Secondary Constituents

5.2.5 Secondary Constituents

Secondary constituents of importance for most natural groundwaters of drinking water quality include metals, fluoride, and organic matter. The term heavy metals (or trace metals) is applied to the group of metals and semimetals (metalloids) that have been associated with contamination and potential toxicity or ecotoxicity; it usually refers to common metals such as copper, lead, or zinc. Some define a heavy metal as a metal with an atomic mass greater than that of sodium, whereas others define it as a metal with

a density above 3.5 to 6 g/cm 3 . The term is also applied to semimetals (elements such as arsenic, which have the physical appearance and properties of a metal but behave chemically like a nonmetal) presumably because of the hidden assumption that “heav- iness” and “toxicity” are in some way identical. Despite the fact that the term heavy metal has no sound terminological or scientific basis, it has been widely used in scientific environmental literature (van der Perk, 2006). Heavy metals commonly found in natural fresh groundwater include zinc, copper, lead, cadmium, mercury, chromium, nickel, and arsenic.

Heavy metals occur naturally as part of many primary and secondary minerals in all types of rocks. In natural waters, they are present mainly at low concentrations (usually much less than 0.1 mg/L), and as cations, although some semimetals such as arsenic

may occur as oxyanions (e.g., arsenate AsO 3− 4 ). Their generally low concentrations in groundwater are due to the high affinity of heavy metals to adsorption and precipita- tion in soils and aquifer porous media. The maximum natural concentrations of heavy metals are usually associated with ore deposits and oxidized, low-pH water. In general,

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many solids control the fixation (immobilization) of heavy metals, namely, clay minerals, organic matter, iron, manganese, and aluminum oxides and hydroxides for adsorption, and poorly soluble sulfide, carbonate, and phosphate minerals for precipitation (Bourg and Loch, 1995; from van der Perk, 2006). The pH of groundwater is the most important factor controlling the fate and transport of heavy metals in the subsurface. In general, decreasing pH results in higher mobility of heavy metals and vice versa. More on general characteristics, hydrochemistry, and mobility of heavy metals in the subsurface can be found in Bourg and Loch (1995), Appelo and Postma (2005), and van der Perk (2006).

Arsenic has emerged as one of the most widespread natural contaminants in ground- water in various regions of the world. Since it can occur both naturally and as a result of anthropogenic contamination, arsenic is covered in more detail later as a groundwater contaminant.

The element fluorine is used by higher life forms in the structure of bones and teeth. The importance of fluoride; its anion, in forming human teeth; and the role of fluoride intake from drinking water in controlling the characteristics of tooth structure was rec- ognized during the 1930s (Hem, 1989). Since that time the fluoride content of natural water has been studied extensively. Although intake of fluoride is necessary for promot- ing strong healthy teeth, at high concentrations it may cause bone disease and mottled teeth in children (MCL for fluoride in the United States is 4 mg/L). Although fluoride concentrations in most natural waters are small, less than 1 mg/L, groundwater exceed- ing this value has been found in many places in the United States, in a wide variety of geologic terrains (Hem, 1989). Fluorite and apatite are common fluoride minerals in magmatic and sedimentary rocks, and amphiboles and micas may contain fluoride that replaces part of the hydroxide. Rocks rich in alkali metals have a higher fluoride content than most other magmatic rocks. Fresh volcanic ash may be rather rich in fluoride, and ash that is interbedded with other sediments could contribute significantly to fluoride concentrations. Fluoride is commonly associated with volcanic or fumarolic gases, and, in some areas, these may be important sources of fluoride in groundwater (Hem, 1989). Fluorine is the most electronegative of all the elements, and its F − ion forms strong solute complexes with many cations, particularly with aluminum, beryllium, and ferric iron. Anthropogenic sources of fluoride include fertilizers and discharge from ore-processing and smelting operations, such as aluminum works.

In addition to inorganic (mineral) substances, groundwater always contains natural organic substances, and almost always some living microorganisms (mainly bacteria), even at depths of up to 3.5 km in some locations (Krumholz, 2000). Organic matter in surface and groundwater is a diverse mixture of organic compounds ranging from macromolecules to low-molecular-weight compounds such as simple organic acids and short-chained hydrocarbons. In groundwater, there are three main natural sources of organic matter: organic matter deposits such as buried peat, kerogen, and coal; soil and sediment organic matter; and organic matter present in waters infiltrating into the subsurface from rivers, lakes, and marine systems (Aiken, 2002). Various components of naturally occurring hydrocarbons (oil and gas) and their breakdown products formed by microbial activity are a significant part of groundwater chemical composition in many areas throughout the world. A very large number of artificial organic chemicals have become part of groundwater reality in recent years because, if analyzed using the latest available analytical methods, they are often detected.

Organic matter in groundwater plays an important role in controlling geochemical processes by acting as proton and electron donors-acceptors and pH buffers, by affecting

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the transport and degradation of pollutants and by participating in mineral dissolution and precipitation reactions. Dissolved and particulate organic matter may also influence the availability of nutrients and serve as a carbon substrate for microbially mediated reactions. Numerous studies have recognized the importance of natural organic matter in the mobilization of hydrophobic (“water-hating”) organic species, heavy metals, and radionuclides. Many contaminants that are commonly regarded as virtually immobile in aqueous systems can interact with dissolved organic carbon (DOC) or colloidal or- ganic matter, resulting in migration of hydrophobic chemicals far beyond the distances predicted by the structure and activity relationships (Aiken, 2002).

A number of significant, although poorly understood, mechanisms can be responsible for the transport or retention of organic molecules in the subsurface. Once in the system, organic compounds, whether of anthropogenic or natural origin, can be truly dissolved, associated with immobile or mobile particles. Mobile particles include DOC, DOC-iron complexes, and colloids. Positively charged organic solutes are readily removed from the dissolved phase by cation exchange, which can be a significant sorption mechanism. Organic solutes that may exist as cations in natural waters include amino acids and polypeptides. Hydrophilic neutral (e.g., carbohydrates and alcohols) and low-molecular- weight anionic organic compounds (e.g., organic acids) are retained the least by aquifer solids. Hydrophobic synthetic organic compounds interact strongly with the organic matter associated with the solid phase of porous media. These interactions are controlled, in part, by the nature of the organic coatings on solid particles, especially with respect to its polarity and aromatic carbon content. Interactions of hydrophobic organic compounds with stationary particles can result in strong binding and slow release rates of these compounds (Aiken, 2002).