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Groundwater provides about 30% of water requirements in Ontario, but farm families depend almost entirely on private wells. Major potential contaminants on farms are nitrate (NO ), pathogenic micro-organisms, pesticides and petroleum derivatives. A survey of farm 3 drinking water wells was conducted throughout the Province of Ontario, Canada, in 1991 and 1992 and tested for these contaminants. Main objectives of the survey were to determine the quality and safety of drinking water for farm families, and determine the effect of agricultural management on groundwater quality at a provincial scale.Four farm wells were chosen in each township where >50% of the land area was used for agricultural production. Elsewhere one well per township was usually sampled. Within each township the types of farming activity and dominant soils were additional criteria for selection.The network comprised 1292 of the estimated 500,000 water-wells in Ontario, and the study conformed to a stratified random survey. A subset of 160 wells, chosen by farm type, soil, and the presence or absence of a fuel storage tank, was investigated for the presence of petroleum derivatives: benzene, toluene, ethyl benzene, and xylene.About 40% of farm wells tested contained one or more of the target contaminants above the maximum acceptable concentration; 34% of wells had more than the maximum acceptable number of coliform bacteria, 14% contained NO -N concentrations above 10 mg L limit and 3 -1 about 7% were contaminated with both bacteria and NO . Only six wells contained pesticide 3 residues above the interim maximum acceptable concentration (IMAC), but pesticides were detected in 7% of wells in winter and in 11% in summer. No wells contained detectable petroleum derivatives. These results for NO contamination were not significantly different from 3 3 those reported for a survey of Ontario wells for the period 1950-1954, but the frequency of contamination by Escherichia coli was greater in the present study.None of the point sources investigated contributed significantly to the NO contamination. The 3 percentage of wells contaminated by coliform bacteria decreased significantly with increasing separation of the well from the feedlot or exercise yard on livestock farms. A full statistical model including the type of well construction, depth, age and soil hydrologic group was developed to describe the frequency of NO contamination.3

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