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ELSEVIER
ENVIRONMENTAL
POLLUTION
Environmental Pollution 141 (2006) 387-395
www .elsevier.com/locate/envpol
Invited paper
Woody biomass phytoremediation of contaminated brownfield land
Christopher J. French a, Nicholas M. Dickinson a,*, Philip D. Putwain b
b
"School of Biological and Earth Sciences , Lil'e1pool Jolin Moores Unil·ersity, Byrom Street, Li1•eJpool L3 3AF, UK
Ecological Restoration Consultants (ERC), Ness Botanic Gardens, Uni1•ersity of Lil·e1pool, Ness, Cheshire CH64 , UK
Received 6 May 2005; received in revised form 17 August 2005; accepted 28 August 2005
Field trials show short-rotation coppice provides effective risk management and remediation solutions to hotspots
of residual metal and As contamination of brownfield land.
Abstract
Economic and environmental regeneration of post-industrial landscapes frequently involves some element of re-afforestation or tree planting.
We report field trials that evaluate whether woody biomass production is compatible with managing residual trace element contamination in
brownfield soils. Large-scale mapping of contamination showed a heterogenous dispersion of metals and arsenic, and highly localised within-site hotspots. Yields of Sali.r, Populus and Alnus were economically viable, showing that short-rotation coppice has a potentially valuable
ro le in community forestry. Mass balance modelling demonstrated that phytoextraction potentially could reduce contamination hotspots of
more mobile elements (Cd and Zn) within a 25-30-year life cycle of the crops. Cd and Zn in stems and foliage of Sali.r were 4-13 times higher
than EDTA-extractable soil concentrations. Lability of other trace elements (As, Pb, Cu, Ni) was not increased 3 years after planting the coppice;
woody biomass may provide an effective reduction of exposure (phytostabi lisation) to these less mobile contaminants.
© 2005 Elsevier Ltd. All rights reserved.
Keywords: Brownfield ; Heavy metals; Trees; Coppice
1. Introduction
Increased tree cover offers low cost and effective solutions
to derelict landscapes and degraded environments (Cannell,
1999; Dickinson, 2000b; Paulson et al., 2003). Regeneration
and urban greening of former industrial landscapes in lowland
England have been achieved through substantial tree planting;
in North-West England, the first 15 years of a national Community Forest programme is halfway towards transforming
a landscape from 4% to 12% tree cover, with about
250 ha annum - t of trees being planted within the Mersey
Forest region that surrounds Liverpool (Anon., 2003; Putwain
et al., 2003; Rawlinson et al., 2004). An increasing proportion
of this tree planting is taking place on an estimated 11,000 ha
of brownfield land in the region, much of which has residual
* Corresponding author. Tel. : +44 151 2312190; fax: +44 151 2073224.
E-nwil address: n.m .dickinson@livjm.ac.u k (N.M. Di ckin son).
0269-7491/$ - see front matter © 2005 Elsevier Ltd. All rights reserved.
doi: 10.10 16/j.envpol.2005.08.065
contamination from former land use (Fig. I). It has been speculated that tree planting may resolve soil contamination issues,
either through extraction or immobilisation (Lepp and
Dickinson, 1998; Vangronsveld and Cunningham, 1998;
Bardos et al., 1999; Dickinson, 2000a; Pulford and Watson,
2003; Berndes et al., 2004). In particular, there is growing
evidence that clean-up of cadmium can be achieved using
short-rotation coppice (SRC) Salix (willow) clones for phytoextraction (Dickinson and Pulford, 2004). However, despite
significant experimental data from laboratory and pot experiments, there is still a real paucity of supportive field evidence.
On most brownfield land in the UK, contamination levels
are not excessively high and, in fact, may not meet the statutory definition of contaminated land (Environment Agency,
2002). Risk assessment criteria for human health and controlled waters, based on a pollutant- pathway-receptor model,
are perceived to be less onerous for soft end-use remediation ,
compared to situation with more obvious linkages; for example, where land is used to grow food crops (DEFRA, 2002;
388
C.J. French eta/. I Environmental Pollution 141 (2006) 387-395
Fig. I. A typical brownfield site at Cromdale Grove (CRM) with amenity grassland on shallow soil (I 0-30 em depth) overlying a former landfill site that received
domestic, industrial and chemical waste (including colliery spoil , ash and rubble) in the 1960s and 1970s, surrounded by residential buildings and light industry and
remains of previous industrial activity. The photograph was taken as the two experimental plots in the present study were being established on the landfill site (plots
and site are marked on the photograph).
DEFRA and EA, 2002). This may be somewhat contradictory
because environmental improvements brought about by landscape restoration and urban greening frequently proceed
alongside residential redevelopment, leading to increased
recreational usage of the land and thus potentially increased
human exposure. The actual risk posed from this type of recreational land use cannot be easily assessed under the present
system, which may lead practitioners towards using inaccurate
or inappropriate guidance. There is also a possibility of harm
being caused to humans or other animals, or of pollution of
controlled waters, through planting trees on contaminated
land; establishment of woodland may alter the lability of
soil contaminants, for example due to biological activity
associated with leaf fall or the rhizosphere, although there is
currently little evidence that this is the case (Dickinson and
Lepp, 1997; Dickinson et a!., 2000).
The objectives of the current research project are to understand what happens to residual contamination of brownfield
land and whether tree planting provides an effective longterm management of risk from soil contaminants. Results presented here are one part of 7 years of experimental fieldwork
at more than 50 trial plots at 18 sites in NW England. In this
paper, we focus on contamination at five brownfield sites and
the potential for (i) site clean-up by tree harvest and (ii) longterm stabilisation by establishing a tree cover. The aim of this
experimental work was to quantify dispersion and mobility of
metals and As over a 3-year growth cycle of mixed woody biomass planting and harvest.
2. Materials and methods
2.1. Sites
Five brownfield sites in the region of Liverpool and St. Helens in NW
England were selected on the basis that they were perceived to be contaminated
by the Loca l Authority or other landowners, following previous desk studies
and Phase I surveys (Table I). The 5 sites had differing former land uses (landfill, industrial waste , sewage-sludge) and thus a likely existence of a varied
range of trace element contamination.
Nine trial plots (each 30m X 30m or 21m X 21 m, with 1-3 plots per
site) were located across the sites and rabbit-proof fencing was installed
around the plots, except at MER where 3-wire public-exclusion fencing was
used (in the absence of rabbits locally). Soils at 2 sites (SUG and CRM)
were crawler ripped to 2 m, in an attempt to resolve perceived drainage problems, and total cultivation was carried out at all sites by turning the soil (using
a tracked excavator) to an approximate depth of 30 em , followed by (tractormounted) rotovation and a glyphosate weed-control treatment. Further, weed
control using a combination of strimming and herbicides was required during
the first 2 years of cultivation.
2.2. Plot characterisation
Dispersion patterns of trace elements were measured and mapped, with
soil sampling (0-30 em depth , 2.5 em auger) designed to identify 1% hotspots
(72 sampling points per 900 nl plot) using a herringbone sampling design
(DoE, 1994). Soils were later re-sampled , following harvest of plants after 3
years, from points within the plots identified as hotspots. After sampJjng, soils
were air-dried , ground and sieved to < 2 mm. Trace elements were determined
as pseudo-total concentrations following HNOiHCl microwave digestion
(US-EPA, 1994). Additionally, a sequential extraction procedure was used:
0.0 I M CaC1 2 (3 g soil in 30 ml , 2 h) followed by centrifuging, filtering and
C.J. French et a/. I Environmental Pollution I 4 I (2006) 387-395
389
Table I
Locations of experimental trials and outline site descriptions
Site
Abbreviation
NGR
Details
Sugar Brook
SUG
SJ 392960
Fazakerley
FAZ
SJ 387963
Kirby Moss
KIR
SJ 445984
Merton Bank
MER
SJ 527960
Cromdale Grove
CRM
SJ 534947
A 3.8-ha urban derelict site adjacent to busy road in a commercial and residential location. Formerly
allotments, but neglected grassland and scrub prior to onset of study.
Formerly a sewage farm but now adjacent to a modern sewage treatment works. Substantial recent
re-engineering of landscape at ca. 15 ha of the site. Sewage-sludge and sewage cake treatments
applied to one experimental plot prior to planting.
34 ha landfill site within an agricultural landscape. Livestock grazing on poor grassland ( 15 em
topsoil depth) since closure of the landfill site in the 1970s.
6.6 ha of intensively mown amenity grassland, used for public recreation, within a residential and
industrial area. Shallow soils (10-30 em). An alkali works from 1873 within an industrial landscape,
and subsequently an industrial waste site.
8.8 ha of mown ameruty grassland with shallow soil overlying former landfill site in a residential
and industrial area (see Fig. I).
Three of the sites were old-style landfills (Dickinson, 2002) with inadequate capping and restoration by modern standards.
0 .05 M EDTA (30 ml , I h). Soil trace elements were then determined using
AAS (Year I ) and ICP-OES (Year 3) following standard methodology. Other
physico-chemical parameters were detennined from bulked samples for each
plot, using standard methods (French, 2005) .
Reference Material (NCS DC73349, bushes, branches and leaves) was included
in all analytical batches.
3. Results
2.3. Planting and l/lanage111ent
Plots were planted with 5 taxa of Salix, 2 Populus hybrids, Alm1s, Bet11ia
and Larix (Table 2) in a fully randomised block design (I 0 blocks per taxa,
12 plants per block, double rows 1.5 m X 0.5 m spac ing). Selection of species
and varieties was based on UK Forestry Commission guidance (Tabbush and
Parfitt, 1999; Tubby and Armstrong, 2002) and earlier work within o ur own
project (Punshon and Dickinson , 1999; Rawlinson et al. , 2004). Following
the standard practice for management of short-rotation coppice management,
Salix and Populus, but not the other species, were manually cut back after I
year. Above-ground biomass of all Salix, Populus and Alnus was then manually harvested after a further 2 years. Standardised foliar sampling (top third
of the inner 8 trees per block) for metal determination was carried out after
the first growing season and at the end of the third year; at the latter time, sampling focussed on trees planted within contamination hotspots and stem samples were also collected at harvest. Plant samples (foliage, and I 0 em section s
of stem removed from every 50 em of total stem height) were washed in deionised water, dried at 80 oc and ground prior to trace element determination,
using HNO/H 2 0 2 microwave digestion and ICP-OES in all cases. Certified
Table 2
Woody species and varieties planted across all plots in a randomised block
design
Taxa
Abbreviation used in text
Larix X euro/epis Henry
Betula pendula Roth
Alnus incana (L.) Moench
Populus deltoides X nigra 'Ghoy '
Populus trichocmpa 'Trichobel '
Salix caprea X cinerea X viminalis
LAR
BET
ALN
GHY
TRI
'Calodendron'
Salix vimina/is 'Orm'
Salix caprea X vimina/is ' Coles '
Salix bwjatica 'Germany'
Salix \'imina/is X schwerinni 'Tora '
CAL
ORM
CLS
GER
TOR
Salix and Populus were planted as unrooted 25 em pegs; Larix, Betula and
Alnus as 20-30 em cell grown stock. Each repli cated block contained 12
plants (2 rows of 6 plants) with 0 .5 m spacing within rows, 1.5 m between
rows and 4-10 replicates of each taxa per plot. Blocks were planted contiguously, with no additional spaci ng between blocks.
Soils at all of the sites were highly disturbed historically or
else derived from various imported substrates through former
land uses. They were largely defined as clay loams or silt
loams with a range of 5.6-13% organic matter (LOI),
0.14-0.3% total N, pH 6.8-7.4 (except SUG, with soil pH
5.4), and are described more fully elsewhere (French, 2005).
There was little evidence of contamination at one of the sites
(SUG), apart from 7% of soil samples with marginally elevated
Zn (Mean 181 mgkg- t, Range 128-373 mgkg- t) . The other
4 sites showed evidence of contamination with at least one
trace element (Table 3). The MER site was highly As-contaminated, with some additional concern over Pb concentrations.
Comparison of these data with UK Soil Guideline Values
(SGV) suggests that contamination may be somewhat marginal
at the other sites, particularly for the more zootoxic elements
(Cd and Pb). However, at all sites, there was a wide range of
variation for all elements. CLEA guidance (DEFRA, 2002)
utilises statistical tools or 'mean value tests' when examining
heterogeneously contaminated land. If the calculated upper
95th percentile (US 95 ) is less than the SGV, then contamination may be judged not to be an issue for purposes of human
health risk assessment. Using this criterion, all three plots at
MER were significantly As-contaminated and one plot was
significantly contaminated with Pb. Of the plots at the other
4 sites, two would be judged to be contaminated with Cd,
two with Ni and another with Pb. When these contaminated
plots are viewed in spatial profile, all but one possessed
a highly heterogenous dispersion of trace elements (Fig. 2).
Uniform contamination of the land surface appears to be unusual , and the existence of hotspots is typical. Analysis of
soil samples collected from identified hotspots showed very
low recovery rates using CaC1 2 as an extractant (Table 4) .
Substantial proportions of Cd, Zn, Cu and Pb were apparently highly mobile and thus potentially bioavailable in the
soils. EDTA extraction of Ni was proportionately low, indicating low mobility of Ni in the hotspots. Obvious care
C..!. French eta/. I Environmental Pollution 14 1 (2006) 387-395
390
Table 3
Ranges of soil variables across the 10 plots and 5 sites in the study, and in the context of UK gu idance on Contaminated Land Exposure Assessment (DEFRA
and EA , 2002)
Variable
Cd
Zn
Cu
Ni
Pb
Asc
pH
LOI (%)
Total N (%)
Range in soi l samples
·across all plots (mg kg0-7.9
2.8-1 300
10-880
10-109
45- 1770
4.9-5266
4.9-8.2
1
)
Range of plot
means (mgkg 0.3-6.1
121-329
5.9-307
24-50
99-720
312-615
6.1-7.6
5.6-13.1
0.14-0.30
1
)
Soil guidance value
(SGV)" (mg kg - 1)
Range of the percentage
of samples exceeding
SGV across all plots
Sites where US 95
exceeded SGV
1-8
(300)h
( 130)b
50-75
450
20
0-100
6-72
0-100
1-49
0-59
99-100
FAZ, KJR
n/a
nla
CRM, FAZ
FAZ, MER
MER
n/a - SGYs not published for Zn and Cu, so not applicable.
" Soil guideline values from current UK CLEA Guidance (DEFRA and EA, 2002).
b Elements not included in current UK CLEA Guidance; former UK Guidance (ICRCL, 1987) thresholds are used here.
c Data refer to three plots at one si te (MER) only.
FAZ 1 Cd
KIR 1 Cd
FAZ 1 Ni
FAZ 1 Pb
CRM 1 Ni
Fig. 2. Spatial heterogenei ty of metals (Cd, Ni, Pb) and As (0- 30 em depth) within samp ling plots that were sign ifi cantly contaminated at four of the study si tes
(CRM , FAZ, KIR , MER). Sampling points are indi cated on the maps (Note: MER2 and FAZ I are smaller plots of 2 1 m X 2 1 m, with fewer sampling points).
Colour differential is arbitrari ly defined by SURFER mapping software, with overall concen trati ons ranges of 0-8 mg Cd kg - 1, 0-180 mg Ni kg - 1,
0-2000 mg Pb kg - I and 0-5000 mg As kg - I.
C.J. French eta/. I Environmental Pollution 141 (2006) 387-395
Table 4
Pseudo-total, EDTA-extractable and CaC]z-extractable concentrations of trace
elements in soil samples collected from identified hotspots within plots
Trace
element
(site)
Cd (KIR)
Cd (FAZ)
Zn (CRM)"
Cu (FAZ)b
Ni (CRM)
Ni (FAZ)
Pb (FAZ)
Pb (MER)
As (MER)"
Mean
pseudo-total
(mg kg- 1)
2.7
6.4
496
323
76.1
54.1
684
2088
1386
EDTA-extractable
Mean
(mg kg0.4
2.3
80.4
151.1
2.2
2.9
303.7
645.6
35.4
1
)
CaClrextractable
Percentage
of pseudototal
Mean
(mgkg-
14.7
35.1
16.2
46.8
2.9
5.4
44 .4
30.9
2.6
0.1
0.2
4.8
1.1
1.0
0.4
0.1
0.1
3.1
1
)
Percentage
of pseudototal
1.9
3.5
1.0
0.3
1.3
0.8
< 0.1
< 0 .1
0.23
Proportions represented by the more readily extractable fractions are shown in
italics.
1
a Thirty-eight percentage of samples exceeded 300 mg Zn kg (max .
827 mgZnkg- 1) .
b All samples exceeded 130 mg Cu kg- 1 (max. 399 mg Cu kg- 1).
c Mean of all three plots at MER.
must be taken in relating EDTA-extractable concentrations to
assumed bioavailability, particularly for this range of trace
elements (Nolan et a!., 2003; Meers et a!., 2005). EDTA
acts by complexing cations and, as such, is an inappropriate
extractant for As which exists in soils in anionic fmm as arsenate or arsenite. However, more detailed analyses reported
earlier for soil from the same site (MER) showed As mobility from short- and long-term leaching tests to be 0.55% and
20% of the total, respectively (Hartley et a!., 2004).
Overall mortality of plants, apart from Larix (LAR), was low
(mean 11%) and was not related to contamination, except at the
site with highly elevated As (MER). LAR mortality was very
high (20-77%) at all sites. Mortality of Betula increased
from 7% to 29% between Years 1 and 3, visibly due to competition from other faster-growing taxa. Plants yields calculated
using data from the Year 3 harvest (Fig. 3) showed considerable
variation between the Salix and Populus taxa, and comparably
high yields of Alnus. There was also wide variation between
sites ( <2-9 tha- 1 annum- 1) and plots (e.g. at MER).
Foliar and stem content of Pb (2-5 mg kg- 1 and 02 mg kg- 1 , respectively) was low at all sites, as was Ni in
both plant parts (1.5- 3.0 mg kg - 1). Uptake of As by plants at
the As-contaminated site (MER) was practically insignificant
( <2 mg kg- 1), with the exception of Larix where up to
14 mg As kg- 1 was recorded in foliage. There was less variability in plant Cu concentrations than of other trace elements, but
four of the Salix clones and Larix appeared to have elevated
concentrations in stems (Fig. 4). In contrast, Cd and Zn uptake
was more vruiable into foliage and stems with notably higher
concentrations in some of the Salix clones (CAL, CLS, GER,
TOR). For Cu, there was no correlation between plant and
soil concentration, but there was a very cleru· relationship in
the cases of Cd and Zn. For Cd in particular, uptake into stems
proportionately exceeded the amount in soil (Fig. 5).
Paired t-test compru·isons of EDTA-extractable trace element concentrations in soil at the onset of the experimental
391
12
)':
'ro 10
.r:
1-
8
8
(j)
2
'iii
6
~
(/)
~
E
r-
4
0
m
ffi
2
Q)
:2
0
ALN
CAL
CLS
GER
GHY
ORM
TOR
TRI
Species
12
)':
'ro 10
.r:
10
0
8
(j)
Q)
' (3
Q)
c.
6
(/)
~
~
ro
E
4
0
m
c
2
ro
:2
1-n
Q)
0
CRM 1 FAZ 1 FAZ 2 KIR 1 MER 1 MER 2 MER 3 SUG 2 SUG 1
Site
Fig. 3. Mean biomass in ODT (oven-dry tonnes) ha- 1 yr- 1 of the 8 most
productive taxa and across the 9 experimental plots (Abbreviated codes as
in Tables 1 and 2). Error bars represent 95 % confidence interval.
work and after 3 yeru·s, showed a statistically significant
decline in soil hotspots of Cd at KIR (p < 0.05), Zn at
CRM (p < 0.05), Cu at CRM (p < 0.05) and As at MER
(p < 0.05), although differences in mean concentrations
were hru·dly different (French, 2005). There was no evidence
of an increase of metal concentrations in soil hotspots at any
of the plots following planting with woody biomass.
4. Discussion
4.1. Soil contamination
The brownfield sites selected for the present study are
typical of urban land in NW England and similar ru·eas with
diffuse historic sources of contamination. Detailed knowledge
is usually lacking of varied former land uses, waste disposal,
spillages, aerial fallout and land disturbance. This has provided brownfield sites that required detailed sampling and field
investigation. In some cases perceptions of contamination
C.J. French eta/. I Environmental Pollution 141 (2006) 387- 395
392
100
-
FoliarYr2
~ FoliarYr3
80
-
stemYr3
'7
Ol
.:.:.
Ol
.s
'7
~
60
Ol
E
Q)
.:.:.
.l9
0.
::::>
::::>
20
"0
0
E
40
$
0
(f)
10
20
0
25
5
10
Total Soil Cd mg kg- 1
15
5
10
EDTA Soil Cd mg kg- 1
15
20
'7
Ol
.:.:.
Ol
.s
15
Q)
.:.:.
ro
c.::::>
'7
Ol
10
.:.:.
20
Ol
"0
E
0
"0
5
~I ni
I
0
0
E
Q)
Ci5 10
1400
1200
'7
1000
.:.:.
Ol
Ol
.s
Fig. 5. Relationship between cadmium concentrations in stems of Salix taxa
and total soil Cd (upper graph) and EDTA-extractable Cd (lower graph) for
TOR (0), ORM (L'>), GER (X), CLS (.).CAL (e) (listing in Table 2).
800
Q)
.:.:.
.l9
0.
600
::::>
c
N
400
.
200
0
m.-Ih
[ ~ l
T
~ I
CAL LAR ALN CLS ORM TRI GHY BET GER TOR
Fig. 4. Cu, Cd and Zn concentrations in foliage (Years 2 and 3) and stems
(Year 3) within each plant taxa. Values are means for all sites combined.
Abbreviations for plant taxa as in Table I. Error bars are 95% Cis.
are probably incorrect, particularly if modem risk assessment
criteria are properly applied. However, it is clear that highly
vaJiable spatial dispersion of metals is revealed by detailed
sampling; hotspots are likely to be masked when small numbers of samples are taken and when samples are bulked prior
to analysis. Best practice is appropriately discussed elsewhere
(Rams~y and Argyraki, 1997; Sastre et al., 2001; CL:AIRE,
2004). It would appear that contamination of many brownfield
sites is likely to be restricted to hotspots and this may have
considerable relevance to appropriate forms of management,
planning of site remediation, treatment and after-use.
Low recovery concentrations of metals using CaC1 2 as an
extractant were recorded, comparable to the findings of previous studies (Pulford et al., 2002; Pulford and Watson, 2003). Although this may become a standard extractant for contaminated
soils in Europe (Pueyo et al., 2003), metals were less easily detectable in soils with lower levels of contamination as others
have reported previously (Houba et al., 1996; Laureysens
et al., 2004). EDTA was a more useful general extractant for
the range of trace elements in the present study, although in
some cases (notably Pb) it provided a poor guide to the amount
of uptake by plants. Nevertheless, in many cases, EDTA may
provide cheaper, easier and equally reliable descriptor of site
contamination than total or pseudo-total analysis of trace elements and is probably more relevant to understanding bioavailability and risk, in the absence of more appropriate routine
methodology to describe speciation (Nolan et al., 2003).
4.2. Productivity of coppice
In the UK it has been considered that economic
retum is achieved when woody biomass yields exceed
C.J. Fren ch eta/. I Environmental Pollution 141 (2006) 387-395
393
Surface 30cm
Surface 1Ocm
6
Ol
..1<:
Ol
5
.sc
4
ts
3
0
::J
'0
Q)
0::
2
'0
0
·o
(f)
10 20 30 40 50 60 70 80 90 100
0
10 20 30 40 50 60 70 80 90 100
10 20 30 40 50 60 70 80 90 100
Years
0
10 20 30 40 50 60 70 80 90 100
Years
100
";'
Ol
..1<:
Ol
80
0
60
'0
40
.sc
t5::J
Q)
0::
c
N
20
·o
(f)
Fig. 6. Number of years to lower concentrations of Cd and Zn in soil at a brownfield site through harvest of Salix (5 taxa; 0 S. X ca/odendron) and Populus
(2 hybrids, • <>) stems and foliage [ e, all taxa combined] . Mean total soil concentrations were 6 mg Cd kg - l and 250 mg Zn kg - l Data are for the whole
plot without targeting hotspots. Constant off-take throughout the period is assumed (see text).
8-10tha- 1 annum- 1 (Mitchell et al., 1999; Tubby and
Armstrong, 2002). In the present study, three sites (CRM,
FAZ, KIR), four Salix taxa (CLS, GER, ORM, TOR), and
Alnus were either close to, or exceeded, this threshold. Clearly,
there is an oppmtunity to increase yield beyond those achieved
in the present study, or at least to maintain yields in the longerterm through fertilization and irrigation (Moffat et al., 2001).
It has also previously been shown that SRC yields may be
substantially higher during subsequent 3-year cutting cycles
(Tubby and Armstrong, 2002). In the FAZ plot, a sewage
cake treatment (600 tha- 1) increased yield by up to 56% in
Salix (GER), 81% in Populus (TRI), 120% in Betula and 6%
in Alnus (French, 2005). The reason for poor establishment
of Larix across all sites and plots is uncettain but may
have been caused by poor quality planting stock or inappropriate cultivation, rather than necessarily being unsuitable for
planting on brownfield land. Betula was visibly slower growing than the other broadleaves and was a poor competitor in
the dense mixed coppice. Whilst there was considerable yield
variation between the Salix and Populus clones, there is likely to be an advantage in planting polyclonal stands rather
than monocultures to reduce disease incidence, particularly
from Melampsora rusts (McCracken et al., 2001). The wide
variation between plots and sites (Fig. 3) was due to local
site conditions rather than climatic vruiables. At MER, for
example, soils were known to be pruticularly shallow and
poor, but aspect and slope of the 3 plots also varied
considerably.
4.3. Uptake of metals
Lead uptake by plants was low in the present study, as is
typical for this relatively immobile metal. Despite a relatively
large proportion (20-45%) of soil Pb being extractable in
EDTA, very little was available to plants. In soils with a pH
above 5.5, as at the present study sites, Pb solubility is controlled by phosphate and carbonate precipitates (Blaylock
et al., 1997). Arsenic in the MER soil is likely to be primarily
in relatively immobile fmms, particularly arsenate (AsV), and
thus persistent. A recent leaching and amendment study on the
same soil found that 0.6-2% of the existing As could be
leached from MER soils and that mobility was strongly influenced by iron oxides (Hartley et al., 2004). Whilst these authors demonstrated the feasibility of immobilising As using
vru·ious Fe amendments, other soil factors such as organic matter, waterlogging and raised pH actually may increase mobility
(O'Neill, 1990). Nickel was taken up by plants in small
amounts in the present study, although this metal has been
found to be taken up by trees in lru·ger quantities from soil
where concentrations ru·e much higher (Pulford et al., 2002).
In the present study, Cd and Zn were found to be pruticulru·ly
mobile in the soil-plant continuum. Both are widespread contaminants that commonly occur in urban environments, but Cd
is a much more zootoxic metal and of more concern to human
health and to food chains (Adriano, 2001; Dickinson and
Pulford, 2004). Salix accumulated Cd in concentrations that
substantially exceeded soil concentrations. The taxa CAL
394
C.l. French eta/. I Environmental Pollution 141 (2006) 387- 395
and GER contained 7-9 times more Cd in stems than EDTAextractable soil concentrations, and 9-13 times more in foliage. These data can be combined with biomass yield data
in predictive models of longer-term metal off-take (Fig. 6).
These are speculative models that assume continued metal lability, consistent soil physico-chemical properties, the same
productivity and the same rates of uptake. Obviously, they
should be treated with caution. However, over a typical 20year life cycle of the crop this would amount to a reduction
of 5.6 mg Cd kg- 1 and 96 mg Zn kg- 1 from the soil by the
most efficient taxon (Soli X calodendron).
scales for clean-up using phytoextraction are long, but there is
now gathering evidence that this can provide a significant contribution to the integrated management of brownfield land.
Acknowledgements
We are grateful to our collaborative partners, particularly
Paul Nolan and the Mersey Forest, Local Authorities and other
landowners, and to NWDA for financial support. Professor
Nicholas Lepp provided helpful comments on earlier drafts
of this manuscript.
5. Conclusions
Despite considerable advances in contaminated land exposure assessment in recent years, there are still obvious difficulties in identifying whether or not and under what conditions
soil pollution should be a matter of concern. Elevated concentrations of heavy metals do not necessarily equate to an episode or occurrence of contamination, but frequently there is
sufficient assessment of risk to justify some degree of cleanup or a reduction of soil concentrations. Disposal of this
type of soil to landfill is obviously unnecessary and wasteful,
and is not tenable under current regulations in Europe; in situ
remediation and risk management are the only realistic options
(Fox et al., 1998; Wong and Bradshaw, 2002). The present
study has shown that, in the first instance, a satisfactory quantification and mapping of trace element dispersion is required.
The occurrence of hotspots may be masked by less detailed
sampling and analysis. EDTA extraction of trace elements
from soils is as informative as determination of total concentration and is likely to be more cost-effective, but soil guideline values and regulatory limits are still based on the latter.
Elevated concentrations of Cd, in particular, frequently
cause concern on brownfield land. Phytoextraction using
shm1-rotation coppice may provide an efficient and cost-effective method of clean-up. Whilst the same applied to Zn, this
metal is less zootoxic and seldom significantly limits plant
growth on urban contaminated soils (Dickinson et al., 2000);
woody biomass may, however, have relevance to clean-up of
sewage-sludge contaminated agricultural soils (Rosselli
et al., 2003; Berndes et al., 2004). Potential risks associated
with phytoextraction of metals, such as biomass combustion
and food chain transfer of metals, have been extensively discussed elsewhere (Dickinson et al., 2000; Pulford and Watson,
2003). Other elements may have little risk associated with
human exposure (e.g. Ni and Cu) or are insufficiently mobile
in soil-plant systems (e.g. Pb, As). In these situations, the
limited evidence from the present study suggests that planting
woody biomass does not increase the lability of metals or their
mobility to the wider environment, at least during the first 3
years. Ground cover with trees is likely to reduce the reentrainment of particulates and contamination of the wider
environment. Thus, woody biomass may provide an effective
form of phytostabilisation or monitored natural attenuation.
Cultivation of woody plants for biomass provides aesthetic improvement and economic benefits (Paulson et al., 2003). Time
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