Total Dissolved Solids
A measurement of total dissolved solids (TDS) from aqueous samples such as stream or lake water can give a general overview of water quality. Primary sources of TDS include runoff from agricultural, industrial, and residential areas, sewage treatment facilities, and single-point discharges or spills. While there are no exact TDS levels set to determine the health of an aqueous environment in the field, the EPA has set a secondary water quality standard of 500 ppm for potable water. A few general definitions of water types based on TDS levels are:
- < 1000 ppm TDS = fresh water
- 1000 – 10,000 ppm TDS = brackish water
- 10,000 – 30,000 ppm TDS = saline water
- > 30,000 ppm TDS = brine
The most common units for TDS are parts-per-million (ppm), and TDS can be measured in different ways.
In the field, for example, samplers use a hand-held combination meter to get an on-site reading of TDS. This value is calculated from an electrical conductivity reading from the meter. In the lab, however, we use a gravimetric method to determine TDS, and so the field meter TDS and laboratory TDS readings are generally different from one another. Despite these differences, we usually see similar trends in TDS values – that is, as TDS readings from the hand-held meter increase, so do the laboratory TDS measurements.
Method for Determining TDS
- A 100-mL portion of unacidified water sample is filtered through a 0.45-μm filter and placed into a dried, pre-weighed porcelain dish.
- The sample is then gently heated on a hot plate (not boiled), until all of the water has evaporated and the dish is completely dry.
- The empty dry dish is then weighed again, and any dissolved solids left behind will cause an increase in the mass of the dish.
- In other words, water with TDS values > 500 ppm should not be consumed.