Page 149 - Soil and water contamination, 2nd edition

136 Soil and Water Contamination

mobility of mercury(II) in soil and water is largely controlled by adsorption reactions with
fixed or mobile adsorbents.
Soil mercury is poorly available to plants and there is a tendency for mercury to
accumulate in the roots, indicating that the roots serve as a barrier to mercury uptake. The
mercury concentration in aboveground parts of plants appears to largely depend on foliar
uptake of elementary mercury volatilised from the soil (Lindqvist et al., 1991). Uptake
of mercury is often plant-specific; organic mercury compounds are taken up in larger
amounts than elemental mercury. Factors affecting plant uptake include soil or sediment
organic content, cation exchange capacity , oxide and carbonate content, redox potential,
and the total metal content. Methyl mercury is lipophyllic and can build up in the fat of
certain fish. For this reason, low levels of mercury in lakes and rivers can contaminate these
fish.
Mercury can easily enter the human body if its vapour is breathed in or if it is eaten
in organic forms in contaminated fish or other foods. Mercury can also enter the body
when food or water contaminated with inorganic mercury is eaten or drunk. Mercury in
all forms may also enter the body directly through the skin. Once mercury has entered the
body, it may be months before all of it leaves. Long-term exposure to organic or inorganic
mercury can permanently damage the brain, kidneys, and developing foetuses (ATSDR,
2013). The form of mercury and the way humans are exposed to it determine which of these
health effects will be more severe. For example, organic methyl mercury is a neurotoxin and
may cause greater harm to the brain and developing foetuses than to the kidney, whereas
inorganic mercury ingested with contaminated food or water may cause greater harm to the
kidneys. Metallic mercury vapour that enters the body via the lungs may cause greater harm
to the brain (ATSDR, 2013).
In the mid 1950s, severe neurological disorders were diagnosed in the fishing population
living around the Minamata Bay in Japan. Mercury containing liquid wastes were discharged
into this bay, and the inorganic mercury was transformed into methyl mercury through the
action of bacteria in the bay sediments. The methyl mercury accumulated in the food chain
of fish in the bay, which resulted in fish catches containing high mercury concentrations.
The large daily fish consumption of the local population resulted in a daily intake of
approximately 2 mg mercury per day. This led to ataxia, limited range of vision, loss of
hearing, speaking disorders, trembling, stiffness, and psychological disorders in the affected
population (Copius Peereboom, 1976). According to the Japanese government, 2955 people
contracted this so-called Minamata disease, of whom 1784 died.
Mercury is introduced to the environment by natural and anthropogenic emissions. Natural
emissions of mercury form two-thirds of the input while man-made releases form about one-
third, although the amounts released from anthropogenic sources have fallen greatly since 1970.
Because mercury is rather volatile, it can be dispersed over great distances in the atmosphere.
The amounts escaping to the atmosphere in smelting and fossil fuel combustion have
probably enhanced the mercury levels in the environment above pre-industrial background
levels. In addition to the burning of fossil fuel, a major present-day source of mercury in
the western world is the production, consumption, and final waste disposal of materials
containing mercury. Metallic mercury is used in thermometers, barometers, cell batteries, and
other common consumer products. One of the most important sources of contamination in
agricultural soils used to be the use of organomercuric compounds as a seed coating to prevent
fungal diseases in germinating seeds. Together with several other agricultural applications of
mercury, this was banned in the 1960s. Important sources of water and sediment pollution by
mercury are its uses in electrolysis cells for the production of chlorine and sodium hydroxide
from sodium chloride brine, and as a catalyst in the production of some plastics. Another
source of mercury pollution of surface waters in its use in small-scale artisanal gold mining in
East Africa, the Philippines, and the Amazon region.

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