Test for bacteria and nitrates annually. If results indicate 5 mg nitrate per liter or more, further testing is recommended.

General chemistry (e.g. iron, manganese, sodium chloride) testing is recommended approximately every three years.

Also test the well water if:

the well is new
the well has been out of service for awhile
there is a change in taste, odour, or colour
a neighbour's well is found to be unsafe
there is a chemical spill nearby
spring runoff might have gotten into your well, or a neighbours well
flooding has covered the wellhead
there has been a significant change in nearby land use
users have recurrent stomach illness or diarrhoea
a baby is expected