(Updated January 1991)
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An aesthetic objective of ≤500 mg/L has been established for total dissolved solids (TDS) in drinking water. At higher levels, excessive hardness, unpalatability, mineral deposition and corrosion may occur. At low levels, however, TDS contributes to the palatability of water.
Total dissolved solids (TDS) comprise inorganic salts and small amounts of organic matter that are dissolved in water. The principal constituents are usually the cations calcium, magnesium, sodium and potassium and the anions carbonate, bicarbonate, chloride, sulphate and, particularly in groundwater, nitrate (from agricultural use).
Total dissolved solids in water supplies originate from natural sources, sewage, urban and agricultural runoff and industrial wastewater. In Canada, salts used for road deicing can contribute significantly to the TDS loading of water supplies. Concentrations of TDS in water vary owing to different mineral solubilities in different geological regions. The concentration of TDS in water in contact with granite, siliceous sand, well-leached soil or other relatively insoluble materials is usually below 30 mg/L.Footnote 11 In areas of Precambrian rock, TDS concentrations in water are generally less than 65 mg/L.Footnote 22 Levels are higher in regions of Palaeozoic and Mesozoic sedimentary rock, ranging from 195 to 1100 mg/LFootnote 22 because of the presence of carbonates, chlorides, calcium, magnesium and sulphates.Footnote 11,Footnote 33Concentrations of TDS in some streams and small lakes in the arid western regions of Canada and the United States are often as high as 15 000 mg/L.Footnote 33,Footnote 44Concentrations of TDS, expressed as the sum of its constituents, were below 500 mg/L in 36 of 41 rivers monitored in Canada.Footnote 55 In a survey of the Great Lakes, TDS levels ranged from 61 to 227 mg/L.Footnote 66 The levels of TDS in all of the Great Lakes except Lake Superior increased between 1900 and 1970. A threefold increase in chlorides and a twofold increase in sulphates, sodium and potassium in Lakes Erie and OntarioFootnote 77 increased the TDS concentration in those lakes by 50 to 60 mg/L.Footnote 66,Footnote 8
Concentrations of TDS in drinking water in Canada are generally below 500 mg/L but are considerably higher in some locations, particularly the arid western regions. Levels of TDS in Newfoundland and Labrador were below 500 mg/L in 96% of 103 communities sampled from 1969 to 1989 (range 10 to 2263 mg/L; average 146 mg/L).Footnote 1111 In Quebec, samples of distributed water taken at 19 plants between 1987 and 1989 contained TDS at mean concentrations ranging from 58 to 213 mg/L.Footnote 1212 Concentrations of TDS in distributed water from 31 plants in Ontario during 1987 and 1988 ranged from 91 to 470 mg/L.Footnote 1313 In Manitoba, TDS concentrations measured during 1988 in the treated water of 168 communities ranged from 56 to 2510 mg/L; concentrations were less than 500 mg/L in 19% of these communities.Footnote 1414 Levels of TDS in 1978 samples of community drinking water taken between 1970 and 1989 in Saskatchewan ranged from 6.5 to 5376 mg/L.Footnote 1515 Concentrations of TDS in 54% of 1042 communities surveyed in Alberta in October 1989 were below 500 mg/L (range <100 to 1000 mg/L).Footnote 1616 In British Columbia, concentrations of TDS in individual well water supplies ranged from 120 to 4662 mg/L; those in community (generally surface water) supplies were commonly less than 500 mg/L.Footnote 1717
The method most commonly used for the analysis of TDS in water supplies is the measurement of specific conductivity with a conductivity probe that detects the presence of ions in water. Conductivity measurements are converted to TDS values by a factor that varies with the type of water.Footnote 1818,Footnote 1919 The practical quantitation limit for TDS in water by this method is 10 mg/L.Footnote 2020 High TDS concentrations can also be measured gravimetrically, although this method excludes volatile organics.Footnote 2121 The constituents of TDS can also be measured individually.
Total dissolved solids are not appreciably removed using conventional water treatment processes. In fact, the addition of chemicals during conventional water treatment generally increases the TDS concentration.Footnote 2222 Certain treatment processes, such as lime-soda ash softening and sodium exchange zeolite softening, may slightly decrease or increase the TDS concentration, respectively.Footnote 2323 Demineralization processes are required for significant TDS removal. Although the technology is available to reduce TDS levels significantly, the economic cost may be a major constraint.Footnote 2323 Reverse osmosis and electrodialysis would probably be the most economical processes for removing TDS from public water supplies.Footnote 2424
Recent data on health effects associated with the ingestion of TDS in drinking water have not been identified; however, associations between various health effects and hardness, rather than TDS content, have been investigated in many studies. These data are discussed in the section on hardness. As well, some of the individual components of TDS can have effects on human health. Effects that can be attributed to specific constituents are discussed in separate reviews for those constituents.
In early studies, inverse relationships were reported between TDS concentrations in drinking water and the incidence of cancer,Footnote 2525 coronary heart disease,Footnote 2626 arteriosclerotic heart diseaseFootnote 2727 and cardiovascular disease.Footnote 2828,Footnote 2929 Total mortality rates were reported to be inversely correlated with TDS levels in drinking water.Footnote 2929,Footnote 3030
Conversely, a summary of an Australian study reported that mortality due to all categories of ischaemic heart disease and acute myocardial infarction was increased in a community with higher levels of soluble solids, calcium, magnesium, sulphate, chloride and fluoride, alkalinity, total hardness and pH, when compared with a community in which levels were lower.Footnote 3131 No attempts were made to relate mortality due to cardiovascular disease to other potential confounding factors. The results of a limited epidemiological study in the former Soviet Union indicated that the average number of "cases" of inflammation of the gall bladder and gallstones over a five-year period increased with the mean level of dry residue in the groundwater.Footnote 3232 It should be noted, however, that the number of "cases" varied greatly from year to year in one district, as did the concentration of dry residue in each district, and no attempt was made to take into account possible confounding factors.
The presence of dissolved solids in water may affect its taste.Footnote 33-42 The palatability of drinking water has been rated, by panels of tasters, according to TDS level as follows: excellent, less than 300 mg/L; good, between 300 and 600 mg/L; fair, between 600 and 900 mg/L; poor, between 900 and 1200 mg/L; and unacceptable, greater than 1200 mg/L.Footnote 3737 Water with extremely low TDS concentrations may also be unacceptable because of its flat, insipid taste.
In addition to palatability, certain components of TDS such as chlorides, sulphates, magnesium, calcium and carbonates also affect corrosion or encrustation in water distribution systems.Footnote 2121 High TDS levels (above 500 mg/L) result in excessive scaling in water pipes, water heaters, boilers and household appliances such as tea kettles and steam irons.Footnote 4343 Such scaling can shorten the service life of these appliances.Footnote 4444
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