PB92-209378
EPA/600/R-92/138
June 1992
HIGHER PLANT ACCUMULATION OF
ORGANIC POLLUTANTS FROM SOILS
by
Robert M. Bell
Environmental Advisory Unit
University of Liverpool
Merseyside Innovation Centre
131 Mount Pleasant
Liverpool L3 5TF, UK
Cooperative Agreement CR812845
Project Officer
P. R. Sferra
Water and Hazardous Waste Treatment Research Division
Risk Reduction Engineering Laboratory
Cincinnati, Ohio 45268
RISK REDUCTION ENGINEERING LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U. S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
REPRODUCED BY
U.S. DEPARTMENT OF COMMERCE
NATIONAL TECHNICAL
INFORMATION SERVICE
SPRINGFIELD, VA 22161
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before campleti
1. REPORT NO.
EPA/600/R-92/138
2.
3,
P892-209378
4. TITLE AND SUBTITLE
Higher Plant Accumulation of Organic
Pollutants from Soils
5. REPORT DATE
June 1992
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Robert M. Bell
8. PERFORMING ORGANIZATION REPORT NO
i. PERFORMING ORGANIZATION NAME AND ADDRESS
Environmental Advisory Unit
University of Liverpool
Merseyside Innovation Centre
Liverpool L3 5TF, UK
10, PROGRAM ELEMENT NO,
11. CONTRACT/GRANT NO,
CR812845
12. SPONSORING AGENCY NAME AND ADDRESS
Risk Reduction Engineering Laboratory—Cincinnati, OH
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, OH 45268
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
EPA/600/14
15. SUPPLEMENTARY NOTES
Project Officer: P.R. Sferra
Telephone: 513-569-7618
A The purpose of this work was to determine the effect of higher plants on
sites polluted by organic chemicals and to discuss the potential of using
plants as an in situ cleanup treatment. This work is based primarily on literature
review but also includes greenhouse experiments and field testwork. It is concerned
with the behavior of organic pollutants in the plant-soil environment, plant uptake
and accumulation of organic pollutants, and variation in uptake by different plant
species in different conditions.
The literature review involved keyword searches into suitable databases
and review of over 750 scientific publications for information. Within this report
greater emphasis was placed on the few reports where sufficient details concerning
experimental methods to make comparisons is provided.
The greenhouse experiments were undertaken to investigate the actual extent
of plant uptake of pollutants from soils under known environmental conditions. The
field testwork was undertaken to quantify natural effects.
This report is not concerned with foodchain effects and is not concerned
with effects of the pollutant on the plant itself.
7.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.iOENTIFIEBS/OPEN ENDED TERMS
COSATl Field/Group
Plant metabolism
Plant nutrition
Plant physiology
Plant tissue
Con t am inan t s
Soil chemistry
Soil dynamics
Soil chemistry
Soils
Plant accumulation of
organic pollutants.
8. DISTRIBUTION STATEMENT
Release to public
19, SfcCVRn Y CLASS (This Report!
Unclassified
21. NO. OF PAGES
138
20. SECURITY CLASS (Thispage/
Unclassified _____
22, PRICE
EPA
2220-1 (R«v. 4-775 PREVIOUS EDITION is OBSOLETE-
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NOTICE
The information in this document has been funded wholly or in part by
the United States Environmental Protection Agency under assistance agreement
CR812845 to the University of Liverpool. It has been subject to the Agency's
peer and administrative review and has been approved for publication as an EPA
document. Mention of trade names or commercial products does not constitute
endorsement or recommendation for use.
n
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FOREWORD
Today's rapidly developing and changing technologies and industrial
products and practices frequently carry with them the increased generation of
materials that, if improperly dealt with, can threaten both public health and
the environment. The U.S. Environmental Protection Agency is charged by
Congress with protecting the Nation's land, air, and water resources. Under a
mandate of national environmental laws, the agency strives to formulate and
implement actions leading to a compatible balance between human activities and
the ability of natural systems to support and nurture life. These laws direct
the EPA to perform research to define our environmental problems, measure the
impacts, and search for solutions.
The Risk Reduction Engineering Laboratory is responsible for planning,
implementing, and managing research, development, and demonstration programs
to provide an authoritative, defensible engineering basis in support of the
policies, programs, and regulations of the EPA with respect to drinking water,
wastewater, pesticides, toxic substances, solid and hazardous wastes, and
Superfund-related activities. This publication is one of the products of that
research and provides a vital communication link between the researcher and
the user community.
This final report provides the results of a project funded by RREL
through a cooperative agreement with the University of Liverpool to determine
the potential of using plants as means of cleaning up hazardous waste sites.
The work investigated the fate of organic pollutants as they are affected by
the interactions that go on between the plant and the soil, the ability of
plants to take up and accumulate these pollutants, and variations in uptake by
different plant species in different conditions. Those wishing additional
information on this project are urged to contact the author or the EPA Project
Officer.
E. Timothy Oppelt, Director
Risk Reduction Engineering Laboratory
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ABSTRACT
The purpose of this work was to determine the effect of higher plants on
sites polluted by organic chemicals and to discuss the potential of using
plants as an in situ cleanup treatment,
In situ cleanup systems have many advantages when compared with other
cleanup techniques. These systems treat polluted soils, without excavating
the bulk of the polluted material, by detoxifying, neutralizing, degrading,
immobilizing or otherwise rendering harmless the contaminants where they are
found.
The first steps in the development of an in situ plant cleanup system
for organically polluted soils include the determination of the technical
feasibility and cost effectiveness of the method, the determination of the
availability of suitable plant species or varieties, the determination of
whether the site possesses optimal soil conditions, the conduction of
greenhouse scale confirmatory uptake tests, and confirmation that the plant
materials that have extracted the contaminants can be disposed of in an
environmentally safe manner and that the plant mass and harvesting mechanics
are realistically manageable.
This work is based primarily on literature review but also includes
greenhouse experiments and field testwork. It is concerned with the behaviour
of organic pollutants in the plant-soil environment, plant uptake and
accumulation of organic pollutants, and variation in uptake by different plant
species in different conditions.
The literature review involved keyword searches into suitable databases
(including Water Resources Abstracts, Biosis Previews, Chemical and Biological
Abstracts, Agricola, Phytotox) and review of over 750 scientific publications
for information. Within this report greater emphasis has been placed on the
few reports where sufficient details concerning experimental methods to make
comparisons is provided.
The greenhouse experiments were undertaken to investigate the actual
extent of plant uptake of pollutants from soils under known environmental
conditions. The field testwork was undertaken to quantify natural effects.
This report is not concerned with foodchain effects where the plant may
accumulate pollutants and animals feeding on the plant may receive high doses
of the pollutant for subsequent effect. Nor does this report address effects
of the pollutant on the plant itself. As will be seen, these effects result
from interactions between pollutant concentrations and a variety of
environmental effects.
iv
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CONTENTS
Foreword ,. i i i
Abstract i v
Figures viii
Tables ix
1. Introduction 1
References 3
2. Behaviour of Pollutants in the Plant-Soil
Environment .5
Sorption and Desorption 8
Theoretical aspects ......8
Case studies 16
Volatilization and Diffusion 17
Theoretical aspects 17
Case studies 19
Degradation 20
Theoretical aspects .....20
Case studies .22
Effects of Water 23
Theoretical aspects 23
Case studies ......25
Wi nd Blow and Mass Transfer .26
Conclusions 26
References .27
3. Plant Uptake of Organic Pollutants .....33
The PI ant Transport System 33
Root Uptake and Translocation
of Pollutants . 36
Modelling 42
PI ant Uptake by Vapour .44
Whol e PI ant Uptake .... 47
Behaviour of Pollutants in Plants 49
Partitioning 49
Degradation 54
References 58
4. Variations in Pollutant Uptake by Different
Plant Species 63
References 69
5. PI ant Uptake of Pol 1 utants ....... 71
Pesticides 72
Polyhalogenated Biphenyls 75
Halogenated Aliphatics ..79
Halogenated Ethers 79
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CONTENTS (Continued)
Monocycl i c Aromat i cs 79
Phthalate Esters 79
Polycyclic Aromatics . 80
Mi seel 1aneous Compounds 86
Conclusions ....88
References .88
6. Experimental Investigations 93
Introduction 93
Experiment to Determine the Accumulation
and Phytotoxieity of Hexachlorobenzene
(HCB) in Radish and Carrot Grown in HCB
Pol 1 uted Soil .97
Methods 97
Results 97
Conclusions .98
Experiment To Determine the Accumulation
and Phytotoxicity of Trichloroethane
(TCE) in Radish Grown in TCE Polluted
soil .99
Methods 99
Results 100
Conclusions 100
Experiment To Determine the Accumulation
and Phytotoxicity of Phenol in Radish
and Carrot Grown in Phenol Polluted
Soil ....101
Methods 101
Results .101
Conclusions .102
Experiment To Determine the Accumulation
and Phytotoxicity of Toluene in Radish
and Carrot Grown in Toluene Polluted
Soil ....103
Methods 103
Results 103
Conclusions 104
Experiment To Assess the Effect of
Different Soil Organic Matter Contents
on the Accumulation of HCB in Radish
Grown in HCB Polluted Soil 104
Methods 104
Results 104
Conclusions ,. 105
Experiment To Determine the Effect of
Plant Age on the Accumulation of HCB
in Radish Grown in HCB Polluted Soil 107
Methods ...........107
Results 107
Conclusions 108
VI
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CONTENTS (Continued)
Experiment To Assess the Effect of
Volatilization of HCB from HCB
Polluted Soil on the Accumulation
of HCB in Radish Plants ...109
Methods ......109
Results 110
Conclusions 110
Experiment To Assess the Uptake of
HCB by Different Plant Species
When Grown in HCB Polluted Soil Ill
Methods ...Ill
Results 112
Conclusions ......114
Conclusions , .114
References ..116
7. Field Testwork - An Investigation into Plant
Uptake of 2,3,7,8-Tetrachlorodibenzo-p-dioxin
in the Field ..117
Introduction 117
Methods 119
Field sampling 119
Analytical methods ..119
Results 120
Discussion 120
References . 123
8. Discussion 125
References 127
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FIGURES
Number Page
2.1 Interrelationships and transformations
of pollutants in soils , , . ,...,.5
2.2 The relationship between the organic matter
sorption of a soil and the n-octanol/water
partition coefficient of the pollutant . ,..13
2.3 Errors in observed values of Koc compared
with calculated values 14
2.4 The loss of Aroclor 1254 from itself at
different temperatures 19
3.1 The time course of uptake of two chlorinated
benzenes by bar! ey 45
3.2 Correlation of barley foliar uptake after
1 week exposure with volatilization
from soil 46
3.3 Correlation of barley concentration factors
(based on soil concentration) with molecular
weights after 1 week exposure 48
3.4 Time course for Stem Concentration Factor 50
3.5 The relationship between Stem Concentration
Factor of chemicals in barley and their
n-octanol/water partition coefficients ...52
3.6 Chemical distribution patterns within whole
soybean plants, shown as a percent of
total chemical in the plants with time 53
4.1 Time course of uptake of hexachlorobenzene
from soil by barley and cress 68
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TABLES
Page
2,1 Pesticide residues bound in soil,
measured in percent of applied amount 16
2.2 Extent and description of the rhizosphere of
18-day-old Blue Lupin seedlings 22
2.3 Comparison of the colony counts of bacteria
in the rhizosphere of various crop plants
and in root free soil 23
3.1 Prevalence of stomata on the surfaces of
leaves of some representative crop plants 35
3.2 Typical values for TSCF and RCF for barley
from a series of herbicides 38
3.3 Labelled herbicides in roots and shoots
of bar!ey seedl ings 39
3.4 The relationship between the n-octanol/water
partition coefficient and RCF and TSCF for
the uptake of o-methylcarbamolyoximes and
substituted phenylureas by barley from
nutrient solution , 40
3.5 A comparison of the potential for root and
shoot uptake of different pollutants from
the soil solution of a soil of 2% organic
matter and 15% water content as measured
by the TSCF and the RCF 43
3.6 The effect of lipophilicity on RCF and net
root uptake for a soil 43
3.7 The effect of lipophilicity on TSCF and
net uptake for a soil 44
3.8 Thickness, weight and wax content of
cuticles isolated from peach, apple, and
orange 1 eaves 47
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TABLES (continued)
3.9 Pesticide residues bound in plants, in
percent of total residues in the plant .49
3.10 Metabolic and other degradation products
from pesticide residues in plants
and soi 1 s 56
3.11 The behaviour of organic chemicals applied
to cell cultures of soybean and wheat ....58
4.1 Average maximum rooting depths of plant species
of the pineywoods and the prairies .....64
4.2 Average maximum rooting depths for different
pi ant types 65
4.3 Linuron and atrazine absorption by different
pi ant speci es 66
4.4 Maximum reported plant CF following growth on
soils polluted by a range of organic compounds 67
5.1 The physical and chemical parameters of
pesticides recognised as priority pollutants ...73
5.2 Plant uptake, translocation, and metabolism of
pesticides from soils 75
5.3 The physical and chemical parameters of those
polychlorinated biphenyls recognised as
priority pollutants 76
5.4 PCBs (Aroclors) in soil and in vegetables
grown in soil amended with contaminated
sediments 78
5.5 The physical and chemical parameters of those
halogenated aliphatic hydrocarbons recognised
as priority pollutants , 82
5.6 The physical and chemical parameters of those
halogenated ethers recognised as priority
pol 1 utants 83
5.7 The physical and chemical parameters of those
monocyclic aromatic hydrocarbons recognised
as priority pollutants 84
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TABLES (continued)
5.8 The physical and chemical parameters of those
phthalate esters recognised as priority
pollutants , ,85
5.9 The physical and chemical parameters of those
polycyclic aromatic hydrocarbons recognised
as priority pollutants ..86
5,10 The physical and chemical parameters of those
miscellaneous compounds recognised as priority
pollutants 87
6.1 Environmental conditions within the
experimental greenhouses 96
6.2 Germination of the seed types, per soil
concentration of HCB, 30 days after sowing 97
6.3 Soil and radish concentrations of HCB
following radish growth within the
polluted soil for 50 days 98
6.4 Soil and carrot concentrations of HCB
following carrot growth within the
pol 1 uted soi 1 for 50 days 98
6.5 Germination of radish seed, per soil
concentration of TCE, 30 days after sowing ...100
6.6 Soil and radish concentrations of TCE
following radish growth in the polluted
soi 1 for 55 days 100
6.7 Germination of the seed type, per soil
concentration of phenol, 30 days after sowing 101
6.8 Fresh weight of carrot grown for 112 days,
and radish grown for 80 days in phenol
polluted soil 102
6.9 Germination of carrot and radish, % of applied
seeds, per soil concentration of toluene 103
6.10 Fresh weight of carrot grown for 116 days,
and radish grown for 81 days in toluene
pol 1 uted soi 1 103
XI
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TABLES (continued)
6.11 Germination rates of radish taken
at harvest time, after being sown
in HCB polluted soils of varying
organic matter contents , , 104
6.12 Initial and final soil concentrations of HCB
following radish grown to maturity in
soils of different organic matter contents ..101
6,13 Accumulation of HCB in radish roots and leaves
after being grown to maturity in soils of
different organic matter contents 101
6.14 Soil, plant root, and leaf concentrations
of HCB, with time; seed sown in soil
containing 5.7% organic matter and an
estimated 1000 mg HCB/kg 107
6.15 Soil, plant root, and leaf concentrations
of HCB, with time; seed sown in soil
containing 5.7 % organic matter and an
estimated 5000 mg HCB/kg .... 107
6.16 Plant root accumulation of HCB with time;
greenhouse grown in a soil containing
5.7% organic matter .108
6.17 Soil concentrations of HCB after being
exposed for various time intervals,
with or without established vegetation,
in a greenhouse ... .108
6.18 The effects of covering the soil surface on
concentrations of HCB in radish grown in a
greenhouse for 35 days in a soil of 5.7%
organic matter and polluted by 1000 mg
HCB/kg 110
6.19 The effects of covering the soil surface on
the accumulation of HCB in radish grown in
a greenhouse for 35 days in a soil of 2%
organic matter and polluted by 1000 mg
HCB/kg 110
6.20 Final plant root concentrations, greenhouse
grown in soil containing 2% organic matter
and two concentrations of HCB ..112
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TABLES (continued)
6.21 Plant accumulation of HCB, greenhouse grown
in a soil containing 2% organic matter
and 600 or 3000 mg HCB/kg 113
6.22 Final soil concentrations of HCB after the
growth and harvesting of various plant
species in a greenhouse ...113
, 7.1 The concentration of TCDD found in the soil
and vegetation collected from the Minker Site 122
7.2 The range of root and shoot concentration
factors found for TCDD for different
plant species ......123
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SECTION 1
INTRODUCTION
The historical disposal of toxic and hazardous chemicals has caused, in
recent years, increased pollution problems and become a threat to our
environment and well being. This problem is most acute and has reached large
proportions in the industrialised countries. In the United States, for
example, a recent estimate includes 25,000 potentially hazardous waste sites
(EPA, 1987). In Wales, a survey of potential contaminated sites, based on
former use, identified 703 sites, or approximately 0.2% of the total land area
(Welsh Office, 1984).
Although many of these contaminated sites contain a wide range of
organic and inorganic pollutants, the behaviour and effect of the organic
compounds, often the predominant pollutants, have received the least study.
This is understandable; inorganic compounds are relatively inexpensive to
assess, are quite straightforward to work with, and are few in number. The
behaviour of the organic compounds, occurring as pollutants needs to be under-
stood, however, so that successful remedial actions can negate the hazards
arising from their presence. The potential health hazard of some of these
compounds has been identified (Dacre, 1980).
Organic compounds may have very low water solubility; some do not
degrade readily and have long half lives in soil. They may have a very high
affinity for lipids, bioaccumulating in tissue, and may accumulate and
translocate in the food chain. They can be highly toxic to mammals - many are
carcinogenic.
The purpose of this report is to determine the effect of higher plants
on sites polluted by organic chemicals and to discuss the potential of using
plants as an in situ cleanup treatment. Higher plants are those that
reproduce by seeds and do not have two conspicuous stages in their life cycle.
This definition includes grasses, trees, shrubs, etc., but excludes the ferns,
mosses, liverworts, etc.
In situ cleanup systems have many advantages when compared with other
cleanup techniques. These systems treat polluted soils, without excavating
the bulk of the polluted material, by detoxifying, neutralizing, degrading,
immobilizing or otherwise rendering harmless the contaminants where they are
found (Stief, 1985). As the polluted materials are not excavated, the
workforce is not exposed to the pollutants, and the pollutants do not migrate
from the site during excavation.
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The first steps in the development of an in situ plant cleanup system
for organically polluted soils have been identified as
1. determine whether vegetative extraction of pollutants from the
contaminated soil has a high probability of being the most technically
feasible and cost effective approach at the specific site, realizing that this
approach will require a substantial time period and intensive agronomic
management over that time,
2. determine whether suitable plant species (or varieties within a
species) are available to accomplish the desired contaminant extraction,
•
3. determine whether the site possesses, or can be readily modified to
possess, soil conditions that will support optimal growth of the selected
plant materials,
4. conduct greenhouse scale confirmatory uptake tests, and
5. confirm that the plant materials that have extracted the
contaminants can be disposed of in an environmentally safe manner and that the
plant mass and harvesting mechanics are realistically manageable (Banning,
1985).
Using vegetation as an in situ cleanup technique has received little
study although there would appear to be many potential benefits from such a
technique were it available. A plant cleanup system for organically polluted
soils may prove suitable for a number of different pollutant situations, e.g.,
when soils are contaminated to shallow depths, i.e., less than 2 meters, which
is around the maximum depths plant roots normally penetrate the soil. Such
surface situations are commonly encountered from spills or leaks, when the
source of the contamination is at, or near, the soil surface. It also occurs
on many former tipping (dumping) sites.
Plant cleanup systems, that use the ability of plants to accumulate
pollutants and then to metabolize them to simple units, would essentially be
inexpensive to establish and maintain. It could, therefore, prove extremely
useful when vast volumes of soil and sediment materials are polluted but not
to an immediately hazardous extent. These materials may be polluted by dust
blow or surface erosion resulting from adjacent contaminated sites.
The ability of plants to remove and accumulate compounds from the soil
is an essential function of the plant. Uptake of compounds from land
treatment sites, for example, has been recognized as important in the
management of these sites for a number of reasons. For example, plants remove
much applied nitrogen and phosphorus while they grow and thereby protect the
groundwater from these nutrients. Also, plants may be able to extract
excessive levels of some microelements from polluted sites and thereby
rehabilitate the site for more normal crops (Chaney, 1983).
This report, which is based primarily on literature review but also
includes greenhouse experiments and field testwork, is concerned with
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1. the behaviour of organic pollutants in the plant-soil environment,
2. plant uptake and accumulation of organic pollutants, and
3. variation in uptake by different plant species in different conditions.
The literature review involved keyword searches into suitable databases
(including Water Resources Abstracts, Biosis Previews, Chemical and Biological
Abstracts, Agricola, Phytotox) and review of over 750 scientific publications
for information on the above topics. Unfortunately, despite this extensive
data base, few reported investigations provided sufficient details concerning
experimental methods to make comparisons. Within this report, therefore,
greater emphasis has been placed on the few reports where such information is
provided.
The greenhouse experiments were undertaken to investigate the actual
extent of plant uptake of pollutants from soils under known environmental
conditions. The field testwork was undertaken to quantify natural effects.
This report is not concerned with foodchain effects where the plant may
accumulate pollutants and animals feeding on the plant may receive high doses
of the pollutant for subsequent effect. Nor does this report address effects
of the pollutant on the plant itself, i.e., stunted growth, delay in
germination, enhanced senescence, etc. As will be seen, these effects result
from interactions between pollutant concentrations and a variety of
environmental effects.
REFERENCES
Chaney, R.L. 1983. Plant uptake of inorganic waste constituents. In: Land
treatment of hazardous wastes. Parr, J.F., P.B. Marsh, and J.M. Kla (eds«).
Noyes Data Corp. Park Ridge, NJ. Pp. 50-76.
Dacre, J.C. 1980. Potential health hazards of toxic organic residues in
sludge. In: Sludge - health risks of land application. Britton, G., R.L.
Damron, G.T. Edds, and J.M. Davidson (eds.). Ann Arbor Sci., Ann Arbor, MI.
Pp. 85-102.
EPA, 1987. Superfund: Looking back, looking ahead. April 1987. EPA-87-007,
Washington, D.C.
Sanning, D.E. 1985. In situ treatment. Chapter 4. In: Contaminated land.
Reclamation and treatment. M.A. Smith (ed.). Plenum Press. New York and
London.
Stief, K. 1985. The long term effectiveness of remedial measures. Chapter 2.
In: Contaminated Land. Reclamation and treatment. M.A. Smith (ed.). Plenum
Press. New York and London.
Welsh Office, 1984. Survey of contaminated land in Wales. Welsh Office,
Cathays Park, Cardiff, Wales, UK.
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SECTION 2
BEHAVIOUR OF POLLUTANTS IN THE PLANT-SOIL ENVIRONMENT
The behaviour of a compound in the soil depends upon both the physical
and chemical properties of the compound and of the soil. As either the soil
or the compound varies, so does the eventual fate and effect of the compound
and so does the effect on plants growing in the soil.
Once an organic compound, or indeed any other compound, is within the
soil, it tends to spread throughout the soil. Once spread within the soil,
further processes are known to determine the fate of an organic compound.
They include such physical, chemical, and biological processes as leaching,
adsorption, desorption, photo-decomposition, oxidation, hydrolysis, and
metabolism.
As any number of these processes can be acting on any given chemical at
the same time, an assessment of the environmental fate of the compound is
extremely complicated. The interrelationships of these processes is shown in
Figure 2.1.
EROSION/RUNOFF / OEGRAaMTON
Figure 2.1. Interrelationships and transformations of pollutants in soils,
Preceding page blank
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The migration of any pollutant within the soil is affected by both soil
factors and the properties of the compound itself. The processes of spread
(excluding mechanical action where man or other animals may pick up the
chemical and place it elsewhere) include molecular diffusion and various kinds
and combinations of flow.
Molecular diffusion occurs continuously and spontaneously wherever there
is a concentration gradient in an attempt to even out that gradient to achieve
a uniform distribution. Flow, on the other hand, is not spontaneous but
occurs from a combination of external forces.
A compound's solubility and volatility, then, directly control its
distribution between undissolved solid, liquid, and vapour phases and controls
the extent of its adsorption to the soil matrix. This latter factor is also
influenced by the soil itself and is discussed at length below.
Although some of the above factors are influenced by climate, e.g.,
temperature, soil water content, etc., others are influenced by the very
nature of the soil medium itself. Soil is normally heterogeneous and is
formed from weathered bedrock. A silt loam surface in good condition for
plant growth consists of 20% to 30% by volume of air, 20% to 30% water, 45%
minerals, and 1% to 3% organic matter. Soil is generally characterized by
three layers referred to, from top to bottom, as the A, B, and C horizons.
The A horizon, usually called the topsoil, is normally higher in organic
matter than the other horizons and is subjected to leaching of soluble
materials. The B horizon, or subsoil, is usually deeper than A, has a greater
clay content, and is a zone of accumulated leached material. The C horizon is
usually called the substratum and consists of weathered bedrock and other
parent materials (Lee et al., 1985).
The way in which compounds partition differently among the different
soil horizons depends on their individual constituents and, obviously, the
site of their disposal.
Texture, compaction, organic matter, and clay minerals are the major
physical soil characteristics affecting chemical behaviour within the soil.
1. Soil texture is a measure of the proportion of different particle
size range groups within the soil, e.g., sand, silt, and clay. When sand is
the dominant particle, the soil is described as "light" and water infiltrates
rapidly. When silts and clays dominate, then the soil is "heavy" and slow to
erode.
2. Soil compaction is a measure of the density of the soil. Highly
compressed soils have low pore space thereby preventing the movement of air
and water within the soil.
3. Soil organic matter is a mixture of plant and animal residues in
various stages of decomposition, of chemicals synthesized chemically and
biologically from their breakdown products, and of microorganisms and small
animals and their decomposing remains (Lee et al., 1985). It is the most
active area of the soil. Quantitively it consists of two fractions, humic and
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nonhumie, Humic organic matter is the transformed remains of the plants,
animals, and microorganisms in the soil; the nonhumie is the unaltered
remains. The former includes fulvic acid, humic acid, and humins; the latter
includes cellulose, starch, proteins, and fats.
4. Clay minerals are minerals with sheet silicate structures. They are
markedly influenced by the time and type of the weathering process of the
parent material from which they were derived.
The soil's acidity and cation exchange capacity are major chemical soil
constituents affecting chemical behaviour in the soil.
m
1. Soil acidity, a measure of the hydrogen ion concentration in the
soil, influences all soil reactions and biological activity.
2. Cation exchange capacity refers to the capacity of the soil to hold
or sorb exchangeable cations. This property arises in the soil's organic
matter and clay minerals and influences biological activity and the potential
of the soil to be leached.
Vegetation growing on a soil can significantly affect many of these
characteristics and reponses. Depending on the nutrient source, for example,
plant roots can make the soil near them either more acidic or more alkaline
than the soil at a distance from the root. This is because the root exchanges
anions or cations with the soil as part of the root's uptake of essential
plant nutrients. Smiley (1974) measured the rhizosphere pH (pHr) of field and
container-grown wheat plants and compared it with the nonrhizosphere pH (pHb).
The pHr was generally lower than pHb when ammonium was supplied as a
fertilizer, higher when nitrate was supplied, and remained relatively
unchanged when both forms were added together. Differences in pHr of up to
2.2 pH units were recorded in laboratory experiments and up to 1.2 units under
field conditions.
As a plant root grows into the soil, it will also affect the bulk
density of the soil and thus will affect the rate of ion diffusion within the
soil. As the plant root grows, it occupies space that was previously occupied
by soil pores or soil particles. Since root diameters are of the order 0.1 to
3 mm and soil pores are of the order 0.002 to 0.2 mm, the bulk density of the
soil near the plant root automatically increases. Greacen et al. (1968),
showed these increases in soil, i.e., an initial density of 1.5 g/cm3
increased to 1.6 to 1.7 g/cm3.
When ions within the soil diffuse towards the root, they do so along the
concentration gradient, which is measured per unit volume of soil. Hence as
the soil is compacted around the root, the concentration of the ion per unit
of soil volume is increased and steeper concentration gradients are obtained.
In addition, the proportion of clay and organic matter is higher in the volume
of soil near the root than at some distance from it. This is because of the
differential packing of the soil particles around the root. As clays and
organic matter are the prime sites for nutrient and pollutant sorption, the
-------
plant root is directly exposed to higher concentrations of these chemicals
than would be found throughout the soil as a whole.
The amount of physical change the root actually exerts on the soil
varies with the density of the roots in the soil and with the root morphology.
When it becomes involved with the soil medium, the organic pollutant
will distribute between the solid phase (either undissolved particles or
adsorbed onto receiving surfaces), the liquid phase (normally dissolved in the
soil water), and the vapour phase. In the solid phase, it can be regarded as
inert, because it cannot migrate throughout the soil mass and is probably
unavailable to plants. Once in the liquid or vapour phase, however, the
pollutant can become active and begin to biologically or chemically affect its
immediate surroundings. The properties of the pollutant, therefore, influence
its effect within the soil environment.
To determine a compound's mobility in the soil, it is essential to know
how a given quantity of the applied compound will partition between these
three phases. The following discussion is concerned with the various factors
acting on an organic pollutant when it is present in the soil. Each section
contains a theoretical discussion of what happens in the soil and case studies
involving the effect of higher vegetation on the system.
SORPTION AND DESORPTION
Theoretical aspects
Sorption represents the single most important factor affecting behaviour
of chemicals in the soil and is an important mechanism for immobilizing soil
pollutants. Sorption also reduces the activity of pesticides, retards their
movement, and delays their degradation. Sorption has, therefore, been the
subject of considerable research.
Adsorption is the accumulation of the pollutant at an interface such as
a solute to solid, i.e., on the surface of a solid at the clay colloid/water
interface. Absorption is the movement of solutes from one place to another
through an interface of a two component mixture, including penetration into
plant or animal cells and microorganisms. Desorption is the release of the
sorbed molecules.
Once a compound is applied to the environment, a dynamic equilibrium is
established between the adsorbed and solution phase, which is known as the
partitioning effect. Sorption can be either chemical or physical. Anionic or
cationic compounds are chemically sorbed, whereas nonionic compounds are
physically sorbed (Kaufman, 1983). Adsorption will affect the rate of
volatilization, diffusion, or leaching as well as the availability of the
chemical to biological uptake and attack.
Sorption can be specific or nonspecific. The former occurs when
specific sites on the surface of soil particles exert forces on a particular
unit of a molecule at a certain configuration; the latter is more general and
8
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occurs where any molecule, at any configuration of the soil particle, is
sorbed. There are major forces that make sorption possible.
1. Van der Waals forces are weak, but additive, electrostatic forces
caused by an uneven distribution of electrons around a molecule. They become
significant for large and organic molecules.
2. Hydrophobic bonding is actually a partitioning between a polar
solvent {e.g., water) and a nonpolar adsorbent surface (e.g., soil humus).
This is related to temperature and is the prime reason for the relationship
between the organic carbon content of the soil and the n-octanol/water
partition coefficient of the soil-borne chemical discussed below.
3, Hydrogen bonding is a weak electrostatic bond that occurs between
hydrogen and two atoms of high electronegativity. As with Van der Waals
forces, this bonding becomes more significant with larger molecules.
4, Charge transfer is formed between bonds or lone pairs of electrons
when there is a partial overlap of the electron orbits of two molecules. It
is an important form of bonding for organic chemicals.
5. Ligand exchange is a replacement of one or more ligands of a
molecule by a further, stronger chelating, absorbent agent,
6. Chemisorption involves the chemical bonding between adsorbent and
adsorbate in an exothermic process.
The adsorbed-liquid partitioning is expressed through an adsorption
isotherm, which is often calculated. At low concentrations, the shape of this
isotherm may frequently be approximated to a straight line, giving rise to the
equation
Cs = Kd (C,)
where Cs is adsorbed concentration (g/kg soil), C, is solution concentration
(g/m3 soil solution), and Kd (m3/kg) is the slope of the adsorption isotherm
of the distribution coefficient.
Several workers have reported that adsorption is highly correlated with
the organic matter content and cation exchange capacity of the soil (Rhoades
et al., 1970; Bailey and White, 1964); whereas it is independent of substrate
pH, textural composition, and clay mineralogy (Means et al., 1982). Lambert
(1968) goes so far as to suggest classifying soils according to "corrected" or
active organic matter content with Ripperdan sandy loam with 1% organic matter
as a standard soil.
Since this distribution coefficient (for nonionic pesticides at least)
primarily represents adsorption to organic matter, variability between soils
may be reduced to an extent by defining an organic carbon distribution
coefficient as
-------
K -Jil
where foc is the fraction of organic carbon in the soil and Koc represents the
adsorption per unit of organic carbon.
Where measured adsorption values are not available, reasonably good
correlation has been found between Koe and the n-octanol/water partition
coefficient (Kow) of the compound or between Koc and the solubility and
melting point of the compound (Jury et a!., 1983).
The n-octanol/water partition coefficient is defined as the ratio
of the chemical concentration in octanol to that in water when an aqueous
solution is intimately mixed with octanol and then allowed to separate.
Dawson et al. (1980) report that this value reflects the bioaccumulation
potential of the chemical; this, in turn, is defined as the concentration of
the chemical in an aquatic organism compared with the concentration of the
chemical in the water to which the organism is exposed (Brown et al . , 1983).
The bioaccumulation factor is, in fact, a partition of the pollutant between
water and the lipid and protein phases of the organism (Briggs, 1981).
The predictive aspects of the n-octanol/water partition coefficient with
the organic matter content of a soil has resulted in mathematical models
describing the plant uptake of organic pollutants from soils (Norton and
Chrostowski , 1986). These models present uptake as a simple partitioning
process. Norton and Chrostowski's model uses three pollutant specific
parameters (organic carbon partition coefficient, the n-octanol/water
partition coefficient, and Henry's Law constant) and addresses partitioning
into the roots, shoots, leaves, and seeds. The authors note that the model is
useful only for pollutants that are not products of normal plant metabolism.
These models, although useful for their intended applications of
assessing the potential food chain transfer of new and existing chemicals,
tend to simplify the living biological system in which plants exist. The use
of the n-octanol/water coefficient and the other partitioning constants for
example should be regarded as a very generalized approach (Dragun, 1986).
The n-octanol/water partition coefficient has proved useful as a means
to predict soil adsorption, biological uptake, lipophilic storage, and
biomagnification. Unfortunately, the partition coefficients of many chemicals
are not available. By definition, the partition coefficient expresses the
equilibrium concentration ratio between an organic liquid, e.g., n-octanol,
and water. This partitioning is in essence equivalent to partitioning an
organic chemical between itself and water. Chiou et al . (1977) showed a
correlation between the partition coefficient and its aqueous solubility
covering eight orders of solubility (from 10"3 to 104 ppm) and six orders of
partition coefficient (from 10 to 10 ).
10
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Factors that affect the aqueous solubility of an organic chemical will
also affect its partition coefficient. For example, there is about a 25%
change in either the partition coefficient or the aqueous solubility for every
10 degree variation in temperature. Temperature can then affect the
absorption or accumulation of a pollutant in the roots of a plant.
Goring (1962) also recognized the close correlation between Kd, the
soil/water distribution, and the organic matter content of the soil, and
proposed a soil organic matter/water distribution, Kom, for each chemical
_ 100 Kd
om ~ -~~
For 2-chloro-6-(trichloromethyl ) pyridine, he found Kom in the range of 86 to
262, with a mean of 155, for 10 soils with an organic matter content range of
0.3% to 32.2%. Graham-Bryce (1967) and Graham-Bryce and Etheridge (1970)
reported similar values of Kom in different soils, with mean values of 491 for
disulfoton and 5 for dimethoate. As the range of Kom values obtained from
these experiments with different soils was small, Briggs (1981) suggested that
despite the complexities of, and variations in, soil organic matter, Kom for a
particular chemical is a constant across all soil types.
Briggs (1973) then showed that the behaviour of 30 chemicals, within
four different agricultural soils, allowed a relationship between the organic
matter/water distribution and the n-octanol/ water partition coefficient of
the chemical to be obtained. This equation
log Kom = 0.521og Kow + 0.62
follows a linear regression equation where 0.52 and 0.62 are data-fitted
coefficients.
Many investigators have since assessed this equation with different
groups of environmentally active chemicals and varying soil types (Karickhoff,
1984).
Means et al . (1982) report a broadly similar relationship between the
organic matter content of the soil and the n-octanol/water partition
coefficient of the chemical with
log Koc = log Kow - 0.317
for the sorption of 22 nonpolar compounds (mainly substituted aromatic
hydrocarbons) by 14 soils and sediments having an organic matter content range
of 0.11 to 2.38%. This equation tended to under estimate the organic matter
sorption for two amines tested but could be corrected if the % organic
carbon/% montmorillonite clay ratio of the substrate was taken into account.
The authors suggest that the enhanced sorption of these two amines must
involve specific interactions of the amine functional group with components of
11
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the associated clay minerals, when the ratio of organic matter to clay mineral
is low, at less than 0.1. Reactions of this type are known to occur with
other aromatic amines.
Schwarzenbach and Westall (1981) reported
log K00 = 0.721og Kow + 0.49
from their experiments with 13 methylated and halogenated benzenes.
Rao et al. (1982) reported
log Koc = 1.0291og Kow - 0.18
from their experiments with 13 pesticides; and Karickhoff (1981) reported
log Koc = 0.9891og Kow - 0.346
from his work with five polyaromatic hydrocarbons.
Brown and Flagg (1981) investigated the adsorption of nine chloro-s-
triazines and dinitroaniline compounds and, by combining other published data,
produced the relationship
log Koc = 0.9371og Kow - 0.006
that covered a broad range of compounds within a broad range of soils and
sediments. They found that the uncertainty in this equation increased with
the addition of data sets on more hydrophobic compounds, where the interaction
of specific functional groups with adsorption sites, is not accounted for by
the K00 treatment. The addition of data from polar compounds only changed the
relationship slightly. They concluded that the practical utility of this form
of relationship is maximized when a broad range of chemicals is being
investigated. Better precision can be obtained by fitting separate
relationships to each distinct group of chemicals.
The above relationships are shown in Figure 2.2. They are surprisingly
similar to one another considering they cover the behaviour of over 100
organic chemicals as well as a large number of soils and sediments. Some
criticisms, however, have been made of the inappropriate use of these
equations.
1. They are valid only with molecular weights of less than 400. Above
this figure, van der Waals forces govern soil adsorption of the pollutant and
these forces are not dependant upon soil organic matter content,
2. They assume hydrophobicity is the only factor governing adsorption.
3. They ignore the mineral content of a soil and thus any mineral
adsorption of a pollutant to a soil. If the % clay/% organic matter is
12
-------
greater than 30, the mineral surfaces in the soil are significant for
pollutant adsorption.
4. They assume K00 or K^ for a soil is constant and thereby ignore
rhizosphere influences and the difference between top and subsoil.
5. They assume all soil organic matter behaves the same, which it
undoubtedly does not.
6. They assume linear isotherms for soil adsorption of pollutants over
all pollutant concentrations (Dragun, 1986; Kariekhoff, 1984).
3-
1 -
o!
4 5
too Kow
Figure 2.2. The relationship between the organic matter sorption of a soil
(Koc) and the n-octanol /water partition coefficient (Kow) of the pollutant.
(Regressions from 3 - Briggs, 1973; 4 - Means et al., 1982; 5 - Schwarzenbach
and Westall, 1981; 6 - Rao et al., 1982; 7 - Karickhoff, 1981; 8 - Brown and
Flagg, 1981.)
These linear regression equations relating the organic matter partition
to the n-octanol/water partition of the organic chemical also have inherent
errors that result in variability when observed values are compared with
calculated values. These have been discussed by Karickhoff (1984) and are
shown in Figure 2.3.
13
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Some efforts have been made to relate the structure of a chemical to its
sorptive potential. Hance (1969), for example, related the parachor of a
molecule, which is a measure of its volume, and the number of sites on the
molecule that could participate in the formation of a hydrogen bond to the
adsorption of 29 aromatic compounds by two soils.
4-
Sleric and/or—
Kinetic limitations
Nonfyxepriobic contributions
Isotherm •nonitnearity
it
Figure 2.3. Errors in observed values of KQO compared with calculated values.
(from Karickhoff, 1984.)
Considerable effort has been expended to determine the relative
importance of clay and organic matter in chemical sorption. Both materials
sorb chemically and physically and both can exhibit a wide variation in their
properties. Wahid and Sethunathan {1978 and 1979) concluded that organic
matter was the most important factor governing the sorption of both the
isomers of hexachlorocyclohexane (HCH) and parathion by 12 soils. In the
absence of organic matter, parathion sorption was correlated with clay content
and free iron oxides, whereas the separate isomers of HCH behaved differently.
Now, however, it is generally accepted that organic matter is more
important than clays except in dry soils or where the organic carbon content
of the soil is low (Kaufman, 1983).
14
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Soil pH and moisture are further critical factors affecting sorption of
some chemicals in soils. The pH affects the dissociation constant of the
chemical, which, in turn, affects its chemical bonding potential. All
chemicals are adsorbed strongly at low pH; anionie compounds are adsorbed
negatively at slightly basic conditions and nonionic compounds are moderately
adsorbed. Sorption of organic chemicals is more complete in dry soils than in
wet soils because surfaces that would normally preferentially adsorb water
become available to the chemical. Yaron and Saltzman (1972) demonstrated this
experimentally with the organophosphorus insecticde, parathion. As their soil
water content increased, so parathion sorption decreased.
Temperature may also influence adsorption both through its effects on
the solubility and vapour pressure of the compound and because adsorption
processes are exothermic (Harris and Warren, 1964), Generally, an increase in
soil temperature leads to decreased adsorption; however, there are exceptions
where higher temperatures result in lower soil water conditions.
To obtain many of the above relationships, it is necessary to assume
that pollutant adsorption to soil is reversible, i.e., the pollutant will be
released from the organic fraction of the soil into the soil solution in the
direction of the concentration gradient. The ease of desorption appears to
depend on the actual strengh of the adsorption process. Harris and Warren
(1964) found that various herbicides could be recovered from soils and
sediments with low organic fractions more readily than they could from muck
soils. Graham-Bryce (1967) found that the adsorption of the systemic
herbicide disulfoton was fully reversible if desorption took place immediately
after uptake when the sorbing soils were still wet. Desorption, or release of
the herbicide from the sorbing soils, was modified if the soils were allowd to
dry thoroughly between adsorption and desorption (Sims et al., 1984).
Error is introduced into the relationships via the formation of
irreversibly bound residues, which occur in soils, plants, and animals.
Although the presence of these residues has been known for some time, only the
increasing recent use of radiolabelling techniques has led to a quantification
of their extent (Klein and Scheunert, 1982). These bound, or non-extractable
residues are defined as chemical species that are unextracted by methods that
do not significantly change the chemical nature of the compound. They exclude
fragments recycled through metabolic pathways leading to natural products.
Table 2,1 gives a few examples to demonstrate the significance of these bound
residues.
15
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TABLE 2.1, PESTICIDE RESIDUES BOUND IN SOIL, MEASURED IN PERCENT
OF APPLIED AMOUNT. (Adapted from Klein and Scheunert, 1982.)
Chemical class Time of exposure
% Residue bound
Free phenols
Anilines
Triazines
Urea herbicides
Carbamates
Organophosphates
Cyclodiene
1 vegetation period
1 vegetation period
4-12 months
1 vegetation period
30-32 days
7-84 days
1 vegetation period
50-18
31-58
56-65
28-41
17-57
18-80
1-8
Di Toro and Horzempa (1982) also reported the concept of irreversibly
bound residues from their sediment sorption studies with 2,4,5,2',4',5'-
hexachlorobiphenyl, They suggested that it may be inappropriate to treat this
and PCB isomer adsorption reactions as either completely reversible or totally
irreversible.
In most instances, sorption can be regarded as advantageous, because
sorbed compounds in the soil are environmentally less active. Fairbanks and
O'Connor (1984) concluded that adding sludge containing PCBs to soil did not
increase the environmental hazard of the PCBs because of the additional
sorptive capacity of the added sludge.
Case studies
The actual type of soil, and thus its sorptive capacity and
characteristics, highly influences plant uptake of pollutants from soil. This
is because pollutants are sorbed more tightly, become biologically
inactivated, and thus less available to the plant roots as the organic matter
content of the soil increases (Finlayson and MacCarthy, 1973). Adams (1971),
for example, reported tenfold differences in the bioactivity of pesticides in
soils with different organic matter contents.
Attempts to relate the extent of adsorption of a given herbicide to soil
properties have shown a correlation between adsorption and organic matter
content, clay content, and cation exchange capacity of the soil. Harrison et
al. (1976) investigated the effects on oats of atrazine, chloramben,
fluometuron, propachlor, and trifluralin in 10 North Carolina soils. Some 15
soil properties were measured and correlated with fresh weight inhibition
levels. Organic matter content of the soils was the soil variable most
closely related to the herbicide phytotoxicity, and there was an inverse
relationship between the herbicide water solubility and its inactivation by
this organic matter.
16
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VOLATILIZATION AND DIFFUSION
T h e o r e t ic a1 asp e ct s
Volatilization and diffusion are further processes by which organic
compounds can be spread throughout the soil. The vapour partitioning of a
compound is important because this spread is considerably greater than that by
the solution phase. Even for chemicals with relatively low vapour densities,
this transport route has been shown to be significant (Mayer et al., 1974).
The rate at which a compound vapourizes in the soil, and from a soil,
will be influenced by its inherent volatility, the ambient and soil
temperature, dilution with inert biological material, and any restraints
placed on the compound by the soil. The vapourized molecules will tend to
diffuse outward primarily through the air spaces in the soil and, while
diffusing, will tend to dissolve in the soil water. The partition of a
chemical between the soil water and soil air is in a constant flux or
equilibrium. The diffusion within the soil is best described by Pick's Law;
the liquid-vapour partition is generally represented through Henry's Law.
Goring (1967) suggested that Pick's first law of diffusion was probably
applicable to all diffusion processes in soil, including movement of organic
chemicals through air, water, and organic matter. This means that the rate of
movement of a chemical is directly proportional to the concentration of the
chemical and its diffusion coefficient, which is a quantitative expression of
the diffusion rate through different media.
Pick's Law is expressed
_F
I
—
where F is the amount of substance diffusing per unit of time across area, A,
normal to the direction, X, and dC/dX is the concentration gradient in that
direction. The negative sign is required because diffusion takes place in the
direction of decreasing concentration (Hartley and Graham-Bryce, 1980),
The diffusion rate of a gas is influenced by molecular weight,
temperature, presence of other diffusing gases, continuity of soil air spaces,
and the distribution of the chemical into the different phases of the soil.
Because this distribution depends on the characteristics of the soil, the
extent of volatilization and diffusion of a particular chemical is site
specific. Insecticides diffuse more slowly in soil than in air because the
pathway through the soil pores is restricted and because the chemical may be
sorbed by the soil. The exact geometry of the soil pathway depends on the
nature of the pore space formed by the particles in a given soil and on the
soil moisture content (Graham-Bryce, 1969).
The division of a compound between the soil solution and the air spaces
in the soil is often described by Henry's Law and the extent of partitioning
is described by Henry's Constant. Henry's Law is expressed
17
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CB = Kh (C,)
where Ca Is the concentration of chemical in the vapour phase (g/m3 soil air);
Kh is Henry's Law Constant, which has no dimensions; and C, is the solution
concentration (g/ra3 soil solution) (Jury et al,, 1983).
Those compounds with a high vapour pressure, which would be reflected in
a high Henry's Constant, will easily move from the soil solution into the soil
air, and will be quickly spread throughout the soil. There have,
unfortunately, been no all- embracing studies to determine the level at which
Henry's Constant describes the predominant transport route. Graham-Bryce
(•1984) reports that a partition between vapour and aqueous phase of 10"4 is
normally sufficient to ensure vapour effects when working with pesticides.
This is based on experience with particular compounds rather than broad-
ranged, detailed investigations. It remains uncertain what level of Henry's
Constant for organic priority pollutants should be taken to be descriptive of
vapour phase transport.
Volatilization, itself, is the loss of the compound to the air mass. It
is influenced by many soil and compound parameters. The position of the
compound in the soil will, for example, affect its chance of being volatilized
and, thereby, of leaving the soil. For example, only 3% of applied heptachlor
was recovered from surface-treated soil, 3 to 4 months after application
compared with 15% of the insecticide incorporated into the soil after 5 months
(Saha and Stewart, 1967).
Soil type also affects the degree of potential volatilization of a
chemical from the soil. Guenzi and Beard (1970) report that volatilization
rates for DDT and lindane from soil containing moisture greater than 15 bar
tensions were dependent on temperature, adsorptive characteristics of the
soil, and concentration of the chemical. For DDT and lindane at 30°C, the
rate of volatilization from soil was in the order of Valentine loamy sand >
Hand loam > Raber silty clay loam > Promise clay, which had 0.6%, 1.63%,
3.06%, and 3.57% organic matter contents, respectively.
Harris and Lichtenstein (1961) showed an increase in the rate of aldrin
volatilization from soil, with increases in aldrin concentrations in the soil
and increases in the rate of air movement over the soil surface. The soil
surface and air over the soil are connected by a layer or boundary of stagnant
air through which water vapour and chemical vapour move by diffusion.
Guenzi and Beard (1970) also found higher losses of both DDT and lindane
at temperatures of 55°C than at 30°C, Haque et al. (1974) investigated the
vapour behaviour of the PCB Aroclor 1254. They found that the loss of the
chemical, via its vapour phase, was negligible from soil but quite significant
from sand. As temperature increased, vapour loss increased, and isomers with
fewer chlorine atoms showed greater loss than those having more chlorine
atoms.
The loss of Aroclor 1254 from itself is shown in Figure 2.4. Although
the loss is rather small at 26°C, it is substantial at 60°C.
18
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Farmer et al. (1972) reported that the vapour density of a chemical in
soil is the main factor controlling volatilization, and that the vapour
density increases with increasing soil concentrations until a peak is reached.
Their laboratory studies gave maximum volatilization rates from Gila silt loam
of 202, 22, and 5 Kg/ha/year for the pesticides lindane, dieldrin, and DDT,
respectively. These rates decreased rapidly with decreasing pesticide
concentrations, but they suggested the decrease could account for a
significant proportion of these applied chemicals that are lost under normal
field conditions.
In fact, losses of organic chemicals from soil surfaces through
volatilization have been reported for the s-triazines and phenylureas, which
have very low vapour pressures (Kaufman, 1983).
.002
Grrs
.006
.010
?C» f
-o-o-o- -a= • ._ jj__ _s2.__™. ^—
PCS 1254 \fapcr Loss
»**
10
30
20
Time {days}
Figure 2.4. The loss of Aroclor 1254 from itself at different temperatures.
(From Haque et al. 1974.)
Case studies
Investigations have aimed at assessing the rate of volatilization, and
thereby loss, of chemicals from the soil. Much of this work has been carried
out with soil fumigants, or herbicides that act through their vapour phase.
These compounds normally have high vapour pressures and low water solubilty
(Harvey, 1974; Goring, 1962).
Harvey (1974) investigated the soil adsorption and volatility of 12
dinitroaniline herbicides in a Piano silt loam and their respective effects on
the growth of foxtail millet, Setarfa italica. High correlations between the
effects of the herbicide vapours on growth of the millet under laboratory
conditions and the relative effectiveness of seven of the herbicides under
field conditions suggested that absorption of the vapours of the
19
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dinitroaniline herbicides by plants may be more important than absorption of
the herbicides from soil solutions.
DEGRADATION
Thi egreM c a la s p ec t s
Degradation of a compound in the soil can occur by biological or
nonbiological processes and can occur whether the compound is in a solid,
liquid, or vapour phase. The concentration of most organic compounds in the
soil, therefore, decreases with time providing that no further additions are
made and that the compound is not being synthesized via the degradation of
other compounds. The rate at which degradation occurs is generally considered
to be first order and can be described as
M{t) = M(0) exp. (-ut)
where M(t) is the quantity of the compound remaining in the soil at time, t,
(Jury et al., 1983), The rate of degradation of individual compounds is
normally described by half life, i.e., the time taken for one half of the
initial concentration of the compound to be degraded.
The half lives of many compounds have been published (Ryan, 1976; Smith
and Dragun, 1984; Jury et al., 1983). Actual half lives are, however, very
dependent on local environmental conditions and published material can only be
taken as a guide. Half lives of many organic pollutants are included in
Section 5.
Nonbiological transformations of compounds are more numerous than
biological transformations. They include those brought about by light as well
as reactions such as hydrolysis, oxidation, or reduction, which may be
catalyzed by soil colloids. Photodegradation is the action of sunlight
chemically altering and degrading an organic chemical in the soil. It usually
depends on the absorption spectrum of the compound.
Biodegradation is generally defined as the molecular degradation of an
organic substance resulting from the complex action of living organisms. A
substance is said to be biodegraded to an environmentally acceptable extent
when its undesirable environmental properties are lost (Rochkind et al.,
1986).
Many biological agents with the capacity to directly degrade, or
otherwise change organic compounds, are in the soil.
1. Bacteria, of which there are many different groups and species, are
normally classified according to their energy source for survival. The
heterotrophSj which decompose organic materials, are thereby more important in
degrading organic pollutants than are the autotrophs which use light as their
energy source either through photosynthesis or the oxidation of inorganic
materials.
20
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2. Fungi, which constitute a large fraction of the microbial biomass of
a soil, also depend on heterotropic metabolism. They are very resistant to
unfavourable environmental conditions and many grow at temperatures below
10°C.
3. Algae, which need oxygen, are usually restricted to the soil
surface. At the surface they may be present in appreciable numbers and may
enhance the photodecomposition of some organic chemicals.
4. Higher life forms, include the protozoa, nematodes, insects, and
worms who graze on the other life forms in the soil or feed on detrital
matter. They may directly degrade organic compounds or assist in their
degradation by mixing the soil and contributing to its aeration.
5. Higher plants, which often form the soil surface cover and through
plant roots may reach soil depth of up to 3m (Foxx et al., 1984), are
discussed at length in Section 3.
There are, of course, many interactions between and within each of these life
forms and between them and the various soil constituents including organic
compounds. Microbial activity, for example, may cause desorption of a
compound from the soil organic matter and thus may increase chemical mobility
and the potential for degradation within a soil. Perhaps the greatest
interactions among the life forms results from competition for readily
assimilable substrates, so that growth rates are generally restricted. This
encourages those life forms that can use substrates other organisms cannot
use.
Although biological degradation refers to degradation that occurs from
all biological agents in the soil, the literature indicates that by far the
larger volume of research has been carried out with micro, rather than macro,
biological agents. Generally, degradation seems related to the overall level
of microbiological activity rather than the presence of any one particular
agent. Biological degradation tends to occur more in soils with relatively
high organic matter contents, (providing soil sorption does not protect the
target compound), than in those with low organic matter content. Adding
nutrients often increases the rate of degradation and this is now referred to
as "enhanced degradation." It is of potential interest as a soil cleanup
technique.
Some herbicides actually induce microbial populations with particular
ability to degrade them (Audus, 1964). Most herbicides,however, do not induce
this sort of behaviour and it is assumed that they are degraded incidently in
the rapidly metabolizing soil environment. Lower concentrations of herbicides
in the soil are normally degraded faster than higher concentrations.
Microorganisms in soils and soil water convert many synthetic organic
chemicals to inorganic products. Other compounds are transformed only by
cometabolism. Microbial processes may lead to environmental detoxication, the
formation of new toxicants, or the biosynthesis of persistent products. Some
organic chemicals are resistant to microbial attack.
21
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The soil environment is, by its very nature, heterogeneous, with soil
particles ranging from meters to nanometers. It is within this environment
that many of the biological agents of degradation have to work, and therefore,
results tend to vary considerably.
Components of the microscopic population are affected to different
degrees when exposed to pollutants (Alexander, 1969). The compound may exert
a general depressive effect on most or essentially all components; on the
other hand, it may act upon a very limited group. This normally depends on
both the type and concentration of the compound and the nature of the
population.
Case studies
Microorganisms are more abundant in the soil surrounding plant roots
than in soil remote from the root (Table 2.2) (Rovira and Davey, 1971). This
zone of soil in which the microorganisms are influenced by the root is called
the "rhizosphere." This normally poorly defined zone has a microbiological
gradient (Table 2.3) where the greatest numbers of microorganisms occur
directly adjacent to the root and decline with distance away from it. Direct
microscopy of roots grown in soil show that over the older root portions
bacteria can be 10 to 40 cells deep, whereas the younger root tip is often
bacteria free. Other organisms, e.g., the mycorrhizal fungi, actually
penetrate the root cortex several cells while also remaining in the soil.
The root associated microorganisms, i.e., the bacteria, fungi, and
actinomycetes of the rhizosphere affect their host plant through their effect
on such factors as the availability of plant nutrients and subsequent nutrient
uptake. Their presence also causes an increased rate of decomposition of soil
organic matter.
TABLE 2.2. EXTENT AND DESCRIPTION OF THE RHIZOSPHERE OF 18-DAY-OLD BLUE
LUPIN (Lupinus angusti'folium) SEEDLINGS. (From Rovira and Davey, 1971)
Distance from Microorganisms (lOOOs/g oven-dried soil)
root, mm. Bacteria Streptomycetes Fungi
0 159,000 46,700 355
0-3 49,000 15,500 176
3-6 38,000 11,400 170
9-12 37,400 11,800 130
15-18 34,170 10,100 117
80 27,300 9,100 91
22
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TABLE 2.3. COMPARISON OF THE COLONY COUNTS OF BACTERIA IN THE RHIZOSPHERE OF
VARIOUS CROP PLANTS AND IN ROOT FREE SOIL (From Rovira and Davey, 1971)
Colony Counts, 106/9 soil
Crop Root free Rhizosphere
Red Clover Tri folium pratense
Oats Avena sativa
Flax Linum usitatissimum
Wheat Tri tt cum aestivum
Corn Zea mays
Barley Hordeum vulgare
143
184
184
120
184
140
3,255
1,090
1,015
710
614
505
The rhizosphere microflora can be adapted by inoculation of the seeds or
roots of the plant by the required microorganism. Much more basic research is
needed, however, before the inoculated population will automatically develop
into the dominant component of the microflora in what is an extremely
competitive environment.
Chemicals, such as pollutants or herbicides, applied to the soil, either
accidentally or deliberately, bring about changes not only in the soil
microflora but also in the relationships between plants and rhizospheric
organisms (Taleve and Stoimenova, 1984). The nature of these changes has not
received much study. Sandman and Loos (1984) found that the most probable
number of 2,4-D degrading organisms in the rhizosphere of African clover was
greater than the number within the rhizosphere of sugarcane. They suggest
that this increased rhizosphere population could play a part in protecting
various types of plants against soil-applied chemicals.
EFFECTS OF WATER
Theoretical aspects
The impact of water on a site containing hazardous chemicals can be
effectively managed by man, and adverse effects through off-site transfer of
pollutants need never occur. Mass flow of pollutants however often occurs by
surface runoff and water infiltration of the soil mass causes leaching of
chemicals to the underlying groundwater.
Surface runoff is an effect of rainwater on the soil surface. As the
slope of the soil surface increases, there is a corresponding rise in the
velocity of surface water runoff, which in turn results in greater erosion.
Long, unbroken slopes allow surface runoff to build up and concentrate in
narrow channels producing rill and gully erosion. Land grading techniques are
required to arrest this runoff and subsequent erosion and are commonly used on
a variety of sites.
23
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Leaching is of major importance in determining the fate of any compound
when it is in the soil. Leaching is the rapid removal of the soil water
component of the soil sorbed/soil water equilibrium, which thereby moves the
equilibrium towards the desorption of the compound.
Chemicals are moved through the soil by rainfall or irrigation, or both.
This is governed by three processes: desorption of the chemical from the soil,
diffusion of the chemical through water, and hydrodynamic dispersion. The net
result of these processes is the increased spread of the organic chemical
throughout the soil, moving with the concentration gradients.
The rate of water flow through the soil is, on the microscopic scale,
very heterogeneous and is controlled by the size of the soil pores. Within
the soil pore, flow rate increases concomitantly with distance from the pore
wall, but may be almost zero in pores away from the continuous pathways of
flow. Differences in flow rate result in the gradual spread of the pollutant
in a band through the soil, called convective dispersion.
In temperate regions, the movement of pollutants through soils via
leaching shows a seasonal distribution (Leistra, 1980). In the spring much of
the rainfall evaporates from the soil surface or is taken up from the upper
layers of the soil by plant roots. In the winter, both rainfall is larger and
plant usage less, so that leaching may become more significant. Complications
in this may be experienced when soils containing much clay show large cracks
on drying. When intense rain falls on such a soil, part may rapidly enter the
deeper reaches of the soil before the upper soil layers are fully moistened.
This water may carry with it some of the pollutant to these deeper layers, but
the main portion of the compound would leach less then would be expected.
Although the movement of pollutants with the mass flow of water may be
significant as far as their environmental effects are concerned, as a pathway
of loss this appears insignificant. Herbicides, for example, are rarely found
beneath the plough layer of a soil.
Water moves both upward {Bailey and White, 1970) and downward in the
soil, and thus any pollutant within the soil water follows the same route.
Vegetation on the soil surface with its associated water requirement for
transpiration can draw considerable quantities of water to the soil surfaces
from great depth, especially in those areas where high evapo-transpiration
ratios are prevalent. This influence of vegetation on the movement of a
compound in the soil increases as the mass of the crop increases (Leistra,
1980).
Bell and Parry (1984) have reported on the upward transport of heavy
metal and various anions, including sulphates and complex cyanides, from
polluted soils that were overlaid by 600 mm of clean soils, primarily through
the translocation pull of overlying vegetation.
The potential of any chemical to be leached from the soil is influenced
by its own persistence. Some chemicals are rapidly broken down in the soil
and are, therefore, unlikely to be present long enough in the soil to be
leached.
24
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Case studies
There is evidence from pot experiments that herbicides enter plants more
or less concomitantly with water. Therefore it would be expected, and indeed
was the case, that increasing the water dose always resulted in a higher total
insecticide residue in beets grown on sandy loam or sandy clay loam soils and
a greater plant weight. Uptake of the insecticide by the plant was also
increased if more of the compound was available in the active root zone for
the plant to take up. If there is not enough water, then not enough chemical
may reach the root; if there is too much water, then polar metabolites in
particular will be leached away from the active rhizosphere and result in less
uptake (Dejonckheere et al., 1982).
Wax and Behrens (1965) investigated the effects of temperature and
relative humidity on the root and foliar uptake of radiolabeled atrazine in
quackgrass (Agropyron repens). They found that uptake and trans!ocation
increased as temperature increased but also as relative humidity decreased.
This follows the response of transpiration to these environmental factors.
The phytotoxicities of atrazine, simazine, linuron, lenacil, and
aziprotryne were increased as the moisture content of the soil increased.
These increases were a result of differences in the concentration of the
herbicides that were accumulated by the plants, with total uptake being
directly proportional to water uptake (Walker, 1971).
Moyer et al. (1972) began with the premise that herbicide uptake by a
plant was related to the volume of water transpired, as shown by Sheets (1961)
and Shone and Wood (1972). They established a system containing soil on which
the herbicide diuron was adsorbed to see if plants are able to take up equiva-
lent amounts of herbicide from media having different quantities of adsorbed
herbicide but equal concentrations of herbicides in solution. After 7 days
growth in the system, the amount of herbicide supplied to the exposed barley
plants by mass flow was calculated. The values obtained from the different
media were all similar, indicating that 50 to 60% of the diuron supplied to
the root was accumulated by the shoot, whether the root was in contact with
amended nutrient solution, with solution plus peat having approximately 7.7
#g/g adsorbed diuron, or with solution plus loam soil having about 1.3 /ig/g of
adsorbed diuron. Diuron uptake seems, therefore, to be controlled by the soil
solution concentration and the volume of water transpired.
Transpiration rates for crops are commonly in the order of 200 to 270 kg
water per kg dry matter yield and some crop yields are as great as 2000 kg dry
matter/ha. This indicates the vast quantity of water that actually passes
through plants that are growing on soils and suggests the potential for plant
accumulation of soil-borne pollutants. Walker (1983), however, concludes that
this uptake by plants will only make a minor contribution to herbicide loss
from the soil.
25
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WIND BLOW AND MASS TRANSFER
The movement of organic chemicals over the soil surface can be as
important as their movement through the soil, especially in terms of pollution
potential and environmental effect. There are two main types of overland
flow: where the chemical is in solution in water and where the chemical is
adsorbed onto particulate matter being carried along in the overflowing water.
Good management of a site area will considerably reduce the potential for
these transport processes to occur.
Abandoned or inactive hazardous waste sites have been recognised as
candidates for vegetative stabilization to reduce fugitive emissions (Turner
et a!., 1984). The primary control mechanism is direct stabilization of the
soil surface by the plant roots binding the soil and the stems and leaves
forming a protective cover preventing particles from becoming airborne.
CONCLUSIONS
The above factors (sorption and desorption, volatilization and
diffusion, degradation, water, and wind) that determine the behaviour of a
compound applied to the soil, make identifying the role of vegetation
difficult. Each factor interacts not only with one another but with other
processes that naturally occur in the soil. Without carefully considering
many factors, the environmental fate of any one compound can not be
determined. An even more complex situation occurs when a range of compounds
are present within a soil and the compounds are competing with one another
for, say, sorption sites.
The sorptive capacity of soils and sediments and response of many
compounds, albeit primarily complex organic herbicides, to sorption has been
well described. The organic matter content of the soil best describes the
soil's ability to sorb compounds, whereas the n-octanol/water partition
coefficient best describes the ability of the compound to be sorbed. The
greater the soil's organic matter content and the higher the Kow of the
compound, then the more sorbed and the less environmentally active will be the
compound.
Vapour transport in soils and sediments has received less study.
Transport will be affected by both physical and chemical aspects of the soil
as well as of the compound. A need still exists to understand the
relationships between the physicochemical properties of a compound and
potential for vapour phase transport.
Other factors and processes affect a compound in the soil and in the
region of the soil affected by the plant root. Degradation of compounds,
through biological, physical, and chemical means, is continually progressing,
and the concentration of a particular chemical in the soil reduces with time,
regardless of the action of higher plants. The spreading of compounds through
wind or water erosion also reduces concentrations in one area but increases
them in another. Further work is needed on all of these processes,
26
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particularly those related to the behaviour of organic compounds that occur as
pollutants on many of our uncontrolled hazardous waste sites, so that remedial
actions can be properly designed.
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•
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Dejonckheere, W., G. Melkebeke, W. Steurbaut, and R.H. Kips. 1982, Uptake and
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Di Toro, D.M., and L.M. Horzempa. 1982. Reversible and resistant components of
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Dragun, J. 1986. The soil chemistry of hazardous materials; basic concepts
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Farmer, W.J., K. Igue, W.F. Spencer, and J.P. Martin. 1972. Volatility of
organochlorine residues from soil: Effect of concentration, temperature, air
flow and vapor pressure. Soil Sci. Soc. Amer. Proc. 36:443-447.
Foxx, T.S., G.D. Tierney, J.M. Williams. 1984. Rooting depths of plants as
related to biological and environmental factors. Los Alamos Nat. Lab. LA
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Goring, C.A.I. 1967. Physical aspects of soil in relation to the action of
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Graham-Bryce, I.J. 1969. Diffusion of organophosphorous insecticides in
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soils. Soil Sci. Soc. Amer. Proc. 34:443-447.
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Harris, C.I., and G.F. Warren. 1964. Adsorption and desorption of herbicides
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Harris, C.R., and E.P. Lichtenstein. 1961. Factors affecting the
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Hartley, G.S., and I.J. Graham-Bryce. 1980. Physical principles of pesticide
behaviour. Academic Press, London.
Harvey, R.G. 1974. Soil adsorption and volatility of dinitroaniline
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564.
Karickhoff, S.W. 1981. Semi empirical estimation of sorption of hydrophobic
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Klein, W., and I. Scheunert. 1982. Bound pesticide residues in soils, plants
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Lee, C.R., J.G. Skogerboe, K. Eskew, R.A. Price, and N.R. Page. 1985.
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SECTION 3
PLANT UPTAKE OF ORGANIC POLLUTANTS
*•
Chemicals in the soil enter plants through at least four main pathways,
1. Root uptake into conduction channels and subsequent trans!ocation by
the transpiration stream.
2. Uptake from vapour in the surrounding air.
3. Uptake by external contamination of shoots by soil and dust,
followed by retention in the cuticle or penetration through it,
4. For oil containing plants, e.g. cress, carrot, and parsnip, uptake
and transport in oil cells {Topp et al., 1986).
In nearly all cases, a combination of all of these pathways or events
will reflect the total pollutant concentration in the plant.
THE PLANT TRANSPORT SYSTEM
In general, plant roots are the most important site of uptake of
chemicals from the soil (Finlayson and MacCarthy, 1973). This is the logical
point of chemical entry since the actual function of the roots is to give
support to the plant and to absorb water and mineral salts. The most active
site for such uptake is 20 to 40 mm above the root cap in the zone of the root
hairs.
The transport or conduction of liquids within the plant is known as
trans!ocation. It may occur upwards or acropetally, downwards or basipetally,
or laterally, within specialized transport vessels, the xylem and phloem.
The function of the young roots is to absorb water and solutes and the
protective covering to these absorbing zones offers little resistence to water
flow to the transport system, or apoplast, within the plant. The main absorb-
ing regions are relatively restricted and may extend only a few centimeters
from the root tip. The branching of most root systems, however, ensures a
multiplicity of absorbing points. Older roots are suberised and thereby
impervious to water.
In the absorbing zone, the surface area to volume ratio of the root may
be greatly increased by the presence of many thousands of root hairs, which
33
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grow out of the epidermal cells of the root. In a classical study of rye
(SecaJe cereals), Dittmer (1937) showed that a single rye plant exposed 4300
ft2 of surface to the soil and estimated the number of root hairs on a 4-month
rye plant at 14 million.
Any chemical taken up by a plant root, or conversely taken in through
the plant leaf, and transported throughout the plant passes along the path
provided by the symplast and apoplast. The path provided by the symplast
consists of the living plant tissue that is bounded by the plasmalemma and
connected via plasmodesmata. It is a reactive environment that places
chemicals in proximity to enzymes and other reactants. Movement within the
conductive portion of the symplast or phloem occurs by mass flow and dif-
fusion. It is a slow process, with rates of a few millimeters or, at best, a
few centimeters per hour. It is responsible for movement of sugars, hormones,
metabolic solutes, and in some instances pollutants, to the growing and
storage tissues.
The apoplastic system includes all the dead portions of the plant. Cell
walls and xylem form a water-permeable continuum through which both short- and
long-distance solute transport occurs by diffusion and mass flow,
respectively. The system often works under positive pressure or under tension
created from the leaf's need for water, which can result in quite fast rates
of water transport of up to 100 m/hr. All substances that enter the plant do
so via the apoplast, which also protects the symplast from the destructive
forces of desiccation, abrasion, and even pressure sufficient to cause its
collapse.
Any pollutant entering the plant through the roots initially enters into
the free space of the root tissue, followed by movement across the plasmalemma
into the endodermis. Entrance into the stele, and the vascular tissues with-
in, then occurs through the cellular membranes. Movement within the xylem is
determined by diffusion, which is being controlled by the energy potential of
the solute, and mass flow, which is controlled by the energy potential
gradient of the continuous solvent system. Movement within the phloem depends
on diffusion, membrane transport, and cytoplasmic streaming, and requires
metabolic energy.
Some chemicals appear to be restricted to either transport in the
apoplast or in the symplast, whereas others termed, ambimobile, move in both
systems. Presently, chemicals can not be classified according to their mode
of transport.
Although the most common pathway by which nutrients, other solutes, and
pollutants enter the plant is through the roots, another possible route of
entry of pollutants into plants is through the stomata of the leaves and stems
in the form of gasses (Table 3.1). Little research has been done on this
route of pollutant entry, partly because most systemic herbicides are not very
volatile (Edgington and Peterson, 1977).
Two processes precede the penetration of chemicals in the soil into leaf
tissue via the air; firstly, volatilization from the soil itself which is
34
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dependent on the soil type (see section 4.1); secondly, deposition from the
air onto the leaf surfaces. Uptake via vapour is, therefore, related to both
the volatility of the pollutant itself and its deposition velocity from the
air to the plant surface (Topp et al., 1986).
TABLE 3.1. PREVALENCE OF STOMATA ON THE SURFACES OF LEAVES OF
SOME REPRESENTATIVE CROP PLANTS
Number per cm2
Leaf Surface
Plant Upper leaf Lower leaf
Bean
Plum
Oat
Corn
Pea
40
0
25
94
101
281
253
23
158
216
The initial barrier to the penetration of volatilized chemicals into the
leaves and other aerial parts of the plant is the cuticle and its associated
structures. The cuticle, which is composed of four regions (the epicuticular
wax, the cutin, the pectin, and the outer wall of the epidermal cell), can be
described as a nonliving, noncellular membrane that covers all the outer
aerial plant surfaces. Its main function is to control the loss of water from
the leaves of the plant, a function achieved by wax platelets imbedded in the
upper surfaces of the cutin matrix. The area available for water loss, and
thus the area available for the entry of aqueous solutions depends on the
spacing of the wax platelets in the leaf.
Plants can regulate this spacing; if the loss of water from the leaf
surface exceeds the supply from the roots, the cutin will contract to bring
the platelets closer together and reduce the area available for water exchange
(Crafts, 1964).
Certain regions of the cuticle may act as sites of preferential entry
for gaseous pollutants, i.e., stomata, trichomes, the cuticle over leaf veins,
anteclinal walls, leaf bases, etc. The cuticle varies not only in thickness
and chemical composition according to species, leaf maturity, and surface and
position on the leaf, but also according to environmental conditions, e.g.,
temperature, relative humidity, and light. Vapour phase uptake of organic
pollutants is, therefore, likely to vary considerably between species. The
ecotoxicological importance of plant cuticles as a lipophilic sorption agent
has recently been pointed out {Riederer and Schonherr, 1985). Cuticular
material occurs 1n considerable amounts in both natural and agricultural plant
communities (180 to 1500 kg/ha.).
The older parts of plants, particulary perennials, are normally covered
with bark. This is a heavily suberized multicellular layer that provides an
35
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impervious covering except where the surface is damaged or is penetrated by
specialized structures like the lenticels. Although aqueous solutions can
enter uninjured bark to a small extent, it seems unlikely that this form of
entry is in any way significant when compared with root and shoot entry.
ROOT UPTAKE AND TRANSLOCATION OF POLLUTANTS
Uptake of most chemicals by plant roots from polluted soils is a passive
process. At its simplest, then, movement of pollutants from soil to plant
leaves can be regarded as a series of consecutive partitions between soil
solids and soil water, root and soil water, and then the transpiration stream
and the tissues of the plant root, plant stem and plant leaves. At any stage
in this transport, the chemical can be bound within the plant or metabolized.
Uptake has been seen to occur in two phases: firstly, chemical
partitioning onto the external root surfaces with rapid accumulation into the
free space of the root, and secondly, the slower process of moving across the
living cells of the cortex to reach the vascular system of the plant (Crowdy
and Jones, 1956).
Factors affecting the translocation of the chemical from root to shoot,
although being influenced by the water potential gradient created by the
leaves, are poorly understood (Brown et a!., 1983). The effect of many
herbicides on plants is known, however, to depend on transpiration rate
(Sheets, 1961).
Polar chemicals may pass through the lipid membranes in the root with
difficulty, whereas the unhindered passage of water results in the selective
rejection of these chemicals at the membrane barriers. Lipophilic chemicals,
on the other hand, being reversibly sorbed by the root solid, might be
expected to pass to the xylem unhindered once equilibrium is reached.
Plant roots are, additionally, not discriminating towards small organic
molecules with a molecular weight of less than 500, except on the basis of
polarity. If the molecule is nonpolar, it tends to adsorb to the roots'
surfaces rather than pass through the epidermis. The more polar the molecule
the more readily it will reach the root, pass through the epidermis, and be
translocated.
Because the first stage of uptake into the root is a passive process, it
can occur in both living and dead root tissue to equal degrees. Tames and
Hance (1969) investigated the extent of root sorption of herbicides by freshly
killed roots of a number of plant species. With atrazine, diuron, linuron,
monolinuron, and 6S14260, bean roots showed the greatest adsorptive capacity
on both a dry and fresh weight basis. There was variation among the other
tested plant species, including oat, pea, cucumber, and radish, with some
species adsorping more of different types of herbicides than did others.
There was no relationship between the adsorptive capacity of the roots and the
susceptibility of the plants to the tested compounds.
36
-------
This variation in compound uptake was shown earlier by Crafts (1964).
He investigated the uptake and translocation of several labeled herbicides
and, using autoradiographs, concluded that this probably resulted from some
organic chemicals being bound within the roots and, therefore, not available
for translocation.
Crowdy et al. (1956) reported that translocation of a chemical continued
even after the plant had been removed from the chemical so that the root must
be providing a reservoir of the chemical. No measurement of the new
equilibrium between the root concentration and the outside solution
concentration was made however, and this possibly was the cause of the
Recorded uptake.
Crowdy (1973) then demonstrated an inverse relationship between
translocation of a number of grisofulvin derivatives to the shoots of broad
beans and the partition coefficients of these compounds between hexane and
water. Crowdy concluded that there may be an optimum lipid/water partition
coefficient for maximum translocation, which is likely to vary between
different compounds, species of plant, and pathways of entry into the plant.
Shone et al. (1973) undertook similar research into herbicide uptake and
thus its effect. They investigated the absorption and translocation of the
herbicide simazine by 6 day old barley plants, in either 24- or 48-hour
experiments in water culture. To describe the relationship between simazine
transport and water uptake, they calculated the Transpiration Stream
Concentration Factor (TSCF), which was defined as
_ M9 simazine in shoots per ml water transpired
/ig simazine per ml uptake solution
In these experiments, water was taken up preferentially to simazine, since the
TSCF was always less than unity and there was no evidence of loss of, or
breakdown of, the parent compound. The concentration of simazine in the plant
roots, on a fresh weight basis, however rapidly reached a value greater than
unity probably as a result of physical adsorption of the herbicide on the root
tissue.
TSCF was assessed indirectly from the mass of chemical accumulated in
the shoots for a known volume of water transpired. TSCF is, therefore,
affected by those environmental conditions that directly affect translocation,
e.g., temperature, light intensity, and humidity, and if required, could be
increased or decreased by changing these environmental conditions.
For a nonpolar solute, which should not be affected by gradients of
electrical potential, values of the TSCF greater than unity would imply a
direct dependence of transport of the solute on metabolism.
In line with the definition of the TSCF, Shone and Wood (1974) proposed
that the uptake of a chemical into roots can be described by its Root
Concentration Factor. This is simply defined as
37
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concentration in root
concentration in external solution
Briggs et al . (1983) proposed the Stem Concentration Factor (SCF) as
concentration in stem
concentration in external solution
Shone and Wood (1974) undertook a series of investigations using
radiolabeled herbicides in solution culture with barley seedlings. They
showed that the quantity of the herbicide transported to the shoots could not
be inferred from the extent to which it is bound in the roots (Table 3.2). In
addition, although the RCF of some of the tested herbicides exceeded 1, actual
uptake was not affected by temperature, this suggests that the root retained
the compounds by physical sorption. Translocation from the roots to the
shoots did not then take place until the root was saturated by the compound.
TABLE 3.2. TYPICAL VALUES FOR TSCF* AND RCFf FOR BARLEY FROM A SERIES
OF HERBICIDES (Abbreviated from Shone and Wood, 1974)
Herbicide TSCF RCF
2,4-D at pH 4
Simazine
Diuron
Atratone
Atrazine
Hydroxyatrazine
3.12
0.90
0.81
0.78
0.75
0.26
88.4
4.5
3.1
1.3
1.9
2.2
2,4-D at pH 6.5
Ethirimol
0.14
0.09
8.1
0.7
'Transpiration stream concentration factor.
fRoot concentration factor.
Herbicides in solution culture at pH 5 to 6.5 unless otherwise stated.
For atratone, atrazine and hydroxyatrazine, the uptake solution contained
1 ppm. For the remainder, the solution concentration was 0.2 ppra.
This also worked in reverse. Shone et al. (1974) transferred the barley
seedlings from solution culture containing the herbicides under test to unpol-
luted cultures and found that RCF was decreased before TSCF was affected by
the change (Table 3.3). This suggests that the lipophilic herbicides, which
38
-------
appear to penetrate the cortical cells of the root, tend to reach the shoots
more readily than do the lipophobic herbicides, which may be largely confined
to the free space in the roots. With the exception of 2,4-D, the uptake and
translocation of all the tested herbicides was passive and was compatible with
passive movement in the transpiration stream. Uptake of 2,4-D at pH 4 ap-
peared related to the metabolic activity of the plant.
TABLE 3.3. LABELED HERBICIDES IN ROOTS AND SHOOTS OF BARLEY SEEDLINGS
(Adapted from Shone et al.» 1974)
Herbicide
After 24 hrs in 0.2 ppm
labeled herbicide solution
Shoots
Roots
After a further 24 hrs
in clean solution
Shoots
Roots
Simazine
Diuron
2,4-D at pH4
Ethirimol
0.23
0.25
1.49
0.07
0.08
0.08
1.92
0.02
0.22
0.25
1.35
0.09
0.03
0.01
1.10
0.01
Concentrations are given as #g herbicide in shoots and roots.
Hawxby and Easier (1976) reported that the more water soluble herbicide
dinitramine was translocated within the plant to a greater extent than the
less soluble profluralin. This is expected on the basis of the relationship
between water solubility and the n-octanol/water partition coefficient.
Uchida et al. (1982) found that the mobility of different classes of
pesticides in rice plants also correlated well with water solubility and n-
octanol/water coefficients.
These attempts at relating root uptake to some physicochemlcal parameter
of the test compound continued with Briggs et al. (1982) working with barley
roots. Root accumulation of different herbicides was directly related to n-
octanol/water coefficient of the tested compound whereas transpiration stream
concentrations showed a bell shaped dependence on Kow with a broad maximum
around 1.8 (Table 3.4). The general explanation of this is probably that at
Kow values below optimum, translocation is limited by root concentration; at
Kow levels higher than optimum, translocation is limited by the rate of re-
lease of the chemical into the transpiration stream.
All the TSCF in this experiment were below unity; this indicated that
the tested chemicals moved passively into the shoot with the transpiration
water and were not taken up against a concentration gradient. There appeared
an optimum lipophilicity for maximum uptake to shoots by translocation
centered on a log Kow of around 1.8.
39
-------
TABLE 3.4. THE RELATIONSHIP BETWEEN THE /7-OCTANQL/WATER PARTITION COEFFICIENT
(Kow) AND RCF" AND TSCFf FOR THE UPTAKE OF o-METHYLCARBAMOLYOXIMES AND
SUBSTITUTED PHENYLUREAS BY BARLEY FROM NUTRIENT SOLUTION
(Adapted from Briggs et al., 1982)
Chemical
Log Kov
RCF
"Root concentration factor
transpiration stream concentration factor
TSCF
o-Methylcarbamoyloximes
-0.57
-0,47
-0.13
1.08
1.49
2.27
2.89
3.12
4.6
Substituted phenyl ureas
Log KQW
-0.12
0.80
1.04
1.57
1.80
1.98
2.64
2.80
3.7
0.66
0.91
0.95
0.94
1.48
2.8
5.61
8.72
81.1
RCF
0.73
1.20
1.10
0.94
2.00
3.17"
5.86
7.08
34.9
0.19
0.21
0.28
0.54
0.67
0.94
0.51
0.26
0.06
TSCF
0.05
0.47
0.47
0.22
0.50
0.55
0.37
0.47
0.11
If the n-octanol/water partition coefficient and the RCF are
logarithmically regressed against each other, then equations similar to those
describing pollutant sorption in soils are obtained for the
methylcarbamoyloximes:
log RCF = 0.5591og Kow - 0.833
with a correlation coefficient of 0.96.
For the substituted phenylureas
log RCF = 0.5891og Kow - 0.812
40
-------
with a correlation coefficient of 0.85.
If the data sets are combined, then
log RCF = 0.571og Kow - 0.814
with a correlation coefficient of 0.917.
Briggs et al. (1982) report
log RCF (macerated roots) = 0.771og Kow - 1.52
for the best fit from seven chemicals, which were sorbed to a measurable
extent, with a correlation coefficient of 0.981. They continued by assuming
that RCF can be explained by a partitioning to lipophilic root solids and some
compound being translocated, the amount of which was taken to be constant for
all compounds tested. This gave
log (RCF - 0.82} = 0.771og Kow - 1.52
where the mechanism of the small uptake, contributing 0.82 to the RCF value,
arises from the equilibrium of the chemical between the external solution and
the water contained in the roots, both within the free space and within the
cells. If such an equilibrium were complete, a contribution of about 0.9 to
the RCF value would be expected for roots containing 90% water by weight. The
measured value of 0.82, therefore, suggests that the equilibrium was nearly
complete.
Unfortunately, few other attempts have been made to relate plant uptake
to either physical or chemical properties of the pollutant. Topp et al.
(1986) also related log RCF to log Kow following the exposure of barley
seedlings for 7 days to various pollutants with a log Kow range from 2 to 6.
Their equation
log RCF = 0.631og Kow - 0.959
with a regression coefficient of 0.896, is very similar to that of Briggs et
al. (1982) above.
Much of this early work, primarily designed to improve the performance
of herbicides, has not been repeated with those organic chemicals that
currently exist as pollutants. This is urgently needed so that the
environmental behaviour of new organic chemicals, or those currently existing
as pollutants, can be determined without the need for complex and time
consuming investigations.
The above discussion is concerned with uptake from nutrient solution.
Walker (1972) highlighted the problems of measuring plant uptake of chemicals
from the soil. He found that the concentrations of the herbicide atrazine in
the shoots of wheat plants growing in 12 different soils were directly
41
-------
proportional to the soil solution concentration of the herbicide estimated
from slurry adsorption measurements. A large discrepancy existed between the
total uptake of herbicide and the amount theoretically supplied by mass flow
in reponse to transpiration, since only the nonadsorbed portion of the
pollutant dissolved in the soil water was available to the plants. In an
experiment where only one soil was used, and the half life of the chemical was
taken into account, Walker could make a much closer prediction of atrazine
uptake.
The potential for uptake of a chemical from the soil solution is then
the same as that from a nutrient or hydroponic solution. Moyer et al. (1972)
started out with the premise that a plant's herbicide uptake was related to
the volume of water transpired (as shown by Sheets, 1961, and Shone and Wood,
1972) and, in turn, showed this reponse with the herbicide diuron.
Uptake from the soil is, therefore, related to the pollutant
concentration in the soil solution and not to the concentration in the soil,
per se. Since, for nonionic compounds, adsorption is largely on the organic
matter in soil, the uptake of chemicals into plant roots should be dependant
also on the organic matter content of the soil.
Model 1i ng
If the above is taken at face value, to cover the behaviour of all
nonionic organic pollutants, then the behaviour of a pollutant in the soil can
be related to its behaviour in the plant.
For a given compound, the organic matter/water distribution (see Section
2.1) which is relatively constant from soil to soil, is related to the n-
octanol/water distribution by;
log Kom = 0.521og Kow + 0.62
As log Kow increases, adsorption increases and, hence, soil solution concen-
tration decreases for a given concentration of pollutant (Briggs et al. 1976).
For a soil with a given organic matter and water content, the fraction of the
pollutant in the soil solution can be calculated from its log Kow. The
product of this fraction and the TSCF appropriate to the value of log Kow
gives the relative ease of translocation from soil. This is shown in Table
3.5 for a soil with 2% organic matter and 15% water content.
The optimum for uptake from soil is given by compounds with a log Kow
around 0.5, which is lower than the optimum log Kow value of 1.7 for uptake
from solution (Briggs et al., 1976).
42
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TABLE 3.5. A COMPARISON OF THE POTENTIAL FOR ROOT AND SHOOT UPTAKE OF
DIFFERENT POLLUTANTS FROM THE SOIL SOLUTION OF A SOIL OF 2% ORGANIC MATTER AND
15% WATER CONTENT AS MEASURED BY THE TSCF* AND THE RCFf
(From Briggs et al., 1976).
log K0
TSCF
RCF
0
1
2
3
4
5
6
0.65
0.95
1.00
0.70
0.05
0.01
<0.01
1.0
2.6
5.7
13.8
33.0
79.0
190.0
"Transpiration stream concentration factor
TRoot concentration factor
If the calculations are continued, the effects of increasing log Kow can be
determined, i.e., the effects of increasing lipophilicity upon the net root or
shoot uptake from a particular soil. This is shown in Tables 3.6 and 3.7.
TABLE 3.6. THE EFFECT OF LIPOPHILICITY ON RCF* AND NET ROOT UPTAKE FOR A
SOIL1
(Calculated from Briggs et al., 1976.)
log K0
RCF
% chemical in water
net uptake
0
1
2
3
4
5
6
7
1
2.6
5.7
13.8
33.1
79.4
190
457
65
35
14
5
1
0.25
0.10
0.04
(RCF x % in
65
84
80
70
33
20
19
18
water)
"Root concentration factor.
'Soil containing 2% organic matter with a 15% water content.
43
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TABLE 3.7. THE EFFECT OF LIPOPHILICITY ON TSCF* AND NET UPTAKE FOR A SOILf
(From Briggs et al., 1976.)
log Kow TSCF % chemical in water net uptake
(TSCF x % in water)
-0.5
0
0.5
1
2
3
4
5
6
0.2
0.65
0.85
0.95
1.0
0.7
0.05
ca 0.01
<0.01
80
65
50
35
14
5
1
0.25
0.1
16
42
43
33
14
3.5
0.05
0.0025
<0.001
"Transpiration stream concentration factor.
TSoil containing 2% organic matter with a 15% water content.
Achieving these values would take time because they are the end points of
various equilibria. For example, there is an equilibrium between the pollu-
tant concentration adsorped on the soil organic matter and that in the soil
solution. The pollutant concentration of the soil solution is, in turn, in
equilibrium with that within the plant. These equilibria will move toward the
plant as the pollutant is translocated away from the root and soil and into
the shoot.
Topp et al. (1986) reported that, under environmental conditions, an
equilibrium of chemical concentration between soils and plants is reached only
very slowly or not at all. Figure 3.1 shows the time course of the uptake by
barley of two chlorinated benzenes from soil in outdoor boxes under field
conditions.
Figure 3.1 illustrates that the two chemicals reach an equilibrium,
i.e., a constant concentration factor, only after 100 days or more, at the
time of harvest when growth stops and the plants become dry. In the earlier
growth stages, concentration factors decrease due to dilution by the increase
in plant mass and through the loss of the chemical to transpiration exceeding
continuing uptake from the soil.
PLANT UPTAKE BY VAPOUR
For some of the more volatile herbicides and pollutants, diffusion in the
vapour phase and subsequent uptake by the shoot, both before and after emer-
44
-------
gence, may be an important route of chemical entry to the plant (Parker 1966;
Prendeville 1968). Barrows et al, (1969), for example, said that aerial
contamination explained the differences found in their experiments with diel-
drin uptake by corn grown in either the field or the greenhouse.
•t BARLEY CONC. FACTOR
70-
6-0-
5-0-
4-0-
3-0-
2-0-
1-0-
0-1-
Figure 3.1.
x PENTACHLOROBENZENE
0 HEXACHLQROBENZENE
10
50
too
TIME
(DAYS)
The time course of uptake of two chlorinated benzenes by barley,
(From Topp et al.» 1986.)
Seal! and Nash (1971) developed a method to discriminate between a
pesticide's movement through the plant vascular system and a vapour phase
movement. They found soybean shoots were contaminated by soil-applied
dieldrin, endrin, and heptachlor largely via root uptake and subsequent
trans!ocation. Vapour phase movement, however, dominated for DDT and was
nearly seven times greater than for root sorption. Foliar contamination from
vapour sorption of residues from all four insecticides was of the same order
of magnitude, about 6.5 ppm plant dry weight, whereas contamination from root
sorption varied from 38 ppm to 1 ppm, depending upon the insecticide.
Using a similar method Fries and Marrow (1981) found that PCBs reached
the shoots of plants via the vapour phase rather than from root uptake and
trans!ocation.
In a laboratory greenhouse study investigating the uptake of 16 organic
chemicals into barley, Topp et al. (1986) found that foliar uptake of the
volatilized chemical from the air far exceeded translocation of the substance
taken up by the roots to the shoots. The relationship between barley foliar
uptake and volatilization from soil after 7 days exposure was
FU = 46.11 - 28.951og VOL
45
-------
where FU was foliar uptake in percent of total 14C-uptake, and VOL was the
organic 14C trapped from the air plus that sublimated on the exposure chamber
walls in percent of 14C originally applied. The correlation coefficient for
this relationship was 0.988 (Figure 3.2). Eleven of the tested chemicals
fitted the curve; four did not. These latter chemicals were substances whose
14
C was taken up preferably after mineralization to CO
VOL,
Io4 VOUmUZAHOM FROM SOIL. VOL.
-15
-to -as -ai 04
Figure 3.2. Correlation of barley foliar uptake after 1 week exposure with
volatilization from soil. (From Topp et al., 1986).
Shone and Wood (1976) investigated the reverse, that is, the potential
for triazine herbicides to be lost from the leaf surface by volatilization.
They applied labeled simazine, atrazine, and atraton to the hypocotyls of
radish plants. After 24 hours, a negligible quantity had been translocated to
the roots and over 95% of all three compounds could be recovered from within
the plant. A similar experiment, under the same conditions and using the same
concentration of atrazine, showed that over 85% applied to a glass rod was
lost, presumably by volatilization.
If vapour phase uptake of volatilized chemicals is related to cutin and
wax composition and to cuticle thickness, then vapour phase uptake will be
species dependent as these factors are variable among different species. Such
differences in the foliar absorption characteristics of different species have
been reported. Riederer and Schonherr (1985) investigated the diffusion of
2,4-D across plant cuticles from 10 species and found it to vary. Leece
(1976) related the foliar absorption of chemicals by peach, apple, and orange
to cuticle thickness, weight, surface wax, and embedded wax content, and to
surface wax wettability, ultrastructure, and composition (Table 3.8).
Peach surface waxes were more difficult to wet than were orange waxes,
and although more polar than orange waxes, they may be more resistant to water
penetration because they are rich in hydrocarbons and triterpenoids.
46
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TABLE 3.8. THICKNESS, WEIGHT AND WAX CONTENT OF CUTICLES ISOLATED FROM PEACH,
APPLE, AND ORANGE LEAVES*. (Adapted from Leece, 1976.)
Cuticle
Peach Apple Orange
Thickness, urn
adaxial
abaxial
Weight, tig/cm2
adaxial
abaxial
Surface wax, ng/cm2
adaxial
abaxial
Embedded wax, tig/ 'cm2
adaxial
abaxial
1.6
2.0
238
270
35
71
70
50
2.1
2.9
347
560
31
47
65
68
4.1
3.9
448
430
18
12
43
48
"Peach leaf area was 61 cm2; apple, 40 cm2; orange 29 cm*1
Topp et al. {1986} reported results showing that qualitative composition
is probably more important than thickness for cuticle penetration, cutin, and
wax. Surface wax concentration correlated well with resistance to foliar
absorption.
In general, any modification of the molecular structure that results in
increased "lipid solubility will tend to enhance cuticular or membrane penetra-
tion; however, this is not always the case. Normally, the nonpolar deriva-
tives of a variety of chemicals penetrate the cuticle or other membranes more
readily than do polar ones.
WHOLE PLANT UPTAKE
Any plant part exposed to an organic pollutant has the potential of
sorbing and or translocating that pollutant to other parts of the plant. This
is also the case with the seed of the plant. Much work has been done on both
the uptake of chemicals from the soil by the seed coat and the transfer of the
chemical from the parent plant to the seed to ultimately affect the develop-
ment of the offspring (Edgington and Peterson, 1977).
However, many chemicals apparently are significantly phytotoxic when
applied to the seed; the concentration is too high for the germinating
seedling. Thus, it is very difficult, if not impossible, for the seed to
accumulate enough pollutant to make any significant difference to the total
47
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soil loading. Any chemical that is sorbed by the seed will, in turn, be
diluted by the plant mass as the plant grows.
As both root and foliar uptake of pollutants actually implies membrane
penetration by the pollutant, a process related to molecule size, it should
also be possible to relate uptake to pollutant molecule size. Topp et al.
(1986) reported
log CF - 5.943 - 2.3851og M
with a correlation coefficient of 0.949, and where CF is the whole plant
Concentration Factor from barley seedlings being exposed to the pollutants for
7 days, and M is the molecular weight of the pollutant. This relationship
(Figure 3.3), applied to volatile as well as nonvolatile compounds including
extremely complex pigments, was good enough for the authors to conclude that
molecular weight is probably more suitable for predicting plant uptake than is
log Kow. This relationship was, however, based on compounds with a relatively
narrow range of molecular weights, from 400 to 800; further investigation with
a wider range of chemicals is needed.
leg CF= 5-343-2-385 x tog M
n=U
, -0.1 T
2.0
log MOLECULE WEC-KT, M
Figure 3.3. Correlation of barley concentration factors {based on soil
concentration) with molecular weights after 1 week exposure.
(From Topp et al., 1986.)
48
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BEHAVIOUR OF POLLUTANTS IN PLANTS
Partitioning
Pollutants or pollutant residues absorbed and translocated in plant
tissues may be present in three possible forms: freely extractable residues,
extractable conjugates bound to natural components of plants, and
unextractable or bound residues incorporated into the plant constituents. The
latter may be considered analogous to the bound residues in soil (Khan 1982)
and could be the cause of many underestimates of herbicide uptake by plants
when inefficient extraction procedures were used (Lichtenstein, 1980; Wheeler
ft a!., 1969).
Haque et al. (1978), for example, planted rice seedlings into soil
containing radioactive pentachlorophenol and found that after 1 week, the
plants had absorbed about 3% of the applied radioactivity, of which half was
bound within the plant and could not be removed by normal extraction
procedures. In fact, when these plants were fed to mammals the residues were
excreted nearly quantitatively, which suggests that the residues were also
unavailable to the mammals.
Klein and Scheunert (1982) concluded that plant growth conditions are an
important factor affecting the formation rate of bound pesticide residues in
plants. Optimum growth conditions which gave high growth yields, resulted in
high levels of bound residues; conversely, poor growth conditions resulted in
low levels of bound residues. Most of these bound residues were located and
localized in the lignin.
Examples of the extent of bound residues are shown in Table 3.9,
TABLE 3.9. PESTICIDE RESIDUES BOUND IN PLANTS, IN PERCENT OF TOTAL RESIDUES
IN THE PLANT. (Adapted from Klein and Scheunert, 1982.)
Chemical class Time of exposure % Residue bound
Free phenols
Anilines
Triazines
Urea herbicides
Cyclodiene
1 vegetation period
1 vegetation period
20 - 100 days
48 - 105 days
1 vegetation period
29 -
87 -
20 -
46 -
1 -
38
90
63
72
2
Once uptake of a chemical is under way it can occur at quite a rapid
rate. Sheets (1961), for example, reported that because of root uptake and
translocation, the herbicide simazine was found throughout seedling oat plants
only 3 hours after being placed within a simazine-amended nutrient solution.
Actual accumulation of the herbicide in the leaf tips of the plants began
within 48 hours of exposure and depended on the translocation rate of the
plants.
49
-------
Briggs et al. (1983) proposed the concept of the Stem Concentration
Factor as
SCF =
concentration in stem
concentration in external solution
to assist in explaining their results following uptake, over 96 hours, of four
nonionized chemicals in nutrient solution into barley shoots. The four chemi-
cals were selected to span a wide range of lipophilicity. They found that,
with the exception of the most lipophilic chemical studied, the amount of
chtmical in the basal and central stem section remained constant after 24 to
48 hours, whereas the chemical concentration in the top portion of the shoots
continued to increase for all compounds up to 72 to 96 hours (Figure 3.4).
tor (fl)
u.
-------
positively related to lipophilicity, i.e., the more lipophilic chemical took
longest to start to decline.
These results suggest that the barley stem is acting as "a rather
inefficient chromatography column," with the concentration of the chemical in
the lower portions of the stem being determined by a reversible partition
between the xylem sap and the adjacent stem. Although small amounts of chemi-
cal reach the tops of the leaves before the stem chemical concentration reach-
es a constant maximum, it is only after the partition requirements of the stem
are filled that the chemical being translocated from the roots really begins
reaching the tops of the leaves and accumulating there as water is transpired.
Although water is also transpired by the leaf sheaths, the translocated chemi-
cal does not appear to accumulate there, probably because the chemical equi-
librium between stem and xylem sap is rapid.
In a further experiment Briggs et al. (1983) related the SCF of
macerated stem material to the n-octanol/water partition coefficients of 15
chemicals by
log SCF (macerated stems) = 0.951og Kow - 2.05
with a regression coefficient of 0.98. This equation is similar to that
discussed earlier within this report for absorption by macerated roots.
For the more lipophilic chemicals, absorption by the stem solids
increases rapidly with increasing lipophilicity. The time taken for the whole
of the stem to equilibrate with the concentration of the chemical entering in
the transpiration stream is also positively related to increasing
lipophilicity. Although the TSCF determines the chemical concentration in the
transpiration stream it does not affect the time taken to reach equilibrium as
this is independent of concentration. If SCF is plotted against log KQW
(Figure 3.5), a maximum of 6 for the SCF is reached at a log Kow of around
4.5. This arises as the increasing absorption potential for the more
lipophilic chemicals is balanced out against their decreasing potential to be
translocated.
Very few studies have been carried out to determine whether a pollutant
within a plant is adsorbed onto the cell surface or is collected internally
within the cell. Ware et al, (1968) found high concentrations of DDT and its
related degradation products in alfalfa roots grown in the field. On further
inspection, they found that the thin epidermal layer of the root contained
nearly five times the level found in the whole roots and approximately six
times that found in the cortex. This suggests that the DDT or its degradation
products, or both, reach the root surface and either become bound to the
epidermis and thus cannot be moved inward, or that the root structure is such
that it actually filters or screens out the pollutant.
Similar results were found by Saha and Stewart (1967) who reported that
the peel of rutabaga contained 98% of the total plant residue resulting from
soil-applied heptachlor. In addition, Smith et al. (1967) showed that Dursban
is not absorbed into the root but accumulates on the root surface.
51
-------
Beestman et al. (1969) investigated the translocation and sites of
accumulated radio!abeled dieldrin in corn. They found the stalks below the
fifth node were the primary site of dieldrin localization and contained 70% to
90% of the total dieldrin. Accumulation was in the order: lower stalk >
lower leaves > upper stalk > upper leaves > ears.
When C-dieldrin was translocated up from the roots into the shoots, it
was found to be located in the apoplast tissues {i.e., mechanical tissue cells
or on their cell walls) and in the xylem tissue. None of the soil-applied
carbon labeled dieldrin was found in the phloem vessels or symplast. In
addition, the dieldrin concentrations increased as the edge of the leaf was
approached (Cotner et al., 1968).
10-Qr
Log
Figure 3.5. The relationship between the Stem Concentration Factor of chemi-
cals in barley and their n-octanol/water partition coefficients
(as log Kow). Values are the mean of 24 and 48 hour measure-
ments. (From Briggs et al., 1983.)
O'Donovan and Vanden Born (1981) used a microautoradiographic technique
to determine the distribution of 14C-labeled picloram in soybean tissue
following root uptake. Little radioactivity was retained in the roots and
this appeared to be mainly associated with the protoplasm of the root cortical
52
-------
cells. In the stem, radioactivity was primarily present in both the xylem and
phloem.
Considerably more radioactivity was found in the young apical leaves,
both in the xylera and phloem, than in the older primary leaves, where
radioactivity was present only in the xylem.
HcFarlane (1986, personal communication) investigated the distribution
of four 14C ring-labeled test chemicals in soybeans grown under amended
hydroponic conditions for 3 days. Bromacil was originally distributed in the
plant roots and then, with time, moved into the stems and leaves. Its
concentration in the leaves and roots then remained approximately equal and
was probably in equilibrium. A similar early pattern was found for
diclobenil, but with time the chemical concentration in the root decreased
whereas that in the leaves increased. This indicates movement of the chemical
from the roots for deposition in the leaves. Nitrobenzene and dinitrobenzene,
the other two chemicals, remained primarily associated with the plant roots
and, possibly because of the short time period available for translocation,
were not moved upwards to any significant extent (Figure 3.6).
toQ.
!\
T5? \
I j W
a I v
u so- V-
c
u
a
BROMACIL
!
0%.
toots
N8
ion
100-
25 50 75 100 0
SO
HOURS
Figure 3.6. Chemical distribution patterns within whole soybean plants, shown
as a percent of total chemical in the plants with time. The
compounds were bromacil, DCBN - dichlorobenzonitrile, DNB -
dinitrobenzene, and NB - nitrobenzene. (From McFarlane, 1986,
personal communication.)
53
-------
Degradation
Because knowledge of the metabolism of herbicides within plants has been
of great importance in ensuring the efficiency of herbicides (Geissbuhler et
al., 1963), much basic research has been undertaken to investigate this.
Unfortunately because there has been little impetus for research into
pollutant metabolism by plants, there is a corresponding lack of information.
Research into herbicide metabolism goes back many years. Nash (1968),
for example, reported diuron was metabolized within a plant to its monomethyl
derivative 3-(3,4-dichlorophenyl)-l-methylurea. Hamilton and Moreland (1961)
reported that simazine is converted to a detoxified hydroxysimazine in vivo by
corn plants and in vitro by corn extracts. Ware et al. (1968) reported DDT
metabolism to DDE by plant tissue, and Smith et al. (1967) showed that 3,5,6-
trichloro-2-pyridinol undergoes metabolism in cranberry plants with the liber-
ation of chlorine and the formation of several decomposition products.
Freed and Montgomery (1963) and Crafts (1964) have reviewed the
metabolism of herbicides in plants under sections of the major herbicide
types, phenoxy acids, the carbamates, and symmetrical triazines,
1, The phenoxy acids are some of the earliest commercial herbicides and
have a wide range of weed control in grains and grasses. There is much evi-
dence that the 2,4-D molecule, the most common member of this group, can be
metabolized in plants, and in many cases, the evolution of 14C02 has been
shown after treatment of plants with radiolabeled 2,4-D. Differences in
response to 2»4-D in red and black currants and varieties of apples and straw-
berries were strongly correlated with differences in the abilities of the
plants to oxidize the carboxyl and methylene carbons from the side chain of
the molecule. Red currant which is tolerant of 2,4-D, oxidized up to 50% of
the carboxyl carbon and 20% of the ethylene carbon of the 2,4-D, whereas the
nontolerant black currant only oxidized 2%, under the same conditions. The
tolerant apple variety, Cox, could decarboxylate 57% of the applied 2,4-D in
92 hours, whereas the sensitive Bramley Seedlings metabolized only 2%. Simi-
lar studies have shown that bean stems, peas, and cucumber species are able to
metabolize 2,4-D probably along metabolic lines similar to those of bacteria,
which partially degrade the benzene ring without cleaving the phenol ether
linkage. Plant roots are generally more efficient in the decarboxylation of
phenoxyacetic acids than are shoots.
2. The carbamates, recognised as mitotic poisons, inhibiting the Hill
reaction of photosynthesis, have their lethal action in root meristems. The
herbicide EPTC was rapidly taken up by resistant plants and metabolized so
that an applied radio!abel was incorporated into various plant constituents,
e.g., cystine.
3. Symmetrical triazines, simazine and atrazine being the most popular,
have a wide spectrum of selective biological activity. Again, there appears
to be a metabolic difference between resistant and susceptible, plant species,
with the resistant species having a metabolic pathway that effectively
detoxifies the herbicide. This pathway may result from the presence of a
54
-------
cyclic hydroxamate that reacts with the triazine in the sap of resistant
plants. Montgomery and Freed (1961) reported that appreciable amounts of
C02 were given off by intact corn plants after being exposed to the
radiolabeled herbicide. In addition, the corn was able to rupture the benzene
ring, where the label had been applied, to completely oxidize the fragments.
In summary, both groups of workers concluded that metabolism of most of
the major groups of herbicides occurs within plants following their uptake and
that such metabolism may play a major part in the biological activity of such
compounds.
Finlayson and MacCarthy (1973) have compiled the metabolic and other
degradation products of pesticide residues in plants and soils (Table 3,10).
Eastin and Easier (1977) reviewed the techniques available for assessing the
absorption and translocation of pesticides.
The potential of different plant species to degrade herbicides seems to
be related to their tolerance or susceptibility to the herbicide, with
tolerant species generally having some form of metabolic protection (Mottley
and Kirkwood, 1978).
Shimabukuro (1968) compared the metabolism of the herbicide atrazine in
the two resistant species, corn (Zea mays) and sorghum (Sorghum vulgare), and
found even different metabolic pathways. In corn, atrazine was metabolized
via both the 2-hydroxylation and N-dealkylation pathways; in sorghum, only the
latter pathway was used. Considerably more research is needed in these areas
of breakdown and associated plant protection through genetic tolerance to the
herbicide.
Plants also use a variety of reactions to reduce more complex aromatic
structures to simpler units. Typical steps Include demethylation, ft oxid-
ation, and decarboxylation, (Ellis 1974). Plants are known to accumulate
large quantities of aromatic compounds, principally phenolics, ranging in
structure from simple phenols to polymers such as lignins; some are known to
be ring cleavage substrates in microbial metabolism. Ellis and Towers (1970),
using sterile cultures, showed that Ruta graveoleus and MeliTotus alba were
able to cleave the aromatic ring of both phenylpropanoid and indole compounds
but this cellular reaction may not be typical of that occurring in the intact
plant (Berlin et al., 1971).
Many disagree on the role of polyaromatic hydrocarbons in plants. As
early as 1966, Graf and Diehl published results showing the existence of
several polyaromatic hydrocarbons in various plants. They suggested that
these compounds were naturally synthesized in the plants and may even act as
growth hormones. Wagner and Siddiqi (1970, 1971), however, do not believe
that plants can form benzo(a)pyrene or benzo(6)fluoranthene. They did not
find any aromatic compounds in lettuce, rye, soybean, or tobacco grown in
carefully filtered air, but did find these compounds in plants grown in the
field. They suggested, therefore, that these chemicals were taken from the
air.
55
-------
TABLE 3.10. METABOLIC AND OTHER DEGRADATION PRODUCTS FROM PESTICIDE RESIDUES
IN PLANTS AND SOILS. (From Finlayson and MacCarthy, 1973.)
Pesticide
Substrate
Plants Soil
Products
Aldicarb 4-
Aldrin +
Amitrole +
Captan +
Carbaryl +
Carbofuran +
DDT +
Disulfoton +
Endosulfan +
Fensulfothion +
Heptachlor +
Hexachlorocyclohexane +
Linuron +
Pentachloronitrobenzene +
Phenoxyacetic compounds +
Phorate +
Trifuralin +
Vegadex +
Zectran +
+
+
+
?
+
?
+
+
+
+
sulfoxide, sulphone, oxime
dieldrin
several
thiophosgene
a-naphthol
3-hydroxy, 3-keto, others
DDE and others
sulphoxide, sulphone
sulfate, others
sulfone, oxygen analog
epoxide
pentachlorocyclohexane
3,4-dichloroaniline
pentachloroanil ine
several
sulfoxide, sulfone
several
lactic acid
several
Avocado fruit has also been shown to metabolize the relatively inert
hydrocarbons benzene and toluene, both of which occur naturally within the
fruit. Toluene is metabolized to a greater extent than benzene by fruit
exposed to hydrocarbon vapour, but both were metabolized, to a small but
significant extent, to C02 (Jansen and Olsen, 1969), Durmishidze and
Ugrekhelidze (1969), quoted in Sims (1982), demonstrated the cleavage of the
carbon atoms in the benzene ring into organic and amino acids. The first
stable products.of benzene metabolism in tea plants were organic acids, which
accumulated in the plant part where the benzene was introduced (roots and
stems). A pathway for benzene metabolism in plants was proposed as
benzene-->phenol-->pyrocatechol-->o-benzoquinone-->muconic acid.
Ourmishidze et al. (1973), also quoted in Sims (1982), studied the
assimilation and conversion of 3,4-benzpyrene-l,2-14C with 14 day old, sterile
corn and bean seedlings in a sterile nutrient solution. When the radio!abeled
compound was introduced into the leaves or roots of the plants, approximately
50% of the label was assimilated into organic acids, which were concentrated
at the site of assimilation. In a follow-up experiment with a wider range of
plant species, also quoted in Sims (1982), organic acid radioactivity ranged
from 5.4% to 56.5% of the total radioactivity of the root biomass and from
2.1% to 62.2% of the total radioactivity in the leaf biomass. Radioactivity
56
-------
was also incorporated into amino acids (up to 18% of the total radioactivity)
and into carbon dioxide (up to 9%).
A series of investigations at the Cold Regions Environmental Engineering
Labs investigated the response of terrestrial plants to 2,4,6-trinitrotoluene
(TNT), Following the growth of yellow nutsedge, Cyperus esculentus, in hydro-
ponic solution containing various concentrations of TNT, TNT and its metabo-
lites 4-amino-2,6-dinitrotoluene (4-ADNT) and 2-amino-4,6-dinitrotoluene (2-
ADNT) were found throughout the plants. Since TNT was the only compound in
the nutrient solution, the metabolites must have been formed within the plant.
Levels of 4-ADNT exceeded those of 2-ADNT and TNT itself, ranging up to 2200
ing/kg in the roots of plants grown in 20 mg/1 of TNT. Increasing the solution
concentration of TNT increased the concentrations of all three compounds in
the plants (Palazzo and Leggett, 1986).
There is little good evidence that plants can degrade organo-chlorine
chemicals. Davis (1984), however, suggests that this is a possibility and
that there may be many more chemicals available for degradation than was once
considered. He concluded the following.
1. Plants can sometimes transform chemicals more extensively than can
other organisms, possibly because of longer exposure periods than other organ-
isms,
2. Dechlorination has been listed as a mechanism of metabolism for
several pesticides, although specific pathways involved have not been eluci-
dated.
3. Dehalogenation bond cleavages have been attributed to peroxidases in
plant tissues. Peroxidases are ubiquitous in the plant kingdom and are found
throughout the plant cell. This enzyme has also been found in increased quan-
tities in cells selected for increased tolerance to paraquat.
4, Plants synthesize many aromatic compounds and are also capable of
degrading them. Plants are capable of the ring fusion reactions required to
complete catabolism of aromatic nuclei to C02.
5, Cell suspensions of purple cockle, carrot, clover, tobacco, lettuce,
and parsley were found to metabolize lindane. Carrot cultures metabolized up
to 6.8% in 12 to 68 days. The main metabolite was tentatively identified as a
glucoside of trichlorophenol.
Harms and Langebartels (1986) reported on a rapid bioassay technique
designed to investigate plant cell breakdown of various chemicals in cell
suspension culture. Their results from the bioassay (Table 3.11) showed that
even the most persistent chemicals were catabolized and metabolized in
suspensions of either soybean or wheat cells. The predominant fractions found
were polar conjugates and nonextractable, i.e., bound residues, and there were
differences between the species. From this, it can be seen that if a chemical
enters the plant, either by the roots or shoots, and then enters a plant cell,
it will be changed.
57
-------
TABLE 3.11. THE BEHAVIOUR OF ORGANIC CHEMICALS APPLIED TO CELL CULTURES OF
SOYBEAN AND WHEAT (Adapted from Harms and Langebartels, 1986.)
Chemical
% of recovered radioactivity
SoyJ)eaji Wheat
Cell-Totalf Changed1 Cell-Total7 Changed1
2 , 4-di chl orophenoxy-
acetic acid
4-chloroanil ine
3,4-dichloroaniline
pentachlorophenol
di ethyl hexyl phthal ate
perylene
benzo(a)pyrene
31
21
24
59
63
55
39
16
19
19
40
12
52
36
97
86
90
93
92
91
91
90
83
81
93
20
10
37
"Applied at 1 mg/1, for 48 hours to the logarithmic growth phase
f'Cell Total' refers to the amount of radioactivity recovered from the cell
mass within the suspension culture, as a percent of the total recovered from
cell mass plus nutrient solution.
''Changed' refers to the percent of the total recovered that was not present
in the cell in its original applied form, i.e., the percent that had been
metabolized or catabolized by the cells.
A common criticism of much of this plant degradation work is that there
is virtually no assured method of eliminating microorganisms from the
experimental system and this could lead to false conclusions (Ellis and
Towers, 1970). Many chemicals, however, are liable to degradation if they can
be collected within a plant cell. Such degradation could be a valuable
technique in destroying long-lived pollutants and deserves considerable
further research investigation.
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Sheets, T.J. 19"61. Uptake and distribution of simazine by oat and cotton
seedlings. Weeds, 9(1):1-13.
Shimabukuro, R.H. 1968. Atrazine metabolism in resistant corn and sorghum.
Plant Physiol. 43:1925-1930.
Shone, M.G.T., D.T. Clarkson, J. Sanderson, and A.V. Wood. 1973. A comparison
of the uptake and translocation of some organic molecules and ions in higher
plants. In: Ion transport in plants, W.P. Anderson(ed.). Academic Press,
London & N.Y. Pp 571-582.
Shone, M.G.T., B.O. Barlett, and A.V. Wood. 1974. A comparison of the uptake
and translocation of some organic herbicides and a systemic fungicide by
barley; ii Relationship between uptake by roots and translocation to shoots.
J. Exp. Bot. 25(85}:401-409.
61
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Shone, M.G.T., and A.V. Wood. 1976. Uptake and translocation of some pesti-
cides by hypocotyls of radish seedlings. Meed Res. 16:229-238.
Shone, M.G.T., and A.V. Wood. 1974. A comparison of the uptake and transloca-
tion of some organic herbicides and a systemic fungicide by barley: I Absorp-
tion in relation to physico-chemical properties. J. fxp. Bot. 25(85):390-400.
Sims, R.C. 1982. Land treatment of polynuclear aromatic compounds. Ph.D.
Thesis, N. Carolina University, Raleigh, N.C.
Smith, G.N., B.S. Watson, and F.S. Fisher. 1967. Investigations on Dursban
insecticide. Metabolism of 0,0-diethyl 0-3,5,6~trichloro-2-pyridyl phosphoro-
tMioate and 3,5,6-tr1chloro-2-pyridinol in plants. J. Agric. Food Chem.
15{5):870-877.
Tames, R.S., and R.J. Hance. 1969. Plant Soil 30:221-226.
Topp, E., I. Scheunert, A. Attar, and F, Korte. 1986, Factors affecting the
uptake of 14C-labelled organic chemicals by plants from soil. Ecotoxicol.
Environ. Safety 11:219-228.
Uchida, M., H. Nishizawa, and T. Suzuki. 1982. Hydrophobicity of buprofezin
and flutolanil in relation to their soil adsorption and mobility in rice
plants. J. Pesticide Sci. 7:397-400.
Wagner, K.H., and I. Siddiqi. 1971. Die speicherung von 3,4-benzfluoranthen im
sommerweizen und sommerroggen, Z. Pflanzenernahr, Bodenkd. 130:241-243.
Wagner, K.H., and I. Siddiqi. 1970. Der stoffwechsel von 3,4-benzpyren und
3,4-benzfluoranthen im sommerweizen. Z. Pflanzenernahr, Bodenkd, 127:211-219.
Walker, A. 1972. Availability of atrazine to plants in different soils. Pes-
tic. Sci. 3:139-148.
Ware, G.W., B.J. Estesen, and W.P. Cahill. 1968. An ecological study of DDT
residues in Arizona soils and alfalfa. Pestic, Monit. Jour. 2(3):129-132.
Wheeler, W.B., H.A. Hoye, C.H. van Middenlem, and N.P. Thompson. 1969. Resi-
dues of endrin and DDT in turnips grown in soil containing these compounds.
Pestic. Monit. Jour. 3(2):72-76.
62
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SECTION 4
VARIATIONS IN POLLUTANT UPTAKE BY DIFFERENT PLANT SPECIES
Oust as different pollutants are sorped in soils and accumulated in
plants to different degrees, so will different plant species accumulate the
same pollutant to different degrees. This variation in reponse has great
significance in the selection of plants to use in any In situ plant cleanup
system of polluted soils. Obviously, if one species accumulates a greater
quantity of pollutant from the soil in a shorter time period than do other
plant species, it has many benefits and could be used to optimize a cleanup
system.
Any organic compound or pollutant in the soil affects vegetation in two
broad ways.
1. At low concentrations, the compound partitioned into the soil
solution or into the gaseous phase is available for uptake by the vegetation.
2. At high concentrations, a phytotoxic response may occur.
The magnitude of both of these plant responses depends on both the organic
chemical and the vegetation species.
Phytotoxicity can occur (a) when the foliar portion of the vegetation is
exposed to the organic compound either directly through its application or
indirectly through its volatilization from the soil, or (b) when the root
system contacts the compound. The applied concentrations at which
phytotoxicity occurs is typically different for foliar versus root contact, as
well as different for different vegetation species.
Phytotoxicity can influence the effectiveness of a plant cleanup system.
If the plant is killed, or its growth is restricted, then it is likely that
pollutant accumulation and metabolism by the plant, will be reduced. The
level of a pollutant within a particular soil that causes a phytotoxic
response could therefore be used to identify materials that may be suitable
for this system.
The longer the plant is exposed to the pollutant, the greater is the
accumulation of the pollutant (Section 3 and Section 6). On polluted soils,
vegetation may be permanently present (e.g., forest or grassland),- or it may
be planted and harvested in cycles (e.g., crops), or it may just be present as
undesirable volunteer or weed species to be disposed of when site cleanup
63
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occurs. It would be expected that each of these different types of vegetation
would accumulate different amounts of pollutants from the soil.
The amount of a pollutant that each plant species can accumulate must,
in some way, be related to the efficiency of the pollutant collecting system,
i.e., the plant root system. This efficiency would, in turn, be a factor of
both the total root mass available for pollutant sorption and the extent of
the root penetration throughout the soil. Only limited information is
available on this.
Russel (1969) includes figures for the fresh weights of roots in the top
10 era of soil -- figures that range from 6000 kg/ha for Agropyron cristatum to
1500 kg/ha, for wheat. Foxx et al. (1984a, 1984b) investigated the maximum
rooting depths of a number of plant species growing on low level waste sites
(Tables 4.1 and 4.2).
TABLE 4.1. AVERAGE MAXIMUM ROOTING DEPTHS OF PLANT SPECIES OF THE PINEYWOODS
AND PRAIRIES {Adapted from Foxx et al.,1984a.)
Species
Common name
Depth cm.
Trees
^Icer spp.
Car/a spp.
Juglans spp.
Pinus n'gida
Pinus spp.
Quercus spp.
Grasses and forbs
Andropogon gerardli
Andropogon scoparius
Axonopus spp.
Cynodon dactylon
Eragrostis spp.
Lespedeza spp.
Nedicago spp.
Panicum spp.
PaspaJum spp.
Sorgastrum nutans
Sporobolus spp.
Maple 113.8
Hickory 152.0
Walnut 173.8
Short!eaf pine 78.1
Pine 181.6
Tree oaks 672.1
Big bluestem 207.1
Little bluestem 165.4
Carpet grass 76.0
Bermuda grass 126,6
Love grass 127.4
Bush clover 244.0
Alfalfa
Panic grass 220.7
Dallis grass 143.4
Indiangrass 159.0
Dropseed 241.1
The earlier sections indicated at least four routes by which chemicals
in the soil can enter a plant: root uptake into conduction channels, uptake
from vapour, uptake by external contamination, and uptake and transport in oil
cells. If uptake between different plant species is to be compared, it is
important that this should occur within one uptake route. Unfortunately,
there are extremely few reports of plant uptake of soil-borne organic
pollutants where each uptake route has been considered separately, or even
64
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where the influence of the separate routes has been noted. This type of
information is urgently needed.
TABLE 4.2. AVERAGE MAXIMUM ROOTING DEPTHS FOR DIFFERENT PLANT TYPES. DATA
LISTED ARE PERCENT OF PLANTS HAVING ROOTING DEPTHS, IN CM, OF LESS THAN THE
INDICATED DEPTHS (Adapted from Foxx et al., 1984.)
VEGETATION
Annual grasses
Biennial forbs
Annual forbs
Perennial forbs
Subshrubs
Perennial grasses
Evergreen trees
Deciduous grasses
Shrubs
90
75
65
65
42
41
40
33
7
10
180
100
100
88
71
85
79
80
52
47
DEPTH
275
97
85
96
94
86
70
60
360
100
93
96
99
86
78
72
450
97
96
99
86
80
77
Investigations into plant uptake of organic chemicals from soils, and
comparisons between different plant species, go back many years. Beall and
Nash (1969) grew soybean, wheat, corn, alfalfa, bromegrass, and cucumber for 3
to 4 weeks in a variety of soils polluted with the herbicides endrin, DDT,
dieldrin, and heptachlor. If the concentration in the plant leaf is expressed
as a ratio to the soil concentration (that is, as a Concentration Factor, CF),
then bromegrass had the highest CF for endrin and heptachlor whereas wheat had
the highest for DDT and dieldrin. The highest CF from this experiment was
2.43 for the aerial shoots of bromegrass growing on soil polluted by 5 ppm
heptachlor.
Harris and Sans (1969) found that sugar beet roots took up more dieldrin
from a clay soil containing 1.2 ppm dieldrin than did carrots, potatoes, and
sugar beet tops. The uptake by corn, oats, and alfalfa were all less than the
other crops, with approximately 0.2 ppm dieldrin in them. Davis et al. (1964)
found that soybean took up more atrazine per gram of fresh weight from
nutrient solution than did corn or cotton. Conversely, Nash et al. (1970)
reported that cotton took up a greater amount of heptachlor from soils than
did soybean, although they note that there were considerable differences in
biomass produced and that this would have affected their results.
Hermanson et al. (1970) reported that carrots normally take up more
organochlorine insecticide residues than do other root crops such as potatoes,
radish, turnip, and beet.
Walker and Featherstone (1973) investigated the absorption and
translocation of atrazine and linuron by carrot, parsnip, lettuce, and turnip
seedlings in culture solutions. They found marked differences between the
species in the distribution of the herbicides within the plants. A high
65
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proportion of the linuron absorbed by the carrot and parsley seedlings was
retained in their root systems, whereas in lettuce and turnip over 60% was
translocated to the shoots (Table 4.3). With atrazine, differences were also
apparent but were less marked. Examination of the extracts of the different
plant species showed that up to 45% of the linuron translocated in parsnip and
carrot was present as metabolites, but that little metabolism had occurred in
the shoots of lettuce and turnip or in the roots of any of the species. They
suggested that the tolerance of parsnip and carrot seedlings to linuron
resulted from a combination of root fixation and metabolism in the shoot.
TABLE 4.3. LINURON AND ATRAZINE ABSORPTION* BY DIFFERENT PLANT SPECIES
(Adapted from Walker and Featherstone, 1973.)
Concentration in plant tissue in//g/g.
Vegetation
Parsnip
Carrot
•lettuce
Turnip
Plant
part
shoot
root
shoot
root
shoot
root
shoot
root
Linuron
0.219
2.267
0.379
1.218
0.769
0.660
1.338
0.739
Atrazine
0.749
0.590
0.544
0.328
0.909
0.405
2.128
0.382
"After 12 days in a nutrient solution containing 0.05 jwg/ml of the
radio!abeled herbicide.
Bristow et al. (1972) grew cotton, soybean, bean, pea, corn, cucumber,
muskmelon, onion, oat, and wheat on a sandy loam polluted by 25 ppm of
pentachloronitrobenzene for seven days. The CFs, based on fresh weight of
roots, ranged from 0.75 for cotton to 0.08 for wheat. The order of CFs was as
the species are listed above.
Overcash (1983), in a review, quoted maximum CFs of 26 for grass; 9.9,
lettuce; 2.9, radish; 2.6, parsley; 1.9, carrot; 0.72, potato; 0.39,
sugarbeet; and 0.12, watercress grown in hexachlorobenzene-polluted soils. No
information was given on the plant part analyzed.
Many further attempts have assessed uptake by different plant species.
Some 150 data sets have been assessed to produce Table 4.4. This .table
compares the maximum CFs of various plants grown on soils polluted by various
66
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pollutants, as decribed by their n-octanol/water partition coefficient. The
variation in results and lack of pattern highlight the need for further work.
TABLE 4.4. MAXIMUM REPORTED PLANT CF" FOLLOWING GROWTH ON SOILS POLLUTED
BY A RANGE OF ORGANIC COMPOUNDS1
log
KQW
Max.
CF Plant Part
Authors
-0.14 22.4 Pea root Lichtenstein et al., 1967
1.83 7.4 Barley shoot Klosowski,et al., 1981
2.72 8.1 Barley shoot Klosowski, et al., 1981
2.9 42.6 Corn root Beestman, et al., 1969
2.9 396.0 Pea root Lichtenstein, et a!., 1968
3,2 648.0 Pea root Lichtenstein, et al., 1967
3.72 8.7 Ryegrass root Voerman and Beserner, 1975
5.2 2.7 Barley shoot Klosowski, et al., 1981
5.57 2.6 Bean root Bristow, et al., 1972
6.04 17.0 Wheat stem Sims and Overcash, 1983
6.18 39.0 Grass Smelt and Leistra, 1974
"Concentration Factor
'Described by their n-octanol/water partition coefficients (log Kow).
Some of this work must investigate the role and extent uptake plays in
the four possible uptake routes described earlier. Perhaps the least
investigated uptake and transport route is that within the oil cells of
adapted plants. In this group of plants with high root lipid contents are
plants like cress, carrot, and parsnip. The relationships outlined above in
Section 3 are probably not completely valid for these plants because, for
lipophilic chemicals, additional effective uptake mechanisms result from the
root lipids.
Topp et al. (1986) compared the uptake of hexachlorobenzene, which has a
low water solubility, by an oil containing plant, cress, with that of a non
oil containing plant, barley (Figure 4.1). The uptake by cress was higher
throughout the experimental period. It is quite possible that increased
uptake of lipophilic chemicals is related to the root lipid concentration of
the plant, but there is no work to substantiate this.
Not only are these variations in the extent of pollutant accumulation
between plant types and species, there are also reported differences between
cultivars and individuals of the same species. In an experiment to
investigate the effects of maturity and varietal differences in carrot on the
uptake of endrin residues from soil, Hermanson et al. (1970) found that, in
general, endrin residues in the root appeared to decline with increasing
maturity. Although the actual carrot variety exposed to the endrin could
67
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affect CFs by as much as four times, 50 to 80% of this endrin was always
removed by peeling the carrot.
10-
5-
1-
PLANT CONC. FACTOR
x BARLEY
o CRESS
10
50
100
TIME
(DAYS)
Figure 4.1. Time course of uptake of hexachlorobenzene from soil by barley
and cress. (From Topp et al., 1985,}
Various attempts have been made to utilize genetic variation within
plants by selecting for tolerance for environmental stresses. Davis (1984)
reported on a program design to obtain plants that could degrade
organochlorine molecules via cell tissue-culturing techniques. The
experiments were only partially sucessful; cell lines of milkweed could be
selected for tolerance to up to 7.5 ppm pentachlorophenol and 5 ppm lindane,
but the plants could not be regenerated.
In conclusion, the use of plants to collect and degrade pollutants is
still in its infancy. The potential for this technique is, however, vast; in
one reported instance, pea roots contained over 600 times the soil
concentration of the pollutant under study. If this collection system could
be harnessed to a plant able to degrade the pollutant, then cleanup would be
achieved. Considerable further work is needed, and in some cases is under
way, to assess the full potential of the variation between plant species,
cultivars, and indeed individuals so that the optimum use of plants on
polluted soils can be made.
68
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REFERENCES
Beall, M.L., and R.G. Nash. 1969. Crop seedling uptake of DOT, dieldrin,
endrin and heptachlor from soils. Agr. Jour. 61:571-575.
Beestman, G.B., D.R. Keeney, and G. Chesters. 1969. Dieldrin uptake by corn as
affected by soil properties. Agronomy J. 61:247-250.
Bristow, P.R., J. Katan, and J.L. Lockwood. 1972. Control of Rhizoctonia
sol am' by pentachloronitrobenzene accumulated from soil by bean plants.
Phytopath. 63:808-813.
Davis, D.E., J.V. Gramlich, and H.H. Funderburk Jr. 1964. Atrazine absorption
and degradation by corn, cotton, and soybeans, Weeds. 252-255.
Davis, M.E. 1984. Development of photosynthetic plants genetically adapted to
degrade organochlorine compounds. Batelle Columbus Laboratories, Columbus, OH,
Contract number 68-02-3169.
Foxx, T.S., G.D. Tierney, and J.M. Williams. 1984a. Rooting depths of plants
as related to biological and environmental factors. Los Alamos Nat. Lab. LA
10254 MS.
Foxx, T.S., G.D. Tierney, and J.M. Williams. 1984b. Rooting depths of plants
on low level Waste sites. Los Alamos Nat. Lab. LA 10253 MS.
Harris, C.R., and W.W. Sans. 1969. Absorption of organochlorine insecticide
residues from agricultural soils by crops used for animal feed. Pestic. Mom't.
J, 3(3):182-185.
Hermanson, H.P., L.D. Anderson, and F.A. Gunther. 1970. Effects of variety and
maturity of carrots upon uptake of endrin residues from soil. J. Econ.
Entomol. 63{5):1651-1654.
Kloskowski, R., I. Scheunert, W. Klein, and F. Korte. 1981. Laboratory
screening of distribution, conversion and mineralization of chemicals in a
soil-plant system adn comparison to outdoor data. Chemosphere. 10(10):1089-
1100.
Lichtenstein, E.P., T.W. Fuhremann, and K.R. Schulz. 1968. Use of carbon to
reduce the uptake of insecticidal soil residues by crop plants; effects of
carbon on insecticide adsorption and toxicity in soil. J. Agr. Food Chem.
16(2):348-355.
Lichtenstein, E.P., T.W. Fuhremann, N.E.A. Scopes, and R.F. Skrent. 1967.
Translocation of insecticide from soils into pea plants; Effects of the
detergent LAS on translocation and plant growth. J. Agr. Food Chem. 15(5}:864-
869.
Nash, R.G., M.L. Beall, and E.A. Woolson. 1970. Plant uptake of chlorinated
insecticides from soils. Agron* Jour. 62:369-372.
69
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Overcash, M.R. 1983. Land treatment of municipal effluent and sludge :
Specific organic compounds. Utilization of Municipal Wastewater and Sludge.
Pp. 199-231.
Russel, E.J. 1969. Soil condition and plant growth. Longmans, London.
Sims, R.C., and M.R. Overcash. 1983. Fate of polynuclear aromatic compounds
(PNAs) in soil-plant systems. Residue Reviews, 88:2-68.
Smelt, J.H., and H. Leistra. 1974. Hexachlorobenzene in soils and crops after
soil treatment with pentachloronitrobenzene. Agric. and the Environ, 1:65-71.
Topp, E., I. Scheunert, A. Attar, and F. Korte. 1986. Factors affecting the
uptake of 14C-labelled organic chemicals by plants from soil. Ecotoxicol.
Environ. Safety. 11:219-228.
Voerman, S., and A.F.H, Besemer. 1975. Persistence of dieldrin, lindane, and
DDT in a light sandy soil and their uptake by plants. Bull. Environ. Contain.
Toxicol. 13(4}:501-505.
Walker, A., and R.M. Featherstone. 1973. Absorption and translocation of
atrazine and linuron by plants with implications concerning linuron
selectivity. J. Exp. Bot. 24(79):450-458.
70
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SECTION 5
PLANT UPTAKE OF POLLUTANTS
Within each of this Section's subsections the cited literature deals
directly with plant accumulation of particular pollutants within pollutant
groups, rather than the more general concepts of pollutant behaviour in the
soil or higher plant uptake of pollutants as discussed earlier. In many
instances, the route of the pollutant into the plant part has not been
reported and no attempt has been made to distinguish between plant
accumulation by different uptake routes. Some contradictory data are,
therefore, evident. Each subsection also includes a listing of typical
pollutants within a group from "The water related environmental fate of 129
priority pollutants" (U.S. EPA, 1979).
The listings include various data that will assist in determining the
potential environmental fate of the compound. These data include the n-
octanol/water-partition coefficient of the compound (KDW), its Henry's Con-
stant (He), its organic carbon partition coefficient (Koc), and its half life
(Tm).
By definition, for example, the higher the log Kaw for a particular
pollutant, the more water insoluble it is and the less of it that is likely to
be present in the soil solution. In turn, a high log KDW also infers that
less of the compound is likely to be translocated from the plant roots to the
plant shoots and more of it is likely to be physically sorbed to the plant
root.
The higher the Henry's Constant for a particular compound, the more it
will partition from the liquid phase to the vapour phase. As vapour phase
transport through the soil is considerably faster than liquid movement, it
would be expected that these compounds will be the first to be accumulated by
a plant. Vapour transport and the loss of compounds from the soil means that
the plant leaves are exposed to gaseous compounds that can then be
accumulated.
To be available for plant uptake, the compound must remain in the soil
long enough to come into contact with the plant roots or must be volatilized
from the soil to come into contact with plant leaves. From this, one would
expect more information would be available on plant uptake and accumulation of
organic compounds with long half lives than actually exists.
71
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The following discussion reveals that contradictory data and conclusions
have been reported. Much of this arises from not recognizing that four quite
distinct uptake routes are available for a compound to enter a plant. There
are also many pollutants whose accumulation into plants has not been studied.
This leaves many large gaps in the available data and thus, large gaps in the
conclusions from this data base.
PESTICIDES
For discussion purposes, pesticides can be divided into the targets they
are used to control, i.e., insecticides, herbicides, and fungicides.
Insecticides can then be further divided on the basis of their chemical
structure into the halogenated hydrocarbons, the organophosphates, the
carbamates, and the inorganic insecticides like lead, arsenic, and mercury.
The inorganic insecticides will not be discussed here; the former will be
discussed according to their chemical structures. A description of some,
physical and chemical parameters of pesticides recognized as priority
pollutants is included in Table 5.1.
72
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TABLE 5.1. THE PHYSICAL AND CHEMICAL PARAMETERS OF PESTICIDES RECOGNIZED AS
PRIORITY POLLUTANTS
Compound
log K0
•1/2
Hcs log K0
Report t
1/2
Acrolein
Aldrin
Chlordane
ODD
DDE
DDT
Dieldrin
Endosulfan
Endrin
Heptachlor
Heptachlor epoxide
Hexachl orocycl ohexane
Lindane
Isophorone
TCDD
Toxaphene
-0.09
-0.14
2.78
5.99
5.69
4.89
2.9
3.55
5.6
3.9
3.9
3.8
3.72
1.70
6.14
2.9
8
C
C
C
B
C
C
C
C
A
C
B
C
nd
C
C
3.3720
0.0022
0.0001
0.0000
0.0009
0.0190
0.0003
1.2222
0.0000
0.3410
nd
0.0003
0.0006
nd
nd
1.2518
-0.273
-0.324
2.681
5.984
5.675
4.852
2.804
3.473
5.582
3.833
3.833
3.370
3.648
1.569
6.138
2.804
nd
l-4y»a
2-4y,a
nd
nd
3-10y,a
l-7y,a
nd
4-8y,a
7-12y,a
nd
2y,a
nd
nd
iy,a
10y,a
fLog KDW are taken from U.S. EPA (1979).
fHalf lives are assessed as A = less than 10 days, B = 10-50 days, and
C = greater than 50 days, based on the prinicipal fate process in the
environment (EPA, 1979).
*Henry's Constant (dimensionless) has been calculated according to Thibodeaux
(1979) from data supplied within U.S. EPA (1979).
5Log «„ has been calculated according to Rao et al. (1982).
^Reported half lives are in days unless followed by y = years, followed by
a = Dacre (1980); b - Jury et al-. (1984); c - Ryan (19860); e - U.S. EPA
(1979), which is based on the predominant environmental process thought to
determine fate.
Biological magnification of halogenated hydrocarbons has been well
documented and is of great concern. Most research exists on plant sorption of
DDT, probably because it is one of the oldest of the chlorinated insecticides
and has had very wide use. The amounts of DDT found in plant material range
from not detectable to 7.5 ppm in root crops and from "not detectable" to 10
ppm in the leaves of crop plants. These figures wiH result from the sum of
the possible uptake routes, as previously discussed (Nash, 1974). Several re-
searchers have observed that DDT is distributed throughout the plant, and this
primarily results from vapour phase uptake (Beall and Nash, 1971).
Aldrin and heptachlor, two chlorinated cyclodiene insecticides, have
also received considerable attention. Both are readily converted to their
more stable epoxides, i.e., dieldrin and heptachlor epoxide, in soils and
73
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plants. Plant residues have ranged from 0 to 150 ppm for aldrin plus
dieldrin, and 0 to 10 ppm for heptachlor plus heptachlor epoxide. The high
values were obtained from the fibrous roots of plants. A few experiments have
demonstrated clearly that both aldrin and heptachlor and their epoxides are
sorbed by plant roots and translocated throughout the plant even to the seeds.
Plant species and varieties affect the amount of pesticides residues
found within a plant. Lichtenstein et al. {1965} reported differences in
absorption of aldrin and heptachlor by five carrot varieties -- differences of
22% to 80% of the soil concentration. Residues of the same pesticides in
peanut, soybean, oat, barley, and corn seeds were directly related to the fat
content of the seed (Bruce et al., 1966). Residues also vary within the
plant, with the top half of the stem normally containing much less residue
than the lower half of the stem, as the distance for trans!ocation is
decreased (Beall and Nash, 1971).
Soil pH, temperature, organic matter, and clay affected the amount of
diuron and its metabolites in the roots and shoots of 14-day-old oat seedlings
grown in a modified Lakeland sandy loam soil. An increase in soil pH resulted
in greater amounts of diuron in the shoots. To a lesser extent, increased
soil organic matter and reduced soil temperature also influenced herbicide
content of shoots. In contrast, herbicide content of roots was independent of
pH, organic matter, or temperature. The compounds actually identified in the
shoots included both the parent compound and monomethyl derivatives (Nash,
1968).
To study the transfer of soil residues to crops, a sandy soil and a silt
loam were each treated with one of six insecticides of different water
solubilities (0.001 ppm to 320 ppm) and plants were grown in the soil. The
amounts of 14C compounds that penetrated into the plant tissue depended on the
water solubility of the insecticide and the soil type, i.e., most of the 14C
compounds were picked up from a sandy soil that had been treated with the most
water soluble insecticide. The amount of 14C recovered from soils and plants
was similar with DDT (water solubility 0.001 ppm) and carbofuran (water
solubility 320 ppm) recovered. With DDT, however, most of the insecticide
remained in the soil; with carbofuran most of the recovered insecticide
residues plus metabolites were associated with the greens. The amounts of 14C
bound to plant tissue as well as the amounts of detoxification products in
plant tissue increased with increasing water solubilities of the insecticides
(Lichtenstein, 1980).
In 1974, Nash summarized the state of knowledge on plant uptake of
pesticides from soils, their translocation, and their metabolism within the
plant (Table 5.2). The list is short, and remains short, considering the vast
number of pesticide formulations currently used. The results indicate that
plants sorb and can metabolize most pesticides.
74
-------
TABLE 5.2, PLANT UPTAKE, TRANSLOCATION, AND METABOLISM OF PESTICIDES
FROM SOILS. (Adapted from Nash, 1974.)
Compound
Root uptake Trans!ocation
Metabolism
Aldrin
Dieldrin
Isodrin
Endrin
Heptachlor
Heptachlor
epoxide
Chlordane
Endosulfan
Toxaphene
BHC
Lindane
DDT
Diazinon
Dimethoate
Dlsulfoton
Phorate
Parathion
Chloroneb
yes
yes
yes
yes
yes
yes
yes
yes
probable
yes
yes
yes
yes
yes
yes
yes
yes
yes
yes
yes
probable
yes
yes
yes
improbable
yes
improbable
yes
yes
probable
yes
probable
yes
yes
probable
yes
yes
probable
yes
yes
yes
unknown
unknown
unknown
unknown
yes
yes
yes
probable
probable
yes
yes
unknown
yes
Lindane, DDT, and aldrin are absorbed into crops; the degree depends on
the crop, the soil type in which the crop had grown, and its concentration
within the soil. Carrots absorbed more insecticide than any other crop (Iwata
et al., 1974) and, in the case of lindane, accumulated more than was in the
soil. The insecticides,were most readily absorbed from a sandy loam and least
from a muck. The amounts absorbed by the same crop from the same soil were
not in direct proportion to the concentration recovered from the soil;
relatively less was recovered from the more polluted soils.
No comprehensive reported investigations relate plant uptake and
accumulation of pesticides with their physical and chemical parameters
although this may be expected because of the economic/confidential nature of
this information. The actual efficiency of herbicides depend on their
characteristics and their ability to affect the growth of plants and, in some
cases, to be accumulated.
POLYHAL06ENATED BIPHENYLS
Discussions of PCBs are often complicated by differences in the chemical
and biological terras that describe them. In the chemical industry, PCBs are
normally described by their Aroclor number, where the first two digits repre-
sent the carbon number of the hydrocarbon and the last two digits are the mean
percent chlorine on the carbon molecule. Aroclors are, therefore, mixtures of
75
-------
chemicals. Aroclor 1254 is a mixture of chemicals with 12 carbon atoms and
54% chlorine. Plants and other biological systems, however, see the Aroclor
mixture as its individual congeners. Some physical and chemical parameters of
PCBs recognized as priority pollutants are included in Table 5.3.
The fate of PCBs in soil and their potential for uptake by plants has
received little attention, partly because their use did not involve
application to agricultural land. In recent years, however, they have gained
research popularity because of their detrimental effect upon the environment.
TABLE 5.3. THE PHYSICAL AND CHEMICAL PARAMETERS OF THOSE POLYCHLORINATED
BIPHENYLS RECOGNIZED AS PRIORITY POLLUTANTS.
Compound
log
He*
log K0
Report t.
2-chl oronaphthal ene
Arochlor 1016
Arochlor 1221
Arochlor 1232
Arochlor 1242
Arochlor 1248
Arochlor 1254
Arochlor 1260
4.
5.
4.
4.
5.
6.
6.
6.
12
58
09
54
58
11
72
11
C
C
C
c
c
c
c
c
0
0
0
0
0
0
0
0
.0221
.0132
.0048
.0350
.0172
.1477
.1134
.3038
4
5
4
4
5
6
6
6
.060
.562
.029
.492
.562
.107
.735
.107
14y,
4y,
4y,
4y»
4y,
4y,
4y,
4y,
a;
a;
a;
a;
a;
a;
a;
a;
i
i
i
i
i
i
i
i
,b
,b
,b
,b
,b
,b
,b
,b
log Kow are taken from U.S. EPA (1979).
'Half lives are assessed as A = less than 10 days, B = 10-50 days,
and C = greater than 50 days, based on the prinicipal fate process in the
environment (EPA, 1979);
'Henry's Constant (dimensionless) has been calculated
according to Thibodeaux (1979) from data supplied within U.S. EPA (1979).
sLog Koc has been calculated according to Rao et al . (1982).
Reported half lives are in days unless followed by y = years, followed by
a = Dacre (1980); b = Jury et al . (1984); c - Ryan (19860); e - U.S. EPA
(1979), which is based on the predominant environmental process thought to
determine fate. If more than one reference occurs for the half life they are
separated by ;. nd = no data, i = infinity.
Iwata et al . (1974) evaluated PCB uptake by carrots from a low-organic-
matter sandy soil, Aroclor 1254 was applied at 100 ppm to the top 15 cm of
soil in the field. "Goldinhart" carrots were grown using normal agricultural
practices. For the persistent 5 and 6 chlorine isomers, unpeeled fresh
carrots contained about 5% of the soil level. Peeling the root removed 14% of
the carrot fresh weight but 97% of the PCBs, leaving the peeled carrot with
only 0.16% of the PCB soil level. Chromatographic analysis showed a
preferential uptake of isomers of low chlorination which is in line with their
K. Iwata et al. concluded that carrots are outstanding in scavenging
QW.
organochlorine pesticide residues from soil.
76
-------
Suzuki et al. (1977) showed that PCBs in soil can be transferred to a
plant via plant roots. When they grew soybean sprouts for 14 days in soil
containing 100 ppm PCS, they found maximum concentrations of 150 //g/kg.
Translocation rates of PCB isomers differ, largely depending on the degree of
chlorination with related degrees of water solubility. Their experiments were
primarily designed to test their methods rather than to obtain accurate uptake
concentrations.
Moza et al. (1979) found that the uptake of a radio!abeled tri- and
pentachlorobiphenyl was greater in the high-oil carrot than the low-oil sugar
beet. For the trichlorobiphenyl, only 32.5% of the applied radioactivity was
recovered in the plants and soil after the first exposure season;
volatilization loss was 67.5%, carrot plant uptake was 3.1%, and sugar beet
uptake was 0.2%. For the pentachlorobiphenyl, total recovery was 58.5%;
volatilization loss was 41.5%, crop uptake was 1.4%, and conversion was less
than 1%.
Mrozek and Leidy (1981) collected and transplanted Spartina aHerniflora
plants into Aroclor 1254-amended soils under estuary-like conditions. After a
90 day growth period, the plants were harvested and separated into aerial and
below-ground portions. Following analysis, a comparison of the increasing
absence of the higher chlorinated congeners in the aerial tissue with those in
the below ground tissue and those in the soil suggested that uptake was selec-
tive for the lesser chlorinated congeners. This has also been shown by Iwata
et al. (1974). The mean CFs across the various congener peaks, and with a
mean soil concentration of 0.039 ppm, were 14.4 for the below-ground tissue
and 0.56 for the aerial tissue.
As a result of their review, Fries and Marrow (1981) concluded that no
study demonstrated beyond doubt that plant root uptake and subsequent
translocation of PCBs do actually occur. The residues that were reported in
many of the reviewed papers could have arisen from direct adsorption to the
roots or surface adsorption to the aerial parts by volatilized PCB. This is
one of the first experiments where the authors recognized different uptake
routes. Fries and Marrow, therefore, grew soybean plants in specially
constructed pots to determine the residue concentrations in the plant tops
from either surface or subsurface soil-applied tri-, tetra-, or
pentachlorobiphenyls. The plants were harvested at 52 days and divided into
top stem, bottom stem, top leaves, bottom leaves and seed pod for analysis.
They found that the concentration of residue in the plant increased with
increasing chlorination and that detectable residues were only found in the
lower leaves of plants where the biphenyl was in the surface soil.
Some of this literature on PCB uptake by higher plants is reviewed and
placed into an environmental context by Strek and Weber (1982). They do not,
however, attempt to resolve the root uptake versus vapour uptake arguments.
Sawhney and Hankin (1984) then found the reverse of most other workers
in that the leaves of beet grown on Aroclor-amended soil contained higher
concentrations of the pollutant than did the roots. Beet roots contained 15,
16, and 35 //g/kg, repectively, of the Aroclor 1248, 1254, 1260, whereas the
leaves contained 22, 94, and 52 //g/kg, respectively, (Table 5.4). Similar
77
-------
results were also found with turnips also grown on the amended soil. CFs
reached a peak of 0.4 for turnip leaves with the least chlorinated Aroclor,
1248. In line with other authors, greater plant tissue contamination
occurred with the lesser chlorinated Aroclors giving a ranking for uptake of
1248 >1254 >1260. Sawhney and Hankin, however, make no attempt to separate
potential vapour uptake from root uptake and translocation.
TABLE 5.4. PCBs (Aroclors) IN SOIL AND IN VEGETABLES GROWN IN SOIL AMENDED
WITH CONTAMINATED SEDIMENTS. (From Sawhney and Hankin, 1984.)*
Afoclor Soil Beet root Beet leaf Turnip root Turnip leaf
//g/kg //g/kg CF //g/kg CF //g/kg CF //g/kg CF
1248 80 IB .187 22 .275 30 .375 32 .400
1254 1880 16 .008 94 .050 16 .008 40 ,021
1260 14440 35 .002 52 .004 20 .002 27 .002
Total 16400 66 168 66 99
*CF = Concentration Factor calculated by dividing the amount in the plant
part by the amount in the soil.
Jacobs et al. (1976) amended a loamy sand, having an organic matter
content of 1.1%, with various concentrations of PBBs to investigate uptake
into Nordstem orchard grass (Dactylis glomerata) and Spartan Delite carrot
(Caucus carota) in a greenhouse. No PBBs were detected in repeated clippings
of the tops of the grass, either in the grass roots or in the carrot tops.
CFs in the carrot roots did not exceed 0.002 and they concluded that PBBs are
unlikely to be taken up into vegetation to any significant degree.
The same authors continued their work (Chou et al., 1978) and found that
when soybean and corn seedlings were grown in a nutrient solution containing
radiolabeled PBBs for about 7 days, plant root contained PBB residues but none
was found in the leaves.
These compounds all have high log Kow values and would therefore be
expected to be sorbed to plant roots to concentrations many times those in the
surrounding soil or sediment media. This has not been reported.
The time scale of many of these investigations has been short. When a
PCB Is within the soil, it is partitioned between the organic content of the
soil and the liquid phase. The plant root can only sorb the compound in the
liquid phase. Attachment to the organic faction is strong, however, and
.considerable time is needed to adjust the equilibrium of the compound to the
liquid phase. Further work on longer term experiments is therefore required.
78
-------
HALOGENATED ALIPHATICS
No relevant information on plant uptake and accumulation of this large
group of compounds was found in the literature searches. Most of the
compounds have low log Kow and short half lives, as shown in Table 5.5. They
are unlikely, therefore, to be present in the soil environment for long
periods, but if they were, they would be translocated within plants,
HALOGENATED ETHERS
No information was found on this group of compounds other than the
chemical and physical parameters described in Table 5.6. They exhibit a range
of log Kow and log Koc and so would be expected to be both accumulated by the
plant root and translocated within a plant following uptake.
MONOCYCLIC AROMATICS
These cyclic compounds, described in Table 5,7, have multiple double
bonds. Plants appear to be able to metabolize the benzene ring to organic
acids, which then accumulate in the plant part where the benzene was
introduced (Sims, 1982).
Toluene, for example, has been reported to be absorbed and detoxified by
roots, foliage, and fruits, with organic acids being the primary products of
cleavage of the toluene aromatic ring. The oxidation of toluene and its meta-
bolites to C02 was more rapid in corn and bean seedlings than in perennial
plants. Regardless of how the toluene was taken up by plant roots or leaves,
however, the same metabolites were found in all species. An enzyme system in
plants is thus capable of degrading the benzene ring and transforming aromatic
into aliphatic compounds. The toluene that enters the plant is not bioaccu-
mulated or translocated as such, but it is readily metabolized and assimilated
into the plant cell components and C02 (Overcash et al., 1982).
Although this is a large group of compounds with a wide range of
physical and chemical properties, most have relatively short half lives and
would, therefore, not present a long-term pollution problem.
PHTHALATE ESTERS
Again, little information is available on these compounds other than
that described in Table 5.8. They would be expected to be sorped to plant
roots and may also be transported via the vapour phase. Considerably more
investigations are needed to determine the potential for plant accumulation of
the phthalate ester group of chemicals.
Overcash et al. (1982) reported on the effects of di-n-butyl phthalate
on the plant mass and height of soybean, corn, and fescue. Unfortunately, no
uptake concentrations were assessed.
79
-------
POLYCYCLIC AROMATICS
The chemical and physical parameters of the polycyclic aromatic
compounds are included in Table 5,9. Many research efforts have centered on
the risk arising from plant accumulation of polycyclic aromatic hydrocarbons
(PAHs) because some of these compounds have been shown to be carcinogenic.
Polyaromatic compounds are composed of multiple fused benzene rings and
include compounds such as naphthalene and anthracene.
Some of the earliest experiments to investigate the uptake of PAHs added
to* soil were undertaken in Germany by Wagner and Siddiqi (1970, 1971). They
observed that, at high concentrations, soil-borne PAH contamination of wheat,
rye, maize, barley, and carrot affected the development of both roots and
shoots. Their results, however, were not consistent because the control
plants also became contaminated, possibly by aerial contamination. CFs for
both roots and shoots were generally less than 0.1 on a dry matter basis.
Higher CFs for PAHs have been reported by Ellwardt (1977) and Muller
(1976) from their work with potato tubers and radish and carrot roots. In
these instances, the PAH remained in the outer portion of the root and
concentrations in the leaves of the plants remained very low, again suggesting
that aerial contamination is the only method by which plant leaves become
contaminated by soil-borne PAHs.
Harms and Sauerbeck (1984) found PAH contamination of potato tuber,
radish, and carrot when direct contact with the soil allowed transfer of the
compound. Concentrations in the above ground portions of the plants remained
low, however.
In Muller's study (1976), 3,4-benzopyrene was taken up and translocated
in both carrot and radish. If the composted solid town waste and soil mix
contained 2 ppm, red radish tubers contained 3.5 ppb, and the leaves contained
20 ppb. In other experiments using pure quartz sand as the substrate, much
more pollutant was taken up but actual concentrations in the carrot roots
declined with successive crops. Uptake of benzopyrene, however, strongly
depends on the chemical composition and physical nature of the substrate.
When, in a study by Sims and Overcash (1983), the soil concentration of
B(a)P was increased, the plant concentration remained approximately the same.
Biomagnification of B(a)P for seed, stem, and straw was demonstrated. With
3,4-benzfluoranthene, the pattern was not the same. Plant concentrations were
generally much higher with smaller differences between the plant parts. Also
an increase in plant 3,4-benzfluoranthene content correlated with an increase
in soil content. Thus the two PNAs exhibited different behaviour under simi-
lar soil and crop conditions but both showed biomagnification.
A quantitative determination of the uptake and plant effects of all PAHs
is practically impossible and many researchers have restricted their work to
the most toxic compounds, e.g., benzo(a)pyrene. Tracer experiments have
confirmed that this compound can be taken up by the roots and translocated
80
-------
upwards and, if shoot-applied, can move basipetally and disappear from the
plant (Grosse, 1978).
Harms (1975) proposed that the uptake rate of PAHs into plants depended
on their molecular size. The highly condensated ring structures of
benzo(a)pyrene and dibenz(a,/))anthracene resulted in low CFs whereas the lower
ring numbers of benzanthracene and anthracene resulted in higher CFs. Topp et
al. (1986) proposed molecular size as a major influence on plant uptake. In
turn, molecular size is broadly related to water solubility and n-
octanol/water partition coefficients.
Data provided in Sims (1982), described the assimilation and conversion
of 3,4-benzpyrene-l,2-14C with 14-day-old sterile corn and bean seedlings in a
sterile nutrient solution. When the radiolabeled PAH was introduced into the
leaves or roots of the plants, approximately 50% of the label was assimilated
into organic acids, which were concentrated at the site of assimilation. In a
follow-up experiment, also described in Sims (1982), with a wider range of
plant species, organic acid radioactivity ranged from 5.4% to 56.5% of the
total radioactivity of the root biomass, and 2.1% to 62.2% of the total
radioactivity of the leaf biomass. Radioactivity was also incorporated into
amino acids, up to 18% of the total radioactivity, and carbon dioxide, up to
9%.
Plants can use a variety of reactions to reduce more complex aromatic
structures to simpler units. Typical steps include demethylation, ft oxi-
dation, and decarboxylation. Recently, plants have been shown to be capable
of the ring-fission reactions needed to permit complete catabolism of aromatic
nuclei to carbon dioxide. The existence of such catabolic routes makes it
likely that the accumulation of secondary metabolites in plants is a dynamic
process of definite significance in the life of the plant (Ellis, 1974).
Experiments with sterile plants and with plant cell suspension cultures have
shown that benzo(a)pyrene can be metabolized into oxygenated derivatives
(Harms et al., 1977). Although some of these derivatives are known to be more
toxic than the original compound, they appear to be polymerized into the
insoluble plant lignin fraction, which may be an important mechanism for their
detoxification.
81
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TABLE 5.5. THE PHYSICAL AND CHEMICAL PARAMETERS OF THOSE HALOGENATED
ALIPHATIC HYDROCARBONS RECOGNIZED AS PRIORITY POLLUTANTS
Compound
log K0
-1/2
He*
log KQ
Report t1/2
Chloromethane
Di chl oromethane
Tri chloromethane
Tetrachl oromethane
Chi oroethane
1,1-Dichloroethane
1,2-Dichloroethane
1,1,1 -Trichl oroethane
1 , 1 , 2-Tr i chl oroethane
1,1,2,2 -T.etrachl oroethane
Hexachl oroethane
Chloroethene
1,1-Dichloroethene
1,2-Trans-dl chloroethene
Tri chloroethene
Tetrachl oroethene
1,2-Dichloropropane
1,3-Dichloropropene
Hexachl orobutadiene
Hexachl orocycl opentadi e
Bromomethane
Bromodl chl oromethane
Dibromochl oromethane
Tribromomethane
Di chl orodi f 1 uoromethane
Tri chl orof 1 uoromethane
0.91
1.25
1.9
2.64
1.54
1.79
1.48
2.17
2.17
2.56
3.34
0.60
1.48
1.48
2.29
2.88
2.28
1.98
3.74
3.99
1.10
1.88
2.09
2.30
2.16
2.53
C
B
B
nd
B
B
B
nd
nd
A
nd
A
A
A
A
A
nd
A
C
A
B
nd
nd
nd
C
nd
1.619
0.1276
0.1200
0.96
0.6152
0.1774
0.0380
1.4606
0.0308
0.1580
0.1037
151.69
7.8377
1.7682
0.3739
0.8471
0.0962
0.0563
1.0708
1.5027
8.1969
nd
nd
0.0434
109.36
4.5637
0,756
1.106
1.775
2.537
1.405
1.662
1.343
2,053
2.053
2.454
3,257
0.437
1.343
1.343
2.176
2.784
2.166
1.857
3.668
3.926
0.952
1.755
1.971
2.187
2.043
2.423
120, e
100, e
i,b;50,e
i,b
nd
45, e
90,e
l-8y,e
nd
nd
nd
i,b;l,e
l,e
l,e
4,e
10,6
nd
nd
nd
nd
i,b
nd
nd
nd
30y,e
10y,e
log Kow are taken from U.S. EPA (1979).
THalf lives are assessed as A = less than 10 days, B = 10-50 days, and C =
greater than 50 days, based on the prinicipal fate process in the environ-
ment (EPA, 1979).
*Henry's Constant (dimensionless) has been calculated according to Thibodeaux
(1979) from data supplied within U.S. EPA (1979).
5Log Koc has been calculated according to Rao et al. (1982).
^Reported half lives are in days unless followed by y = years, followed by
b = Jury et al. (1984); e = U.S. EPA (1979), which is based on the pre-
dominant environmental process thought to determine fate. If more than
one reference occurs for the half life they are separated by ;.
nd = no data, i = infinity.
82
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TABLE 5.6. THE PHYSICAL AND CHEMICAL PARAMETERS OF THOSE HALOGENATED ETHERS
RECOGNIZED AS PRIORITY POLLUTANTS
Compound
log K0
-1/2
He1
log Kfl
Report t
1/2
Bis(chloromethyl) ether
Bi s (2-chl oroethoxy)methane
2-Chloroethyl vinyl ether
Bi s(2-chloroethyl )ether
Bi s(2-chl oroi sopropyl ) ether
4*-Chlorophenyl phenyl ether
4-Bromophenyl phenyl ether
-0.38
I.Z6
1.28
1.58
2.58
4.08
4,28
A
C
A
nd
nd
nd
nd
0.0086
0.0000
0.0104
0.0005
0.0047
0.0101
nd
-0.571
1.117
1.137
1.446
2.475
4.018
4.224
l,e
0.5-2y,e
l,e
nd
nd
nd
nd
"Log Kow are taken from U.S. EPA (1979).
rHalf lives are assessed as A = less than 10 days, B = 10-50 days,
and C = greater than 50 days, based on the prinicipal fate process in the
environment (EPA, 1979).
*Henry's Constant (dimensionless) has been calculated according to Thibodeaux
(1979) from data supplied within U.S. EPA (1979).
sLog KQO has been calculated according to Rao et al, (1982),
"Reported half lives are in days unless followed by y - years; e - U.S. EPA
(1979), which is based on the predominant environmental process thought to
determine fate.
nd = no data."
83
-------
TABLE 5,7. THE PHYSICAL AND CHEMICAL PARAMETERS OF THOSE MONOCYCLIC AROMATIC
HYDROCARBONS RECOGNIZED AS PRIORITY POLLUTANTS
Compound
Phenol
Nitrobenzene
2,4-Dinitrotoluene
2»6-Dinitrotoluene
Benzene
2-Chlorophenol
Toluene
2,4-Dichlorophenol
Chlorobenzene
2,4,6-Trichlorophenol
Ethyl benzene
1,2-Dichlorobenzene
1,3-Dichlorobenzene
1 , 4-Di chl orobenzene
1, 2, 4-Tri Chlorobenzene
Hexachl orobenzene
Pentachlorophenol
2-Nitrophenol
4-Nitrophenol
2,4-Dinitrophenol
2, 4- Dimethyl phenol
p-Chloro-ra-cresol
4,6-Dinitro-o-cresol
log KOW*
1.46
1.85
2.01
2.05
2.13
2.17
2.69
2.75
2.84
3.38
3.15
3.38
3.38
3.39
4.26
6.18
5.01
1.76
1.91
1.53
2.50
2.95
1.85
^1/2
A
C
nd
nd
A
nd
A
A
nd
nd
A
nd
nd
nd
A
C
A
C
B
C
nd
nd
nd
He*
0.0000
0.0005
0.0000
nd
nd
0.0005
0.2703
0.0002
nd
0.0135
0.2672
0.0832
0.1505
nd
0.1387
0.0251
0.0001
0.0036
0.0010
nd
0.0000
nd
nd
log C
1.322
1.724
1.888
1.929
2.012
2.053
2.588
2.667
2.742
3.298
3.061
3.298
3.298
3.308
4.204
6.179
4.975
1.631
1.785
1.394
2.393
2.856
2.753
Report t1/2fi
i,b;2,c
i ,b
nd
nd
2,e
nd
2-7,c;l,e
3-28,c;6,e
i ,b-
nd
1,6
nd
nd
nd
nd
nd
nd
nd
16, c
nd
1-2, c
30, c
nd
"Log Kow are taken from U.S. EPA (1979).
THalf lives are assessed as A = less than 10 days, B = 10-50 days,
and C = greater than 50 days, based on the prinicipal fate process in the
environment (EPA, 1979).
'Henry's Constant (dimensionless) has been calculated according to Thibodeaux
(1979) from data supplied within U.S. EPA (1979).
5Log K00 has been calculated according to Rao et al. (1982).
"Reported half lives are in days unless followed by y = years, followed by
b = Jury et al. (1984); c = Ryan (1986); e = U.S. EPA (1979), which is
based on the predominant environmental process thought to determine fate.
If more than one reference occurs for the half life they are separated by ;.
nd = no data, i = infinity.
84
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TABLE 5.8. THE PHYSICAL AND CHEMICAL PARAMETERS OF THOSE PHTHALATE ESTERS
RECOGNIZED AS PRIORITY POLLUTANTS
Compound
log K0
"1/2
He'
log Kj
Report t
1/2
Dimethyl phthalate
Di ethyl
Di-n-butyl
Di-n-octyl
Bjs(2-ethylhexyl)
Butyl benzyl
2.12
3.22
5.20
9.21
8.73
5.80
B
B
B
B
B
B
0.0000
0.0006
0.0884
0.9884
0.5351
nd
2.001
3.133
5.171
9.287
8.803
5.788
nd
4,c
nd
nd
14, c
nd
'Log Kow are taken from U.S. EPA (1979).
THalf lives are assessed as A = less than 10 days, B = 10-50 days,
and C = greater than 50 days, based on the prinicipal fate process in the
environment (EPA, 1979).
*Henry's Constant (dimensionless) has been calculated according to Thibodeaux
(1979) from data supplied within U.S. EPA (1979).
'Log Koc has been calculated according to Rao et al. (1982),
^Reported half lives are in days unless followed by y = years, followed by
c = Ryan (1986), which is based on the predominant environmental process
thought to determine fate. If more than one reference occurs for the
half life they are separated by ;. nd = no data.
85
-------
TABLE 5.9. THE PHYSICAL AND CHEMICAL PARAMETERS OF THOSE POLYCYCLIC AROMATIC
HYDROCARBONS RECOGNIZED AS PRIORITY POLLUTANTS.
Compound log KQW"
Acenaphthene 4.33
Acenaphthylene 4.07
Fluorene 4.18
Naphthalene 3.37
Anthracene 4.45
Fluoranthene 5.33
Phenanthrene 4.46
8enzo[a]anthracene 5.61
Benzo[6]fluoranthene 6.57
Benzo[/5rffluoranthene 6.84
Chrysene 5.61
Pyrene 5.32
Benzo[g/?i]perylene 7.23
Benzo[a]pyrene 6.04
Dibenzo[a]anthracene 5.97
Indeno[123-cd]pyrene 7.66
t1/2<
C
C
C
c
c
c
c
c
nd
C
C
C
C
C
C
C
He'
0.0025
0.0021
0.0032
0.0100
0.0434
0.0004
0.0045
0.0000
nd
nd
0.0006
0.0005
0.0000
0.0000
0.0000
nd
log Kj
4.276
4.008
4.121
3.288
4.399
5.305
4.409
5.593
6.581
6.858
5.593
5.294
7.260
6.035
5.963
7.702
Report tme
nd
nd
nd
i,b;0. 1-125, c
nd
44-1825c
i ,b
15-6250, c
67-130, c
nd
5-10, c
nd
nd
2-694, c
21-190, c
200-600, c
"Log Kow are taken from U.S. EPA (1979).
THalf lives are assessed as A = less than 10 days, B = 10-50 days,
and C = greater than 50 days, based on the prinicipal fate process in the
environment (EPA, 1979).
^Henry's Constant (dimensionless) has been calculated according to Thibodeaux
(1979) from data supplied within U.S. EPA (1979).
§Log Koc has been calculated according to Rao et al. (1982).
^Reported half lives are in days unless followed by y = years, followed by
b = Jury et al. (1984); c = Ryan (1986), which is based on the predominant
environmental process thought to determine fate. If more than one reference
occurs for the. half life they are separated by ;.
nd = no data, i = infinity.
MISCELLANEOUS COMPOUNDS
The physical and chemical parameters of other compounds are described in
Table 5.10. Because nitrosamines are another group of chemicals found in
sewage wastes they have been investigated for plant contamination. Although
plants can accumulate nitrosamines from nutrient solution and soil (Brewer et
al., 1980; Dean-Raymond and Alexander, 1976), nitrosamines appear to be
rapidly degraded in both environments. Sander et al. (1975) cultivated cress
plants (Lepidium sativum) over nutrient solutions containing the nitrosamines,
nitrosodiethyl amine, nitrosomorpholine, and dinitrosopiperazine. They found
that the plant roots quickly absorbed each of the nitrosamines but that the
86
-------
concentration within the plant was quickly depleted. The losses in the plant
could have resulted from the effect of sunlight on the chemicals,
vapourization, or metabolic breakdown within the plant.
TABLE 5.10. THE PHYSICAL AND CHEMICAL PROPERTIES OF MISCELLANEOUS
COMPOUNDS RECOGNIZED AS PRIORITY POLLUTANTS
Compound log Kow* t1/2r He1 log KQCS Report t1/28
Dimethyl nitrosamine
Diphenyl nitrosamine
Di-/?-propylnitrosamine
Benzidine
3,3-Dichlorobenzidine
1,2-Diphenylhydrazine
Acrylonitrile
0.06
2.57
1.31
1.81
3.02
3.03
-0.14
nd
nd
nd
A
A
nd
A
nd
nd
nd
nd
nd
nd
0.0040
-0.087
2.465
1.168
1.683
2.928
2.938
-0.324
nd
nd
nd
l,e
l,e
nd
nd
"Log KDW are taken from U.S. EPA (1979).
f Half lives are assessed as A = less than 10 days, B = 10-50 days, and
C - greater than 50 days, based on the prinicipal fate process in the
environment (EPA, 1979).
1 Henry's Constant (dimensionless) has been calculated according to
Thibodeaux (1979) from data supplied within U.S. EPA (1979).
*Log Koc has been calculated according to Rao et al. (1982).
9 Reported half lives are in days unless followed by y = years, followed
by e = U.S. EPA (1979), which is based on the predominant
environmental process thought to determine fate. If more than one
reference occurs for the half life they are separated by ; nd = no
data.
Kearney et al. (1980) investigated the uptake and translocation of 14C-
labeled W-nitrosodipropylamine (NDPA) and /V-riitrosopendimethalin (NP) in
soybeans grown in the field in Matapeake silt loam amended with the
pollutants. No residues of NDPA were found in the beans or other plant parts.
Low levels of 14-C were found in plants grown on NP amended soil but, again,
not in the beans.
Another group of chemicals investigated because of their carcinogenic
properties are the aflatoxins. Hertz et al. (1980) exposed 12-day-old maize
seedlings to Hoagland's solution adulterated with aflatoxin Bl. After 7 days,
the seedlings were transferred to aflatoxin-free solution or soil to determine
the concentration of the toxin absorbed and retained-within the plant tissue.
Two days after transfer, there was a 75% and 50% reduction in root and leaf-
stem concentration. After four days, the tissue concentrations appeared to
increase slightly, probably as a result of reabsorption of previously desorbed
87
-------
toxin. After 13 days, the concentration of the toxin had been reduced to 80%
and 96% of the original in the root and leaf stem tissue, respectively.
In a follow up experiment, Mertz et al. (1981) transplanted 2-day-old
lettuce seedlings to a clay loam adulterated with the aflatoxin Bl. Following
a growth period of 7 to 12 days, the aflatoxin recovered from leaf stem and
root tissue represented less than 1% of the original soil addition.
CONCLUSIONS
At the start of this rather extensive collation and review of reported
plant uptake experiences, it was anticipated that much useful information
concerning levels of uptake, differences between liquid and vapour phase
uptake, and differences between pollutants could be obtained. This has not
been the case.
Because many authors approached uptake from one specific point of view,
all the experimental details or plant details, etc., are not included in their
reports. This has made a broader interpretation of the collected data almost
impossible.
The above data do illustrate one rather major point; no research on
plant accumulation has been undertaken for large groups of chemical compounds.
REFERENCES
Beall, M.L., and R.G. Nash. 1971. Organochlorine insecticide residues in
soybean plant tops: Root uptake vs. vapour sorption. Agron. J. 63:460-464.
Brewer, W.S., A.C. Draper, and S.S. Wey. 1980. The detection of dimethylni-
trosamine and diethylnitrosamine in municipal sludge applied to agricultural
soils. Environ. Pollut. 16:37-43.
Bruce, W.N., G.C. Decker, and J.G. Wilson. 1966. The relationship of the
levels of insecticide contamination of crop seeds to their fat content and
soil concentration of aldrin, and heptachlor. J. Econ. Entomol. 59(1):180-182.
Chou, S.F., L.W. Jacobs, D. Penner, and J.M. Tiedje. 1978. Absence of plant
uptake and translocation of polybrominated biphenyls(PBBs). Environ, Health
Perspectives. 23:9-12.
Dacre, J.C. 1980. Potential health hazards of toxic organic residues in
sludge. In: Sludge - Health risks of land application. Britton, G., R.L.
Damron, G.T. Edds, and J.M. Davidson (eds.). Ann Arbor Sci., Ann Arbor, MI
Dean-Raymond, D., and M. Alexander. 1976. Plant uptake and leaching of di-
.methylnitrosamine. Nature. 262:394-396.
Ellis, B.E. 1974. Degradation of aromatic compounds in plants. Lloydia.
37(2) .-168-184.
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Ellwardt, P. 1977. Variation in content of polycyelic aromatic hydrocarbons in
soil and plants by using municipal wastes composts in agriculture. Proc.
Series - Soil organic matter studies. 2:291-298.
Fries, S.F., and G.S. Marrow. 1981. Chlorobiphenyl movement from soil to soy-
bean plants. J. Agr, Food Chen. 29:757-759.
Grosse, B. 1978. Zur analytik von 3,4 benzpyren und dessen aufnahme uber
wurzel und sprob hoherer pflanzen. Dissertation TU, Munich.
Harms, H, 1975. Metabolisierung von benzo(a)pyren in pflanzlichen zellsus-
pensionskulturen und weizenkeimpflanzen. Landbauforsch 25:83-90.
Harms, H., and D. Sauerbeck.. 1984. Toxic organic compounds in municipal waste
materials: Origin, content, and turnover in soils and plants. Angew. Botam'k.
58:97-108.
Harms, H., W. Dehren, and W. Monch. 1977. Benzo{a)pyrene metabolites formed by
plant cells. Z. Naturforsch, 32:321-326.
Iwata, Y., W.E. Gunther, and F. A. Westlake. 1974. Uptake of a PCB (Aroclor
1254} from soil by carrots under field conditions. Bull. Environ. Contam.
Toxicol. 11(6):523-528.
Jacobs, L.W., S.F. Chou, and J.M. Tiedje. 1976. Fate of polybrominated biphe-
nyls (PBBs) in soils. Persistence and plant uptake. J. Agn'c. Food Chem.
24(6):1198-1201.
Jansen, E.F., and A.C. Olsen. 1969. Metabolism of 14C-labelled benzene and
toluene in avocado fruit. Plant Physiol. 44:786-787.
Jury, W.A., W.F. Spencer, and W.J. Farmer. 1983. Behaviour assessment model
for trace organics in soil; 1. Model description. J. Environ, Qual. 12(4}:558-
564.
Jury, W.A., W.J. Farmer, and W.F. Spencer. 1984. Behavior assessment model for
trace organics in soil; 2. Chemical classification and parameter sensitivity.
J. Environ. QuaT. 13:567-572.
Kearney, P.C., J.E. Oliver, A. Kontson, W. Fiddler and J.W. Pensabene. 1980.
Plant uptake of dinitroaniline herbicide related nitrosamines. J. Agric. Food
Chem. 28:633-635.
Lichtenstein, E.P. 1980. Bound residues in soils and transfer of soil residues
in crops. Residue Rev. 76:147-153.
Lichtenstein, E.P., G.R. Myrdal, and K.R. Schulz. 1965. Absorption of insecti-
cidal residues from contaminated soils into five carrot varieties. J. Agr.
Food Chem. 13(2):126-131.
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Hertz, D., T. Edward, D. Lee, and M. Zuber. 1981. Absorption of aflatoxin by
lettuce seedlings grown in soil adulterated with aflatoxin 81. J. Agric. Food
Chem. 29:1168-1170.
Hertz, D., D. Lee, M. Zuber, and E. Lillehoj. 1980. Uptake and metabolism of
aflatoxin by Zea mays. J, Agric. Food Chem. 28:963-966.
Hoza, P., I. Scheunert, W. Klein, and F. Korte. 1979. Studies with 2,4,5-tri-
ch1orobiphenyl-14C and 2,2',4,4',6-pentachlorobiphenyl-14C in carrots, sugar-
beets, and soil. J. Agric. Food Chem. 27{5}:1120-1124.
Hpozek, E., and R.B. Leidy. 1981. Investigation of selective uptake of PCB by
Spartina alterniflora. Bull. Environ. Contain. Toxicol. 27:481-488.
Huller, V.H. 1976. Aufnahme von 3,4-benzpyren durch nahrungspflanzen aus
kunstlich angereicherten substraten. Z. Pflanzenernaehr. Bodenkd. 6:685-695.
Nash, R.G. 1968. Plant uptake of 14C diuron in modified soil. Agron. Jour.
60:177-179.
Nash, R.G. 1974. Plant uptake of insecticides, fungicides and fumigants from
soils. In: Pesticides in soil and water. W. Guenzi (ed.) Chapter 11. Pp.257-
313.
Overcash, M.R., J.B. Weber, and M.L. Miles. 1982. Behaviour of organic prior-
ity pollutants in the terrestrial system: Di-/j-butyl phthalate ester, toluene
and 2,4-dinitrophenol. Water Resources Research Inst. 171:94.
Rao, P.S.C., and J.M. Davidson. 1982. Retention and transformation of selected
pesticides and phosphorous in soil-water systems. A critical review. U.S. EPA.
600/3-82-060. 341 pp.
Ryan, J. 1986. The land treatment of appendix VII organics presented in petro-
leum industry wastes. In: Land treatment- A hazardous waste management alter-
native. Loehr, R.C., and J.F. Malina (eds.),
Sander, J., M. Ladenstein, J. Labar, and F. Schweinsberg. 1975. Experiments on
the degradation of nitrosamines by plants. In: N-Nitroso compounds in the
environment. Bogovski, P., and E.A. Walker {eds.}. Pp. 205-210.
Sawhney, B.L., and L. Hankin. 1984. Plant contamination by PCBs from amended
soils. J. Food Prod. 47(3):232-236.
Sims, R.C. 1982. Land treatment of polynuclear aromatic compounds. Ph.D.
Thesis, N. Carolina University, Raleigh, N.C.
Sims, R.C., and M.R. Overcash. 1983. Fate of polynuclear aromatic hydrocarbons
(PNA's) in soil-plant system. Residue Revs. 88:2-68.
Strek, H.J., and J.B. Weber. 1982. Behaviour of polychlorinated biphenyls
(PCB's) in soils and plants. Environ. PoTlut. 28:291-312.
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Suzuki, M.} N. Aizawa, G. Okano, and T. Takahashi. 1977. Trans!ocation of PCBs
in soil into plants: A study by a method of culture of soybean sprouts. Arch.
Environ. Contain. Toxicol. 5:343-352.
Topp, E,, I. Scheunert, A. Attar, and F. Korte. 1986. Factors affecting the
uptake of 14C-labelled organic chemicals by plants from soil. EcotoxicoT.
Environ. Safety. 11:219-228.
U.S. EPA. 1979. The water related environmental fate of 129 priority pollu-
tants. P880-204373. Office of Water Planning and Standards, Washington, D.C.
Wagner, K.H., and I. Siddiqi. 1971. Die speicherung von 3,4-benzfluoranthen im
sommerweizen und sommerroggen. 2. PfTanzenernahr. Bodenkd. 130:241-243.
Wagner, K.H., and I. Siddiqi. 1970. Der stoffwechsel von 3,4-benzpyren und
3,4-benzfluoranthen im sommerweizen. Z. Pflanzenernahr. Bodenkd. 127:211-219.
91
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-------
SECTION 6
EXPERIMENTAL INVESTIGATIONS
INTRODUCTION
A range of experiments was designed to investigate some of the
hypotheses previously reported on plant uptake of nonionic hydrocarbons from
soils. The experiments can be described simply as assessments of plant uptake
under a range of conditions, using different soil types and different plant
species. Uptake has been generally taken to include both root uptake and
translocation and vapour uptake following volatilization from the soil
surface.
Each of the experiments can be related to one another through controls
so that climatic variations, occurring through the use of a greenhouse without
air conditioning, could be assessed. The experiments were not designed to be
definitive statements of uptake but descriptions of potential.
The following experiments were conducted.
1. An introductory experiment for familiarization with the greenhouse
and operational conditions and to assess the potential for plant uptake of
hexachlorobenzene (HCB), phenol, toluene, and trichloroethane (TCE).
2. An assessment of plant uptake of HCB from different soil media.
3. An assessment of plant uptake of HCB with plant age.
4. An assessment of the effects of volatilization of HCB on total plant
uptake concentrations.
5. An assessment of the uptake of HCB from soil by different plant
species.
Each of the experiments was similar in a number of respects.
1. Pollutants were mixed within the soil and the plants were sown as
seed and harvested as adults from each pot.
2. The 5-inch-diameter plastic pots were completely lined with
aluminium foil so that the polluted soil did not come into contact with the
sides of the pot.
93
Preceding page blank
-------
3. The soil medium consisted of a mixture of cactus soil (Franks Nurs-
ery) and silica sand (Wedron Silica Co.) to achieve various soil organic
matter levels.
4. Each pot contained 600 g of dry soil material to which analytical
grade chemical was added to achieve the desired pollutant concentrations,
5. The pots were watered to excess so that water collected in saucers
beneath each pot.
6. The pots were spaced apart from one another.
7. Fertilizer was added at various intervals throughout experiments as
a liquid feed.
8. At harvesting, watering stopped and actual harvesting did not begin
until all excess water in the saucer beneath each pot had been drawn back into
the pot.
9. Harvesting consisted of collecting plant roots and plant tops as
separate items and taking fresh weights.
10. Polluted soil was carefully removed from plant roots.
11. All treatments were replicated at least three times.
Analyses were conducted according to U.S. Environmental Protection Agency
approved methods (U.S. Environmental Protection Agency, 1982).
Time constraints may have reduced the overall value of these
investigations. Although the results would have been more complete without
the limitations, the experimental strategy was to assess the potential for
plant uptake. It is unlikely, then, that the limitations listed below are
highly significant.
1. Biological activity within the experimental pots was not assessed.
Although sterilized soil materials were used in setting up the pots, there
were visual signs of fungal growth towards the end of some experiments. No
doubt bacterial activity occurred in the pots. This activity could have
reduced the concentration of the pollutant under test through biological
degradation and may not have occurred equally throughout all test pots.
2. The biological variation of the plant material under test and indi-
vidual plant and species variation remain unassessed. For example, the
variation in germination rate between one batch of seeds of a particular
species could vary significantly from a further batch of the same seed purely
through genetic difference. This difference becomes more important in the
experiment where many different species are used. Wherever possible,
replicates were used to attempt to overcome this variation.
94
-------
3. Because harvesting both soil and plant material was destructive and
thereby terminated the investigation, there was no monitoring of uptake with
time.
4. No analytical scans investigated degradation products arising from
biological breakdown of the pollutant under test. This may cause underesti-
mates of soil and plant concentrations.
The experiments were all conducted in the greenhouses at the U.S.
Environmental Protection Agency's T & E Facility in Cincinnati, OH, The
environmental conditions in the greenhouses were monitored for temperature
(°«F) and relative humidity; results throughout the experimental investigations
are outlined below (Table 6.1).
95
-------
TABLE 6.1. ENVIRONMENTAL CONDITIONS WITHIN THE EXPERIMENTAL GREENHOUSES
Temperatures Relative Humidity
Week commencing Mean High Low Mean
10 Aug 1986
17 Aug
24 Aug
31 Aug
07 Sep
14 Sep
21 Sep
28 Sep
05 Oct
12 Oct
19 Oct
26 Oct
02 Nov
09 Nov
16 Nov
23 Nov
30 Nov
07 Dec
14 Dec
21 Dec
28 Dec
04 Jan 1987
11 Jan
18 Jan
25 Jan
01 Feb
08 Feb
15 Feb
22 Feb
01 Mar
08 Mar
15 Mar
22 Mar
29 Mar
05 Apr
12 Apr
19 Apr
26 Apr
03 May
10 May
17 May
24 May
31 May
07 Jun
14 Jun
80
85
75
75
75
70
75
80
75
70
65
65
65
65
65
65
70
65
70
65
70
65
65
70
75
75
75
70
75
75
75
70
75
75
75
75
70
80
80
85
85
85
85
100
100
102
100
102
95
100
115
98
100
93
100
95
83
83
__
95
100
90
95
93
80
90
88
100
105
100
100
93
105
105
100
85
100
105
110
105
110
110
105
105
110
no
no
60
68
50
52
54
52
64
66
55
45
55
50
__
33
55
55
--
55
55
55
55
57
55
53
50
55
50
53
58
58
55
60
60
60
60
60
55
60
55
60
55
65
70
65
70
65
60
50
55
55
65
60
60
65
45
60
50
--
45
45
45
__
35
45
45
35
35
35
35
20
25
25
15
20
30
15
15
15
25
25
40
35
40
25
25
25
30
30
30
40
96
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EXPERIMENT TO DETERMINE THE ACCUMULATION AND PHYTOTOXICITY OF HEXACHLORQ-
BENZENE (HCB) IN RADISH AND CARROT GROWN IN HCB POLLUTED SOIL
Methods
A growth medium (1:1 weight ratio of cactus soil [Frank's Nursery] and
silica sand [Wedron Silica Co.] resulting in an organic matter content of
5,70%) was placed in aluminum foil-wrapped 5 inch pots. Analytical grade HCB
(Fischer Scientific) was mixed with the soil medium to give soil-borne
concentrations of 0, 1, 10, 100, 250, 500, 1000, 2500, 5000, and 10000 mg
HC8/kg of dry growth medium.
Ten radish seeds and 15 carrot seeds were planted per pot in growth
medium containing a range of HCB concentrations.
The experiment began on August 12, 1986. On September 29, after 48 days
of growth, the radishes were harvested, and on October 31, after 80 days of
growth, the carrots were harvested. Each treatment had three replicates.
During harvesting, the vegetation was cut from the roots and vegetation
samples were analyzed. The pots were allowed to stand for 3 days to allow the
soil to dry sufficiently to separate the roots and tubers from the soil.
Results
TABLE 6.2. GERMINATION OF THE SEED TYPES, PER SOIL CONCENTRATION OF HCB,
30 DAYS AFTER SOWING
Proposed Soil % Seeds germinated
concentration
mg HCB /kg Radish Carrot
0
1
10
100
250
500
1000
2500
5000
10000
77.0
47.0
27.0
33.0
3.0
17.0
13.0
47.0
3.0
10.0
91.3
__
--
28.7
--
--
42.0
48.7
62.0
97
-------
TABLE 6.3. SOIL AND RADISH CONCENTRATIONS* OF HCB FOLLOWING
RADISH GROWTH WITHIN THE POLLUTED SOIL FOR 50 DAYS
Concentrations in soil
Initial Final
Concentrations in vegetation
Root Leaf Radish
(edible part)
0.00
5.15
2.01
2188
3994
7216
10150
5770
18200
18890
0.03
0.41
1.54
6.81
13.08
35.30
56.60
91.70
119.90
101.00
0.28
6.03
8.10
19.97
26.7
37.5
24.5
21.3
46.6
0.00
0.07
0.02
0.03
0.01
0.12
0.05
0.67
0.28
0.08
0.00
0.62
0.02
0.63
3.09
381.00
665.00
776.00
508.00
604.00
"All concentrations in mg HCB/kg soil
TABLE 6.4. SOIL AND CARROT CONCENTRATIONS OF HCB FOLLOWING CARROT GROWTH
WITHIN THE POLLUTED SOIL FOR 50 DAYS
Concentration in soil
Initial Final
Concentration in vegetation
Root Leaf Carrot
(edible part)
2188
10150
18200
18890
51.8
88.4
2396.6
3668.0
140.9
328.6
6957.0
6440.0
5.16
33.43
19.23
7.50
27.18
27.08
76.22
93.52
"All concentrations in mg HCB/kg soil.
Conclusions
This simple experiment highlighted many of the problems associated with
work of this nature.
1. The germination rates of both radish and carrot seeds varied
greatly, from only 3% to 77% for radish and 28.7% to 91.3% for carrot,
98
-------
suggesting that the greater the number of seeds grown, the greater
experimental reproducibility would be achieved.
2. This variation in germination masks any effects of the pollutant on
germination, but it appears that HCB concentrations up to 1% in soil of 5,7%
organic matter content do not affect radish or carrot seed germination.
3. The analytical testwork had poor reproducibility; the concentrations
measured at the beginning of the experiment were not verified by the analysis.
The final soil concentrations appeared unrelated to the initial.
4. When radish was grown in HCB-polluted soils, the highest concentra-
tions (7216 mg/kg) of HCB occurred in the edible portion of the radish, with
least being found in the plant leaves. Generally, concentrations in the plant
tissue increased with increasing soil concentrations.
5. When carrot was grown in HCB-polluted soils, the highest HCB concen-
trations occurred in the root of the carrot as distinct from its edible por-
tion, and, again, lowest concentrations were found in the plant leaves.
6. HCB concentration in the carrot root reached a maximum of 69i7 rag
HCB/kg following growth in soil, whose initial concentration was 18,200 mg
HCB/kg and whose final soil concentration of 2396.6 mg HCB/kg.
7. This experiment highlights one of the problems in using the Root or
Leaf Concentration Factor as a term describing plant accumulation of pollu-
tants from soil. Soil concentrations of pollutants rarely remain stable, and
in this experiment, a consistent trend was shown with the concentration of HCB
in soil being reduced throughout the experiment, probably as degradation
occurred.
HCB is a long lived pollutant, with a log n-octanol/water partition
coefficient of 6.18 and a Henry's Constant of 0.0251. Therefore HCB would be
expected to be strongly sorbed to the organic matter in the soil, and in due
course to the plant root, and migrate through the soil pores in the vapour
phase. Controlling the vapour migration of HCB wastes has been addressed in
U.S. Environmental Protection Agency (1980).
EXPERIMENT TO DETERMINE THE ACCUMULATION AND PHYTOTOXICITY OF
TRICHLOROETHANE (TCE) IN RADISH GROWN IN TCE POLLUTED SOIL
Methods
The methods outlined in the Section above, were replicated to establish
this experiment. Soil concentrations of TCE were 0, 500, 1000, 2500, 5000,
and 10000 mg/kg, and 5%'and 10%. The soil organic matter content was 9.4% by
weight.
The experiment began on August 29, 1986. On October 22, 1986, after 54
days of growth, the radishes were harvested.
99
-------
Results
TABLE 6.5. GERMINATION OF RADISH SEED, PER SOIL CONCENTRATION OF TCE,
30 DAYS AFTER SOWING
Proposed soil
concentration % Seed germination
0 83
500 97
1000 87
2500 100
5000 87
10000 87
5% 83
10% 73
TABLE 6.6, SOIL AND RADISH CONCENTRATIONS OF TCE (mg/kg FRESH WEIGHT)
FOLLOWING RADISH GROWTH IN THE POLLUTED SOIL FOR 55 DAYS
Concentration in soil Concentration in vegetation
Initial Final Leaf Roots
43.1
236.0
323.0
481.0
1373.0
957.0
4104.0
0.086
0.084
1.316
1.576
0.839
1.652
9.995
9.473
0.713
0.484
0.180
0.153
1.003
0.330
0.162
0.122
0.274
0.174
0.094
0.374
1 . 533
4.792
4.457
2.685
Conclusions
TCE is not an ideal pollutant to investigate under these experimental
conditions. It has a log K<,w of 2.17 and a high Henry's Constant of 1,46,
indicating its volatility. TCE is known to be degraded by various systems
(Wilson and Wilson, 1985). The experiment indicated:
1. Even soil concentrations of TCE up to 10%, in a soil of organic
matter content of 9.4%, did not affect the germination rate of the radish
seeds. Possibly higher concentratins of TCE than these would prove toxic to
germinating seeds but these concentrations actually delayed germination until
the soil concentrations had been reduced to a level that did not prove toxic.
100
-------
2. The soil concentration of TCE throughout the 55 day experiment was
drastically reduced so that maximum concentrations experienced by the plants
at the end of the experiment were only up to 10 mg/kg.
3. TCE was found in both the leaves and roots of radish grown in the
polluted soils. There was no trend of increasing plant concentrations with
increasing soil concentrations, probably because of the drastic reduction in
soil concentrations discussed above and TCE's volatility that allowed all
plants to be exposed to TCE to a similar extent.
EXPERIMENT TO DETERMINE THE ACCUMULATION AND PHYTOTOXICITY OF
PHENOL IN RADISH AND CARROT GROWN IN PHENOL POLLUTED SOIL
Methods
The methods outlined above were replicated to result in a wide range of
soil concentrations of phenol. The soil organic matter content was 9.4% by
weight.
The experiment began on November 3, 1986. On January 22, 1987, after 80
days of growth, the radishes were harvested, and on February 16, 1987, after
112 days of growth, the carrots were harvested.
Results
TABLE 6.7. GERMINATION OF THE SEED TYPE, PER SOIL CONCENTRATION OF PHENOL,
30 DAYS AFTER SOWING
Presumed soil
concentration Radish Carrot
0
50
100
500
1000
2500
5000
10000
83
93
90
70
53
0
0
0
58
45
40
42
9
0
0
0
101
-------
TABLE 6.8. FRESH WEIGHT OF CARROT GROWN FOR 112 DAYS, AND RADISH GROWN FOR
80 DAYS IN PHENOL-POLLUTED SOIL
Initial soil
concentration
Carrot,
total production
Leaf Root
Radish,
total production
Leaf Root
5.7
25.6
6.5
193.5
587.0
1408.0
1232.0
13823.0
18.4
15.2
18.5
16.7
4.9
0
0
0
33.1
26.2
26.0
25.8
12.3
0
0
0
34.3
32.7
40.7
28.6
24.5
0
0
0
6.4
9.9
6.3
7.8
10.9
0
0
0
"Fresh weight in g, concentration in mg/kg.
Conclusions
Phenol has not appeared to be the ideal pollutant to investigate under
these environmental conditions. Its log n-octanol/water partition coefficient
is 1.46, indicates a water solubility and it is degraded relatively easily by
a variety of routes. Detailed conclusions follow.
1. Germination of both plant species was reduced as concentrations of
phenol in the soil reached 1000 mg/kg. No seeds of either species germinated
at concentrations above 2500 mg phenol/kg soil.
2. Total production, assessed as fresh weights of both species, was
determined at harvesting. Carrot produced more root than leaf during its 112-
day exposure, whereas radish produced more leaf than root. Overall production
of both species per pot was remarkably similar at approximately 40 mg fresh
material per pot.
3. Due to analytical problems, which could not be overcome, phenol
concentrations in vegetation were not determined and so uptake rates could not
be assessed.
102
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EXPERIMENT TO DETERMINE THE ACCUMULATION AND PHYTOTOXICITY OF TOLUENE
IN RADISH AND CARROT GROWN IN TOLUENE POLLUTED SOIL
Methods
The methods outlined were replicated to establish this experiment. Soil
concentrations of toluene were 0, 50, 100, 500, 1000, 2500, 5000, and 10000
mg/kg. The soil organic matter content was 9.4% by weight.
The experiment began on November 4, 1986. On January 22, 1987, after 81
days of growth, the radishes were harvested, and on February 20, 1987, after
116 days of growth, the carrots were harvested.
Results
TABLE 6.9. GERMINATION OF CARROT AND RADISH, % OF APPLIED SEEDS,
PER SOIL CONCENTRATION, mg/kg, OF TOLUENE
Presumed soil
concentration
Radish
Carrot
0
50
100
500
1000
2500
5000
10000
87
87
73
87
83
90
67
30
35
25
45
47
40
29
49
2
TABLE 6.10. FRESH WEIGHT OF CARROT GROWN FOR 116 DAYS AND RADISH GROWN FOR 81
DAYS IN TOLUENE-POLLUTED SOIL
Presumed soil
concentration
Carrot
Total production
Shoot Root
Radish
Total production
Shoot Root
0
50
100
500
1000
2500
5000
10000
13.7
10.3
15.7
12.7
16.9
18.3
13.9
1.6
36.5
27.5
32.2
32.8
37.2
40.1
24.8
6.7
23.5
21.4
25.2
23.6
22.6
25.1
17.2
13.7
19.0
13.6
11.0
9.2
8.7
8.1
6.8
7.6
"Fresh weight in g, presumed concentration in mg/kg.
103
-------
Conclusions
Analysis of soils and vegetation for toluene was also very difficult and
no analytical results were obtained. The experiment has shown that the
germination rate of both carrot and radish was affected by concentrations of
toluene between 5,000 and 10,000 mg/kg in soil containing 9.4% organic matter.
These concentrations did not prove totally phytotoxic.
EXPERIMENT TO ASSESS THE EFFECT OF DIFFERENT SOIL ORGANIC MATTER CONTENTS
ON THE ACCUMULATION OF HCB IN RADISH GROWN IN HCB POLLUTED SOIL
»
Methods
This experiment used five growth media: pure silica sand, pure cactus
soil, pure peat (Transcontinental Peat Moss Co.), and various mixtures of
these materials to give a range of soil organic matter contents. These were
pure sand 0.0 %
5% peat, 95% sand 3.1 %
5:1 sand:peat 10.2 %
pure cactus soil 16.7 %
pure peat 96.1 %
'0
Each of these soil media was treated with five concentrations (0, 100, 250,
500, and 1000 mg HCB/kg) of HCB, with three replicates of each treatment.
Each treatment was sown with 10 seeds.
The experiment began on December 6, 1986, and on January 13, 1987, after
radish growth of 67 days in sand and 80 days in the other growth media, rad-
ishes were harvested on February 11 and 22 and on March 10, 1987.
Results
TABLE 6.11. GERMINATION RATES'QF RADISH TAKEN AT HARVEST TIME, AFTER BEING
SOWN IN HCB POLLUTED SOILS OF VARYING ORGANIC MATTER CONTENTS
Presumed soil Organic matter contents
concentrations 96% 17% 10% 3% 0%
0
100
250
500
1000
47
33
47
47
60
83
83
90
87
87
50
50
53
30
23
77
60
43
60
80
30
67
27
40
33
*At maturity, 67 days after sowing in sand and 80 days
after sowing in the other growth media
104
-------
TABLE 6.12. INITIAL AND FINAL SOIL CONCENTRATIONS* OF HCB FOLLOWING RADISH
GROWN TO MATURITY IN SOIL OF DIFFERENT ORGANIC MATTER CONTENTS
Percent Organic Matter Contents
0%
Init Final
0.5 2
203 385
270 644
457 697
456 859
3%
Init Final
4 2
74 155
340 231
598 489
1167
10%
Init
3
158
233
532
692
Final
1
150
251
277
328
17%
Init Fi
0.6
207
495
670
589
nal
1
150
252
280
327
96%
Init Final
4 0.2
335 190
243 249
514 668
1060 1242
"Concentrations as rag HCB/kg.
TABLE 6.13. ACCUMULATION OF HCB IN RADISH ROOTS AND LEAVES AFTER BEING GROWN
TO MATURITY IN SOILS OF DIFFERENT ORGANIC MATTER CONTENTS
Organic Matter Contents
0% 3% 10% 17% 96%
Root Leaf Root Leaf Root Leaf Root Leaf Root Leaf
0.6
39.2
26.8
31.8
31.2
0.00
0.00
0.02
0.01
0.01
0.01
3.9
5.4
10.1
9.2
0.00
0.01
0.03
0.01
_ - -
0.03
5.6
6.0
10.8
7.4
0.00
0.00
0.01
0.01
0.01
0.1
5.0
6.4
17.4
10.4
0.00
0.00
0.01
0.01
0.00
0.1
1.5
2.3
3.1
4.4
0.00
0.00
0.00
0.00
0.00
'Accumulation as mass by concentration in mg.
Conclusions
This successful experiment highlights the importance of soil organic
matter in sorping pollutants within the soil. Such sorption reduces the
amount of any soil borne pollutant available for plant uptake and
accumulation.
105
-------
Detailed conclusions include the following.
1. As established previously, HCB concentrations of up to 1000 mg/kg
had no effect upon the germination rate of the test species. This is the case
even when the pollutant is present in pure sand or pure peat, with soil
organic matter contents ranging from 0 to 100%,
2. When the presumed soil concentrations of HCB (i.e., those attempted
by carefully weighing out and mixing of materials) were compared with those
actually found at the start and termination of the experiment, several prob-
lems became apparent,
«
a. Some HCB added to the soils could not be found in the soil
after just 2 hours, i.e., the estimated time between
establishing the pots and sampling the soil. This could be
explained by the concept of irreversibly bound residues, as
discussed in Section 2 of this report.
b. Generally, HCB concentrations in all soils decreased with time,
so that at the end of the experiment, when plant mass would be
greatest, pollutant concentrations available for plant uptake
would be least.
3. When the accumulation of HCB, calculated as the plant-part mass
times its concentration, in the radish roots and leaves are considered, the
following are concluded.
a. Considerably more HCB accumulated in the plant root than in
the plant leaf.
b. The plant root accumulated more HCB as HCB concentrations in
the soil medium increased.
c. Considerably more HCB was accumulated in plant roots grown in
pure sand, with a soil organic matter of 0%, than in pure peat,
with a soil organic matter content of 100%. In sand, sorption
of HCB by soil organic matter would be absent.
d. A maximum of 39.2 mg of HCB was accumulated by radish roots
grown in sand polluted by an initial soil concentration of 202
mg/kg. In a pot containing 600 g of soil, this approximates
33% of the total soil borne pollutant. If the attempted soil
concentration of 250 mg/kg was, in fact, achieved, this percent
changes to 26%. If the final soil concentration of 385 mg/kg
is used, plant root accumulation accounted for 17% of the HCB
in the pot.
106
-------
EXPERIMENT TO DETERMINE THE EFFECT OF PLANT AGE ON THE ACCUMULATION
OF HCB IN RADISH GROWN IN HCB POLLUTED SOIL
Methods
Two concentrations of HCB {1000 and 5000 mg/kg) were made up in soil
with an organic matter content of 5.7%. Each pot was sown with 10 radish
seeds on March 19, 1987, and each treatment had four replicates. Plants were
harvested every 2 weeks for 14 weeks, with the final harvest being taken on
May 7, 1987.
Results
TABLE 6.14. SOIL, PLANT ROOT, AND PLANT LEAF CONCENTRATIONS OF HCB,
(mg/kg), WITH TIME; SEED SOWN IN SOIL CONTAINING 5.7% ORGANIC MATTER
AND AN ESTIMATED 1000 mg HCB/kg.
Time, days
after sowing
Soil
Root
Leaf
14 1034
22 1889
31 1126
37 2059
43 . 2347
49 2398
222
314
242
271
363
159
778 {
1054
1453
4782
3342
4095
164} 233 (
; 84) 75
184) 88
866) 44
485) 103
1088) 147
159)
45
15
11
41
46
"Mean of 4 replicates (Standard Error)
TABLE 6.15. SOIL, PLANT ROOT, AND PLANT LEAF CONCENTRATIONS OF HCB,
(mg/kg), WITH TIME; SEED SOWN IN SOIL CONTAINING 5.7% ORGANIC MATTER
AND AN ESTIMATED 5000 mg HCB/kg
Time, days
after sowing
Soil
Root
Leaf
14
22
31
37
43
49
3193
4742
4407
6987
7174
5912
(839
(1015
605
592
817
765
1103
3215
2707
9004
5032
13249
(57) 155 (47)
(900
(339
(1064
(413
(3498
186
115
195
52
622 (
63
14
72
36
83
"Mean of 4 replicates (Standard Error)
107
-------
TABLE 6.16. PLANT ROOT ACCUMULATION OF HCB (mg) WITH TIME;
GREENHOUSE GROWN IN A SOIL CONTAINING 5.7% ORGANIC MATTER
Time, days Soil concentration mg HCB/kg
after sowing 1000 5000
Root Leaf Root Leaf
14
22
31
37
43
49
3.
3.
10.
108.
269.
264.
7
4
8
0
2
9
(0
(0
(0
(9
(41
(42
.7)"
.3}
• 8)
.9)
.9)
.6)
0
0
1
1
3
3
.7
.6
.2
.2
.6
.3
5
13
29
131
421
634
.9
.3
.2
.6
.9
,0
(0.8)
(2.9)
(3.0)
(10.0)
(60.9)
(131)
0
1
1
4
1
12
.5
.5
.7
.3
.6
.9
* Accumulation as mass by concentration in mg.
"Mean of four replicates (Standard Error)
TABLE 6.17. SOIL CONCENTRATION OF HCB (rag/kg) AFTER BEING EXPOSED FOR VARIOUS
TIME INTERVALS, WITH OR WITHOUT ESTABLISHED VEGETATION, IN A GREENHOUSE
Time, days
after sowing With vegetation Without vegetation
14
22
31
37
43
49
1034 (222)*
1889 (314)
1126 (242)
2059 (271)
2347 (363)
2398 (159)
1640 (145)
2505 (462)
861 (187)
1834 (230)
1771 (189)
2432 (371)
*Mean of 4 replicates (Standard Error)
Conclusions
This was another successful experiment investigating the changes in
plant accumulation; of soil-borne pollutants with actual time of exposure.
Detailed conclusions follow.
1. Unfortunately, soil concentrations appeared to have increased with
time for .both the pots with 1000 and those with 5000 mg HCB/kg soil. This is
obviously not possible and must result from some analytical problem. The
108
-------
concentration in the 1000 mg/kg pots had increased to 2398 mg/kg after 49 days
and the 5000 mg/kg pots had increased to 5912 mg/kg,
2. Plant root concentrations also increased with time in both sets of
pots. There was a 5-fold increase in the 1000 mg HCB/kg pots and a 12-fold
increase in the 5000 mg/kg pots, from 14 days to 49 days exposure.
3. Plant leaf concentrations of HCB remained low throughout the experi-
ment .
4. A large variation in the replicate pots of each treatment, as as-
sgssed by the standard error, suggests that more replicates should have been
used.
5. The concentration of HCB in soil that was not planted with radish
generally reflected the changes in HCB concentration found in the correspond-
ing pots sown with radish. At harvesting, the corresponding soil concentra-
tions were 2398 mg/kg in the pots with radish and 2432 mg/kg in the pots
without.
6. Of most significance is the fact that both plant roots and leaves,
grown in 1000 or 5000 mg HCB/kg soil, accumulated more HCB with increasing
time of exposure. HCB mass within the radish root increased approximately 70
times, from 14 days to 49 days exposure in soil containing 1000 mg HCB/kg.
The corresponding increase in the 5000 mg/kg pots was 110 times.
7. The'maximum plant root accumulation of the pollutant was apparently
reached after 49 days' growth in the 1000 mg/kg pots; the accumulation after
43 and 49 days' exposure remained relatively constant. That this was not the
case for the 5000 mg/kg pots could reflect the equilibrium between the soil
sorbed and plant available pollutant. In the 1000 mg/kg pots, all pollutant
available for plant uptake had in fact been taken up by the 49th day. In the
5000 mg/kg pots, because of the larger concentration available for
uptake, this position had not been reached.
8. The maximum accumulation of the pollutant by the plant roots in the
1000 and 5000 mg/kg pots, based on the corresponding soil concentration, ac-
counted for 19% and 18%, respectively, of the added HCB.
EXPERIMENT TO ASSESS THE EFFECT OF VOLATILIZATION OF HCB FROM HCB POLLUTED
SOIL ON THE ACCUMULATION OF HCB IN RADISH PLANTS
Methods
Two concentrations of HCB (0 and 1000 ppm) and nine replicates of each
concentration were established. Radish seeds were sown into the polluted
soils through 3/8-inch plastic tubing for all the pots, to isolate the lower
leaves from the soil. The pots were then covered with aluminium foil so that
only the upper portions of the tubes were exposed. Each pot was sown with
four radish seeds. The experiment began on March 3, 1987, and radishes were
harvested after 35 days. The pots were kept adjacent to one another.
109
-------
Controls, containing no HCB and left uncovered to the atmosphere, were also
established.
Results
TABLE 6.18. THE EFFECTS OF COVERING THE SOIL SURFACE ON CONCENTRATIONS OF
HCB IN RADISH GROWN IN A GREENHOUSE FOR 35 DAYS IN A SOIL OF
5.7% ORGANIC MATTER AND POLLUTED BY 1000 mg HCB/kg
Plant part/
Soil
Leaf
Root
Soil
1000
Covered
157 (33)*
1045 (215)
1553 (189)
mg HCB/ kg
Uncovered
242 (49)
1610 (340)
1289 (149)
0 mg HCB/kg
Control
117 (30)
97 (114)
26 (6)
"Results from 9 Replicates (Standard Error)
TABLE 6,19. THE EFFECTS OF COVERING THE SOIL SURFACE ON THE ACCUMULATION
(mg) (CONCENTRATION BY MASS) OF HCB IN RADISH GROWN IN A GREENHOUSE FOR
35 DAYS IN A SOIL OF 2% ORGANIC MATTER AND POLLUTED BY 1000 mg HCB/kg
Plant part Covered Uncovered
Leaf
Root
0.997 (0.23)
9.02 (1.15)
0.896 (0.17)
6.36 (1.23)
Conclusions
This experiment was specifically designed to quantify the volatilization
of HCB from the experimental pots and to investigate the significance of the
tranlocation of HCB from the plant roots to the leaves. The experimental
design should have stopped gaseous pollutant uptake by the plant leaves. It
was expected that the leaves of plants grown isolated from the soil surface
would contain less HCB than those where gaseous uptake may have occurred. De-
tailed conclusions include the following.
1. The plant leaf contained higher concentrations of HCB when the soil
surface was left uncovered than when it was covered. This was expected as the
uptake of volatilized pollutant was stopped by the aluminium foil in those
covered pots. This indicated that gaseous phase uptake by plant leaves is
significant for HCB.
110
-------
2. The plant root concentration was highest in the pots with uncovered
soil surfaces. Reasons for this are uncertain. Possibly vapour phase
transport in the soil is also significant for root sorption of the pollutant,
and volatilized pollutant has been kept in the vicinity of the root, available
for uptake, by the presence of the soil surface cover.
3. The soil concentration at the end of the experiment was, as expect-
ed, lowest in those pots with uncovered soil surfaces. Volatilization of the
pollutant had reduced soil concentrations where the vapour phase pollutant had
been allowed to leave the soil.
4. Surprisingly, large concentrations of pollutant were recovered in
soil, plant roots, and leaves from pots that originally had received no
pollutant loading. This clearly demonstrates the importance of vapour phase
transport of HCB. Plant leaves, the first surface to be exposed to gaseous
HCB from adjacent pots, showed highest concentrations.
5. The plastic tubing around the plant increased both plant root and
leaf yield and the trends discussed above are reversed when accumulation of
the pollutant is considered.
EXPERIMENT TO ASSESS THE UPTAKE OF HCB BY DIFFERENT PLANT SPECIES
WHEN GROWN IN HCB POLLUTED SOIL
Methods
Two concentrations of HCB {1000 and 5000 ppm) with five replicates of
each concentration were sown with 12 different plant species. The experiment
began on March 19, 1987. Plants were harvested at various dates depending on
the growth of the crop, which, in turn, partially depended on its suitability
for greenhouse growth. The plants used and their harvest or exposure times
were
Festuca ovina (Sheeps fescue) 97 days
Kentucky Bluegrass (Argyle) 77 days
Taraxacum officinale (dandelion) •- 63 days
Lathyrus latifolius (perennial sweet pea) --89 days
Sweet corn (Golden hybrid blend) 36 days
Medicago sativa Ladak (Alfalfa) 82 days
Green bean (Tendergreen) 36 days
Carrot (Danver) 82 days
Radish (Crimson Giant) - 36 days
Tn'folium pratense (red clover) — 63 days
Beet (Early Wonder Green Top) -- 63 days
Fescue/Clover mix -- -- 77 days
111
-------
Results
TABLE 6.20. FINAL PLANT ROOT CONCENTRATIONS (mg/kg dry weight), GREENHOUSE
GROWN IN SOIL CONTAINING 2% ORGANIC MATTER AND TWO CONCENTRATIONS OF HCB
600 mg HCB/kg 3000 mg HCB/kg
Initial soil 1243 (41)* 4581 (170)'
Bean
Fescue
Clover
Radish
Bluegrass
Corn
Dandel ion
Alfalfa
Carrot
Beet
Pea
Fescue/Clover
Radish (no foil)
3291
1101
3089
3479
1228
2471
883
1253
1974
3372
807
950
2217
(166)
(144)
(625)
(684)
(92)
(143)
(80)
(246)
(693)
(410)
(60)
(73)
(365)
4614
3880
4184
8792
5366
4468
2897
4048
5827
4358
4506
4629
6366
(406)
(336)
(353)
(821)
(421)
(359)
(991)
(492)
(892)
(290)
(208)
(500)
(656)
"Five replicates (Standard Error)
112
-------
TABLE 6.21. PLANT ACCUMULATION (nig) OF HCB, GREENHOUSE GROWN IN A SOIL
CONTAINING 2% ORGANIC MATTER AND 600 OR 3000 mg HCB/kg
600 mg HCB/kg 3000 mg HCB/kg
Root Leaf Root Leaf
Bean
Fescue
Clover
Radish
Bluegrass
Corn
Dandelion
Al f al fa
Carrot
Beet
Pea
Fescue/Clover
Radish (no foil)
36
5
44
46
10
37
26
12
7
36
7
12
31
0.7
0.0
0.1
0.6
0.0
0.3
0.8
0,0
0.1
0.2
0.0
0.0
0.2
52
10
46
106
39
56
60
34
15
46
41
25
81
0.7
0.6
1.2
0.8
0.1
1.0
1.0
0.2
0.2
0.4
0.0
0.1
0.9
"Five replicates
TABLE 6.22. FINAL SOIL CONCENTRATIONS (mg/kg) OF HCB AFTER THE GROWTH
AND HARVESTING OF VARIOUS PLANT SPECIES IN A GREENHOUSE
Initial soil 1243 (41)* 4581 (170)'
Bean
Fescue
Clover
Radish
Bluegrass
Corn
Dandelion
Alfalfa
Carrot
Beet
Pea
Fescue/Clover
Without vegetation
Without foil
1162
805
655
1172
786
1111
667
637
688
668
775
720
737
1075
(118)
(30)
(43)
(74)
(58)
(173)
(37)
(37)
(47)
(29)
(28)
(30)
(35)
(40)
3699
4143
3628
3812
4532
5212
3632
4012
4314
4094
4631
5582
4330
4523
(241)
(161)
(257)
(129)
(128)
(303)
(326)
(191)
(211)
(224)
(262)
(170)
(104)
(120)
"Five replicates (Standard Error)
113
-------
Conclusions
This successful experiment was designed to assess plant species
differences in both concentrations and accumulation of soil-borne pollutants
in plant leaves and roots. Detailed conclusions are as follows.
i. Species response to growing in HCB polluted soil varied widely. For
example, after 89 days of exposure, the roots of perennial sweet pea still had
a lower HCB concentration than either of the surrounding soil media. Dande-
lion and the fescue/clover mix were similar. On the other hand, radish root,
after 36 days of exposure, contained significantly higher HCB concentrations
than did the surrounding soil. Bean and carrot were similar,
2. The majority of the roots of the vegetation under test finished the
experiment with higher concentrations of HCB than did the surrounding soil
medium. The roots in the more polluted soils tended to finish the experiment
with less HCB than did the surrounding soil.
3. When the accumulation of HCB by the different plant species is con-
sidered, radish and clover roots accumulated most from the soil where 600 mg
HCB had been added; radish and dandelion roots accumulated most from the soil
where 3g HCB had been added. Maximum plant root accumulation accounted for 8%
of the added pollutant.
4. Plant leaves also accumulated some of the added HCB although this
was always less than that accumulated by the plant roots. There were differ-
ent trends between the two pollutant levels; bean, dandelion, and radish
leaves accumulated most from the soil containing 1000 mg HCB/kg, whereas
clover, corn, and dandelion accumulated most from the soil containing 5000 mg
HCB/kg. There is no obvious reason for this. Plant leaf accumulation was
always less than 1% of the pollutant added to the soil.
5. The soil concentrations after vegetation growth and removal also
show a wide range. An initial soil concentration of 1243 mg HCB/kg was re-
duced to 655, 637, and 667 mg HCB/kg by the growth and removal of clover,
alfalfa, and dandelion, respectively. Similarly, the initial soil concentra-
tion of 4581 tug/kg was reduced to 3699, 3628, and 3632 mg/kg by bean, clover,
and dandelion. The comparative control concentrations, where the soil was
left unvegetated within the greenhouse, were 737 and 4330 mg HCB/kg. This
reduction is likely to arise from a combination of factors including degrada-
tion and volatilization of the HCB.
CONCLUSIONS
For each experiment, detailed conclusions have been given. The aim. of
this subsection is to present some of the broader conclusions from the
experimental work.
It should be recognized that working in a greenhouse environment and not
in the field presents many drawbacks. Uptake rates from microecosystems, as
114
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in a greenhouse, have been reported up to 50 times those actually experienced
in the field under normal climate conditions (Fuhr and Mittlestaedt 1982;
Kloskowski et al.» 1981). This is partially because the plant root is in
constant contact with the pollutant and is not able to grow away from it, as
would occur in the field.
In this Section, the investigation of the potential for plant accumula-
tion of HCB from soils with different organic matter contents is reported.
Because pollutants in soils are sorbed to the organic fraction of the soil
and, thereby, made environmentally unavailable (Karickhoff, 1984), those soils
with most organic fraction should allow least plant uptake of the pollutant,
T«his was demonstrated. Radish accumulation of HCB decreased with increasing
organic matter contents of the soil, all other environmental factors being
equal. There was a substantial decrease in accumulation with a modest increase
in organic matter content, from 0 to 3%. Accumulation did occur in all soils,
even in those containing 96% organic fraction, but this was extremely low.
After a growth period of 67 days, maximum plant accumulation of HCB
represented approximately 33% of that measured in the soil at the start of the
experiment.
This investigation followed radish accumulation of the soil-borne HCB
with time. There have been few similar investigations although Topp et a!.
(1986) showed that plant CFs decreased with time when barley was grown in HCB
and pentachlorobenzene polluted soils. Because the concept of the CF includes
a measure of plant mass, as the plant grows, the concentration of the pollut-
ant within the plant becomes increasingly less.
The reported investigation showed that accumulation of the pollutant
from the soil by the plant increased with time and that, even after reaching
maturity, (about 49 days for radish), there was still further potential for
root sorption of the pollutant. This accumulation with time probably reflects
the time taken for the equilibrium in the soil (between the sorbed and liquid
phases) to shift to the liquid phase and depends on the concentration of the
pollutant in the soil. Plant leaf accumulation of HCB from the soil also
increased with time.
Although HCB has been identified as primarily moving through soils in
its vapour phase (USEPA, 1980), plant leaf accumulation of HCB was extremely
low in all experiments. It would be expected that volatilization of the
compound from the soil surface would cause it to accumulate in the lower
leaves of the plant. This was not investigated because of the insufficient
mass of lower leaves produced during the experiments.
The difference between radish accumulation of HCB when the soil surface
was covered with that when it was left uncovered is reported. Leaf concen-
tration was higher 1h those plants where the soil surface was left uncovered,
suggesting that volatilization of the pollutant and subsequent uptake by
plants actually does occur. Surprisingly, however, these plants also con-
tained the highest root concentrations. The only obvious explanation for this
is that more of the pollutant was being kept in the vicinity of the plant root
for possible sorption.
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In the study that investigated uptake of HCB from soil by a range of
different plant species, the species were selected for being able to grow in
the experimental greenhouse conditions rather than on the basis of other
reported experiences of uptake. The plants under test showed a wide range in
both their abilities to accumulate the pollutant and to clean up the soil in
which they were growing. Alfalfa, for example, reduced a soil concentration
of 1243 mg/kg to 637 mg/kg after 82 days growth.
Greater variation could be shown if further plant species were tested
for their abilities to clean up soil. It remains possible, and indeed likely,
that further test species could show greater affinity for soil cleanup.
These experiments have highlighted many areas where further work is
needed to understand the behaviour of pollutants in soils and the influence of
plants on the soil. They have also shown that the use of plants to clean up
soils remains a valid concept. In one experiment, the plant accumulated about
33.3% of the applied pollutant from the soil within 67 days.
As was discussed at the initiation of this investigation, the greenhouse
facilities were not ideal for the experiments but this has not prevented the
collection of extremely valuable information.
REFERENCES
Fuhr, F., and W. Middlestaedt. 1982. Influence of experimental and certain
environmental factors on the uptake of soil applied herbicides. Proc, Int.
Pestic. Chem. 29:757-759.
Karickhoff, S.W. 1984. Organic pollutant sorption in aquatic systems. J. Hy-
draulic Eng. 110(6):707-735.
Kloskowski, R., I. Scheunert, W. Klein, and F. Korte. 1981. Laboratory
screening of distribution, conversion, and mineralization of chemicals in a
soil plant system and comparison to outdoor data. Chemosphere, 10(10}:1089-
1100.
Topp, E., I. Scheunert, A. Attar, and F. Korte. 1986. Factors affecting the
uptake of 14C labelled organic chemicals by plants from soil. Ecotoxicol.
Environ. Safety 11:219-228.
U.S. Environmental Protection Agency. 1980. Land disposal of Hexachlorobenzene
waste. Controlling vapour movement in soil, EPA-600/2-80-119,
U.S. Environmental Protection Agency. 1982. Test methods for evaluating solid
waste. Physical/chemical methods. SW846.
Wilson, J.T., and B.H. Wilson. 1985. Biotransformations of trichloroethylene
in soil. App. Environ. Hicrobiol. 49(1):242-243.
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SECTION 7
FIELD TESTWORK - AN INVESTIGATION INTO PLANT UPTAKE OF
2,3,7,8-TETRACHLORQDIBENZQ-P-DIQXIN IN THE FIELD
INTRODUCTION
Considerable research has investigated the behaviour of pesticides in
the environment and the interactions between pesticides in soil and plants
(Talekar et al., 1983; Dejonckheere et al., 1976), From the work of Briggs
and others (Briggs et al., 1976, 1982; Shone and Wood, 1974; Shone et al.,
1974), vegetation apparently accumulates organic chemicals front nutrient
solutions via root uptake, and translocation can be related to, and assessed
from, the n-octanol/water coefficient of the chemical. This coefficient is
defined as the ratio of the chemical concentration in octanol to that in water
when an aqueous solution is well mixed with octanol and then allowed to
separate.
Plant root accumulation of organic chemicals from solution increases as
the log n-octanol/water coefficient {log Kow) of the chemical increases and
can reach many times unity. Plant shoot accumulations following root uptake
and translocation reach their maximum at a log Kow of around 1.8, and, as they
never reach unity, this indicates passive movement of the chemical with the
transpiration water {Briggs et al., 1982).
In the field, plant accumulation can be related to the concentration of
the chemical in the soil solution rather than in the soil as a whole. This
results from sorptive effects of the soil organic matter (Karickhoff, 1981),
which can also be related to the log Kow of the chemical, with the more water
insoluble {or lipophilic) chemicals being sorbed to the greatest extent. The
quality of soil solution is dependent upon both the water holding capacity of
the soil {and, thus, the amount of water in the soil) and the amount of time
the water is in contact with the sorbed pollutant.
Plant root and soil organic matter thereby compete as sites for
pollutant sorption within the soil. Although it is known that pollutant
sorption to soil organic matter is in equilibrium and can, therefore, be
reversed (Graham-Bryce, 1967), similar information is not available concerning
the behaviour of pollutant sorbed to plant roots. Therefore, with time,
pollutant sorption to plant roots will possibly increase whereas that to soil
organic matter will decrease.
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This section is concerned with higher plant uptake of TCDD from dioxin
polluted soil in the field. The aim of the investigation is to assess the
extent of species variation and the degree of plant uptake of TCDD from soil
under field conditions. This knowledge is essential in understanding the
environmental cycling of soil borne pollutants and thus in designing remedial
actions to limit their impact.
The actual concentration of any chemical in the soil does not remain
constant over any length of time. Various forces remove the chemical from its
immediate sphere of influence (e.g., dust blow, volatilization, and leaching)
and forces that break the chemical into a simpler unit (e.g., bio- and
photodegradation). Pollutant concentrations in the soil, therefore, generally
decrease with time if no further pollutant additions are made. The methods
most likely to be responsible for the disappearence of TCDD from soil include
photodegradation, wind and water movement of contaminated particles,
volatilization, microbial degradation, and biomass removal (Young, 1981).
2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a member of the dioxin
group of chemicals and is one of the most water insoluble or lipophilic
chemicals found as a pollutant on hazardous waste sites. It has a log Kow of
6.14 (EPA, 1979) and is one of the most long-lived pollutants with an
estimated half life of 10 years (EPA, 1985). Its effect on the environment
through suspected animal health effects is great and has caused much concern
(Kimbrough et al.,1984; Tucker et al., 1981).
Few investigations have assessed the potential for plant uptake of TCDD
from soil. Young (1981) reported typical data for TCDD concentrations in
grasses, (roots = 710 ppt; crown = 270 ppt; leaves - 10 ppt) and broadleaves
(roots = 760 ppt; stem and leaves = 75 ppt) when grown outside in soil
containing approximately 0.6 ppb (600 ppt) of TCDD. The levels of TCDD in
roots and in soil are broadly similar. Although the above ground portions of
vegetation were contaminated, the test species were perennial species and the
levels of TCDD may reflect soil particle contamination.
Helling et al. (1972) undertook further investigation of plant uptake of
dioxins. Lakeland sandy loam with a low adsorptive capacity was treated with
radiolabeled pollutants at the rates of 0.07 ppm 2,4-dichlorophenol (DCP),
0.10 ppm DCDD, and 0.06 ppm TCDD. Oats and soybean were grown in this soil
and their tops were harvested at intervals to maturity. Maxima of 0.21% of
the DCP and 0.15% of the DCDD and TCDD present in the soil were translocated
to oats or soybean tops. Mature oats and soybean tops contained 10 and 20 ppb
DCP, and less than 1 ppb DCDD or TCDD. Concentrations in oat grain or soy
bean were undetectable.
The tissue content of DCP, DCOD, and TCDD decreased as the age of
soybeans and oats increased. Total content increased for 15 to 20 days and
then remained relatively constant. Tissue dilution (due to growth) could
account for this relationship and indicated that no further uptake occurred
after 15 to 20 days. Several other processes, however, such as metabolism,
volatilization from the tissue, and translocation back into the roots, could
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also be occurring. It is impossible to determine from this experiment which,
if any, of these processes are actually taking place.
METHODS
Field Sampling
Plants growing on dioxin-polluted soil at the Minker Site, St. Louis,
Missouri, were collected on November 24, 1986. The safety precautions taken
during the plant and soil sampling included the use of Tyvek suits, inner and
outer boots and gloves, and full-face respirators. These precautions follow
normal EPA procedures.
The plants were excavated from the dioxin-polluted soil using a
stainless steel trowel, which was steam-cleaned between each use. The upper
portions of the plant, including stems, leaves, and shoots were carefully cut
from the lower root portions of the plant with the use of stainless steel
scissors, which were also steam cleaned between each use. The roots were
separated from the polluted soil so that the plant root sample contained only
adsorbed soil particles. The soil associated with the plant roots, i.e., that
in the rhizosphere, was sampled for analysis and represented the collected
soil sample.
Wherever possible, three individuals were collected to represent each
different plant species sampled from the site. The individuals were
collected at some distance from one another in an attempt to obtain different
plant genotypes, as well as different soil concentrations from the
heterogeneous site. Some 10 different plant species were assessed, which
resulted in a total of 90 samples (30 plant shoots, 30 plant roots, and 30
soil samples) being forwarded for analysis.
Plant species were identified according to Steyermark, 1977.
Analytical methods
Samples were analyzed at the THS Analytical Laboratories, Indianapolis,
according to EPA QA/QC procedure. The analysis procedure was modified from
one used as EPA Region VII to determine TCDD in soil and sediment samples with
the use of gas chromatography and tandem mass spectrometry.
Five grams of anhydrous sodium sulfate was placed in a 10 ml serum vial
with cap and septum and weighed. Approximately 5 grams of a soil sample was
added and the vial reweighed. The sample was then spiked with internal and
surrogate standards of isotopically labeled 2,3,7,8-TCDD. Following mixing by
shaking, and extracting with acetonitrile/dichloromethane in the. closed vial,
an aliquot of the extract was taken and, after separation from acetonitrile,
the dichloromethane was used directly for GC/MS/MS analysis.
A cleanup procedure was included for those samples that did not meet
quality assurance criteria. The extract may concentrate to reduce the minimum
detectable concentration. Quantification of the TCDD was based on the
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response of native TCDD relative to the isotopically labelled TCDD internal
standard. Performance was assessed based on the results for surrogate
standard recoveries, EPA performance evaluation samples, spike recovery tests,
and method and field blanks. The procedure was varied for analysis of the
vegetation samples. For these samples, the extraction was carried out by
Soxhlet extraction, to scale up the option A column cleanup step by a factor
of 3. Solvent washes of the Option D column cleanup step before eluting the
TCDD was also used.
RESULTS
*
Table 7,1 relates the rhizosphere soil concentration of TCDD to that in
the plant root and plant shoot associated with that in the soil.
DISCUSSION
As can be seen from Table 7.1, there was a wide range of concentrations
of the dioxin TCDD in soil, plant root, and plant leaf. Unfortunately, the
sampling area was not as polluted as was originally believed and some samples
did not contain TCDD. This reduced the value of the set of data.
The Concentration Factors (CF) of the different parts of the different
plant species, as defined by Shone and Wood (1972), are given (Table 7.2).
These results show TCDD being accumulated in the plant root from the
surrounding soil, in one case up to 15 times. In addition, on five occasions
TCDD was recovered from the plant root but not from the soil surrounding the
plant root.
Plant leaf CFs were generally zero; this showed that TCDD was usually
not recovered or accumulated in the plant leaf. In two cases, TCDO was
recovered from the plant leaf of Allium vineale, the crow garlic, although not
recovered from the surrounding soil. It is not known whether this resulted
from contamination or whether this fleshy, stalk like leaf of the plant
related to the common onion actually showed TCDD accumulation.
As no real history of the vegetation of the site is known, e.g.,
seeding dates, the variation in TCDD accumulation between different plant
species can not be accurately discussed, other than to say that such
differences in both root and leaf accumulation existed. Plant accumulation of
these long-lived pollutants depends on the time of exposure (Topp et al.,
1986) as well as the local soil conditions.
The results show conclusively that plant roots sorb high levels of
lipophilic pollutants from the soil. Such sorption is in competition with
that occurring on the soil organic matter and could, eventually, collect all
the pollutant from within the soil. From a sample size of 30, this occurred
on five occasions. The use of plant root sorption as a positive soil cleanup
technique through the ability of the root to collect and hold pollutants, and
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the environmental fate of TCDD, needs and warrants considerable further
investigation.
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TABLE 7.1. THE CONCENTRATION OF TCDD FOUND IN THE SOIL AND VEGETATION
COLLECTED FROM THE MINKER SITE"
Sample Plant species
Concentration, ppbf
Soil* Root1 Leaf*
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18 k
19
20
21
22
23
24
25
26
27
28
29
30
Festuca elatior
Festuca rubra
Festuca rubra
Festuca rubra
Alii urn vineale
Alii urn vineale
Festuca elatior
Festuca elatior
Carex bland a
Car ex bland a
Carex blanda
Glachoma hederacea
Glachoma hederacea
Glachoma hederacea
Taraxicum officinale
Taraxicum officinale
Taraxicum officinale
Daucus carota
Daucus carota
Rosa multiflora
Rosa multiflora
Rubus spp.
Geum canadense
Geum canadense
Geum canadense
Setaris viridis
Juniperus virginiana
Acer saccharinum
Acer saccharinum
Acer saccharinum
2.710
0.234
ND
0.393
ND
ND
0.884
0.449
0.402
0.281
0.530
ND
ND
1.307
8.340
0.287
7.208
0.384
0.175
ND
0.384
0.044
0.044
ND
ND
ND
ND
0.764
0.466
0.135
7.857
0.575
0.201
6.06
0.255
1.554
3.274
0.867
1.088
0.673
0.894
ND
0.426
4.884
10.081
0.295
0.168
4.743
0.295
0.063
0.395
0.033
ND
ND
ND
ND
ND
0.409
0.898
0.363
0.775
ND*
ND
ND
0.784
0.344
1.218
ND
ND
ND
ND
ND
ND
ND
ND
ND
0.495
ND
ND
ND
ND
ND
ND
ND
ND
ND
NO
ND
ND
ND
"St. Louis, Missouri, Nov. 24, 1986
Detectable limit varied, about 0.3 ppm
*Dry weight
5Not detectable
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TABLE 7.2. THE RANGE OF ROOT AND SHOOT CONCENTRATION FACTORS FOUND
FOR TCDD FOR DIFFERENT PLANT SPECIES*
Plant species Range of root CFf Range of shoot CFf
Festuca elatior
Festuca rubra
Alii urn vineaJe
Carex blanda
Glachoma hederacea
Taraxicum officinale
Daucus car ota
Rosa multi flora
Rubus spp.
Geum canadense
Setaria viridis
Juniperus virgim'ana
Acer sacchan'num
2.9,
2.5,
-------
Helling, C.S., Jsensee, A.R., Woolson, E.A., Ensor, P.D.J., Jones, G.E.,
Plitnmer, J.R., and P.C, Kearney. 1972. Ghlorodioxins in pesticides, soils and
plants. J. Environ. Qua!. 2(2):171-178.
Karickhoff, S.W. 1981. Semiempirical estimation of sorption of hydrophobic
pollutants on natural sediments and soils. Chemosphere, 10:833-846.
Kimbrough, R.D., Falk, H., Stehr, P., and G. Fries. 1984. Health implications
of 2,3,7,8-tetrachlorodibenzodioxin (TCDD) contamination of residential soil.
J. Toxicol. Environ. Health 14:47-93.
Shone, M.G.T., and A.V. Wood. 1972. Factors affecting absorption and
translocation of simazine by barley. J. Exp, Bot, 23(74):141-151.
Shone, M.G.T., and A.V.Wood. 1974. A comparison of the uptake and
translocation of some organic herbicides and a systemic fungicide by barley. I
Absorption in relation to physicochemical properties. J. Exp. Bot.,
25(85):390-400.
Shone, M.G.T., Barlett, B.O., and A.V. Wood. 1974. A comparison of the uptake
and translocation of some organic herbicides and a systemic fungicide by
barley. II Relationship between uptake by roots and translocation to shoots.
J. Exp. Bot, 25(85):401-409.
Steyermark, J.A. 1977. The Flora of Missouri. Iowa State University Press.
Talekar, N.S.," Chen, J.S., and H.T. Kao, 1983. Long term persistence of
selected insecticides in subtropical soils; their adsorption by crop plants.
J. Econ. Entomol., 76(2):207-214.
Topp, E, Scheunert, I., Attar, A., and F. Korte. 1986. Factors affecting the
uptake of 14-C labelled organic chemicals by plants from soil. Ecotoxicol.
Environ. Safety 11:219-228.
Tucker, R.E., Young, A.L., and A.P. Grey (eds.). 1981. Human and environmental
risks of chlorinated dioxins and related compounds. Plenum Press, New York and
London.
U.S. EPA, 1979. Water related environmental fate of 129 priority pollutants.
Volume 1. PB80-204373.
U.S. EPA, 1985. Health assessment document for polychlorinated
dibenzo-p-dioxins. EPA/600/8-84/014F.
Young, A.L. 1981. Long term studies on the persistence and movement of TCDD in
a natural ecosystem. In Human and environmental risks of chlorinated dioxins
and related compounds. Tucker, R.E., Young, A.L., & A.P. Gray (eds.). Plenum
Press, New York.
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SECTION 8
DISCUSSION
This project investigated the behaviour of organic pollutants in soils
and plant uptake and accumulation of these soil-borne pollutants. The overall
aim was to assess the potential of higher plants to clean up polluted soil.
Sanning (1985) identified the first steps in developing an in situ plant
cleanup system for organically polluted soils.
1. Determine whether vegetative extraction from the contaminated soil
has a high probability of being the most technically and cost effective
approach at the specific site, realizing that this approach will require a
substantial time period and intensive agronomic management over that time.
2. Determine whether suitable plant species (or varieties within a
species) are available to accomplish the desired contaminant extraction.
3. Determine whether the site possesses, or can be readily modified to
possess, soil conditions that will support optimal growth of the selected
plant materials.
4. Conduct greenhouse-scale confirmatory uptake tests.
5. Confirm that the plant materials that have extracted soil
contaminants can be adequately disposed of in an environmentally safe manner
and that the plant mass and harvesting mechanics are realistically manageable.
From the work discussed in this report such a cleanup system, although
requiring considerable further investigation, has much merit and has a high
potential. Unfortunately, the time and resources required to realize the goal
of soil cleanup by higher plants is likely to be great.
This report has collated much information showing that higher plants do
accumulate many times the concentration of some pollutants that actually occur
within the soil. There is strong evidence to suggest that those pollutants
with high log /)-octanol/water partition coefficients (log KQW) are sorbed to
the plant root in direct relationship to their Kow. From a nutrient solution,
plants can sorb up to 600 times the concentration of the pollutant occurring
in the solution. Plant leaf concentrations appear to increase as the log Kow
of the pollutant approaches 1.8. The concentration of the pollutant in the
leaf, however, never reaches that in the nutrient solution.
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Evidence from soil studies is much more complex, partly because the soil
system itself is much more complex. Only limited evidence is reported that
plant roots accumulate more pollutant, as a concentration, than exists in the
soil within which it is growing. The evidence from the simple experimental
investigations undertaken as part of this project, however, collaborate the
relationship between root sorption and Kow. In many cases, plant roots
contained four to five times the concentration of hexachlorobenzene than
existed within the soil. From field data collected from a dioxin-polluted
site in St. Louis, on two occasions plant root contained over 10 times the
amount of dioxin found in the adjacent soil. On five more occasions, dioxin
was found in the plant root but not in the adjacent soil. These results, from
f total of 30, indicate that plant root accumulation of almost any pollutant
can occur if time is allowed.
Higher plants can also accumulate pollutants that move in the vapour
phase within and from the soil. This ability is described by Henry's
Constant. Almost no data from compounds other than that from pesticides
describe the importance of this mode of soil transport. Vapour phase
transport is estimated to be some 30 times faster than is aqueous phase
transport, so that the pollutant should reach the plant root or lower plant
leaf much faster in the vapour phase than in the liquid phase. No research
has been undertaken to assess plant root uptake of chemical vapours.
Once within a plant, a pollutant can be transported to many places,
although some pollutants probably never leave the plant root. The extent of
pollutant transport within a plant also appears related to the KQW of the
pollutant. Increasing evidence suggests that a wide range of compounds can be
degraded by plant cells; to what extent and with what compounds this actually
occurs, however, remains uncertain. Considerable further effort is needed to
understand routes of plant degradation and routes of transport of pollutants
within plants.
From the experimental research undertaken here, those sites likely to
prove most amenable to the use of a higher-plant-accumulation system can be
described.
* They would have sandy soil with low organic fractions.
• They would have high water tables so that the soils are often
saturated.
* They would be polluted by long lived pollutants, with high Kows.
• They would be amenable to a long-term solution because the plant's
accumulation of the pollutant increases with increasing time.
Although the species selected would depend on the site, etc., having the
species able to degrade the pollutant would be desirable. The range of plant
species from which selection can occur is vast.
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The aim of this project has suffered throughout from a basic lack of
other reported work. Many recognized priority pollutants have not been
investigated for their potential to be accumulated or degraded by plants.
Much of the published material is old, originating in the 1960s, and refers to
the behaviour of pesticides. The dearth of information on this subject is an
anomaly in our modern society.
In view of this, further research is needed:
• to determine the importance of vapour phase transport and investigate
plant root uptake of vapours,
•
• to investigate the relationship between RCF, TSCF, and Kow for
compounds that occur today as pollutants,
• to investigate plant degradation of pollutants following both root
uptake and translocation and vapour phase collection,
• to undertake further greenhouse trials aimed at identifying species,
cultivar and individual variation in uptake and accumulation to
enable selection of optimums, and
* to undertake field trials on polluted sites.
REFERENCES
Sanning, D.E. 1985. In situ treatment. Chapter 4. In Contaminated Land. Recla-
mation and treatment. M.A. Smith (ed.). Plenum Press. New York and London.
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