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1989 - 1994

Physical Chemistry Parameters That Control Pesticide Persistence And Leaching In Watershed Soils

Donald S. Gamble
Centre for Land and Biological Resources Research,
Agriculture and Agri-Food Canada,
Research Branch, Ottawa, Ontario K1A 0C6

June 27, 1994

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1 - Rationale/Objective

There is ample evidence that the Great Lakes basin is contaminated with pollutants due to agricultural and industrial activities and municipal discharges. Recent human health, wildlife and laboratory studies lend further support that exposure to persistence toxic substances is the most significant problem facing the Great Lakes region (International Joint Commission, 1994). The 35 million people living around the lakes are being affected by the toxic substances coming from diverse sources such as atrazine and PCB's. Children's ability to learn and adults' ability to reproduce now are in danger more than ever (International Joint Commission, 1994).

It would be at least technically possible for most industries to prevent the escape of the toxic chemicals that they must use into the environment. The problem for agriculture is fundamentally different from those of other industries. The protection of crops from pest damage requires the introduction of at least some chemical pesticides directly into the environment. As a consequence, one of the few available strategies for protecting the Great Lakes from agricultural pesticides is to find means by which the delivery of the pesticides from cultivated land into the Great Lakes can be prevented or minimized.

This can only be done by developing and using methods for risk assessment and for the management of pesticide applications in the field. For such methods to be effective at tolerable costs, they must be based on a working knowledge of the processes that control the persistence and transport of pesticides in soils. In the present case, this means the soils of the Great Lakes watersheds. The three categories of processes that must be accounted for are the physics of water movement, chemical changes and interactions, and microbiological breakdown.

The Great Lakes Water Quality Program (GLWQP) is an agreement between the Governments of the United States and Canada to "Restore and maintain the chemical, physical and biological integrity of the water of the Great Lakes Basin Ecosystem." The first objective of the project was to produce more realistic and relevant information about the chemical changes and interactions. The second objective was to improve the reliability of the computer simulations by integrating this improved chemical information into computer simulation models that already use information about water movement. The project supported the GLWQP initiative in solving the contamination problem on four points:

  1. Laboratory test methods that have been developed in the project measure pesticide-soil parameters that control the persistence and transport of the pesticides;

  2. a multidisciplinary computer model has been developed in collaboration with Professor S. O. Prasher, a hydrology engineer as an improved technology for risk assessment and for the control of field operations (Clemente and Prasher, 1993). The project developed a new technology by which contaminant species concentrations can be determined at greater accuracy.This will decrease the pesticide load on the Great Lakes watershed;

  3. a pesticide-soil interactions mechanism data base has been created, to support the computerized risk assessment and the computer control of field operations;

  4. the project has supported two other Great Lakes projects. They are the Microbiological Pathways project and the Sediments project; and

  5. the project has trained post doctoral fellows, a graduate student, and technicians in an environmental chemistry project.

Because the types of empirical data that are currently used as chemical input for hydrology computer models and risk assessment do not account for cause and effect relationships, they do not support predictive calculations very effectively. There is a need for predicative calculation to assess the contamination problem properly. The objectives of the project are to: a) investigate the fundamental processes of the interactions of soils with contaminants; b) use the predictive physical chemistry constants as input for hydrology computer models to improve prediction of contaminant persistence and fate in the environment under field conditions; c) develop new analytical test methods to determine bioavailable concentrations for risk and hazard assessment, and provides information about chemical speciation of the contaminants in the environment; and d) train qualified scientific people meeting high academic professional standards in a combination of environmental disciplines.

2. Background

There are many studies on atrazine interactions with the soil, ranging from field experiments (Bowman, 1989; Graham et al., 1992) to laboratory experiments (Bailey et al., 1968; Weber et al., 1969; Weber, 1970; Karickhoff and Morris, 1985; Gamble and Khan, 1992). Gamble and Khan (1990) have demonstrated that the reverse phase High Performance Liquid Chromatography-Microfiltration Technique (HPLC-MFT) can be used to track over time labile and non-labile concentrations, as well as other chemical species of the analyte. The labile surface sorption is the uptake process that leaves some of the pesticide on sorption sites from which it can be quickly recovered back into solution. It can be distinguished experimentally from the uptake processes that produce a type of bound residue. These bound residues are physically trapped so that quick removal is prevented, although slow release can subsequently occur.

3. Sorption of Atrazine

Atrazine is frequently applied to moderately dry soil in a low volume of water. Under such conditions, one expects a rapid wetting of soil aggregates by water molecules (mass flow) which transport atrazine molecules with them to soil particles surfaces. Within a short time small concentrations of surface-sorbed atrazine might develop on soil particles surfaces. This may result in pesticide molecules being retained by soil particles as water proceeds in the soil. The movement of the sorbed atrazine molecules is then determined by forces of interactions between the soil and atrazine. The phenomenon of atrazine-soil interactions resembles that seen in the thin layer chromatography in which organic compounds are separated according to their differential movements through a porous media under the influence of a moving solvent. After the sorbed water reaches some sort of distribution equilibrium, there will be a redistribution of atrazine molecules over the soil particles surfaces, and sorption-desorption processes become effective.

The sorption may be regarded as occurring in three steps, diffusion of the free atrazine molecules from aqueous solution to soil particles, adsorption onto the surface, and diffusion into the interiors of the soil particles. Many authors have reported that the initial adsorption is followed by a second stage that is relatively slower. The pesticide taken up by this second process cannot be recovered by a quick extraction (Talbert, 1965; Mill, 1980; Macalady and Wolfe, 1984; Karickhoff and Morris, 1985). Hamaker et al. (1966) have suggested that organic chemicals slowly diffuse into the interiors of the soil particles. The surface adsorption and diffusion into the bulk of the soil particles can be distinguished experimentally by the HPLC-MFT. The method is based on exhaustive extraction of the pesticide from the soil by the mobile phase, inside the pressurized liquid system of the instrument. That portion of the pesticide that cannot be extracted by the mobile phase is a type of bound residue. The experimental evidence indicates that it is trapped by retarded intraparticle diffusion into solid matrices (Gamble and Khan, 1992).

4  Study Conclusions.

The test methods and multidisciplinary model developed for the project can be used to relate standards and operational guidelines to the chemical species that influence bioavailability and toxicity hazards, instead of simply to total concentrations. Multidisciplinary computer models are more reliable for risk assessment and the control of field operations than conventional single discipline models are.

Cost estimates indicate that the technology developed by the project will permit a more cost effective combination of bench scale laboratory tests, computer simulations and field trials, than has been previous practice. The cost estimates are attached.

The net result of the new technology is that it offers the opportunity of reducing the pesticide loading onto the Great Lakes watershed, of cutting the costs of computer model input, and of cutting the costs of crop protection.

Early validation studies indicate that this combination of new methods gives better agreement with not only laboratory column experiments, but also with filed experiments, than the conventional methods do.

We now have the opportunity to reduce the delivery of agricultural pesticides into the Great Lakes by using cheaper, faster, and more reliable methods for risk assessment and the management of field operations.

It is anticipated the future advances in soil microbiology and macropore flow research, together with an expansion of the new pesticide-soil parameter data base will make the projects' contribution even more useful.

5  New technologies and benefits.

The HPLC-MFT method is a novel technique developed in this laboratory as an experimental component of the GLWQP research. It can be used in any laboratory that has an HPLC and a constant temperature water bath. With this technology it is now possible to determine contaminant chemical species concentrations over time. This generates invaluable information about the mechanism of fate and persistence of contaminants in the environment. It can be used to complement other methods that are being used by regulatory agencies to prevent or reduce the contamination problem.

6  Implications for Great Lakes Basin ecosystem.

The methods developed by the project permit the effective determination of chemical species concentrations related to risks, instead of the currently used ambiguous total concentrations. With this technology it is possible that the maximum acceptable limit of a contaminant can be determined effectively and reliably based on the chemical speciation and bound residues. The new test methods can be used in regulation and risk assessment to protect the Great Lakes ecosystem in particular and the environment in general.

7  Technology Transfer Potential.

It is anticipated that both the new bench scale test methods and the new type of multidisciplinary computer model can be used by commercial consulting and contracting companies, by Federal Government registration and regulatory agencies, by Provincial Government ministries, and by agricultural extension officers.

8  Gaps/needs for Future Research.

It is frequently pointed out that the macropore flow problem still remains unresolved, and this might limit the usefulness of the rest of the research results. The first relevant point is that some successful field and soil column validation has been obtained in spite of this. The second is that other laboratories at the University of Calgary and at the Environmental Chemistry Laboratory of the USDA-ARS in Beltsville Maryland are currently applying new NMR imaging methods to the investigation of the macropore problem. Future research should also be conducted to expand the mechanisms data base, seek trends in the effects of the types and amounts of chemical materials and the types and amounts of surfaces on the mechanisms parameters that are used as computer model input, establish the effects of pesticide chemical structure and reactivity on the above mechanisms parameters, investigate the problems of microbiological degradation, macropores, and field variability of the above parameters, and apply and adapt the new technology to pesticide regulation, regulatory work, and practical field operations.




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