Dissolved Solids

Abstract

The total dissolved solids (TDS) concentration in freshwater is determined by passing water through a 2 $μ$m filter, evaporating the filtrate to dryness, and reporting the weight of the solids remaining after evaporation in milligrams per liter. The TDS concentrations in natural freshwaters typically range from about 20--1,000 mg/L; the solids consist mainly of bicarbonate (and carbonate at pH above 8.3), chloride, sulfate, calcium, magnesium, potassium, sodium, and silicate. The concentration of TDS in inland waters is controlled mainly by geological and climatic factors. The most weakly mineralized waters are found in areas with high-rainfall and heavily leached or poorly developed soils. The most strongly mineralized waters usually occur in arid regions. Examples of TDS concentrations in different regions are presented, and reasons for differences in TDS concentrations among regions and sources of water are discussed. Although the major dissolved inorganic substances in natural water are essential for life, minor dissolved constituents in water often have the greatest effect on aquatic organisms. The main effect of TDS concentration on animals and plants usually is related to osmotic pressure that increases with greater TDS concentration. The average TDS concentration in seawater is about 35,000 mg/L. However, it is more common to use salinity or specific conductance as an indicator of the degree of mineralization of saline and marine waters.