What Are Total Dissolved Solids?
Total Dissolved Solids (TDS) is the combined measure of all inorganic and organic substances dissolved in water, expressed in milligrams per liter. In hot springs, TDS typically ranges from under 200 mg/L in dilute mountain springs to over 30,000 mg/L in highly mineralized brines. High-TDS springs often have a distinctly mineral taste and may leave a residue on the skin after soaking.
Why TDS Matters for Hot Springs
TDS is the single best indicator of a spring's overall mineral richness. Springs with TDS above 1,000 mg/L are generally considered "mineral springs" and have historically been the most sought-after for therapeutic bathing. The higher the TDS, the greater the concentration of dissolved calcium, magnesium, sodium, and other ions available for transdermal absorption.
High-TDS springs tend to create more dramatic mineral deposits around their outlets, including travertine terraces, silica sinter formations, and colorful bacterial mats. These geological features are often the most visually striking aspects of hot spring landscapes.
Therapeutic Tradition
European balneology has long classified mineral waters by their TDS content, with formal thresholds for therapeutic recognition. German spa regulations, for example, require a minimum of 1,000 mg/L TDS for water to be designated as a "Heilwasser" (healing water). Research published in Rheumatology International has shown that bathing in high-TDS mineral waters may reduce joint pain and improve mobility in osteoarthritis patients.
Disclaimer: The information on this page is for educational purposes only and does not constitute medical advice. Always consult a healthcare provider before using hot springs for therapeutic purposes.