Article http://pubs.acs.org/journal/aesccq
Mercury Pollution in Amapá, Brazil: Mercury Amalgamation in Artisanal and Small-Scale Gold Mining or Land-Cover and Land-Use Changes? Rebecca Adler Miserendino,†,‡ Jean Remy Davée Guimaraẽ s,§ Gary Schudel,‡ Sanghamitra Ghosh,‡ José Marcus Godoy,∥ Ellen K. Silbergeld,† Peter S. J. Lees,† and Bridget A. Bergquist*,‡ †
Department of Environmental Health Sciences, Johns Hopkins University Bloomberg School of Public Health, 615 N. Wolfe Street, Baltimore, Maryland 21205-2103, United States ‡ Department of Earth Sciences, University of Toronto, 22 Russell Street, Toronto, Ontario M5S 3B1, Canada § Inst. de Biofísica Carlos Chagas Filho, Universidade Federal do Rio de Janeiro, Bloco G, CCS, Ilha do Fundão, 21949-902 Rio de Janeiro, Brazil ∥ Pontifícia Universidade Católica do Rio de Janeiro, Rua Marquês de São Vicente 225, 22453-900 Rio de Janeiro, Brazil S Supporting Information *
ABSTRACT: Mercury (Hg) poses a public health burden in the Amazon and worldwide. Although usually attributed to Hg used in artisanal and small-scale gold mining (ASGM), the primary source of elevated Hg in Amazonian aquatic ecosystems is contested since there have not been tools to differentiate between Hg from ASGM and Hg from other sources such as increased soil erosion associated with landcover and land-use change. To directly assess Hg contamination from ASGM, stable Hg isotope analyses were applied to sediment cores, surface sediments, and soils from two aquatic ecosystems in Amapá, Brazil, one downstream of ASGM activities and one isolated from ASGM. Downstream of the ASGM sites, the Hg isotope data is consistent with elevated Hg coming dominantly from increased erosion of soils and not from Hg used during gold extraction. Although these two sources represent different pathways of contamination to downstream ecosystems, ASGM may contribute to both land-cover and land-use change and local contamination of soils. Accordingly, these findings demonstrate that in some regions of the Amazon effective Hg mitigation strategies need to address land-use practices in addition to ASGM. KEYWORDS: Artisanal gold mining, gold mining, mercury, mercury isotopes, soil erosion, deforestation, Amazon, trace metal geochemistry
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INTRODUCTION
(ASGM) and also from increased soil erosion resulting from land-cover and land-use change (LCLUC), which is principally associated with deforestation. During ASGM gold is extracted through amalgamation with elemental mercury (Hg0), and some portion of this Hg can enter the environment where it may subsequently be methylated within aquatic systems. Although numerous studies generally attribute elevated downstream Hg concentrations in fish and sediment to ASGM,9−11 others suggest elevated Hg in some Amazonian aquatic systems results also from mobilization of soil Hg associated with LCLUC.12−22 The latter conclusion is based on comparisons of
Mercury (Hg) is a highly toxic pollutant emitted by natural and anthropogenic processes.1 While Hg can be toxic in all its forms, it is the organic form monomethylmercury, which is formed mostly through the biological methylation of inorganic mercury in aquatic ecosystems, that bioaccumulates in aquatic food webs and poses the most widespread health risk via fish consumption.1,2 The majority of global primary Hg emissions and environmental Hg is now attributable to anthropogenic sources;1,3 however, the importance of direct contributions from specific local anthropogenic activities relative to regional and global deposition of Hg and remobilization of natural and historic anthropogenic Hg in any one location is often unclear.4−6 In the Amazon, Hg sources are often debated and there are no direct tools to differentiate sources.7,8 There, Hg is released from Hg amalgamation during artisanal small-scale gold mining © XXXX American Chemical Society
Special Issue: Global Cycling of Mercury Received: Revised: Accepted: Published: A
August 7, 2017 September 16, 2017 September 19, 2017 September 19, 2017 DOI: 10.1021/acsearthspacechem.7b00089 ACS Earth Space Chem. XXXX, XXX, XXX−XXX
Article
ACS Earth and Space Chemistry
Figure 1. Sampling schematic with HgT, δ202Hg and Δ199Hg ranges for different sample types. Mine status reflects 2010 activity. Isotopic compositions are reported according to Blum and Bergquist.36 The 2σ errors for δ202Hg are ±0.14‰ and for Δ199Hg are ±0.04‰.
Hg concentrations and the geochemical composition of fine particulate matter in streams and sediments downstream ASGM locations12−16 and in soils before and after slash-andburn cultivation and conversion of land to pasture.17−20 While highly suggestive, these findings do not definitively identify Hg source and result in debates regarding Hg sources, especially since in many areas both ASGM and LCLUC activities are present. Differentiating between Hg derived from use during ASGM and from LCLUC is critical for understanding Hg transport into aquatic systems and for devising effective strategies to reduce environmental Hg and subsequent exposures in vulnerable populations. Measurements of Hg isotopic composition in natural samples show a large range of mass-dependent fractionation (MDF) and mass-independent fractionation (MIF), including Hg in sediments, soils, and ores.23−25 The discovery that specific biogeochemical processes, particularly photoreduction, impart large MIF signatures26 greatly expanded the usefulness of Hg isotopes. The combined interpretation of MDF and MIF allows for the application of Hg isotopes to determine the origin and postdepositional processing of Hg. Because ore-derived Hg and the Hg used for gold amalgamation are isotopically very different from the Hg in most soils,23−25,27−30 there is potential for Hg isotopes to distinguish between these two Hg sources, particularly in cases where environmental processes do not obscure the isotopic differences during transport. Ores are typically isotopically heavier than soils and tend not to exhibit MIF,23,31,32 whereas soils are isotopically light and tend to have negative MIF.27−30 In this study, stable Hg isotope analyses were used to identify the source of elevated Hg in the Tartarugalzinho Region of Amapá, Brazil. Sediment cores, surface sediments, and soils from two aquatic ecosystems were compared in two ecosystems in Amapá, Brazil, one downstream of ASGM activities (Lake
Duas Bocas ecosystem in Tartarugalzinho) and one isolated from ASGM (Lake Pracuúba ecosystem in Pracuúba). Since the late 1970s when gold prices increased, an influx of small-scale gold miners began operations in the Lake Duas Bocas area. These pursuits continued to grow, and in the mid-1990s the Brazilian Government launched an investigation to determine the source of Hg following concerns over observed high levels of environmental Hg as well as high levels in human populations. These investigations that were based on Hg concentration data concluded that Hg0 use during ASGM was responsible for the elevated Hg.10 Despite these conclusions and although Hg use is illegal, ASGM continues in this region and artisanal miners still use Hg-based technologies. However, it may not be so clear that direct use of Hg for amalgamation during ASGM is responsible for Hg delivery to downstream ecosystems in this region since the floodplains surrounding Lake Duas Bocas became increasingly exploited for buffalo and cattle production beginning at the same time as ASGM activities in the early 1970s. According to statistics collected by the Brazilian Institute of Geography and Statistics (IBGE), the prevalence of buffalo ranching increased by over 20-fold from the early 1970s to the present in the Lake Duas Bocas region (Figure S1).33 To create sufficient pasture for increased numbers of buffalo, ranchers have converted the land bordering rivers into pasture. The goal of this study was to assess whether Hg isotopes could distinguish between Hg used during ASGM and Hg mobilized by LCLUC in downstream ecoystems. It should be stated that ASGM activities themselves also contribute to LCLUC and may also contribute to local contamination of the soils, which are repositories for local, regional and global Hg deposition. Thus, although Hg use during ASGM and LCLUC are different sources of Hg to downstream ecosystems, Hg derived from erosion of soils should not be considered natural. B
DOI: 10.1021/acsearthspacechem.7b00089 ACS Earth Space Chem. XXXX, XXX, XXX−XXX
Article
ACS Earth and Space Chemistry
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MATERIALS AND METHODS Description of the Field Area. Tartarugalzinho is located in Amapá, Brazil approximately 200 km north of the state capital, Macapá (1° 30′ 21″ N/50° 54′ 43″ W) and is centered on the Lake Duas Bocas Ecosystem. Just north of Tartarugalzinho at 1° 44′ 37″ N/50° 47′ 4″ W, Pracuúba is similar to Tartarugalzinho in the socioeconomic characteristics of its population and is dependent on Lake Pracuúba. With high unemployment rates in both Tartarugalzinho and Pracuúba, most families survive through subsistence fishing in Lake Duas Bocas and its tributaries or in Lake Pracuúba, respectively. A smaller portion of the population exports fish to sell in the Macapá fish markets.10,34 Overall, the majority of the populations rely on the integrity of the aquatic ecosystems for their survival and livelihood. The mineralogy of the catchments for both Lake Duas Bocas and Lake Pracuúba are also similar, consisting of quartz, kaolinite, and mica, which occur in similar proportions in the sediments of both lakes.10 However, the gold resources in Tartarugalzinho, near Lake Duas Bocas, have supported an additional economic avenue in the form of ASGM that has not occurred in Pracuúba. There is no ASGM in the Pracuúba area or in the areas surrounding Lake Pracuúba. Additionally, since the dominant winds are northeasterly, atmospheric transport of Hg from ASGM from Lake Duas Bocas to Lake Pracuúba is unlikely (Figure S2). As described in the introduction, the floodplains surrounding Lake Duas Bocas are also exploited for buffalo and cattle production,33 which has increased over the same time period as ASGM. Since Lake Pracuúba does not have large tributaries, buffalo production in its immediate proximity has been limited as this environment does not provide a good habitat for animals to wallow. In contrast to larger scale industrial mining facilities that are zoned, permitted, and in operation for decades at a time, ASGM sites are typically informal, not geographically defined, nor necessarily long-lived. Thus, while all of the samples collected from our field area from 1996 and 2010 can be used to help understand the impact of Hg use during ASGM activities on the environment, the relative position of the samples to the ASGM sites differs based on the time of sample collection. In 1996, for example, the active mining sites were located on the Tartarugalzinho River. Whereas in 2010, the active mines were located on the Tartarugal Grande and mostly away from the rivers (Figure 1 and Figure S2). Thus, for this study, samples were compared based on their relative position (upstream or downstream) in relation to ASGM sites at the time of sample collection. Sample Collection. Archived sediment samples were collected in the dry season of 1996 as previously described.10 Briefly, sediment cores were acquired from Lake Duas Bocas and Lake Pracuúba, both of which have a water depth of approximately 2−3 m, using acid-washed Perspex tubes and a custom-made remote corer. Cores were sliced into 2.0 cm sections. River sediment grab samples were collected both upstream and downstream of the ASGM sites using an Eckman dredge along the Tartarugal Grande and Tartarugalzinho River systems. The 2010 grab samples and sediment core were similarly collected, though the core was collected using an Uwitec coring device (Uwitec Sampling Equipment, Austria). Mining samples were collected in 1996 and 2010 from both active and inactive ASGM sites from the Lake Duas Bocas ecosystem. In particular, sediment samples were collected from
active mines on the sluice box, fresh tailings, tailing ponds, as well as from tailing drainage (See Figures 1, S2, and S3). Surface soils (0−10 cm) were also collected in 2010 to capture potential differences in Hg concentration and isotopes between forested and recently deforested sites (i.e., deforested within five years). After removing the organic litter at the soil surface consisting of leaves and twigs, the soil sample was taken using a stainless-steel shovel. Both the forested and deforested sites were located similar distances away from ASGM sites to prevent potential differences in atmospheric deposition from local ASGM activities. All sediment, mining, and soil samples were stored in Whirlpak plastic bags or acid-washed polyethylene large-mouth bottles and kept frozen until analysis. The 1996 archived surface and core sediments were wet sieved to collect the
