Reducing Risks to Water Quality

An estimated 20 million kilograms of pesticides are applied to crops on the Canadian Prairies every year. Their positive effects on crop yield are well-documented, but much less is known about how they migrate from their intended target, and what impacts they have on water quality.

Researchers at Environment Canada's National Water Research Institute (NWRI) are investigating the pathways agricultural chemicals take as they move through and over soil. The idea is to help develop practices that reduce contaminant migration and minimize damage to prairie water quality.

The contamination of groundwater by herbicides is a threat to the prairie environment. Soil scientists agree that certain preferential flow routes, such as fractures in the ground, wormholes and other pathways, can cause herbicides to leach more rapidly into groundwater than might be inferred from their physical and chemical properties. To better understand how this occurs, NWRI recently conducted a study in which several pesticides were applied simultaneously to a field irrigated by sprinklers and tile-drained at an average depth of two metres.

Researchers monitored differences in the natural leaching rate of the chemicals, and tested the effect of a single tillage pass in reducing their movement through soil pathways into the groundwater. In analyzing the drain effluents, they found that almost all of the herbicides tested leached through preferential flowpaths. The tillage reduced the flow of water to tile-drain depth only slightly, but it substantially reduced the amount of herbicides transported, especially in the case of more soluble, readily leached herbicides. This suggests that groundwater contamination could be reduced by tillage; however, the soil erosion caused by excessive tillage must also be considered if tillage is to take place.

The trend toward larger-scale hog operations has led to waste disposal difficulties, and using nutrient-rich manure as fertilizer is a possible solution. What is not clear, however, is how much can be used without causing other problems. If more nutrients are applied than a crop requires, they may travel to surface water through runoff or to groundwater through leaching, and possibly harm water quality. To address these concerns, NWRI researchers are in the third year of a six-year project to establish whether manure can be applied to agricultural fields without adversely affecting water resources, and to determine sustainable rates of application. They are working at the landscape scale in drainage basins ranging in size from one to seven hectares, and comparing two rates of application—7000 and 10 000 gallons per acre—with a control plot treated with an inorganic fertilizer.

Preliminary results showed that levels of phosphorus and ammonia in snowmelt runoff were higher in the drainage basin receiving the 10 000-gallon treatment than in the control basin. They also showed elevated concentrations of ammonia in the runoff from the 7000-gallon treatment. However, there was no indication that using hog manure as fertilizer led to higher coliform counts in runoff. Work will continue in the fall to see if different methods of applying the manure will make a difference to the surface runoff water quality.

This research, along with another four-year study investigating the leaching of inorganic fertilizer and pesticides to groundwater under intensive crop irrigation, is giving scientists a clearer picture of the effects of current agricultural practices on the environment, and leading the way to better methods of protecting Canada's water quality.

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