IsTotal Dissolved Solids the Best Water Quality Indicator ?

October 10th, 2012

Those of us who are concerned with the quality of our drinking water frequently turn to the total dissolved solids (TDS) measurement to determine whether our water is contaminant-free or not. But do we all really understand what total dissolved solids are and what this measurement tells us about drinking water quality? It seems worthwhile to dig a little deeper and discover what TDS is all about.

What is TDS?

The term, total dissolved solids (TDS), is used to describe the amount of materials dissolved in water. Most of these materials are dissolved salts or ions that are naturally occurring like calcium, magnesium, sodium, carbonate, bicarbonate, and so on. For example, molecules like calcite (CaCO3) are present in some types of rock, like shale, and dissolve easily into calcium and carbonate ions when coming into contact with water. Unfortunately industrial waste, sewage, urban and fertilizer runoff, and countless other sources may also contain many materials like lead, arsenic, or nitrates that easily dissolve in the water and contribute to TDS as well.

How is TDS measured?

There are several different ways to measure TDS but the most common method usually used by household TDS meters is based on conductivity which is the ease with which electric current flows through the water. As the concentration of ions increases, the conductivity increases as well. The conductivity measurement is converted to a concentration measurement which is usually described by parts per million (ppm).

What can the TDS reading tell us?

Any TDS reading greater than zero ppm indicates that ions are present in the water however we cannot conclude from this reading alone that the water poses a risk to human health since we do not know the type of ions present. The reading could simply be due to a healthy mineral such as calcium which can actually improve the taste of the water. In fact, the World Health Organization (WHO) reported that water with up to 300ppm TDS, consisting of healthy mineral ions, was rated as having excellent taste by study participants. Furthermore TDS levels are not even regulated in the United States. The EPA instead only recommends these levels do not exceed 500 ppm to avoid aesthetic issues like color or unpleasant taste.

Nevertheless, a high TDS reading raises concerns with many people. If you are getting an elevated reading, it is possible that harmful contaminants are contributing to TDS as well. It is best to think of TDS as a “screening test” that informs you that something is in your drinking water. From this point, the Colorado State University (CSU) Cooperative Extension System and WHO recommend that you have the water tested to identify the specific ions that are causing the elevated TDS reading.

If you would like to read more about TDS, the CSU Cooperative Extension System website and the WHO report about total dissolved solids in drinking water are very useful.


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