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1989 - 1994

Fate of agricultural chemicals in soil, ground water and agricultural drainage water under farm conditions

N.K. Patni, L. Masse, P.Y. Jui, and B.S. Clegg

Associate Investigators
R. De Jong, D. Gamble, J. Millette, D. Reynolds, W. Smith,
E. Topp, G. Wall, A. Hamill, J. Gaynor, T. Dickinson,
R.K. Gupta, and R.P. Rudra

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1.0  RATIONALE/OBJECTIVES

Environmental concerns have been raised because of the presence of fertilizer and pesticide residues in streams and ground water. Information is required on the contribution of agriculture to surface and subsurface water pollution under different cropping management practices in order to control and reduce loading of chemicals to water systems in the Great Lakes Basin.

The popularity of conservation tillage including zero or no tillage (NT) has grown steadily in Canada and the USA, mainly because of reduced costs and lower potential for surface water pollution compared to conventional tillage (CT). The high residue cover left on non-tilled land and the improved soil aggregate stability associated with an increase in organic matter content under NT can reduce soil erosion by 50 to 90%, and thus decrease the transport of sediment-bound pollutants such as pesticides and phosphorus to nearby streams (Logan et al., 1987). Absence of tillage, however, will generally favour the formation of macropores or preferential flow channels, the continuity of which is not disturbed by cultivation. Macropores and reduced runoff under NT could permit an increased downward movement of water and thus increase leaching of water-borne pollutants to tile drains or ground water. In Ohio, Schwab et al. (1985) reported increased atrazine and reduced nitrate losses in tile effluents under NT compared to CT. Most of the reported research on tillage effects is based on plot studies. Conclusions from such research need to be validated in field studies.

Computer simulation models could be useful in the selection and development of the most promising and cost-effective remedial action plans for the abatement of pollutant transport in the Great Lakes Basin. Several models have been proposed to predict water and chemical transport/behaviour in soil-water systems. However, field results required to calibrate, verify, and adapt the various transport models to Canadian conditions are very limited at present. Model validation also requires an improved understanding of the field variability of the input parameters, and of processes by which applied agricultural chemicals and their degradation products are entrained, transformed, and transported through soil-water systems in the field.

This study was aimed at determining the long-term (1990-1993) fate of two commonly used herbicides, metolachlor and atrazine, and two pollutants of concern, nitrate and phosphorus, in tile drained, loam soil corn fields under CT and NT treatments. Chemical concentrations were determined in tile effluents and in soil and groundwater at various depths. Surface drainage water quality was also determined. Additional information required for model validation, such as water flow rates, water table elevations, precipitation, and various soil properties, was also obtained. Results from this field-scale study will complement other simultaneous studies at the detailed laboratory and plot scale, and less-detailed watershed scale, under the Great Lakes Action Plan of the Government of Canada.

This report describes study methodology, findings, conclusions, new technologies and benefits, implications for the Great Lakes basin ecosystem, technology transfer potential, and needs for future research. Publications based on this study are also listed. Some analysis of samples and data is still in progress at the time of writing this report.

4.0 STUDY CONCLUSIONS

Tile drainage flow was higher under NT than CT during the spring/snowmelt period but was not significantly different in the two treatments during the growing season and in the fall. Atrazine and its degradation product deethylatrazine were almost always present in tile effluent, with concentrations mostly below 5 g/L, the Canadian drinking water guideline for atrazine. Concentrations exceeding this limit were observed in both treatments after rainfall-induced flow events within a few weeks after herbicide application. Metolachlor was detected in a few tile effluent samples only at concentrations well below the Canadian drinking water guideline of 50 g/L. In contrast, nitrate-nitrogen concentrations in tile water were above the drinking water limit of 10 mg/L in over 93% of the samples and were higher under CT than NT except during the spring period. Under both treatments, soluble total phosphorus exceeded the Ontario objective of 30 g/L for total phosphorus in surface water, in about 25% of the 330 samples that were analysed. Annual loss in tile effluent ranged between 0.02 and 0.33% of the amount applied for herbicides, and between 7 and 32% for nitrogen in the nitrate form.

In ground water, atrazine concentrations met the drinking water guideline in over 99% of the 920 samples analysed. The maximum concentration in any sample was 5.4 g/L. Atrazine and deethylatrazine concentrations were consistently higher under NT than CT up to 3.0 m depth. Metolachlor was detected in 25% of the samples and concentrations were well below the drinking water guideline. Under both tillage treatments, nitrate-nitrogen concentrations decreased with depth and were above the drinking water standard in over 80% of the samples collected at depths up to 3.0 m. Soluble total phosphorus concentrations exceeded 30 g/L in over 50% of the samples collected up to 3.0 m depth in the two treatments.

5.0 NEW TECHNOLOGIES AND BENEFITS

  1. Establishment of a long-term, large data base on field-scale movement of water, herbicides and nutrients: This field site was established in 1987 when field data collection was started. Long-term field data on chemical (atrazine, metolachlor, nitrate, and phosphorous) transport, soil characteristics and transformation, precipitation, and water regime has been collected for two tillage practices, conventional and no tillage. These data are essential for the calibration and verification of computer simulation models for agri-chemical transport and for providing information to the environmental protection and regulatory agencies.

  2. Establishment of a long-term monitoring site to study field-scale transport of agri-chemicals in soil and water under different management practices. As far as can be established, this is the only highly-instrumented field-scale site in Canada to simultaneously study the effects on groundwater, tile water and surface drainage water. It includes: piezometric wells to monitor water table fluctuations and to sample shallow ground water at different depths; calibrated H-flumes (for flow monitoring) and automatic samplers for surface as well as tile drainage water; soil moisture probes and soil solution samplers at different depths in the soil; and heated shelters for flumes and samplers to enable year-round monitoring and sampling of drainage water. Since all possible avenues for the transport of water and chemicals are monitored, the site is suitable for a complete mass balance analysis under different management practices.

  3. Establishment of a network of multi-disciplinary experts and training of graduate students. A collaborative research network, with multidisciplinary expertise has emerged which is evident in the names of associate investigators for this study. One Ph.D. and two Master's degree candidates have used the data collected at the site for their dissertation. In addition, four students from France have used the site for training in field research methodology.

6.0  IMPLICATIONS FOR GREAT LAKES ECOSYSTEM

  1. Reduced and minimum tillage systems are being increasingly recommended for use in the Great Lakes Basin. Results from this study suggest a greater movement of atrazine and metolachlor to shallow ground water and tile drainage water under no tillage compared to conventional tillage. However, at the recommended rates of application, contamination of tile drainage or ground water by atrazine and metolachlor in excess of acceptable limits for drinking water would be unlikely under either tillage practice.

  2. Nitrate-N concentrations in excess of the drinking water limit are likely to occur in tile effluent and shallow ground water in corn fields even at agronomically recommended N application rates. Subsurface drainage water could carry enough nitrate to cause a substantial loss of valuable nitrogen. Management practices which reduce nitrogen loss should be encouraged. For example, drainage water could also be retained in a surface or subsurface reservoir to be used subsequently as nitrogen-rich irrigation water. Nitrification inhibitors and winter catch crops could also be useful.

7.0  TECHNOLOGY TRANSFER POTENTIAL

  1. The long-term field-scale data base will be available to other researchers for calibration and verification of computer simulation models and to test different hypothesis. Model predictions would then be useful in the development of remedial measures to abate pollution in the Great Lakes Basin.

  2. This well-instrumented site will be available to others for research on pollutant movement on field-scale. Also, it will be an educational site for students and researchers interested in environmental research. It is already being used for this purpose.

  3. Technical knowledge and experience acquired on field installations and instrumentation - electronics, machinery, data collection and analysis - has been, and is being made available to other researchers.

8.0  GAPS/NEEDS FOR FUTURE RESEARCH

  1. A large proportion of the applied herbicides (atrazine and metolachlor) cannot be accounted for in ground water, tile drainage water and surface flow. The long-term fate of the parent compound as well as the degradation products of applied herbicides, including the formation and release of bound (nonlabile) residues needs to be determined more precisely than what is known at present.

  2. Improved understanding is required of chemical transformations and transport in the period from harvest in the fall to chemical application in the spring, when most of the annual surface and tile flow and chemical loading occurs. The role and importance of macropores in chemical leaching also needs better understanding.

  3. Information is required on pollutant transport to tile effluent and groundwater under crops other than corn in the Great Lakes Basin.

  4. Strategies for manure utilization under reduced and no tillage systems need to be developed. Manure incorporation in normally recommended to control odour emission and reduce nitrogen loss as ammonia, and some tillage is therefore necessary.

 

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