TDS in Well Water: Taste, Deposits, and the 500 mg/L Line

By SubcontractorHub Editorial Team·Published October 2026

Dealers meet TDS in well water as one number on a panel that also has hardness, chloride, sulfate, and nitrate. The U.S. Geological Survey describes dissolved solids as the sum of substances dissolved in the water, and says calcium, magnesium, sodium, potassium, bicarbonate, sulfate, chloride, nitrate, and silica typically make up most of that sum. The number is a picture of the mix. It is not a prescription by itself.

How a single-home well sits outside the public-water program is private well water regulations. The annual panel that includes this line is the well water testing guide. SubcontractorHub keeps the test and the equipment on one dealer record. That is a filing claim, not a removal percentage.

Read the Line Before You Name a Device

What the 500 mg/L Figure Is

EPA's secondary drinking-water standards table lists total dissolved solids at 500 mg/L. Above that secondary level, EPA names hardness, deposits, colored water, staining, and salty taste. The same page says secondary standards are guidelines for aesthetic, cosmetic, or technical effects, and that EPA does not enforce those secondary maximum contaminant levels. The contaminants are not described as a human-health risk at the secondary level.

USGS uses the same 500 mg/L figure as a recommended drinking-water value and says concentrations greater than that can give water an unpleasantly salty taste, and that elevated dissolved solids can contribute to corrosion of plumbing fixtures. Say “public guideline” when you repeat it. Do not tell a private-well owner they are over a federal limit. State or county rules can still apply. Quote the rule you actually have.

TDS in Well Water Is Not Hardness

Hardness is calcium and magnesium, usually reported as calcium carbonate. Dissolved solids include those ions and also sodium, chloride, sulfate, nitrate, and the rest of the mix. A house can be high in dissolved solids because of salt, not because of hardness. A house can be hard and still sit under 500 mg/L. The two lines answer different questions.

Chloride has its own secondary line and its own page, chloride in well water. Sulfate is separate again. If you only talk about the sum, you will sell a softener for a salty anion or a membrane for hardness spots. Split the panel before you split the price.

A Meter Is Not This Lab Line

USGS says dissolved solids can be monitored in real time by specific conductance, a surrogate that rises as dissolved ions rise. A pocket meter that prints “TDS” is often that surrogate with a factor built in. The factor is not on the USGS page fetched for this article, and this page does not invent one. When the lab prints a residue or a constituent list, that sheet is the record. The meter is a screen.

How to keep the two from being written as the same number is conductivity and TDS. If you have the ions and want to add only the ones on the sheet, use the TDS constituent sum. Call the result a partial sum. Do not relabel it as the laboratory total.

What EPA Names, and What a Softener Is

EPA's secondary guidance says non-conventional treatments such as distillation, reverse osmosis, and electrodialysis are effective for chloride, total dissolved solids, and other inorganic substances. It also says those technologies are fairly expensive and may be impractical for smaller public systems. Repeat that cost remark as a statement about public systems. Do not turn it into a kitchen-unit price.

A cation softener is built to exchange hardness. It is not in that sentence as the dissolved-solids process. Sodium or potassium leaves with the treated water while calcium and magnesium stay on the resin until regeneration. The sum can change shape. It is not, by that exchange, the removal EPA is describing. Say so on the proposal.

1. Write the number and the method. Residue, a sum of ions, or a meter. Say which.
2. List the ions that dominate. Hardness, chloride, sulfate, nitrate, and anything else the lab flagged.
3. Name the tap. A point-of-use reverse osmosis unit is the drinking faucet, not the showers.
4. Keep the softener labeled as hardness. If you also sell it, it is a different stage.
5. Retest the tap you treated. A raw-well sample does not grade the kitchen device.

Scale, Staining, and the Rest of the Panel

EPA groups total dissolved solids with scale and sediment as well as with taste and color. Spots on glass can be hardness. Color can be iron or organics. A salty taste can be chloride or sulfate or the mix. The dissolved-solids line tells you the load is high enough to notice. The other columns tell you which complaint you are looking at.

Alkalinity and pH still matter if you are judging scale. They are not this number. The Langelier saturation index calculator asks for a dissolved-solids input as one term in a planning index. Paste the lab figure, and say if it was a meter. Do not average a field cup and a lab bottle and call it one sample.

What to Put on the Quote

The lab date, the milligrams per liter, whether it was a sum or a meter, the ions you are actually treating, the tap the device serves, and the retest. If the public 500 mg/L line is the comparison, label it as EPA's secondary guideline for public systems. Leave removal percentages off unless the equipment listing you are selling states one for that model, and even then quote the listing, not this article.

A water treatment dealer who files that note can answer “why this device” on the next visit without rebuilding the story from memory. No national installed price belongs on this page. The price is the quote.

When TDS in Well Water and a Neighbor's Well Disagree

Two houses on the same road can print different dissolved-solids numbers. Depth, a water softener already in the house, a reverse-osmosis tap, and a sample taken at the well versus the kitchen are enough to explain a gap. Do not average the two addresses into one “area TDS.” The proposal is for the sample you actually have.

Ask where the bottle was filled, whether a softener or a drinking-water unit was in the path, and whether the lab reported a residue or a list of ions. If the customer hands you a meter photo from a different day, file it as a meter photo. The public 500 mg/L line is still a taste and deposits guideline for public systems. It does not become a neighborhood statute because two wells disagree. Retest the tap you intend to treat, and keep the first sheet so the change has a starting point.

Frequently Asked Questions