M ercury pollution and exposure in South India and pathways of mercury transfer to selected human and environmental receptors

Subhavana, K L and Qureshi, Asif (2019) M ercury pollution and exposure in South India and pathways of mercury transfer to selected human and environmental receptors. PhD thesis, Indian institute of technology Hyderabad.

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Abstract

Mercury (Hg) is a toxic element of global concern. Emitted from anthropogenic and natural sources, it cycles continuously and reversibly between different environmental compartments (air, water, sediments, biota and vegetation) and is bio-accumulated in the aquatic food chain. Exposure to aquatic biota may lead to adverse health effects in humans and wildlife. India is the second largest emitter of mercury to the atmosphere. However, in contrast to other countries, the number of studies of mercury in India are extremely limited. This thesis uses a combination of field sampling, lab experiments and mathematical modeling to bridge this information gap. In this work, the as yet largest database on the occurrence of mercury in humans in India has been developed (Chapter 3). Hair in 668 volunteers from participants in three cities were analyzed: (i) a city on the east coast with four active coal-fired thermal power plants (Nellore), (ii) a city on the west coast with no major mercury source (Vasco da Gama) and (iii) a metropolitan city in the interior with no major mercury source (Hyderabad). Geometric mean of total mercury in hair of the entire study population was 0.14 μg/g (95% confidence interval, CI: 0.13 – 0.15 μg/g). Age, fish consumption rate, occupation such as dental studies, subsistence fishing, and artisanal goldsmithing were the statistically significant predictor variables. The results supported the hypothesis that living near a coal-fired power plant (Nellore) may have higher mercury in hair than those in cities with no major mercury source (Kruskal Wallis test, p < 0.05). Nellore was found an interesting place as it had active mercury sources like coal-fired power plants, and aquatic systems with conditions conducive to bioaccumulation of mercury, such as rice fields, and rivers, aquaculture ponds and coastal waters. Field sampling and experimental analysis was conducted to compare the concentrations of mercury in the environmental receptors (soil and air) of Nellore with other regions of India and a remote region, Antarctica (Chapter 4). Concentrations of total mercury in Nellore (1.69 – 38.2 µg/kg) were much higher than that found in Schirmacher Hills, Antarctica (concluded to be a global background site) but lower than some other locations in India. As fish consumption is the most common pathway for exposure to mercury, methyl mercury (MeHg) and total mercury (THg) concentrations were measured in fish and shellfish caught viii offshore, in freshwaters and in aquaculture ponds from Nellore (Chapter 5). The concentrations of MeHg varied in the range 1.67 – 105.25 ng/g ww (wet weight). These concentrations are not high enough to suggest that the fish species themselves are contaminated (according the United States Environmental Protection Agency, USEPA, limit of 1000 ng/g, and the Food Safety and Standards Authority of India, FSSAI, limit of 500 ng/g). However, it is found that even consuming a fish of 105 ng/g ww at frequencies currently consumed in Nellore may lead to a more than 19% probability of exceedance of the reference dose for mercury (0.1 µg/kg body weight/day) for Nellore residents. One-box pharmacokinetic modeling to describe mercury intake, metabolism and elimination kinetics in humans suggests that fish consumption can account for all the mercury observed in human hair in the Nellore population surveyed in Chapter 3. Finally, the sources of mercury incorporated in the fish species (fish feed or directly water) were aimed to be explored. However, coastal water and rivers are environmental systems with open boundaries and ‘complex’. They were considered out of the scope of this thesis. Instead, aquaculture ponds were considered as they are environmental systems with closed boundaries, and perhaps easier to analyze compared to coastal waters and rivers. A previously published model describing mercury bioaccumulation in fish was adopted to analyze if any of the potential sources could reproduce the observed mercury in the fish caught in the aquaculture pond. A case study was conducted to simulate the concentrations of MeHg in cultivated fish (Labeo Rohita) in one of the aquaculture pond in Nellore. Algae from the pond and the fish feed fed to the cultivated Rohu fish were experimentally analyzed. As the method of detection of ultra-low quantities (order of pg/L) of mercury in water was not yet developed in the lab (and probably anywhere in India), a range of MeHg concentrations in water from literature were simulated. In all cases (fish feed or only water as source), the predicted concentrations of MeHg in fish were lower compared to the observed MeHg. However, a 30-40% increase in the concentration of mercury in fish feed resulted in a better prediction. Mercury in pond water was not able to explain the observed concentrations of MeHg in fish even if the concentrations of mercury in them were increased several times. Overall, the concentration of MeHg in fish seemed to be determined by the concentration of MeHg in fish feed, and probably only to a minor extent by water. Similar conclusion has been made elsewhere in the world. ix Overall, this thesis provides new information on levels of mercury and its exposure among general populations in South India, and makes an attempt to track the sources that cause this exposure. While coal-fired power plants seem to be a source of mercury to which general population are exposed, media such as feed given to fish cultivated in aquaculture ponds are important for people who almost exclusively eat aquaculture cultivated fish. However, more extensive modeling or experimental analysis such as that using stable isotopes of mercury are required to better understand the exact sources of mercury that ends up in the human body.

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