Aquatic Pollution and Coastal Management

Unit 5 : Heavy metal pollution

5.5.1.Mercury
Mercury is a global pollutant found in the environment in elemental, organic and inorganic forms with varying degrees of toxicity. Most of the mercury in the atmosphere is elemental vapour and inorganic mercury, while the mercury in water, soil and living organisms is primarily organic mercury (methyl mercury).
Mercury occurs naturally in the environment form the degassing of earths crust and volcanic emissions.

Anthropogenic Sources of Mercury include:
Mining and smelting operations, chloralkali plants, paper and pulp industries, leather tanning, electroplating, chemical manufacturing, application of fertilizers and pesticides, disposal of solid waste including batteries and thermometers.
Waste incineration and fossil fuel combustion.

Fate and Transport of Mercury:
The global cycling of mercury is a complex process. Mercury evaporates from soils and surface waters to the atmosphere, is redeposited on land and surface waters and then is absorbed by soil or sediments.
After redeposition on land and water, mercury is commonly volatilized back to the atmosphere as a gas or as adherents to particles. Atmospheric mercury is dispersed across the globe by winds and return to the earth in rainfall. The long atmospheric lifetime of one year of its gaseous form means the emission, transport and deposition of mercury are a global issue. Once released into the environment, inorganic mercury is converted to organic mercury (methyl mercury) which is the primary form that accumulates in fish and shellfish, biomagnifies up the food chain as it is passed from lower food chain level to a subsequently higher food chain level through consumption of prey organisms or predators. Fish at the top of the aquatic food chain (predation) bioaccumulate methyl mercury approximaltely 1,000 – 10,000 times greater than found in surrounding waters.

Effects:
In aquatic organism, mercury adversely affects reproduction, growth, behavior, osmoregulation and oxygen exchange. Mercury also causes kidney lesions, reduced food intake leading to weight loss and progressive weakness in wings and legs in marine birds.

Public Health:
Potential sources of human exposure to mercury include food contamination with mercury, inhalation of mercury vapours in ambient air and exposure to mercury through dental and medical treatments. Dietary intake is by far the dominant source of exposure to mercury for the general population.
Acute toxicity to methyl mercury may result in impaired central nervous system function, kidney damage and failure, gastro-intestinal damage, cardiovascular collapse, shock and death.
Two Major Episodes:

• Early 1950s (1952) – The first incidence of mercury poisoning appeared in the population of Minimata Bay, Japan – effluents from chemical factory.
• 1965 – Occurred near Nigata, Japan