Contamination in Ontario farmstead domestic wells and its association with agriculture:: 2. Results from multilevel monitoring well installations

Abstract

Multilevel monitoring wells (MLWs) were installed at 144 farms which were part of a province-wide survey of farm drinking water wells conducted in 1991–1992 in Ontario, Canada. The multilevel sites were selected in areas characterized by coarse-textured soils, on farms typifying local agricultural enterprises. The MLWs were installed in cultivated fields adjacent to the location of the drinking water wells on each farm (within 200 m). On 16 of these farms, MLWs were also installed in a woodlot adjacent to the field site. Water samples were collected on two occasions (winter and summer) and analyses were conducted for nitrate (NO 3 −), typical bacteria and a selected suite of common pesticides. At 23% of the sites, concentrations in 50% or more of the monitored intervals exceeded the provincial drinking water standard (MAC) for NO 3 −-N during both sampling periods. Significantly higher frequencies of total coliform contamination were encountered in the winter (66%) than in the summer (36%). Very few pesticide detections were recorded. The average concentration of NO 3 − with depth in multilevel wells decreased from approximately 10 mg N l −1 near the water table to 3 mg N l −1 at a depth of about 6.5 m. Bacteria concentrations remained more uniform with depth but decreased significantly in the summer. For most analytes, contaminant frequency was similar for both the drinking water wells and multilevel wells. The occurrence of elevated levels of contamination in the water wells appeared to be associated more with activities on the cultivated fields than with on-farm point sources of contamination. Groundwater quality determined using drinking water wells was consistent with conclusions drawn from multilevel monitoring wells, indicating the utility of water well survey data for assessing groundwater quality within the rural environment. No correlations were observed between the type of cropping practice and the frequency of groundwater contamination. Farms where manure spreading occurred, however, had significantly higher occurrences of contamination from NO 3 − and bacteria than farms where manure was not applied along with commercial N-fertilizer. The uncultivated conditions beneath the woodlot areas appeared to provide an environment conducive to attenuation of nitrate but not bacteria.