Environmental pollution is defined as ‘the direct or indirect introduction of substances or heat, as a result of human activity, into water or land which may be harmful to human health or the quality of the aquatic or terrestrial ecosystems, which directly result in damage to material property or which impair or interfere with amenities and other legitimate uses of the environment.’ (The European Union Water Framework Directive, 2000). Many elements are essential to life, some are toxic; when increased concentrations of essential as well as toxic elements are present in the environment, they acquire the ability to become a pollutant.
Mercury, given the symbol Hg, is a natural occurring toxic heavy metal (Adler-Miserendino, 2012). Mercury has an atomic mass of 200.59 grams per mole and is the only metal possessing the ability to occur as a liquid at room temperature. (Environment and Climate Change Canada, 2013). Mercury can occur naturally in three valence states within the environment: elemental, inorganic and organic mercury, all classified as toxic (University of Minnesota, unknown date). Inorganic mercury compounds are subject to chemical and biological changes by bacteria into methyl-mercury (University of Minnesota, unknown date). Organic mercury compounds are soluble in water and accumulate in the tissues of living organisms allowing transport through the marine ecosystem trophic levels through a process called bioaccumulation (Environment and Climate Change Canada, 2013). Mercury is persistent within the environment due to its non-biodegradable properties coupled with the ability of biogeochemical reactions able to occur whereby elemental mercury has a long atmospheric residence time of six months to a year (Environment and Climate Change Canada, 2013). These properties make high mercury concentrations within a spatially restricted area a significant threat to the environment and humans (Adler-Miserendino, 2012).
Global mercury pollution, emissions and demand are expected to decline in response to the Global Mercury Partnership whereby reduction of mercury use through targets and action plans are to be put into place globally (UNEP, 2013). However, where gold prices are rising, small-scale artisanal mining is expected to rise, this is shown by Table 1 whereby all processes and products using mercury declined from 2000 to 2005 where small-scale gold mining increased by 60% (González-Carrasco et al., 2011).
A source is a ‘place or object in which something originates from’ (Oxford Dictionaries, 2017). There are numerous sources of mercury from both human influences and natural processes, all sources are of a concern due to mercury having a high persistence within the environment (New Hampshire Department of Environmental Services, 2003). Mercury can be introduced into the environment in three ways. Firstly, through natural emissions into the atmosphere through the weathering of Hg-bearing rocks and soils as well as volcanic eruptions. Secondly, mercury can enter the environment from a series of human activities. There are three processes that commonly use high concentrations of mercury, accounting for 1230 tonnes of mercury emissions per year; small-scale artisanal gold mining, the production of vinyl chloride monomer and chlor-alkali production (Hill, 1997). Other human activities that contribute to emissions are fossil-fuel burning, incineration of waste and the use of mercury in common household products such as batteries, thermostats, thermometers, electrical devices and fluorescent lights. Thirdly, mercury can be re-introduced into the environment via natural processes including out gassing from the oceans and other surface waters as well as re-mobilization allowing uptake by soil, plants and sediments (Hill, 1997; New Hampshire Department of Environmental Services, 2003; Adler-Miserendino, 2012; Minnesota Pollution Control Agency, 2013).
There are several of transport pathways that mercury can take from its source, significant pathways being air, water, soil and ingestion. When mercury is emitted into the atmosphere, over time atmospheric reactions result in a transformation into mercury compounds, e.g. mercuric oxide. These compounds can then be deposited on the earth’s surface via precipitation and snowstorms or adhesion to other particles. Deposition is random where elemental mercury in the atmosphere can travel a significant distance away from its source, shown by sediment records. Drainage from mines or industrial areas can have an output straight into a fluvial or marine system allowing transport throughout these systems. However, some outputs occur on land e.g. mine tailings. Trace elements of mercury can become mobilized and infiltrate through the sub-surface when high events of precipitation occur, mercury can therefore be found in groundwater (Hill, 1997; New Hampshire Department of Environmental Services, 2003; Adler-Miserendino, 2012).
There are many receptors of mercury pollution including but not limited to: humans, aquatic life and birds. The effects of mercury on humans can range from minor to fatal. Effects humans can incur are toxicity to the nervous system through chronic inhalation of elemental mercury vapor over a varied period of time leading to symptoms such as hearing and eyesight impairment. High levels of mercury can cause adverse effects within hours including irreversible brain damage or death. Within the marine environment, mercury biomagnification in organisms occurs where mercury binds to sediment where bacteria convert into methylmercury which algae and other plankton take up as part of their metabolism. The level of mercury accumulates higher through the trophic levels where mercury concentrations can be toxic when birds, mammals and humans consume the fish (Hill, 1997).
A significant global source of mercury is small-scale artisanal mining, it is responsible for 18% of total global emissions and 30% of the world’s gold produce (Hill, 1997). Small-scale artisanal mining is typically defined as mining practised by individuals, groups or communities with minimal mechanization often in the informal (illegal) market in developing countries (Hentschel et al., 2003; Fraser Institute, 2012). However, currently there is no internationally recognised definition of artisanal mining established.
Mercury pollution is particularly a problem in Portovelo, Ecuador. Portovelo is a small town of 35km2 located towards the Southeast of the El Oro Province of Ecuador which falls along the confluence of two rivers: El Amarillo and La Calera (González-Carrasco et al., 2011). This is one of the oldest gold mining towns in Ecuador whereby artisanal mining still uses mercury during the extraction of gold (González-Carrasco et al., 2011). Artisanal mining has been used to recover gold in southwestern Ecuador for many centuries; it increased in popularity in the 1970s whereby rising international gold prices and an influx of unemployment occurred causing local communities to find an alternative source of income (United Nations Environment Programme, 2012; Waldick, unknown date).
Extensive gold mining is concentrated in the Portovelo region where over 100 processing plants are located along this stretch of the Puyango River (González-Carrasco et al., 2011; United Nations Environment Programme, 2012). At these processing plants, gold is extracted using a process that relies on mercury: mercury amalgamation (Fraser Institute, 2012). This process involves the ore being ground and sifted, then being combined with mercury (Velásquez-López et al., 2010). The mercury bonds with the gold to form an amalgamation, HgAu, which can then be heated to release Hg vapor leaving behind gold as the product (Fraser Institute, 2012).
The problem with this method of gold mining is that high concentrations of mercury are being released into the environment causing environmental contamination and harm to human health (Fraser Institute, 2012; González-Carrasco et al., 2011). Around 1.5 tonnes/annum of mercury are released in the Portovelo region through mine tailings accounting for 30% of emissions which are a chemical mixture of many elements including mercury, lead and cyanide as well as evaporation accounting for 70% (Velásquez-López et al., 2010). In the Portovelo region, air pollution caused by the intensive use of mercury has become a threatening problem to communities respiratory and neurological health (Fraser Institute, 2012).
Mine tailings are compromised when there is a meteorological event such as high precipitation causing an overflow and runoff of these bodies or mobilization of mercury allowing it to infiltrate through the subsurface eventually meeting with groundwater. Mine tailings eventually enter the fluvial systems of the rivers El Amarillo and La Calera through groundwater output whereby aquatic life are poisoned and water becomes contaminated as well as polluting local soils (González-Carrasco et al., 2011). Mercury that is dissolved with cyanide in these agitated tanks is more bioavailable than metallic mercury allowing bioaccumulation to occur in organisms (Velásquez-López et al., 2010).
In freshwater aquatic ecosystems, mercury reactivity is driven by various organic and inorganic ligands including parameters such as pH. Mercury, Hg, can bind to dissolved organic matter (DOM) readily due to its high reactivity in the water column. Bacteria then convert the mercury into methylmercury, MeHg, via metabolic processes allowing algae, plankton and benthic organisms to take up the MeHg. Methylmercury is then transmitted from the plankton up the trophic levels via a process called bioaccumulation (Adler-Miserendino, 2012). Bioaccumulation is when ‘an organism readily uptakes a potentially toxic substance at a rate faster than at which the substance is lost by catabolism and excretion’ (Oxford Dictionaries, 2017). Humans can then be further affected by mercury pollution via the ingestion of fish affected by mercury toxicity.
Not only does the use of mercury within Portovelo have an effect on local communities, the effects are widespread throughout the communities within southwestern Ecuador, typically populations along the Puyango River which meets the marine system over one hundred kilometres away near the Peruvian border (Hill, 1997).