Most matter dissolved in water occurs as electrically-charged ions, with ion concentrations dependent on the pH and EH ( redox potential ) of the water, and the composition and concentration of different negatively- and positively- charged ions that may react to form inert solids.
Total dissolved solids ( TDS ) is defined as the quantity of dissolved material in water, and depends mainly on the solubility of rock and soil minerals in contact with waters within normal pH and EH ranges.
For instance, water that flows through limestone or gypsum dissolved calcium, carbonate and sulphate, resulting in high levels of TDS - Total dissolved solids.
Conductivity is a measure of the capacity of water to pass an electrical current and is affected by the presence of dissolved solids. As the level of TDS rises, the conductivity will also increase.
Conductivity change may indicate contamination. For instance, contamination of a water supply with sewage can raise the conductivity because of the the presence of chloride, phosphate, and nitrate.
Intensive use of fertilisers will have similar consequences.
Conductivity also increases with water temperature, so measurements must be presented in terms standarised at either 20°C or 25 °C. Normally, the measuring devices used in the field automatically make this conversion.
Conductivity differences, like temperature variations, may indicate zones of pollution, mixture or infiltration.
Surface water conductivities are normally below 1500 µS/cm.
In practice, acceptance problems for consumers appear above 1500µS/cm, but this factor depends a lot on the habits of the population concerned ( the value of 1.5 g/l of TDS, which is equivalent to around 2000 µS/cm, is the drinking water standard in France ).
Conductivity is measured in micro-siemens per centimetre ( µS/cm ) , and TDS in milligrams per litre ( mg/l ).
The relation between the tow parameters is not linear.
Analysis should be done in situ with an electrical-conductivity meter.
## WHO recommendation : < 1000 mg/l or 1400 µS/cm. ##
