Environmental radioactivity and trace metals in surficial sediments from estuarine systems in Ghana (Equatorial Africa), impacted by artisanal gold-mining

Highlights

• Sediments sampled from the Ankobra, Pra and Volta estuaries in Equatorial Africa.

• ²³⁸U and ²³²Th series and ⁴⁰K in the range of concentrations in the Earth's crust.

• Enrichments of As, Sb, Cu, Cs and Cr by gold-mining in the Ankobra and Pra basins.

• They can be associated to Fe-rich compounds in sulphide ores (such as FeAsS).

• ²¹⁰Pb informing on local sedimentary conditions and pollution pathways.

Abstract

This paper reports concentrations of γ-emitter radionuclides (⁴⁰K, ¹³⁷Cs, ²¹⁰Pb, ²²⁶Ra, ²²⁸Ra, ²²⁸Th and ²³⁴Th) and some metals (Al, Cr, Fe, Co, Ni, Cu, Zn, As, Sr, Cd, Sb, Cs, Pb, Th and U) in surficial sediments from the Ankobra, Pra and Volta estuaries, in Ghana. Artisanal gold-mining in the Ankobra and Pra basins promoted moderate enrichments of As, Sb, Cu, Cs and Cr in their estuarine sediments, with respect to the reference background of the Volta Estuary. Radionuclide concentrations were in the range found in the Earth's crust. Present data do not support any conclusion on their potential enrichments due to gold-mining activities. Radionuclide isotopic ratios revealed a transfer of ²²⁸Ra from sediments to the water column. Pearson correlation coefficient matrices showed different patterns, which were reasonably understood after novel approaches: i) inter-estuaries comparison of slopes in the linear regressions of element-concentrations vs Al, Fe and Cs; ii) study of Al-normalized concentrations of elements; iii) excess ²¹⁰Pb informing on local sedimentary conditions. The metal enrichments observed in the Ankobra and Pra estuaries are associated with the Fe-rich compounds in sulphide ores (such as FeAsS) transported along the river course and deposited in the estuary.

Introduction

Anthropogenic activities are increasingly altering the landscape and affecting sediment fluxes to the aquatic environments. Estuaries are effective sediment traps due to their particular hydrodynamics and they can represent one of the ultimate sinks for pollutants discharged into the aquatic environment.

Measuring concentrations of potentially harmful elements, such as some heavy metals, is in the basis for the environmental risk assessment of the sediment compartment (Tarazona et al., 2014). Sediment quality guidelines (SQGs), based upon the definition of reference toxicity values, can be used for screening purposes (Macdonald et al., 1996). Other pollution indexes, such as the enrichment factor (EF) and the geo-accumulation index (I_geo) are based upon the comparison with suitable reference levels (Loring and Rantala, 1992). The radionuclide content in sediments can pose radiological hazards, and there are several established indexes for assessing the radiation dose to humans (e.g., Botwe et al., 2017) and to a series of reference organisms (Larsson, 2008).

Multielemental and isotopic analysis, among other sediment properties, can support the sediment fingerprinting technique, allowing for the identification of sediment sources and the understanding of some features of sediment dynamics within river basins (Koiter et al., 2013; Reese et al., 2019). Natural occurring radionuclides have been used for assessing anthropogenic impacts in river basins and estuaries, such as the releases from the phosphate industry (Travesi et al., 1997), the magnitude and impact of acid mining drainage and industrial effluents (Hierro et al., 2012; Villa et al., 2011), or conventional metal mining activities (Manjón et al., 2019).

The study in sediment cores of the ²¹⁰Pb found in excess with respect to its parent radionuclide, ²²⁶Ra, has shown to provide useful insights on the functioning of the sedimentary systems at a centennial time scale (Robbins, 1978; Mabit et al., 2014). Independent chronostratigraphic information can be provided by ¹³⁷Cs, a man-made radionuclide first introduced in the environment after the atmospheric nuclear weapon tests (Robbins and Edgington, 1975; Sánchez et al., 1992; Abril, 2003; Abril et al., 2018). The radiometric dating of estuarine sediments allows estimating sediment accumulation rates and the reconstruction of temporal records of pollution (e.g., Álvarez-Iglesias et al., 2007; Costa-Böddeker et al., 2018; Sun et al., 2019).

Apart from the radiometric dating, the combined study of radionuclides and trace metals in estuarine sediments is unusual. At present, one of the main limitations of the sediment fingerprinting approach is the non-conservative behaviour of the properties of transported sediments, which limits the sole use of statistical analyses (Koiter et al., 2013). In studies on the spatial distribution of pollutants in surficial estuarine sediments some of the sampling sites may be subject to transient and dynamic sedimentary conditions (e.g., close to areas with sand bars dynamics, recoiling shorelines, or localized entries of terrigenous flows from the banks) involving the continuous and/or episodic reallocation of large amounts of sediments which are carrying a metals load which may be decoupled from the recent pollution history in the estuary. The conclusions of the study could be then biased or limited by the number and distribution of samples with achronological sedimentary conditions. Handling a large number of dated sediment cores is not feasible in many cases, but the measurement of ²¹⁰Pb_exc concentrations in surficial sediments can provide some valuable insights on the local sedimentary conditions, as shown in this paper with the support of historical satellite photographs. This can help to a better understanding on the pathways and dynamics of pollution by heavy metals and other hazardous materials. Apart from the obvious radiological interest, including radionuclides in geochemical studies also can contribute, among others, to give insights on varying digestion yields when applying pseudo-total methods and to reinforce the physic-chemical meaning of some statistical correlations.

In Ghana, Equatorial Africa, the Volta Estuary is affected by the anthropogenic fingerprint of the Ada Foah city and by upstream damming, while the Ankobra and Pra estuaries are affected by artisanal gold-mining activities (Donkor et al., 2006; Klubi et al., 2018). The use of Hg for amalgam in small-scale gold mining was legalized in Ghana in 1989, leading to an increase in these activities which nowadays provides employment to over one million people and makes Ghana to account for half of the total gold production in the region. Mercury pollution is then a major threat for environmental and human health. Other major impacts are land degradation and the huge amount of terrigenous inputs injected into the water courses, with mining tailings carrying high concentrations of As, Cr, Zn, Cu and Pb, among other potentially harmful elements (Donkor et al., 2006; Fashola et al., 2016). Thus, Klubi et al. (2018), in their study of these three estuaries, found a moderate to significant contamination of As, Ag and Cu in sediments from the Pra and Ankobra estuaries.

It is known that gold-mining tailings may contain high concentrations of natural radionuclides, as it has been documented in South Africa (Winde and Sandham, 2004). The available data suggest that this is also the case in Ghana (Doyi et al., 2013).

This paper is aimed at studying environmental γ-emitter radionuclides (⁴⁰K, ¹³⁷Cs, ²¹⁰Pb, ²²⁶Ra, ²²⁸Ra, ²²⁸Th and ²³⁴Th), along with some major and trace metals (Al, Cr, Fe, Co, Ni, Cu, Zn, As, Sr, Cd, Sb, Cs, Pb, Th and U) in surficial sediments from three estuarine systems (Ankobra, Pra and Volta) in Equatorial Africa. The work focuses on the radionuclide fingerprint of the estuaries, on whether the gold-mining activities in the Ankobra and Pra basins could affect their concentrations, and on some novel aspects of the combined use of the multielemental analysis by ICP-MS and gamma spectrometry for assessing environmental pollution, particularly on the use of ²¹⁰Pb_exc as an indicator of local sedimentary conditions. Gamma measurements and some complementary analysis were performed for this work, while for the ICP-MS analytes we use the data generated in the work by Klubi et al. (2018).