Total dissolved solids (TDS) in well water: what your number means
TDS is the total amount of minerals and salts dissolved in your water. It isn't a health limit on its own. Above 500 mg/L (EPA and Ontario) water can taste salty or bitter and leave scale, so look at which individual results are driving it.
- US (EPA)
- 500 mg/L cosmetic guideline.
- Ontario
- 500 mg/L aesthetic objective.
What it is
TDS adds up everything dissolved in the water: calcium, magnesium, sodium, chloride, sulfate, bicarbonate and more. Most of it comes from the rock and soil the water passes through. Labs report it in mg/L, which is the same as parts per million (ppm).
What your number means
- Within limitsBelow 500 mg/L
Typical for many wells. No taste or scale problems expected from TDS.
- Worth a look500 to 1,000 mg/L
Water may taste salty, bitter or flat, and leave white scale on fixtures and kettles.
- Worth a lookAbove 1,000 mg/L
Often noticeably salty. Check sodium, chloride and sulfate, and ask your health department whether something nearby, like road salt or a septic system, could be adding to it.
What to do
- Look at the other results on your report. Sodium, chloride, sulfate and hardness usually explain a high TDS.
- If TDS has jumped since your last test, ask your health department about road salt, a septic system or other nearby sources.
- For drinking water, reverse osmosis or distillation lowers TDS. A water softener doesn't: it swaps hardness for sodium.
How often to test
With a full panel every 3 to 5 years, and sooner if the taste changes.
Common questions
Is high TDS dangerous?
Not by itself. TDS is a total, so check the individual results on your report, such as sodium, chloride, sulfate, nitrate and hardness, to see what is actually in the water.
Can I use a TDS meter instead of a lab test?
A handheld TDS meter is handy for spotting changes over time, but it only estimates the total. It can't tell you what is dissolved or whether any one thing is above a health limit.
Sources: EPA secondary standards; Ontario: testing and treating private wells. Limits checked November 2026.