Susanne M. Ullrich

Mercury in the Aquatic Environment: A Review of Factors Affecting Methylation

Mercury is one of the most hazardous contaminants that may be present in the aquatic environment, but its ecological and toxicological effects are strongly dependent on the chemical species present. Species distribution and transformation processes in natural aquatic systems are controlled by various physical, chemical, and biological factors. Depending on the prevailing environmental conditions, inorganic mercury species may be converted to many times more toxic methylated forms such as methylmercury, a potent neurotoxin that is readily accumulated by aquatic biota. Despite a considerable amount of literature on the subject, the behavior of mercury and many of the transformation and distribution mechanisms operating in the natural aquatic environment are still poorly understood. This review examines the current state of knowledge on the physicochemical behavior of mercury in the aquatic environment, and in particular the environmental factors influencing its transformation into highly toxic methylated forms.

Mercury in the river Nura and its floodplain, central Kazakhstan I: river sediments and water

The River Nura in Central Kazakhstan has been heavily polluted by mercury originating from an acetaldehyde plant. Mercury in the riverbed is mainly associated with power station fly ash, forming a new type of technogenic deposit. A systematic survey of the bed was carried out to establish the location, extent and nature of the contaminated sediments, and to evaluate the potential for sediment transport. The bed sediments were found to contain very high concentrations of mercury, particularly in the first 15 km downstream of the source of the pollution. Average total mercury concentrations in this section of the river are typically between 150 and 240 mg/kg, falling rapidly with increasing distance downstream. The estimated total volume of silts in the riverbed between Temirtau, the origin of the pollution, and Intumak Reservoir, located 75 km downstream, has been calculated as 463500 m3, containing an estimated 9.4 tonnes mercury. Forty-six percent of the total volume of contaminated silts containing almost 95% of the mercury are located in the upper 25 km of the river, however. The data clearly support the hypothesis that large quantities of polluted sediment are not transported long distances downstream but are removed from the aquatic environment in times of flood and deposited on the low-lying lands adjacent to the river. This process, however, does not stop mercury moving further downstream in the water column.

Burdens of mercury in residents of Temirtau, Kazakhstan I: Hair mercury concentrations and factors of elevated hair mercury levels

Mercury (Hg) is released either naturally in the environment or by anthropogenic activities. During its global circulation, Hg presents in a diversity of chemical forms and transforms between each other. Among Hg species, methylmercury (MeHg) is readily absorbed by humans via the aquatic food chain and thus it is very neurotoxic to exposed populations including fetuses due to perinatal exposure. In 2005, a survey was carried out in Temirtau, an Hg-contaminated site in North Central Kazakhstan, to investigate Hg concentrations in the hair samples of the residents and the relationship between Hg exposure levels and the related factors. Among the 289 hair samples, Hg concentrations ranged from 0.009 to 5.184µg/g with a mean of 0.577µg/g. Nearly 17% of the population exceeded 1µg/g for hair Hg, which corresponds to the reference of dose (RfD) 0.1µg/kg body weight/day developed by the United States Environmental Protection Agency (USEPA). Subgroups of males, people aged over 45 and fishermen or anglers were found to have elevated Hg exposure levels in their hair. A positive correlation was found between Hg concentrations in hair and frequencies of river fish consumption. As a result, the finding that people were exposed to high levels of Hg was expected due to the frequent consumption of fish caught from the polluted River Nura or the neighbouring lakes. A regression model showed that approximately 41% of variance of Hg concentrations in the study populations hair was attributed to the variables of gender, residential location, age and fishery occupation. The model implied that demographic characteristics together with dietary behaviour should be taken into account in studies associated with Hg exposure risk, in order to clearly define the group potentially sensitive to Hg exposure.

Burdens of mercury in residents of Temirtau, Kazakhstan II: Verification of methodologies for estimating human exposure to high levels of Hg pollution in the environment

A considerable amount of work has been conducted developing exposure estimate models for quantitative evaluation of mercury (Hg) intake and human health risks, but few have assessed the applicability and the validity for evaluating the risks posed by Hg in the environment and have achieved very mixed results. The present study focused on verifying estimated daily Hg intake using exposure equations with either the deterministic or probabilistic (the Monte Carlo) approaches. The simulated daily Hg intake doses were compared with those established from measured Hg concentrations in the hair of 289 participants. The results showed that the single-value deterministic method for simulating Hg exposure levels overestimated the level of risk by a factor of 1.5 when compared with the highest concentration of Hg observed in the hair of the study population. Contrarily, the average daily Hg intake doses simulated using the probabilistic simulation were similar in distribution to the biomarker data. When the reference dose (RfD) of 0.1 microg/kg body weight/day was adopted as the acceptable dose for daily intake rate, there were approximately 19 percent estimated to have potential Hg exposure risks based on the Monte Carlo simulation. This percentage was favourably similar to the 17 percent determined from Hg concentrations in the hair samples. The difference between the probabilistic simulation and the data derived from hair Hg levels was considered mainly due to the uncertainties in unconfirmed questionnaire-based survey data, small sampling sizes and the surrogates used in the exposure models. The findings implied that the existing exposure models together with the probabilistic approach were appropriate for research of human exposure to Hg.

Quantification of the risk associated with high levels of mercury pollution at Temirtau, Kazakhstan

This research aims to establish the risks of human exposure to high environmental mercury levels, based on a quantitative risk modelling approach. The study site was located on the River Nura in central Kazakhstan. From the 1950s until its closure in the mid 1990s, an acetaldehyde production plant located in the industrial city of Temirtau released Hg-containing wastewater into the river. River sediments are highly contaminated with Hg up to a distance of 25 km and beyond. In additon, a local power station released an estimated 6 million tonnes of fly-ash into the water and the Hg has become quite tightly associated with the ash deposits. River water, fish and agricultural land in the floodplain are also contaminated with mercury, yet the risks posed to the local population have not been evaluated to date. In June and July 2005, we took samples of soil, interior and exterior dust, drinking water, and food from individual households, communal areas and markets. Additionally, water and sediment samples and fish were taken from the river. Interviews were conducted with householders to establish their age and body weight, general living conditions and sources of irrigation and drinking water. A food frequency questionnaire (FFQ) was designed to investigate the frequencies of consumption of several common regional food items, including fish from the river and/or local market. Human hair samples were also collected to estimate the Hg bioburden and to enable the validity of the modelling approach to be established. The paper expands on the main pathways of contamination and looks at linkages between exposure pathways and Hg concentrations found in peoples’ hair. Uncertainties inherent in risk analysis as well as their influence on the relative importance of different exposure routes are also discussed. The analysis showed that the risk was less than originally expected, the most likely cause being the ability of the fly-ash to reduce the bioavailability of the mercury.

Mercury concentrations in a contaminated river system in Kazakhstan and biomonitoring of the local population

The River Nura in central Kazakhstan has been heavily contaminated by mercury (Hg). […] Humans are exposed to Hg primarily via the consumption of contaminated fish from the river. A survey was undertaken in June/July 2005 to investigate Hg concentrations in river water, drinking water, sediments and fish. To estimate the risk posed to the local population, approximately 300 hair samples were collected from people living in villages near the most contaminated section of the river, at a distance of between 5 and 30 km downstream of the acetaldehyde plant, and their dietary habits were recorded. Mercury concentrations in river water ranged from 2-3 ng/L upstream of the source of the pollution to 348 ng/L downstream of the factory outfall pipe. Some drinking water wells close to the river were contaminated, but deeper wells had Hg concentrations below the detection limit. An earlier investigation conducted by us in 2002 found that average Hg concentrations in fish are between 0.4 and 0.5 mg/kg within 30 km from the outfall. Fish collected in 2005 also contained up to 0.5 mg/kg of Hg in their tissue. Mercury concentrations in human hair samples were found to be strongly linked with fish consumption. Overall, Hg concentrations in human hair ranged from 0.01 to 5.18 mg/kg, with a mean of 0.58 mg/kg (compared to 0.03-0.9 mg/kg for samples taken from a control group in an uncontaminated area). Mean Hg concentrations in hair were found to be twice as high for males (0.84 mg/kg, n=115) compared to females (0.41 mg/kg, n=175). Most significantly, people living in a village ~30 km downstream had a higher Hg bodyburden (as estimated from hair samples) than people living in villages in the most contaminated section of the river. This could be due to higher rates of MeHg production in less contaminated areas and has important implications for planned remediation work.