Selenium occurs commonly in water, it is also added to selenium deficient soil as a supplement for domestic food producing animals. Selenium can be methylated under microbiological action to form dimethyl selenide.
Whilst little is written about selenides as tainting compounds, they are believed to have similar properties to sulphur compounds and dimethyl selenide is reported to have a garlic odour. So the investigation switched to the source of the water. It was concluded that microbial contamination at the water source was responsible.