Impact of gold-mining activity on trace elements enrichment in the West African estuaries: The case of Pra and Ankobra rivers with the Volta estuary (Ghana) as the reference
Graphical abstract
Introduction
Estuaries are effective sediment traps due to their particular hydrodynamics and to the salinity induced sediment flocculation (Postma, 1967; Allen et al., 1980). Contamination of estuarine sediments is of broad concern since they represent one of the ultimate sinks for pollutants discharged into the aquatic environment. They reflect the proximity of the contamination sources and can preserve the fingerprint of the range of chemical, oceanographic, geological and anthropogenic factors, which govern the distribution of trace elements (Bryan and Langston, 1996; Olmos and Birch, 2008; Magesh et al., 2011; Mil-Homens et al., 2014; Xu et al., 2014). This makes these aquatic environments of particular interest for assessing anthropogenic impacts (Pan and Wang, 2012).
Among estuaries, those affected by mining activities are of special concern since they can be severely polluted aquatic environments (López-González et al., 2006; Doe et al., 2017). One of the major impacts comes from the artisanal and small-scale gold mining, where the use of mercury (Hg) for amalgamation is common, with associated yearly discharges to the environment of this metal of 650–1000 tons (Adjei-Kyereme et al., 2015). In West Africa, Ghana is the most representative case of this exploitation of natural resources, with half of the total gold production in the region (World Bank, 2012; Chuhan-Pole et al., 2017).
In May 1989, the use of Hg in small-scale gold mining was legalized in Ghana (Provisional National Defence Council Law 218). This has increased these mining activities, which nowadays provides employment to over one million people (Donkor et al., 2006). The greater part of gold production (about 81%) in Ghana comes from goldfields in its southwestern region (Fig.1), an area of about 40,000 km2 which is drained by three main rivers: Tano, Ankobra, and Pra (Donkor et al., 2006).
To separate the gold (Au), Hg is mixed with the mineral bearing rock to form an amalgam. The burning of the amalgam leads to vaporization and spreading of Hg into a toxic plume (Pfeiffer and Lacerda, 1988). Mercury also reaches the streams of water, where it poses a great threat for the environment and human health (Oduro et al., 2012). A brief review on health effects due to exposures to Hg can be seen at the US-EPA website (www.epa.gov/mercury). Although much attention has been paid to gold-mining related Hg pollution, this is not the only concern derived from this activity.
Tailings are a mixture of finely ground rock resulting from the retrieval of gold and water used in the processing. They may contain significant concentrations of potentially toxic elements. Arsenic (As) in mine tailings usually exists as sulfide minerals such as arsenopyrite [FeAsS], realgar [As2S2] and orpiment [As2S3]. Zinc (Zn) and lead (Pb) occur in gold ore bodies in the form of sphalerite [ZnS] and galena [PbS], respectively, while copper (Cu) appears in sulphides, arsenites, chlorides and carbonates, and chromium (Cr) in chromates [FeCr2O4] (Fashola et al., 2016).
In the mining tailings exposed to the air, oxidation of sulfide minerals results in the release of As and other potentially toxic elements, thus promoting the contamination of the surrounding soil and waters (Lim et al., 2009). Tailings spillages are more drastic pathways of pollution which can occur by flood damages and ground subsidence (Bempah et al., 2013).
Environmental risk assessment of the sediment compartment is a complex task which is subject to intense debate (Tarazona et al., 2014). It is necessary a proper problem definition (hazardous elements, target organisms, and level of protection required) and a conceptual model for exposure and effect assessments. This requires a detailed knowledge on the sedimentological and biotic characteristics and on the dynamics of the studied system. Sediment quality guidelines (SQGs), based upon the definition of reference toxicity values, can be used for screening purposes.
A series of reference concentrations in sediments have been defined for a wide set of elements (Macdonald et al., 1996), such as the lower 10th-percentile concentration associated with observation of biological effects (effect range low, ERL) and the probable effects level (PEL). These screening levels are widely used in the scientific literature to assess the consequences of pollution (e.g., Suresh et al., 2012; Pereira et al., 2015; Ennouri et al., 2016). Other pollution indexes are based on the comparison with suitable reference levels (Loring and Rantala, 1992).
Current data available in West Africa are mostly focused on Hg and As (Amonoo-Niezer et al., 1996; Bannerman et al., 2003; Donkor et al., 2006; Asante and Ntow, 2009; Rajaee et al., 2015; Adjei-Kyereme et al., 2015). Donkor et al. (2005) measured Hg, Al, Fe, As, Pb, Cu, Cr, Ni, Mn, Co, V, and Zn in sediments sampled along the course of the Pra River. In spite of above referred studies, there is a lack of knowledge on how the gold mining activities in West Africa may increase the concentrations of a wide set of harmful trace-elements in waterbodies and estuarine environments.
The aim of this study was the assessment of major and trace-metal concentrations in surface sediments from three representative estuarine environments in Ghana, in order to explore their natural and anthropogenic inputs. The target elements were Be, B, Al, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Sr, Mo, Ag, Cd, Sb, Cs, Ba, Tl, Pb, Th, U and Hg. In the absence of site-specific sediment quality guidelines, this work adopts reference values and SQGs from scientific literature as a preliminary and logical step. The core of the proposed methodology is however the comparison at regional scale of sediments from estuaries potentially affected (Pra and Ankobra) with other free of gold mining activities (the Volta estuary).
Section snippets
The studied estuaries
This study was conducted in three major estuaries (Volta, Pra and Ankobra) in the coastal belt of Ghana (see Fig. 1). There are no official logs on gold-mining sites in Ghana. Fig. 1 shows some of them in the Pra and Ankobra basins, based on data by Donkor et al. (2005) and on a survey conducted with local communities for this study. In this rural south-western portion of Ghana the lack of industrial activities makes small-scale gold mining the main source of water resource contamination with
Granulometric characterization of samples from the Ankobra estuary
The Rosin-Rammler particle size distributions for sediment samples from the Ankobra estuary (Fig. 3) led to mean grain sizes of 77.6, 86.8, 91.4 and 78.0 μm, with modal values of 70, 91, 96 and 76 μm for sediment samples SA-1 to SA-4, respectively (see sampling sites in Fig. 2). Discussion on the implications of granulometry will be readdressed in Subsection 3.6.
Multielemental analyses
Results from the multielemental analyses of the surficial sediment samples in the three studied estuaries are summarized in Table 2,
Conclusions
The mean concentrations of most of the studied trace elements were higher in the two gold-mining impacted estuaries (Pra and Ankobra) than in the Volta.
The application of Igeo with reference levels from literature served for stablishing as suitable local reference the concentrations measured in the Volta estuary, unaffected by gold mining activities.
These reference levels served to estimate Igeo, EF(Al) and EF(Fe) for all the studied elements in the Pra and Ankobra estuaries, which revealed
Acknowledgements
The authors would like to extend gratitude to the International Atomic Energy Agency (IAEA) for supporting the Regional Project RAF7009 and the fellowship GHA-16025, and to the Spanish Ministry of Economy and Competitiveness and the European Regional Development Fund of the European Union through the National Research, Development and Innovation Program for complementary funding (Plan Nacional I + d + i, Project AGl2014-57835-C2-1-R). Authors appreciate the technical support by Dr Oliva
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Prof. Elvis Nyarko is also the Vice Chancellor of the Regional Maritime University, Accra, Ghana.