Mercury is a toxic bioaccumulative pollutant, with the atmosphere being the primary pathway for global distribution. Although atmospheric mercury cycling has been extensively monitored and modeled across the Northern Hemisphere, there has long been a dearth of mercury data for the Southern Hemisphere. Recent efforts in Australia are helping to fill this gap, with new observational records that span environments ranging from cool temperate to warm tropical climates and near-source to background conditions. Here, we review recent research on atmospheric mercury in Australia, highlighting new observational constraints on atmospheric concentrations, emissions, and deposition and, where possible, comparing these to model estimates. We also provide our best estimate of the current Australian atmospheric mercury budget. Ambient mercury observations collected to date show unique features not captured at other observing sites across the Southern Hemisphere, including very low concentrations at inland sites and a monsoon season drawdown in the tropical north. Previously compiled estimates of Australian anthropogenic mercury emissions differ substantially due to both methodological differences (e.g., assumptions about mercury control technology in coal-fired power plants) and recent closures of major Australian mercury sources, and none are appropriate for modern-day Australia. For mercury emissions from biomass burning, new measurements from Australian smoke plumes show emission factors for both savanna and temperate forest fires are significantly lower than measured elsewhere in the world, and prior estimates based on non-Australian data are likely too high. Although significant uncertainties remain, our analysis suggests that emissions from terrestrial sources (both newly released and legacy) significantly exceed those from anthropogenic sources. However, recent bidirectional air-surface flux observations suggest this source is likely balanced by deposition and surface uptake at local scales. Throughout, we highlight lingering uncertainties and identify critical future research needs for understanding Australian atmospheric mercury and its role in Southern Hemisphere mercury cycling.

Waterways in the Southern Hemisphere, including on the Australian continent, are facing increasing levels of mercury contamination due to industrialization, agricultural intensification, energy production, urbanization, and mining. Mercury contamination undermines the use of waterways as a source of potable water and also has a deleterious effect on aquatic organisms. When developing management strategies to reduce mercury levels in waterways, it is crucial to set appropriate targets for the mitigation of these contaminated waterways. These mitigation targets could be (1) trigger values or default guideline values provided by water and sediment quality guidelines or (2) background (pre-industrialization) levels of mercury in waterways or sediments. The aims of this study were to (1) quantify the differences between existing environmental guideline values for mercury in freshwater lakes and background mercury concentrations and (2) determine the key factors affecting the spatial differences in background mercury concentrations in freshwater lake systems in Australia. Mercury concentrations were measured in background sediments from 21 lakes in Australia. These data indicate that background mercury concentrations in lake sediments can vary significantly across the continent and are up to nine times lower than current sediment quality guidelines in Australia and New Zealand. This indicates that if waterway managers are aiming to restore systems to ‘pre-industrialization’ mercury levels, it is highly important to quantify the site-specific background mercury concentration. Organic matter and precipitation were the main factors correlating with background mercury concentrations in lake sediments. We also found that the geology of the lake catchment correlates to the background mercury concentration of lake sediments. The highest mercury background concentrations were found in lakes in igneous mafic intrusive regions and the lowest in areas underlain by regolith. Taking into account these findings, we provide a preliminary map of predicted background mercury sediment concentrations across Australia that could be used by waterway managers for determining management targets.

Mercury (Hg) contamination is an environmental concern as a by-product of legacy mining in Australia. Here we investigate the spatial and temporal distribution of Hg in the Molonglo River system in New South Wales, Australia, and assess the physical and chemical factors influencing that distribution. Mercury concentrations in sediment cores were measured in conjunction with 210 Pb and 137 Cs dating to establish historical contamination. This was done at the source mine site of Captains Flat, New South Wales, and the system’s sink in Lake Burley Griffin, Australian Capital Territory. Additionally, surficial sediment Hg concentrations along the Molonglo River were analyzed to determine the spatial distribution of Hg. Analytical results showed the primary physical and chemical factors influencing Hg dispersion to be distance, total organic matter, and the presence of iron oxides and oxyhydroxides. The highest Hg concentrations were near the mine site at Captains Flat and decreased significantly with distance. Sediment core analyses in both Captains Flat and the lake showed reductions in Hg concentrations toward surficial sediment layers. It is suggested government-funded rehabilitation programs are playing a part in reducing the release of metal contamination.

Health and ecological risks associated with the use of mercury in gold mining are well known, with much recent attention focussed on contemporary small-scale artisanal mining. Legacy tailings from historical gold mining may also present ongoing risks, as the industry used large quantities of mercury with minimal environmental regulation to limit its discharge. This occurred in both alluvial (placer) mining and in processing auriferous ores. Analysis of historical data on mercury use in the mining industry in Victoria, Australia, indicates that at least 131 tonnes of elemental mercury were discharged into the environment as mine tailings between 1868–1888, with the total amount lost over the historic mining period likely to be much higher. The processing of pyritic ores also concentrated mercury losses in a small number of mining centres, including Bendigo, Ballarat, Castlemaine, Clunes, Maldon and Walhalla. This analysis provides a basis for further research needed to support improved management of legacy mine tailings.

Terrestrial air-surface exchange of mercury (Hg) forms an important component of the global Hg cycle, with drivers varying across spatial and temporal scales. These drivers include substrate properties, atmospheric chemistry, and meteorological factors. Vegetation uptake represents the dominant pathway of atmospheric Hg deposition to terrestrial surfaces. This study investigated the drivers of net ecosystem exchange of gaseous elemental mercury (Hg 0 ) across multiple seasons in order to gain an understanding of the influence of vegetation and other environmental parameters on the Hg 0 air-surface exchange. Measurements were made continuously using a micrometeorological aerodynamic flux gradient method at a low-vegetated background site in south-eastern Australia, over 14 months. Mean Hg fluxes and atmospheric concentrations across the entire study period were 0.002 ng m –2 h –1 (SD ± 14.23 ng m 2 h –1 ) and 0.68 ng m –3 (SD ± 0.22 ng m –3 ), respectively. Variability was observed across seasons, with the highest average rate of emissions occurring in austral summer (December, January, February) (0.69 ng m –2 h –1 ) and the highest rate of deposition observed in autumn (March, April, May) (–0.50 ng m –2 h –1 ). Vegetation uptake dominated Hg flux during the winter and spring when meteorological conditions were cold and light levels were low. This is supported by CO 2 flux data, with a daytime winter mean of 0.80 µmol m –2 h –1 and a spring daytime mean of 1.54 µmol m –2 h –1 . Summer Hg fluxes were dominantly emission due to higher solar radiation and temperature. Climatic conditions at Oakdale allowed plant production to occur year-round, however the hot dry conditions observed in the warmer months increased evasion, allowing this site to be a small net source of Hg 0 to the atmosphere.

Since the onset of hydraulic gold mining in California’s Sierra Nevada foothills in 1852, the environmental damage caused by displacement and storage of hydraulic mining sediment (HMS) has been a significant ecological problem downstream. Large volumes of mercury-laden HMS from the Yuba River watershed were deposited within the river corridor, creating the anthropogenic Yuba Fan. However, there are outstanding uncertainties about how much HMS is still contained within this fan. To quantify the deep storage of HMS in the Yuba Fan, we analyzed mercury concentrations of sediment samples collected from borings and outcrops at multiple depths. The mercury concentrations served as chemostratigraphic markers to identify the contacts between the HMS and underlying pre-mining deposits. The HMS had mercury concentrations at least ten-fold higher than pre-mining deposits. Analysis of the lower Yuba Fan’s volume suggests that approximately 8.1 × 10 7 m 3 of HMS was deposited within the study area between 1852 and 1999, representing ~32% of the original Yuba Fan delivered by 19 th Century hydraulic gold mining. Our estimate of the mercury mass contained within this region is 6.7 × 10 3 kg, which is several orders of magnitude smaller than what was estimated to have been lost to the mining process. We suggest that this discrepancy is likely due to a combination of missing (yet to be found) mercury masses stored upstream, overestimated losses during mining, and high delivery of mercury to the lowland Sacramento Valley and to the San Francisco Bay-Delta system, where it poses a great risk to sensitive ecosystems.

The largest source of global mercury (Hg) anthropogenic inputs to the environment is derived from artisanal and small-scale gold mining (ASGM) activities in developing countries. While our understanding of global Hg emissions from ASGM is growing, there is limited empirical documentation about the levels of total mercury (THg) and methylmercury (MeHg) contamination near ASGM sites. We measured THg and MeHg concentrations in soil (n = 119), sediment (n = 22), and water (n = 25) from four active ASGM villages and one non-ASGM reference village in Senegal, West Africa. Nearly all samples had THg and MeHg concentrations that exceeded the reference village concentrations and USEPA regulatory standards. The highest median THg concentrations were found in huts where mercury-gold amalgams were burned (7.5 μg/g), while the highest median MeHg concentrations and percent Hg as MeHg were found in river sediments (4.2 ng/g, 0.41%). Median river water concentrations of THg and MeHg were also elevated compared to values at the reference site (22 ng THg/L, 0.037 ng MeHg/L in ASGM sites). This study provides direct evidence that Hg from ASGM is entering both the terrestrial and aquatic ecosystems where it is converted in soils, sediment, and water to the neurotoxic and bioavailable form of MeHg.

Mercury contamination in the Great Lakes continues to have important public health and wildlife ecotoxicology impacts, and atmospheric deposition is a significant ongoing loading pathway. The objective of this study was to estimate the amount and source-attribution for atmospheric mercury deposition to each lake, information needed to prioritize amelioration efforts. A new global, Eulerian version of the HYSPLIT-Hg model was used to simulate the 2005 global atmospheric transport and deposition of mercury to the Great Lakes. In addition to the base case, 10 alternative model configurations were used to examine sensitivity to uncertainties in atmospheric mercury chemistry and surface exchange. A novel atmospheric lifetime analysis was used to characterize fate and transport processes within the model. Model-estimated wet deposition and atmospheric concentrations of gaseous elemental mercury (Hg(0)) were generally within ∼10% of measurements in the Great Lakes region. The model overestimated non-Hg(0) concentrations by a factor of 2–3, similar to other modeling studies. Potential reasons for this disagreement include model inaccuracies, differences in atmospheric Hg fractions being compared, and the measurements being biased low. Lake Erie, downwind of significant local/regional emissions sources, was estimated by the model to be the most impacted by direct anthropogenic emissions (58% of the base case total deposition), while Lake Superior, with the fewest upwind local/regional sources, was the least impacted (27%). The U.S. was the largest national contributor, followed by China, contributing 25% and 6%, respectively, on average, for the Great Lakes. The contribution of U.S. direct anthropogenic emissions to total mercury deposition varied between 46% for the base case (with a range of 24–51% over all model configurations) for Lake Erie and 11% (range 6–13%) for Lake Superior. These results illustrate the importance of atmospheric chemistry, as well as emissions strength, speciation, and proximity, to the amount and source-attribution of mercury deposition.

The past and future of cities are inextricably linked, a linkage that can be seen clearly in the long-term impacts of urban geochemical legacies. As loci of population as well as the means of employment and industry to support these populations, cities have a long history of co-locating contaminating practices and people, sometimes with negative implications for human health. Working at the intersection between environmental processes, communities, and human health is critical to grapple with environmental legacies and to support healthy, sustainable, and growing urban populations. An emerging area of environmental health research is to understand the impacts of chronic exposures and exposure mixtures—these impacts are poorly studied, yet may pose a significant threat to population health. Acute exposure to lead (Pb), a powerful neurotoxin to which children are particularly susceptible, has largely been eliminated in the U.S. and other countries through policy-based restrictions on leaded gasoline and lead-based paints. But the legacy of these sources remains in the form of surface soil Pb contamination, a common problem in cities and one that has only recently emerged as a widespread chronic exposure mechanism in cities. Some urban soils are also contaminated with another neurotoxin, mercury (Hg). The greatest human exposure to Hg is through fish consumption, so eating fish caught in urban areas presents risks for toxic Hg exposure. The potential double impact of chronic exposure to these two neurotoxins is pronounced in cities. Overall, there is a paradigmatic shift from reaction to and remediation of acute exposures towards a more nuanced understanding of the dynamic cycling of persistent environmental contaminants with resultant widespread and chronic exposure of inner-city dwellers, leading to chronic toxic illness and disability at substantial human and social cost.