NITRATE CONTAMINATION OF GROUNDWATER - MEASUREMENT AND PREDICTION

Agriculture makes a significant contribution to the diffuse source con tamination of surface and groundwater resources, particularly contribu ting to the NO3- contamination of groundwater. Two approaches were ado pted to evaluate management practices (within the context of the whole farming system) for their impacts on the environment : (1) measuremen t of the quality of groundwater under different fanning systems, and ( 2) comparison of predictions of the impact of farming systems on water quality, obtained using whole farm N budgets, with measured values. T he Ontario Farm Groundwater Quality Survey evaluated the rural groundw ater quality in Ontario, with respect to common contaminants including NO3-. Approximately 1300 domestic farm wells were sampled, and wells were drilled in some fields of farms involved in the study. NO3- was p resent at concentrations above the maximum acceptable for drinking wat er (10 mg N l(-1)) in 14% of wells, including 7% of wells that also ha d unacceptable concentrations of coliform bacteria. Significant levels of NO3- contamination were observed under most agricultural land use practices investigated. Calculation of N budgets was simplified by ass uming that there was no net change in the N content of farm assets. Th e N inputs to agricultural systems considered were: purchases from off -farm suppliers, N-2 fixation and atmospheric deposition. Symbiotic N- 2 fixation was estimated from empirical relationships between crop yie ld and N-2 fixed. The N outputs were in sales of plant and animal prod uce, gaseous and leaching losses. Gaseous loss was assumed to result o nly from volatilization of ammonia, estimated to be 39% of total manur e N. We have identified one cash crop farming system where there was a true balance. The rotation included corn soybeans and wheat, with two years of soybean always being grown before corn. Many livestock farms , including two organic farms, gave large imbalances of N which might indicate that these operations were not in equilibrium. The relationsh ip between measured and predicted values of NO3--N expected in the gro undwater under the different management systems showed that the simpli fied N budget overestimated the NO3--N concentration by about one thir d. However, the budget approach appeared to identify farms where conta mination was likely even if the actual amount was over estimated. Simp lified budgets could therefore be used to compare the potential of dif ferent farming systems for causing environmental contamination.