The amount of TDS is the sum of the concentrations of all substances dissolved in water. The term ‘‘salinity’’ as applied to fresh water is often used synonymously with TDS. Another term used to described water quality is total alkalinity, but this is not as good an indication of water quality as is TDS. Total alkalinity is the sum of the concentrations of alkali metals, which are primarily sodium and potassium, but may also include lithium, rubidium, cesium, and francium. The hydroxides of these metals are alkaline; i.e., in water they neutralize acids. The total alkalinity of water is always less than its TDS, or salinity, since TDS and salinity include the sum of the concentrations of all substances dissolved in water, and total alkalinity includes only the sum of the concentrations of alkali metals. Salinity and TDS should not be confused with hardness. Highly saline waters may contain low levels of the minerals responsible for hardness. Water ‘‘hardness’’ indicates the tendency of water to precipitate soap or to form a scale on heated surfaces. Hardness is generally expressed as the sum of calcium and magnesium reported in equivalent amounts of calcium carbonate. Other substances, such as strontium, iron, zinc, and manganese, also contribute to hardness.

Sodium, potassium, calcium, magnesium, iron, chloride, and sulfate in water are not toxic, but high concentrations decrease water palatability. In contrast, a number of other substances, which may be present in water, are quite toxic if sufficiently high concentrations are present. Toxic concentrations of water contaminants, excluding pesticides and herbicides, most commonly occur as a result of stagnant or runoff water that contains disease-producing organisms, or from industrial wastes. Some potentially toxic substances do not reduce water palatability and, therefore, water intake. Thus, they are potentially even more harmful than those that do decrease palatability. In addition to these contaminants, drinking water containing some bacteria and algae may be harmful.

Some species of blue-green algae, which grow on pond and lake water, may result in poisoning; therefore, water with heavy algae growth should be avoided. Heavy algae growth occurs most commonly during summer and fall in shallow, still water rich in organic nutrients. These nutrients may be increased, and thus algae growth promoted, by runoff of nitrogen or phosphate from slurry lagoons, or of fertilizers applied to fields. Steady prevailing winds may concentrate the algae at one end of the pond or lake, increasing the risk of poisoning. The algae may be visible on the water surface or mixed with the water.

Blue-green algae poisoning in domestic livestock may cause sudden death or else photosensitization, tremors, weakness, bloody diarrhea, and convulsions. Clumps of algae may be found in the gastrointestinal contents of animals that die suddenly. Copper sulfate added to pond water up to a concentration of 1 ppm (1 mg/L) has been used successfully to kill algae blooms, but will probably be harmful to other types of aquatic life.

Water high in bacteria is usually also high in nitrates as a result of surface contamination from manure and barnyard runoff. However, high nitrate water levels may come from other nitrate sources, such as crop fertilizers, and not be high in bacteria. Nitrates may build up in well water by leaching down through the soil. Water nitrate levels may fluctuate widely; they are generally highest following wet periods, and lowest during dry periods of the year. Since nitrates dissolve in water, they cannot be filtered out; however, commercially available anion exchange units remove both nitrates and sulfates. Nitrate toxicosis, however, is rare in horses, if it occurs at all, and in livestock is most often associated with high nitrate levels in forage, not water. Water sulfate concentrations exceeding 1000 ppm may cause diarrhea, although animals develop a tolerance to a constantly high level of sulfates and can tolerate two to three times this concentration after a period of time. Water with low levels of sulfates, however, may have an odor and reduced palatability.

In most areas and situations, bacteria in water pose a greater threat than the contaminants previously discussed. Most infectious diseases can be transmitted from contaminated water to animals. If water nitrate or phosphate concentrations are low, the water probably does not contain excessive bacteria. However, if either is high, bacterial levels may be elevated and should be checked. The accepted criterium for the sanitary quality of water is the absence of coliform bacteria. Although all coliform bacteria are not disease producing, many are, and their presence indicates that other infectious bacteria and viruses may be in the water. The U.S. Public Health Service considers water containing coliform bacteria (M.P.N.) of 9 or more coliforms per 100 ml unsafe for human consumption. In some countries, levels of 50 coliforms/100 ml are acceptable. What amount is safe for horses isn’t known but, of course, also depends on which organisms are present.

Salmonella species is generally the bacterial contaminant in water most likely to cause disease in farm animals. Giardia is the most common cause of water-related illness in people, partly because it survives chlorination. Although giardiasis is rare in farm animals, it can cause diarrhea in young animals. The most common method of destroying bacteria in a water supply is chlorination, although iodine, ozone, exposure to ultraviolet rays or ultrasonics, or filters may be used. Objectionable chlorine taste and odor can be removed from water by an activated carbon filter.

In summary, flowing surface water is most likely to have bacterial contamination, pond or lake water is most likely to contain blue-green algae, and well water, particularly in arid areas, is most likely to have high mineral concentrations. Coliform counts and measures of total dissolved solids are the main indications of water quality.

This article is from Feeding and Care of the Horse, second edition, by Lon D. Lewis, Lippincott Williams & Wilkins, 1995. Reprinted with permission from the publisher.

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