Halides
Halides are anion forms of halogen atoms, which are located in Group 7 of the periodic table. Common halides found in natural water sources include fluoride, chloride, and bromide. Halides exist in natural water sources, such as rivers, lakes, and streams, due to their high solubility in water. Sources of halides include the natural bedrock of running water, and industrial runoff. For example, increased chloride levels in urban areas are almost exclusively caused by runoff from the deicing of roadways.
Fluoride is strongly attracted to soil particles, and can be dissolved into running water sources from nearby soil samples. Aquatic animals can be exposed directly to fluoride in natural water sources, while terrestrial animals are also at risk due consumption of vegetation containing high fluoride levels. High fluoride concentrations are harmful to all organisms, resulting in stunted growth, bone degradation, birth defects in animals, and low crop yields. The Environmental Protection Agency (EPA) has set 2 parts per million (ppm) as the maximum acceptable concentration of fluoride in drinking water.
Chloride is found in natural waterways from inorganic chloride containing salts, such as sodium chloride (table salt), and chlorinated tap water. They may also be introduced through drainage, precipitation, runoff, soil and clay sources, pollution and waste disposal. Once these ions come into contact with natural water, it is possible and likely to form chloroform and other carcinogens by reacting with naturally-occurring compounds. Due to its high reactivity, it also contributes to the corrosion of many metals, including man-made structures, such as dams. The EPA standard for chloride concentration in drinking water is 250 ppm.
Bromides exist naturally as inorganic salts, both in mines and in natural water sources. Industrial dumping and runoff can contribute to increased levels. High bromide concentrations can have lethal effects on aquatic organisms such as flatworms, snails, shrimp, and minnows. Bromide can also be absorbed by plants, with adverse effects such as stunted growth and poor germination.
These analytes can be tested for in natural water samples by a variety of laboratory instruments, including ion chromatography and the use of ion selective electrodes. Most freshwater sources tend to reflect the halogen concentration due to the local precipitation, but likely have elevated levels if human activity occurs nearby. Freshwater lakes and ponds typically contain 100-300 mg/L of chloride, while streams contain less than 100 mg/L. The concentration of chloride in residential and urban areas fluctuates greatly, with concentrations ranging from 200 to 700 mg/L being common. The importance of bromide is in combination with chloride as a Cl-/Br- ratio which can be used to identify the history of freshwater groundwater systems based on the pollution present. This is done because the ratio is conserved in potable groundwater until a source of water with another ratio is added. This allows researchers to follow the source of pollution to an origin point in groundwater systems.
References
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Content in this section authored by Ben Winoker and Luke Vista, Wilson Lo, Tyler Weaver & Andrew Zurlinden