TDS in Drinking Water: The 500 mg/L Secondary Standard and What Total Dissolved Solids Mean

Total dissolved solids, or TDS, is the water-quality number most people meet through side effects rather than a lab report: a white crust on faucets and showerheads, spots left on glassware, a salty or mineral taste, or scale that clogs a water heater. It is common in groundwater, and it is governed by one number — the 500 mg/L secondary maximum contaminant level in 40 CFR § 143.3 — a number that is about taste and scale, not health. If you are not sure what your system's TDS runs, check your water system's compliance status free at orevant.com before you read on.

The 500 mg/L secondary standard is about taste and scale, not health

The secondary maximum contaminant level (SMCL) for total dissolved solids is 500 milligrams per liter, set in 40 CFR § 143.3 under the National Secondary Drinking Water Regulations. A secondary standard is not federally enforceable the way an MCL is. It is the level above which water starts to taste salty, bitter, or metallic and to leave scale and spots — the complaints that make customers call. TDS has no primary MCL, because at the levels found in drinking water it is not itself a health hazard; the standard exists to protect taste, appearance, and plumbing. See the difference between an MCL and an action level for how the federal rulebook sorts enforceable limits from advisory numbers.

What TDS actually is

TDS is the sum of the inorganic salts dissolved in the water — chiefly calcium, magnesium, sodium, potassium, bicarbonate, chloride, and sulfate — plus a small amount of dissolved organic matter. It is not one contaminant. It is a yardstick for how much mineral content the water is carrying overall. A high TDS reading is the clue that the water is mineral-heavy; the individual minerals inside that total are what decide whether there is also a compliance issue in the mix. See how iron and manganese, two common contributors, behave on their own.

Why high TDS shows up in groundwater systems

TDS climbs where water spends a long time in contact with rock and soil. Groundwater that moves slowly through limestone, gypsum, or salt-bearing formations picks up dissolved minerals along the way, so well systems and small groundwater systems carry the bulk of high-TDS complaints. The same slow, mineral-rich path that raises TDS can also carry naturally occurring metals — see how naturally occurring metals enter groundwater. Evaporation concentrates it further, which is why some arid and agricultural regions run noticeably higher TDS in both groundwater and surface supplies.

What a small system actually does about it

Treatment depends on how high the number is and what is driving it. For taste and scale complaints the proven options are reverse osmosis, ion exchange (water softening for hardness), and distillation, with blending as a lower-cost fix where a second, lower-TDS source is available. The first step is knowing your raw-water TDS, whether it is climbing, and which minerals are doing the driving — which a monitoring calendar tracks. See how to know if your water system is compliant.

Know the number before a customer calls about scale

TDS rarely triggers an enforceable violation, which is exactly why it gets ignored until a customer calls about a white crust on the tap or spots on the glassware. But a system that knows its TDS number can head off the complaint, decide whether it crosses the 500 mg/L secondary standard, and pick the right treatment before the water becomes a visible problem. The $199 compliance scan maps every monitored contaminant on your system, including TDS and its secondary-standard neighbors, against the numbers you are actually up against, so you know what to fix first.

Run the $199 Compliance Scan — see your TDS level and the exact step to fix it.

Prefer to see the public record first? Check your water system free at orevant.com.

FAQ

What is a good TDS level for drinking water?

Under 500 mg/L is generally considered acceptable for taste and appearance, which is why 40 CFR § 143.3 sets 500 mg/L as the secondary standard. Water under that level is not automatically perfect, and water over it is not automatically unsafe — TDS is a taste and scale measure, not a health standard.

Is high TDS water safe to drink?

TDS itself is not a health hazard at drinking-water levels, which is why it carries no primary MCL. A high reading is worth investigating because the minerals behind it, such as sodium or sulfate, can matter to some customers, and because it can indicate naturally occurring contaminants in the same source.

How do I lower TDS in my water?

Reverse osmosis, ion exchange, and distillation all remove dissolved minerals. Blending a high-TDS source with a lower-TDS source is a lower-cost fix where available. The right choice depends on how high the number is and which minerals are driving it.

Is TDS the same as hardness?

No. Hardness is specifically calcium and magnesium, while TDS is the total of all dissolved minerals including hardness, sodium, chloride, and sulfate. Hard water is one contributor to TDS, not the whole number. See how iron, another common groundwater mineral, is handled separately.