11 Halogens, Sulfur, Metals, and Metalloids specific trace elements. A thorough, updated review of the scientific literature is recommended for project design. A number of substances are considered measures of water quality, but are seldom of concern as pollutants to be treated in constructed wetlands. These include common metals (., sodium, potassium, calcium, and magnesium) as well as halogens (., fluorine, chlorine, bromine, and iodine). Together with sulfate, these compounds often dominate the total ion content of natural waters and wastewaters. In total, they form the major part of EC and TDS. Sulfate is of special importance, because of its active biogeochemical. | 11 Halogens Sulfur Metals and Metalloids In addition to the pollutants discussed in earlier chapters wastewaters typically contain many other substances. Some of these elements can cause problems when discharged to receiving waters and their removal must be considered during design. These additional materials include salts acids bases macronutrients micronutrients and heavy metals and may be categorized in a number of ways. Salts include compounds that readily dissociate in water to form charged ions that may or may not be used as nutrients for plant and animal growth. Common examples of salts are sodium chloride NaCl and gypsum CaSO4 . Acids release a hydrogen ion when they dissociate . hydrochloric acid HCl and bases release a hydroxyl ion . ferric hydroxide Fe OH 3 . Specific environmental conditions determine whether the cations positively charged ions and anions negatively charged ions formed when a salt acid or base is dissolved in water are chemically or biologically active. Collectively ionic materials contribute to the electrical conductivity EC of the water. When ionic materials are combined with dissolved nonionic materials the result is the total dissolved solids TDS content of the water. Nitrogen and phosphorus discussed in Chapters 9 and 10 are examples of macronutrients which have strong biogeochemical cycles in a wetland. Sulfur also is typically present in variable but potentially high concentrations and has just as powerful influences on wetland functioning. The magnitude of these influences is just emerging as a controlling factor on wetland performance for a number of other pollutants. Most obvious is the role of sulfides in immobilizing trace metals. Iron aluminum and manganese are ubiquitous in wetlands but are present at elevated concentrations in mine drainage waters and the wetlands constructed to treat them. A trace metal can be either a required micronutrient or toxic depending on the concentration. For example copper and zinc are .
