View source for Earth:Total dissolved solids

{{Short description|Measurement in environmental chemistry}}
[[File:Stilles Mineralwasser.jpg|thumb|upright|Bottled [[Chemistry:Mineral water|mineral water]] usually contains higher TDS levels than tap water.]]

'''Total dissolved solids''' ('''TDS''') is a measure of the dissolved combined content of all [[Chemistry:Inorganic compound|inorganic]] and [[Chemistry:Organic compound|organic]] substances present in a liquid in [[Physics:Molecule|molecular]], [[Physics:Ion|ion]]ized, or micro-granular ([[Chemistry:Sol (colloid)|colloidal sol]]) suspended form. TDS are often measured in parts per million (ppm). TDS in water can be measured using a digital meter.<ref>{{Cite web|url=https://theberkey.com/blogs/water-filter/what-is-theacceptable-total-dissolved-solids-tds-level-in-drinking-water|title=What Is The Acceptable Total Dissolved Solids (TDS) Level In Drinking Water?|website=The Berkey|access-date=2020-02-22|archive-date=2020-02-22|archive-url=https://web.archive.org/web/20200222212218/https://theberkey.com/blogs/water-filter/what-is-theacceptable-total-dissolved-solids-tds-level-in-drinking-water|url-status=live}}</ref>

Generally, the operational definition is that the solids must be small enough to survive filtration through a filter with 2-micrometer (nominal size, or smaller) pores. Total dissolved solids are normally discussed only for freshwater systems, as [[Earth:Salinity|salinity]] includes some of the ions constituting the definition of TDS. The principal application of TDS is in the study of [[Earth:Water quality|water quality]] for [[Earth:Stream|stream]]s, [[Earth:River|river]]s, and [[Earth:Lake|lake]]s. Although TDS is not generally considered a primary [[Chemistry:Pollutant|pollutant]] (e.g. it is not deemed to be associated with health effects), it is used as an indication of aesthetic characteristics of drinking water and as an aggregate indicator of the presence of a broad array of chemical contaminants.

Primary sources for TDS in receiving waters are [[Earth:Surface runoff|agricultural runoff]] and [[Earth:Urban runoff|residential (urban) runoff]], clay-rich mountain waters, leaching of [[Chemistry:Soil contamination|soil contamination]], and point source [[Earth:Water pollution|water pollution]] discharge from industrial or [[Earth:Sewage treatment|sewage treatment]] plants. The most common chemical constituents are [[Chemistry:Calcium|calcium]], [[Chemistry:Phosphate|phosphate]]s, [[Chemistry:Nitrate|nitrate]]s, [[Chemistry:Sodium|sodium]], [[Chemistry:Potassium|potassium]], and [[Chemistry:Chloride|chloride]], which are found in [[Earth:Nutrient|nutrient]] runoff, general [[Earth:Stormwater|stormwater]] runoff and runoff from snowy climates where road [[Chemistry:De-icing|de-icing]] salts are applied. The chemicals may be cations, anions, [[Physics:Molecule|molecule]]s or agglomerations on the order of one thousand or fewer molecules, so long as a soluble micro-granule is formed. More exotic and harmful elements of TDS are [[Biology:Pesticide|pesticide]]s arising from [[Earth:Surface runoff|surface runoff]]. Certain naturally occurring total dissolved solids arise from the weathering and dissolution of rocks and soils. The United States has established a secondary water quality standard of 500&nbsp;mg/L to provide for palatability of drinking water.

Total dissolved solids are differentiated from [[Earth:Total suspended solids|total suspended solids]] (TSS), in that the latter cannot pass through a sieve of 2 micrometers and yet are indefinitely suspended in solution. The term ''settleable solids'' refers to material of any size that will not remain suspended or dissolved in a holding tank not subject to motion, and excludes both TDS and TSS.<ref>{{cite book |first=John |last=DeZuane |title=Handbook of Drinking Water Quality |publisher=John Wiley and Sons| year=1997 |isbn=0-471-28789-X |edition=2nd}}</ref> Settleable solids may include larger particulate matter or insoluble molecules.

Total dissolved solids include both volatile and non-volatile solids. Volatile solids are ones that can easily go from a solid to a gaseous state. Non-volatile solids must be heated to a high temperature, typically 550&nbsp;°C, in order to achieve this state change. Examples of non-volatile substances include salts and sugars.<ref>Wetzel, R. G. (2001). Limnology: Lake and river ecosystems. San Diego: Academic Press.</ref>

==Measurement==
[[File:Tds-meter.jpg|thumbnail|right|Conductivity based TDS meter in a cup of water]]
The two principal methods of measuring total dissolved solids are [[Chemistry:Gravimetric analysis|gravimetric analysis]] and [[Physics:Conductivity (electrolytic)|conductivity]].<ref>{{cite web | url = http://www.epa.gov/quality/total-dissolved-solids-tds-epa-method-1601-gravimetric-dried-180-degrees-c |title=Total Dissolved Solids (TDS): EPA Method 160.1 (Gravimetric, Dried at 180 deg. C) |author=<!--Staff writer(s); no by-line.--> |date=1999-11-16 |publisher=U.S. Environmental Protection Agency (EPA) |location=Washington, D.C.|archive-url=https://web.archive.org/web/20160223132222/http://www.epa.gov/quality/total-dissolved-solids-tds-epa-method-1601-gravimetric-dried-180-degrees-c |archive-date=2016-02-23 }}</ref> Gravimetric methods are the most accurate and involve evaporating the liquid [[Physics:Solvent|solvent]] and measuring the mass of residues left. This method is generally the best, although it is time-consuming. If inorganic salts comprise the great majority of TDS, conductivity-based methods are appropriate.

Conductivity of water is directly related to the concentration of dissolved ionized solids. These ions allow the water to conduct [[Physics:Electric current|electric current]]. This electric current can be measured using a conventional conductivity meter or '''TDS meter'''. When correlated with laboratory TDS measurements, [[Physics:Conductivity (electrolytic)|conductivity]] provides an approximate value for the TDS [[Chemistry:Concentration|concentration]], with around 10% accuracy.

The relationship of TDS and specific conductance of groundwater can be approximated by the following equation:

:''TDS = k<sub>e</sub>EC''

where TDS is expressed in mg/L and EC is the electrical conductivity in microsiemens per centimeter at 25&nbsp;°C. The conversion factor ''k<sub>e</sub>'' varies between 0.55 and 0.8.<ref>{{cite journal |last1=Atekwanaa |first1=Eliot A. |last2=Atekwanaa |first2=Estella A. |last3=Roweb |first3=Rebecca S. |last4=Werkema Jr. |first4=D. Dale |last5=Legalld |first5=Franklyn D. |date=2004 |title=The relationship of total dissolved solids measurements to bulk electrical conductivity in an aquifer contaminated with hydrocarbon |url=http://www.epa.gov/esd/cmb/pdf/JAG-TDSpublished.pdf |journal=Journal of Applied Geophysics |publisher=Elsevier |volume=56 |issue=4 |pages=281–294 |doi=10.1016/j.jappgeo.2004.08.003 |access-date=15 February 2016 |bibcode=2004JAG....56..281A  |archive-url=https://web.archive.org/web/20140801000000/http://www.epa.gov/esd/cmb/pdf/JAG-TDSpublished.pdf |archive-date=1 August 2014 |url-status=dead}}</ref>

Some TDS meters use an electrical conductivity measurement to the [[Parts per million|ppm]] using the above formula. Regarding units, 1&nbsp;ppm indicates 1&nbsp;mg of dissolved solids per 1000 g of water.<ref name="Frequently Asked Questions">{{cite web|url= http://www.tdsmeter.com/faqs|title=Frequently Asked Questions|access-date=23 May 2017| archive-url=https://web.archive.org/web/20170618073154/www.tdsmeter.com/faqs| archive-date= 2017-06-18| url-status= unfit }}</ref>

==Hydrological simulation==
[[File:Pyramid Lake sat.jpg|thumb|left|Pyramid Lake, Nevada, receives dissolved solids from the Truckee River.]]

Hydrologic transport models are used to mathematically analyze movement of TDS within river systems. The most common models address surface runoff, allowing variation in land use type, [[Earth:Topography|topography]], [[Chemistry:Soil|soil]] type, [[Biology:Vegetation|vegetative]] cover, precipitation, and land management practice (e.g. the application rate of a [[Chemistry:Fertilizer|fertilizer]]). Runoff models have evolved to a good degree of accuracy and permit the evaluation of alternative land management practices upon impacts to stream water quality.

Basin models are used to more comprehensively evaluate total dissolved solids within a catchment basin and dynamically along various stream reaches. The DSSAM model was developed by the U.S. Environmental Protection Agency (EPA).<ref name="hogan-papineau">C.M. Hogan, Marc Papineau et al. ''Development of a dynamic water quality simulation model for the Truckee River'', Earth Metrics Inc., Environmental Protection Agency Technology Series, Washington D.C. (1987)</ref> This hydrology transport model is actually based upon the pollutant-loading metric called "Total Maximum Daily Load" (TMDL), which addresses TDS and other specific chemical pollutants. The success of this model contributed to the Agency's broadened commitment to the use of the underlying TMDL protocol in its national policy for management of many river systems in the United States.<ref>EPA. [http://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=00001KIO.txt "Guidance for Water Quality-Based Decisions: The TMDL Process."] {{Webarchive|url=https://web.archive.org/web/20170706075111/https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=00001KIO.txt |date=2017-07-06 }} Doc. No. EPA 440/4-91-001. April 1991.</ref>

==Practical implications==
[[Image:bristol.zoo.aquarium.arp.jpg|thumb|right|Aquarium at Bristol Zoo, England. Maintenance of filters becomes costly with high TDS.]]
When measuring water treated with water softeners, high levels of total dissolved solids do not correlate to hard water, as water softeners do not reduce TDS; rather, they replace magnesium and calcium ions, which cause hard water, with an equal charge of sodium or potassium ions, e.g. Ca<sup>2+</sup> ⇌ 2&nbsp;Na<sup>+</sup>, leaving overall TDS unchanged<ref>{{cite web |last1=W. Adam Sigler |first1=Jim Bauder |title=TDS Fact Sheet |url=http://waterquality.montana.edu/docs/homeowners/tds_fact_sheet.shtml |publisher=Montana State University |access-date=23 January 2015 |archive-url=https://web.archive.org/web/20150429145631/http://waterquality.montana.edu/docs/homeowners/tds_fact_sheet.shtml |archive-date=2015-04-29}}</ref> or even increased. [[Physics:Hard water|Hard water]] can cause scale buildup in pipes, [[Engineering:Valve|valve]]s, and filters, reducing performance and adding to system maintenance costs. These effects can be seen in aquariums, spas, [[Earth:Swimming pool|swimming pool]]s, and [[Chemistry:Reverse osmosis|reverse osmosis]] [[Earth:Water treatment|water treatment]] systems. Typically, in these applications, total dissolved solids are tested frequently, and filtration membranes are checked in order to prevent adverse effects.

In the case of hydroponics and aquaculture, TDS is often monitored in order to create a water quality environment favorable for organism productivity. For freshwater [[Biology:Oyster|oyster]]s, [[Biology:Trout|trout]]s, and other high value seafood, highest productivity and economic returns are achieved by mimicking the TDS and [[Chemistry:PH|pH]] levels of each [[Biology:Species|species]]' native environment. For hydroponic uses, total dissolved solids is considered one of the best indices of nutrient availability for the aquatic plants being grown.

Because the threshold of acceptable aesthetic criteria for human drinking water is 500&nbsp;mg/L, there is no general concern for [[Chemistry:Odor|odor]], taste, and [[Color|color]] at a level much lower than is required for harm. A number of studies have been conducted and indicate various species' reactions range from intolerance to outright toxicity due to elevated TDS. The numerical results must be interpreted cautiously, as true toxicity outcomes will relate to specific chemical constituents. Nevertheless, some numerical information is a useful guide to the nature of risks in exposing aquatic organisms or terrestrial animals to high TDS levels. Most aquatic ecosystems involving mixed fish fauna can tolerate TDS levels of 1000&nbsp;mg/L.<ref>{{cite book|title=Water Quality: An Introduction |first=Claude E. |last=Boyd |publisher=Kluwer Academic Publishers Group |location=The Netherlands |year=1999 |isbn=0-7923-7853-9}}</ref>

[[Image:Daphnia magna01.jpg|thumb|left|''[[Biology:Daphnia|Daphnia magna]]'' with eggs]]

The [[Biology:Leuciscinae|fathead minnow]] (''Pimephales promelas''), for example, realizes an {{LD50}} concentration of 5600&nbsp;ppm based upon a 96-hour exposure. LD50 is the concentration required to produce a lethal effect on 50&nbsp;percent of the exposed [[Social:Population|population]]. ''[[Biology:Daphnia magna|Daphnia magna]]'', a good example of a primary member of the food chain, is a small [[Earth:Plankton|plankton]]ic [[Biology:Crustacean|crustacean]], about 0.5&nbsp;mm in length, having an LD50 of about 10,000&nbsp;ppm TDS for a 96-hour exposure.<ref>''Position Paper on Total Dissolved Solids'', State of Iowa, IAC 567 61.3 (2)g et sequitur updated March 27, 2003</ref>

[[Biology:Spawn|Spawning]] [[Biology:Fish|fish]]es and juveniles appear to be more sensitive to high TDS levels. For example, it was found that concentrations of 350&nbsp;mg/L TDS reduced spawning of [[Biology:Striped bass|Striped bass]] (''Morone saxatilis'') in the San Francisco Bay-Delta region, and that concentrations below 200&nbsp;mg/L promoted even healthier spawning conditions.<ref>Kaiser Engineers, California, ''Final Report to the State of California, San Francisco Bay-Delta Water Quality Control Program'', State of California, Sacramento, CA (1969)</ref> In the Truckee River, EPA found that juvenile [[Biology:Lahontan cutthroat trout|Lahontan cutthroat trout]] were subject to higher mortality when exposed to [[Physics:Thermal pollution|thermal pollution]] stress combined with high total dissolved solids concentrations.<ref name="hogan-papineau"/>

For terrestrial animals, poultry typically possess a safe upper limit of TDS exposure of approximately 2900&nbsp;mg/L, whereas dairy cattle are measured to have a safe upper limit of about 7100&nbsp;mg/L. Research has shown that exposure to TDS is compounded in toxicity when other [[Biology:Stressor|stressor]]s are present, such as abnormal pH, high [[Biology:Turbidity|turbidity]], or reduced dissolved oxygen with the latter stressor acting only in the case of Animalia.<ref>{{cite journal |url=http://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=9101UVQ5.txt |title=Statistical Prediction of Dynamic Thermal Equilibrium Temperatures using Standard Meteorological Data Bases |publisher=EPA |last1=Hogan |first1=C. Michael |last2=Patmore |first2=Leda C. |last3=Seidman |first3=Harry |date=August 1973 |access-date=2016-02-15 |archive-date=2016-02-23 |archive-url=https://web.archive.org/web/20160223135442/http://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=9101UVQ5.txt |url-status=live }} Environmental Protection Technology Series. Document No. EPA-660/2-73-003.</ref>
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In countries with often-unsafe/unclean tap water supplies, the TDS of drinking water is often checked by technicians to gauge how effectively their RO/Water Filtration devices are working. While TDS readings will not give an answer as to the amount of microorganisms present in a sample of water, they can get a good idea as to the efficiency of the filter by how much TDS is present.

== Water classification ==
<ref>{{Cite web|url=https://www.usgs.gov/special-topic/water-science-school/science/saline-water-and-salinity?qt-science_center_objects=0#qt-science_center_objects|title=Saline Water and Salinity &#124; U.S. Geological Survey|access-date=2020-02-12|archive-date=2020-08-18|archive-url=https://web.archive.org/web/20200818104859/https://www.usgs.gov/special-topic/water-science-school/science/saline-water-and-salinity?qt-science_center_objects=0#qt-science_center_objects|url-status=live}}</ref>
Water can be classified by the level of total dissolved solids (TDS) in the water:
*[[Earth:Fresh water|Fresh water]]: TDS is less than 1,000&nbsp;ppm
*[[Earth:Brackish water|Brackish water]]: TDS = 1,000 to 10,000&nbsp;ppm
*[[Chemistry:Saline water|Saline water]]: TDS = 10,000 to 35,000&nbsp;ppm
*Hypersaline: TDS greater than 35,000&nbsp;ppm
Drinking water generally has a TDS below 500&nbsp;ppm.  Higher TDS Fresh Water is drinkable but taste may be objectionable.

==See also==
* [[Earth:Acid rain|Acid rain]]
* [[Earth:Surface runoff|Surface runoff]]
*Regarding meters:
**EC meter
**[[Chemistry:PH meter|pH meter]]
**[[Engineering:Salinometer|Salinometer]]

==References==
{{Reflist|2}}

==External links==

{{Wastewater}}
{{Chemical solutions}}

[[Category:Environmental chemistry]]
[[Category:Water pollution]]

{{Sourceattribution|Total dissolved solids}}