TDS is one of the most commonly misunderstood well water test results. A high TDS reading does not automatically mean your water is unsafe — but it does tell you something important. Here is how to interpret yours.
Total dissolved solids (TDS) is a measure of all dissolved minerals, salts, and metals in your water — expressed in milligrams per liter (mg/L) or parts per million (ppm). It is a summary measurement, not a test for any specific contaminant. High TDS can come from naturally occurring minerals (calcium, magnesium, sodium) that are harmless, or from contaminants (nitrates, arsenic, lead) that are not.
| TDS Level (mg/L) | Classification | Typical Source |
|---|---|---|
| Under 300 | Excellent | Very pure groundwater |
| 300–600 | Good | Normal mineral content |
| 600–900 | Fair | Elevated minerals; taste may be affected |
| 900–1,200 | Poor | High mineral content; further testing recommended |
| 1,200+ | Unacceptable | Very high minerals or contamination |
The EPA's secondary standard for TDS is 500 mg/L — based on taste, not health. Many rural areas have naturally high TDS groundwater with no health concerns. Hard water (high calcium and magnesium) will show high TDS. A water softener or RO system reduces TDS significantly.
If your TDS is significantly elevated (over 1,000 mg/L) and you have not previously tested your well comprehensively, order a full contaminant panel to rule out nitrates, arsenic, and other dissolved contaminants that contribute to TDS. A TDS meter alone cannot tell you what is dissolved — only that something is.
Most people can taste a difference at TDS levels above 500–600 mg/L. Very high TDS (1,000+ mg/L) produces a distinctly mineral, salty, or bitter taste. A point-of-use reverse osmosis system typically reduces TDS to 20–50 mg/L — producing noticeably cleaner-tasting water.
TDS meters are useful screening tools but not replacements for lab tests. A $15 TDS meter from Amazon tells you TDS level in seconds but cannot identify specific contaminants. Use it for quick checks and as a trigger for more comprehensive testing when levels are unexpectedly high.
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