Background concentrations of mercury in Australian freshwater sediments: The effect of catchment characteristics on mercury deposition

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.


Introduction
Mercury is a highly toxic compound, especially in its methylated form (Ullrich et al., 2001). Anthropogenic activities such as fossil fuel combustion , industrialization (Navarro et al., 1993), mining (Davies et al., 2018), and agricultural intensification (Denton and Breck, 1981) have led to increasing levels of mercury contamination in aquatic environments. Mercury is currently listed as one of the top 10 chemicals or groups of chemicals of major public health concern by the World Health Organization (2010). Previous studies of centennial-scale metal changes in levels in Australian freshwater systems have shown that metal concentrations in sediments had low variability over time prior to European settlement, the development of mines, and industrialization (Lintern et al., 2015). These studies indicate that it was only with European settlement and industrialization that the metal concentrations in aquatic sediments started to fluctuate significantly. These preindustrialization and pre-European concentrations of metals in aquatic sediments are often assumed to represent "background" levels of metals in aquatic sediments.
There are many waterways throughout the world where the mercury concentrations in aquatic sediments are elevated compared to pre-industrialization concentrations (Balogh et al., 1999(Balogh et al., , 2009Cooke et al., 2020). Australian aquatic systems such as Reedy Creek (Churchill et al., 2004), Lerderderg River (Bycroft et al., 1982), the Gippsland Lakes , Molonglo River (Stinton et al., 2020), the Connewarre Complex (Reeves et al., 2016), and Newell Creek , all show elevated concentrations of mercury in postindustrialization sediments. This is of concern due to the toxic impacts of elevated concentrations of mercury on humans (Castilhos et al., 2006), as well as on biodiversity and ecosystem survival (Selin, 2009). As such, efforts are currently being made to reduce mercury levels in waterways. These methods include controlling and reducing mercury sources within catchments, increased regulations to reduce atmospheric emissions of mercury, treatment of wastewater, and stabilization, dredging, or capping of mercury-contaminated sediments (Wang et al., 2004;Hylander and Goodsite, 2006;Mathews et al., 2013).
When remediating a polluted aquatic environment using these methods, target sediment and water toxicant levels are required to calculate a pollutant's enrichment factor compared to its background concentrations. Target toxicant levels are also needed to establish mitigation measures and restoration programs. Therefore, it is of crucial importance to identify the optimal sediment and water quality guidelines that should be used when restoring waterways affected by mercury contamination in Australia. Water and sediment quality management frameworks in many parts of the world, including Australia, state that pollution reduction targets should be based on the background conditions of the aquatic system (Canadian Council of Ministers of the Environment, 2003; Water Quality Australia, 2018). Water quality management frameworks also often provide guideline values calculated using ecotoxicity data (the probability of a toxic effect of a certain concentration of the chemical on benthic organisms), which can be used when background levels cannot be quantified (MacDonald et al., 2000;Water Quality Australia, 2018). The current default guideline value (DGV) for mercury in Australia and New Zealand is 150 ng/g. This represents the value below which there is a low risk of toxic environmental effects due to mercury. The guidelines also provide an upper guideline value (GV-high) of 1,000 ng/g for mercury, which represents the value above which we are likely to observe some toxic effects of mercury on the environment (Water Quality Australia, 2018). When the GV-high is exceeded, there is a high probability that the level of mercury in the sediments is toxic to benthic organisms. While GV-high threshold values provide an indication of concentrations at which toxicityrelated adverse effects are expected to be observed, the DGV threshold value is used for guidance on the remediation of contaminated sediments. Our paper focuses on the lower threshold value, the DGV, and aims to provide a guideline for restoration of polluted sites.
We currently have very little understanding of the difference between background conditions of freshwater lakes and existing guideline values (Lintern et al., 2016). This is largely because quantitative data of background conditions are unavailable due to a lack of water and sediment quality monitoring until the late 20th century (Alexander et al., 1998). Understanding the quantitative differences between background mercury levels and guideline values provided by sediment quality guidelines is critical in guiding waterway managers to select the most appropriate water quality and sediment quality targets for remediation projects. The water or sediment quality target selected will depend on the objectives of the project, which could range from restoring the aquatic environment to "pre-pollution" levels, protecting key aquatic species, to protecting human health.
The aim of this study is to determine the difference between background mercury concentrations and existing sediment quality guideline values in aquatic systems. We had the additional objective of determining the spatial variability in background mercury concentrations and predicting background mercury concentrations across Australia using our understanding of the key factors driving background mercury concentrations. In this study, we do not intend to assess the health risks of mercury to aquatic environments. Rather, we use sediment cores from 21 Australian lakes as a case study to identify background concentrations of mercury across the region (Förstner and Salomons, 1980;Dubois et al., 2017). Lake sediment cores have been used in previous studies to understand longterm historical trends in metal pollution of aquatic environments (Latimer et al., 2003;Lintern et al., 2015). Although a data set larger than 21 lakes would result in more robust statistical findings, these data provide an initial insight into background mercury concentrations in lakes in Australia and the Southern Hemisphere. Little is known about mercury in Australia, and this lack of evidence for national levels of risk contribute to the difficulties Australia has been facing in ratifying the Minamata Convention (Sinclair and Schneider, 2019). This study is the first step in filling our knowledge gap of background mercury levels in Australia and the Southern Hemisphere, which will provide evidence that may advance the call for ratification of this international convention.

Methodology
Site selection and sediment subsampling Sediment cores with known age-depth models from previous studies were selected for this study. This included cores from 21 freshwater lakes, the majority of which were located in Southeast Australia ( Figure 1). The characteristics of these sites have been summarized in Table 1. From the 21 cores listed in Table 1, one to four sediment subsamples were obtained from sediments deposited prior to industrialization and European settlement in Australia in 1788 CE (Powell, 1994). These sediment samples were taken from depths corresponding to approximately 2,000; 4,000; 6,000; and 8,000 calibrated years before present (BP). For the rest of this study, we use ka to represent thousands of calibrated years BP. It should be noted that not all cores had available sediments corresponding to each of these depths. A detailed inventory of the subsampling depth of each core and the approximate ages of the sediment subsamples are provided in the Supplementary Materials (Table S1). Seventy one subsamples were collected in total, each having a mass greater than 4 g wet weight.

Analysis of sediments
Sediment subsamples were taken to the Palaeoworks Lab at the Australian National University and freeze-dried using a FreeZone Plus 6 freeze-drier (Labconco, Kansas City, MO) for 48 h. Samples were then homogenized and crushed into a fine powder, and 100 mg of the sample was analyzed for total mercury concentration using the USEPA method 7473 (US EPA, 1998) and a Milestone Direct Mercury Analyzer (DMA-80 Tricell; Milestone, Bergamo, Italy). The DMA uses thermal decomposition, amalgamation, and atomic absorption spectrometry to identify total mercury concentrations in solid materials. A pair of blanks and a pair of certified reference materials (WQB-1 Lake Ontario sediment from the National Water Research Institute in Canada and 2711a Montana II soil from the National Institute of Standards and Technology in the USA) were analyzed for every 36 samples. A duplicate sample was run for every three samples, and results for these were within 10% of the original sample and reported as the mean between the duplicates. When the two duplicates had a difference in concentration higher than 10%, a triplicate was run.
All sediment samples were also analyzed for additional metals including aluminum (Al), arsenic (As), chromium (Cr), copper (Cu), lead (Pb), nickel (Ni), and zinc (Zn). These analyses were conducted at ALS in Scoresby,Victoria, a commercial laboratory accredited by the National Association of Testing Authorities. Sediments were first ground into a fine powder and then were digested using aqua regia (hydrochloric and nitric acids), and the digested sediments were analyzed using inductively coupled plasma mass spectrometry following USEPA SW846 (US EPA, 2007). Limits of reporting in dry weight are 5 mg/kg. All laboratory blanks were below the detection limit, and duplicate runs were within 6.8% difference of each other for all metals. Recovery rates were between 80.7% and 116% for all metals.
Particle size distribution of all sediment samples was determined at the Coastal Lab (University of Melbourne, Parkville, Victoria) using 0.25 g wet weight of the sediment sample. The sediment samples were first digested to remove all organic matter by heating the samples and slowly adding 30% hydrogen peroxide until all organic matter was removed. One ml of tetra-sodium pyrophosphate decahydrate was then added to disperse each sample, and samples were ultrasonicated for 30 minutes. The particle size distribution of sediments was then analyzed in a Beckman Coulter LP 13320, ensuring an obscuration of at least 7%.
Total organic carbon of the sediment samples was determined by weight loss on ignition at the ANU Palaeoworks Lab, following the protocols of Wang et al. (2011). After freeze-drying samples, shells and other visible organic materials were removed. Sediment was weighed to approximately 1 g per sample and then heated in

Data analysis
We calculated the median, minimum, and maximum concentrations of mercury prior to industrialization for each lake. These distributions were then compared to sediment quality guideline values (DGVs) in Australia (Water Quality Australia, 2018) to quantify the differences between background mercury concentrations and the DGVs. We attempted to explain the spatial variability in background mercury concentrations by identifying relationships between background mercury concentrations, sediment characteristics and lake catchment characteristics. The sediment characteristics assessed included grain size (% clay) and organic matter content. The lake catchment characteristics assessed included elevation, current average annual temperature, current average annual rainfall and evapotranspiration and lithology. These catchment characteristics were obtained from national spatial data sets (Geoscience Australia, 2011Australia, , 2012. For all categorical catchment characteristics, we used the Kruskal-Wallis chi-squared test (a ¼ .05) to assess whether statistically significant differences in background mercury concentrations were explained by catchment characteristics. For numerical catchment characteristics, the Spearman Correlation Coefficient (Spearman, 2010; a ¼ .05) was used to assess whether there was a statistically significant relationship between the catchment characteristic and background mercury concentration. These analyses were completed in RStudio Version 3.5.2 (RStudio Team, 2015).
Finally, we developed predictions of background mercury concentrations in waterways across Australia. We used the statistical distributions of background mercury concentrations from each geological formation for which background sedimentary mercury concentrations were available. We log-transformed all background mercury concentrations. We then determined the median, 2.5th and 97.5th percentiles of the distributions of background log-transformed mercury concentrations for each geological formation.

Mapping
A map with the predicted background log-transformed mercury concentrations across Australia was produced using Arc Map 10.6.1. The Join Feature tool was used to combine a layer of Australian lithology (Geoscience Australia, 2012) with the 2.5th, 50th, and 97.5th percentiles of mercury background distribution calculated for the four lithologies related to the lakes in this study: igneous mafic intrusive, igneous mafic volcanic, regolith, and sedimentary siliciclastic. Areas where lithological data were not available were left blank.

Results and discussion
Background mercury concentrations in sediments compared to sediment quality guidelines Background mercury concentrations in the 21 Australian lakes ranged from 15.6 ng/g to 249 ng/g. DGVs for mercury derived using ecotoxicological data are 150 ng/g in Australia (Water Quality Australia, 2018). In some Australian lakes, background mercury concentrations were considerably lower than the existing DGVs. Figure 2 indicates that 19 of the 21 lakes had median background mercury concentrations that were less than the existing DGVs derived from ecotoxicological data. These results indicate that existing sediment quality guidelines in Australia can overestimate Australian background sediment mercury concentrations by at least nine times (Figure 2). Two lakes (Hartz and Perry, all located in Tasmania) had median background mercury concentrations higher than the default mercury guideline values (Figure 2). This suggests that under current guideline values, these lakes would potentially be considered contaminated even prior to industrialization. This indicates the importance of identifying the background sediment concentrations of aquatic systems and using these to help determine restoration targets.
Background concentrations of other metals (As, Cr, Cu, Pb, Ni, and Zn) were mostly below the detection limit of 5 mg/kg, so no data analyses were performed with these data (presented in Supplementary Table S1).

Variability in background sediment mercury concentrations
As indicated in Figure 2, there is considerable spatial variability in background mercury concentrations in lakes across Australia. Background mercury concentrations ranged from 12 ng/g at Tareena Billabong in New South Wales to 206 ng/g at Hartz Lake in Tasmania.
From the 21 lakes studied, the spatial variability is considerably greater than the temporal variability in background mercury concentrations. The spatial variability in background mercury concentrations (represented by the mercury concentration range divided by the median) is 433% between the 21 lakes but ranges from 5% to 114% for the temporal variability (also represented by the mercury concentration range divided by the median) in background concentrations. In Figure 3, black dots represent the background median values (calculated using the samples taken from 2 ka to 8 ka). The error bars represent the ranges (i.e., temporal variability) in background mercury concentrations between 2 ka and 8 ka for each site.
This graphical representation demonstrates that, between 2 ka and 8 ka, spatial variability in background concentrations is greater than temporal variability. This agrees with previous works indicating consistent background levels of metals in lake cores prior to European settlement and industrialization of Australia (Hollins et al., 2011;Lintern et al., 2015). Note that lakes from mainland Australia have less variability of mercury concentration than sites in Tasmania.

Relationship between background mercury concentrations and catchment characteristics
There is a strong positive correlation between mercury concentrations and organic matter (r ¼ 0.90, P < 0.05), as well as a positive correlation between mercury (r ¼ 0.56, P < 0.05), elevation and rainfall (r ¼ 0.32, P < 0.05; Figure 4). Sediment grain size and catchment evapotranspiration and temperature are not significantly correlated to spatial differences between the background mercury concentrations (Figure 4).
The high correlation of mercury with organic matter agrees with previous studies (Bengtsson and Picado, 2008;Chakraborty et al., 2015). As for other locations worldwide, mercury in these 21 freshwater catchments of Australia is highly influenced by reduction and complexation with organic matter (Chakraborty et al., 2015). As such organic matter content of sediments influences spatial differences in mercury concentrations. The correlation between rainfall and elevation can be explained by the process of wet deposition in which mercury aerosol and reactive gaseous forms of Hg(II) (RGM) are efficiently scavenged by precipitation events. This enables the deposition of atmospheric mercury to the earth's surface and freshwater catchments Art. XX, page 6 of 12 Lintern et al: Background concentrations of mercury in Australian freshwater sediments (Guentzel et al., 2001). Catchment elevation is likely correlated to mercury in this study due to its correlation with rainfall ( Figure 4). The highest lakes in this study are located in Tasmania, one of the wettest regions in Australia (BOM, 2019).
The Kruskal-Wallis H test showed mercury concentrations were significantly different depending on the catchment lithology of the lakes w 2 (2) ¼ 24.678 P ¼ 1.8Â10 -5 ( Figure 5). Post hoc comparisons using the Bonferroni test indicated that "igneous mafic intrusive" was  the catchment rock type with highest mercury concentrations, significantly different from "sedimentary siliciclastic", "regolith" and "igneous mafic volcanic" rock (P ¼ 1.4Â10 -2 , P ¼ 5.8Â10 -5 , and P ¼ 1.8Â10 -4 , respectively). The largest mercury concentrations were measured in igneous mafic intrusive formations and the lowest concentrations in the regolith. The higher mercury concentrations in the igneous mafic intrusive formations is a result of mercury being sourced from volcanic activities (Gustin et al., 2000). The most important and efficient natural source of mercury is volcanic activity, which liberates mercury via quiescent degassing events that overwhelm the atmospheric budget of mercury (Higueras et al., 2013). Thus, igneous mafic intrusive formations in Australian freshwater catchments play important roles on mercury concentration in freshwater lakes in the country.
We suggest the use of this link between lithology and background mercury concentrations in lake sediments as a useful way to predict background mercury concentrations in lakes where background sediment samples are not available. In addition, this assessment of catchment lithology and distribution of mercury may identify potential hot spots for natural mercury emissions in Australia. Currently, there is no published material available on natural emissions of mercury from soils in the country. The link between rock formation and mercury concentrations provided in this study is the first step to understand the natural mercury emissions in the country and provides guidance to future studies on soil emission fluxes. This is particularly the case for mercury emissions from fire, as bushfires promote volatilization of mercury in soil .

Predicted background mercury concentrations across Australia
Predicted background mercury concentrations (log-normalized) across Australia are presented in Figure 6. Regions across eastern Tasmania and northwestern Australia are likely to present the highest levels of background mercury concentrations in freshwater sediments (with a predicted log-normalized median of 4.8 ng/g), with the lowest background mercury concentrations likely in inland areas in eastern Australia (with a log-normalized median of 3.03 ng/g). This is a concern for mercury management in Tasmania, considering that several current and legacy mining sites, which mobilize mercury from the earth's crust to the atmosphere, are located in this area of the country (Unger et al., 2012).
While these maps could be used as a preliminary guide when determining background mercury concentration for water quality and sediment quality management purposes, further analysis is recommended to refine and expand these maps and validate results with fine-scale analyses. First, as only 21 lakes were used in this analysis, there are several lithologies that were excluded from Figure 5, including sedimentary carbonates, organic-rich rocks, and argillaceous detrital sediments. The inclusion of cores from lakes with these geologies in the analysis could expand the coverage of our predicted background mercury concentrations across Australia. Second, some of the distributions of background mercury concentrations are based on as little as 12 data points (e.g., for the igneous mafic intrusive and igneous mafic volcanic formations). The inclusion of a larger number of lake core samples could further refine the statistical distributions available for background mercury concentrations for waterways with these geologies. Further, the risk of mercury contamination to society and the environment suggests a more comprehensive program of background analyses based on pre-European sediment cores is warranted. Finally, the estimates provided in these maps should not be considered in areas in which cinnabar deposits have been recorded (McQueen, 2011).

Conclusions
This study aims to compare background lake sediment concentrations of mercury in Australia to existing sediment quality guidelines. We defined "background" lake sediments as those deposited prior to industrialization, mining, and intensification of agricultural activities in Australia in the early 19th century. Mercury concentrations in background lake sediments from 21 lakes across Australia indicate that a large proportion of lake sediments have mercury concentrations that are lower than the Australian sediment quality guidelines. Sediment quality guidelines for mercury were nine times larger than background mercury sediment concentrations. This indicates that, for mitigation or waterway management projects aiming to restore waterways polluted by mercury, "background" levels should be considered in addition to the existing Australian sediment quality guideline values. An understanding of site-specific background mercury concentrations is necessary for setting these restoration targets. Furthermore, this suggests that adoption of the existing sediment quality guidelines for mercury in Australia should not be used as an absolute value to protect some ecosystem services (e.g., benthic organisms or vegetation) native to the local region. Studies have reported that mercury toxicity tolerance is linked to exposure history. Flora and fauna that live in high mercury concentration sites are able to tolerate higher dosages of mercury than those that have no previous exposure (Chang, 1977;Singh, 2005). In this context, the DGV value should be used more carefully at sites with lower background mercury concentrations. We recognize that determining background mercury concentrations for every waterway in Australia is not feasible. Using our understanding of the background mercury concentrations in 21 lake systems across Australia, we have identified that these background concentrations are mainly driven by organic matter, precipitation, and underlying geology of the region. We have produced maps that predict background mercury concentrations, based on geology, across Australia. These maps offer an initial baseline against which current sediment mercury concentrations can be compared. Further studies using a greater number of sites, particularly from northern and western parts of the continent, would provide more robust predictions of background mercury concentrations in waterway sediments. We recommend that future researchers and the Australian government work together to develop a coordinated program of analyzing pre-European sediments to improve the robustness of this map. Regardless, this study provides the first inventory of mercury concentrations in sediments in Australia, which could be used as restoration targets in remediating Australian waterways with elevated mercury levels. Furthermore, the results of this study provide baseline guidance for future mercury studies in the freshwater lake systems in Australia.

Data Accessibility Statement
All data generated in this study are available in the supplementary materials.

Supplemental file
The supplemental files for this article can be found as follows: Table S1. Background mercury concentrations used to calculate median and range of background mercury concentrations, and other metals measured in 21 lake sediments in Australia. Docx.