Total Dissolved Solids (TDS) refers to the concentration of dissolved inorganic and organic substances in water. While TDS meters measure these substances, their utility in determining overall water safety and potability is limited. TDS provides insights into some water characteristics but has significant limitations regarding harmful contaminants.
Understanding Total Dissolved Solids
Total Dissolved Solids (TDS) encompasses the combined content of all inorganic and organic substances in water small enough to pass through a fine filter. These include minerals like calcium, magnesium, sodium, and potassium, as well as salts, metals, cations, and anions. TDS originates from natural sources, such as the dissolution of rocks and soil, or from human activities like agricultural runoff, industrial discharges, and urban runoff.
TDS is commonly measured in parts per million (ppm) or milligrams per liter (mg/L). A TDS meter estimates this value by measuring the electrical conductivity of water. Since dissolved ions increase water’s ability to conduct electricity, the meter applies a voltage between electrodes and converts the resulting current into an approximate TDS value. Pure water, with very few dissolved minerals, conducts electricity poorly.
What TDS Indicates About Water
A TDS reading offers insights into the aesthetic and physical characteristics of water. High TDS levels can influence the taste, odor, and appearance of drinking water, sometimes leading to a metallic, bitter, or salty taste. The Environmental Protection Agency (EPA) has established a secondary drinking water standard of 500 mg/L for TDS, primarily as a guideline for aesthetic qualities rather than health concerns.
TDS also indicates water hardness, as minerals like calcium and magnesium contribute to the overall TDS level. High TDS can lead to scale buildup in pipes, fixtures, and appliances, reducing their efficiency and lifespan. Extremely low or high TDS levels can signal issues with water treatment processes or the source water, prompting further investigation.
Limitations of TDS as a Water Quality Indicator
Despite its utility for aesthetic and physical properties, a TDS reading has significant limitations as a comprehensive indicator of water quality and safety. A TDS meter primarily detects mobile charged ions, meaning it does not identify specific contaminants, nor does it distinguish between beneficial minerals and harmful substances. For instance, water with high TDS might contain harmless minerals that contribute to its taste, while water with low TDS could still harbor dangerous pollutants.
A TDS reading does not indicate the presence of many harmful contaminants that pose serious health risks. It cannot detect microorganisms such as bacteria, viruses, or cysts, which can cause waterborne diseases, as these are typically uncharged or too large to be registered by the conductivity measurement. Similarly, a TDS meter will not reveal the presence of toxic heavy metals like lead or arsenic, nor will it identify organic pollutants such as pesticides, herbicides, pharmaceuticals, or volatile organic compounds (VOCs). These non-ionic or uncharged compounds do not significantly affect electrical conductivity and thus go undetected. Even water with a very low TDS reading, approaching pure H₂O, could still be contaminated with hazardous substances.
Comprehensive Water Quality Assessment
To truly assess water quality for safety and health, a comprehensive approach extending beyond a simple TDS reading is essential. For instance, well water, which is not regulated by municipal standards, should undergo regular professional laboratory testing for common harmful substances. This includes testing for coliform bacteria, nitrates, lead, arsenic, and VOCs, especially if there are nearby agricultural or industrial activities.
For municipal water supplies, consumers can typically access annual water quality reports, also known as Consumer Confidence Reports, which detail the results of tests for regulated contaminants. If concerns arise about specific contaminants not covered by these reports, or if using a private well, professional lab testing remains the most reliable method for a thorough analysis. These detailed tests provide a complete picture of water quality, identifying the presence and concentration of various inorganic, organic, and microbial contaminants that a TDS meter cannot detect.