measured soil water concentrations of cadmium and zinc in plant pots and estimated leaching outflows...

MEASURED SOIL WATER CONCENTRATIONS OF CADMIUM ANDZINC IN PLANT POTS AND ESTIMATED LEACHING OUTFLOWS

FROM CONTAMINATED SOILS

P.E. HOLM1, T.H. CHRISTENSEN1,� S.E. LORENZ2, R.E. HAMON2,H.C. DOMINGUES3, E.M. SEQUEIRA3 and S.P. MCGRATH2

1 Department of Environmental Science and Engineering/Groundwater Research Centre, TechnicalUniversity of Denmark, Building 115, DK 2800 Lyngby, Denmark.2 Soil Science Department,IACR- Rothamsted, Harpenden, Herts, AL5 2JQ, UK.3 Departamento de Pedologia, Estacao

Agronomica Nacional, Quinta do Marques, P 2780 Oeiras, Portugal

(Received 23 April, 1996; accepted 8 October, 1996)

Abstract. Soil water concentrations of cadmium and zinc were measured in plant pots with 15contaminated soils which differed in origin, texture, pH (5.1 – 7.8) and concentrations of cadmium (0.2– 17 mg Cd kg�1) and zinc (36 – 1300 mg Zn kg�1). The soil waters contained total concentrationsof 0.5 to 17�g Cd L�1 and 9 to 3600�g Zn L�1, which were dominated by free metal ions asmeasured by an ion exchange-resin method. Annual leaching outflows were estimated from soil waterconcentrations to be 0.5 – 17 g Cd ha�1 y�1 and 9 – 3600 g Zn ha�1 y�1 per 100 mm of net percolation,corresponding to 0.1% per year of the total soil content of cadmium and zinc. The measured soilwater concentrations of cadmium and zinc did not correlate linearly with the corresponding soilconcentrations but correlated fairly well with concentrations measured in Ca(NO3)2 extracts of thesoils and with soil water concentrations estimated from soil concentrations and pH. Such concentrationestimates may be useful for estimating amounts of cadmium and zinc being leached from soils.

Key words: cadmium, contaminated soils, leaching, soil extracts, solute species, zinc

1. Introduction

Total soil concentrations are often used in the management of Cd and Zn contam-inated soils in terms of soil quality criteria. However, total soil concentrations arenot directly related to the availability and mobility of the metals and do not provideaccurate information with which the leachability of Cd and Zn from soils can becharacterized. Leaching of Cd and Zn from soils has only obtained little focus andhas not been quantified adequately in past research. Therefore much uncertaintyexists about the amount of metal loss from soils and of the contribution of leachingto the overall budgets of Cd and Zn in both soils and groundwater. As well as thedependence on solution concentrations, the leachability or outflow of Cd and Znfrom soils is related to the amount of water percolating through the soil.

The purpose of this study was to provide estimates of the Cd and Zn leachabilityfor a range of soils, and to develop simple relations to be applied for prediction ofCd and Zn leachability from contaminated soils.

� Corresponding author.

Water, Air, and Soil Pollution102: 105–115, 1998.c 1998Kluwer Academic Publishers. Printed in the Netherlands.

106 P. E. HOLM ET AL.

2. Materials and Methods

2.1. SOILS

Soils were sampled from the field plots of Long-Term Liming Experiments (WoburnL soils, which were relatively uncontaminated) and of the Market Garden Experi-ments (Woburn S soils, which had received sludge, McGrath, 1984) in the UK andfrom 5 other locations in the UK, France and Germany. The soils were sampled toa depth of 22 cm using an mild steel auger, sieved through a 3 mm mild steel sieveand stored moist in plastic bags in darkness at 4�C.

2.2. SOIL EXTRACTS

Ca(NO3)2 extracts were obtained by equilibrating 10.0 g of soil and 30 mLCa(NO3)2 solution (10�2 M) in 50 mL polyethylene (PE) screw cap bottles for4 days in an end-over-end rotator. After equilibration, the pH was measured andsoil and solute were separated by centrifugation.

2.3. SOIL WATER COLLECTION

Approximate volumes of moist soil, each equivalent to either 0.75 kg or 1.0 kg dryweight (DW), were placed in plant pots (13 cm in diameter) with nylon mesh intheir base. Water holding capacities (WHCs) were determined from 2 pots of eachsoil. The soil water content of three replicate pots of each soil was adjusted to 60%of WHC by watering from above with deionised water. The pots were then enclosedin loosely-sealed black polyethylene bags in order to reduce evaporation, and soilwater contents were subsequently readjusted to 60% WHC whenever necessary.Soils were allowed to equilibrate for two weeks in a growth chamber (8 h at 15�C,16 h at 20�C). Then soil water solutions were obtained from the three replicate potsof each soil by displacement with water (Lorenzet al., 1994). Sixteen hours beforesampling, soil water contents were raised to WHC. For solution displacement,deionized water was added to the soil surface at a rate of 5 mL every 5 min. Afterdiscarding the first 5 mL of leachate, 40 mL of solution from each pot was collectedinto polystyrene centrifuge bottles. Preliminary tests on these soils, using Cl� asa tracer, had shown that no breakthrough of unequilibrated added water occurreduntil after 40 mL of solution had been collected. Solutions were centrifuged inorder to remove particulate matter. The solutions obtained from the three replicatepots were bulked so enough solution was available for speciation analysis.

2.4. SPECIATION OF DISSOLVEDCD AND ZN

The fraction of free divalent Cd and Zn (Cd2+ and Zn2+) in the soil water sampleswas determined using the speciation method described by Holmet al.(1995). Thisprocedure involved a batch equilibrium experiment with a cation exchange resin

MEASURED SOIL WATER CONCENTRATIONS OF CADMIUM AND ZINC 107

(Amberlite CG 120), for both the actual sample (25 mL) and a correspondingreference solution of identical ionic strength, Ca- and mg-activity and resin weightto sample volume as the sample of interest. The reference experiment containedCd and Zn solely as Cd2+ and Zn2+. The reference experiment was identical to thesample experiment, apart from the ligands present in the actual sample, providingthe information on the distribution of Cd2+ and Zn2+ ions between the resin andthe solution, required for calculating the degree of complexation in the sample(Holm et al., 1995).

2.5. ANALYSIS

Soil texture was analyzed using the pipette method described by Day (1965).Organic C was measured using the Walkley-Black method (Walkley, 1947). Cationexchange capacity (CEC) was determined by extracting oven-dry (105�C) soilsground to approximately 75�m (agate ball mill, Fritsch Pulverisette) with 1NNH4OAc at pH 7.0 (Schollenberger and Simon, 1945). Concentrations of exchange-able ions were analyzed by flame-atomic absorption spectrometry (Flame-AAS,Perkin Elmer 5000).

Soil pH was measured on 10 g of air dried soil suspended in 25 mL deionizedwater, using a pH meter (Radiometer PHM 62). Total concentrations of Cd and Zn insoil were determined by flame atomic absorption spectrophotometry (Perkin-Elmer5000) after digestion of soils with a mixture of HNO3, HClO4 and HF (Pratt, 1965).Soil water concentrations of Ca, mg and K were analyzed by inductively-coupledplasma atomic emission spectrometry (ICP-AES, ARL 34000). Cadmium and Zn insoil water and Ca(NO3)2-extracts were determined after solvent extraction (1.0%Na diethyldithiocarbamate, trihydrate in 4-methylpentan-2-one) by graphite fur-nace atomic absorption spectrophotometry (Perkin-Elmer 5000, deuterium back-ground correction, HGA 400 graphite furnace, AS-1 automatic sample injectionsystem). All samples were acidified to 10�2 M HNO3 before solvent extraction.

2.6. GENERAL

All chemicals used were analytical grade (Merck, pro analysis) except the standardsfor Cd and Zn analysis which were prepared from Cd or Zn BDH Spectrosol1000 mg L�1 standard solutions. All materials used in the experiments were cleanedand conditioned before using by soaking in 2 M HNO3, rinsing with deionized waterand drying at 45�C in a convection oven.

2.7. LEACHABILITY CALCULATIONS

Leaching outflow of Cd and Zn, expressed in g of metal per hectare per year, wascalculated as:

OF = 0.01 P� C


where:– OF (g ha�1 y�1) is the annual outflow per hectare (10000 m2) land of either

Cd or Zn– P (mm y�1) is the net water percolation through the soil– C (�g L�1) is the concentration of Cd and Zn in the soil water.

The annual outflow OF of Cd and Zn relative to the total metal content in theploughing layer T per hectare was calculated as:

OF/T = OF/(10000 D� B � S)

where:– D (m) is the depth of the ploughing layer– B (tonnes m�3) is the dry bulk density of the soil– S (mg kg�1) is the total concentration of Cd and Zn in the soil.

3. Results and Discussion

3.1. SOIL CHARACTERISTICS

The studied soils (Table I) differ in origin, texture, pH (5.1 – 7.8) and metal content(Cd: 0.18 – 16.5 mg kg�1 Zn: 36 – 1317 mg kg�1). Except from the 4 Woburn L soilswhich contain background concentrations, the metal concentrations were in therange of soil metal concentrations typically found in contaminated soils (Hornburgand Brummer, 1993). The source of contamination was sewage sludge, exceptfor Arras and Avonmouth soils, which were contaminated from aerial depositionnear zinc smelters. The soils from the Woburn field experiments in the UK havesimilar physical characteristics but different pH-values (Woburn L soils) and metalcontents (Woburn S soils).

Soil extracts with 10�2 M Ca(NO3)2 gave Cd concentrations of 0.1 to 18�gCd L�1 and Zn concentrations of 10 to 2450�g Zn L�1. The soils used covereda range of approximately two orders of magnitude both with respect to total andextractable soil concentrations of Cd and Zn.

3.2. CADMIUM AND ZINC LEACHING OUTFLOWS

The concentrations of Cd and Zn in soil water are shown in Table II together withthe leaching outflows of Cd and Zn calculated for a ploughing depth of 0.25 m,a dry bulk density of 1.7 tonnes m�3 and an annual net infiltration of 100 mm.The leaching outflow of metals is proportional to the net infiltration which variesbetween locations. In order to give infiltration leaching outflows independent of thespecific infiltration a net infiltration of 100 mm was chosen as a basis for furthercalculations of different specific infiltrations. The soil water concentrations were0.5 to 17�g Cd L�1 and 9 to 3600�g Zn L�1 and the leaching outflows were


Tabl

eI

Soi

lcha

ract

eris

tics

Soi

lS

oilt

extu

reC

lay

Org

.CC

EC

Soi

lpH

Soi

lcon

cent

ratio

nsS

oile

xtra

cts

%g

kg

�

1m

eq(1

00g)�

1m

gC

dkg

�

1m

gZ

nkg

�

1�

gC

dL�

1

�

gZ

nL�

1

Wob

urna

L40

sand

ylo

am10

.75.

211

.37.

80.

1841

0.1

10W

obur

nL

45sa

ndy

loam

10.7

5.6

10.9

7.6

0.18

430.

150

Wob

urn

L56

sand

ylo

am10

.75.

49.

65.

10.

1840

4.5

400

Wob

urn

Lm

ixsa

ndy

loam

10.7

5.3

10.0

5.9

0.18

360.

790

Wob

urn

S56

sand

ylo

am9.

616

.015

.36.

610

.339

716

2370

Wob

urn

S58

sand

ylo

am9.

614

.114

.96.

71.

312

23.

041

0W

obur

nS

60sa

ndy

loam

9.6

11.2

13.5

6.6

0.77

811.

414

0W

obur

nS

62sa

ndy

loam

9.6

12.9

15.2

6.7

4.7

218

9.1

1550

Wob

urn

S66

sand

ylo

am9.

619

.216

.16.

65.

930

210

1610

Wob

urn

S67

sand

ylo

am9.

614

.112

.46.

73.

120

34.

467

0B

orde

aux,

Fsa

nd4.

615

.66.

46.

31.

732

04.

624

50B

raun

schw

eig,

Glo

amy

sand

7.1

9.0

11.8

6.1

1.5

247

4.3

2320

Bon

n,G

loam

14.5

16.8

19.2

7.6

0.85

340

0.6

180

Arr

as,F

sand

ylo

am14

.88.

610

.87.

816

.51

317

1814

70A

vonm

outh

,UK

sand

ylo

am15

.524

.627

.57.

11.

237

00.

621

0

aT

heW

obur

nso

ilsar

efr

omth

eU

K.


Tabl

eII

Soi

land

soil

wat

erco

ncen

trat

ions

ofC

dan

dZ

nan

dca

lcul

ated

drai

nage

loss

es

Soi

lTo

tals

oilw

ater

Soi

lwat

erco

nten

tof

Yea

rlyle

achi

ngou

tflow

per

100

mm

conc

entr

atio

nsco

ncen

trat

ions

free

diva

lent

ions

neti

nfiltr

atio

na

Cd

Zn

Cd

Zn

Cd

Zn

Cd

Zn

Cd

Zn

mg

kg

�

1m

gkg

�

1

�

gL�

1

�

gL�

1%

ofto

tal

%of

tota

lg

ha�

1y�

1g

ha

�

1y�

1pe

rth

ousa

ndbpe

rth

ousa

ndb

Wob

urn

L40

0.18

410.

79

–c–

0.7

90.

500.

05W

obur

nL

450.

1843

0.5

16–

–0.

516

0.40

0.09

Wob

urn

L56

0.18

407.

119

0–

–7.

119

06.

31.

2W

obur

nL

mix

0.18

362.

056

––

2.0

561.

70.

39W

obur

nS

5610

.339

714

.910

5066

100

14.9

1050

0.36

0.66

Wob

urn

S58

1.3

122

2.7

360

8110

02.

736

00.

550.

74W

obur

nS

600.

7781

1.8

290

8685

1.8

290

0.59

0.90

Wob

urn

S62

4.7

218

12.5

1000

9373

12.5

1000

0.67

1.1

Wob

urn

S66

5.9

302

11.1

1040

9110

011

.110

400.

470.

86W

obur

nS

673.

120

34.

445

091

100

4.4

450

0.35

0.55

Bor

deau

x,F

1.7

320

10.3

3000

100

100

10.3

3000

1.5

2.3

Bra

unsc

hwei

g,g

1.5

247

11.3

3600

7910

011

.336

001.

93.

6B

onn,

g0.

8534

00.

935

6181

0.9

350.

260.

03A

rras

,F16

.513

1717

.024

074

100

17.0

240

0.26

0.05

Avo

nmou

th,

UK

1.17

370

0.6

7794

970.

677

0.14

0.05

aY

early

neti

nfiltr

atio

nof

100

mm

isas

sum

ed.T

heou

tflow

sof

Cd

and

Zn

are

dire

ctly

prop

ortio

nalt

oth

ene

tinfi

ltrat

ion.

bY

early

met

alle

achi

ngou

tflow

from

the

plou

ghin

gla

yer

(0.2

5m

dept

h)in

per

thou

sand

ofto

talm

etal

soil

cont

ent.

cN

otde

term

ined

.


calculated to be 0.5 to 17 g Cd ha�1 y�1 and 9 to 3600 g Zn ha�1 y�1 from thesoils. These outflows constitute about 0.1% of the total soil content of Cd and Znin the ploughing layer. Hardly any reliable data are available in the literature withrespect to concentrations of Cd and Zn in soil solutions. Campbell and Beckett(1988) found concentrations of Zn between 7 to 130�g L�1 in soil solutionsfrom untreated and sewage sludge treated slightly acid loamy soil. Soil solutionconcentrations of Cd were below an unspecified detection limit.

The annual outflows may be affected by temporal variations in the soil solutionconcentrations of Cd and Zn. Very few investigations have monitored changes overtime of heavy metal concentrations in soil solutions. Campbellet al. (1989) foundthat concentrations of both minor and major cations in soil solution varied no morethan by a factor of 3 over the year in a temperate climate. Metal distribution maybe substantially altered in the rhizosphere during the summer due to changes inthe CO2 pressure and pH from nutrient uptake and respiration by roots and micro-organisms. However, the rhizosphere solution is likely to be of minor importancewith respect to leaching of metals, because the net infiltration of water is expectedto be very limited during growth of crops in the field, due to high evapotranspirationby the plants at this time of the year. Furthermore, the rhizosphere usually occupiesonly around 1% of volume of the topsoil (Barber, 1984). Therefore, variations inCd and Zn concentrations in the rhizosphere were not considered for estimating theleaching outflows of Cd and Zn. The estimated annual leachability outflows givenin Table II were calculated on the basis of 100 mm net water infiltration and soilwater concentrations of Cd and Zn from non-rhizosphere soils.

Net infiltration differs among areas but in temperate climates it is typicallyseveral hundred mm’s per year. In Denmark for example, an annual net infiltrationof about 300 mm is fairly common. Multiplying the outflows presented in Table II bya factor of 3 yields outflows of 1.5 to 51 g Cd ha�1 y�1. Tjell and Christensen (1992)estimated Cd outflows in Denmark of approximately 1 g ha�1 y�1, which is in thelower range of the estimates presented here and in agreement with expectationsfor less contaminated soils. Zinc outflows equivalent to 300 mm net infiltration peryear are 27 to 10800 g ha�1 y�1. This would amount to Cd and Zn outflows ofabout 0.3% of the total soil content of Cd and Zn in the ploughing layer. Thus,only a very small fraction of the total metal content of the soil is leached per year,and Cd and Zn are retained in the topsoils for several hundred years as suggestedby others, e.g. McGrath and Lane (1989). The fractions of free divalent ions ofCd and Zn in the soil water samples are shown in Table II for all the soils exceptthe Woburn L soils in which the metal concentrations were too low for analyticaldetermination of species. Since free divalent metal ions dominate the metal insolution, dissolved complexes apparently are not very important in these soils withrespect to leachability of metals. Thus, the leachability of Cd and Zn seemed to beunaffected by complexation for the soils studied.


Table IIIConcentrations of K, Ca and mg in the soil watersamples

K Ca Mgmg L�1 mg L�1 mg L�1

Woburn L40 23 693 20Woburn L45 18 650 24Woburn L56 17 533 42Woburn Lmix 18 382 25Woburn S56 28 521 45Woburn S58 75 427 56Woburn S60 47 420 52Woburn S62 25 575 56Woburn S67 20 490 47Woburn S66 29 545 49Bordeaux 90 667 92Braunschweig 95 355 26Bonn 33 388 29Arras 63 487 17Avonmouth 43 392 55

3.3. ESTIMATION OF CADMIUM AND ZINC SOIL WATER CONCENTRATIONS

The difficulty in obtaining leachability data for Cd and Zn in soil is primarilyrelated to the fact that methods for sampling soil solution are laborious and time-consuming. This makes it desirable to estimate Cd and Zn concentrations fromsoil parameters or from concentrations measured in soil extracts. We examinedtwo approaches for estimating soil water concentrations of Cd and Zn: Estimationon the basis of the actual soil concentrations of Cd and Zn and their distributioncoefficients estimated from soil pH, as suggested by published regression equationsand, secondly, estimation on the basis of measured Cd and Zn concentrations inCa(NO3)2 soil extracts.

Correlations of measured concentrations of Cd and Zn in the soil water sampleswith the corresponding concentrations in the soils were found to be not significant.pH is considered to be the soil parameter mainly controlling the distribution of Cdand Zn between soil and soil water (Anderson and Christensen, 1988; Boekholdet al., 1992, 1993). This led to the idea of obtaining independent estimates of thedistribution coefficients (soil concentration/ soil water concentration) for Cd andZn using the logKd – pH equations presented by Anderson and Christensen (1988):

Cd: logKd = 0.64 pH – 1.53 (r2 = 0.776)

Zn: logKd = 0.89 pH – 3.16 (r2 = 0.906)


Figure 1. Concentrations of Cd and Zn in 10�2 m Ca(NO3)2 soil extracts and estimated concentrationscalculated from the soil concentration combined with a logKd – pH relation plotted as a function ofmeasured soil water concentrations of Cd and Zn.

Estimates of the soil water concentrations of Cd and Zn can then be obtainedby dividing the corresponding soil concentration by the distribution coefficientestimated from pH. The pH depends on the ionic composition of the solution inwhich it is measured. The ionic composition of the soil water samples in terms ofconcentrations of the major cations K, Ca and mg is shown in Table III. Calciumis the cation present at the highest concentrations, typically in the range of 350 to550 mg L�1. Thus, pH measured in 10�2 M Ca(NO3)2 (corresponding to 400 mg CaL�1) was used in the regression equations. Additionally, previous studies (e.g. forCd by Christensen, 1984) have shown that second to pH, the solute concentrationof Ca is a very important factor for the distribution between soil and soil water ofdivalent metals like Cd and Zn. Therefore, the measured Cd and Zn concentrationsin the 10�2 M Ca(NO3)2 soil extracts were used as estimates of the soil waterconcentrations of Cd and Zn.

Measured concentrations of Cd and Zn in Ca(NO3)2 soil extracts, and estimatedconcentrations calculated from soil concentrations and the logKd – pH relationare presented in Figure 1 as a function of the measured concentrations of Cd andZn in soil water. For both Cd and Zn, the concentrations in the estimates were ofthe same order as those measured in soil water. The regression lines were forcedthrough origin. With respect to Cd, the estimated concentrations correlated wellwith the measured concentrations (r2 above 0.8 for both correlations) but theconcentrations estimated from pH slightly overestimated those measured in soilwater. With respect to Zn, soil extract concentrations correlated weakly (r2 = 0.54)


with the measured concentrations, whereas the logKd – pH relation gave a better(r2 = 0.61) correlation with the measured concentrations. Since the measured soilwater concentrations of Cd and Zn covered a range of two orders of magnitude,estimates deviating up to a factor of 2 to 3 from measured concentrations wouldbe acceptable for many cases. Thus, if measured soil water concentrations for Cdand Zn are not available, it seems reasonable to use estimated concentrations eitherbased on measured Cd and Zn concentrations in Ca(NO3)2 soil extracts, or on totalsoil concentrations combined withKd values from the logKd – pH relationshippresented by Anderson and Christensen (1988).

4. Conclusions

Leaching outflows of Cd and Zn from contaminated soils, estimated from soil waterconcentrations measured in plant pots, were calculated to be in the range of 0.5 to17 g Cd ha�1 y�1 and 9 to 3600 g Zn ha�1 y�1 per 100 mm of net infiltration.This corresponds to outflows of typically 0.1% of the total Cd and Zn in soiland retention times in the soil of several hundred years. When actual soil waterconcentrations of Cd and Zn are not available for estimation of leaching losses, fairestimates can be obtained (within a factor of 2 or 3) using either measurement ofCa(NO3)2 soil extracts or from a combination of total Cd and Zn concentrations inthe soil and the solute pH.

Acknowledgement

This work was partly carried out on a contract from the Science and Technologyfor Environmental Protection programme of the Commission of the EuropeanCommunities. The technical assistance by Jakob Futtrup and Nikolaj Lehmann,Department of Environmental Science and Engineering, Technical University ofDenmark is gratefully acknowledged.

References

Anderson, P. R. and Christensen, T. H.: 1988,J. Soil Sci.39, 15.Barber, S. A.: 1984,Soil Nutrient Bioavailability. A Mechanistic Approach,John Wiley & Sons Ltd,

New York, N.Y.Boekhold, A. E. and Van der Zee, S. E. A. T. M.: 1992,Soil Sci. Soc. Am. J.56, 747.Boekhold, A. E., Temminghoff, E. J. M. and Van der Zee, S. E. A. T. M.: 1993,J. Soil Sci.44, 85.Bolton, J.: 1977,J. Agr. Sci.89, 81.Campbell, D. J. and Beckett, P. H. T.: 1988,J. Soil Sci.39, 283.Campbell, D. J., Kinniburgh, D. G. and Beckett, P. H. T.: 1989,J. Soil Sci.40, 321.Christensen, T. H.: 1984,Water, Air, and Soil Pollut.21, 105.Day, P. R.: 1965, in: Black, C.A.et al. (eds.),Methods of Soil Analysis. Part 1: Physical and

Mineralogical Properties, Including Statistics of Measurement and Sampling, American Societyof Agronomy, Madison, WI, pp. 545-567


Holm, P. E., Christensen, T. H., Tjell, J. C. and McGrath, S. P.: 1995,J. Environ. Qual.24, 183.Hornburg, V. and Brummer, G. W.: 1993,Z. Pflanzenernahr. Bodenk.156, 467.Lorenz, S. E., Hamon, R. E. and McGrath, S. P.: 1994,Eur. J. Soil Sci.45, 431.McGrath, S. P.: 1984,J. Agr. Sci.103, 25.McGrath, S. P. and Lane, P. W.: 1989,Environ. Pollut.60, 235.Pratt, P. F.: 1965, in: Black, C.A.et al.,Methods of Soil Analysis. Part 2: Chemical and Microbiological

Properties.American Society of Agronomy, Madison, WI, pp. 1019–1021.Schollenberger, C. J., and Simon, R. H.: 1945,Soil Sci.59, 13.Tjell, J.C. and Christensen, T.H.: 1992, in: Vernet, J.-P. (ed.),Impact of Heavy metals on the Envi-

ronment, Elsevier, Amsterdam, NL, pp. 273–286.Walkley, A.: 1947,Soil Sci.63, 251.

measured soil water concentrations of cadmium and zinc in plant pots and estimated leaching outflows...

Documents