&EPA
United States
Environmental Protection
Agency
Municipal Environmental Research EPA-600 2-80124
Laboratory August 1980
Cincinnati OH 45268
Research and Development
Adsorption,
Movement, and
Biological
Degradation of Large
Concentrations of
Selected Pesticides
in Soils
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further deveJopment and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the ENVIRONMENTAL PROTECTION TECH-
NOLOGY series. This series describes research performed to develop and dem-
onstrate instrumentation, equipment, and methodology to repair or prevent en-
vironmental degradation from point and non-point sources of pollution. This work
provides the new or improved technology required for the control and treatment
of pollution sources to meet environmental quality standards.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/2-80-124
August 1980
ADSORPTION, MOVEMENT, AND BIOLOGICAL
DEGRADATION OF LARGE CONCENTRATIONS
OF SELECTED PESTICIDES
IN SOILS
by
J. M. Davidson
P. S. C. Rao
L. T. Ou
W. B. Wheeler
D. F. Rothwell
Soil Science Department
University of Florida
Gainesville, Florida
Grant No. R-803849
Proj ect Officer,
M. H. Roulier
Solid and Hazardous Waste Research Division
Municipal Environmental Research Laboratory
Cincinnati, Ohio 45268
MUNICIPAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U. S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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DISCLAIMER
This report has been reviewed by the Municipal Environmental Research
Laboratory, U. S. Environmental Protection Agency, and approved for publi-
cation. Approval does not signify that the contents necessarily reflect
the views and policies of the U. S. Environmental Protection Agency, nor
does mention of trade names or commercial products constitute endorsement
or recommendation for use.
11
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FOREWORD
the Environmental Protection Agency was created because of increasing
public and government concern about the dangers of pollution to the health
and welfare of the American people. Noxious air, foul water, and spoiled
land are tragic testimony to the deterioration of our natural environment.
The complexity of that environment and the interplay between its components
require a concentrated and intergrated attack on the problem.
Research and development is that necessary first step in problem solu-
tion and it involves defining the problem, measuring its impact, and search-
ing for solutions. The Municipal Environmental Research Laboratory develops
new and improved technology and systems for prevention, treatment, and man-
agement of wastewater and solid and hazardous waste pollutant discharges
from municipal and community sources, for the preservation and treatment of
public drinking water supplies, and to minimize the adverse economic,
social, health, and aesthetic effects of pollution. This publication is
one of the products of that research; a most vital communication link
between the researcher and the user community.
This report presents the results of a study on the behavior of selected
pesticides in soils when the pesticide is present at large concentrations.
The work was conducted because of uncertainty whether the existing low-
concentration agricultural data base could be extrapolated to predict the
behavior of large concentrations of pesticides in soils at disposal sites.
Francis T. Mayo, Director
Municipal Environmental Research
Laboratory
111
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ABSTRACT
Because of the importance of soil in biologically reducing the quantity
and retarding the rate of pollutant movement into groundwater, this labora-
tory study was initiated to evaluate the adsorption, mobility, and degrada-
tion of large concentrations of the pesticides atrazine, methyl parathion,
terbacil, trifluralin, and 2,4-D in soils representing four major soil orders
in the United States.
Equilibrium adsorption isotherms of the non-linear Freundlich type were
obtained for all pesticides and the four soils. Pesticide solution concen-
trations used in the study ranged from zero to the aqueous solubility limit
of each pesticide. The mobility of each pesticide increased as the concen-
tration of the pesticide in the soil solution phase increased. These results
were in agreement with the equilibrium adsorption isotherm data. Pesticide
degradation rates and soil microbial populations generally declined as the
pesticide concentration in the soil increased; however, some soils were able
to degrade a pesticide at all concentrations studied, while others remained
essentially steril throughout the incubation period (50 to 80 days). As
shown by measurements of 14C02 evolution, total CO-2 evolution was not always
a good indication of pesticide degradation. Several pesticide metabolites
were formed and identified in various soil-pesticide systems. The quantities
of trifluralin and atrazine "bound" to the soil at the end of the incubation
period were measured and in some cases appeared to be related to types of
metabolites formed during biological degradation.
The observed increase in pesticide mobility for large pesticide concen-
trations in the soil invalidates, in many cases, the usefulness of the
existing low concentration data base for designing pesticide waste disposal
sites. Owing to the increased mobility and lower microbial decompostion
rates of many pesticides when introduced into soils at waste disposal con-
centrations, the potential for groundwater contamination is increased sig-
nitifantly. The data presented in this report should be given consideration
when designing waste disposal sites for pesticides. The low concentration
data base for pesticide mobility may be reasonable when considering pesticides
with low aqueous solubilities; however, this should not be accepted without
verification.
This report was submitted in fulfillment of Grant No. R-803849 by the
University of Florida, Gainesville, under the partial sponsorship of the U. S.
Environmental Protection Agency. This report covers the period August 1, 1975
to October 31, 1977, and work was completed as of October 31, 1977.
IV
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CONTENTS
Foreword iii
Abstract iv
Figures vi
Tables ix
Acknowledgements xi
1. Introduction 1
2. Conclusions 3
3. Recommendations 5
4. Materials and Methods 6
Soils 6
Pesticides 6
Adsorption Isotherms 9
Pesticide Displacement through Soils 10
Pesticide Degradation and Soil Respiration 11
Soil Microbial Populations 11
Metabolite Identification 12
5. Results and Discussion 15
Adsorption and Mobility 15
Adsorption Experiments 15
Column Displacement Experiments 26
Simulation of Pesticide
Mobility in Soils 32
Infiltration Experiments 35
Microbial Acitvity and Degradation 42
Atrazine 42
Methyl Parathion 45
Trif luralin 58
2,4-D , 61
Metabolites 90
Atrazine; 90
Trif luralin 90
2,4-D 99
6. Implications of Project Results With Regards to
Pesticide Disposal 101
References 103
Appendix: List of Publications from this project 109
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FIGURES
Figure
1 Adsorption isotherm for 2,4-D amine and Webster soil. Data
obtained using soil:solution ratios of 1:2, 1:5 and 1:10.. 17
2 Adsorption isotherm for 2,4-D amine and Webster soil. 2,4-D
was applied to soil in distilled water or 0.01N CaCl2 18
3 Adsorption isotherm for 2,4-D amine and Cecil soil. 2,4-D
was applied to soil in distilled water or 0.01N CaCl2 19
4 Adsorption isotherms for atrazine and Webster, Cecil, Glen-
dale and Eustis soils. Freundlich constants (K and N) for
each isotherm, determined by least-squares fit to the data,
are also shown 21
5 Adsorption isotherms for methyl parathion and Webster, Cecil,
Glendale and Eustis soils. Freundlich constants (K and N)
for each isotherm, determined by least-squares fit to the
data, are also shown 22
6 Adsorption isotherms for terbacil and Webster, Cecil, Glendale
and Eustis soils. Freundlich constants (K and N) for each
isotherm, determined by least-squares fit to the data, are
also shown 23
7 Adsorption isotherms for trifluralin and Webster, Cecil, Glen-
dale and Eustis soils. Freundlich constants (K and N) for
each isotherm, determined by least-squares fit to data, are
also shown 24
8 Adsorption isotherms for 2,4-D amine and Webster and Cecil
soils. Freundlich constants (K and N) for each isotherm,
determined by least-squares fit to data, are also shown... 25
9 Effluent breakthrough curves for 2,4-D amine (C = 50 and
5,000 yg/ml) and for tritiated water displacement through
Webster soil column 28
10 Effluent breakthrough curves for 2,4-D amine (C = 50 and
5,000 yg/ml) and for tritiated water displacement through
Cecil soil column 29
VI
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Figure Page
11 Effluent breakthrough curves for 2,4-D arnine (C = 50 and
5,000 yg/ml) and for tritiated water displacement through
Eustis soil column 30
12 Effluent breakthrough curves for atrazine (C = 5 and 50
yg/ml) and for tritiated water displacement through
Eustis soil column 31
13 Measured and simulated breakthrough curves for 2,4-D amine
displacement through Cecil soil column. Parameter values
used to calculate the solid lines were obtained from a
2-parameter fit, and those for dashed lines were estimated
from a 4-parameter fit to C =50 yg/ml data 37
14 Soil-water content (solid line) and 2,4-D, terbacil and
atrazine concentration distribution in Eustis soil follow-
ing infiltration of water to approximately 30 cm. Soil was
initially air dry and herbicide was in the top 1.5 cm seg-
ment of the soil (2,000 yg/g of soil) 40
15 Soil-water content (solid lines) and terbacil concentration
distributions in Eustis, Cecil and Webster soils following
infiltration of water to approximately 30 cm. Soils were
initially air dry and herbicide was in the top 1.5 cm seg-
ment of the soil (2,000 yg/g of soil) 41
16 CC>2 evolution rate from the Cecil soil receiving various
concentrations of atrazine. (A) Technical grade; (B)
Formulated atrazine 43
17 CC>2 evoltuion rate from the Web,ster soil receiving various
concentrations of atrazine. (A) Technical grade; (B)
Formulated atrazine , 44
18 C02 evolution rate from the Cecil soil receiving various
concentrations of trifluralin. (A) Technical grade; (B)
Formulated trifluralin 59
19 C02 evolution rate from the Webster soil receiving various
concentrations of trifluralin. (A) Technical grade; (B)
Formulated trifluralin 60
20 C02 evolution rate from the Webster soil receiving various
concentrations of 2,4-D. (A) Technical grade; (B) Formu-
lated 2,4-D 71
21 Percent 1IfC02 evolved from the Webster soil receiving various
concentrations of 2,4-D. (A) Technical grade; (B) Formu-
lated 2,4-D , 73
vii
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Figure Page
22 C02 evolution rate from the Cecil soil receiving various
concentrations of 2,4-D. (A) Technical grade; (B)
Formulated 2,4-D 74
23 Percent 14C02 evolved from the Cecil soil receiving various
concentrations of 2,4-D. (A) Technical grade; (B) Formu-
lated 2,4-D 75
24 C02 evolution rate from the Terra Ceia soil receiving 5,000
and 20,000 ppm of technical grade and formulated 2,4-D.... 76
25 Percent 14C02 evolved from the Terra Ceia soil receiving
5,000 and 20,000 ppm of technical grade and formulated
2,4-D 77
26 Percent of total 14C-activity bound to Webster and Cecil soil
after receiving 10, 1,000 and 20,000 ppm of technical grade
and formulated atrazine 91
27 Percent of total 14C-activity bound to Webster and Cecil soil
after receiving 10, 1,000 and 20,000 ppm of technical grade
and formulated trifluralin 98
28 Metabolic pathways for trifluralin metabolism. Code numbers
correspond with those in Tables 26 and 27 100
viii
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TABLES
Table Page
1 Physical and chemical properties of the soils used in this
study ....... * .............................................. 7
2 Properties of pesticides used in this study .................. 8
3 Solvents used for pesticide extraction from soil ............. 12
4 Thin layer chromatography systems ............................ 14
5 Freundlich constants calculated from equilibrium adsorption
isotherms for various soil -pesticide combinations .......... 16
6 Comparison of model parameter values estimated from the 2,4-D
amine breakthrough data (C = 50 yg/ml) for Cecil soil by
varying either two or four°parameters in the nonlinear least-
squares curve-fitting procedure ............................ 36
7 Physical data for infiltration experiments . . ^ ................ 39
8 Effect of technical grade and formulated atrazine on bacterial
populations in Cecil and Webster soils . . . . : ................ 46
9 Effect of technical grade and fromulated atrazine on fungal
populations in Cecil and Webster soils ..................... 49
10 Effect of technical grade and formulated atrazine on actino-
mycete populations in Cecil and Webster soils .............. 52
11 Total 0)2 evolution from methyl parathion treated Webster
soil ............................. . ......................... 55
12 Total C02 evolution from methyl parathion treated Cecil
soil [[[ 56
13 Percent of ^C -methyl parathion evolved as ll*C02 in Webster
and Cecil soils ............................................ 57
14 Effect of technical grade and formulated trifluralin on
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Table Page
15 Effect of technical grade and formulated trifluralin on fungal
populations in Cecil and Webster soils ..................... 55
16 Effect of technical grade and formulated trifluralin on actino-
mycete populations in Cecil and Webster soils .............. 68
17 Comparison of the percentage of 2,4-D degradation from the
Webster soil derived from lkCQ2 and total C02 evolution during
the second peak period and the total experimental period. . . 78
18 Degradation rates for the three soils receiving 5,000 ppm of
technical grade and formulated 2,4-D during exponential degra-
dation period .............................................. 79
19 2,4-D degradation in the Cecil soil receiving 5,000 ppm of the
herbicide and various nutrient treatments during 60 days of
incubation ................................................. 81
20 Soil pH of the Webster soil recieving technical grade and
formulated 2,4-D ........................................... 82
21 Soil pH of the Cecil soil receiving technical grade and
formulated 2,4-D ........................................... 82
22 Effect of 2,4-D on bacterial populations in Cecil soil ....... 84
23 Effect of 2,4-D on fungal populations in Cecil soil .......... 86
24 Effect of 2,4-D on actinomycete populations in Cecil soil.... 88
25 Percentage of applied trifluralin mineralized and volatilized
after 83 days of incubation ................................ 93
26 R values for trifluralin and metabolite references
27 Chromatographic and mass spectral parameters of products
isolated from trifluralin treated soil ..................... 95
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ACKNOWLEDGEMENTS
The assistance of Mr. R. E. Jessup (University of Florida] in develop-
ing numerical solutions to the conceptual mathematical models used to de-
scribe pesticide adsorption and mobility in soils is gratefully acknowl-
edged. Also, the cooperation and overall project coordination by Dr. M. H.
Roulier (EPA, Municipal Environmental Research Laboratory, Cincinnati,
Ohio) as the Project Officer is appreciated.
The project investigators wish to express their appreciation to the
Center for Environmental Programs in the Institute of Food and Agricultural
Sciences at the University of Florida for financial support during the
project.
The following companies are acknowledged for providing pesticidal
materials used in this work:
CIBA-Geigy for Atrazine (technical material, formulation,
11+C-atrazine, authentic metabolites);
Dupont for Terbacil (technical material, formulation,
llfC-terbacil);
Eli Lilly for Trefluralin (technical material, formula-
tion, 14C-trifluralin, authentic metabolites);
Kerr-McGee for Methyl Parathion (formulation);
Monsanto for Methyl Parathion (technical material);
Thompson-Hayward for 2,4-D (formulation).
XI
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SECTION 1
INTRODUCTION
Because of a continued increase in the number and quantity of pesticide
compounds being placed on the market, the safe disposal of surplus and/or
waste pesticide materials has become an acute problem (von Rumker, et al.,
1974). Incineration, encapsulation, isolation in underground caves and
mines, chemical stabilization, land spreading and landfills are some of the
procedures being considered for the disposal of pesticides and other
hazardous wastes (Schomaker, 1976; von Everdingen and Freeze, 1971; Wilkin-
son, et al., 1978). Of these methods, disposal by landfills and land
spreading appear to be more common and economical (Fields and Lindsey,
1975; Lindsey, et al., 1976). Placing hazardous wastes in the land has
come under attack recently (Atkins, 1972; Rouston and Wildung, 1969) because
there is no guarantee that the hazardous chemicals disposed of in this
manner will not migrate from the disposal site to potable water supplies.
In general, pesticide applications associated with agricultural pro-
duction have had very little adverse effect on the soil microbial activity
(Cole, 1976; Hubbell, et al., 1973; Jensen, 1962; Kaiser, et al., 1970;
Newman and Downing, 1958; Roslycky, 1977). However, reports on soil micro-
bial activity where large concentrations were used have been contradictory
and inconclusive. For example, Ou et al. (1978a) observed 2,4-D (2,4-di-
chlorophenoxyacetic acid) degradation at concentrations of 5,000 and 20,000
yg/g of soil (ppm) for one soil type and no degradation at the same concen-
trations for another soil. Soil respiration and bacterial, fungal and
actinomycete populations were significantly reduced in the soil unable to
degrade 2,4-D. They concluded that the physical and chemical properties of
the soil as well as the 2,4-D concentration were important factors in
governing microbial activity and pesticide degradation in soils receiving
large pesticide concentrations.
Trifluralin (a,a,a-trifluro-2,6-dinitro-N, N-dipropyl-p-toluidine) and
atrazine (2-chloro-4-ethylamine-6-isopropylamino-s-triazine) are commonly
used herbicides. Trifluralin applications of 3 kg/ha (1.4 ppm) have been
shown not to influence soil bacterial, fungal and actinomycete populations
significantly (Tyunaeva, 1974). However, when 1.1 kg/ha (0.5 ppm) was
applied per year over a five year period, bacterial populations were in-
hibited while fungal and actinomycete populations were enhanced (Breazeale
and Camper, 1970). When analytical grade trifluralin was incorporated into
the soil at concentrations of 5,000 ppm, C02 evolution and bacterial popu-
lations were inhibited while streptomycete populations were stimulated.
Stojanovic et al. (1972) has shown that formulated trifluralin stimulated
C02 evolution and streptomycete populations while inhibiting bacterial
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populations in soil. Atrazine was shown not to inhibit soil respiration at
concentrations associated with agricultural production (Kaiser, et al.,
1970; Eno, 1962). Cole (1976) and Voets et al. (1974) have shown that soil
bacterial and fungal populations were not affected at rates below 4 kg/ha
(1.8 ppm). However, Stojanovic et al. (1972) reported that atrazine iiihib-
ited soil respiration and bacterial populations at 5,000 ppm but had no
effect on fungal populations.
A thorough understanding of the various processes that influence the
persistence, retention, and leaching of pesticides in soils is required to
develop technology for the selection and management of pesticide disposal
sites involving soils. The fate of pesticides in soils when applied at
concentrations similar to those associated with agricultural practices has
been well-documented in several reviews (Bailey and White, 1970; Sanborn,
et al., 1977). However, the direct extrapolation of this data base to
systems containing large pesticide concentrations, such as those occurring
at or below disposal sites, may not be feasible (Davidson, et al., 1976).
Laboratory experiments were initiated to investigate the physical,
chemical and microbiological behavior of five pesticides in five soils when
the pesticide was present at large concentrations. The objectives of the
study were to: 1) Measure and describe pesticide adsorption in selected
soil-water systems over a wide range of chemical concentrations (zero to
water solubility), 2) Measure mobility and distribution of pesticides in
selected soils, when applied or initially present in soil at large concen-
trations, 3) Measure chemical and microbial degradation rate, and identify
metabolites produced in soil-pesticide systems receiving large pesticide
concentrations, and 4) Measure the influence of large pesticide concentra-
tions on soil microbial activity and respiration rate for selected soil-
pesticide systems.
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SECTION 2
CONCLUSIONS
1. Equilibrium adsorption isotherms for pesticides and soils consider-
ed in this study were nonlinear. The Freundlich equation described the
full range of pesticide solution concentrations (zero to aqueous solubility
limit) studied. The adsorption "sites" for the pesticide were apparently
never saturated in any soil-pesticide system investigated.
2. Pesticide mobility was inversely related to the pesticide concen-
tration in the soil solution phase when the adsorption isotherm was non-
linear (N < 1.0). 2,4-D at 5,000 yg/ml was nearly as mobile as the chloride
ion. The dependence of the mobility on pesticide solution concentration
could be predicted and described by the equilibrium adsorption isotherm.
3. 14C02 evolution rate from a 14C-ring labeled pesticide was a good
indicator of pesticide degradation rate in soils containing large pesticide
concentrations. Total C02 evolution, however, did not always represent the
actual degradation of a pesticide. For this reason, the biological activity
of soils containing low pesticide concentrations may, in many cases, be
unrelated to the behavior and degradation of pesticides at large concentra-
tions in the soil.
4. Moderately persistent pesticides such as atrazine and trifluralin,
degrade slowly forming a number of metabolites when present in soils at
high and low concentrations. These pesticides and metabolite products have
the potential to become "bound" to the soil matrix. Bound residue was de-
fined in this study to be the 14C-labeled material remaining in the soil
after recommended extraction procedures had been employed.
5. The less persistent pesticides such as 2,4-D and methyl parathion
may or may not degrade when applied to a soil at high concentrations.
Factors that determine the degradation rate were pesticide concentration,
chemical formulation, nutrients in the soil, soil type, soil pH, temperature,
soil-water content, soil organic matter content, texture, and the presence
of a microbial population capable of degrading the pesticide. If the
pesticide was degraded, the time required for degradation to begin was
longer for large concentrations (>50 ppm) than for low concentrations. The
lag period was directly related to pesticide concentration.
6. Pesticide mobility and degradation were described using adsorption
and biological degradation parameters measured during this study. Con-
ceptual or process based mathematical models were used to describe pesticide
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mobility. The models were well suited for estimating the concentration
distribution and mobility of pesticides associated with waste disposal
sites.
7. Bound residues were formed at all concentrations studied (10 to
20,000 ppm). The percent of binding was higher for low concentrations;
however, the amount bound (atrazine or trifluralin) increased as the pesti-
cide concentration increased. The toxicity and chemical nature of the
"bound residues" were not studied in detail.
8. At low pesticide concentrations (<50 ppm), microbial activity
(indicated by total C02 evolution, and bacterial, fungal and actinomycete
population) was generally not affected. Microbial activity may be enhanced
or inhibited when a large amount of pesticide is applied to a soil. When
degradation occured at large concentrations, microbial activity was enhanced.
Moreover, formulation chemicals may stimulate microbial activity by serving
as energy or nutrient sources. If pesticide degradation does not occur,
microbial activity may be either enhanced or inhibited.
9. Based on the results of this study, groundwater contamination may
be a significant problem when highly soluble pesticides are placed in waste
disposal sites subject to considerable leaching. The potential for ground-
water contamination, on the other hand, is not as great for pesticides with
low solubilities.
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SECTION 3
RECOMMENDATIONS
1. Total C02 evolution or soil respiration should not be used as the
only measurement to estimate pesticide degradation in soils receiving or
containing large pesticide concentrations.
2. Because this study considered only systems involving the presence
of one pesticide, additional studies need to be conducted to establish the
degradation, adsorption, and movement of pesticides in systems containing
mixtures of several pesticides at high and low concentrations. This type
of information is more relevant to actual pesticide disposal sites.
3. Identify procedures for stimulating pesticide degradation in waste
disposal sites involving soils.
4. Establish the major soil parameters and properties which determine
the potential for a given soil to degrade a high concentration of a given
pesticide. Factors to consider are microbial population and species, soil
pH, organic matter content, cation exchange capacity, soil-water content,
temperature, etc.
5. Determine the mechanisms responsible for the formation of "bound"
residues. Evaluate stability and/or phytotoxicity of bound residues.
6. Develop protocols for site evaluation and selection for pesticide
disposal using surface soils or landfills. Protocols should consider
single as well as mixtures of pesticides.
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SECTION 4
MATERIALS AND METHODS
SOILS
Soils used in this study were selected on the basis of their geographic
and taxonomic representation of major soil orders in the United States.
State Conservationists for the Soil Conservation Service in selected regions
were asked to identify the major soil series in their area for a selected
soil order and to ship 735 kg (air-dry) of the Ap horizon of that soil. A
soil profile description, sample site location, previous crop and management
history, and climatic conditions at the site were provided with each soil
series selected and studied. Soils selected were: Webster silty clay loam
(Typic Haplaquolls) from Iowa, Cecil sandy loam (Typic Hapludults) from
Georgia, Glendale sandy clay loam (Typic Torrifluvents) from New Mexico,
Eustis fine sand (Typic Quartzipsamments), and Terra Ceia muck (Typic
Mediasaprista) from Florida.
The soils were air-dried and sieved to pass a 2-mm screen prior to
being stored. Selected physical and chemical properties of the mineral
soils are given in Table 1. Terra Ceia muck is characterized by 81% organic
matter, 19% total mineral content, CEC of 350 meq/lOOg and pH of 6.4.
PESTICIDES
The pesticides used in this study were selected based upon their
present and anticipated usage as well as their different chemical properties.
The production and use of herbicides has increased significantly in the
past few years and at the present time herbicides represent the largest
group of pesticides on the market. Because of this market shift in pesticide
production, the herbicides were identified as a group of chemicals requiring
major attention. The following description of each pesticide used in the
study illustrates the diversity in chemical properties and toxicity of the
selected compounds. It was believed that this range in chemical properties
(Tabel 2) provided the necessary information needed to evaluate the problems
associated with introducing large pesticide concentrations into the soil
environment.
1) Atrazine (Herbicide): (2-chloro-4-ethylamino-6-isopropylamino-s^-
triazine).Low solubility in water (Table 2), but highly soluble in
chloroform, methanol, and ether. Losses due to chemical and microbial
degradation are significant. Leaching from soils may be limited due
to adsorption on certain soil constituents. Acute oral LD50 to rats
3080 mg/kg.
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TABLE 1. PHYSICAL AND CHEMICAL PROPERTIES OF .THE MINERAL SOILS USED IN THIS STUDY
PARTICLE
SOIL Sand
Webster 18.4
Cecil 65.8
Glendale 50.7
Eustis 93.8
SIZE FRACTION
Silt
45.3
19.5
16.4
3.0
(*)
Clay
38.3
14.7
22.9
3.2
pH (1:
Water
7.3
5.6
7.4
5.6
1 paste)
IN KC1
6.5
4.8
6.5
4.1
CEC
(meq/lOOg)
54.7
6.8
35.8
5.2
Organic C
(%)
3.87
0.90
0.50
0.56
Base
Saturation
(%3
91
31
90
10 _
Extractable
Acidity
(meq/lOOg)
5.15
4.68
3.74
4.68
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TABLE 2. PROPERTIES OF PESTICIDES USED IN THIS STUDY
Common
Name
Atrazine
Methyl
parathion
Terbicil
Trifluralin
2,4-D
2,4-D Amine
Molecular
Weight
215.7
263.2
216.7
335.3
221.0
266.1
Vapor
Pressure
(mmHg x 106)
1.4 (30° C)
9.5 (20° C)
0.48 (29.5° C)
199 (29.5° C)
n.d.*
0.001 (28° C)
Aqueous
Solubility
Cg/100 ml H20)
0.0033 (27° C)
0.0055-0.0060
(25° C)
0.071 (25° C)
1 x lO"1* (27° C)
0.09 (25° C)
300 (20° C)
Melting
Point
173-175° C
35-36° C
175-177° C
48.5-49.0° C
135-138° C
85-87° C
*not determined
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2) Methyl Parathion (Insecticide): (0-0-dimethyl-0-p-nitrophenyl phos-
phorothioate).Technical grade is a liquid. Low solubility in water
(Table 2), but soluble in most organic solvents. Highly toxic and
degrades readily in a soil environment producing some metabolites that
are equally toxic. Acute oral LDso to rats is 9-25 mg/kg.
3) Terbacil (Herbicide): (3-tert-butyl-5-chloro-6-methyl uracil).
soluble in water (Table 2) and soluble in most organic solvents.
mobile in soils due to relatively low adsorption. Acute oral LD50 for
rats between 5,000-7,000 mg/kg.
4) Trifluralin (Herbicide): (a,a,a-tri£luoro-2,6-dinitro-N, N-dipropyl-
p-toluidine).Almost insoluble in water (Table 2) but very soluble
in most organic solvents. Volatile unless incorporated into the soil
immediately following application. Microbial and photo-degradation
play a significant role in dissipation from soil. Adsorption
on organic matter and clay colloids retards leaching from soils.
Acute oral LD^o for rats greater than 10,000 mg/kg.
5) 2,4-D (Herbicide): (2,4-dichlorophenoxyacetic acid). Somewhat soluble
in water (Table 2), and very soluble (30-60%) in acetone and alcohols.
Undergoes microbial degradation in soils, but losses due to photodecom-
position are minimal. Because of low adsorption in soils, it is
readily leached. "Acute oral LD50 for formulations are in the range of
300-1000 mg/kg rats, guinea pigs, and rabbits.
Formulated and technical grade materials fortified with 14C materials
were used to study mobility, adsorption-desorption, microbial degradation
and accumulation of metabolites in soils at various pesticide concentrations.
Stock solutions of each pesticide were prepared in 0.01N CaCl2 using the
commercial stock solution made up to the aqueous solubility limit of
the pesticide. Solutions of lower concentrations were prepared by succes-
sive dilutions of the original stock solution. A mixture of antibiotics
consisting of Penicillin G at 1 iug/ml and Polymixin B sulfate at 5 yg/ml
(Sigma Chemical Co., St. Louis, MO) was added to all pesticide solutions to
prevent microbial degradation during storage and use.
The commercial formulation of 2,4-D was Ded-Weed 40 (Thomson-Hayward
Chemical Co., Kansas City, MO), a dimethylamine salt of 2,4-D (41% acid
equivalent). The stock solution of commercially formulated atrazine was
prepared using AATREX SOW (80% wettable powder; Ciba-Geigy Corp., Greens-
boro, NC). A concentrated xylene solution of methyl parathion (80% solu-
tion; Monsanto Co., Agricultural Division, St. Louis, MO) was diluted in
0.01N CaCl2 to give the desired stock concentration. The commercial form
of trifluralin was Treflan (Blanco Products Co., a division of Eli Lilly
and Co., Indianapolis, IN; 44.5% trifluralin and 55.4% inert ingredients).
The technical grade form of each pesticide was at least 97% pure. All
pesticide solutions were spiked with the appropriate uniformly ring-labeled
™C compound (except for trifluralin) to give specific activities in the
range of 2-5 nCi/ml. Trifluralin was 14C-labeled at the -CF3 position.
ADSORPTION ISOTHERMS
Equilibrium adsorption isotherms for all soil-pesticide combinations
were 'measured using the batch procedure. Equilibrium was achieved by
-------
shaking duplicate samples of 5 or 10 g of soil with 10 ml of pesticide
solution in Pyrex serew-cap glass test tubes for 48 hrs. Preliminary
experiments had indicated that there was no measurable increase in pesti-
cide adsorption beyond this time. Following equilibration, the test tubes
were centrifuged at 800 g for 10 minutes and the ^C-activity in 1-ml
aliquots of the clear supernatant solution was assayed by liquid scintilla-
tion counting. Decreases in pesticide solution concentration were attri-
buted to adsorption by the soil. All adsorption experiments were performed
at a constant temperture (23 ± 1° C).
PESTICIDE DISPLACEMENT THROUGH SOILS
Pesticide movement through water-saturated columns of Webster, Cecil,
and Eustis soils was studied using the miscible displacement technique
described by Davidson et al. (1968). Air-dry soil was packed in small
increments into glass cylinders (15 cm long; 45 on2 cross-sectional area).
Medium porosity fritted glass plates served to retain the soil in the
column. The soil was initially saturated with 0.01 N CaCl2 solution. A
known volume of pesticide solution at a desired concentration was intro-
duced into the soil at a constant flux using a constant-volume peristaltic
pump. After a specific volume of pesticide solution had been applied, the
pesticide solution was subsequently displaced through the soil column with
0.01 N CaCl2 at the same flux. Effluent solutions were collected in 5 or
10 ml aliquots using an automatic fraction collector. A pulse of 3H20
(specific activity = 5 nCi/ml) was also displaced through each soil column
to characterize the transport of non-adsorbed solutes. The activity of 11*C
and 3H in effluent fractions was assayed by liquid scintillation. The
counting efficiencies exceeded 90% for 1!*C and 50% for 3H in all cases.
The column experiments consisted of displacing 2,4-D amine solutions
at two concentrations (50 and 5,000 vg/ml) through columns of Cecil, Eustis
and Webster soils, and 5 and 50 pg/ml solutions of atrazine through a
Eustis soil. All displacements were performed at a Darcy flux of approxi-
mately 0.22 cm/hr to ensure near-equilibrium conditions for pesticide
adsorption during flow. The total volume of water held in the soil column
was gravimetrically determined at the end of each displacement by extruding
the soil from the glass cylinders and oven-drying. The number of pore
volumes (V/V ) of solution displaced through the column was calculated by
dividing the cumulative outflow volume (V) by'total water volume (V ) in
the soil column. Effluent pesticide concentrations are expressed a§ rela-
tive concentrations (C/C ), where C and C are, respectively, effluent and
input concentrations. Plots of C/C versus V/V are referred to as break-
through curves (or ETC). ° °
Air-dry soil -was packed into 3.2-cm diameter lucite cylinders composed
of 1-cm sections supported by a V-shaped container that permitted observa-
tion of the wetting front position with time. Technical or analytical
grade pesticide was dissolved in benzene and was spiked with lkC-labeled
compound. The benzene solution was mixed with air-dry soil (to give 200
or 2,000 yg pf pesticide/g of soil and 10 nCi/g soil) and the benzene
evaporated. In order to simulate a waste disposal site, the pesticide-
spiked soil was packed into the top 1.5 cm of the soil column. Infiltra-
10
-------
tion of water into horizontal columns of soil was controlled by maintaining
the soil surface at a negative pressure (-4 on of water) using a fritted-
glass plate and a constant-head burette. The fritted-glass plate apparatus
was filled with 0.01 N CaCl2 and the desired negative pressure applied
before it was placed in contact with the soil. Measurement of time in all
experiments commenced the instant contact was established between the
fritted-glass plate and soil surface. Water entering the soil was measured
volumetrically using a constant-head burette connected to the fritted-glass
plate apparatus. Measurements of distance to the wetting front (zero at
the contact plane between soil and plate) were visually observed. When
flow had proceeded for the desired time (i.e., until the wetting front had
advanced to about 30 cm), the water supply was discontinued and the soil
column was immediately cut into 1-cm segments. About one-half of the soil
contained in each 1-cm segment was oven-dried at 105° C for 24 hours to
determine the gravimetric soil-water content. The remaining one-half of
the soil from each 1-cm-segment was dried in a vacuum desiccator over P20s
or H^Oij for a 24-48 hour period. About 0.5-0.7 g of the desiccator-dried
soil was then combusted in a Packard Model 306B sample oxidizer; the ll*C02
evolved by combustion was trapped in a premixed organic amine-fluor cocktail
and assayed by liquid scintillation. The pesticide concentrations were
calculated using the specific activity (dpm/yg pesticide) of the pesticide-
spiked soil sample. The pesticide concentrations determined in the above
manner represent the sum of adsorbed and solution-phase concentrations and
were expressed as yg pesticide/g oven-dry soil. '
PESTICIDE DEGRADATION AND SOIL RESPIRATION
Each soil was initially wet to a soil-water content corresponding to
30% of the 0.33 bar soil-water tension and incubated for one week at 25° C.
Following incubation, each soil was then mixed thoroughly with a specific
quantity of pesticide fortified with lkC-pesticide at 1 yCi/100 g of soil
and sufficient water was added to bring the soil up to 0-33 bar soil-water
tension. For pesticide degradation and soil respiration experiments, 100 g
(oven-dry basis) of each pesticide-treated soil was placed in a 250 ml
Erlentneyer flask. Special care was taken when mixing the pesticide with
the soil to ensure a uniform distribution of the pesticide within the soil.
Technical grade and formulated materials of each pesticide were used. The
flasks were then connected to a plexiglass manifold and C02-free water-
saturated air passed through the manifold into the falsks at a flow rate of
10 ml/min per flask. For pesticides with high vapor pressures, the air
leaving the flask was passed through 40 ml of ethylene glycol to absorb the
volatilized pesticide. The air leaving each flask was also bubbled through
a KOH solution (0.1-0.5N) to absorb the evolved C02. At frequent intervals,
the KOH solutions were replaced with fresh KOH solutions and the total C02
concentrations in the KOH solutions were determined by titration. lltC02
activity in the KOH solution was determined by liquid scintillation count-
ing.
SOIL MICROBIAL POPULATIONS
For the microbial enumeration experiments, the experimental set-up
was essentially the same as that for the degradation experiment, except
11
-------
250 g of each pesticide-treated soil (10 to 20,000 yg/g) was placed in a
500 ml Erleraneyer flask and no ltfC-labeled pesticide was used. Ten gram
soil samples were withdrawn weekly from each fla'sk. Bacterial, fungal and
actinomycete populations in the samples were determined using a dilution
plate count method. The number of microorganisms were expressed as colony
forming units (cfu) per gram of oven-dry soil. Bacterial populations were
determined using a TGY medium consisting of 5 g tryptone, 5 g of glucose, 4
g of yeast extract, and 18 g of agar in 1,000 ml of distilled water. The pH
of the medium after autoclaving was 7.0. Bacterial colonies in the plates
were counted after 48 to 52 hours at 28° C. Fungi were enumerated in
Martin's (Martin, 1950) Rose Bengal-streptomycin agar (RBS agar). The pH
of the fungal medium after autoclaving was 6.0. Fungal colonies were
determined after 60 to 72 hours at 28 C. Actinomycetes were plated on
starch-casein agar (SC agar) supplemented with antibiotics cycloheximide 50
yg/ml and nystatin 50 yg/ml (Williams and Davis, 1965). The pH of the
actinomycete medium after autoclaving was 7.2. The actinomycete colonies
were counted after 12 to 14 days at 25° C.
TABLE 3. SOLVENTS USED FOR PESTICIDE EXTRACTION FROM SOIL
Pesticide Solvent
2,4-D Ether Ethyl
Trifluralin 1) Benzene/Ethyl Acetate
(3:1)
2) MeOH
Atrazine MeOH (soxhlet)
Methyl Parathion Hexane: Ace tone (80:20)
METABOLITE IDENTIFICATION
Soil treatments and incubation were essentially the same as for the
pesticide degradation experiments, except 150 g of soil were used and
2 yCi lkC-pesticide per 100 g soil were added. Ten gram soil samples were
withdrawn biweekly. The soil samples were extracted three times with the
appropriate solvent (Table 3). The 20,000 yg/g samples were extracted four
times because of their high pesticide concentrations. The extracted soil
was then air dried and stored in a cold chamber prior to analysis for
"bound" ltfC. The extracts were concentrated to 10 ml on a rotary evaporator,
or in a Danish Kuderna evaporator, then further concentrated using a gentle
stream of N2 and an aliquot was assayed for 1IfC. The extracts were further
concentrated with N2 to one ml and an aliquot equivalent to 15,000 dpm
of each sample was streaked on a thin layer plate (see Table 4). The TLC
plates were developed and placed on Kodak on-screen x-ray film (NS-57)
12
-------
for one month. Radioactive streaks on the TLC plates were scraped and the
radioactivity eluted with two ml of solvent (Table 3) . The percentage of
radioactivity in each separate radioactive component on the TLC plates was
determined by liquid scintillation counting.
The unextractable "bound" portion of the 14C- residue was determined by
oxidizing the extracted soil samples in a stream of 02 at 800° C according
to the modified method of Watts (1971) . The air dried extracted soil was
placed directly in the oven at 800° C and the 02 stream which passed through
the soil was bubbled directly into 15 ml of phenethylamine C02- trapping
cocktail solution. Complete combustion took approximately five minutes.
Then N2 was purged through the system to eliminate 02 and the trapped lkC02
was assayed.
A Varian Aerograph Series 2100 gas chromatograph (GC) with one flame
ionization and three electron capture detectors was used for the gas chroma-
tography work. 2,4-D analysis consisted of quanitfication of the buto-
xyethyl ester of 2,4-D which was prepared as follows: 1) Five ml of acetyl
chloride was added dropwise to cold butoxyethanol (final volume equal to
100 ml) ; 2) 0 . 1 ml of this solution was then added to a test tube containing
the extracted 2,4-D. The tube was sealed and held at 80° C for 30 minutes;
3) The tube was cooled and 2.0 ml of hexane was added; 4) The hexane solu-
tion was then extracted three times with 0.2 M K2HPOit and 5) A small amount
of anhydrous sodium sulfate was then added to the hexane solution to dry
the sample. The GC column was 1.8mx2mmi.d. glass packed with 31 QF-1
80/100 on Gas Chrom Q and operated at 200° C. The injection port and
detector temperatures were 250° and 300° C, respectively. The same column
was used for trifluralin but was operated at 140° C. Trifluralin and its
metabolites were assayed without modification.
Atrazine and its dealkylated metabolites were assayed using a 51 CBWX
20 M 60/80 mesh Gas Chrom Q packed inal.8mx2mmi.d. glass column and
utilizing a flame ionization detector. The hydroxy metabolites of atrazine
were methylated and then assayed on the CBWX column using a flame ioniza-
tion detector. Injector, column and detector temperatures were 250 , 200
and 300° C, respectively.
Mass spectral analyses for trifluralin were performed using a Finnigan
Model 1015C chemical inozation quadruple mass spectrometer equipped with a
Varian 1400 gas chromatograph as the inlet source without a separator. The
mass spectrometer (MS) was interfaced to a System Industries 150 computer
data acquisition system. The 1.8mx2mmi.d. glass column was packed
with 31 QF-1 on 100/120 mesh Gas Chrom Q. Methane was used as the carrier
gas at a flow rate of 18 ml/min and passed directly into the ion source
where the pressure was maintained at 1.0 torr. The injector, column and
transfer line temperatures were 240°, 140° and 230 C, respectively.
In addition to mass spectra, the computer data system provided recon-
structed gas chromatograms (RGC) and limited mass range searches (LMS) .
RGC's represent the normalized total ion current plotted versus the spec-
trum number.
13
-------
TABLE 4. THIN LAYER CHROMATOGRAPHIC SYSTEMS
Pesticide
TLC Solvent Systems
TLC Plates*
TCL Scraping
Eluting Solvent
2,4-D
Trifluralin
Atrazine
Methyl Parathion
n-Butanol:Acetic Acid:H20
(4:1:1.8)
Hexane:MeOH (97:3)
:Acetone:Acetic Acid
(9:1:1)
Hexane:Acetone (80:20)
Silica gel 60F-254
(20 x 20 x 0.2 mm)
Silica gel 60F-254
(20 x 20 x 0.2 mm)
Aluminum Oxide
(20 x 20 x 0.2 mm)
Silica gel 60F-254
(20 x 20 x 0.2 mm)
Ether
Ethyl Acetate
CHCl3:Acetic Acid
(9:1)
*A11 were Brinkman
-------
SECTION 5
RESULTS AND DISCUSSION
ADSORPTION AND MOBILITY
Adsorption Experiments
Equilibrium adsorption isotherms were determined for each soil-pesti-
cide combination by measuring pesticide adsorption at five or more concen-
trations ranging from zero to the pesticide's aqueous solubility. All
adsorption isotherms considered in this study, with the exception of 2,4j%D
and the Glendale soil, were described by the Freundlich equation (S = KCj,
where K and N are constants, and S and C are adsorbed (yM/kg soil) and
solution (yM/£) phase pesticide concentrations. The values of the Freund-
lich adsorption constants, K and N, for each soil-pesticide combination
studied were obtained using a least-square fit procedure to the adsorption
data. These values are presented in Table 5.
Equilibrium adsorption isotherms should be independent of the soil to
solution ratio employed in the batch adsorption procedure. This has not
always been true for some published data (Grover and Hance, 1970; Hance
1977). The results presented in Figure 1 illustrate that no difference was
observed between the 2,4-D amine and Webster soil adsorption isotherms
conducted at soil:solution ratios ranging from 1:2 to 1:10. The indepen-
dence of the adsorption isotherm to soil solution rate used in the batch
adsorption procedure was shown to be consistent for the other soil-pesti-
cide systems considered in this study. These results are in agreement with
those of Green and Obien (1969), Nearpass (1967), and Dao and Lavy (1978).
This concept is important when using the Freundlich equation and its
constants, K and N, to predict pesticide partitioning between soil and
solution phases in systems where soil:solution ratios are known but not
constant (e.g., soil profile, runoff water containing sediments, etc.).
The type and quantity of specific electrolytes in the water frequently
influence the adsorption of pesticides. The sensitivity of pesticide ad-
sorption characteristics to electrolytes depends upon the adsorption mechan-
ism between the pesticide and solid phase. For example, 2,4-D amine adsorp-
tion increases in the presence of calcium (Figures 2 and 3) for both Cecil
and Webster soils. This phenomenon, however, is not consistent for all
soil-pesticide systems (Abernathy and Davidson, 1971). Based upon the
results presented in Figures 2 and 3 one needs to be aware of the type and
quantity of electrolytes present in the water and their influence on pesti-
cide adsorption.
15
-------
TABLE 5. FREUNDLICH CONSTANTS CALCULATED FROM EQUILIBRIUM ADSORPTION
ISOTHERMS FOR VARIOUS SOIL-PESTICIDE COMBINATIONS
PESTICIDE
Atrazine
Methyl
Parathion
Terbacil
*
Trifluralin
2,4-D Amine
, ... .- .-• .
SOIL
Webster
Cecil
Glendale
Eustis
Average + % CV*
Webster
Cecil
Glendale
Eustis
Average ± 1 CV
Webster
Cecil
Glendale
Eustis
Average ± \ CV
Webster
Cecil
Glendale
Eustis
Average ± % CV
Webster
Cecil
Glendale
Eustis
Average ± % CV
f
9.12
0.84 '
0.69
0.85
2.87 ± 145
18.67
4.81
6.05
3.30
8.21 ± 86
2.96
0.39
0.42
0.15
0.98 ± 135
2.49
0.43
1.31
0.23
1.11 ± 92
7.27
0.84
--
1.14
3.08 ± 118
KG*
6.03
0.89
0.62
0.62
2.04 ± 131
13.39
3.95
3.57
2.72
5.91 ± 85
2.46
0.38
0.38
0.12
0.83 ± 130
2.93
0.46
1.60
0.24
1.31 ± 94
4.62
0.65
--
0.76
2.01 ± 112
N
0.73
1.04
0.93
0.79
0.87 ± 16
0.75
0.85
0.61
0.86
0.77 ± 15
0.88
0.99
0.93
0.88
0.92 ± 6
1.15
1.05
1.18
1.06
1.11 ± 6
0.70
0.83
—
0.73
0.75 ± 9
KJ
155.8
98.9
124.0
110.7
122.3 ± 20
346.0
438.6
714.5
486.4
496.4 ± 32
63.6
42.2
76.0
21.4
50.8 ± 47
75.7
50.7
177.8
43.2
86.8 ± 72
119.4
72.2 .
—
135.7
109.1 ± 30
Freundlich constant when solution and adsorbed phase concentrations are
j- expressed as pM/1 and pM/kg of soil.
KG Freundlich constant for solution and adsorbed phase concnetrations are
as pg/ml and pg/g of soil [Kf = K-CMW/l.OOO)1^], when MW is the pesti-
cide's molecular weight.
Freundlich constant for solution and adsorbed phase expressed as pg/ml
and pg/g of organic carbon.
is the coefficient of variation, I CV = (standard deviation/average x
100.
*cv
16
-------
1000
en
o
E
to
u 100
z
O
U
Q
U
ffl
CC
O
to
< 10
_ Webster Soil
2,4-D Amine
•5=10.55 c
P.62
SOIL:SOLN.
o 1
. 1
•»• 1
2
5
10
10 100 1OOO
SOLUTION CONC, C (jjmoles/1)
Figure 1. Adsorption isotherm for 2,4-D amine and Webster soil. Data ob-
tained using soil:solution ratios of 1:2, 1:5 and 1:10.
17
-------
104
O)
"5
i/)
u
z
o
u
10
Q
LU
m
a:
O
WEBSTER SOIL
2,4-D Amine
s = 5.5? c
.07
o CaCI2
10'
H2O
101 i i i 11 nil i i i i n nl i i i 11 nil i i i 11 MI
SOLUTION
icr 10
CONC., C (jjmol/l)
1CT
Figure 2. Adsorption isotherm for 2,4-D amine and Webster soil. 2,4-D was
applied to soil in distilled water or 0.01 N CaCl-.
18
-------
10
O)
o
to
u
z
O
U
Q
U
m
cr
O
CO
Q
10
10'
CECIL SOIL
2,4-D Amine
0.78
10
2 —
I I i i i ml i i i i i iifl i i i i i ml i i i i i
10 10 10
SOLUTION CONG., C (jjmol./!)
Ill
10
Figure 3. Adsorption isotherm for 2,4-D amine and Cecil soil. 2,4-D was
applied to soil in distilled water or 0.01 N CaCl2.
19
-------
Adsorption isotherms for atrazine, methyl parathion, terbacil, tri-
fluralin, and 2,4-D amine and four soils are presented in Figures 4-8.
Because of the large quantity of pesticide adsorbed by the Terra Ceia muck,
it was not included in these figures. The Freundlich adsorption constants
which describe the data for each isotherm are shown adjacent to the best-
fit line (least-squares) through the data. Note that for all pesticide-
soil systems studied, with the exception of trifluralin, N in the Freundlich
equation is less than one.
To achieve the high solution concentrations of trifluralin shown in
Figure 7, it was necessary to use a 1% solution of a nonionic surfactant
(Triton X-100; polyoxyethylene ethers). Decreasing solution concentrations
of the surfactant due to its adsorption on the soil (Valoras et al., 1969),
could lead to a reduction in pesticide solubility and promote precipitation
of the trifluralin, especially at the higher concentrations. In this
study, the amount of pesticide adsorbed by the soil was measured from the
change in solution concentration. Thus, any decrease in pesticide concentr-
ation in the solution phase due to precipitation would have been erroneously
attributed to adsorption by the soil. If precipitation had occurred primar-
ily at the higher trifluralin concentrations, a value of N greater than 1.0
would be anticipated. It should also be recognized that the presence of
surfactants can drastically increase or decrease pesticide adsorption on
soils (Hugenberger, et al., 1972). The surfactant may compete with the
pesticide for adsorption sites on the soil, or it could enhance the lipo-
phillic nature or the soil surface and thereby increase pesticide adsorption.
The adsorption data for 2,4-D amine and the Glendale soil are not in-
cluded in Figure 8 or Table 5 because a calcium salt of 2,4-D was formed in
this soil and precipitated out of solution making it impossible to measure
2,4-D adsorption at large herbicide concentrations. The Glendale soil con-
tains a large quantity of calcium carbonate. The quantity of 2,4-D amine
precipitated increased as the herbicide concentration in solution prior to
contact with the soil increased. The precipitation of 2,4-D as a calcium
salt in the Glendale soil was confirmed through analytical procedures (mass
spectroscopy).
Two important conclusions can be made based on the data presented in
Table 5. First, the-fact that the Freundlich equation describes all pesti-
cide adsorption isotherms over a wide concentration range suggests that
adsorption sites were not saturated at any concentration considered in this
study. The amount of pesticide adsorbed by the soil continued to increase,
at a decreasing rate, with each increase in solution concentration. This
behavior may not, however, hold for other pesticide adsorbents (Weber and
Usinowicz, 1973). Second, contrary to a frequent assumption, pesticide
adsorption isotherms are generally nonlinear, that is, N is greater or less
than one (Table 5). Linear adsorption isotherms have been generally accept-
ed for low pesticide concentrations because it simplified computer simula-
tion modeling (Davidson, et al, 1968; Davidson and Chang, 1972; Hugenberger,
et al., 1972; Kay and Elrick, 1967).
Because soil organic carbon content generally correlates well with
pesticide adsorption, the use of an adsorption paratition coefficient based
20
-------
io
h ATRAZINE
O)
^
o
E
u
a
u
CO
a:
O
CO
a
10'
1O
10'
WEBSTER
CECIL
I i i i I
10
SOLUTION CONC. (jumol/l)
Figure 4. Adsorption isotherms for atrazine and Webster, Cecil, Glendale
and Eustis soils. Freundlich constants (K and N) for each iso-
therm, determined by least-squares fit to the data, are also
shown.
21
-------
O)
_*:
"o
E
D
(J
u
Q
UJ
CD
CC
O
to
Q
- METHYI PARATHION
WEBSTER
ECIL
EUSTIS
GLENDALE
i i f i i i I
I i
i i i i i i 1
10° 101
SOLUTION CONC. (pmol./l)
10'
Figure 5. Adsorption isotherms for methyl parathion and Webster, Cecil,
Glendale and Eustis soils. Freundlich constants (K and N) for
each isotherm, determined by least-squares fit to the data, are
also shown.
22
-------
O)
.*
o
E
O
o
Q
LU
CO
o:
O
10T
10J
TERBAC
10'
WEBSTER
EUSTIS
I i i i I i i
1O
SOLUTION C O NC. (jjmol./!)
Figure 6. Adsorption isotherms for terbacil and Webster, Cecil, Glendale
and Eustis soils. Freundlich constants (K and N) for each iso-
therm determined by least-squares fit to data, are also shown.
23
-------
O)
_*
"o
E
3
U102
o
u
Q
LU
CD
go'
(/)
Q
1OC
u TRIFLURALIN
WEBSTER
GLENDALE
i i i i i i
10° 1O1 1O2
SOLUTION CONC.(jumol./l)
10J
Figure 7. Adsorption isotherms for trifluralin and Webster, Cecil, Glendale
and Eustis soils. Freundlich constants (K and N) for each iso-
therm, determined by least squares fit to data, are also shown.
24
-------
10T
E
u
z
O
u
Q
LU
CD
Q:
O
to
Q
10'
h 2,4-D Amine
CECIL
10
l i i I i IIII i i i i 1111
i i I i 11 ll I l I I i 111
1O" 10J 10* 1O
SOLUTION CONC. (jjmoL/l)
Figure 8. Adsorption isotherms for 2,4-D amine and Webster and Cecil soils.
Freundlich constants (K and N) for each isotherm, determined by
least-squares fit to data, are also shown.
25
-------
upon organic carbon content rather than total soil mass has been proposed
by Lambert (1968) and Hamaker and Thompson (1971). Using this procedure,
the amount of pesticide adsorbed was expressed as yg/g organic carbon and
the Freundlich constant ifKp.p) for each adsorption isotherm was computed.
These values are also presented in Table 5. It is apparent that the values
of IC™ for a given pesticide are much less variable (smaller percent CV)
among the four soils studied than are the K values uncorrected for organic
carbon. These results are in general agreement with the observations of
Hamaker (1975) where the K^ values for a given pesticide were nearly inde-
pendent of soil type. It should be recognized, however, that other factors
such as soil pH, clay content, and cation exchange capacity may also play a
significant role in determining pesticide adsorption by soils (Bailey and
White, 1970). On the basis of the IC^, values listed in Table 5, the extent
of pesticide adsorption in soils was in the order of terbacil < trifluralin
< 2,4-D amine < atrazine < methyl parathion.
COLUMN DISPLACEMENT EXPERIMENTS
The partial differential equation generally assumed to describe the
movement of pesticides and other adsorbed solutes through soils under
steady-state water flow conditions is (Van Genuchten, et al., 1974):
3C n 82C 9C £ 3S
3t 3X2" 3X " 6 8t [1]
where t is time (days), D is dispersion coefficient (cm2/day) , x is dis-
tance (on) , v is average pore-water velocity (cm/ day) , p is soil bulk
density (g/on3) , e is volumetric soil-water content (on3/cm3) , and C and
S are solution and adsorbed pesticide phase (yg/ml and yg/g) , respectively.
When the adsorption isotherm obeys the Freundlich equation, the convective-
dispersive solute transport model (Equation 1) reduces to:
- '[2]
where ,
R(C) = [1 + pKNC^'1] [3]
The retardation term R(C) is a quantitative index of the pesticide's
mobility in that its value is equal to the ratio of the positions of the
adsorbed and nonadsorbed solute fronts in soil. The value of the adsorp-
tion coefficient K in Equation [3] for nonadsorbed solutes (e.g., chloride
or 3H20) is equal to zero; hence, R(C) = 1. For adsorbed solutes, R(C) is
greater than one because the value of K is larger than zero. Thus, larger
values of R(C) indicate reduced pesticide mobility in soils. It may be
noted from Equation [3] that for the case of nonlinear adsorption isotherms
(N < 1) , the value of the retardation term increases with decreasing solu-
tion concentration C, while for a linear isotherm (N = 1), R(C) is indepen-
dent of pesticide solution concentration. Thus, the mobility of pesticides
and other adsorbed solutes through soils is directly influenced by the
shape of the equilibrium adsorption isotherms.
26
-------
Effluent breakthrough curves (ETC) were measured for 2,4-D amine at
two input concentrations (C = 50 and 5,000 pg/ml) and tritated water
(3H20) using columns of Webster, Cecil and Eustis soils. These ETC are
shown in Figures 9, 10 and 11. Tritiated water represents a nonadsorbed
solute and serves as a reference for the adsorbed solutes (2,4-D amine in
this case). A shift of the ETC for adsorbed solutes to the right of 3H20
ETC is due to an adsorption-induced retardation. The greater the right-
hand shift of the ETC, the greater the adsorption; thus, a decrease in
mobility. It is apparent from the data presented in Figures 9, 10 and 11
that the mobility of 2,4-D amine was significantly increased as the input
concentration (C ) increased from 50 to 5,000 yg/ml. Note that for the
5,000 yg/ml inpu£ concentration, 2,4-D amine was nearly as mobile as was
3H20. The effect of increased mobility at high concentration was more
pronounced in the Webster soil (Figure 9) than in the Cecil (Figure 10) or
Eustis soil (Figure 11). These column results are consistent with Equation
[3] and the measured nonlinear adsorption isotherms (Table 5) for 2,4-D
amine.
Breakthrough curves for the displacement of atrazine through the
Eustis soil at two herbicide input concentrations (C = 5 and 50 yg/ml) and
3H20 are presented in Figure 12. The trend of increased mobility at higher
atrazine solution concentrations again is evident. However, the differ-
ences in pesticide mobility between the two concentrations were not as ,
large for atrazine (Figure 12) as they were for 2,4-D amine (Figures 9-11).
Deviations of the adsorption isotherms from linearity, i.e., constant
retardation term, increase exponentially as the concentration differences
become larger and/or as N approaches zero (Davidson, et al., 1976; Hamaker
and Thompson, 1972). In this study, atrazine concentrations varied only by
ten-fold while the 2,4-D amine concentrations varied by 100-fold. Further-
more, the isotherm for 2,4-D amine adsorption in Webster soil was more
nonlinear than that for the Eustis soil-atrazine system (Table 5).
The position of the ETC for an adsorbed solute is governed by the
nature of the equilibrium adsorption isotherm (Equation 3), whereas the
shape of the ETC (i.e., symmetry or lack of it) is defined by nonlinearity
of the adsorption isotherm and the kinetics of adsorption-desorption pro-
cesses. Symmetrical ETC are obtained when adsorption is an instantaneous
process and the adsorption isotherm is linear. For nonequilibrium adsorp-
tion conditions during flow, asymmetrical ETC are generally obtained (Van
Genuchten, et al., 1974; Rao, et al., 1979). All of the pesticide ETC
measured in this study (Figures 9-12) were asymmetrical in shape with
extensive "tailing" observed as C/C approached 1.0 or zero. Tailing was
absent in 3H20 breakthrough curves.0 The extent of the asymmetrical shape
of each pesticide ETC exceeded that which could be attributed to the non-
linear nature of the adsorption isotherms. Hence, much of the asymmetry
measured for the pesticide ETC may be attributed to nonequilibrium condi-
tions which.exist in the soil columns during flow. Rao et al. (1979)
presented an evaluation of two conceptual models where nonequilibrium
during flow was attributed to either kinetics-controlled or diffusion-
controled adsorption-desorption processes.
27
-------
i
D
~ 0.8
u
z
o
<-> 0.6
LU
> 0.4
£0.2
o.o
8J*-C0=5000jjg/ml
o
o
HO
o
o ,
• "
'°Oc|oO o oOol no
WEBSTER SOIL
2,4-D Amine
A 6 8
PORE VOLUMES (V/V)
10
12
14
Figure 9. Effluent breakthrough curves for 2,4-D amine (Co = 50 and 5,000
yg/ml) and for tritiated water displacement through Webster soil
column.
28
-------
1.0
u
NmiX
U
z
O
U
08
0.6
UJ 0.4
5 0.2
UJ
a:
0.0
*+ Ow00oo0ood0oooo0oo0<*>o
B<* c
•~c»
o •
X
0 = 5000jug/ml
+
I I
CECIL SOIL
2,4-D Amine
I
3456
PORE VOLUMES (V/VJ
8
Figure 10. Effluent breakthrough curves for 2,4-D amine (Co = 50 and 5,000
yg/ml) and for tritiated water displacement through Cecil soil
column.
29
-------
y°
u
1.0
00.8
O
U0.6
>°'4
I-
< 0.2
LiJ
ct:
— o
EUSTIS SOIL
2,4-D Amine
000°
oooooo
*»
50 jug/ml
C0= 5000jug/ml-v»o
o
o
I
I
f *
2468
PORE VOLUMES (V/VJ
10
Figure 11. Effluent breakthrough curves for 2,4-D amine (Co = 50 and 5,000
vg/ml) and for tritiated water displacement through Eustis soil
column.
30
-------
6 8
VOLUMES
(V/V0
Figure 12. Effluent breakthrough curves for atrazine (C0 = 5 and 50 ug/ml)
and for tritiated water displacement through Eustis soil column.
31
-------
The illustrated increase in pesticide mobility at high concentrations
limits the usefulness of the present low concentration data base for devel-
oping safe management practices for pesticide disposal in the soil. However,
underestimation of pesticide movement by assuming linear adsorption isotherms
may not be severe for pesticides with low aqueous solubilities. Ou et al.
(1978a, 1978b) showed that for high loading rates, up to 20,000 yg 2,4-D/g
soil, there was a significant decrease in the pesticide degradation rate
with a concomitant depression of total microbial activity in the soil.
Thus, due to rapid leaching and minimal microbial decomposition of pesticides
at high concentrations, the potential for groundwater contamination with
pesticides is increased.
SIMULATION OF PESTICIDE MOBILITY IN SOILS
Several conceptual models have been proposed and evaluated for describ-
ing the solute adsorption-desorption term (9S/3t) in Equation [1]. Because
many of these models are based on the assumption of instantaneous adsorption
and linear isotherms, they fail to describe experimental data (Davidson, et
al., 1976). Models based on first-order or other reversible kinetic adsorp-
tion-desorption processes were found to simulate experimental data reason-
ably well at low pore-water velocities but failed at high pore-water veloci-
ties (Davidson and McDougal, 1973).
Recent attempts to model the asymmetry or "tailing" in experimental
breakthrough curves thought to be associated with nonequilibrium adsorption
may be classified into two groups. In the first group, physical processes
are assumed responsible for the observed tailing. In these models, the
soil-water regime was divided into mobile and immobile regions. Although
the solute adsorption-desorption was assumed to be instantaneous, the rate
at which adsorbent molecules approached a fraction of the adsorption sites
was governed by diffusion through the stagnant soil-water region (Skopp and
Warrick, 1974; Van Genuchten and Wierenga, 1976). The assumption that a
wide-range in pore-water velocities would result in the observed tailing
has been evaluated by Rao et al. (1976), using a capillary bundle model.
In a variation of the latter approach, Skopp et al. (1977) considered
convective-dispersive solute transport in a system consisting of two soil-
water phases where the mass transfer of the solute between these phases
obeyed a first-order kinetic process.
In the second group of conceptual models, the observed excessive
asymmetry in the solute breakthrough curves was attributed to chemical
processes. Adsorption-desorption isotherms for these models were assumed
nonsingular (Van Genuchten, et al., 1974; Hornsby and Davidson, 1973).
Because these models assume instantaneous equilibrium between the adsorbed
and solution phases, they failed to describe experimental data for high
pore-water velocities. Furthermore, the physical and/or chemical justifi-
cation for the nonsingularity in the adsorption-desorption isotherms has
been questioned. More recently, Selim et al. (1976) and Cameron and
Klute (1977) have proposed to two-site adsorption-desorption model for
describing asymmetrical breakthrough data. Because nonsingular adsorption
isotherms observed earlier could be simulated using this model (Davidson,
et al., 1976; Selim, et al., 1976) and because justifications for such a
32
-------
conceptualization may be found in the chromatography literature (Giddings,
1965) , a thorough evaluation of the two-site adsorption-desorption model
appears warranted.
Agreement between model simulations and experimental data is generally
used as a criterion for verification of conceptual models. Davidson et al.
(1976) discussed the limitations of such an approach when model parameters
were estimated by "best-fit" to experimental data and not by independent
measurements. However, due to the present inadequacy of experimental tech-
niques, independent determination of model parameters may not always be
feasible. However, for curve- fitting procedures to be valid for model
verification purposes, the same set of parameters estimated from a given
experiment should be used to predict experimental results obtained under
different conditions (e.g., column length and input concentrations, etc.).
In the model proposed by Selim et al. (1976) to describe the adsorption
term 3S/3t in Equation [1J , two groups of adsorption sites were assumed to
be responsible for solute adsorption by soils; one group of sites achieved
instantaneous equilibrium (type-1) while the other group was time- dependent
(type -2). Using this approach, the time rate of change in the adsorbed
phase concentration (3S/3t in Equation 1) may be expressed as,
p. 3S = fpKiNCj 3C +
e 9t e at e [4]
where KI and N are constants associated with instantaneous adsorption on
type-1 sites, lq and k2 are, respectively, forward and backward rate coeffi
cients (day *) for kinetic adsorption on the type- 2 sites, S2 is adsorbed
phase concentrationon the kinetic sites, and other terms are as defined
previously. Note that at equilibrium, total adsorption is the sum of the
two types of sites and described by the Freundlich equation:
S = Si + S2 = K^ + K2CN = KC^ [5]
where K = (K]. + K2) , Klf K2, and N are Freundlich constants, Sj and S2
(yg/g) are adsorbed phase concentrations on type-1 and type -2 sites, res-
pectively, and.K and S are defined on the basis of total solute adsorbed.
Note that the exponent N in Equation [5] was assumed identical for both
sites. Assuming that type-1 sites represent some fraction F of the total
available adsorption sites, the following relationships may be stated:
KI = FK [6a]
K2 = (l-F)K [6b]
By assuming a linear equilibrium adsorption isotherm (i.e., N = 1 in
Equations [4] and [5]), an analytical solution to the two-site adsorption
model (i.e., Equation [4] coupled with Equation [1]), has been obtained by
Cameron and Klute (1977) . Numerical solutions for nonlinear adsorption
isotherms as well as the more general case when adsorption on both sites is
kinetic-controlled were presented by Selim et al. (1976).
33
-------
In the conceptual model (Equation [1]), the values of the experimental
variables K, N, 6, p, and v are known. The dispersion coefficient, D, for
each soil was estimated from tritiated water breakthrough curves using the
method proposed by Rose and Passioura (1972). Experimental methods are
unavailable to independently measure the values of F, KI and K2. It may be
noted from equations [4], [5] and [6] that,
K2 = (1 - F)K = (6k1/pk2) [7]
and
k2 = 6k!/(1 - F)PK [8]
Since the value of k2 can be calculated given K, F, and kl5 the problem now
reduces to that of estimation of the two unknown parameters F and ki.
The model parameters were estimated using a nonlinear least-squares
(NLLS) optimization procedure (Meeter and Wolfe, 1968) to fit the model
prediction to measured ETC at low input concentrations. This iterative
technique is based on minimizing the differences between simulated and
measured ETC data by successive refinement of the initial parameter values;
the estimates at each iteration are obtained by a combination of Gaussian
method and steepest descent method described by Marquardt (1963). These
estimates of model parameters were then used to verify the conceptual
models by comparing the predicted and the measured ETC obtained at a higher
input concentration. Conversely, the model parameters obtained by fitting
to high concentration ETC data were also used to predict low concentration
ETC data. Model verification procedures similar to this approach have been
employed by Gaudet et al. (1977), O'Connor et al. (1976), and Van Genuchten
and Wierenga (1977).
A finite-difference scheme was used to solve the model (Equations [1]
and [4]) subject to the following initial and boundary conditions:
C = 0, S = 0, 0£X£L, t = 0 [9a]
vCQ, x = 0, t £ t:
[9b]
0, x = 0, t > ti
ax'"' X = L> t>0 [9C]
These conditions are applicable for a soil column of L length (cm), initially
void of pesticide, to which a pesticide solution of C (yg/ml) concen-
tration is applied at an average pore-water velocity of v (on/day) for a
period of ti days, and then leached through the soil with a pesticide-free
solution. All computations were performed on an AMDAHL 470 V/6-11 digital
computer (soft-ware compatible with IBM 370/165 systems) with the aid of
computer programs written in FORTRAN IV language.
34
-------
Measured and simulated ETC for 2,4-D amine displacement through a
water-saturated Cecil soil column are presented in Figure 13. Pesticide
mobility was greater for high input concentrations as indicated by the
left-hand shift of the ETC for C = 5,000 yg/ml compared to that for C =
50 yg/ml. This increased pesticide mobility at higher concentrations i§ due
to the nonlinearity of the adsorption isotherm (Rao et al., 1979). The
simulated curves, shown as solid lines in Figure 13, were calculated using
the two-site model, where the parameters F and kx were estimated by curve-
fitting to the C = 50 yg/ml ETC data. Independently measured values of K
and N (Table 5) along with the estimated values of F, k1} and k2 were used
to predict the ETC for C = 5,000 yg/ml. The agreement between the simula-
tions (solid lines in Figure 13) and the measured ETC is only fair. There-
fore, the model parameters K, N, F, ki, and k2 were re-estimated using a 4-
parameter fit procedure and the C = 50 yg/ml ETC data. Improved prediction
(dashed lines in Figure 13) of the1 measured ETC using the four parameter
fit procedure is evident.
The values of the model parameters K, N, F, ki, and k2 estimated from
2,4-D amine data for C = 50 yg/ml by varying two parameters (F and kj or
four parameters (K, N,°F and ki) in the NLLS procedure are presented in
Table 6. More than a two-fold increase in K value and a small decrease in
N over that obtained from an equilibrium adsorption isotherm was necessary
to describe the ETC data for both input concentrations. Furthermore, the
two estimates of the kj and k2 values were also different. Note that about
60 to 70% of the total adsorption sites were required to be kinetic (i.e.,
type-2 sites) in order to describe the extensive tailing in the measured
ETC;
unlike the ETC for the pesticides, those for 3H20 displacement through
the three soils were symmetrical and sigmoidal in shape (see Figures 9, 10,
and 11). Excellent descriptions of the ETC were obtained using the convec-
tive-dispersive transport model (Equation 1) where all the soil-water was
assumed to be mobile. A conceptual model where the soil and soil-water are
partitioned into mobile and immobile phases failed to describe the ETC for
^H20 for the. three soils studied (Rao et al., 1979). These results suggest
that while Equation [1] correctly represented nonadsorbed solute transport
processes, the nonequilibrium adsorption-desorption phenomenon were not
adequately described by Equation [4]. Additional studies need to be carried
out to identify the causes of nonequilibrium conditions for pesticide
adsorption-desorption during transport in soils.
INFILTRATION EXPERIMENTS
Pesticide transport in soils during transient, unsaturated, and one-
dimensional water flow was investigated using hand packed horizontal soil
columns. In order to simulate a waste disposal site, the top 1.5 cm of
each soil column was packed with pesticide-treated soil (2,000 yg pesti-
cide/g soil), and 0.01 N CaCl2 was infiltrated into the soil at a constant
negative head (-4 cm of water). During infiltration, the rate of wetting
front advance was recorded by visual observations. The soil column was
cut into 1-cm segments at the end of the infiltration. In each soil seg-
ment, the total amount of pesticide (sum of solution, adsorbed, and solid
35
-------
TABLE 6. COMPARISON OF MODEL PARAMETER VALUES EXTIMATED FROM THE 2,4-D
AMINE BREAKTHROUGH DATA (C = 50 yg/ml) FOR CECIL SOIL BY
VARYING EITHER TWO OR FOUR°PARAMETERS IN THE NONLINEAR LEAST-
SQUARES CURVE-FITTING PROCEDURE
CECIL -2,4-D AMINE
PARAMETER
N
K
ki
F
kiCday"1)
k2(day-i)
klA2
2 -parameter Fit
0.83*
0.664*
0.324
0.488
0.271
0.222
. 1.220
4-parameter Fit
0.73
1.604
0.425
0.265
0.515
0.122
4.221
*These values of the Freundlich constants were obtained independently
from the equilibrium adsorption isotherm.
36
-------
1.0
u
z
o
u
0.8
0.6
U
K
CECIL SOIL
2,4-D Amine
*S
TWO-SITE MODEL
K=0.664, N = 0.83; F= 0.488
=0.271 day4. k2= 0.222 day"1
{:
<=1.604 , N = 0.73,- F=0.265
f 0.515 day1. k2= 0.122 day"1
ooo 50 jug/ml
•••• 5000
VOLUMES
Figure 13. Measured and simulated breakthrough curves for 2,4-D amine dis-
placement through Cecil soil column. Parameter values used to
calculate the solid lines were obtained from a 2-parameter fit,
and those for dashed lines were estimated from a 4-parameter
fit to C0 = 50 pg/ml data.
37
-------
phases) was determined by combustion, while the soil-water content was
measured by oven-dry ing.
The depth (X ) to which the pesticide front moved in a soil due to
water infiltration* was dependent upon the wetting front depth (XJ, the
pesticide adsorption isotherm constants (K and N), and the aqueous solubi
lity (C ) of the pesticide. The relationship between these variables may
be exprlssed as:
Xw pl
w s 1 + —^_
e [10]
where, p is the soil bulk density (g/cm3) and e is the average soil-water
content (cm3/cm3) in the wetted zone behind the wetting front. It should
be noted that for the case of a linear adsorption isotherm (N = 1), Equa-
tion [10] is exact and the retardation of the pesticide front due to adsorp-
tion is independent of concentration, while for the nonlinear isotherm
case, Equation [10] is only an approximation.
Assuming that Darcy's law is valid for unsaturated water flow in
soils, the rate of the advance of the wetting front is given by (Kirkham
and Powers, 1972):
Xw = mt1/2 [11]
where, m is a constant and t is time (min). In the present study, Equation
[11] described (r2 >_ 0.95) the advance of the wetting front for all soil
columns considered. The values of m and other pertinent data for the
infiltration experiments are summarized in Table 7.
Measured pesticide concentration profiles and soil-water content
distributions at the end of infiltration in Eustis,'Cecil, and Webster soil
columns are shown in Figures 14 and 15. Because the final wetting front
position in each soil column was different, for ease of comparison the
ordinate in Figures 14 and 15 is plotted as soil depth relative to the wet-
ting front depth (i.e., X/XJ. Except for the 2,4-D-Eustis and the terbi-
cil-Eustis data, the relative mobilities of the pesticides are in general
agreement with those anticipated from the equilibrium adsorption isotherms
and pesticide aqueous solubilities. The measured mobility of terbacil and
2,4-D in the Eustis soil (Figure 14) was nearly the same although the
adsorption coefficient for 2,4-D is greater than that for terbacil in
Eustis soil (see Tables .5 and 7). The importance of aqueous solubility is
demonstrated by the atrazine data in the Eustis soil (Figure 14 and Table
7). Note that the .volume of water infiltrated into the soil column could
solubilize and transport only about 4% of the total pesticide present in
the top 1.5 cm segment; thus, most of the atrazine does not appear to have
moved. The retardation factors, (X/X ), calculated by Equation [10] are
generally larger than those measured in the infiltration experiments
(Table 7). Similar results were reported by Wood and Davidson (1975) for
transient-flow studies and by Davidson and Chang (1972) for saturated flow
experiments. The kinetics of pesticide adsorption-desorption in soils are
38
-------
TABLE 7. PHYSICAL DATA FOR INFILTRATION EXPERIMENTS
vo
SOIL
Eustis
Eustis
Eustis
Cecil
Webster
PESTICIDE
Atrazine
2,4-D
Terbacil
Terbacil
Terbacil
CY
_ P .A.
S W
(pg/ml) (g/cm3) (cm)
33
650
710
710
710
1.67
1.69
1.64
1.51
1.40
21.3
30.0
29.5
28.4
24.1
m*
\/\
(on/min%) Measured
3.195
3.854
4.039
0.705
0.543
2.
1.
1.
1.
4.
50
05
11
49
16
Calculated*
3.
1.
1.
3.
4.
03
89
37
03
55
Volume
of water
applied
* (ml)
44
68
63
68
67
Pesticide
Recovery
(*)
>100
>100
91
93
99
v
*see Equation [11]
**see Equation [10]
-------
Pesticide Concentration, jjg/g soil
250 0 250 0 250
Q2
0.4
JC
+->
Q.
1.0
-------
TERBACIL CONCENTRATION, jjg/g soil
500
75O
0.2
O.3 0.1 0.2
SOIL-WATER
0.3 0 0.1
CONTENT , cm3/cm3
0.2
O3
0.4
Figure 15. Soil-water content (solid lines) and terbacil concentration
distributions in Eustis, Cecil and Webster soils following
infiltration of water to approximately 30-cm. Soils were
initially air dry and herbicide was in top 1.5-cm of soil
(2,000 yg/g of soil).
41
-------
not understood well enough at this time (Rao et al., 1979) to describe
modeling the nonequilibrium conditions for pesticide adsorption-desorption
during transient soil-water flow. Additional studies are needed in this
area.
MCROBIAL ACTIVITY AND DEGRADATION
Total C02 evolution (respiration) is generally a good indicator of
soil microbial activity. This procedure was used by Stojanovic et al.
(1972) to estimate pesticide degradation rates in soil systems receiving
large pesticide concentrations. A more direct procedure for determining
pesticide degradation, however, is to measure ^CC^ evolution from uniformly
ring-labeled (llfCF3 position for trifluralin) pesticides. In this study,
the accuracy of total C02 evolution as an estimate of pesticide degradation
was compared with that obtained using ll*C02 evolution.
ATRAZINE
Soil respiration rates from Cecil soil treated with 10 ppm of technical
grade atrazine generally were not different from the soil without atrazine
during the entire incubation period of 82 days (Figure 16). Soil respiration
rates for the 1,000 and 20,000 ppm treatments were generally smaller than
the untreated soil with the inhibition being the greatest for the 20,000
ppm treatment. Total C02 evolution from the Cecil soil receiving 10, 1,000
and 20,000 ppm of formulated atrazine was enhanced, especially during the
first 16 days of incubation and the stimulation was greatest for the 1,000
and 20,000 ppm treatments. Both technical grade and formulated atrazine at
concentrations of 10, 1,000 and 20,000 ppm enhanced the C02 evolution from
the Webster soil (Figure 17), with C02 evolution being greater for the
largest herbicide concentration.
For incubation periods up to 70 days, 80-90% of the 1£tC-activity
extracted from the Webster soil was in the form of intact atrazine; the
remainder of the * ^-activity existed as metabolites. More extensive
break-down occured in the soil treated with 10 ppm of technical grade
atrazine and then only after incubation exceeding 80 days. A small portion
(less than 1%) of the applied 1£*C from all treatments was lost as lkCQ2
during the entire incubation period.
The breakdown of atrazine in Cecil soil was more rapid and more exten-
sive than it was in the Webster soil. However, the amount degradated de-
clined with increasing concentrations. In the case of the 10 ppm treatment,
there was a substantial breakdown of atrazine. After 63 days of incubation
of the technical material, 14% of the llfC remained as the parent compound;
for the case of formulated atrazine incubated for 75 days, 40% was intact
atrazine. Cecil soil treated with 1,000 ppm of atrazine exhibited somewhat
less breakdown. After 55 to 63 days of incubation with the technical grade
material, 70 to 82% of the ll*C extracted was parent atrazine; and after 75
days of incubation with the formulated herbicide, 50% of the herbicide
was unchanged. In the Cecil soil fortified with 20,000 ppm, even less
breakdown occurred; after incubation for 75 days, 80% of the lltC remaining
was atrazine. Similar to the Webster soil, only a small quantity of the
42
-------
_ A
o
O)
O
o
6
E 4
o
cvj
8a
o
20
o—o o Ppm
•—• 10 ppm
°—a 1,OOO ppm
120,OOO ppm
40
60
Days
8
0
1 I
B
20
O ppm
10 ppm
a—° 1,OOO ppm
•—• 2O.OOO ppm
40
Days
60
Figure 16. C02 evolution rate from the Cecil soil receiving various concentrations of atrazine.
(A) Technical grade; (B) Formulated atrazine.
-------
3
m
O)
O
O
D)
o
CM
O
u
o—o O ppm
•—• 10 ppm
t>-a 1,000 ppm
•—• 2O.OOO ppm
O
20
40
Days
60
B
o—o 0 ppm
•—• 1 0 ppm
o—a 1.OOO ppm
•—• 20,000 ppm
0
2O
40
Days
60
Figure 17. C0£ evolution rate from the Webster soil receiving various concentrations of atrazine.
(A) Technical grade; (B) Formulated atrazine.
-------
total lkC-activity (less than 1%) was degraded to 14C02 in the Cecil soil.
Technical grade and formulated atrazine at concentrations of 10, 1,000
and 20,000 ppm did not have a significant effect on bacterial populations
in the Cecil or Webster soils, except for the 20,000 ppm formulated atrazine
concentration in the Cecil soil (Table 8). The bacterial population in
this treatment was nearly four times higher than the untreated soil following
eleven weeks of incubation.
The number of fungi in the technical grade treated Cecil soil at 10,
1,000 and 20,000 ppm were not significantly different from the untreated
soil following eleven weeks of incubation (Table 9). Fungal populations in
the technical grade treated Webster soil at the same application rates
were, however, significantly higher (p < 0.01). Fungal populations in all
the formulation atrazine treated soils, but one, were not significantly
different. Fungal populations in the Cecil soil treated with 20,000 ppm of
formulated atrazine were reduced significantly (p < 0.01) during the first
seven weeks of incubation.
Technical grade and formulated atrazine at concentrations of 10, 1,000
and 20,000 ppm showed no consistent effect on the actinomycete populations
in the Cecil or Webster soils, except for the Cecil soil which received
20,000 ppm of formulated atrazine (Table 10). In this treatment, actino-
mycete populations were reduced significantly (p < 0.01).
METHYL PARATHION
Total C02 evolution from the Webster soil containing 24.5 and 10,015
ppm of technical grade and formulated methyl parathion were generally not
different from an untreated soil (Table 11), except during the first few
days of incubation. C02 evolution was somewhat greater for the treatments
receiving a pesticide during the first few days of incubation than for the
untreated soils. Unlike the Webster soil, total C02 evolution from the
Cecil soil containing 10,015 ppm of methyl parathion was reduced (Table
12). The reduction was somewhat greater for the Webster soil receiving the
formulated material. Apparently, the formulation chemicals at this concen-
tration exhibited an inhibitory effect on soil respiration in the Cecil
soil, but not for the Webster soil. C02 evolution from the Cecil soil re-
ceiving 24.5 ppm of technical grade or formulated methyl parathion was not
different from the untreated soil.
Both technical grade and formulated methyl parathion at 24.5 ppm were
degraded rapidly to C02, H20 and simple inorganic ions in the Webster and
Cecil soils as indicated by 14C02 evolution from uniformly ring labeled
1HC-methyl parathion (Table 13). More than half of the ltfC -methyl parathion
added was mineralized to ll*C02 in 10 days in the Webster and Cecil soils.
Degradation rates were greater in the Webster soil than the Cecil soil.
Degradation rates from the soil receiving the technical grade material were
somewhat greater than those receiving the formulated material. At the end
of 52 days of incubation, 75% to 85% of 1IfC-methyl parathion in the 24.5
ppm treatments was degraded to 14C02. Lichtenstein, et al. (1977) reported
that up to 43% of the 14C-ring labeled methyl parathion applied to a soil
45
-------
.£>.
ON
TABLE 8. EFFECT OF TECHNICAL GRADE AND FORMULATED ATRAZINE ON BACTERIAL POPULATIONS IN CECIL AND
WEBSTER SOILS
Concentrat ion
of atrazine cfu-g'1 soil (x 10"6)
(vg*g l) Time (Weeks)
0.1 1 23
0
10
Cecil
Webster
Cecil
Webster
Cecil
26.0a(26.4)b
47.6(53.8)
24.9(29.9)
58.4(56.2)
30.1(30.6)
16.2(20.5)
31.7(30.7)
21.9(23.3)
33.6(32.2)
18.0(20.5)
17.3(19.6)
28.3(22.4)
20.1(19.2)
31.7(28.9)
18.0(24.9)
14.4(13.0)
ND (28.6)
12.5(16.4)
ND (28.3)
10.3(14.8)
19.2 (ND)
20.5(24.7)
13.3 (ND)
22.1 (30.9)
10.9 (ND)
15.5(18.7)
18.6 (ND)
14.1(14.8)
24.3 (ND)
17.3(15.0)
1,000
Webster 62.7(30.4) 33.8(38.2) 22.9(25.7) ND (20.3) 22.6(34.8) 17.7 (ND)
Cecil 32.4(69.1)d 19.6(111.0)d 22.1C(121.0)d14.4(70.9)d 16.2 (ND) 18.5(51.3)d
20,000 A A A
Webster 52.0(50.7) 29.9(70.7)a 22.6(59.3)° ND (47.1)a 24.2(28.1) 16.1 (ND)
^irst column, technical grade atrazine.
Second column, formulated atrazine.
cSignificant at p < 0.05.
Significant at p < 0.01 . . (continued)
-------
TABLE 8. (continued)
Concentration
of atrazine- cfu-g"1 soil (x 10~6)
Cvg'g"1) Time (Weeks)
6 7 8 9 10 11
Cecil ND (15.7) 15.2(16.0) 13.9 (ND) 17.7(13.5) 16.3(13.7) 18.5(16.2)
0
Webster 13.5(23.4) 18.2(22.8) 16.1(18.5) 19.2(20.0) 20.0(21.1) 19.5(25.2)
Cecil ND (20.9) 12.1(14.7) 17.7 (ND) 10.9C(11.4) 12.0(14.8) 13.7(13.9)
10
Webster 18.5(33.8) 27.0(16.4) 20.0(19.8) 26.8(18.7) 24.4(21.6) 19.8(19.5)
Cecil ND (11.6) 11.4(15.0) 17.6 (ND) 13.7(11.9) 11.9(16.9) 17.1(13.0)
1,000
Webster 21.8(19.8) 19.0(19.2) 21.1(27.3) 13.8(23.1) 23.9(21.8) 19.2(27.0)
Cecil ND (49.6)d 13.5(56.3)d 13.9 (ND) 19.8(52.0)d 16.0(36.3)d 19.2(57.2)d
20,000
Webster 17.6(30.4) 18.7(28.9) 20.8(32.2) 15.3(23.4) 19.5(25.1) 17.4(23.1)
rirst column, technical grade atrazine.
Second column, formulated atrazine.
Significant at p < 0.05.
Significant at p < 0.01 (continued)
-------
TABLE 8. (continued)
OO
Concentration
o£ atrazine
Cvg-g"1)
0
10
1,000
20,000
•ill* « •••Him • . — mm - -IP— «l • •!••
cfu-g'1 soil (x 10 ~6)
Time (Weeks)
Cecil
Webster
Cecil
Webster
Cecil
Webster
Cecil
Webster
Average
17.3(17.3)
23.0(26.5)
15.7(17.9)
27.9(26.9)
16.0(17.4)
25.3(26.2)
18.7(67.8)d
23.1(38.1)
•MMmlVIV^V.^l^HIMvmllKBA^^^B^^MM'
column, technical grade atrazine.
Second column, formulated atrazine.
Significant at p < 0.05.
Significant at p < 0.01.
-------
VO
TABLE 9. EFFECT OF TECHNICAL GRADE AND FORMULATED ATRAZINE ON FUNGAL POPULATIONS IN CECIL AND
WEBSTER SOILS.
Concentration
of atrazine cfu-g"1 soil (x 10"1*)
(pg'g"1) Time (Weeks)
0.1 1 234
Cecil
0
Webster
Cecil
10
Webster
Cecil
1,000
Webster
Cecil
20,000
Webster
8.0a(8.
15.2(15
8.9(8.
15.9(16
9.4(7.
17.7(10
7.7(3.
17.3(12
6)b
.2)
6)
-3)
9)
-9)d
8)d
.8)°
8.
14.
10.
15.
9.
14.
8.
16.
1(9.1)
0(15.3)
2(8.5)
3(12.7)
6(9.7)
8(14.4)
8(6. 5)d
5(15.3)
9.
8.
9.
14.
8.
14.
9.
11.
0(8.9)
7(12.1)
5(8.9)
4d(12.1)
4(8.4)
2d(13.3)
1(5. 3)d
4C(14.0)
9.0(8.8)
ND (13.3)
10.0(9.1)
ND (12.9)
7.7(9.5)
ND (14.7)
9.3(6.2)d
ND (14.8)
8.
8.
9.
9.
8.
13.
8.
14.
7 (ND)
4(12.2)
6 (ND)
8(15.0)
8 (ND)
3d(17.8)
9 (ND)
1
6a(12.9)
8
7
8
14
9
12
9
14
.9(8.5)
.0 (ND)
.6(9.4)
.4d(ND)
.5(8.9)
.7d(ND)
.7(6. 5)d
•t
.6a(ND)
column, technical grade atrazine.
Second column, formulated atrazine.
Significant at p < 0.05.
Significant at p < 0.01. (continued)
-------
TABLE 9. (continued)
Concentration
of atrazine-
(yg-g"1)
0
10
1,000
20,000
Cecil
Webster
Cecil
Webster
Cecil
Webster
Cecil
Webster
6
ND (9.
6.1(10
ND (9.
9.9d(10
ND (9.
12.8d(ll
ND (5.
13.8d(12
cfu-g l soil (x
Time (Weeks)
7 8
1)
.6)
1)
.8)
7)
.4)
5)d
.4)
8.
6.
8.
12.
8.
13.
8.
13.
2(8.6)
9(13.1)
4(9.1)
Od(10.2)
8(9.2)
3d(14.7)
7(6. 6)d
5d(11.5)
8.
8.
10.
10.
9.
11.
9.
5 (ND)
1(10.2)
8 (ND)
8(11.9)
4 (ND)
6d(14.8)d
10"^)
9
9.1(9.0)
6.5(11.6)
8.8(9.6)
14.2d(12.1)
8.6(8.4)
9.8d(16.4)d
9 (ND) 7.2(9.4)
9C(14.3)d13.7d(13.1)
10
8.6(8.0)
7.1(11.1)
8.9(9.3)
13.3d(11.0)
7.9(8.6)
11.8d(ll.l)
8.0(8.3)
13.3d(15.0)
9
6
8
12
9
10
7
d!3.
11
.3(9.5)
.5(12.7)
.8(8.6)
.5d(10.4)
.0(8.2)
.6d(14.6)
.6(7.5)
ld(16.0)C
colijmn, technical grade atrazine.
Second column, formulated atrazine.
Significant at p < 0.05.
Significant at p < 0.01. (continued)
-------
TABLE 9. (continued)
ui
Concentration
of atrazine
(yg-g"1)
0
10
1,000
20,000
cfu-g"1 soil (x
Time (Weeks)
Cecil
Webster
Cecil
Webster
Cecil
Webster
Cecil
Webster
10"")
Average
8.7(8.8)
8.6(12.5)
9.3(9.0)
A
13.0a(12.3)
8.8(8.9)
A
13.0 (14.0)
8.6(6.6)d
A
14.0a(13.8)
Thirst column, technical grade atrazine.
Second column, formulated atrazine.
Significant at p < 0.05.
Significant at p < 0.01.
-------
TABLE 10. EFFECT OF TECHNICAL GRADE AND FORMULATED ATRAZINE ON ACTINOMYCETE POPULATIONS IN CECIL
AND WEBSTER SOILS
Concentrations
of atrazine cfu-g"1 soil (x 10~5)
(wg-g x) Tijne (Weeks)
0.1 1 2345
Cecil 4.6a(1.8)b 2.3(3.4) 5.5(10.0) 6.6(10.0) 10.7 (ND) 5.9(6.2)
Webster 14.6(17.7) 14.6(12.7) 8.8(7.5) ND (12.7) 9.6(12.5) 14.0 (ND)
10
Cecil 5.2(3.0) 7.3(2.5) 9.8C(4.3)C 9.4(7.5) 11.7 (ND) 3.7(7.3)
Webster 17.4(16.6) 16.4(8.3) 11.2(8.1) ND (13.0) 12.2(8.8) 12.5 (ND)
Cecil 6.0(1.6) 6.4(1.4) 7.5(3.9)C 6.4(5.2)c 7.4 (ND) 6.4(5.5)
1,000
Webster 19.8(10.4)° 14.8(12.7) 8.8(10.1) ND (11.4) 13.5(10.4) 8.6C(ND)
Cecil 3.2(0.2)d 4.8(0.2)d 8.0(1.4)d 11.9d(0.9)d 8.0 (ND) 7.3(0,5)d
20,000
Webster 19.0(13.8) 13.8(10.4) 11.2(10.1) ND (13.0) 13.5(7.5)c 10.7 (ND)
column, technical grade atrazine.
Second column, formulated atrazine.
Significant at p < 0.05.
Significant at p < 0.01. (continued)
-------
TABLE 10. (continued)
in
Concentration
of atrazine
(yg-g'1)
Cecil
0
Webster
Cecil
10
Webster
Cecil
1,000
Webster
Cecil
20,000
Webster
ND
6
(8.4)
ND(10.4)
ND
ND
ND
ND
ND
ND
(7.8)
(10.4)
(11.2)
(7.0)
(0.7)d
(9.6)
1.
12.
2.
8.
2.
13.
3.
15.
d
7
8(5.5)
7(8.5)
7(5.7)
8(7.3)
1(3.0)
3(6.5)
0(1. 4)d
6(6.8)
Pu-g'1 soil (x 10"
Time (Weeks)
8
4.8
13.0
5.2
9.1
5.5
12.5
8.4C
11.7
(ND)
(ND)
(ND)
(ND)
(ND)
(ND)
(ND)
(ND)
1.
9.
1.
10.
2.
14.
2.
12.
5)
9
4(2.7)
9(10.4)
1(2.1)
1(8.1)
1(2.3)
8C(13.8)
3(0. 2)d
2(9.9)
10
3.2(4.3)
ND (11.2)
1.4(4.8)
ND (9.6)
1.1(4.3)
ND (11.4)
2.1(0.5)d
ND (9.1)
11
1.
12.
1.
14.
1.
9.
0.
17.
4(2.1)
0(9.6)
8(0.7)
3(11.4)
4(0.9)
6(11.7)
7(0. 2)c
7(13.5)
column, technical grade atrazine.
Second column, formulated atrazine.
Significant at p < 0.05.
^Significant at p < 0.01.
(continued)
-------
TABLE 10. (continued)
01
Concentration
of atrazine
(yg-g"1)
0
10
1,000
20,000
cfu'g'1 soil (x 10 "!
Time (Weeks)
Cecil
Webster
Cecil
Webster
Cecil
Webster
Cecil
Webster
5)
Average
4.4(5.4)
12.1(11.3)
5.4(5.2)
12.4(10.2)
4.7(3.8)
12.9(10.5)
5.4(0.6)d
13.9(10.4)
column, technical grade atrazine.
Second column, formulated atrazine.
Significant at p < 0.05.
Significant at p < 0.01.
-------
TABLE 11. TOTAL C02 EVOLUTION FROM METHYL PARATHION TREATED WEBSTER SOIL
Time
(day)
2
7
12.5
18.5
25.5
32.5
39.5
47.5
0 ppm
1.81
1.32
1.15
0.86
0.79
0.72
0.51
0.56
24.5
Technical
2.88
1.53
1.25
1.00
0.89
0.82
0.58
0.56
C02-C mg/lOOg
ppm
Formulated
2.40
1.85
0.91
1.24
1.07
0.89
0.58
0.72
soil/day
10,015
Technical
1.99
1.45
1.00
0.93
0.89
0.72
0.54
0.50
ppm
Formulated
2.64
1.73
1.25
0.89
0.82
0.75
0.40
0.61
Total 45.65 54.94 58.18 47.74 52.79
55
-------
TABLE 12. TOTAL C02 EVOLUTION FROM METHYL PARATHION TREATED CECIL SOIL
C02-C mg/lOOg soil/day
Time
(day)
2
7
12.5
18.5
25.5
32.5
39.5
47.5
0 ppm
1.93
1.20
0.86
0.82
0.75
0.65
0.61
0.56
24.5
Technical
1.75
1.12
0.86
0.82
0.61
0.65
0.68
0.40
ppm
Formulated
2.16
1.28
0.86
0.72
0.79
0.65
0.79
0.42
10
Technical
1.34
0.55
0.61
0.37
0.54
0.54
0.58
0.40
,015 ppm
Formulated
0.45
0.67
0.57
0.40
0.51
0.47
0.65
0.40'
Total 44.10 40.95 45.04 29.04 26.74
56
-------
TABLE 13. PERCENT OF ll*C -METHYL PARATHION EVOLVED AS 14C02 IN WEBSTER AND
CECIL SOILS
Concentration
(ppm)
24.5
10,015
24.5
10,015
Technical
Formulated
Technical
Formulated
Technical
Formulated
Technical
Formulated
4
45.9
44.8
0
0
36.7
32.6
0
0
\
10
67.0
64.5
0
0
59.7
55.0
0
0
\ evolved as lt*C02
Time (days)
15 29 43
rVfcJDo LCI s-jUjLjL""--"---- — — - —
73.3 80.6 83.8
69.7 76.2 79.2
0 ~0 <0.1
0 ~0 <0.1
— Lecu sou.
66.6 75.1 78.5
61.9 70.6 73.9
0 ~0 ~0
0 ~0 <0.1
52
85.0
80.3
<0.1
<0.1
79.4
75.0
~0
<0.1
57
-------
exhibited the characteristics of a bound residue after 28 days, and only 7%
of the 14C-activity was extractable.
For the 10,015 ppm concentrations, very little technical grade or
formulated methyl parathion was degraded in Webster or Cecil soils during
52 days of incubation. Less than 0.1% of 14C-methyl parathion was degraded
to 1'»C02 at the 10,015 ppm concentration (technical grade and formulated).
Total C02 evolution from the soils containing 10,015 ppm methyl parathion
was in good agreement with lkC02 evolution. The total amount of methyl
parathion-carbon added to 100 g of soil at 10,015 ppm was 365 mg. If
extensive degradation occurred, a substantial amount of C02 would have
evolved. For example, if 101 of the methyl parathion had mineralized, 36.6
mg of C02-carbon would have evolved. Total C02 evolution from the Cecil
soil receiving 10,015 ppm of methyl parathion was less than the untreated
soil; therefore, it would not be expected that extensive degradation oc-
curred. Total C02 evolution for the Webster soil receiving 10,015 ppm of
methyl parathion was not enhanced, and extensive degradation did not occur.
Microbial populations have been reported to be inhibited five years after
parathion application of 30,000 to 95,000 ppm to a soil (Wolfe, et al.,
1973}.
This project has shown that for low application rates, methyl parathion
was nonpersistent in soils, but the insecticide persisted following appli-
cations of large quantities. Thus, using the results from the low appli-
cation rates to predict the behavior of methyl parathion at high applica-
tions would lead to erroneous conclusions.
TRIFLURALIN
Soil respiration rates from Cecil and Webster soils receiving 10 ppm
of technical grade and formulated trifluralin were not different from un-
treated soils except during the first week of incubation (Figures 18 and
19). Carbon dioxide evolution from the Webster soil receiving 1,000 and
20,000 ppm of technical grade trifluralin were not enhanced except during
the initial incubation period (Figure 19). Soil respiration from the Cecil
soil treated with 1,000 and 20,000 ppm at technical grade trifluralin was
enhanced during the first two weeks of incubation. C02 evolution from the
Cecil and Webster soil receiving 1,000 ppm of formulated trifluralin was
also enhanced during the initial period of incubation, and the enhancement
of C02 evolution was greatest for the Webster soil. C02 evolution in the
Cecil and Webster soils treated with 20,000 ppm of formulated trifluralin
was stimulated in the first 5 days of incubation. This stimulation was
followed by a later increase in C02 production. Stimulation in C02 produc-
tion in the Webster soil treated with 20,000 ppm of formulated trifluralin
was much greater than that in the Cecil soil receiving the same treatment,
and C02 production' in the Webster soil remained 2 to 9 times higher than
for the untreated Webster soil during the entire 83 days of incubation.
Less than 5% of the trifluralin applied to the Cecil and Webster soils
was degraded to ltfC02 after 83 days of incubation. Several metabolites
from the trifluralin were detected and characterized. Information regarding
metabolites will be presented in the metabolite section of this 'manuscript.
58
-------
Ul
vo
0 ppm
1 O ppm
1,000 ppm
20,000 ppm
0
8
B
,
o—o 0 PPm
•—• 10ppm
o—n 1,000 ppm
•—• 20.OOO ppm
20
40
Days
60
Figure 18. C02 evolution rate from the Cecil soil receiving various concentrations of trifluralin
(A) Technical grade; (B) Formulated trifluralin.
-------
0\
o
O
(ft
D)
o
C\J
O
U
Oppm
10 ppm
1,000 ppm
2O.OOO ppm
20
40
Days
60
8
B
o—o
I I I
Oppm
10 ppm
a—a 1,OOO ppm
20,000 ppm
0
20
40
Days
60
Figure 19. C02 evolution rate from the Webster soil receiving various concentrations of trifluralin
(A) Technical grade; (B) Formulated trifluralin.
-------
Bacterial populations in the Webster soil treated with 10 ppm tri-
fluralin (technical grade and formulated) were not significantly different
from the untreated soil (Table 14). However, bacterial populations in the
Webster soil receiving 1,000 ppm of formulated trifluralin were consistently
higher than the untreated soil with the greater stimulation occurring in
the Cecil rather than the Webster soil (Table 14). For 20,000 ppm of
technical grade trifluralin, bacterial populations in the Cecil soil were
greater than those in the Webster soil. Bacterial populations in the Cecil
soil receiving 20,000 ppm of formulated trifluralin were initially inhibited,
but were enhanced significantly after two weeks of incubation; whereas
bacterial populations in the Webster soil for the same treatment were
stimulated during the first 18 hours and the stimulation was greatest
during the third week of incubation.
Fungal populations in the Cecil and Webster soils treated with 10 ppm
of technical grade and formulated trifluralin were generally not signifi-
cantly different from the untreated soil (Table 15). For the 1,000 ppm
treatments, fungal populations in the Cecil soil receiving technical grade
and in the Webster soil receiving formulated trifluralin were not signifi-
cantly affected, whereas the Cecil soil treated with the formulated material
and the Webster soil with the technical grade material showed a significant
effect at the 0.01 and 0.05 levels, respectively. Fungal populations were
significantly reduced in both soils at 20,000 ppm of formulated trifluralin;
whereas fungal populations in the Webster soil receiving the technical
grade material were stimulated during the first four weeks.
Actinomycete populations in Cecil and Webster soils receiving 10,
1,000 and 20,000 ppm of trifluralin. (technical grade or formulation) were
not significantly different from the untreated soils (Table 16) except in
the 20,000 ppm formulated trifluralin treatments. Actinomycete populations
in both soils receiving 20,000 ppm of the formulated material were reduced
significantly.
2,4-D
The rates of total C02 evolution from the Webster soil receiving
various concentrations of 2,4-D are given in Figure 20. A common character-
istic of the C02 evolution rates for 5,000 and 20,000 ppm was the presence
of two response peaks. The initial increase in C02 evolution rate was
followed by a decline and a second increase. This was observed for both
forms of 2,4-D. The C02-carbon (C02-C) produced prior to the appearance of
the second peak was not from the 2,4-D-carbon (2,4-C) because very little
lkCQ2 was measured during this initial time period. For formulated 2,4-D,
the C02-C in the first peak appeared to come from the oxidation of the
formulation ingredients including dimethylamine; and for technical grade
2,4-D, the C02-C probably originated from impurities and soil organic
matter. The appearance of the second peak coincided with the occurrence
of 2,4-D degradation (14C-C02 activity) (Figure 21). Assuming all C02-C
was evolved from 2,4-D-carbon, the extent of degradation during the entire
experimental period (80 days) and during the second peak period were
calculated and are shown in Table 17. These results clearly illustrated
that the majority of the C02-C evolved during the second peak period was
61
-------
TABLE 14. EFFECT OF TECHNICAL GRADE AND FORMULATED TRIFLURALIN ON BACTERIAL POPULATIONS IN CECIL AND
WEBSTER SOILS
Concentration
of trifluralin cfu-g"1 soil (x 10~6)
(yg-g J) Time (Weeks)
0.1 1 2345
Cecil 17.2a(17.0)b 8.7(19.9) 11.4(16.5) ND (14.4) 16.2(14.9) ND (12.8)
0
Webster 39.4(25.2) 21.1(17.0) 18.9(15.9) 20.5(15.5) 15.3(14.4) ND (13.1)
Cecil 20.1(16.4) 12.5(17.1) 12.8(14.9) 10.2(15.4) 11.4(16.0) ND (13.3)
10 , .
^ Webster 46.8(46.5)° 40.0a(28.3)c 31.5a(20.8) 20.7(19.0) 17.6(14.6) ND (15.0)
tsi
Cecil 20.7(40.7)d 11. 5(259. 0)d 10. 7(238. 0)d 11. 1(331. 0)d 9.1(288.0)d ND(246.0)d
1,000 , A A A A
Webster 49..1(38.2)c 29. 6(298. 0)a 26. 4(107. 0)a 23.9(42.8)a 17.9(41.3)a ND (31.2)a
Cecil 27.9d(3.6)d 18.5d(70.3)d 33.3d(173.0)d 21. 0(120. 0)d 21.1(74.6)d ND (62.2)d
20,000 , , -, •, A A
Webster 39.9(63.2)a 28. 3(352. 0)a 27. 0(559. 0)a 19. 0(2030. 0)a 18. 6(1140. 0)a ND(484.0)a
column, technical grade trifluralin.
Second column, formulated trifluralin.
Significant, at p < 0.05.
*d "' •"->
.significant at p < 0.01. (continued)
-------
TABLE 14. (continued)
ON
Concentration
of trifluralin
Cecil
0
Webster
Cecil
10
Webster
Cecil
1,000
Webster
Cecil
20 000
Webster
6
8.6(16
15.7(13
8.3(14
18.3(14
11.0(284
18.9(36.
26.0d(62
17.0(204
.7)
• 9)
• 1)
.4)
,0)d
?)d
• 8)d
A
.0)d
7
11.3
ND
11.9
ND
10.5
cfu-g"1 soil (x 10"6)
Time (Weeks)
8 9
(ND)
(17.9)
(ND)
(20.3)
(ND)
ND(36.4)d
23.7
d(ND)
A
ND(189.0)"
12.5(12.
16.3(14.
10.6(14.
18.5(18.
11.9(160.
18.2(32.
27.7d(37.
15.3(195.
3)
8)
0)
9)
o)d
Dd
7)d
A
0)d
ND (18.
15.3(12.
ND (15.
17.4(15.
ND(141.
19.1(36.
ND(55.1
16.1(147
1)
7)
0)
3)
o)d
6)d
)d
A
.0)d
10
11
16
15
16
11
19
30
14
.4 (ND)
.6(14.0)
.7 (ND)
.5(15.7)
.3 (ND) 9
.4(32. 6)d
.6d(ND)
j
.4(168. 0)a
11
12.4(14.4)
ND (ND)
14.1(11.5)
ND (ND)
. 0(134. 0)d
ND (ND)
31.0(13.1)
ND (ND)
aFirst column, technical grade trifluralin.
Second column, formulated trifluralin.
Significant at p < 0.05.
Significant at p < 0.01.
(continued)
-------
TABLE 14. (continued)
Concentration
of trifluralin
cfu-g"1 soil (x 10"
Time (Weeks)
column, technical grade trifluralin.
Second column, formulated trifluralin.
cSignificant at p < 0.05.
Significant at p < 0.01.
Average
0
10
1,000
20,000
Cecil
Webster
Cecil
Webster
Cecil
Webster
Cecil
Webster
12.2(15.7)
19.9(15.8)
12.8(15.8)
25.3(20.8)
11. 7(212. 0)d
A
25.5(66.6)a
26.1d(67.2)d
A
21. 7(503. 0)a
-------
TABLE 15. EFFECT OF TECHNICAL GRADE AND FORMULATED TRIFLURALIN ON FUNGAL POPULATIONS IN CECIL AND
WEBSTER SOILS
ON
tn
Concentration
of trifluralin
G-ig'g"1)
0
10
1,000
20,000
Cecil
Webster
Cecil
Webster
Cecil
Webster
Cecil
Webster
12.2
14.
13.
16.
13.
14.
13.
13.
0.1
a(14.3)b
0(13.4)
3(14.5)
0(14.6)
8(11.0)
0(12.1)
7(6. 9)d
7(12.6)
9.
9.
13.
16.
11.
14.
10.
15.
1
1(12.7)
3(11.7)
1(10.1)
ld(12.7)
A
4(6. 9)a
Od(12.3)
3(0. 7)d
Od(3.4)d
cfu-g"1 soil (x 10"1*)
Time (Weeks)
2
10.5(13.
9.4(10.
12.2(11.
13.5d(ll.
10.9(13.
15.0d(13.
9.7(0.2)
14.2d(2.1
6)
5)
3)
5)
1)
7)
d
)d
3
11.4(11.7)
10.8(12.0)
10.7(12.4)
13.9(13.8)
A
10. 5(6. 0)d
13.8(13.6)
9.3(0.3)d
13.3(0.3)d
4
10.6(10.3)
8.6(12.5)
10.4(11.1)
14.4d(12.3)
A A
6.1d(6.2)d
13.8d(13.6)
6.5d(0.2)d
13.3d(0.4)d
5
ND
ND
ND
ND
ND
ND
(10.7)
(10.3)
(10.5)
(12.4)
A
(7.9)d
(10.9)
ND(<0.1)d
ND
(0.3)d
Tarst column, technical grade trifluralin.
Second column, formulated trifluralin.
Significant at p < 0.05.
Significant at p < 0.01.
(continued)
-------
TABLE 15. (continued)
o\
Concentration
of trifluralin
(ug-g"1)
Cecil
0
Webster
Cecil
10
Webster
Cecil
1,000
Webster
Cecil
?n nnn
L\) , UUU
Webster
cfu-g l soil (x 10 **)
Time (Weeks)
9
9
6
13
11
14
9
11
6
.8(8.
.5(12
8)
• 3)
.2d(10.4)
.7(9.
.9(6.
.3(13
.7(0.
.2(0.
6)
8)C
•8)
2)d
A
3)d
7
11.2
ND
11.5
ND
11.7
ND
9.2
(ND)
(11.2)
(ND)
(15.1)
(ND)
(13.4)
(ND)
A
ND(0.3)"
11.
9.
19.
12.
12.
12.
11.
10.
8
1(9.9)
6(10.0)
7d(10.3)
4(12.8)
7(5. 8)d
2(15.1)
5(0. l)d
A
3(0. 2)a
.9
ND (13.
1)
9.7(8.9)
ND (16.
11.3(10.
ND(6.3)
12.7(15.
ND(<0.1
10.9(<0.
4)
3)
d
A
0)d
1d
A
1 *\
10
9.9 (ND)
9.2(10.7)
11.4 (ND)
12.9(11.1)
8.9 (ND)
A
14.0(12.9)a
9.0 (ND)
A
10.7(<0.1)a
11
8.9(12.0)
ND (ND)
9.9(8.1)d
ND (ND)
19.7(6.3)d
ND (ND)
9.3(0.0)d
ND (ND)
column, technical grade trifluralin.
Second column, formulated trifluralin.
Significant at p < 0.05.
Significant at p < 0.01.
(continued)
-------
TABLE 15. (continued)
Concentration
of trifluralin
(yg'g *)
cfu-g"1 soil (x 10"4)
Time (Weeks)
aFirst column, technical grade trifluralin.
Second column, formulated trifluralin.
Significant at p < 0.05.
^Significant at p < 0.01.
Average
0
10
1,000
7
20,000
Cecil
Webster
Cecil
Webster
Cecil
Webster
Cecil
Webster
10.5(11.7)
10.0(11.2)
11.8(11.5)
13.8C(12.4)
11.8(7.6)d
13.8C(13.3)
9.8(0.9)d
_ j
12.5c(1.8)a
-------
TABLE 16. EFFECT OF TECHNICAL GRADE AND FORMULATED TRIFLURALIN ON ACTINOMYCETE POPULATIONS IN CECIL
AND WEBSTER SOILS
oo
Concentration
of trifluralin
(ug-g"1)
0
10
1,000
20,000
Cecil
Webster
Cecil
Webster
Cecil
Webster
Cecil
Webster
5
14
5
27.
12
16
7
15
0.1
.2a(11.4)b
.0(11.2)
.2(14.3)
8C(12.5)
.ld(9.8)
.9(13.0)
.1(15.3)
.9(6. 0)C
1
6.8(9.8)
12.0(13.3)
5.7(6.8)
13.5(14.3)
4.6(3.2)d
16.6(13.5)
6.4(7.3)
14.0(2.1)d
cfu-
5
18
6
15
7
13
5
13
g l soil (x
Time (Weeks)
2
.2(8.9)
.5(8.8)
.2(4.6)
.6(12.7)
.5(6.2)
.3(13.3)
.2(1.9)d
.3(1.3)d
10 ~5)
3
ND (7.5)
16.4(18.2)
ND (6.2)
15.1(15.6)
ND (3.4)
15.1(16.1)
ND (l.l)d
15.1(0.7)d
4
11
5
14
5
13
4
14
4
.6(5.0)
.4(14.6)
.5(4.8)
.0(15.6)
.1(5.0)
.3(16.6)
.8(1. 4)d
.0(0.3)d
5
ND
ND
ND
ND
ND
ND
ND
ND
(ND)
(14.6)
(ND)
(13.3)
(ND)
(14.0)
(ND)
(0.2)d
column, technical grade trifluralin.
Second column, formulated trifluralin.
Significant at p < 0.05.
Significant at p < 0.01.
(continued)
-------
TABLE 16. (continued)
Os
. .
Concentration
of trifluralin
Cng'g'1)
0
10
1,000
20,000
Cecil
Webster
Cecil
Webster
Cecil
Webster
Cecil
Webster
4.
14.
2.
14.
3.
14.
5.
15.
6
1(7.8)
2(10.9)
5(6.6)
7(12.0)
9(6.6)
0(16.1)
2(1.8)d
0(0. 2)d
7
4.1
ND
3.4
ND
5.0
ND
3.9
ND
cfu-g l soil (x
Time (Weeks)
8
(ND)
(15.3)
(ND)
(19.5)
(ND)
(18.2)
(ND)
(0.2)d
5.
16.
4.
15.
6.
11.
3.
14.
0(5.5)
9(9.1)
1(4.1)
9(18. 2)d
2(5.2)
4(12.5)
7(0. 4)d
6(0. 2)d
10 ~5)
9
ND (5.7)
11.2(9.1)
ND (5.0)
10.7(10.9)
ND (7.1)
15.1(10.9)
ND(<0.1)d
10.4(0.0)d
5
12
4
13
4
14
3
19
10
.9 (ND)
.2(10.9)
.6 (ND)
.3(13.5)
.8 (ND)
.0(17.9)
.4 (ND)
.2(0. 0)d
11
3.4(7.5)
ND (ND)
3.2(4.8)
ND (ND)
3.7(4.6)
ND (ND)
4.6(0.0)d
ND (ND)
aFirst column, technical grade trifluralin.
Second column, formulated trifluralin.
Significant at p < 0.05.
Significant at p < 0.01. (continued)
-------
TABLE 16. (continued)
Concentration
of trifluralin
Cvg-g"1)
0
10
1,000
20,000
cfu-g l soil (x
Time (Weeks)
Cecil
Webster
Cecil
Webster
Cecil
Webster
Cecil
Webster
10"5)
Average
4.9(7.7)
14.1(12.4)
3.2(6.4)
15.6(14.4)
3.7(5.7)
14.4(14.7)
4.9(3.2)C
14.6(1.0)d
column, technical grade trifluralin.
Second column, formulated trifluralin.
Significant at p < 0.05.
Significant at p < 0.01.
-------
30
20
o
o"
O
o»
10
o—o
A
I
Days
Oppm
SOppm
SOOppm
5,000ppm
20,000ppm
Oppm
SOppm
SOOppm
5,000 ppm
ZO.OOOppm
60
80
Figure 20.
C02 evolution rate from the Webster soil receiving various con-
centrations of 2,4-D. (A) Technical grade; (B) Formulated 2,4-D.
71
-------
from the carbon in the 2,4-D. The C02 evolution rates were generally
higher from the formulated 2,4-D treated soil than from the technical grade
treated soil. Therefore, microbial activity in the soil with formulated
treatments was higher.
Unlike the Webster soil, total C02 evolution was inhibited in the
Cecil soil which received 5,000 and 20,000 ppm of technical grade 2,4-D as
well as 20,000 ppm of formulated 2,4-D (Figure 22). Small stimulations
were noted in the total C02 evolution from the soil treated with 5,000 ppm
of formulated 2,4-D. In contrast to the Webster soil, very little degrada-
tion was observed in the Cecil soil receiving 5,000 and 20,000 ppm (Figure
23). At a rate of 500 ppm, both forms of 2,4-D were degraded slowly. Only
10.5% and 6.3% of the technical and formulated material were degraded, re-
spectively, in the Cecil soil after 80 days of incubation.
The C02 evolution rates from the Terra Ceia soil receiving 2,4-D are
presented in Figure 24. The C02 evolution rate from the untreated organic
soil was approximately four times higher than that from the Webster soil
and 11 times higher than that from Cecil soil. Total C02 evolution from
the organic soil treated with 5,000 and 20,000 ppm of formulated 2,4-D was
high. For the technical grade material, C02 evolution was inhibited initially.
The C02-C produced from 5,000 ppm of formulated 2,4-D after 7 days was at
least partly from the 2,4-D-C because significant degradation occurred
thereafter. For the organic soil treated with 20,000 ppm of formulated
2,4-D, total C02 evolution was enhanced after 36 days. This increase in
C02 evolution was concurrent with the degradation of the herbicide. The
C02 evolution rate for the organic soil treated with 5,000 ppm of the
technical grade 2,4-D increased after 20 days of incubation. This increase
coincided with the period of rapid 2,4-D degradation (Figure 25). Total
C02 evolution was inhibited throughout the 80 days of incubation for the
organic soil treated with 20,000 ppm of technical grade 2,4-D. Similar to
the Webster soil, comparable degradation results were obtained for the
organic soil when total C02 evolution during the second peak rather than
the entire experimental period was used to calculate 2,4-D degradation.
Degradation rates for the formulated 2,4-D in the Webster and organic
soil were greater than for the technical grade material when the applica-
tion rate was 5,000 ppm or higher. Table 18 shows the rates of first order
(exponential) degradation for the three soils receiving 5,000 ppm of 2,4-D.
Exponential degradation was not observed for the Cecil soil.
Attempts were made to stimulate 2,4-D degradation in the Cecil soil
receiving 5,000 ppm of 2,4-D. In addition to 2,4-D (technical grade or
formulated) and ^T-2,4-D, the Cecil soil was amended with various nutrient
sources. Table 19 shows the amount of 2,4-D degradation during 60 days of
incubation. Readily degradable nutrients such as yeast extract (1%) and
glucose (1%) plus urea(0.5%) did not stimulate 2,4-D degradation. Only
a small stimulation, if any, occurred with these treatments with the excep-
tion of the treatment with lime plus 2,4-D degrading bacterium. In this
treatment 28.6 and 3.91 of the formulated and technical grade 2,4-D,
respectively, were degraded in 60 days.
72
-------
too
o—o 50 ppm
•_• 500ppm
A—A 5,000 ppm
20,000 ppm
o .o 50ppm
SOOppm
SpOOppm
20,000 ppm
80
Figure 21. Percent 14C02 evolved from the Webster soil receiving various
concentrations of 2,4-D. (A) Technical grade; (B) Formulated
2,4-D.
73
-------
X.
o
0
3
8 2
is
O
O
A
20
40
B
a—a 0 ppm
•—• 500 ppm
A—A 5,000 ppm
20,000 ppm
60
80
o—D 0 ppm
•—• 500 ppm
£—A 5,000 ppm
A—* 20,000 ppm
20
40
Days
60
80
Figure 22. C02 evolution rate from the Cecil soil receiving various concen-
trations of 2,4-D. (A) Technical grade; (B) Formulated 2,4-D.
74
-------
15
10
i
I
A
500 ppm
SpOOppm
20,000 ppm
20
40
60
80
B
500 ppm
5000 ppm
20pOOppm
60
80
Figure 23.
40
DAYS
Percent 14C02 evolved from the Cecil soil receiving various con-
centrations of 2,4-D. (A) Technical grade; (B) Formulated 2,4-D.
75
-------
15
>»
olO
o
o
N
o»
2
Formulated Technical
5000 ppm A—A 5000 ppm o—o
20000ppm A—A 20,000 ppm •—•
Control a—a
A-A
20
40
DAYS
60
80
Figure 24. C02 evolution rate from the Terra Ceia soil receiving 5,000 and
20,000 ppm of technical grade and formulated 2,4-D.
76
-------
60
CO
O
o
u 20
Formulated
SOOOppm A—A
20pOOppm A—A
if Technical
, SOOOppm o-o
20,000ppm •-•
0
20
40
DAYS
60
80
Figure 25. Percent 14C02 evolved from the Terra Ceia soil receiving 5,000
and 20,000 ppm o£ technical grade and formulated 2,4-D.
77
-------
TABLE 17. COMPARISON OF THE PERCENTAGE OF 2,4-D DEGRADATION FROM THE WEBSTER
SOIL DERIVED FROM ll*C02 AND TOTAL C02 EVOLUTION DURING THE SECOND
PEAK PERIOD AND THE TOTAL EXPERIMENTAL PERIOD
Form of 2,4-D
% degradation % degradation from
Concentration calculated from of ^C-activity
(ppm) total C02 evolution evolved as 14C02
Technical grade
Formulated
5,000
20,000
5,000
20,000
32.7* (87. 0)**
14.2 (18.2)
77.3 (152.0)
21.4 (52.1)
30.3* (56.9)**
11.6 (12.9)
55.5 (60.4)
,
21.0 (22.7)
*The second peak period
**80-day period
78
-------
TABLE 18. DEGRADATION RATES FOR THE THREE SOILS RECEIVING 5,000 ppm OF
TECHNICAL GRADE AND FORMULATED 2,4-D DURING EXPONENTIAL DEGRA-
DATION PERIOD
Exponential ^degradation (% of Rate of degradation
degradation 2,4-D evolved as during this period
period (day) C02-C) (% per day)
Webster soil +
technical grade
2,4-D 10th- 45th
Webster soil +
formulated 2,4-D 19th-42nd
Organic soil +
technical grade
2,4-D 18th-42nd
Organic soil +
formulated 2,4-D 7th- 26th
Cecil soil +
technical grade
2,4-D none
Cecil soil +
frvnmil a-f-Arl 7 A-Tt none
43.1
53.9
32.4
59.0
~0
~0
1.23
2.34
1.35
3.11
~0
~0
79
-------
The soil pH of the Webster and the Cecil soils receiving formulated
2,4-D were not changed appreciably during 11 weeks of incubation (Tables 20
and 21). However, the pH of the two soils decreased after receiving 5,000
and 20,000 ppm of technical grade 2,4-D, especially the Cecil soil. The pH
of the Webster soil receiving 5,000 and 20,000 ppm of technical grade 2,4-D
increased to 7.1 and to 6.7, respectively after the llth week of incubation.
pH changes were small in the Terra Ceia soil receiving the technical grade
and formulated 2,4-D.
At application rates of 5,000 ppm or higher, if 2,4-D degradation
occurred, total C02 evolution was generally enhanced. In this case, the
pattern of the 0)2 evolution rate was generally a two peak response. The
results from 2,4-D degradation indicated that the C02-C from the second
peak appeared to be associated with the 2,4-D-C (oxidation of 2,4-D-C to
C02-C) and the C02-C from the first peak was probably from formulation
compounds, impurities and/or soil organic matter. Thus, using total C02
evolution for the entire experimental period to calculate the extent of
2,4-D degradation may lead to erroneous conclusions.
It was observed that if degradation occurred at high 2,4-D application
rates, the degradation rate for formulated 2,4-D was generally higher than
that for the technical material. This may be attributed to the additional
carbon or nutrients present in the formulation material. The solubility of
the dimethylamine salt of 2,4-D in water is 300 g per 100 g water, whereas,
the solubility of 2,4-D is only 900 ppm (Weed Science Society of America,
1974). Therefore, formulated 2,4-D, even at 20,000 ppm, would remain in
the soil aqueous phase, whereas the majority of technical grade 2,4-D at
concentrations of 5,000 and 20,000 ppm would remain as a solid. Also the
2,4-D in the formulation was neutralized by dime thy 1 amine; thus, the soil
pH did not change appreciably. The pH in the soil receiving technical
grade 2,4-D decreased up to 3 pH units, depending upon the soil type and
2,4-D concentration. This decrease in pH may have an inhibitory effect on
soil microbial activity.
For low application rates, i.e., below 100 ppm, 2,4-D degradation has
been shown to be favored by moisture and organic matter (Foster and McKer-
cher, 1973; Loos, 1969). Based on the information presented in this manu-
script, herbicide degradation in the presence of high concentrations was
also higher in soils with large quantities of organic matter. Numerous
other factors may also influence the rate of degradation. For example,
2,4-D concentration, concentration of formulation material, supplement of
external nutrients, clay, soil pH, arid temperature. Increasing the herbi-
cide concentration may exhibit direct and indirect effects on microbial
activity. For example, bacteria have been reported as being the major
organisms responsible for 2,4-D degradation in soils (Loos, 1975), and the
activity of bacteria will be significantly reduced by a low soil pH. The
buffering capacity of the Webster soil and organic soil was greater than
that for the Cecil soil. The pH of the Cecil soil dropped 1.9 and 2.7 units
when application rates of technical grade 2,4-D were 5,000 and 20,000 ppm,
respectively; whereas, the pH of the Webster soil declined only 0.6 and
1.2 units for 5,000 and 20,000 ppm, respectively. Only a small pH change
was observed in the organic soil receiving 5,000 ppm of technical grade
80
-------
TABLE 19. 2,4-D DEGRADATION IN THE CECIL SOIL RECEIVING 5,000 ppm OF THE
HERBICIDE AND VARIOUS NUTRIENT TREATMENTS DURING 60 DAYS OF
INCUBATION
% Degradation
Treatment Technical Formulated
None
Glucose (1%) + Urea (0.51)
Yeast extract (1%)
Webster soil (21)
Terra Ceia organic soil (2%)
Cow manure (21)
2,4-D degrading bacterium (1 x 106 cells/ g soil)*
Lime (pH 7.5)
Lime (pH 7.5) + Terra Ceia organic soil (2%)
Lime (pH 7.5) + 2,4-D degrading bacterium
(1 x 106 cells/g soil)
0.2
0.2
0.2
0.6
0.3
0.4
0.4
0.9
1.1
3.9
mmimmmm**i^^mm*mmiim-~+immii^^~~~~~~~*~*^*
0.2
0.1
0
0.4
0.4
0.5
0.4
0.6
0.4
28.6
*2 4-D degrading bacterium was isolated from the Webster soil treated with
5,000 ppm of technical 2,4-D. The bacterium was a gram-negative, motile rod.
81
-------
TABLE 20. SOIL pH OF THE WEBSTER SOIL RECEIVING TECHNICAL GRADE AND FORMU-
LATED 2,4-D
pH
Weeks
0 ppm
0 7.3
6 7.3
11 7.2
500 ppm
7.3* (7.3)**
7.3 (7.4)
7.3 (7.4)
5,000 ppm
6.7 (7.3)
7.0 (7.4)
7.1 (7.4)
20,000 ppm
5.1 (7.4)
6.2 (7.4)
6.7 (7.4)
technical grade 2,4-D
**Formulated 2,4-D
TABLE 21. SOIL pH OF THE CECIL SOIL RECEIVING TECHNICAL GRADE AND FORMULATED
2,4-D
Weeks
0
6
11
PH
0 ppm
5.6
5.6
5.4
500 ppm
5.3* (5.9)**
6.0 (6.2)
5.8 (5.9)
5,000 ppm
3.9- (5.9)
3.7 (6.1)
3.8 (5.9)
20,000 ppm
3.7 (6.2)
2.9 (5.9)
2.8 (5.8)
*Technical grade 2,4-D
**Formulated 2,4-D
82
-------
i c Ann inab^y o£ the Cecil soil to degrade 2,4-D at a concentration
of 5,000 ppm or higher appeared to be due to concentration effects of the
herbicide and the formulation chemicals, low PH, low organic matter content,
and low 2,4-D degrading microorganism populations.
The bacterial population in the 500 ppm technical grade 2,4-D treated
Cecil soil was significantly lower than that in the untreated Cecil soil
except during the 6th and 7th week (Table 22). During the 6th and 7th week
there was no significant difference at the 0.05 level. The bacterial
density in the 500 ppm formulated 2,4-D treated soil was also significantly
lower than that of the control (P < 0.05 level), except during the 2nd
week. The overall bacterial population in the 500 ppm technical grade
treated soil during the entire experimental period was 7.99 x 106 cfu per g
soil which was significantly lower (0.05 level) than the 1.11 x 107 cfu
per g soil in the untreated soil. Also, the overall average bacterial
density for 500 ppm formulation treated soil was significantly lower than
the control, i.e., 6.1 x 106 vs 1.0 x 107. As the 2,4-D concentration was
increased to 5,000 and 20,000 ppm, the bacterial populations declined pro-
foundly even after three hours of incubation. Furthermore, the bacterial
populations in the 5,000 and 20,000 ppm treated soil, for both technical
grade and formulated, were all significantly lower (P < 0.01) than from the
untreated soil. The 20,000 ppm treatments had a more detrimental effect
than 5,000 ppm. The overall average bacterial populations for 5,000 and
20,000 ppm technical 2,4-D treated soil were 1.7 x 106 and 7.0 x 105,
whereas the bacterial populations for 5,000 and 20,000 ppm formulated
treated soil were 1.9 x 106 and 8.2 x 10s, respectively.
Both forms of 2,4-D significantly stimulated fungal population (P <
0.01) when the application was at 500 ppm (Table 23). The stimulation was
greater for the formulation treatment. At an application rate of 5,000
ppm, formulated 2,4-D did not stimulate or inhibit fungal population (not
significant at 0.05 level), whereas technical 2,4-D profoundly inhibited
fungal populations throughout the entire experimental period (P < 0.01). At
20,000 ppm, both forms of 2,4-D exhibited inhibition on fungal populations.
Fungal populations were reduced drastically after 3 hours of incubation,
and completely destroyed in 7 and 6 weeks after incubation with technical
and formulated 2,4-D, respectively.
Actinomycete populations in the 500 ppm technical 2,4-D treated soil
were significantly reduced (P < 0.01), except during the 5th week of incu-
bation (Table 24). Whereas, for formulated 2,4-D at the same application
rate, actinomycete populations were significantly lower than in the un-
treated soil (P < 0.01) after the 5th week. At application rates of 5,00.0
and 20,000 ppm, both forms of 2,4-D caused sharp declines in actinomycete
populations. At these concentrations, technical grade 2,4-D appeared to
have a greater inhibitory effect on actinomycetes in Cecil soil. Actinomy-
cetes were not detected in the SC agar after 2 weeks of incubation; whereas,
actonomycetes were not detected after 4 and 5 weeks in soils treated with
5,000 and 20,000 ppm of formulated 2,4-D.
Generally, technical grade 2,4-D had a greater inhibitory effect on
the bacteria/fungi and actinomycetes in the Cecil soil for applications
83
-------
TABLE 22. EFFECT OF 2,4-D ON BACTERIAL POPULATIONS IN CECIL SOIL
Concentration
of 2,4-D
Cvig'g"1)
0
500
5,000
20,000
cfu'g'
'l soil (x 10 ~6)
Time (Weeks)
Technical
Formulated
Technical
Formulated
Technical
Formulated
Technical
Formulated
0(3h)
12.5
12.3
11.0
5.0*
3.2*
2.1*
0.7*
1.3*
1
10.6
15.0
7 fif
/.O
7.6*
1.1*
1.4*
1.1*
0.8*
2
11.9
8.1
7.3f
7.8
2.5*
1.0*
1.1*
1.1*
3
10.3
11.2
4.4*
8.7f
2.9*
1.7*
0.7*
0.5*
4
11.4
9.0
8.1f
5.6f
1.3*
1.0*
0.5*
0.8*
5
14.0
12.7
n.d.
7.6f
n.d.
3.1*
n.d.
0.7*
*Signi£icant at p < 0.01.
Significant at p < 0.05.
(continued)
84
-------
TABLE 22. (continued)
Concentration
of 2,4-D cfu-g'1 soil (x 10"6)
(vg-g *) Time (Weeks)
6 7 8 9 10 11 Average
0
500
5,000
20,000
Technical
Formulated
Technical
Formulated
Technical
Formulated
Technical
Formulated
10.9
10.0
10.9
4.6*
1.6*
1.1*
0.3*
0.7*
8.6
11.5
9.5
6.5*
0.7*
3.4*
0.6*
n.d.
8.1
n.d.
6.9f
n.d.
1.4*
n.d.
0.8*
n.d.
12.9
8.9
7.0f
3.2*
1.0*
2.6*
0.5*
0.9*
10.
10.
7.
4.
1.
1.
0.
0.
9
8
3f
1*
5*
3*
5*
8*
n.
9.
n.
6.
n.
2.
n.
0.
d.
4
d.
•
3f
d.
1*
d.
7*
11.
10.
8.
6.
1.
1.
0.
0.
1
8
Of
1*
7*
9*
7*
8*
*Signi£icant at p < 0.01.
Significant at p < 0.05.
85
-------
TABLE 23. EFFECT OF 2,4-D ON FUNGAL POPULATIONS IN CECIL SOIL
Concentration
of 2,4-D
(yg-g"1)
0
500
•
5,000
20,000
cfu-g
~l soil
(x 10 "^
Time (Weeks)
Technical
Formulated
Technical
Formulated
Technical
Formulated
Technical
Formulated
0(3h)
25.7
16.3
25.9
14.8
1.9*
4.0*
0.2*
1.6*
1
10.5
6.6
24. 4f
71.8*
0.2*
3.2*
0.2*
<0.1*
2
9.1
10.8
48.5*
124*
0.2*
12.3
0.4*
<0.1*
3
10.2
10.8
57.9*
103*
0.9*
10.3
0.2*
0.4*
4
n.d.
12.5
n.d.
133*
n.d.
11.4
n.d.
<0.1*
5
12.8
13.2
70.9*
144*
0.8*
13.6
0.1*
0.1*
*Signi£icant at p < 0.01.
"^Significant at p < 0.05.
(continued)
86
-------
TABLE 23. (continued)
Concentration
of 2,4-D cfu-g'1 soil (x 10"1*)
(yg*g *) Time (Weeks)
6 7 8 9 10 11 Average
Technical 9.1 5.7 9.3 3.4 5.2 n.d. 10.1
0
Formulated 10.9 9.7 9.8 8.8 7.1 10.5 10.6
Technical 70.9* 69.0* 77.1* 33.7* 74.3* n.d. ' 55.3*
500
Formulated 132* 324* 310* 196* 204* 192* 162*
Technical 0.2* <0.1* 0.1* <0.1* 0.2* n.d. 0.4*
5,000
Formulated 31.0* 13.1 13.0 12.5 6.0 7.9 13.8
Technical <0.1* 0* 0* 0* 0* n.d. 0.1*
20,000
Formulated <0.1* <0.1* 0.1* 0* ,0* 0* 0.2*
*Signi£icant at p < 0.01.
Significant at p < 0.05.
87
-------
TABLE 24- EFFECT OF 2,4-D ON ACTINOMYCETE POPULATIONS IN CECIL SOIL
Concentration
of 2,4-D
(vg-g"1)
0
500
5,000
20,000
cfu-g'
~l soil (x 10 "^
Time (Weeks)
Technical
Formulated
Technical
Formulated
Technical
Formulated
Technical
Formulated -
0(3h)
15.4
2.4
14.1
2.4
8.4*
0.3*
0.1*
0.4*
1
14.8
1.2
2.5*
1.1
<0.1*
0.2*
<0.1*
0.1*
2
n.d.
2.4
n.d.
1.1
n.d.
0.2*
n.d.
<0.1*
3
8.9
2.3
1.3*
3.0
0*
<0.1*
0*
<0.1*
4
n.d.
2.1
n.d.
2.3
n.d.
<0.1*
n.d.
0*
5
1.2
5.0
1.9
0.5*
0*
0*
0*
0*
*Signi£icant at p < 0.01.
(continued)
88
-------
TABLE 24. (continued)
Concentration
of 2,4-D
(yg-g l)
Technical
0
Formulated
Technical
500
Formulated
Technical
5,000
Formulated
Technical
20,000
Formulated
cfu*
g"1 soil (x 10 "•*)
Time (Weeks)
6
1.7
1.1
0.2*
0.2*
0*
0*
0*
0*
7
1.7
1.3
0.2*
0.2*
0*
0*
0*
0*
8
1.4
n.d.
0.2*
0.2*
0*
n.d.
0*
n.d.
9
n.d.
2.6
n.d.
0.2*
n.d.
0*
n.d.
0*
10
1.7
1.7
<0.1*
0.5*
0*
0*
0*
0*
11
n.d.
2.0
n.d.
0.2*
n.d.
0*
n.d.
0*
Average
5.9
2.2
2.6*
1.1*
1.1*
0.1*
0.02*
0.1*
*Significant at p < 0.01.
89
-------
of 5,000 ppm and higher. This may be attributed to the fact that the soil
pH dropped from 5.6 to 3.7 and 2.9 in the soil treated with 5,000 and
20,000 ppm of technical grade 2,4-D. The pH in the formulated treated
soils remained unchanged. Also, formulation compounds in the formulated
2,4-D may reduce or buffer 2,4-D toxicity to the microorganisms. Among the
three groups of microorganisms considered, bacteria were the least sen-
sitive to high 2,4-D concentrations (5,000 ppm and higher) and actinomycetes
were the most sensitive for the soils considered. However, for the soils
that degraded 2,4-D at high concentrations, for example, Webster silty clay
loam and Terra Ceia muck (Ou et al., 1978), the response of the soil micro-
flora to high 2,4-D concentrations may be different. Microorganisms in the
20,000 ppm treated Webster soil after 70 days of incubation were nearly all
bacteria, and the number of bacteria was more than 100 times higher than
that in the untreated soil.
METABOLITES
Atrazine
Only tentative indent if icat ions of atrazine metabolites were made.
Structures are based on thin layer and gas chromatographic behavior. Mass
spectral analyses are currently incomplete. In the Webster soil treated
with 10 or 1,000 ppm, 4 to 8 percent of the radioactivity was associated
with a compound or compounds having a relative Rf similar to that of 2-
chloro-4-amino-6-isopropylamino-s-triazine (0.80J and/or 2-chloro-4-ethyl-
amino-6-amino-s-triazine (0.88). One to four percent of the activity was
associated with a component having a relative R^ indistinguishable from
that of 2-hydroxy-4-ethylamino-6-isopropylamino-s-triazine (0.48; hydroxy-
atrazine) . Less than 2 percent of the radioactivity was detected at a
relative Rr similar to that of 2-hydroxy-4-amino-6-isopropyl-amino-s-
triazine (0.10) and 2-hydroxy-4-ethylamino-6-amino-s-triazine (0.10). In
the Cecil soil, all the radioactivity not associated with atrazine had a
relative Rr corresponding to hydroxy- atrazine (0.48). Percentages of
metabolites increased with time and corresponded to a reduction in the
level of the parent compound.
The percentage of unextractable radioactivity was generally higher at
10 and 1,000 ppm for the Cecil soil than for the Webster soil; this differ-
ence was not observed at the 20,000 ppm treatment. There was no apparent
difference in bound residues between the technical and formulated appli-
cations at any of the three concentrations studied (Figure 26) . The per-
centage of bound 14C-activity increased with time and as much as 30% of the
detected in the soil was unextractable.
Trifluralin
Trifluralin degradation in the Webster and Cecil soils measured by
evolution from the 14C-labeled trifluralin decreased on a percentage
basis with an increase in herbicide concentration during the 83 days of
aerobic incubation (Table 25) . Degradation was greater in the Webster soil
for all concentrations (10, 1.000 and 20,000 ppm) than in the Cecil soil.
The percentage volatilized (1(*C-material trapped in ethylene glycol) was
90
-------
80
WEBSTER TECH
WEBSTER FORM
CECIL TECH.
CECIL FORM.
0 10 20 30 40 50 60 70 80
INCUBATION TIME (days)
Figure 26. Percent of total ll*C-activity bound to Webster and Cecil soil
after receiving 10, 1,000 and 20,000 ppm of technical grade
and formulated atrazine.
91
-------
negligible for all treatments except for the 10 ppm formulated application.
In this treatment, 0.11 of the total 11+C-activity was trapped in the ethylene
glycol in the first three weeks of incubation, but no ^C-activity was
detected thereafter. It was apparent, however, that volatilization occurred
to a certain extent based on the orange color observed on the tygon tubing
connecting the soil incubation flask to the ethylene glycol test tube. The
tygon tubing was extracted with acetone and the amount of ltfC-activity
associated with the orange color determined. The percentage of the total
amount of trifluralin evaporated decreased with an increase in herbicide
concentration, although a greater quantity was volatilized at the higher
concentrations (Table 25).
The chromatographic and mass spectral characteristics of the metabo-
lites detected are presented in Tables 26 and 27. Table 27 presents the GC
and TLC behavior and mass spectral fragmentation pattern for each material
isolated. In addition to the trifluralin, four compounds were identified:
(1), a,a,a-trifluoro-2,6-dinitro-N-propyl-p-toluidine (2), a,a,a-trifluoro-
2,6-dinitro-p-toluidine (3), 2-ethyl-7-nitro-l-propyl-5-trifluoromethyl
benzimidazole (4) and 2-ethyl-7-nitro-5-trifluoromethyl benzimidazole (5).
Each compound identified was identical to authentic standards in chromato-
graphic behavior and mass spectral fragmentation patterns. It is interesting
to note that methane chemical ionization mass spectrometry (CI/MS) gave
rise to four distinctive fragments. These were the molecular ion minus 19
(m-19) due to a loss of hydrogen fluoride from the protonated parent molecule
and the m+1, m+9, and Ijl+41 fragments. The last three components result
from the addition of H , C2H5 to the molecular ion and are characteristic
of methane CI/MS. All of the authentic standards received from Eli Lilly
and Co. were subjected to GC/CI/MS and each gave rise to these four charac-
teristic fragments.
In addition to the four metabolites described above, two other com-
ponents designated unknowns A and B were also detected (Table 27). The GLC
retention time and mass spectrum data for unknown A is indistinguishable
from mono-dealkylated, 2. Thin-layer chromatography, however, revealed
that product A possessed" a relative R£ of 0.56 while compound 2 has a
relative R^ of 0.82. This material could be some derivative oF 2 which
yields 2 wnen subjected to elevated temperatures in the GC or GC7MS. The
other unknown component, B, had a relative R,. of approximately 0.1 to 0.2,
a GC retention time of 9.75 minutes and a mass spectrum different, but
typical of the other dinitro aniline derivatives. This material has an
apparent molecular weight of 223 and had the m-19, m+29 and m+41 fragments.
A tentative structure has not been identified.
No differences in metabolic rates or pathways could be detected be-
tween the Webster or Cecil soils or between the formulated or technical
trifluralin. The quantity of "bound" residues of trifluralin is illustrated
in Figure 27. The percentage bound was calculated by measuring the organic
solvent extractable 14Carbon and the unextractable amount as determined by
combustion of the soil after extraction. For the 10 ppm concentration, a
greater percentage of technical trifluralin was bound to Webster soil than
to Cecil. Ten days after application, 10% was bound and by 35 days it had
risen to 30% and after 63 days of incubation, 72% was unextractable.
92
-------
l/J
TABLE 25. PERCENTAGE OF APPLIED TRIFLURALIN MINERALIZED AND VOLATILIZED AFTER 83 DAYS OF INCUBATION
10 ppm 1,000 ppm
On On
Soil Mineralized Volatilized Tygon Mineralized Volatilized Tygon
Tubing Tubing
Webster
Cecil
Technical
Formulated
Technical
Formulated
2.5* (0.04)**
3.1 (0.05)
2.0 (0.03)
1.4 (0.02)
0.01 (.
0.0 (0.
0.0 (0.
0.0 (0.
0015)
0)
0)
0)
7.9
9.8
8.2
8.7
(.12)
(0.4)
(0.12)
(0.13)
0.4
0.5
0.3
0.3
(0.4)
(0.5)
(0.3)
(0.3)
0.0 (0.0)
0.0 (0.0)
0.0 (0.0)
0.0 (0.0)
1.1
1.2
1.2
1.3
(1.1)
(1.2)
(1.2)
(1.3)
*percent
**milligrams
(continued)
-------
TABLE 25. (continued)
*percent
**milligrams
Soil
20,000 ppm
On
Mineralized Volatilized Tygon
Tubing
Technical
Webster
Formulated
Technical
Cecil
Formulated
0.1 (2.0)
0.1 (2.0)
0.1 (2.0)
0.1 (2.0)
0.0 (0.0)
0.0 (0.0)
0.0 (0.0)
0.0 (0.0)
0.1 (2.0)
0.1 (2.0)
0.1 (2.0)
0.1 (2.0)
-------
TABLE 26. R£ VALUES FOR TRIFLURALIN AND METABOLITE REFERENCES
Number R
1 Trifluralin Ca,a,a-Tri£luoro-2,6-dinitro-N,
N-dipropyl-p-toluidine 1.00
2 a,a,a-Trifluoro-2,6-dinitro-N-propyl-p-toluidine 0.82
3 a,a,a-Trifluoro-2,6-dinitro-p-toluidine 0.25
4 2-Ethyl-7-nitro-l-propyl-5-tri£luoromethyl benzimidazole 0.25
5 2-Ethyl-7-nitro-5-tri£luoromethyl bezimidazole 0
6 a,a,a-Trifluoro-N,N-dipropyl-5-nitrotoluene-3,4-diamine 0.60
7 a,a,a-Trifluoro-5-nitro-N-propyltoluene-3,4-diamine 0.22
8 a,a,a-Trifluoro-5-nitrotoluene-3,4-diamine 0
9 a,a,a-Trifluoro-N,N,dipropyltoluene-3,4,5-triamine 0.44
10 a,a,a-Trifluoromethyl-5-nitrotoluene-3,4-diamine
11 a,a,a-Trifluorotoluene-3,4,5-triamine 0
95
-------
TABLE 27. CHRCMATOGRAPHIC AND MASS SPECTRAL PARAMETERS OF PRODUCTS ISOLATED FROM TRIFLURALIN TREATED
SOIL
Compound
Trif luralin (a ,a ,a- trif luoro- 2 , 6-
dini tr o -N , N- dipropyl -p - toluidine)
a,a,a-trifluoro-2,6-dinitro-N-
propyl -p - toluidine
Unknown A
a,a,a-trifluoro-2,6-dinitro-
p- toluidine
2 - ethyl - 7 -nitro - 1 -propyl - 5 -
trifluoromethyl benzimidazole
2 - ethyl - 7 -nitro - 5 -
trifluoromethyl benzimidazole
Unknown B
TLCa
Code R,,
1 1.00
2 0.82
0.56
3 0.25
4 0.25
0.00
0.1-0.2
GLCb
Rt
19.6
24.6
24.6
13.2
30.1
22.7
9.75
M-19
316
275
275
234
281
240
204
Massc
M+l
336
295
295
254
301
260
224
Spec
M+29
364
323
323
282
329
288
252
M+41
376
335
335
294
341
300
264
aSee text for TLC parameters; values given are relative Rf's.
See text for GLC parameters.
GThe four characteristic molecular ions are given. See text for mass spec parameters.
-------
The quantity of formulated trifluralin bound in the Webster soil was only
^S^f ?; ??ofd 84 Jay? jncubation> ^t showed a similar but somewhat
lesser (45 and 51<) amount of bound residue than that measured for the
technical material.
There was less bound trifluralin material in the Cecil soil. Approx-
imately 8< of the technical trifluralin was bound after 7 days. The per-
centage remained relatively constant throughout the sampling period. The
formulated material was somewhat slower in binding, but reached approxi-
mately the same level.
For the 1,000 ppm concentration, the Webster soil also bound more tri-
fluralin than did the Cecil soil and the technical material appeared to be
bound to a greater extent than did the formulated material. Twenty to 351
of the 14C was bound in the Webster soil at 1,000 ppm.
At the 20,000 ppm level, relatively low percentages of trifluralin
were bound; however, these still represent considerable quantities of the
pesticide. Again it appears that the Webster soil binds more than Cecil
soil.
Recent reports by Katan et al. (1976) and Katan and Lichtenstein
(1977) show rapid binding of the parathion amine analogue which suggests
that a similar phenomenon may have occurred with trifluralin in the Webster
soil. The parent compound represented 90% or more of the extractable lkC
while considerable radioactivity was not extracted. Thus, it is possible
that some metabolic products were formed and a substantial portion of them
were bound. This would explain why certain previously reported metabolites
were not detected in the extract or were only found in small quantities.
In an effort to examine this possibility, trifluralin, mono-dealkylated,
and di-dealkylated derivatives were incubated in sterilized Webster soil
for four hours. Percentages of each compound extractable after four hours
were 89, 75 and 57 for the above compounds, respectively. Thus, in the
Webster soil there was a clear relationship between the amount of non-
extractable material and the substitution on the amino nitrogen. This is
consistent with the findings of Katan and Lichtenstein (1977) with amino
analogs of parathion. If one fortifies Webster soil with either trifluralin
or the di-dealkylated derivative and immediately perform an extraction, 93-
95% of both compounds were recoverable. Substantial binding did not occur
for the di-dealkylated derivative in the sandy Cecil soil. Thus, it is
possible in the case of the Webster soil, that metabolites containing
secondary or primary amino functional groups became a part of the "bound"
portion of the residue. The absence of the same magnitude of binding in
Cecil soil could be the result of different kinds and numbers of microbial
populations, etc. This could also influence the formation of microbially
induced amino metabolites in the Cecil soil.
The metabolites detected were isolated from both soils treated at all
three application rates and are characteristic of the aerobic degradation
pathway described by Probst et al. (1975) and shown in Figure 28. Tri-
fluralin 1 underwent dealkylation to mono-dealdylated and subsequently
97
-------
80
60
40
20
o
o
o
u
80
60
40
20
u
r
•«•
Q
O
CQ
80
60
4O
20
0
TRIFLURALIN
10 ppm
1OOO ppm
20jOOO ppm
WEBSTER TECH. •
WEBSTER FORM.-
CECIL TECH.
CECIL FORM.
20 40 60 80
INCUBATION TIME (days)
Figure 27. Percent of total ^C-activity bound to Webster and Cecil soil
after receiving 10, 1,000 and 20,000 ppm of technical grade
and formulated trifluralin.
98
-------
to the di-dealkylated product. Two benzimidazoles, £ (2-ethyl-7-nitro-l-
propyl-5-trifluoromethyl benzimidazole) and 5_ (2-ethyl-7-nitro-5-triflouro-
methyl bezimidazole) were also identified. Thus, very high application
rates of trifluralin resulted in the same degradation pathways that have
been previously reported.
The GC/MS data system also performed limited mass searches (IMS) for
ions characteristic of 6^ (a,a,a-trifluoro-methyl-N,N-dipropyl-5-nitrotoluene-
3,4-deamine), 7_ (a,a,a-trifluoro-5-nitro-N-propyltoluene-3,4-diamine) and 9^
(a,a,a-Trifluoro-N,N-dipropyltoluene-3,4,5-triamine) in soil extracts from
both soils (Figure 28). All LMS's were negative for these compounds. These
materials represent the transitions between postulated aerobic and anaerobic
pathway. Thus, there was no detectable reduction of ring nitro groups. In
the Webster soil, the possibility exists that the amino metabolites, 6_, 7,
8 (a,a,a-trifluoro-5-nitro-toluene-3,4-diamine) and 9, may have been hidcfen
In the "bound" portion of the residue and should notTie overlooked.
2,4-D
In the majority of cases, parent 2,4-D was the only radioactive com-
ponent isolated from the soil. In a few instances, 2,4-dichlorophenol
(based upon thin-layer chromatography) .was detected. It appeared that more
2,4-dichlorophenol accumulated at the 500 ppm treatment after 7 to 10 days
of incubation than for other treatments and was present to a greater extent
in the Webster than in the Cecil soil. Only the parent 2,4-D, however, was
detected after 29 days of incubation.
99
-------
4 ^T 1 '
02N
5^V 2
CF. . OF,
\ / CF3\
\H7C3>NH f
rNH,
Figure 28. Metabolic pathways for trifluralin metabolism: Code numbers
correspond with those in Tables 26 and 27.
100
-------
SECTION 6
IMPLICATIONS OF PROJECT RESULTS WITH REGARDS TO PESTICIDE DISPOSAL
Pesticide applications to control pests associated with agricultural,
industrial and urban environments, in general, have not had an adverse
effect on soil microorganisms or groundwater quality. Because applications
to these environments have involved primarily low concentrations (0.5 to 10
kg/ha), the suitability of these data for designing and managing pesticide
waste disposal sites has been questioned. Therefore, the objectives of
this project were to: 1) Measure the adsorption-desorption and mobility
characteristics of various pesticides in different soil-water systems
receiving high pesticide concentrations; 2) Quantify and describe microbial
degradation rates of various pesticides in soils containing large pesticide
concentrations, and 3) Measure the influence of large pesticide concentra-
tions on soil microbial activity and respiration rates as well as to identify
specific microorganisms which degrade a given pesticide at both low and
high pesticide concentrations.
Two significant conclusions can be made regarding the adsorption re-
sults obtained during this project. First, the Freundlich adsorption equa-
tion described all pesticide adsorption isotherms considered for solution
concentrations up to the aqueous solubility of the pesticide. Thus, the
pesticide adsorption sites for all soils investigated were apparently not
saturated at any concentration considered in this study. Second, contrary
to a frequent assumption, pesticide adsorption isotherms were not linear,
that is, N in the Freundlich equation was generally less than one. The
nonlinearity of the pesticide adsorption isotherm is an important observa-
tion because it explicitly points out that pesticides will be more mobile
in soils containing pesticide concentrations similar to those associated
with pesticide waste disposal sites. This is especially true for pesti-
cides that are very soluble in water (e.g., 2,4-D amine).
The increased pesticide mobility at high pesticide concentrations
limits the usefulness of the available low concentration data base for
developing "safe" management practices for pesticide disposal procedures in
soils. If a linear adsorption isotherm is assumed on the basis of the low
pesticide concentration data, one underestimates the soil depth to which a
pesticide will leach or move for a given water input. The seriousness of
the failure of the low concentration data base to describe the true mobility
of a pesticide as it moves toward the groundwater from a waste disposal
site depends upon the water solubility of the pesticide and nonlinearity of
the adsorption isotherm. For example, the adsorption isotherms for atrazme
and 2,4-D and a Eustis soil were similar (see Table 5); however, 2,4-D
101
-------
moved similar to an unadsorbed chemical as it moved away from a simulated
waste disposal site, while the mobility of the atrazine through the same
soil was about 2.5 times less (Figure 14). To have assumed that the adsorp-
tion isotherms were linear would have resulted in a serious underestimation
of the depth to which each pesticide would have moved for a given water
input.
Many of the pesticides available on the market today are biodegradable
in soils when applied at low concentrations (0.5 to 10 kg/ha). However,
many of these same organic chemicals become persistant when applied to
soils at high concentrations. It has been observed that some soils are
able to biologically mineralize one pesticide (e.g., 2,4-D in Webster
soil), but the same pesticide may be persistant in another soil (e.g.,
Cecil). This study clearly points out that the soil respiration rate of a
soil receiving high pesticide concentrations is not, in general, a reasonable
procedure for measuring pesticide degradation potential. Also, the apparent
persistance of some pesticides may be further confounded by the formation
of metabolites which are "bound" (not extractable by recommended procedures)
to the soil and suggest a greater apparent loss of the original chemical
than what actually occurred.
The contrast between the behavior of soil environments containing
large and small pesticide concentrations illustrates the importance and
potential for management of pesticide waste disposal sites. Many soils
frequently can be altered chemically and/or biologically to enhance their
potential for biologically degrading pesticides. Also, microorganisms
capable of degrading specific chemicals at high concentrations have been
identified and isolated which could be added to a waste disposal site to
enhance the degradation and mineralization of a given chemical. Because of
increased pesticide mobility at high concentrations, the chemical may not,
however, remain in the vicinity of desired biological environment for
degradation; thus, it is important to manage both the water leaching rate
and biological environment for optimum inactivation and efficiency of a
pesticide waste disposal site.
A major limitation in using the results of this project is that only
one chemical was applied to a soil at a time. Combination or mixtures of
pesticides would be the common situation for a pesticide waste disposal
site. Waste disposal sites receiving several pesticides may fail to func-
tion as designed for a specific pesticide because of interactions between
chemicals and their environment. The behavior of a pesticide mixture may
or may not be independent or additive, but rather based upon the influence
of one pesticide and/or the formulation associated with a given pesticide.
Problems which may arise owing to the mixing of pesticide in a waste disposal
site should be considered before site selection and management protocols
are defined by the United States Environmental Protection Agency. This
work should include the evaluation of the major surfactants used with
pesticides and various formulation chemicals.
102
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104
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Katan, J., T. W. Fuhreman, and E. P. Lichtenstein. Binding of
Parathion in Soil: A reassessment of Pesticide Persistence
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Katan, J., and E. P. Lichtenstein. Mechanisms of Production of Soil-
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106
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xy / *f •
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Sorbing Porous Media: I. Analytical Solutions. Soil Sci. Soc.
Amer. J. 40:473-480, 1976.
Van Genuchten, M. Th., J. M. Davidson and P. J. Wierenga. An Evaluation
of Kinetic and Equilibrium Equations for Predicting Pesticide Move-
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Kelso, and F. Horay. Production, Distribution, Use and Environmental
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107
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Williams, S. T. and F. L. Davis. Use of Anitbiotics for Selective Iso-
lation and Enumeration of Actinomycetes in Soil. J. Gen. Microbiol.,
38:251-261, 1965.
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108
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APPENDIX
LIST OF PUBLICATIONS RESULTING FROM THIS PROJECT
1. Davidson, J. M., L.-T. Ou, and P. S. C. Rao. 1976.. Behavior of high
pesticide concentrations in soil-water systems. In:Residual Manage-
ment by Land Disposal: Proc. of the Hazardous Waste Research Symp.
(ed. W. H. Fuller). EPA-600/9-76-015, July 1976. p. 206-212.
2. Rao, P. S. C., J. M. Davidson, and L. C. Hammond. 1976. Estimation
of nonreactive and reactive solute front locations in soils. In:
Residual Management Waste Research Symp. (ed. W. H. Fuller). EPA-
600/9-76-015, July 1976. p. 235-242.
3. Ou, L.-T., D. F. Rothwell, W. B. Wheeler, and J. M. Davidson. 1978.
The effect of high 2,4-D concentrations on degradation and carbon
dioxide evolution in soils. J. Environ. Quality. 7_, 241-246.
4. Davidson, J. M., L.-T. Ou, and P. S. C. Rao. 1978. Adsorption,
movement, and biological degradation of high concentrations of
selected pesticide in soils. In:Fourth Annual Hazardous Waste
Management Symp. (ed. D. ShultzJ. EPA-600/9-78-016, March 1978.
5. Ou, L.-T., J. M. Davidson, and D. F. Rothwell. 1978. Response of
soil microflora to high 2,4-D applications. Soil Biol. Biochem.
10;443-445, 1978.
6. Rao, P. S. C., and J. M. Davidson. 1978. Adsorption and movement
of selected pesticides at high concentrations in soils. Water Res.
(in press).
7. Rao, P. S. C., J. M. Davidson, R. E. Jessup, and H. M. Selim. 1978.
Evaluation of conceptual models for describing nonequilibrium adsorp-
tion-desorption of pesticides during steady-flow in soils. Soil
Sci. Soc. Amer. J. Vol. 43(1), 1979.
8. Ou, L.-T., and J. M. Davidson. High concentration effect of methyl-
parathion on degradation and carbon dioxide evolution in soils. J.
Environ. Quality, (submitted).
9. Ou, L.-T., J. M. Davidson, and D. F. Rothwell. High concentration
effect of herbicides trifluralin and atrazine on microbiota and soil
respiration. Soil Biol. Biochem. (submitted).
109
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10. Wheeler, W. B., G. D. Stratton, R. P. Twilley, L.-T. Ou, D. A. Carlson,
and J. M. Davidson. 1979. Trifluralin degradation and binding in
soils at high concentrations. J. Agr. Food Chem. (submitted).
11. Rao, P. S. C. and J. M. Davidson. Non-equilibrium conditions for
solute transport in soils: Flow interuption experiments. Soil Sci.
(in preparation).
12. Wheeler, W. B., G. D. Stratton, R. P. Twilley, L.-T. Ou, and J. M.
Davidson. Atrazine degradation and binding in soils at high appli-
cations . (in preparation).
110
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TECHNICAL REPORT DATA
(flease read Instructions on the reverse before completing)
EPA-600/2-80-124
3. RECIPIENT'S ACCESSION-NO.
iUBI
ri_E
Adsorption, Movement, and Biological Degradation of
Large Concentrations of Selected Pesticides in Soils
5. REPORT DATE
August 1980 (Issuing Date)
6. PERFORMING ORGANIZATION CODE
P. S. C. Rao
L. T. Ou
W. B. Wheeler
D. F. Rothwell
8. PERFORMING ORGANIZATION REPORT NO
IZATION NAME AND ADDRESS
Soil Science Department
University of Florida
Gainesville, Florida
10. PROGRAM ELEMENT NO.
1DC618
11. CONTRACT/GRANT NO.
R-803849
.12.SP-QNSORING AGENCY NAME AND ADDRESS
Municipal Environmental Research Laboratory—Cin., OH
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
13. TYPE OF REPORT AND PERIOD COVERED
8/1/75 to 10/31/77
14. SPONSORING AGENCY CODE
EPA/600/14
15. SUPPLEMENTARY NOTES
Project Officer: Mike H. Roulier (513) 684-7871
is.ABSTRACT because or-tne importance ot soil in biologically reducing the quantity and
retarding the rate of pollutant movement into groundwater, this laboratory study was
initiated to evaluate the adsorption, mobility, and degradation of large concentrations
of the pesticides atrazine, methyl parathion, terbacil, trifluralinland 2, 4-D in soils
representing four major soil orders in the United States.
Solution concentrations ranged from zero to the aqueous solubility limit for each
pesticide. The mobility of each pesticide increased as its concentration in the soil
solution phase increased. These results were in agreement with the adsorption isothern
data. Pesticide degradation rates and soil microbial populations generally declined
as the pesticide concentration in soil increased; however, some soils were able to
degrade a pesticide at all concentrations studied, while others remained essentially
sterile throughout the incubation period (50 to 80 days). As shown by measurements of
ltfC02 evolution, total C02 evolution was not always a good indication of pesticide
degradation. Several pesticide metabolites were formed and identified. Bound residues
of trifluralin and atrazine at the end of the incubation period appeared to be related
to types of metabolites formed.
The observed increase in pesticide mobility for large pesticide concentrations in
the soil invalidates, .in many cases, the usefuln^o of the existing low concentration
data base for designing pesticide waste disposal sites.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Pesticides
Soil Chemistry
Adsorption
Soils
Mobility
Degradation
Pollutant Migration
Groundwater Pollution
13B
8. DISTRIBUTION STATEMENT
Release to Public
19. SECURITY CLASS (ThisReport)'
Unclassified
21. NO. OF PAGES
123
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
111
4 U.S. GOVERNMENT PRINTING OFFICE: 1980-657-165/0029
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