data (NZ Met. Service, 1985) for the sites are given in Table 1.

The Main Divide bog was the most anaerobic, which is probably due to the higher rainfall in the location and its poor drainage. The Whites Bridge bog, which receives slightly less rainfall, has a less reducing envi- ronment and this bog appeared to be better drained. The Craigieburn site was the least waterlogged, reflect- ing the lower rainfall.

The E h profiles for all three sites show a marked change of slope at around l0 cm depth owing to a sig- nificant change in pH. The zone below 10 cm is more likely to be permanently waterlogged (catotelm) as in-

Table 1. Information on the rainfall and location of the three sampling sites

Site Annual Distance (east) from Distance from rainfall a Main Divide highway

(mm) (km) (m)

Main Divide 4800 0 100 Whites bridge 4400 2 50 Cragieburn 1600 40 400

NZ Met. Service (1985).

Use of peat in historical monitoring of trace metals 245

A

E

,'5,,

800

Whites Bridge ~. Main Divide

Craigieburn 6OO

4OO

2O0

0 0 10 20 30 40

Depth (cm)

Fig. 1. Depth profiles of redox potentials (mV) for the three peat cores.

dicated by the depth of the water table, whereas above 10 cm, air may penetrate at times producing more oxi- dising conditions (acrotelm).

The pH profile for Whites Bridge peat is shown in Fig. 2. The upper 10 cm of the bog has pH values in the range 3.6-4.0, which decrease slightly to 3.4-3.6 below 10 cm. The acidity in peat is most likely due to organic acids released by humidification of plant re- mains. Hemond (1980) found that dissociation of weak organic acids could quantitatively account for the ob- served pH of 3.8 in a Massachusetts bog.

The estimated organic content (per cent weight loss on ignition) is high, and decreases only slowly down the profile, from around 95% at the surface to 85% at 35-40 cm depth. It is considered that the high acidity of the peat inhibits bacterial decomposition of organic matter (Glooschenko, 1986). Plant remains were still visible at the bottom of the cores.

Total metal distributions Depth profiles for total metal concentrations, expressed on a dry-weight basis, are shown graphically in Fig. 3 for the Craigieburn peat. Similar concentration/depth profiles occur for the other two cores, although there are some differences in the actual concentrations of the elements in the different cores, which will be discussed

below. The complete set of data is available on request from one of the authors (J.E.F.). The analytical errors in these results are approximately 10% for lead, copper and cadmium; and 5% for zinc, manganese and calcium. The error involved in subsampling each core was found to be approximately 15%; this was evaluated by analysing six subsamples from a single segment of peat. Because only one core was obtained from each site, the magnitude of the sampling error is unknown. On the whole the authors' results are lower than re- ported levels for the Northern Hemisphere (Clymo, 1983; Coleman, 1985; Livett, 1988), suggesting lower pollution levels.

Lead Lead profiles from the three sites are very similar, despite differences in rainfall and redox conditions. In each case the lead concentrations fell from 10-15 /~g g l above about 10 cm depth to 4-5 /xg g-l below this depth. Thus, lead appears to have been partially removed from the permanently water logged and anaerobic peat. A similar trend has been observed by Damman (1978).

In a reducing environment of high organic content, low pH, and low Eh it is likely that lead (II) would be immobilised by low soluble compounds such as PbS

4,1

4.0

3.9

3.8

~=. 3.7

3.6

3.5

3.4

3.3 i i i

10 20 30

Depth (cm)

Fig. 2. Depth profile of pH for the Whites Bridge peat.

i

40

246 C. Stewart, J. E. Fergusson

t 3 5 7 9

11 13 15 17 19 21 23 25 27 29 31 33 35 37

i i , i

5 lO 15 Concentration ~g/g)

(a)

Lead

20

1 3 5 7 9

11 13 15 17 19 21 23 25 27 29 31 33 35 37

• ".. . . . . . . . ~ . . . . . . . . L ~ - -- ; ; E ~ C 4

~ : ~ |

i i

10 20 Concentration (pg/g)

(b)

Zinc

i

3O

13

1 3 5 7 9

11 13 15 17 19 21 23 25 27 29 31 33 35 37

[ . . . . . . . . . . . . . . . . .

Copper ...-, , --

i , i • i , 1

0 2 4 6 8

Concentration (p.g/g)

(c)

1 3 5 7 9

11 13

~ ~9

23 25 27 29 31 33 35 37

i

lo

tea

[ ] [ ]

o

Manganese

i , i • i i D i

100 200 300 400 500 600

Concentration ~g/g)

(d)

J~

a

1 3 5 7 9

11 13 15 17 19 21 23 25 27 29 31 33 35 37

o.o • i i

o.1 0.2

Concentration (gg/g)

(e)

Cadmium

i i

0.3 0.4

, c

13

t 3 5 7 9

11 13 15 17 19 21 23 25 27 29 31 33 35 37

n . . . . .

_ Calcium

• i - i • i i J • i

200 400 600 800 1000 1200

Concentration (~g/g)

(0

Fig. 3. Concentration depth profiles for (a) lead, (b) zinc, (c) copper, (d) manganese, (e) cadmium and (f) calcium for the Craigieburn peat.

(Garrels & Christ, 1965). The lead, however, does not appear to be retained in the anaerobic peat, suggesting that either there is insufficient sulphide concentration to form PbS, or other species are more important. A different deposition rate may also be a factor. The total sulphur content of samples from the Whites Bridge bog

(32-34 cm depth) and the Main Divide bog (24--26 cm depth) was in the region of 800/xg gl on a dry-weight basis which is sufficient to form PbS depending on the chemical form of the sulphur.

The lead and manganese concentration/depth profiles show similar features which suggests that the lead is

Use of peat in historical monitoring of trace metals 247

associated with the iron(III)-manganese(IV) oxide fraction. Jones (1987) found that about half the lead was associated with this fraction. The strong positive correlation between lead and manganese for all the profiles (r = 0.708, 0-821 and 0.846; n = 20; P < 0.001) supports this explanation. Since Fe/Mn oxides are solubilised under reducing conditions by reduction to Fe (II) and Mn (II), the depletion of lead in the anaerobic peat can be explained by the release of lead from the Fe/Mn oxide fraction. The lead concentration is also strongly related to Eh in the Craigieburn (P < 0.001) and Whites Bridge (P < 0.01) bogs but less strongly so in the Main Divide peat (P < 0.05).

Lead is enriched in the top 10 cm relative to calcium. It is possible that the elevated lead levels are due to aerial deposition of aerosol transported from urban areas (Fergusson & Stewart, 1992). The lead concentra- tion in the two Arthurs Pass bogs, which are closest to the highway, are not any different from those found in the Craigieburn bog. The surface lead concentrations found in this study (around 10-15/xg g ~) are similar to some reported overseas data from remote sites (Damman, 1978; Livett et al., 1979).

Zinc The concentration/depth profiles for zinc display a similar pattern to those for lead, with perhaps a slightly greater proportional reduction in concentration of zinc compared with lead below the water table. Presumably a similar explanation for the distribution may be given. Jones (1987) also found that zinc and lead showed very similar chemical partitioning in peat.

Copper The copper profiles differ from those of lead and zinc as the copper concentration hardly varies down the profile. This is probably due to an association between copper and the organic material of the peat, regardless of the low pH. Also, since peat decays more slowly in the catotelm the slightly elevated values of copper in the deeper levels (below 23 cm, see Fig. 3(c)) may be because of this (Clymo, 1991). The low pH of the peat could be expected to destabilise copper/organic com- plexes. Data reported by Gregor et al. (1988) indicate that for a Cu(II)-fulvic acid system, at pH 3, almost 100% of the copper exists as the free metal ion, whereas at pH 5, around 10-20% free Cu 2÷ remains. In natural systems, however, kinetic factors may often mean that reactions are slow; hence, thermodynamically unstable species can exist for long periods. A combination of the potential mobility of Cu (II) and the formation of stable organic complexes may be the reason for the resulting even distribution down the core. The copper concentra- tion and organic content do not correlate (White Bridges peat), but this is not surprising considering the high organic content of the peat and the low concentra- tion of copper. The affinity of copper for organic matter is well documented (Davies, 1980) and most copper is indeed organically bound and hence relatively immobile within a peat profile (Jones, 1987). Stability

constants for Cu(II)-peat complexes (Coleman et al., 1956) suggest that most Cu (II) in peat exists in a complexed form.

Manganese Manganese distributions within the profiles show a strong relationship to the redox status of the soil. Man- ganese appears to have been removed from the zone below the permanent water table, which is consistent with the formation of soluble Mn (II) from insoluble manganese (III) and (IV) oxy, hydroxy species as the Eh drops. The low pH assists in stabilising the Mn 2+ ion in solution.

In the surface peat, the highest manganese concen- trations (400-600/xg g ~) occur at the Craigieburn site, which has the least reducing environment, and the lowest manganese concentrations (40-60/xg g ~) are in the Main Divide peat which has the most reducing environment. Hence, manganese retention within the cores appears to be strongly controlled by the redox conditions.

Cadmium The cadmium concentration/depth profiles show no dis- tinctive trends, similar to the copper profiles. This is surprising as cadmium is usually relatively mobile in soils, since the free ion (Cd 2+) is favoured under a wide range of pH and Eh conditions. One explanation is that the situation for cadmium is similar to that for copper, that is, a combination of mobility and association with organic matter (Gong et al., 1977). For example, cad- mium has been found to be enriched in soils relative to sediments because the former have a higher organic content (typically 20-25% in soils compared to 5-10% in sediments). Therefore, it seems reasonable to assume that stable Cd/organic compounds are produced to ac- count for the concentration profiles and also the enrich- ment of cadmium in the peat relative to soils. The concentration of cadmium in peat lies in the range 0.1-0.3/xg g-I compared with levels of 0.05-0.1/xg gl in nearby soils (Stewart et al., 1991).

Calcium The calcium concentration/depth profiles found in this study are similar to those reported by Damman (1978) and show a slow decrease with depth. This corresponds to a mobile Ca 2+ ion being removed from the peat by water movement as the depth increases.

Chronology of peat cores Pollen analyses were carried out on each core (Randall, 1990) and samples were ~4C dated at the Institute of Nuclear Sciences, DSIR, Wellington. The results are summarised in Table 2, indicating reasonable agree- ment exists between the two methods.

It is not obvious why the three sites have different rates of peat accumulation. Compared with data re- ported by Livett et al. (1979), the accumulation rates for two of the sites tend to be fast, with the normal range considered to be 1 cm per 5.5 years to 1 cm per

248 C. Stewart, J. E. Fergusson

Table 2. Chronological data for peat cores a

Site Depth Age Technique Calculated peat (cm) (years BP) accumulation rate b

Main Divide 40 180 Pollen 1 cm per 4-5 years 42 <200 C 14C 1 cm per <4-8 years

Whites Bridge 42 500 Pollen 1 cm per 11.9 years 42 496 + 61 14C 1 cm per 11.8 years

Craigieburn 36 120 Pollen 1 cm per 3.3 years

a Randall (1990). b Assuming constant accumulation rate. ¢ Limit of detection for lac dating analysis. BP, Before present.

20 years. A possible explanation for the difference observed in growth rates is that the faster-growing bogs may also have plants that received their nutrients from the mineral substrate. Other possibilities are that the dating from which the accumulation rates are derived are in error (Clymo et al., 1990), or that mixing has occurred in the bogs.

Calculation of metal deposition rates Given the uncertainty in both the knowledge of be- haviour of metals in peat and the establishment of an accurate chronology, any attempt to assign deposition rates must be considered as very tentative. If the following assumptions are made--(a) the dates deter- mined are accurate, (b) there has been a uniform growth rate and compaction, (c) the metals are retained in the surface 10 cm of the peat, and (d) a factor of 11.3 exists between the subsample used for analysis of metal content and the entire section 2 cm deep by 0.0177 m 2 in surface area-- then deposition rates can be calculated.

The total mass of each metal down to 10 cm depth was calculated, and the rate of deposition can be ex- pressed in units o f /xg m -2 day ~. The results are given in Table 3, together with experimental data from a dustfall collector (Fergusson & Stewart, 1992) located approximately 50 km south of the Main Divide and White Bridges bogs. The metal deposition rates calcu- lated from the peat bog data agree both with each other and (with the exception of cadmium) with rates measured by a conventional fallout collector placed at a similarly remote site, to within a factor of 2-3. Whereas this appears satisfactory, there is no justifica-

Table 3. Metal deposition rates estimated from peat bog data

tion for assuming that deposition rates today are similar to the past.

For all the elements, except manganese, deposition rates are higher in the faster-growing Main Divide and Craigieburn bogs than in the Whites Bridge bog. This may be because they receive additional non-atmospheric input from the underlying mineral substrate. Also, the deposition rates for both these peat bogs are higher than the rates measured with the fallout collectors, again suggesting an additional source of the metals. Manganese is an exception, however, which may be due to the strong relationship between manganese con- centration and redox conditions.

Finally, for cadmium the peat deposition rates are much greater than those obtained from the fallout collector by factors of between 3 and 8. As suggested earlier, cadmium may be accumulated by the humic substances in peat.

CONCLUSION

The distributions of the metals lead, zinc, manganese and calcium in the peat bogs appear to be strongly dependent upon the position and fluctuations of the water table, with concentrations being depleted in the permanently waterlogged zone. The most probable explanation is that in the strongly anaerobic conditions prevailing below the water table, Fe(III)/Mn(IV) oxy and hydroxy species become solubilised through reduc- tion to the Fe (II) and Mn (II) forms; the latter would release the associated metals, which are then dissolved in the water. The distributions of copper and cadmium, on the other hand, were found to vary little with in- creasing depth and it is likely that these metals are pre- dominantly organic-bound. An added complication became evident in that two of the peat bogs may also be receiving additional non-atmospheric sources of trace metals.

Thus, in terms of the potential of peat bogs for his- torical monitoring purposes, it seems that the concen- tration profiles of lead, zinc, calcium and manganese are modified by internal factors as well as the external supply of these elements. However, since copper and cadmium appear to be relatively immobile within the peat profile it is possible that analyses of concentration trends of these metals are less complicated and may be used in reconstructing pollution histories.

Element Metal deposition rate Deposition gauge (/xg m 2 day 1) (/zg m -2 day l)a

Main Whites Cragieburn Castle Hill Divide Bridges

Pb 2.7 1.4 2.4 1-8 + 0.2 Zn 2.8 1.8 4.3 1.8 + 2-0 Cu 0.78 0.34 0.93 0.83 + 0.43 Mn 9-4 19.2 69 21 + 11 Cd 0.049 0.018 0.052 0-006 4 + 0.000 7 Ca 366 135 190 - -

a Fergusson & Stewart (1992).

A CK N O W LED G EMEN TS

The authors acknowledge grants for the purchase of equipment, and one of us (C.S.) for a Postgraduate Scholarship from the New Zealand Universities Grants Committee.

R E F E R E N C E S

Clymo, R. S. (1983). Peat. In Ecosystems of the World: Mires, Swamp, Bog, Fen and Moor; General Studies, Vol. 4A, ed. A. J. P. Grove. Elsevier, Amsterdam, pp. 159-224.

Use o f peat & h&torical monitoring o f trace metals 249

Clymo, R. S. (1991). Peat growth. In Quaternary Landscapes, ed. L. C. K. Shane & E. J. Cushing. University of Min- nesota Press, Minneapolis, MN, pp. 76-112.

Clymo, R. S., Oldfield, F., Appleby, P. G., Pearson, G. W., Ratnesar, P. & Richardson, N. (1990). The record of atmo- spheric deposition on a rainwater-dependent peatland. Phil. Trans. R. Soc. Lond., 327B, 331-8.

Coleman, D. O. (1985). Peat. In Historical Monitoring. MARC Report 31, University of London, pp. 155-73.

Czobic, E. J. & Matousek, J. P. (1977). Effects of anions on atomization temperatures in furnace AAS. Talanta, 24, 573-7.

Damman, A. W. H. (1978). Distribution and movement of elements in ombrotrophic peat bogs. Oikos, 30, 480-95.

Davies, B. E. (ed.) (1980). Applied Soil Trace Elements. Wiley-Interscience, New York.

Fergusson, J. E. & Stewart, C. (1992). The transport of air- borne trace elements copper, lead, cadmium, zinc and man- ganese from a city to rural areas. The Sci. Total Environ., 121,247-69.

Garrels, R. M. & Christ, C. L. (1965). Solutions, Minerals and Equilibria. Harper and Row, New York.

Glooschenko, W. A. (1986). Monitoring the atmospheric deposition of metals by use of bog vegetation and peat profiles. In Toxic Metals in the Atmosphere, ed. J. O. Nriagu. Wiley-Interscience, New York, pp. 507-38.

Gong, H., Rose, A. W. & Suhr, N. H. (1977). The geochem- istry of cadmium in some sedimentary rocks. Geochim. Cosmochim. Acta, 41, 168742.

Goodman, G. T. & Roberts T. M. (1971). Plants and soils as indicators of metal in air. Nature, 231,287-92.

Gregor, J. E., Powell, H. K. J. & Town, R. M. (1988). Evi- dence for aliphatic mixed-mode coordination in copper(II)- fulvic acid complexes. J. Soil Sci., 40, 661-73.

Hemond, H. J. (1980). Biogeochemistry of Thoreau's bog, Concord, Massachusetts. Ecological Monographs, 50, 507-26.

Hermann, R. & Neumann-Mahlkau, P. (1985). The mobility

of zinc, cadmium, copper, lead, iron and arsenic in ground- water as a function of redox potential and pH. The Sci. Total Environ., 43, 1-12.

Jones,. J. M. (1987). Chemical fractionation of copper, lead and zinc in ombrotrophic peat. Environ. Pollut., 48, 131--44.

Livett, E. A. (1988). Geochemical monitoring of atmospheric heavy metal pollution: Theory and applications. In Ad- vances in Ecological Research Iiol. 18, ed. M. Began, A. H. Fitter, E. D. Ford & A. MacFadyen. Academic Press, London, pp. 65-177.

Livett, E. A., Lee, J. A. & Tallis, J. H. (1979). Lead, zinc and copper in analysis of British blanket peats. J. Ecology, 67, 865-91.

Madsen, P. P. (1981). Peat bog records of atmospheric mercury deposition. Nature, 293, 127-30.

New Zealand Geological Survey (1964). Map 18 Hurunui. New Zealand Meteorological Service (1985). Annual rainfall

normal map 1951-1980. Misc. publ. 175, part 6(i). Govern- ment Printing Office, Wellington.

Randall, P. M. (1990). Pollen dispersal across the southern Alps, South Island, New Zealand. MSc thesis, University of Canterbury, Christchurch, New Zealand, and personal communication.

Ruhling, A. & Tyler, G. (1973). Heavy metal deposition in Scandinavia. Water, Air, Soil Pollut., 2, 445-55.

Stewart, C., Norton, D. A. & Fergusson, J. E. (1991). Histor- ical monitoring of heavy metals in kahikatea ring wood in Christchurch, New Zealand. The Sci. Total Environ., 105, 171 90.

Swaine, D. J., Godbeer, W. C. & Morgan, N. C. (1984). Environmental consequences of coal combustion. NERDDP Report EG/84/315. Commonwealth Science and Industrial Research Organisation (CSIRO) Canberra, Australia.

Ward, N. I., Reeves, R. D. & Brooks, R. R. (1975). Lead in soil and vegetation along a New Zealand state highway with low traffic volume. Environ. Pollut., 9, 243-51.