What is a normal TDS for well water?

Freshwater wells typically run 100-400 mg/L. Wells in limestone geology (Midwest, Texas Hill Country, Southeast) often run 300-600 mg/L. Arid regions with high mineral dissolution can run 500-1,500 mg/L. The EPA's secondary standard for TDS in drinking water is 500 mg/L, which is an aesthetic guideline, not a health limit.

Is high TDS well water safe to drink?

TDS level alone doesn't determine safety. Water at 800 mg/L TDS could be perfectly fine if the dissolved solids are calcium, magnesium, and bicarbonate. Water at 200 mg/L could contain arsenic above EPA limits. High TDS may taste salty or mineral-heavy, but for safety questions you need specific contaminant testing.

Does TDS measure arsenic or nitrates in water?

No. TDS measures total dissolved substances by electrical conductivity, but arsenic and nitrates appear at concentrations too low to meaningfully register on a TDS meter, even at levels well above EPA health limits. You need specific tests for arsenic and nitrates.

What TDS level is too high for drinking water?

The EPA secondary standard (non-enforceable, aesthetic only) is 500 mg/L. Above that, water may taste salty, brackish, or leave mineral deposits. The WHO guideline is also 500 mg/L. Above 1,000-1,200 mg/L, most people find the taste unacceptable. Above 3,000 mg/L, water is generally not suitable for livestock.

Does reverse osmosis reduce TDS in well water?

Yes. Reverse osmosis removes 90-97% of dissolved solids, including calcium, magnesium, sodium, and most mineral ions. That's why TDS meters are useful for monitoring RO membrane performance -- a significant rise in output TDS signals that the membrane needs replacement.

TDS in Well Water: What Total Dissolved Solids Actually Measures

TDS stands for total dissolved solids. It measures the total concentration of dissolved substances in water: minerals, salts, metals, everything ionic. The unit is parts per million (ppm) or milligrams per liter (mg/L), which are the same value.

A TDS meter tells you there’s stuff dissolved in your water. It tells you nothing about what that stuff is.

That limitation is the thing most TDS meter marketing glosses over.

What TDS Actually Measures

TDS meters work by measuring electrical conductivity. Water conducts electricity in proportion to how many dissolved ions it contains. The meter converts that conductivity reading to a TDS value in mg/L using a standard conversion factor.

The problem: the meter doesn’t know what ions it’s measuring. Calcium ions and arsenic ions both conduct electricity. A reading of 300 mg/L could represent 300 mg/L of harmless calcium carbonate (common in hard water areas) or it could represent 295 mg/L of calcium plus 5 mg/L of arsenic at a concentration far above the EPA limit of 0.01 mg/L (10 ppb).

The TDS meter cannot tell these apart.

The EPA Secondary Standard

The EPA’s Secondary Maximum Contaminant Level (SMCL) for TDS is 500 mg/L. Secondary standards are non-enforceable guidelines based on aesthetics, not health. They cover taste, odor, and appearance. Water above 500 mg/L may taste salty, brackish, or leave white mineral deposits on fixtures.

Water below 500 mg/L isn’t necessarily safe. It could have arsenic at 50 ppb (five times the EPA health limit of 10 ppb) and still show a TDS of 200.

Typical Well Water TDS Ranges

Where your well falls depends heavily on your geology.

Freshwater wells in most of the eastern U.S. generally run 100-400 mg/L. Limestone regions, including much of the Midwest, the Texas Hill Country, and parts of the Southeast, commonly run 300-600 mg/L because calcium and magnesium carbonate dissolve readily from limestone into groundwater. Wells near saltwater intrusion zones (coastal Florida, parts of the mid-Atlantic) can run 1,000 mg/L or more.

Arid regions with high mineral dissolution rates, including parts of the Southwest and Great Plains, can produce well water at 500-1,500 mg/L. This is not inherently dangerous, but it can produce water that tastes mineral-heavy and leaves significant scale.

What Actually Contributes to Well Water TDS

The bulk of TDS in most freshwater wells consists of: calcium (hardness), magnesium (hardness), sodium, potassium, bicarbonate, sulfate, chloride, and silica. These are the mineral components that dissolve from rock and soil as groundwater moves through the aquifer.

Contaminants like arsenic, nitrates, lead, and PFAS are present in water at much smaller concentrations. Often in the parts-per-billion range. That’s well below what a TDS meter can meaningfully distinguish.

A well water TDS reading tells you almost nothing about whether your water contains these health-relevant contaminants at dangerous levels.

When TDS Actually Matters

TDS above 1,000 mg/L starts causing real effects: visible white scaling on appliances and fixtures, a noticeably salty or brackish taste, and reduced efficiency in water heaters and appliances.

Very high TDS above 2,000 mg/L can damage certain appliances. Above 3,000 mg/L, it’s generally not suitable for livestock. Above 10,000 mg/L, water is considered brackish and not suitable for most uses without treatment.

For RO system monitoring, TDS is genuinely useful. An RO membrane should reduce TDS by 90-97%. If your output TDS starts rising toward your input TDS over months, the membrane needs replacement. This is one practical application for a home TDS meter.

What to Test Instead

If you want to know whether your well water is safe, get a certified lab test for bacteria, nitrates, and the contaminants relevant to your geography: arsenic in the Northeast and Southwest (USGS mapping shows elevated natural arsenic in these regions), VOCs near industrial sites, pesticides near agricultural land, PFAS near military bases or airports.

A TDS reading of 400 mg/L tells you your water has minerals. A certified lab test for arsenic, bacteria, and nitrates tells you whether your water is within EPA health guidelines.

The well water testing guide covers what to test, how to find a certified lab, and what your results mean. For well water with high mineral content, reverse osmosis systems are the most effective way to reduce both TDS and most health-relevant contaminants simultaneously. For a deeper explanation of what TDS measures and what it doesn’t: what is TDS in water.