E. Topp and W.N. Smith
W.D Reynolds, R. de Jong, N.K. Patni, L. Masse, and R.S.
Centre for Land and Biological Resources Research,
Research Branch, Agriculture and Agri-Food Canada,
Ottawa, Ontario, Canada
1.0 Rationale and Objectives
The herbicides atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine),
metolachlor (2-chloro-6'-ethyl-N- (2-methoxy-1-methylethyl) acet-o-toluidine)
and metribuzin (4-amino-6-tert-butyl-3- methylthio-1,2,4-triazin- 5(4H)-one)
are extensively used in Canada for control of a wide variety of broadleaf
and grassy weeds. All three herbicides are relatively mobile and have
been detected in various public and domestic surface and ground water
resources at concentrations greater than drinking water guidelines (Trotter
et al., 1990; Pauli et al., 1990; Kent et al., 1991). Consequently,
in an attempt to improve management practices that will minimize contamination
of our water resources, there has been considerable effort placed on
studying and predicting the environmental fate of herbicides under various
Government and other research institutions are now developing new
and cost effective methodologies for determining pollution potential
of pesticides. Screening models, management models, and expert systems
provide estimates of the suitability of pesticides for various sites.
Another approach, the use of computer simulation models, is gaining
wider acceptance for advisory purposes. Simulation models predict transport
and fate of pesticides according to principles of specific processes
expressed quantitatively. Often the processes are not well understood
and/or have not been extensively validated experimentally (Jury and
Ghodrati, 1989; Wagenet and Hutson, 1990). Also, detailed parameters
are required to describe the processes at specific sites and under various
The processes of degradation and nonlabile residue (bound residue)
formation determine, to a large extent, the persistence of pesticides
in soils. Walker (1976) found that the first-order rate law when fit
to simazine and prometryne degradation data produced correlation coefficients
significant at P=0.001, and thus assumed first order kinetics. A simulation
model, which Walker developed, satisfactorily estimated the persistence
of these herbicides under field conditions. In subsequent years the
modified version of this model adequately predicted field persistence
of several pesticides from laboratory derived first order rate constants
(Walker, 1978; Walker and Zimdahl, 1981; Smith and Walker, 1989; Walker
et al., 1992). Wagenet and Hutson (1990), in their simulations with
LEACHP (Wagenet and Hutson, 1989), found that pesticide persistence
was very sensitive to the first order decay rate.
Hamaker and Goring (1976) suggested a model in which the pesticide
was assigned to an 'unavailable' and a 'labile' compartment. First order
rate constants were assumed for decomposition, for movement to unavailable
sites and for release to labile pesticide. Very few models simulate
formation and release of nonlabile (bound) residues. One reason for
this is the lack of detailed data available in the literature. Most
decay rates of pesticides are determined solely by solvent extractions
with no radioactively labelled parent compounds. Thus the decay rate
constants in literature often include both decay and bound residue formation
as a lump sum (dissipation). Racke and Lichtenstein (1985), and Khan
and Behki (1990) have found that bound residue formation can be a reversible
process and thus it is not accurate to include it in the dissipation
For various pesticides, it has been shown that dissipation rate may
vary greatly, not only with soil type but with temperature and soil
water content (Walker, 1976;Gillian and Hance, 1979;Ou et al., 1982;Walker
and Brown,1985;Walker et al., 1992;Obrador et al., 1993). In many instances,
however, the effects of temperature and moisture on the kinetics of
disappearance of pesticides are not well enough understood to be described
quantitatively by temperature and moisture functions in simulation models.
Also, information is currently lacking on the spatial and temporal behaviour
of decay rate (Wagenet and Hutson, 1990).
The main objectives for this study were as follows:
Determine the kinetics of dissipation and bound residue formation
of widely used pesticides in the Great Lakes area, as influenced
by soil moisture, temperature and soil structure.
Test laboratory derived dissipation and bound residue formation
data by comparing it to dissipation kinetics under field conditions.
Provide decay rates and hydrologic transport parameters for soils
in the Great Lakes Basin as input for simulation models. Also, modify
the pesticide fate and transport model, LEACHP, to provide improved
measures of pesticide dissipation kinetics.
In laboratory incubations, atrazine dissipation was most rapid
in clay soils whereas metolachlor and metribuzin dissipated more
rapidly in sandy soils. There was a greater accumulation of toxic
atrazine metabolites in soil with greater sand content.
In the field lysimeters more leaching of atrazine and metolachlor
occurred in no-tilled soil but for both treatments little herbicide
residues reached the 20 cm depth. In the tilled plot higher clay
content correlated with greater leaching. Conventional and no-tillage
practices had little effect on pesticide dissipation, bound residue
formation, and mineralization.
In the field, during the four month cropping season, greater
than half the atrazine was mineralized. Microbial degradation of
pesticides is often influenced by treatment history. The efficacy
of some soil applied herbicides such as EPTC and insecticides such
as carbofuran can be compromised by the extremely active degrading
microflora that develops upon repeated application. The impact of
treatment history on biodegradation is difficult to predict and
has yet to be incorporated into simulation models. The rate of volatile
loss in our field experiments was much higher than that observed
in laboratory experiments or reported in the literature. From the
environmental perspective this is good news. But clearly this is
an area of research which has to be further explored.
The pesticide fate and transport model LEACHP was modified to
make dissipation responsive to environmental conditions. With respect
to dissipation, the model worked well under field conditions. In
comparison to other models LEACHP did reasonably well in predicting
atrazine and chloride movement in the indoor column leaching experiments.
If anything, LEACHP tended to overpredict atrazine transport which
is a conservative approach to modelling.
5.0 New Technologies and Benefits
In this study the adsorption of atrazine or metolachlor to a
variety of experimental materials was determined. We recommend that
plastics used in equipment or containers coming in contact with
atrazine or metolachlor be chosen accordingly. For that matter,
all organic materials in experimental equipment coming in contact
with pesticides should be tested for their ability to adsorb these
chemicals. Otherwise the validity of the experimental results may
be in question.
Experiments with intact soil core, packed soil core and flask
soil incubations with atrazine and metolachlor have shown that soil
structure need not necessarily be maintained in laboratory soil
pesticide dissipation experiments.
Laboratory intact soil core and field lysimeters systems have
been designed for determining pesticide dissipation kinetics, bound
residue formation kinetics, and leaching. The field lysimeter design
allows for safe experimentation with radioactively labelled compounds.
The pesticide transport models LEACHP and PESTFADE have been
enhanced by introducing new chemical subroutines for degradation
and pesticide sorption.
A database for half lives and bound residue formation kinetics
of herbicides in the Great Lakes Basin is available.
6.0 Implications for Great Lakes Ecosystem
Degradation is the predominant means by which most pesticides
are removed from the environment. Agricultural practices which promote
degradation should therefore be developed and used. Such practices
may include, subsurface drainage and irrigation, tillage practices,
pesticide injection, and various cropping activities.
The ability of the LEACHP model to predict pesticide transport
has been improved. Non-point pesticide contamination of ground water
as predicted by LEACHP was generally low-level (Reynolds et al.,
7.0 Technology Transfer Potential
The influence of temperature and moisture on dissipation of herbicides
in soils in the Great Lakes Basin have been determined, providing
improved measures of pesticide dissipation kinetics for use in computer
simulation models. Temperature and soil moisture functions could
be incorporated into other pesticide fate and transport models.
A data base on the kinetics of dissipation and bound residue
formation has been provided for other researchers and future research.
8.0 Gaps Needing Future Research
The influence of spatial variability on pesticide behaviour needs
to be further examined. On the field scale, geostatistics could
be employed to determine which soil properties affect pesticide
Agricultural practices which potentially promote degradation
of pesticides in the crop rooting zone should be examined.
In this study mineralization rates of atrazine and metolachlor
were much higher under field conditions than those reported in literature.
Mineralization, or complete transformation of pesticides to nontoxic
derivatives, provides an excellent means for decontaminating soils
in agricultural fields. Mechanisms underlying the process of mineralization
in this soil type should be further examined.
The release of bound (nonlabile) residues as influenced by temperature,
soil moisture and soil properties, as well as wetting and drying
cycles needs to be determined for use in pesticide transport models.
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