Total Dissolved Solids, Specific Conductance, and Salinity

5.2.1 Total Dissolved Solids, Specific Conductance, and Salinity

Overall, water is the most effective solvent of geologic materials and other environmental substances—solid, liquid, and gaseous. This quality of water is the result of a unique structure of its molecule, which is a dipole—the centers of gravity and electric charges in the water molecule are asymmetric. The polarity of molecules, in general, is quantitatively expressed with the dipole moment, which is the product of the electric charge and the −

distance between the electric centers. Dipole moment for water is 6.17 × 10 30 Cm (kulon- meters), higher than for any other substance, and explains why water can dissolve more

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solids and liquids than any other liquid. Dissolution of rocks by water plays the main role in continuous redistribution of geologic materials in the environment, at and below the land surface.

Substances subject to dissolution by water (or any other liquid) are called solutes. Some substances are more soluble in water than others. Ionized mineral salts, such as sodium chloride, are very easily and quickly dissolved in water by its dipolar molecules. Synthetic organic substances with polarized molecules, such as methanol, are also highly soluble in water: hydrogen bonds between water and methanol molecules can readily replace the very similar hydrogen bonds between different methanol molecules and different water molecules. Methanol is, therefore, said to be miscible in water (its solubility in water is infinite for practical purposes). On the other hand, many nonpolar organic molecules, such as benzene and trichloroethylene (TCE) for example, have low water solubility.

True solutes are in the state of separated molecules and ions, all of which have very − small dimensions (commonly between 10 6 and 10 − 8 cm), thus making a water solution transparent to light. Colloidal solutions have solid particles and groups of molecules that are larger than the ions and molecules of the solvent (water). When colloidal particles are present in large enough quantities, they give water an opalescent appearance by scattering light. Although there is no one-agreed-to definition of what exactly colloidal − −

sizes are, a common range cited is between 10 6 and 10 4 cm (Matthess, 1982). The amount of a solute in water is expressed in terms of its concentration, usually in milligrams per liter (mg/L or parts per million—ppm) and micrograms per liter (parts per billion—ppb). It is sometimes difficult to distinguish between certain true solutes and colloidal solutions that may carry particles of the same source substance. Filtering and/or precipitating colloidal particles before determining the true dissolved concentration of a solute may

be necessary in some cases. This is especially true for drinking water standards because these, for most substances, are based on dissolved concentrations. Laboratory analytical procedures are commonly designed to determine total concentrations of a substance and do not necessarily provide indication of all the individual species (chemical forms) of it. If needed, however, such speciation can be requested. For example, determination of individual chromium species, rather than the total chromium concentration, may be important in groundwater contamination studies, since hexavalent chromium or Cr(VI) is more toxic and has different mobility than trivalent chromium, Cr(III).

The total concentration of dissolved material in groundwater is called total dissolved solids (TDS). It is commonly determined by weighing the dry residue after heating the

water sample usually to 103 ◦ C or 180

C (the higher temperature is used to eliminate more of the crystallization water). TDS can also be calculated if the concentrations of major ions are known. However, for some water types, a rather extensive list of analytes may be needed to accurately obtain the total. During evaporation, approximately one- half of the hydrogen carbonate ions are precipitated as carbonates and the other half escapes as water and carbon dioxide. This loss is taken into account by adding half of the −

HCO 3 content to the evaporation (dry) residue. Some other losses, such as precipitation of sulfate as gypsum and partial volatilization of acids, nitrogen, boron, and organic substances, may contribute to a discrepancy between the calculated and the measured TDS.

Solids and liquids that dissolve in water can be divided into electrolytes and non- electrolytes. Electrolytes, such as salts, bases, and acids, dissociate into ionic forms (posi- tively and negatively charged ions) and conduct electrical current. Nonelectrolytes, such as sugar, alcohols, and many organic substances, occur in aqueous solution as uncharged

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molecules and do not conduct electrical current. The ability of 1 cm 3 of water to conduct electrical current is called specific conductance (or sometimes simply conductance, although the units are different). Conductance is the reciprocal of resistance and is measured in units called Siemen (International System) or mho (1 Siemen equals 1 mho; the name mho is derived from the unit for resistance—ohm, by spelling it in reverse). Specific conductance is expressed as Siemen/cm or mho/cm. Since the mho is usually too large for most ground- water types, the specific conductance is reported in micromhos/cm or microSiemens/cm (μS/cm), with instrument readings adjusted to 25 ◦

C, so that variations in conductance are only a function of the concentration and type of dissolved constituents present (water temperature also has a significant influence on conductance). Measurements of specific conductance can be made rapidly in the field with a portable instrument, which pro- vides for a convenient method to quickly estimate TDS and compare general types of water quality. For a preliminary (rough) estimate of TDS, in milligrams per liter, in fresh potable water, the specific conductance in micromhos/cm can be multiplied by 0.7. Pure ◦ water has a conductance of 0.055 micromhos at 25

C, laboratory distilled water between

0.5 and 5 micromhos, rainwater usually between 5 and 30 micromhos, potable ground- water ranges from 30 to 2000 micromhos, sea water from 45,000 to 55,000 micromhos, and oil field brines have commonly more than 100,000 micromhos (Davis and DeWiest, 1991).

The term salinity is often used for total dissolved salts (ionic species) in groundwater, in the context of water quality for agricultural uses or human and livestock consumption. Various salinity classifications, based on certain salts and their ratios, have been proposed (see Matthess, 1982). One problem with the term salinity is that a salty taste may be already noticeable at somewhat higher concentrations of sodium chloride, NaCl (e.g., 300 to 400 mg/L), even though the overall concentration of all dissolved salts may not “qualify”

a particular groundwater to be called “saline.” In practice, it is common to call water with less than 1000 mg/L (1 g/L) dissolved solids fresh, and water with more than 10,000 mg/L saline.