Investigations of pesticide contaminations in rural wells, 1979–1984, Ontario, Canada

Abstract

Between the years 1979 and 1984, investigations were conducted into 311 events of suspected contaminations of wells with pesticides. This involved the analyses of water from 359 wells where the suspected contamination originated from (i) spills (ii) spray drift or (iii) surface runoff waters carrying pesticides into wells. Investigations covered 83 spill events involving 104 rural wells; tests revealed 79 of these became contaminated. The contaminations were caused by (i) spills of pesticide concentrates (ii) back-siphoning of spray solutions and/or (iii) spills from overfilling, emptying or rinsing spray equipment. The pesticides either entered directly into the wells or contaminated the area in the vicinity of the wells. In spite of cleanup attempts, difficulty was experienced in decontaminating most well waters and some had to be abandoned. The longest period of monitoring a contaminated well was 1, 117 days; during that time the decline in residue was slow.

Investigations were made into 228 events involving 255 wells where spray drift and/or surface runoff waters with pesticides were observed as entering the well; however, only 55 were found to contain detectable residues. The highest proportion of these events was associated with surface runoff or spray drift from cornfields; of 86 wells involved only 26 contained measurable residues and all involved atrazine. Fifty-seven well investigations were associated with spraying right-of-ways and 16 waters were contaminated with 2,4-D and dichlorprop. The remaining 13 well contaminations were associated with various other land-use activities. It required 45 to 347 days to decontaminate these 55 wells.