Total dissolved solids

Total dissolved solids (often abbreviated TDS) is an expression for the combined content of all inorganic and organic substances contained in a liquid which are present in a molecular, ionized or micro-granular (colloidal sol) suspended form. Generally the operational definition is that the solids must be small enough to survive filtration through a sieve size of two micrometres. Total dissolved solids are normally only discussed for freshwater systems, since salinity comprises some of the ions constituting the definition of TDS. The principal application of TDS is in the study of water quality for streams, rivers and lakes, although TDS is generally considered not as a primary pollutant (e.g. it is not deemed to be associated with health effects), but it is rather used as an indication of aesthetic characteristics of drinking water and as an aggregate indicator of presence of a broad array of chemical contaminants.

Primary sources for TDS in receiving waters are agricultural runoff, leaching of soil contamination and point source water pollution discharge from industrial or sewage treatment plants. The most common chemical constituents are calcium, phosphates, nitrates, sodium, potassium and chloride, which are found in nutrient runoff, general stormwater runoff and runoff from snowy climates where road de-icing salts are applied. The chemicals may be cations, anions, molecules or agglomerations on the order of 1000 or fewer molecules, so long as a soluble micro-granule is formed. More exotic and harmful elements of TDS are pesticides arising from 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 mg/l to provide for palatability of drinking water.

Total dissolved solids are differentiated from total suspended solids (TSS), in that the latter cannot pass through a sieve of two micrometres 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 exclude both TDS and TSS.^[1] Settleable solids may include larger particulate matter or insoluble molecules.

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Measurement of TDS

The two principal methods of measuring total dissolved solids are gravimetry and electrical conductivity. Gravimetric methods are the most accurate and involve evaporating the liquid solvent to leave a residue which can subsequently be weighed with a precision analytical balance (normally capable of .0001 gram accuracy). This method is generally the best, although it is time consuming and leads to inaccuracies if a high proportion of the TDS consists of low boiling point organic chemicals, which will evaporate along with the water. In the most common circumstances inorganic salts comprise the great majority of TDS, and gravimetric methods are appropriate.

Electrical conductivity of water is directly related to the concentration of dissolved ionized solids in the water. Ions from the dissolved solids in water create the ability for that water to conduct an electrical current, which can be measured using a conventional conductivity meter. When correlated with laboratory TDS measurements, electrical conductivity provides an approximate value for the TDS concentration, usually to within ten percent accuracy.

Hydrological simulation

See also: Hydrological transport model

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, topography, soil type, vegetative cover, precipitation and land management practice (such as the application rate of a 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.^[2] 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 Environmental Protection Agency’s broadened commitment to the use of the underlying TDML protocol in its national policy for management of many river systems in the United States.^[3]

Practical implications

  High TDS levels generally indicate hard water, which can cause scale buildup in pipes, valves and filters, reducing performance and adding to system maintenance costs. These effects can be seen in aquariums, spas, swimming pools and reverse osmosis water treatment systems. Typically, in these applications, total dissolved solids are tested frequently and filtration membranes 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 which is favorable for organism productivity. For freshwater oysters, trouts and other high value seafood, highest productivity and economic returns are achieved by mimicking the TDS and pH levels of each 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.

Toxicity issues

Since the threshold of acceptable aesthetic criteria for human drinking water is 500 mg/l, there is no general concern for cancer as most humans will reject consuming drinking water due to odor, taste and color at a level much lower than is required for harm. However, in the outdoor environment, aquatic species as well as terrestrial animals may be unwillingly exposed to high TDS levels from human interference (and, rarely, natural occurrences). A number of studies have been conducted and indicate various species' reactions range from intolerance to outright toxicity due to elevated TDS. Obviously, the numerical results must be interpreted cautiously, since 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 mg/l.^[4]

The Fathead minnow (Pimephales promelas), for example, realizes an LD50 concentration of 5600 ppm based upon a 96 hour exposure. LD50 is the concentration required to produce a lethal effect on 50 percent of the exposed population. Daphnia magna, a good example of a primary member of the food chain, is a small planktonic crustacean, about five millimeters in length, having an LD50 of about 10,000 ppm TDS for a 96 hour exposure.^[5]

Spawning fishes and juveniles appear to be more sensitive to high TDS levels. For example, it was found that concentrations of 350 mg/l TDS reduced spawning of Striped bass (Morone saxatilis) in the San Francisco Bay-Delta region, and that concentrations below 200 mg/l promoted even healthier spawning conditions.^[6] In the Truckee River, the EPA found that juvenile Lahontan cutthroat trout were subject to higher mortality when exposed to thermal pollution stress combined with high total dissolved solids concentrations.^[2]

For terrestrial animals, poultry typically possess a safe upper limit of TDS exposure of approximately 2900 mg/l, while dairy cattle are measured to have a safe upper limit of about 7100 mg/l. Research has shown that exposure to TDS is compounded in toxicity when other stressors are present, such as abnormal pH, high turbidity or reduced dissolved oxygen with the latter stressor acting only in the case of animalia.^[7]

References

See also

• Acid rain
• Surface runoff