The total dissolved mineral content of water. The main constituents are common minerals such as calcium (Ca), magnesium (Mg), sodium (Na), sulphate (SO4), chloride (Cl) and bicarbonate (HCO3). TDS also includes many common but minor and trace minerals such as iron (Fe), fluoride (F), zinc (Zn), copper (Cu), manganese (Mn).
Note on units: Common units are mg/l (milligrammes per litre) and the equivalent ppm (parts per million). Trace minerals may be measured in µg/l (micro-grammes per litre) or ppt (parts per trillion). 100 mg/l = 100 ppm = 100,000 µg/l = 100,000 ppt.
The most common range of TDS for drinking water is between 100 and 600 mg/l. What is ‘acceptable’ is mostly dependent on personal taste. At higher TDS, water starts to taste salty. For some communities, 1,000 mg/l is considered too high, while others tolerate it. At high TDS the water is considered salty (>2,000 mg/l), brackish (>4,000 mg/l) or saline (>15,000 mg/l). Seawater is usually around 35,000 mg/l TDS.
The balance of minerals (ie. which are more dominant) affects the taste of the water.
TDS must be measured in a laboratory. A quick alternative is to measure the electrical conductivity (EC) of the water, which can be measured directly and in-situ with an EC meter. Common units are µS/cm (micro-Siemens/cm). There is an approximate correlation between TDS and EC. Thus EC is a proxy for TDS. For an approximate conversion from EC to TDS, multiply by 0.5. Example: An EC of 1,000 µS/cm represents a TDS of approximately 500 mg/l. But precise calculation depends on temperature and TDS.
