Mercury (with the chemical symbol Hg) is a naturally occurring element. It exists in nature in many different chemical forms. In its elemental state, pure mercury is a liquid at room temperature. It is not common for mercury to be in this pure liquid form in nature. Instead it is generally found dispersed at very low concentrations (e.g. parts per billion) in soils, sediments, natural waters and the atmosphere, as well as plants and animals. The chemical forms of mercury in the environment include adsorbed mercury species, organic (carbon-containing) and inorganic compounds, and mercury vapour.
Elemental or metallic mercury is a shiny, silver-white liquid metal. In this form mercury is expressed as Hg(0) or Hg0, where the 0 denotes the oxidation state of the mercury atom (in this case uncharged). Elemental mercury can evaporate (volatilise) forming mercury vapour when it is exposed to air. Rates of evaporation increase with increasing temperature. Mercury vapour is colourless, odourless, and very toxic.
The inhalation of vapour is the prime route of exposure to elemental mercury (but not other forms) for humans. Concentrations of mercury in ordinary ambient air are too low to pose a significant exposure risk for humans. However, in cases of occupational exposure to elemental mercury, the risk of poisoning can be high. Mercury vapours are readily absorbed into the human body through the lungs and (at high concentrations) can go on to affect the nervous system, causing neurological and behavioural disorders (UNEP, 2008a). Once in the body, elemental mercury can oxidise to inorganic mercury and be retained in body tissues, including the brain and kidneys, for some months.
Traditionally, elemental mercury has been used in thermometers and barometers, dental amalgams, some electrical switches, and as a preservative in some medical preparations. More recently it has been used in lamps, electronics, and skin lightening creams (UNEP, 2008a). Elemental mercury is the most common form of mercury in the atmosphere.
Inorganic mercury compounds are more commonly found in nature than the elemental form of mercury. Mercury is a natural component of the Earth’s crust and exists (at some level) in all rocks, soils and sediments, usually as an inorganic compound. A major source of mercury in soils is from the parent rocks and minerals that are weathered to form the soil.
Mercury is mined as the inorganic mineral cinnabar, which is mercuric sulphide (HgS). Other common inorganic mercury compounds include mercuric oxide (HgO) and mercuric chloride (HgCl2). The mercuric (doubly oxidised) form of mercury, for which there is a single mercury atom and an overall +2 charge (Hg(II)), readily forms salts, binding with negatively charged ions. This form of mercury also forms strong covalent (non-salt) bonds with sulphur, and organic matter, in the latter case leading to organic-mercury compounds of mercuric mercury (such as methylmercury). Some mercuric forms of mercury are soluble in water, such as HgCl2, but mercury oxide and mercury sulphide compounds are insoluble in water. Due to their insolubility in water, these two inorganic forms of mercury act as mercury ‘sinks’ in the environment and effectively remove mercury from cycling within the biosphere.
Mercurous (singly oxidised Hg(I)) compounds form another group of inorganic mercury compounds. The mercurous form is diatomic (i.e. two mercury atoms) with an overall +2 charge (Hg2(2+)).
Inorganic mercury compounds may be less efficiently absorbed by living organisms when compared to elemental mercury vapour, depending on the inorganic compound the mercury atom is bound to. In humans, the majority of inorganic mercury comes from ordinary dietary sources, and absorption of this type of mercury through the gastrointestinal tract is not particularly efficient. Inorganic mercury can be passed to babies via breast milk, and therefore mercury exposure is a major concern for mothers with small infants (UNEP, 2008a).
In additional to ordinary dietary sources, humans may be exposed to inorganic mercury via dental amalgams and skin lightening creams, from products that use inorganic mercury salts, or during certain industrial activities e.g. mining and concrete and steel manufacture (UNEP, 2008a). Inorganic mercury was historically used as a fungicide, but this has been discontinued in many countries, including New Zealand (UNEP, 2008a).
Organic mercury compounds are formed when the mercury atom or inorganic mercury compound is bound to a carbon molecule. The most common form, and the form of most concern in terms of toxicity, is methylmercury (CH3Hg+). Like inorganic mercury compounds, organic mercury compounds can also exist as salts, such as methylmercury chloride (CH3HgCl).
Organic mercury compounds are easily absorbed by fish in the aquatic environment (Hansen and Danscher, 1997). Once absorbed, these compounds can bioaccumulate in the body of fish (bioaccumulation is absorption of a substance by an organism at a greater rate than the organism can remove it). When smaller fish or other organisms containing organic mercury are consumed by a species higher up in the food chain, the concentration of mercury in the higher species (e.g. larger fish) increases, in a process called biomagnification. Of all the mercury forms, organic mercury poses the greatest risk to biota (living species). This is because methylmercury is more readily absorbed through the gastrointestinal tract and once inside, can migrate through cells which normally form a barrier to toxins. Methylmercury compoundscan be transformed into compounds which cross the blood-brain and placental barriers, allowing mercury to react directly with brain and foetal cells (Choi and Grandjean, 2008). Once inside these barriers, mercury can also be oxidised to inorganic forms, effectively trapping it inside.
Humans are mainly exposed to organic mercury (in particular methylmercury), through the dietary consumption of fish. The highest levels of mercury are found in older predatory fish such as swordfish and tuna (UNEP, 2008a).
Elemental mercury and inorganic and organic mercury compounds can be transformed in the environment from one form to another in a series of complex processes (UNEP, 2008a). In general, however, elemental mercury exists mostly in the atmosphere, while inorganic and organic mercury compounds exist mostly in land and water environments. Transformation between different forms is a continual process, and can be caused by natural (e.g. microbial) and anthropogenic activities. The total amount of mercury in the various forms globally does not increase during these transformations, since mercury, as an element, cannot be created or destroyed.
While mercury cannot be destroyed, it can be transformed into non-bioavailable, non-water soluble chemical species (such as mercury sulphide and mercury oxide), and therefore can be removed from the biosphere into ‘sinks’. An important sink is underwater sediments.
The transformations and transportation of mercury between forms and different areas of the environment are examined in further detail in the sections below. It is worth noting that, at present, little information is available for New Zealand on the residence time of mercury in any particular pathway. The chemical form of mercury will influence its residence time in any particular environmental compartment. Some of the transformation and transportation processes are poorly understood, and for some environmental forms of mercury, there are no estimates of residence time.
Mercury moves freely throughout the globe in a complex combination of transformations and transport (Figure I-1). Mercury is emitted to the atmosphere by both anthropogenic and natural processes on land. In addition, mercury is emitted to the atmosphere directly from the ocean, in a process called evasion. Similarly, mercury is re-deposited to both the oceans and land from the atmosphere. Elemental mercury is highly mobile and can be transported and dispersed easily throughout the atmosphere. Elemental mercury is therefore less likely to be deposited close to its source than the ionic mercury compounds.
Ionic forms of mercury in the atmosphere are more reactive and soluble than elemental mercury, and are therefore more readily removed from the atmosphere by precipitation and dry deposition. Ionic forms of mercury tend to be deposited close to their emission sources, while elemental mercury may be transported far from its emission sources.
Mercury also moves from land to water via processes such as sedimentation, runoff and leaching. These processes can be of particular concern in areas of high industrial activity. Greater detail on how mercury is transformed within and between the different areas of the environment (air, land and water) is described in Section I-2.3.
The mercury pathways in New Zealand fit within the global pattern as described above. Fortunately, due to New Zealand’s geographic isolation, it is not subject to the same level of anthropogenic mercury emissions sourced from neighbouring countries as is observed in Europe and North America. However, due to New Zealand’s location on the convergence zone of two tectonic plates (Australian and Pacific), New Zealand has a natural mercury source that many other countries do not have, namely geothermal mercury emissions. White Island and, less frequently, Mt Ruapehu (Figure I-2), are two of New Zealand’s more active volcanoes emitting mercury into the atmosphere. Geothermal areas situated elsewhere throughout the country also contribute to the total mercury emissions to air and water generated from New Zealand’s land mass. The majority of these are in the Taupo Volcanic Zone in the Waikato and Eastern Bay of Plenty regions.
Figure I-1: Global mercury circulation
A picture to illustrate the global mercury pathways.
In the air: elemental mercury emissions flow into the air from mercury deposition on the land and mercury evasion from mercury deposition in the water. Atmospheric transportation of mercury occurs within the air.
On the land: mercury deposition flows into the air as mercury evasion and from the land to water as sedimentary runoff.
In the water: mercury deposition release mercury into the air as mercury evasion and the water receives mercury from the land from sedimentary runoff.
Figure I-2: Mercury pathways in New Zealand
A picture to illustrate the mercury pathways in New Zealand.
In the air: the air receives mercury as emissions of mercury particulates and vapour from geothermal sources such as Mount Ruapehu and White Island and from industrial sources. Mercury moves through the air by atmospheric transport. Mercury is transported back to the land and the ocean as wet deposition of mercury with rain and as dry deposition of mercury.
In the ocean: mercury flows from the ocean to the air as mercury evasion and mercury is transported into the ocean from the land by groundwater and by sedimentation, runoff and leaching. Mercury moves into the sediment from the ocean by sedimentation.
On the land: mercury is received from the air by dry and wet deposition of mercury. Mercury leaves the land into the air by emissions of mercury particulates and vapour and into the ocean through groundwater and sedimentary, runoff and leaching.
Anthropogenic sources of mercury in New Zealand include air emissions from industrial activities, such as cement and steel manufacturing, geothermal power generation and coal combustion. These are all thought to be important sources of mercury into the environment. Additional sources include the incorrect disposal of mercury-containing items such as some electrical switches, medical equipment, electronic equipment, some dry cell batteries, some thermometers, fluorescent tubes, mercury-vapour discharge lamps and compact fluorescent lamps (CFL).
The biogeochemical mercury pathways that occur in the environment are outlined in Figure I-3. A biogeochemical pathway is used to show how mercury moves through the living parts of the earth (biosphere) and non-living parts of the earth (lithosphere, atmosphere, and hydrosphere). An understanding of these pathways is important to understanding how mercury can be a risk to living species, including humans.
Mercury is transformed between different forms by two main types of reactions, oxidation-reduction and methylation-demethylation. Oxidation-reduction reactions refer to the loss of electrons (an increase in the oxidation state of mercury) and gain of electrons (reduction in the oxidation state of mercury). Methylation-demethylation is the process of gaining and losing a methyl group (CH3). Methylation is primarily assisted by microorganisms (bacteria) under oxygen-poor (anaerobic) conditions that exist in sediments and soils (State of Utah, 2009). However, the process of methylation can be carried out by chemical processes that do not involve living organisms (abiotic processes) (UNEP, 2002). Oxidation-reduction reactions can occur in all areas of the environment, whilst methylation-demethylation reactions more commonly occur in the water and land environments.
A description of the transformations and transportation of mercury forms within the atmosphere, land and water environments follows.
There are many different physical and chemical transformations and reactions occurring between the various mercury compounds that exist in the atmosphere. Mercury enters the atmosphere from various natural and anthropogenic sources. Volcanoes, for example, emit particulate mercury in mercury-containing minerals and salts, and elemental mercury as vapour. Industry and mining also emit these forms of mercury, and can also emit other more complex forms of mercury compounds.
Once emitted to the atmosphere, elemental mercury can be readily circulated within it, or oxidised to inorganic (Hg(II)) compounds. These compounds can be similarly reduced back to elemental mercury in the atmosphere. The form mercury adopts in the atmosphere dictates its mobility and distribution potential, and this has consequences for the control of mercury emissions.
Mercury from the atmosphere can be deposited to land or water by either “wet” or “dry” deposition. Wet deposition occurs when mercury is dissolved into water droplets (either by rain or other types of precipitation) and returned to land or water. The process of wet deposition is efficient at removing divalent (soluble) forms of mercury from the air. Dry deposition is where mercury is removed via settling and scavenging processes (such as absorption onto plant foliage or chemical reactions with other gaseous compounds and suspended particulate matter). Dry deposition is more likely to remove particulate forms of mercury from the ambient air, and can also remove gaseous mercury forms.
About 80% of the total mercury in the atmosphere is in the form of elemental mercury vapours (Wang et al, 2004). Due to its high volatility, elemental mercury can remain in the atmosphere for 1 to 2 years, which results in the long range transport of mercury across country borders as part of the global circulation of mercury (Lin and Pehkonen, 1999). Compared to Hg(0), Hg(II) has a much shorter lifetime in the atmosphere; from several days to a few weeks (Lin and Pehkonen, 1999).
Figure I-3: Mercury biogeochemical cycle
A picture showing a volcano, a landfill, farming, industry/mining, trees, soil, a body of water with fish and sediment and the mercury flows between them.
As previously described in Section I-2.1.2, mercury exists naturally in the land environment, for example in the mineral cinnabar (HgS) or as a trace component in all sedimentary, igneous and metamorphic rocks. Mercury compounds in the land/soil environment can also be a result of atmospheric deposition. Mercury compounds in the soil can undergo oxidation-reduction reactions, where oxidised mercury Hg(II) can be reduced to elemental mercury, Hg(0). This reaction can then be followed by volatilisation of elemental mercury, where mercury is re-emitted to the atmosphere, or can be followed by leaching or runoff, where Hg(0) is transported to the water environment.
In the soil layer, mercury can also form complexes with inorganic and organic compounds via a series of reactions including methylation. The formation of mercury complexes can make the mercury less mobile, causing it to remain in the soil for long periods of time.
The mobility of mercury in the soil layer depends on many factors. It can be mobilised if attached to organic matter that is subsequently washed into waterways by runoff (UNEP, 2002), or if it binds to soluble compounds; or conversely, it can be immobilised if it is strongly bound to insoluble compounds. In areas of soils with high levels of mercury accumulation, mercury can continue to be released into other environments for long periods of time (UNEP, 2002). The average residence time of mercury in soil has been estimated as being in the order of 1,000 years (NRC, 1979).
Mercury enters the waterways via deposition of particles or ionic compounds from the atmosphere, runoff and erosion from the land surface, leaching from landfills, geothermal inputs, combustion and industrial discharges. Once in the water environment, mercury undergoes similar oxidation-reduction, sorption-desorption processes on to mineral surfaces and organic matter, and methylation-demethylation reactions as occur in the land environment.
In a similar manner to the land environment, elemental mercury can be evaporated back to the atmosphere directly from water (this process is also called evasion) or it can be oxidised to form Mercury(II). This ionic form of mercury can then be either reduced back to elemental mercury and again evaporate to the atmosphere, or it can form methylmercury. Mercury(II) can also form complexes with organic matter or be sorbed onto suspended particulate matter within the water environment. Mercury(II) complexes are thought to be the dominant form in which mercury is found in natural waters. Mercury(II) complexes can be transported over very long distances (Lin and Pehkonen, 1999).
The methylation reactions in the water environment are of particular concern due to the bioaccumulation of methylmercury in aquatic species. As a result, top-level predators acquire greater body burdens of mercury than the fish they consume. Bioaccumulation and biomagnification within the food chain can result in top-level predators (fish-eating fish, birds and humans) having bioaccumulation factors of the order of 10 million (Sigel and H. Sigel, 1997). In other words, concentrations within the bodies of these predators can be 10 million times higher than the environments in which they live.
Both oxidised mercury(II) and methylmercury can be deposited into the underlying sediments, where similar processes and transformations occur as those described for the land environment. Sediments at the bottom of water bodies can act as mercury sinks from which mercury can be distributed back into circulation for many years after initial deposition (UNEP, 2002).
The residence time of mercury in the oceans is up to 3,200 years, while the average residence time of mercury in oceanic sediments is in the order of 250 million years (NRC, 1979).
Many of the transformations of mercury between the different parts of the environment are caused by anthropogenic or man-made processes and activities. It is estimated that approximately one third of mercury currently emitted to the atmosphere is sourced from anthropogenic or man-made activities (UNEP, 2008b).
Over the years, humans have mined and used mercury in many different products ranging from electrical switches, batteries, thermometers, medical equipment, dental amalgams, fluorescent tubes, discharge lamps and, more recently, in energy-efficient lamps. In addition to these intentional uses, humans have carried out various activities that cause unintentional mercury emissions. Such activities include fossil fuel combustion, mining and metal extraction, chloralkali production (used to produce chlorine gas and caustic soda used in many chemical and industrial processes), and waste incineration. These anthropogenic activities have increased atmospheric concentrations of mercury by a factor of three, on average, since pre-industrial times (UNEP, 2008b).
Greater emissions caused by anthropogenic or man-made activities are of concern when mercury is re-deposited to both the ocean and land environments and converted to toxic organic mercury compounds. As previously described, these transformations (such as methylation forming methylmercury), are mostly assisted by bacteria under oxygen-poor conditions. Such bacterial transformations in landfill soils, for example, can act to mobilise mercury from mercury-containing products such as crushed lamps into water and the food chain, presenting a risk to humans and other living organisms.
The general mercury pathways in New Zealand’s environment were described in Section I-2.2. Mercury emissions in New Zealand are derived from a mix of both anthropogenic and natural sources, with mercury from geothermal activity notable in many places throughout the country. The breakdown of the various sources is discussed in Section III.
As mercury is a toxic, too much mercury can potentially result in a number of adverse effects. These include:
Of these, most global concern has been over the issue of accumulation of anthropogenic mercury from diffuse sources in aquatic ecosystems. This concern is most strongly felt in large continental areas with significant industry, such as the United States and continental Europe. Bioaccumulation and biomagnification of mercury may result in adverse effects on the aquatic animals and associated wildlife, but can also cause increased dietary intakes of mercury in the human population due to higher concentrations of methylmercury in fish. The greatest proportions of anthropogenic loadings of mercury in the global environment are generated by fossil fuels, industrial, and mining activity. When added to natural loadings, these artificial loadings may present a risk to the environment.
In practice, mercury concentrations in the New Zealand environment are sufficiently low that it is rare to encounter the adverse effects listed above. Section III of this report provides more information regarding these risks associated with mercury emissions in New Zealand.
There is limited evidence of problems from mercury in New Zealand. However, with respect to fish the New Zealand Food Safety Authority (NZFSA) has issued precautionary advice for pregnant women to limit their consumption of long-lived and predatory fish species at the top of the marine food chain (e.g. school shark, bluefin tuna), and trout caught from lakes in geothermal areas.
For trout, mercury concentrations in fish caught in North Island geothermally-influenced lakes are significantly greater than those of trout caught in other lakes (e.g. Kim, 1995). A section of at least one lake bed (Lake Waikare in the Waikato region) is contaminated with mercury, to levels that may prevent germination of aquatic plants, as a result of a natural geothermal spring in the lake.
Some geographically localised areas of elevated mercury exists as a result of industrial activities. Some specific areas of potential concern are:
Monitoring of coastal marine sediments in some locations has revealed some areas of elevated mercury concentrations which exceed ANZECC (2000) Interim Sediment Quality Guidelines-low (ISQG-low) values, possibly from run-off from urban areas. These include some areas of the Waitemata and Manukau Harbours in Auckland (McHugh and Reed, 2006) and Wellington Harbour (Stephenson et al, 2008).
It should be noted that there is generally little evidence for actual adverse environmental effects from mercury in most of the cases cited above.
The disposal of mercury-containing consumer products, (especially CFLs) into landfills has been raised as a potential concern. At this stage there is no well-recognised waste disposal facility to deal with used lamps New Zealand. Most commonly, these lamps are disposed of with household refuse to landfill. It is estimated that 98% of mercury-containing lamps are disposed to the 60 landfills that currently operate in New Zealand, with only 2% recycled. This means approximately 5,400,000 lamps are disposed of to landfill each year (Stewardship Solutions, 2008).
The effect of this and past disposal of mercury-containing consumer products into many closed landfills which were not up to current landfill standards, is not known. Much of the mercury in current and closed landfills is probably in bound forms. Modern landfills are also lined to contain leachate and have leachate collection and treatment systems. It is therefore probable that there is very limited release of mercury from closed and operating landfills.
The potential impact on the environment from discharges of mercury from landfills is examined in greater detail in Section III-3.4 of this report.