Mercury (\(\text{Hg}\)) is a naturally occurring heavy metal that has become a widespread environmental pollutant, posing a significant health risk primarily through the consumption of contaminated fish. Its contamination in aquatic systems is a complex issue, originating from both Earth’s natural cycles and substantial human industrial activity. Understanding these pathways is crucial because, once in the water, mercury can be converted into a form that is highly toxic to humans and wildlife.
Primary Global Sources of Mercury Release
The mercury that enters the global environment originates from natural processes and, increasingly, from human activities, which have significantly amplified the amount cycling in the biosphere. Natural sources include volcanic eruptions, geothermal vents, and the slow weathering of mercury-containing minerals in the Earth’s crust. These geological processes contribute to the background levels of mercury in the atmosphere and on land.
However, anthropogenic, or human-caused, sources are now the dominant factor driving the increase in global mercury levels. The largest single industrial source is the combustion of fossil fuels, particularly coal, which releases mercury trapped within the fuel into the atmosphere. This process is responsible for a significant portion of manmade mercury emissions globally.
A second major anthropogenic source is artisanal and small-scale gold mining (ASGM). Miners use elemental mercury to separate gold from ore, and when the resulting gold-mercury amalgam is heated, the mercury vaporizes into a toxic plume. Other industrial activities, such as cement production, non-ferrous metal smelting, and certain chemical manufacturing processes, also contribute to the global mercury pool.
Atmospheric Transport and Global Deposition
Once mercury is released, the atmosphere acts as the primary vehicle for its global dispersal, often transporting it thousands of miles from its source. When emitted from sources like smokestacks or mining operations, mercury is often in the form of elemental mercury vapor (\(\text{Hg}^0\)). This form is relatively inert and can remain in the atmosphere for up to a year, allowing for extensive, long-range transport across continents.
During its atmospheric journey, the elemental mercury vapor undergoes oxidation, converting it into a water-soluble form called ionic mercury (\(\text{Hg}^{2+}\)). This oxidized form is highly reactive and is removed from the atmosphere relatively quickly through atmospheric deposition.
Wet deposition involves the scavenging of ionic mercury by rain, snow, or fog, which then carries the dissolved metal onto land or directly into water bodies. Dry deposition occurs when gaseous ionic mercury or mercury-containing particulate matter settles directly onto surfaces. This global cycling means that mercury released from an industrial source in one region can eventually contaminate a remote lake or ocean in another region.
Direct Water Entry and Localized Runoff
While atmospheric deposition is the main source of mercury for many aquatic systems globally, localized activities also provide direct pathways for the metal to enter the water. Immediate industrial discharge, such as wastewater from certain manufacturing plants, can release mercury directly into nearby rivers or coastal waters. This direct entry bypasses the long-range atmospheric cycle and creates concentrated contamination zones.
Another significant localized source is the runoff from contaminated land, carrying mercury into streams and lakes. This includes areas near historic mining operations where mercury was used or where mercury-containing minerals were processed. Rain and snowmelt water can leach mercury from these contaminated soils and carry it into the aquatic system.
The improper disposal of mercury-containing products, such as old thermometers, fluorescent lights, or batteries, also contributes to localized water contamination. When these items are placed in landfills, the mercury can leach out and enter groundwater or surface water through discharge. These direct pathways introduce inorganic mercury into the aquatic environment, where it is available for further transformation.
The Aquatic Transformation: Methylation and Bioaccumulation
Once inorganic mercury (\(\text{Hg}^{2+}\)) enters a water body, it becomes susceptible to a chemical process called methylation. This transformation is carried out by specific types of anaerobic bacteria, such as sulfate-reducing bacteria, which chemically convert the inorganic form into methylmercury (\(\text{MeHg}\)). Methylmercury is the form that poses the greatest risk to human and ecosystem health, as it is a potent neurotoxin.
Methylmercury is readily absorbed by aquatic organisms, marking the beginning of a process known as bioaccumulation. Organisms like phytoplankton and zooplankton take up the methylmercury from the water or sediment. As smaller organisms are consumed by larger ones, the concentration of methylmercury in the tissue of the predator increases, a process called biomagnification.
This concentration effect is why large, long-lived predatory fish, such as tuna and swordfish, accumulate methylmercury levels significantly higher than the surrounding water. The consumption of these fish is the primary route of human exposure worldwide. Because methylmercury easily crosses the blood-brain barrier, it is particularly harmful to the developing nervous systems of fetuses and young children.