What the number is

USGS defines it plainly: the dissolved solids concentration is “the sum of all the substances, organic and inorganic, dissolved in water.”

A sum. It reports how much is dissolved and never what.

Most of it is ordinary mineral content. USGS lists calcium, magnesium, sodium, potassium, bicarbonate, sulfate, chloride, nitrate and silica as typically making up most of the dissolved solids in water.

Which leads to the first thing the shop demonstration does not mention: “water with high total dissolved solids usually is hard, because calcium and magnesium — the two elements that define hardness in water — are two of the major components of dissolved solids in groundwater.”

A high TDS reading, most of the time, is hard water saying so in a different unit.

The 500 mg/L figure, and what kind of number it is

EPA’s Secondary Maximum Contaminant Level for total dissolved solids is 500 mg/L, above which the noticeable effects are “hardness; deposits; colored water; staining; salty taste.”

Every item on that list is aesthetic. And EPA states it does not enforce Secondary Maximum Contaminant Levels — they are “guidelines established for aesthetic considerations.”

So there is no health-based TDS limit to be above or below. There is a taste and scale guideline, unenforced.

The arithmetic the demo depends on you not doing

Here is why a TDS meter cannot answer the question people buy it to answer. Every figure below is a published limit, and they are all in the same unit.

What it measures Limit Type
TDS guideline 500 mg/L Aesthetic, unenforced
Nitrate 10 mg/L Enforceable health limit
Arsenic 0.010 mg/L Enforceable health limit
Lead action level 0.010 mg/L Utility action trigger
PFOA 0.000004 mg/L Enforceable health limit

PFOA’s figure is a unit conversion of EPA’s stated limit, and it is worth showing rather than asserting:

1 part per trillion = 1 ng/L        (EPA states the MCL "also expressed as ng/L")
1 ng/L              = 0.000001 mg/L (SI prefixes: nano 1E-9, milli 1E-3)

4.0 ppt = 4.0 ng/L = 0.000004 mg/L

Read the gap. Arsenic matters at one fifty-thousandth of the TDS guideline. PFOA matters at roughly one hundred and twenty-five millionth of it.

A meter scaled to read a 500 mg/L guideline cannot resolve those quantities — and even if it could, it reports a total, so it could not say which substance it had found. Bacteria contribute essentially nothing to dissolved solids at all.

A TDS reading of zero would not clear water of lead, PFAS or bacteria. That is the sentence the demonstration is built to prevent you from forming.

The counterweight, because TDS is not meaningless

Debunking pages usually stop at “TDS tells you nothing.” That overshoots.

USGS is careful: “Although high dissolved solids is not in itself a health concern, it can sometimes signal the presence of elevated concentrations of arsenic, uranium, radium, or other trace elements in the groundwater as well.”

So a high reading is a legitimate prompt to test — for the specific things that do have health limits. It is a signal that costs nothing to read and resolves nothing on its own.

It is also genuinely useful for one operational job: on a reverse osmosis system, a rising TDS reading is how you detect a membrane that has stopped working before the taste changes.

The certification detail that closes the loop

Under NSF/ANSI 58, TDS reduction is the only required claim. Arsenic, nitrate, lead and cyst reduction are all optional.

Sit that beside the sales demonstration. The instrument measures the one thing a certified reverse osmosis system is guaranteed to improve, and that one thing is the aesthetic measure. Every health claim on the same unit is optional and must be checked model by model.

→ What each NSF standard actually certifies

The decision path

Your reading What it probably means What to do
Above 500 mg/L, water tastes salty or scales Mineral content, usually hardness Test hardness in gpg; consider softening on cost grounds
Above 500 mg/L, private well Could accompany trace elements Test arsenic, uranium and radium specifically
Below 500 mg/L, worried about lead The reading is silent on lead Test first-draw water; TDS cannot see it
Near zero after an RO unit The membrane is working Track it over time as a maintenance signal
Rising over months on an RO unit Membrane degrading Replace the membrane
A salesman produced the number An aesthetic measure, presented as a safety one Ask which health-based contaminant it detects

Choose to act on a TDS reading if

  • You are tracking an RO membrane’s condition over time.
  • The water tastes salty or leaves scale, and you want to confirm mineral content before pricing a softener.
  • You are on a private well and the reading is high, in which case USGS gives you a reason to test for arsenic, uranium and radium.

Choose to ignore it if

  • Someone is using it to demonstrate that your water is unsafe. It measures aesthetics against an unenforced guideline.
  • You are deciding about lead, PFAS, nitrate or bacteria. None of them are visible to it.
  • The reading is low and you are treating that as clearance.

Mistakes that cost the most money

Reading TDS as a safety score. It is a sum of dissolved substances against an aesthetic guideline EPA does not enforce.

Reading low TDS as clean. Lead at the action level, PFOA at its MCL and bacteria are all effectively invisible to the instrument.

Buying reverse osmosis to fix a number. RO will reduce TDS — that is its only required certification claim. Whether it addresses your actual contaminant is a separate, optional claim.

Treating a demo as a test. A meter reading is not a laboratory analysis and identifies nothing.

Assuming high TDS is harmless. USGS says it can accompany arsenic, uranium or radium. It earns a test, not a shrug.

Removing minerals you were not troubled by. Most TDS is calcium and magnesium. Reducing it is a taste and scale decision, not a health one.

Three situations, and what changes

A salesman at your door with a meter. The number will be real and the inference will not. Ask which health-based contaminant the instrument detects, then request a laboratory test instead.

High TDS on a private well. This is the case where the reading earns attention. USGS links high dissolved solids to trace elements in groundwater, so test arsenic, uranium and radium rather than buying on the reading.

You already own an RO system. TDS becomes a maintenance instrument. Log it monthly; a steady rise means the membrane is failing while the water still tastes acceptable.

The objections worth answering

“My reading was 400 and the bottled water was 20.” Both figures are true and neither is a safety measurement. Most of the difference is dissolved minerals, and EPA’s guideline for the difference is aesthetic and unenforced.

“So TDS is a scam?” No. It is a real measurement being used to answer a question it cannot answer. USGS treats high TDS as a legitimate signal worth following up, and it is a reliable way to watch an RO membrane.

“Shouldn’t I want fewer dissolved solids anyway?” Perhaps, for taste or scale. That is a preference and a running-cost decision. It is not a health argument, and no health-based limit supports it.

Who this is for

Anyone who has been shown a TDS reading and told what it means — and anyone who owns a reverse osmosis system and wants to know what the number on the display is genuinely good for.

What to ask when someone shows you a number

Which contaminant with a health-based limit does this instrument detect, and at what concentration?

There is no answer to that question, which is the point of asking it.

→ What each NSF standard covers, and the one required claim → Hardness, which is most of what TDS is measuring → Micron ratings, for the particles TDS does not measure → Reading the report that does list your regulated contaminants → City water treatment, and what actually carries a health limit → Fluoride, TDS and nitrate all live under NSF/ANSI 58 — the certified set → Microplastics, and the contaminant with a claim but no federal limit → Reverse osmosis: the device certified for exactly this number → The 15 secondary standards, and why none is enforced