Mercury pollution in the oceans
Statement of the Problem
Mercury (Hg) is a non essential heavy metal with no nutritional value, yet can be hazardous even in small quantities if consumed by humans, causing some serious adverse effects. It can be found in all three physical states (liquid, gas and solid particles), which causes even more complications in terms of mercury pollution treatment. The biggest quantity is found in the atmosphere and oceans, which are interconnected by mercury cycling. The atmosphere contains most of the pollution in the form of mercury vapour with a life cycle of one year and can be widely dispersed by wind, precipitation and deposition. An increase in Hg levels in the atmosphere has been observed over the decades with anthropogenic processes becoming more intense. As Hg is found to be volatile, it easily settles in soil, water bodies, sediment and plants in the form of either inorganic mercury salts or organic mercury as methyl-mercury (EPA, 1997). These processes are all followed by mercury leakage into fluvial systems. This allows mercury to be transported into oceans and affects overall aquatic life systems. Fish and other water organisms are prone to accumulating Hg in their tissues (e.g., shells, muscles, skin, and liver), thus compromising the food chain with heavy metal contamination. This could be fatal for humans as they are at the top of food chain. The growing concern with mercury contamination requires more studies to be done in order to find out the best approaches and methods for environmental issue to be solved.
A break out of mercury poisoning epidemics in Japan and Iraq due to high levels of exposure to methyl-mercury raised major concerns about human health. Research has proved that methyl-mercury is almost fully absorbed by the blood and distributed to all tissues with major concentrations in the brain (EPA, 1997; Thompson et al., 1992). A reasonable reference dose as calculated by the US EPA was defined as 0.1 μm/ kg bw/day; however, more RfD analyses are required to avoid all uncertainties in the protection of human health. Fish consumption is the dominant pathway for human and wildlife exposure to hazardous Hg levels. Once the harm of Hg exposure was recognized, scientists started looking for explanations for the mercury cycles and anthropogenic activities were accredited as a major contributor to rising mercury levels in the environment. Primary anthropogenic and secondary anthropogenic sources were defined (Pacyna et al., 2010).
First, all coal-burning plants and factories burning fossil fuels with mercury trace contaminants in conjunction with the ore mining of mercury and other metals (e.g., gold, silver) have been identified as primary anthropogenic sources and major contributors to the increase of mercury levels in the atmosphere (Slemr and Langer, 1992). According to analyses of peat bogs, soils and lake sediment, predictions of doubled atmospheric mercury deposition levels since the nineteenth century (the start of industrialisation) were confirmed. Slemr and Langer (1992) in their study, which took 20 years in order to obtain reliable measurements, showed a significant increase in atmospheric mercury concentrations over the North and South Atlantic Oceans – 1.46 ± 0.17% per year and 1.17 ± 0.16% per year respectively. They concluded that measurements were consistent with other analyses, thus supporting the idea of anthropogenic sources being more responsible for the increased atmospheric Hg levels than natural sources. Further observations of total atmospheric mercury (TAM) in 1996 and 1999–2001 were compared with those obtained by Slemr and Langer (1992) and 1980–1983 measurements obtained by Fitzerald (1995). Further, the global decrease of TAM levels between 1990 and 1996 was noted and explained by a substantial decrease in anthropogenic activities during this period (Temme, 2003).
Artisanal and small-scale gold mining, which are secondary anthropogenic sources, in conjunction with coal burning were responsible for 61% of total annual anthropogenic emissions into the atmosphere during the last decade. Major contributors were Southern and Eastern Asia; due to their economic growth which boosted industrial development, it was estimated that they were responsible for half of the global Hg emissions. The other 30% of global emissions were attributed to sub-Saharan Africa and South America, where developing countries are used to accommodate the large factories of economically strong countries to alleviate the poverty of the developing countries (UNEP, 2013). Unfortunately, an increase in artisanal and small-scale mining emitted mercury levels has been predicted due to the increase in the gold price (Pacyna et al., 2010). Other secondary sources are found in industries where mercury is used intentionally, such as in the production of products containing mercury (i.e. thermometers) (Pacyna, et al., 2010). A study was undertaken in Kodaikanal in India to prove that a thermometer factory had a negative impact on the local environment. Kodai Lake is located north of the factory whereas two comparable lakes, Berijam and Kukkal, are situated in a forest environment. Measurements were taken of total mercury 356-465 ng/l Hg(T) and methyl-mercury 50 ng/l (CH3Hg) in Kodai Lake and compared with measurements from Berijam and Kukkal; the latter showed significantly lower values. Sediment analyses supported the measurements of water samples, as Kodai Lake sediments contained almost double the amount of methyl-mercury 276-350 mg/kg Hg(T) compared with the samples from Berijam 189-226 mg/kg Hg(T) and Kukkal 85-91 mg/kg Hg(T) lakes (Karunasagar et al., 2006).
However, it should be noted that mercury is a naturally occurring chemical element with a natural cycle; thus natural emission sources, such as volcanoes and some fisheries should be taken into account when investigating mercury cycling. Eshlemen et al. (1971) noted that some physical properties of Hg depend on smelting cinnabar for manufacturing, and some microorganisms in water are responsible for Hg conversion into dimethyl-mercury gas. Therefore it is difficult to control the overall mercury cycle.
Recent studies suggest that recently deposited mercury is more likely to be converted into methyl-mercury than mercury that is already present in the environment (Renner, 2002). It could be that if action was taken timeously in terms of reducing mercury emissions, targets could be achieved in reducing the mercury pollution levels (Bridgen and Santillo, 2002).
With precipitation, mercury is deposited onto terrestrial areas. Many studies have indicated that mercury deposition in areas of emission sources, plus ‘en route’ deposition due to transport, have exceeded permissible levels in soils and terrestrial water bodies in those areas (Pacyna and Pacyna, 2001). Once terrestrial and aquatic environments are contaminated by Hg traces, bioaccumulation in soil is activated by microorganisms and their metabolic functions, thus affecting the food chain.
As mercury has been found to be ubiquitous and persistent, the whole Hg cycle and bioaccumulation process has become more complex. Studies have been undertaken in Idrija, Slovenia, where the largest world mercury mining area used to be. The Gulf of Trieste suffered from constant leakage of mercury into the Soča River. It is known as the most polluted are in the Mediterrenean with contributing sewage pollution. After analyses of estuarine and marine water’s sediments and organisms, results showed that even ten years after the mine’s closure, the concentrations of Hg were extremely high, and no potential decrease was observed (Horvat et al., 1999). In another study conducted in Idrija, Slovenia, a downstream increase of Hg(T) was observed, ranging from <3 ng/l to >60 ng/l, with a methyl-mercury concentration of ~0.5% , which proved higher mercury accumulation in movement towards the river mouth (Hines et al., 2000). It was found that Hg was converted into methyl-mercury in most esturies, possibly due to dissolution and recycling; however, severe levels of methyl-mercury, strongly produced by Gulf of Trieste sediments, were found to have been introduced into bottom waters and aquatic life (Hines et al., 2000). Contrary to these findings, more recent study results show that there has been a significant decrease in the last decade in the Idrija region. According to Kotnik et al.’s (2005) findings, levels of Hg concentration in the air have dropped below10 ng/m3, compared with 20 000 ng/m3 in the early 1970s. The difference in these findings might be explained by the development of research methods and improved technologies. Nevertheless, constant measurements should be taken over a prolonged period for more accurate values to be obtained.
The importance of finding potential methods for tackle the mercury pollution is increasing. A whole-system study was conducted to define the response of fish methyl-mercury concentrations to changes in mercury depositions by adding stable mercury isotopes into the watershed of the lake being examined. Measurements were taken for three years and results showed slow movement of mercury into the lake. It was concluded that a reduction in mercury emission levels would have a considerably positive effects on fish methyl-mercury concentrations, thus ensuring a reduction in risk to human health (Harris et al., 2007). In addition, some other methods were suggested for reducing the impact of mercury, such as capping, dredging or natural attenuation (Wang, et al., 2004). Two ways of capping were proposed: in situ (ISC), where the contaminated sediment in aquatic areas is covered with an appropriate cover, and ex situ (ESC), when contaminated sediments are dredged and relocated in a site and covered by multiple isolating layers. Both should be used in order to minimise the possible negative effects on the environment. Such isolation layers would be made of sand and finer particles, as their ability to absorb mercury perfectly has been proved. Leakage of mercury would be avoided and tests have approved such layers as being efficient barriers between Hg contaminated sediments and overlaying water. Hosakawa (1993) found that dredging could decrease the concentrations of mercury in sediment by 5 mg/kg. By comparing various dredging techniques, scientists suggested that a combination of hydraulic and mechanical dredging could be the best method as this avoids sediment resuspension (Hauge et al., 1998). Natural attenuation could be an effective natural remedy for less contaminated areas, as no strict remedial methods would be implemented and aquatic systems would be expected to recover naturally (Wang et al., 2004). Unfortunately, as mercury is known to be very persistant in the environment it will be difficult to achieve targets in mercury concentration reductions, especially in highly contaminated areas. As mercury transportation and transformation in aquatic systems has been found to be highly complicated, sufficient data is required in order to calibrate and validate current methods of mercury transportation (Wang et al., 2004).
The proposed study would investigate mercury pollution levels in the UK’s coastal areas, concentrating on the north-western and south-western coastlines, which are facing the highest exposure of Hg pollution in the Atlantic Ocean.
- Investigation of mercury pollution levels in the air of areas near the factories, harbours and other potential emission sources
- Measure mercury concentrations accumulated in peat bogs, lichens and mosses in studied areas
- Measure Hg and methyl-mercury concentrations in water samples of both terrestrial water and marine water
- Sample the sediment of lakes found in studied areas and measure the concentration of accumulated in it
- Examine water birds and seabirds as bio-indicators of mercury concentration level changes in the environment
This study will look at accumulated mercury concentrations in areas by studying Macroramphosus scolopax (snipefish) and Scomber japonicus (chub mackerel) species as bio-indicators of heavy metal pollution. Both species are commonly found in the North and South Atlantic Ocean and are prone to accumulating Hg in their tissues. Both species are found to be top predators, thus more efficient results could be obtained. Mercury bioaccumulation in mid-Atlantic waters are determined by water chemistry and available food levels. Higher mercury concentrations are expected to be found in fisheries of northern coastal area waters. Fish length and weight would be measured, colour noted and the ratio between Hg and methylmercury calculated, as bioaccumulation of mercury manifests mainly in the MeHg form in fisheries. Cold vapour atomic absorption spectrophotometer would be implemented in determining the concentrations. Collected data would be synthesized by statistical tests, and conclusions reached.
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