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1989 - 1994

Transport And Dissipation Pathways Of Pesticides In
Upland Watersheds Employing Conventional
And Conservation Tillage In Ontario

 G.J. Wall1, B.T. Bowman2, B.A. Grant3 and D.J. King1
1 Land Resource Division, Centre for Land and Biological Resources Research,
Agriculture and Agri-Food Canada, Guelph, ON,
2 Pest Management Research Centre,
Agriculture and Agri-Food Canada, London, ON,
3 Upper Thames River Conservation Authority, London, ON.


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The detection of pesticides in surface waters has caused great concern about human and ecosystem health. Concentrations often exceeding water quality guidelines are frequently detected in surface water throughout the growing season, with the highest concentrations being observed in May and June (Thurman et al. 1992, Isensee, A. R. et al. 1990).

Agricultural pesticides make up the largest percentage of pesticides used in the Great Lakes Basin (Mokley, J. 1989). Significant amounts of pesticides, mostly herbicides are used annually to increase crop yield. As a result of the large amount of pesticides used, they are commonly found in the tributaries of the Great Lakes, but little was known about the transport pathways and processes of herbicides from the point of application in the farmer's field to the point of deposition into the Great Lakes (Buttle, J. M. 1989).

Movement of pesticides from agricultural lands is a complicated process. It depends on several factors including the characteristics of the pesticide (solubility, decay constant etc), and the amount of water travelling through and off the soil from the time the pesticide is applied until degradation (Bowman et al., 1994). Other important associated factors are soil type, soil moisture and soil hydrological properties. The greater the soils ability to hold water the less runoff and percolation will occur resulting in the pesticide remaining in the soil profile for microbial degradation.

Microbial degradation tends to be the most important breakdown route for many soil-applied herbicides, as compared to chemical breakdown processes. Of the factors which influence microbial activity in soil, both moisture and temperature rank amongst the most important. Previous research has demonstrated that tillage practices influence temperature and moisture regimes in soil (Kovar et al., 1992). It is a well-established fact that no-till soils appear to be wetter at planting time than conventionally-tilled soils, perhaps because soil cultivation accelerates surface moisture losses, which in turn might also permit those soils to warm up more quickly.

The increase in conservation tillage in the 1980's raised the concern of the potential increase in pesticide use, and the corresponding potential increase of pesticide loss. It was believed that the trend toward less tillage would result in greater weed problems, therefore resulting in a need to increase herbicide usage. This could result in an increase in pesticide loss to surface water. Other perceptions accompanying the increased use of conservation tillage systems were that increased residue would decrease runoff, thereby reducing pesticide loss by surface flow, but possibly increasing pesticide loss through preferential flow (Hinkle, M.K. 1983).

New technologies and better understandings of pesticide movement were required to move to sustainable productions systems. Non-point-source (NPS) models were thought to be a possible tool in educating farmers about pesticide loss and remedial actions which could reduce the environmental impacts of pesticide movement. Field based models such as CREAMS, GLEAMS and DRAINMOD could be used to show the effects of tillage and application timing on pesticide loss. These models have the ability to simulate the movement of sediment, nutrients and pesticides to surface water, tile water and groundwater (Knisel 1980). Several of the models have been modified and combined to give a complete simulation of the movement of pesticides and nutrients from surface runoff to groundwater. The major problem with the models was the lack of applicable data to calibrate these field scale models for Ontario conditions. It was thought that once the models were calibrated, they would be a useful tool in Environmental Farm Planning.

The objectives of this study were:

  • Determine the impact of tillage practices on soil temperature and moisture profiles, which in turn are important factors in influencing herbicide dissipation patterns in soil.

  • Describe pathways and processes for pesticide transport to surface water supplies.

  • Employ existing models for predicting pesticide transport and fate

  • Recommend remedial measures for reducing pesticide transport to surface and groundwater




  1. Temperature and moisture profiles in adjacent CT and NT-managed fields tended to be measurably different throughout the cropping season. NT soils were slightly cooler and considerably wetter just after planting, largely because of the induced surface drying in the CT soils as a result of cultivation.

  2. The disappearance rates for atrazine and metolachlor were somewhat retarded in NT soils, relative to adjacent CT soils during the first few weeks following herbicide application. After the fourth week disappearance rates were similar under both tillage practices. Because of the large number of factors influencing microbial decomposition of herbicides, the impact of tillage practice upon herbicide decomposition (through temperature and moisture effects) tends to be rather complex and site specific.

  3. The temperature and moisture-monitoring system used in this study was capable of tracking heat and water fluxes through the soil profile, and would be very useful as an aid in ground truthing small-scale field models for water and solute transport. Based on rainfall, air temperature and the soil monitors (temperature, moisture), it would be possible to calculate mass balances of water and heat storage in the soil, and perhaps to even observe the development of surface seals on CT soil during the growing season.

  4. The decrease in the amount of tillage, as the shift was made from CT to NT practices, resulted in significant increases in soil faunal activity (2X increase in population; 3X increase in biomass). Earthworms and other soil animals play an important role in crop residue decomposition and redistribution in the soil profile, as well as providing macrochannels for water and solute transport in the soil profile.


  1. No till systems reduce soil erosion losses significantly, while at the same time, under proper management requiring no more herbicides than conventional cropping systems.

  2. Banded pesticide application technique decrease usage, and decrease pesticide concentrations in surface runoff to the point where they meet water quality guidelines after several weeks following herbicide application.

  3. Even though preferential flow did not contribute to a high percentage of herbicide loss, the herbicide concentrations in the subsurface flow still exceeded water quality guidelines. This indicates that tile drains could still be a significant pathway for herbicide transport to the Great Lakes.

  4. For soil applied herbicides, 85 - 90 percent of the herbicide loss at the time of application is in the aqueous phase and not with the sediment in both surface and sub-surface flow. Therefore it is critical to control surface runoff as well as sediment losses especially closely following herbicide application. As time progresses, herbicides bind to the soil, therefore decreasing the risk of surface transport or leaching to the tiles drains.

  5. Calibrated models have been found to be an important management tool to help determine agricultural areas sensitive to soil erosion and herbicide loss. The ability to show the effects of different management practices on soil erosion and herbicide loss will have great potential for use in Environmental Farm Planning.


The development of pan lysimeters allows the ability to do a mass water balance and determine the effects of tillage on soil hydrological properties and contaminant transport. They also provide the ability to get a better understanding of the seasonal changes in soil hydrological properties. They provide a relatively low cost screening method to study various pesticides and cropping practices, allowing research to focus on agricultural practices that pose a potential threat to water quality.

The availability of automated temperature and moisture (multiplexed TDR) systems now make it possible to accurately track moisture and temperature fluxes in soil profiles. This permits a more precise description of the micro-environment which greatly influences the disposition of applied agrochemicals. Multiplexed, automated TDR systems became available only within that past two years.

The data which was collected over the five year study will be useful for calibrating and evaluating computer models for Environmental Farm Planning. Once the models have been calibrated they can be used to demonstrate the effects of management practices on pesticide movement. They will be valuable tools in showing the farmers the benefits of old techniques such as banding herbicide applications to reduce herbicide and soil loss through conservation tillage.


Pesticide transport form agricultural land is a wide spread problem in the Great Lakes Basin. Up to this point the true pathways and processes of pesticide movement from agricultural lands were not clearly understood. As we move toward more sustainable farming systems, it is imperative that we more fully understand the cropping systems we use. This study has made some important observations regarding some of the changes which take place as a conventionally-tilled system is converted to a no-tillage practice. In order to fully appreciate the impact of these changes, it is necessary to monitor some of the important parameters controlling crop growth and performance, such as soil temperature and moisture. A better understanding of the micro-ecosystem will be an aid in optimizing inputs, and in reducing nutrient and pesticide inputs to the Great Lakes system.

With the information and data collected from this studied researchers will be better able to predict possible areas of concern. Using provincial databases scientist will be able to determine susceptible areas of pesticide movement based on soil type, crop distribution, pesticide use and tile drain information. These areas can than be targeted for educational and incentive programs to increase farmer awareness and decrease the potential pesticide loss. Models calibrated from the data can be used to show the effects of management practices on pesticide movement form agricultural lands.


The study has shown that pesticide movement may be a wide spread problem from both surface runoff and tile flow. It has shown that surface runoff and tile flow are significant pathways for pesticides reaching the Great Lakes ecosystem.

Even though banded applications of herbicides is an old practices, it is not a wide spread practice. The results of this study should be used to increase the awareness and the benefits of banded application of herbicides on conventional and minimum tilled sites.

The highest potential for pesticide loss occurs shortly after herbicide application. This information has to be transferred to the farmers so that they will be better able to decrease herbicide loss through proper timing of application (not prior to a predicted rainfall).


  1. Further multi-year studies of this type need to be conducted on paired tillage sites such as this, but at sites where the NT soil has reached an equilibrium state. Although crop yields were unfortunately not recorded in this study, it was obvious that the corn stand on the NT site was considerably inferior to the adjacent CT site for the first three years when the NT soil was in a transition state. In the final year (1993), the crop stand was at least as good on the NT as on the CT site. Perhaps much of this improved crop quality may be related to increased faunal activity which aided in redistributing surface organic debris, nutrients and water.

  2. Conducting herbicide residue studies at the intermediate plot scale, as in this study, brings with it a number of specific challenges, from a research perspective. From a technology standpoint, further efforts are needed to develop and standardize methodology to deal with these problems:

    1. Herbicide Application
      Once the plot dimensions exceed the size of common sprayer boom lengths (about 1 to 2 m), custom sprayers have to be assembled to provide the required application accuracy and precision on soils varying in both roughness and firmness. Perhaps the only accurate (albeit expensive) solution to this problem may be a motorized sprayer boom which travels on a track straddling the plot, and is propelled at a precise rate. Application consistency problems which totally invalidate the data from a research standpoint would not be even noticed from an agronomic perspective.

    2. Soil Sampling for Herbicide Residues
      The soil sampling strategy for determining herbicide residues is as much as challenge as is their application. Taking relatively large numbers of random soil cores in the plots throughout the growing season does not necessarily guarantee reliable, consistent results. Herbicides applied to raised row areas may well be redistributed to lower areas with heavy rains, and their movement and dissipation behaviour may well vary considerably between wheel track and row positions. Therefore it is necessary to select a consistent sampling strategy to help minimize such sampling errors. One example of this strategy is the grid-type sampling procedures that we adopted in the second year of this study, in which soil samples were confined to in-row positions. It is also advisable to increase the cross-sectional area of the soil core sample, especially for the first few days or weeks when the herbicide residue is largely concentrated in the surface soil layer.

  3. Further research is needed on pesticide application techniques more appropriate for a no till system, such as spot spraying and delayed spraying that would be , and would lead to improved water quality through a reduction in pesticide use, as well as a reduction in pesticide loss. The application technique and timing, would have to include a detailed crop management strategy.

  4. This project studied pesticide loss under a continuous grain corn cropping system. Many farmers use crop rotations to help in weed control, which decrease pesticide use. There is a need to look at several cropping systems and pesticides to determine if the same potential threat to water quality exists under different cropping practices. As well, a more detailed analyses of crop response to herbicide application techniques must be conducted. Farmers need to know the effects on crop yield before they can make educated decisions on pesticide use.

  5. The research was conducted on a silt loam soil. The ability for preferential flow will not be the same between soil types. There is a great necessity to look at different soil types to see if the same conditions apply.

  6. It has been established that preferential flow is an important pathway for surface water contamination on a small plot scale. This information needs to be verified at a field scale level using tile drains to quantify the amount of herbicide loss, and determine and if there is a the potential threat to water quality. Larger plots would also change surface runoff results, thereby possible increasing the potential for preferential flow on a no till system.




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