the concept of controlled afforestation of dredged sediment landfills polluted with heavy metals

Symposium no. 55 Paper no. 1272 Presentation: oral

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The concept of controlled afforestation ofdredged sediment landfills polluted with heavy metals

VANDECASTEELE Bart and DE VOS Bruno

Institute for Forestry and Game Management, Ministry of the Flemish Community,Gaverstraat 4, B-9500 Geraardsbergen, Belgium

AbstractLandfilling of polluted dredged sediment from inland rivers is a current practice in

Belgium. The soil matrix of landfilled dredged sediment is very rich in organic matter,clay and calcium carbonate, which means that leaching of heavy metals is of noenvironmental concern. Because of the strong adsorbing matrix, chemical extraction ofheavy metals is very difficult. Tree growth is favoured by the rich nutrient status of thedredged sediment substrate. Phytoextraction with poplar and willow species on dredgedsediment landfills is characterised by a very low efficiency compared to other pollutedareas. Instead of trying to remediate or clean the soil, the goal of the afforestationresearch is to control the heavy metal fluxes through the ecosystem and to monitor theconsequences for the environment.

The heavy metal cycling is monitored in several forests and other polluted dredgedsediment landfills with different land-uses. A major aspect is the study of the control ofheavy metal uptake and circulation in leaves through a correct choice of tree species.Willow and Poplar species and clones show elevated Cd and Zn foliar concentrations,while this is not the case with for instance Sycamore Maple, Black Alder, CommonAsh, Pendunculate Oak and Wild Cherry. Subsequently, the transfer of heavy metalsthrough foliar insects is screened. Litter layer quality and heavy metal bioavailability forlitter and soil dwelling insects is measured and a rational use of covering layers isproposed. Tree growth is measured in detail and a comparison beween the differentspecies is made. Especially Common Ash and Sycamore Maple shows an excellentgrowth prestation.

In a broader scope several land-use types (meadow, maize monoculture, poplarplantations and afforestations) are evaluated based on leaf samples and soil biotaconsiderations and a concept of well-considered land-use of polluted areas has beendeveloped. The revalorisation of polluted dredged sediment landfills throughafforestation is a useful tool for landscape planning and forest expansion in urbanisedareas.

Keywords: heavy metals, dredged sediments, afforestation, landfill

IntroductionPolluted dredged sediment landfills along the rivers Upper and Sea Scheldt and the

river Leie have a large geographical impact, especially around the city of Ghent(Vandecasteele et al., 2001; Vandecasteele et al., in prep.). In a first attempt for riskassessment of these polluted sites, research focussed on the soil compartment. Researchconcerning the geochemistry of heavy metals in soils derived from landfilled dredged

VANDECASTEELE & DE VOS 17th WCSS, 14-21 August 2002, Thailand

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shows that adverse effects of heavy metals depend upon the field conditions, of whichredox potential and pH are of prime importance. Tack et al. (1996) showed that thesolubility of Cd, Cu, Pb and Zn as a function of pH increased strongly in an oxidisingenvironment compared to the reduced environment. Metal mobility and availability inselected upland disposal sites in Flanders has been studied in considerable detail (Singhet al., 1996; Singh et al., 1998; Tack et al., 1998; Tack et al., 1999). Sequentialextraction revealed low residual fractions compared to total contents. This may reflectan important anthropogenic input of metals in these dredged sediment-derived soils(Zhang et al., 1990; Vicente-Beckett, 1992). Based on DTPA extraction, Zn, Cd and Cuwere estimated to be highly plant-available (Singh et al., 1998). Elevated heavy metalconcentrations in the pore water equally raised concerns for enhanced metal availabilityfor plant uptake (Tack et al., 1998). In contrast, long-term leaching and migration ofmetals to ground water predicted from leaching tests was estimated to be of noenvironmental concern (Tack et al., 1999). Field trials where polluted dredged sedimentwas applied as a topsoil revealed no migration of metals or organic pollutants 16months after application (Ruban et al., 1998). However, depending on the geometry ofthe area, metal transport from the sediment disposal site to surrounding areas by surfacerunoff may be of concern (Singh et al., 2000).

Afforestation of calcareous polluted dredged sediment landfills has severalenvironmental benefits such as soil stabilisation and visual buffering (Luyssaert, 2001).Research about forestation initially focussed on pioneer tree species (willow and poplarspecies), especially for biomass production and fytoextraction and phytoremediation(Vervaeke et al., 2001). Both willows and poplars were found to have elevated Cd andZn concentrations relative to reference situations. However, phytoextraction on thesesites is characterised by a low efficiency and long application periods because thecalcareous dredged sediment substrate is found to have a high binding capacity forheavy metals. Research on forestation of polluted dredged sediment landfills nowfocuses on heavy metal cycling and how the adverse effects of the cycling can beminimised. A major topic is the control of heavy metal uptake and circulation in leavesthrough a correct choice of tree species. Conclusions about forest functioning must bebased on certain critical steps in nutrient and heavy metal cycling. Heavy metal cyclingin trees can be screened by sampling roots, bark, wood, leaves or buds. Trace-metalanalysis of woody plants predominantly focuses on foliar concentrations because offood chain transfers and metal recycling of metals via leaf litter (Nissen and Lepp,1997). The litter layer and especially the litter decomposition is an important parameterfor long-term adverse effects of heavy metal pollution (Martin et al., 1982; Martin &Bullock, 1994; Laskowski et al., 1995). A decreased litter decomposition caused bypollution results in litter accumulation.

In this paper we present the results of ongoing research about the afforestation ofupland dredged sediment disposal sites with a low groundwater influence and anoxidized topsoil status. Based on these results the concept of controlled afforestation ofpolluted dredged sediment landfills is detailed.


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Materials and Methods

Site descriptionThree recent polluted dredged sediment landfills (sites 1, 2 and 3) along the Leie

river in the neighborhood of Deinze which were constructed between 1984 and 1988and were afforested between 1988 and 1991 were screened. The landfills were used fordisposal of dredged sediments of the Leie river near Deinze. Total area of the sites was14 ha. Soil, leaf and litter were sampled (Figure 1).

Figure 1 Schematic overview of the sampled compartments to monitor adverse affectsof heavy metal pollution on the forested dredged sediment landfill.

Soil samplingOne dredged sediment landfill (site 1 = 6 ha) was sampled in detail. On the site, 46

points were sampled to a depth of 2 m. It was found that on the outer margin of the site,a thin oligotrophic covering layer (30-60 cm) was used. When this fact is not taken into


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account, the soil properties of the landfill are very variable (Table 1). But even withouttaking the covering layer data into account, a large variability is found, and thisvariability is due to a minority of samples with deviant properties. The use of thephysicochemical criteria to identify dredged sediment-derived soils as described inVandecasteele et al. (submitted) give raise to a more homogeneous dataset. The reasonfor this is that the deviant soil samples were not recognised as dredged sedimentsamples. These samples had a sandy texture, a low organic carbon content and wereunpolluted. For the two other landfills (sites 2 and 3) sampling points were restricted toapp. 1 point per ha. Soil properties of the samples recognised as dredged sedimentsamples were in the range given in Table 1. He used methods for soil analysis aredescribed in Vandecasteele et al. (2001).

Table 1 Minimum, median and maximum value for the total dataset of aerobic soilsamples (dataset 1), dataset 1 without the covering layer samples (dataset 2),and dataset 2 without the soil samples not identified as dredged sedimentsamples (dataset 3).

dataset 1 dataset 2 dataset 3min. med. max. min. med. max. min. med. max.

Cd (mg kg-1 DM) 0.5 9.5 13.1 0.5 10.6 13.1 7.4 10.9 13.1Cr (mg kg-1 DM) 23 210 310 29 230 310 125 242 310Pb (mg kg-1 DM) 5 171 265 19 193 265 121 206 265Zn (mg kg-1 DM) 3 1045 1915 3 1250 1915 632 1309 1915clay (%) 5 40 52 5 41 52 27 42 52sand (%) 1 18 86 1 16 86 1 15 35

Leaf and litter samplingThe sampled trees were very homogeneous in age and crown form. Leaf samples

were collected by means of an extension crosscut saw. Samples were collected between25 August and 5 September. Approximately 2,000 cm³ of leaf samples was collected.Branches from different heights and expositions in the crown of the young trees weretaken. The sampling strategy proposed by UN/ECE-EC (1998), restricting samples tosun leaves of the upper third of the crown was not followed, because no representativevalues were obtained for Salix fragilis grown on a polluted dredged sediment landfill(Luyssaert, 2001). Relative to older trees the sampling strategy was more representativefor young trees because it gives a relatively large sample compared to the total leafvolume of the tree. Leaf samples were not rinsed with water before drying and analysis.In the area the influence of air pollution on foliar metal concentration is low (Luyssaert,2001). Litter samples were collected in triplicate in the different plots at the intensivesampled site 1 under Pendunculate Oak (Quercus robur L., 12 plots) and SacymoreMaple (Acer pseudoplatanus L., 10 plots). Common Ash (Fraxinus excelsior L.) litteralready had disappeared complety at the time of sampling (February 1999). Besidessampling, general litter decay is screened at several forestations on polluted dredgedsediment landfills (total area = 32 ha). Samples were dried at 40°C during 7 days in aventilated oven. The used methods for leaf analysis are described in Vandecasteele etal., (submitted).

Leaf sample concentrations from the landfilled sites are compared with referenceleaf samples from several unpolluted locations to avoid data handling while neglectingreference situation variability. Locations with similar soil properties relative to dredged


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sediment landfills but without soil pollution are hard to find because of the stronginteraction between the heavy metals and other soil properties in dredged sediments.Therefore a reference dataset from a broad scope of soils was used.

Results

SoilGeneral soil characteristics of the sampled sites is given in Table 1 and Table 2.

Dredged sediment landfills were characterised by high clay and organic matter contents,high nutrient contents and markedly high carbonate contents and electricalconductivities. General nutrition status of the dredged sediment substrate is very good,especially compared to the normal Flemish forest soils characterised by extreme acidconditions. Soil formation of the landfill starts with structural development after gradualdrying and oxidation, with the formation of large cracks because of swelling andshrinking of the clayey dredged substrate.

For 19 points on site 1 sampled in detail on 46 points a reduced sediment layeroccurred under the oxidised top horizon. A comparison of both horizons using thepaired t-test allowed to conclude that for S, an average decrease of about 1.3 g kg-1 DM(p < 0.001) had occurred in the oxidised top layer since 1985. For the other soilcharacteristics, no significant differences in soil properties and elemental contents wereobserved (Table 3). This relates to physico-chemical bulk soil properties and totalcontents only, as it has been well established that the chemistry, reactivity and hencemobility and bioavailability of elements is strongly affected during oxidation of areduced sediment (Gambrell et al., 1991; Tack et al., 1996, 1997; Cauwenbergh andMaes, 1997; Stephens and Alloway, 2001).

Table 2 General soil properties of the 41 aerobic soil samples identified as dredgedsediment substrate samples.

Minimum Median MaximumP (g kg-1 DM) 2.2 3.0 3.3S (g kg-1 DM) 0.9 2.9 4.8N (g kg-1 DM) 0.02 0.03 0.05% CaCO3 5.6 6.4 7.7% OC 2.2 3.5 5.0pH-H2O 7.2 7.4 7.6pH-CaCl2 7.2 7.3 7.5EC (µS cm-1) 196 1100 1625

Table 3 Mean values for soil properties in the oxidised and the underlying reduceddredged sediment layer for 19 points on a landfill. Significant differences (p <0.001) are marked with a *.

Oxidised ReducedS (g kg-1 DM) 2.9±0.9 4.3±0.7N (g kg-1 DM) 0.04±0.01 0.04±0.01% CaCO 6.6±0.5 6.7±0.4% OC 3.9±0.5 4.0±0.8pH-H2O 7.4±0.1 7.6±0.1pH-CaCl2 7.4±0.1 7.5±0.1EC(µS cm-1) 1148±317 1121±228


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Results indicate no distinct relative organic matter decomposition, which can becaused by the low relative effect because of the high initial OM contents, or because ofa hampered organic mattter decomposition due to heavy metal contents being harmfulfor growth of microorganisms (Kabata-Pendias, 2000). In a similar experiment on apolluted dredged sediment landfill from Rotterdam harbour forested in 1970-1971(Peeters and van den Berg, 1999), a significant increase of the originally high organicmatter content even in deeper soil layers was found after 26 years. It was proposed thata hampered organic matter decomposition due to the organic matter pollution and themull litter supply by the trees was responsible for this feature.

Organic matter mineralisation is a major cause of acidification in soils, leading to ahigher availability of heavy metals. Low or reduced organic matter decomposition dueto the pollution status of the soil thus can slow down the soil acidification.

Both organic matter decomposition and CaCO3 status (and the related pH values)are 2 soil features for which a long-term monitoring will be important. A decrease of theCaCO3 content can lead to a higher availability of heavy metals, and can be acceleratedthrough periodical waterlogging (van den Berg and Loch, 2000).

Tree growthThe growth since the tree planting in 1989 and spring 2001 expressed as m³ ha-1

was compared for Pendunculate Oak, Common Ash and Sacymore Maple on site 1. Thegrowth of the latter two species was significantly higher (p < 0.001) than for Oak (77.7m³ ha-1). Between Ash (211.3 m³ ha-1) and Maple (238.2 m³ ha-1) no difference wasfound. Oak is known as a tree species with a slower growth in young plantations. Mostof the measured stands for Oak were situated on plots with a thin covering layer, whichwas not the case for the other 2 tested species. Tree growth can be a valuable forestcondition indicator. However, sylvicultural operations like thinning will have animportant influence on the data.

Leaf samplesFirst results showed high foliar levels of Cd and Zn in leaves of pioneer species like

willows an poplars. For the sampled landfills near Deinze, Poplar foliar concentrationsranged from 6 to 18 mg Cd kg-1 DM and from 460 to 800 mg Zn kg-1 DM. Salix sp.were not sampled in the studied areas, but on other landfills. Cd and Zn concentrationsof 4-27 and 180-1,600 mg kg-1 DM were found. Reference values for both groups aretypically 1-3 mg Cd kg-1 DM and 50-300 mg Zn kg-1 DM. Because of the deviant Cdand Zn foliar concentrations, these species are thus less appropriate for afforestation onpolluted dredged sediment substrates.

On the experimental site, leaf samples of Common Ash, Pendunculate Oak andSacymore Maple were collected. For the latter only plots without covering layer werefound. Other species were only sporadically found on the studied landfills, and weregrouped with samples from trees grown on other polluted dredged sediment landfills.When foliar concentrations for poplar, maple, ash and oak were compared for allsamples from the experimental site 1, the Zn concentration in poplar leaves wheresignificantly higher than for the other trees.

For all maple, oak, ash, alder, cherry and lime leaf samples from the experimentalsites 1, 2 and 3, from the other dredged sediment landfills and from reference nurseries


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and young plantations Cd levels were below the detection limit (< 0.35 mg Cd kg-1

DM).For Common Ash and Pendunculate Oak, four groups were compared: leaf samples

from the experimental site both (1) with or (2) without a covering layer, (3) samplesfrom other polluted landfills and (4) reference samples from nurseries, youngplantations and young forest stands. For Common Ash, the N and Zn concentration wassignificantly higher (p < 0.01) on the dredged sediment landfill compared to thereference samples (Table 4). The thin covering layer didn’t affect the results. The leafsamples from other dredged sediment landfills had Zn and N values inbetween the othergroups. For Pendunculate Oak, both N and Zn were significantly higher (p < 0.001) forthe first three groups compared to the reference samples (Table 5). For SacymoreMaple, only two groups were compared: (1) leaf samples from the experimental sitewithout a covering layer and (2) reference samples from nurseries, young plantationsand young forest stands. No difference between both groups was found for N, but Znconcentrations were significantly higher (p < 0.001) for the dredged sediment landfillgroup (98 vs. 46 mg Zn kg-1 DM).

Table 4 Zn and N foliar concentrations for Common Ash from different situations.Means that are not significantly different are denoted with the same letter(Sidak multiple comparison of means at the 95 % level of significance).

Zn (mg kg-1 DM) N (g kg-1 DM)site 1 without covering layer 66b 0.27bsite 1 with covering layer 67b 0.27bother polluted landfills 35ab 0.23abreference samples 29a 0.22a

In general, slightly elevated Zn concentrations were thus found in the leaves on theexperimental sites. Van den Burg (1985) listed deficient, sufficient, optimal and highranges for several elements for tree species in pot trials, nurseries, young plantationsand stands. For Common Ash, Sacymore Maple and Pendunculate Oak normalconcentrations were in the range 24-156, 21-440 and 16-215 mg Zn mg-1 DM. Themeasured concentrations on the polluted dredged sediment landfills are evaluated to benormal. According to the ranges of Van den Burg (1985) Foliar N concentrations werefound to be optimal for all sampled species, indicating that the landfilled dredgedsediment substrate was supplied with sufficient N to ensure normal growth.

Table 5 Zn and N foliar concentrations for Oak from different situations. Means thatare not significantly different are denoted with the same letter (Sidak multiplecomparison of means at the 95 % level of significance).

Zn (mg kg-1 DM) N (g kg-1 DM)site 1 without covering layer 79b 0.3bsite 1 with covering layer 123b 0.3bother polluted landfills 72ab 0.22abreference samples 37a 0.23a


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For Black Alder, Wild Cherry and Small leaved Lime, no differences between thereference data and the samples from dredged sediment landfills were found for N andZn. However, the amounts of trees sampled on the landfills were rather low.

It is conclude that all measured Cd concentrations for the sample trees exceptwillows and poplars were lower than the detection limit (0.35 mg kg-1 DM). Thedetection limit is rather high, but from an ecological viewpunt the concentration is low,especially compared to soil pollution levels. Zn concentrations were higher in leaves ofthe tested species compared to reference samples, but concentrations are not deviant. Athin oligotrophic covering layer didn’t have a significant influence on the Znconcentrations in the leaves. Zn is an essential element for plants. For Common Ash andPendunculate Oak higher N leaf contents were found, indicating a better nutrientsupply, which can be the explanation for the higher Zn-concentrations.

LitterLitter decomposition efficiency is mainly dominated by tree species and site

characteristics. The decreased litter decomposition rate in polluted forest ecosystemswas reported by several authors. Litter degradation can thus be monitored as aparameter of forest health and was observed in several afforestations (total area 32 ha)on polluted dredged sediment landfills. The litter decay in all stands was very efficient:in spring most litter had disappeared. Only for the oak stands, a slower litter decay wasfound, but no litter accumulation as a sign of adverse pollution effects was found. Oakis generally known to have a lower litter decomposition rate. The general soil propertiesof the dredged sediment substrate are optimal for a fast litter decomposition.

On the landfilled site 1, litter samples were collected in february 1999 forPendunculate Oak and Sacymore Maple. The optimal period for litter sampling ishowever in late fall. The amounts of litter collected were low, especially for SacymoreMaple. The Zn and Cd concentrations were 3.9 + 4.4 mg Cd kg-1 DM and 429 + 224 mgZn kg-1 DM in the oak litter and 3.5 + 2.1 mg Cd kg-1 DM and 483 + 154 mg Zn kg-1

DM in the maple litter. These values are higher than values reported for oak-hornbeamlitter after almost 2 years of litter decomposition by Laskowski et al., (1995), but arelow compared to the concentrations given by Martin and Bullock (1994) for oakwoodlands near an old zinc smelter where a high litter mass accumulation was found:39-112 mg Cd kg-1 DM and 1,900-3,500 mg Zn kg-1 DM.

For Cd and Zn, no differences were found with the Kruskal-Wallis rank sum testbetween both tree species and between plots with or without a thin covering layer. Znand Cd concentrations were both expressed on a dry matter and a dry ash base. The useof dry ash is a better reference base to evaluate concentrations in litter decay research(Claussen, 1990; Luyssaert, 2001). However, litter sampling will be repeated in thefuture on regular time intervals after reorganisation of the sampling strategy to avoidunbalanced experimental designs. Especially the variability within the sampled plotswas very high.

Litter and soil dwelling organismsSchollen (2000) analysed heavy metal contents in the woodlouse Philoscia

muscorum (detrivorous) and the spider Pardosa amentata (carnivorous) on the in detailsampled site 1 on locations with or without a covering layer. The measured Cd, Zn, Cuand Pb concentrations were found to be high, but the lowest concentrations were found


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on the sampled sites without a covering layer. The differences were only significant forCd. A thin covering layer can thus reduce heavy metal uptake by soil dwellingorganisms. The tree species influenced the heavy metal uptake too: the relative slow andacidifying oak litter decomposition resulted in higher heavy metal contents in thesampled organisms, which feed on fungal hyphae living on the litter. For Pardosaamentata it was analogous concluded that a covering layer resulted in a reduced uptakeof heavy metals.

Future researchRecent sampling of 2 groups of organisms started on the experimental plots: on the

one hand earthworms and, on the other, leaf beetles. Beyer and Stafford (1993)calculated that bioconcentration of Cd by earthworms is a risk for higher trophic levelsin the foodweb when concentrations in dredged sediment landfills exceed 10 mg Cd kg-1

DM. This concentration is exceeded in most of the superficial layers of the dredgedsediment landfills along the Scheldt and Leie river. To determine the relative risk ofearthworm contamination, earthworm biomass is measured in detail according to themethod described by Muys (1993) both in dredged sediment landfills and adjacentrelatively unaffected alluvial soils. First results indicate a lower biomass at the dredgedsediment-derived soils relative to the alluvial soils, but even on relatively recentlandfills (14 years after disposal) significant earthworm populations were found.

For poplars and willows, elevated leaf Cd and Zn concentrations were found onpolluted dredged sediment landfills. To determine the risk of heavy metal contaminationof the food chain, two kinds of leaf beetles (Crepidodera aurata Marsh. and Fratoravitellinae L.) were sampled in spring on trees known to have deviant Cd and Zn foliarconcentrations. The samples are not yet analysed.

ConclusionSoil processes on a 14 year old dredged sediment landfill revealed only a

significant decline of the S content relative to the initial reduced soil status. No organicmatter decrease was found during oxidation. Long-term monitoring of both OM andCaCO3 contents will be a valuable tool.

Especially Common Ash and Sycamore Maple show an excellent growthprestation. Leaf samples of oak, ash and maple revealed normal Cd concentrations andslightly higher Zn concentrations than for reference situations. A thin oligotrophiccovering layer didn’t have a significant influence on foliar concentrations. Leaf Nconcentrations revealed that the trees were supplied with sufficient soil N to ensureoptimal growth.

No litter accumulation was found, and litter Cd and Zn concentrations were notdeviant. However long-term monitoring of litter quantity and quality is necesarry toallow for a correct assessment. No quality improvement of the litter layer was foundwhen a thin covering layer was present, but litter and soil dwelling organisms showedlower Cd and Zn contents in contrast to plots without a covering layer.

Establishing a forest on dredged sediment landfills without a thick covering layer orwithout the use of liners allows for re-use of the good nutrient status of the dredgedsediment substrate. Nutrient status favours tree growth and allows for a relative rapidforest creation and thus a visual buffering of dredged sediment landfills. Howevercaution must be kept for adverse effects of heavy metals on forest functioning. Only


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long-term observations of these new forests will lead to a correct assessment of theactual risks, but after 14 years of landfilling and 11 years of afforestation no adverseeffects were found. Most monitoring aspects are based on detailed research, thusrequiring an interdisciplinary approach.

AcknowledgementsThis project was carried out with financial support from the Waterways and Marine

Affairs Administration (AWZ) of the Ministry of the Flemish Community. We aregrateful to Carine Buysse, Els Mencke, Anya De Rop, Athanaska Verhelst and AnCapieau for the accurate soil, litter and foliar analyses and to Rik Delameillieure andKoen Willems for the help during the field survey. Thanks also to Raf Lauriks for thecritical review and to Paul Quataert for the statistical support.

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