CONTAMINATION IN ONTARIO FARMSTEAD DOMESTIC WELLS AND ITS ASSOCIATIONWITH AGRICULTURE - 1 - RESULTS FROM DRINKING-WATER WELLS

Groundwater provides about 30% of water requirements in Ontario, but f arm families depend almost entirely on private wells. Major potential contaminants on farms are nitrate (NO3-), pathogenic microorganisms, p esticides and petroleum derivatives. A survey of farm drinking-water w ells was conducted throughout the Province of Ontario, Canada, in 1991 and 1992 and tested for these contaminants. The main objectives of th e survey were to determine the quality and safety of drinking water fo r farm families, and determine the effect of agricultural management o n groundwater quality at a provincial scale. Four farm wells were chos en in each township where > 50% of the land area was used for agricult ural 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 t ype, soil, and the presence or absence of a fuel storage tank, was inv estigated for the presence of petroleum derivatives: benzene, toluene, ethyl benzene, and xylene. About 40% of farm wells tested contained o ne or more of the target contaminants above the maximum acceptable con centration; 34% of wells had more than the maximum acceptable number o f coliform bacteria, 14% contained NO3--N concentrations above 10 mg l (-1) limit and about 7% were contaminated with both bacteria and NO3-. Only six wells contained pesticide residues above the interim maximum acceptable concentration (IMAC), but pesticides were detected in 7% o f wells in winter and in 11% in summer. No wells contained detectable petroleum derivatives. These results for NO3- contamination were not s ignificantly different from those reported for a survey of Ontario wel ls for the period 1950-1954, but the frequency of contamination by Esc herichia coli was greater in the present study. None of the point sour ces investigated contributed significantly to the NO3- contamination. The percentage of wells contaminated by coliform bacteria decreased si gnificantly 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 wa s developed to describe the frequency of NO3- contamination. (C) 1998 Elsevier Science B.V. All rights reserved.