E. Topp and W.N. Smith
 
Associate Investigators:
W.D Reynolds, R. de Jong, N.K. Patni, L. Masse, and R.S. Clemente
Centre for Land and Biological Resources Research,
Research Branch, Agriculture and Agri-Food Canada,
Ottawa, Ontario, Canada

Download Report  (674 KB pdf)    [graphs & tables included]

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 agricultural conditions.

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 environmental conditions.

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 process.

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:

1. 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.

2. Test laboratory derived dissipation and bound residue formation data by comparing it to dissipation kinetics under field conditions.

3. 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.

4.0 Conclusions

1. 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.

2. 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.

3. 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.

4. 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

1. 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.

2. 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.

3. 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.

4. The pesticide transport models LEACHP and PESTFADE have been enhanced by introducing new chemical subroutines for degradation and pesticide sorption.

5. 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

1. 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.

2. 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., 1994).

7.0 Technology Transfer Potential

1. 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.

2. 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

1. 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 dissipation.

2. Agricultural practices which potentially promote degradation of pesticides in the crop rooting zone should be examined.

3. 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.

4. 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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