Halides are the stable anions of the highly reactive halogens: fluoride (F), chloride (Cl), bromine (Br), and iodine (I). Halides occur naturally in soils and are also present in many industrial waste streams.

Fluoride is present in phosphatic fertilizers, hydrogen fluoride, fluorinated hydrocarbons, and certain petroleum refinery waste. The leaching losses and mobility of fluoride can be large because of the anionic structure of fluoride and the solubility of some of its salt (Bemner and Shaw 1958). Sodium salts of fluoride (NaF) are soluble and result in high soluble fluoride levels in soils low in calcium. Calcium salts of fluoride (CaF2), however, are relatively insoluble and limit the amount of fluoride leached to groundwater. Fluoride solubility depends on the kind and relative quantity of cations present in soil that has formed salts with the fluoride ion (F—). Fluorosis disease can occur in animals who consume water containing 15 ppm of fluoride (Lee 1975).

Chloride (Cl) is present in chlorinated hydrocarbon production and chlorine gas production wastes as well as other wastes. Chloride is soluble and mobile in groundwater because of its anionic structure.

Bromide (Br) is present in synthetic organic dyes, mixed petrochemical wastes, photographic supplies, and pharmaceutical and inorganic wastes. Other forms of bromide such as bromate and bromic acid occur naturally in soils at smaller concentrations. Most bromide salts (CaBr, MgBr, NaBr, and Kbr) are soluble and readily leachable into water percolating through the soil and down to groundwater (U.S. EPA 1983).

Iodine (I) is present in pharmaceutical and chemical industrial wastes. Iodine is only slightly water soluble and tends to be retained in soils by forming complexes with organic matter and being fixed to phosphates and sulfates.

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