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
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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 µg/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 µg/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 µg/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 µg/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 µg/L in over 50% of the samples collected up to 3.0 m depth in the two treatments.
5.0 NEW TECHNOLOGIES AND BENEFITS
6.0 IMPLICATIONS FOR GREAT LAKES ECOSYSTEM
7.0 TECHNOLOGY TRANSFER POTENTIAL
8.0 GAPS/NEEDS FOR FUTURE RESEARCH
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