Turbidity in well water: cloudy water explained
Turbidity is cloudiness, measured in NTU. Ontario's aesthetic objective is 5 NTU. It isn't a health risk by itself, but cloudy water can carry or hide bacteria.
- US (EPA)
- No limit for private wells.
- Ontario
- 5 NTU aesthetic objective.
What it is
Turbidity comes from tiny particles of silt, clay, iron or organic matter. In wells, it often rises after heavy rain, new drilling, pump work, or when surface water gets into the well.
What your number means
- ClearBelow 1 NTU
Clear water. Typical for a sound well.
- Worth a look1 to 5 NTU
Slightly cloudy. Watch for changes after rain.
- Worth a lookAbove 5 NTU
Cloudy. Test for bacteria and check the well cap and casing.
What to do
- If turbidity rises after rain, test for bacteria. It can mean surface water reaches the well.
- Check the well cap and the grading around the wellhead.
- A sediment filter clears particles. Fix the source first if bacteria are present.
How often to test
When the water looks cloudy or changes after storms, together with a bacteria test.
Common questions
Is cloudy well water safe?
Cloudiness itself is not harmful, but it can carry bacteria and makes disinfection less effective. Test for bacteria.
What is NTU?
Nephelometric turbidity units, a measure of how much light the particles in water scatter.
Sources: Ontario Drinking Water Quality Standards (O. Reg. 169/03); Ontario: testing and treating private wells. Limits checked November 2026.