Seasonal variation and mobility of trace metals in the beach sediments of NW Borneo
Graphical abstract
Introduction
Globally the contamination of a coastal ecosystem by trace and heavy metals is a serious environmental problem. Trace metals such as chromium, cobalt, copper, iron, magnesium, and zinc are generally occurring at very small levels in the environment. Living organisms need very lesser quantities of some trace metals, but high levels of the same metals can be toxic. Trace metals originate from both natural (erosion and mineral weathering) and anthropogenic sources (industrial and mining activities) and are transferred into the coastal environment via river runoff, discharge of municipal and industrial effluents through small streams and atmospheric fallout (Hyun et al., 2007; Li et al., 2013; Liu et al., 2018; Nagarajan et al., 2019). Under reasonable environmental conditions, where freshwater is well-oxygenated, the concentration of trace metals is high in the sediments compared to the water column. The partitioning of trace metals between the water column and sediments results from the adsorption and complexation by clay minerals and organic matter in the sediments. Trace metal concentration in the sediments often reflects their lithological source (Govindasamy and Azariah, 1999; Gopinath et al., 2010; Prabakaran et al., 2020) but sometimes anthropogenic contamination (Barath Kumar et al., 2017; Lu and Kang, 2018; Sajimol et al., 2021). The physicochemical changes occurring in the overlying water column and within sediments remobilize the trace metals (Förstner, 1985; Chakraborty et al., 2015; Tiquio et al., 2017; Xie et al., 2019). After remobilization, various physicochemical and biological processes recycle the trace metals (Xu et al., 2017), which undergo bioaccumulation and biomagnification through a complex food web structure that initiates adverse health effects to aquatic organisms and humans (Gleyzes et al., 2002; Barath Kumar et al., 2017; Liang et al., 2018; Anandkumar et al., 2020).
The bioaccumulation and biomagnification depend on the physicochemical availability (mobility) dictated by the carrier phase with which the trace metals are associated in the sediments and the prevailing environmental conditions (Tessier et al., 1979; Salomons and Förstner, 1980; Rauret et al., 1988; Szefer et al., 1995; Chakraborty et al., 2014, 2019, 2019; Tiquio et al., 2017). The labile form of the trace metals primarily exists in five different phases viz., exchangeable, bound to carbonates, adsorbed onto iron and manganese oxides, complexed with natural organic matter, and as sulphides (Gibbs, 1973). The mobility and bioavailability of the trace metals differ across these carrier phases. Thus, measuring the total metal concentrations is inadequate to explore the contamination status of the sediments (Cottenie et al., 1980; Gleyzes et al., 2002). Therefore, earlier researchers have employed different sequential extraction procedures (Tessier et al., 1979; Ure et al., 1993; Morillo et al., 2004; Nemati et al., 2011; Chakraborty et al., 2014, 2021; Wali et al., 2015; Schintu et al., 2016; Lu and Kang, 2018; Prabakaran et al., 2019; Dong et al., 2019; Zhao et al., 2021) of which the Tessier et al. (1979) protocol is widely adopted.
However, the lack of standardized procedures favored the development of the BCR method (Ure et al., 1993). The present work utilized the modified BCR procedure of Rauret et al. (1999). The revised protocol involves the use of an increased concentration of hydroxylammonium chloride and a lower pH. This allows for a more efficient dissolution of the soil/sediment matrix in the reducible fraction, in particular, metals bound to the iron oxyhydroxide phase (Mossop and Davidson, 2003). Many researchers (Morillo et al., 2004; Cuong and Obbard, 2006; Nemati et al., 2011; Alshemmari et al., 2012; Chakraborty et al., 2015; Schintu et al., 2016; Akhbarizadeh et al., 2017) have successfully applied this modified BCR sequential extraction protocol to investigate bioavailability, mobility and toxicity of trace metals in the coastal sediments.
The Miri coast of the Sarawak consists of scenic geological structures, such as rock cliffs, arches, caves, stacks, beach rocks, a recreational waterfront area, and coral reefs (Miri-Sibuti Coral Reef National Park; MSCRNP), which makes them regularly visited hotspot areas for local and foreign travelers. Apart from the coastal ecosystems, three rivers (Baram River: the 2nd longest River in Sarawak and Miri, and Sibuti Rivers are minor rivers) that originate inland of Sarawak and confluence in the South China Sea and deliver a vast amount of sediments to the South China Sea. Compared to these three rivers, the Baram River contributes a much higher sediment flux to the Miri coast. Research on the alluvial sediments on the Miri coastal belts, which delineate the source and nature of these sediments is limited (Nagarajan et al., 2013; Ismail, 1993; Sim et al., 2017; Prabakaran et al., 2019).
An earlier study by Nagarajan et al. (2013) in the tourist beach sediments of Miri revealed possible anthropogenic influence based on the acid leachable trace metals in the sediments and not reported the total trace metals in the beach sediment. Later, Nagarajan et al. (2019) reported the total trace metals including Rare Earth Elements (REEs) and mainly emphasized the source rock characterization, weathering intensity and the geological processes controlling the geochemical signature of the sediments. A high rate of erosion in the catchment area of the Baram River led to enhanced sediment transport to the Miri coast, which resulted in the formation of the sand bar opposite the Baram River mouth (Nagarajan et al., 2015a, 2019). Such a sand bar modulates the alongshore transport and distribution of the sediments delivered by the Baram River to the South China Sea. The enhanced upstream erosion due to changing land use and land cover modulate sediment delivery, and dictate accretion and erosional features of the Miri coast (Anandkumar et al., 2019a), and impact the nearshore Miri-Sibuti Coral Reef National Park (Browne et al., 2019). Studies on trace metal accumulation in biological tissues were also reported from the Miri Estuary of Sarawak (Billah et al., 2016) and the Miri coastal area (Anandkumar et al., 2017, 2018, 2019) which observed a higher concentration of Cu and Zn in different tissue organs of consumable aquatic organisms such as fish, crab, and prawn, etc. Even though there have been some studies on the trace and heavy metals in the sediments of coastal areas of Borneo, a combined study that addresses the nature of the source, the seasonal, environmental, and biological impact of the trace metals, seasonal variations and mobility of the trace metals in the sediments of adjacent coastal belts are limited. In addition, geochemical fingerprinting studies to address the impact on the coral reef ecosystem in the MSCRNP by the local rivers, as well as the adjacent coastal belts, are also limited to evaluate the future threats to the reef due to high suspended sediment input with a significant amount of pollutants (e.g. Browne et al., 2019). Thus, the present study mainly focuses on the sequential extraction of trace metals of various beach sediments of the Miri coast to assess their bioavailability, toxicity, the association of metals in the different sediment fractions, to identify possible sources of the contaminants, and the pollution status based on the sediment quality guidelines (SQGs).
Section snippets
Study area
Miri is the birthplace of the Malaysian oil industries and it is located in the northern part of Sarawak in Borneo Island. The beaches in this region are exposed to several types of natural and anthropogenic influences (Nagarajan et al., 2013, 2019, 2019; Billah et al., 2016; Anandkumar et al., 2017, 2018, 2018; Prabakaran et al., 2019). The Baram, Miri, and Sibuti rivers deliver freshwater and sediments to the coastal area of the South China Sea. The sediment input from the Baram River is much
Sample collection and analysis
A total of 57 beach sand samples were collected during monsoon (MON; December 2013; n = 26) and post-monsoon seasons (POM; June 2014; n = 31) during low tides. Using a clean plastic scoop, sediment samples were collected from 0 to 10 cm depth, transferred to the polyethylene bags, labelled, transported to the laboratory and stored under refrigeration at 4 °C. Before sample processing, all the samples were thawed to attain ambient temperature. After removal of organic debris, the sediments were
Sediment texture
The Miri beach sediments were mainly composed of fine sand and silt; the coarse fraction was lesser than 1%. Based on grain size analysis the study area is divided into two segments – the northeastern (NE) and the southwestern (SW) segments. The fine sand dominates in the NE segment while silt is predominant in the SW segment. The Kualabaram River mouth/Estuary (KBE), Fish Landing Center (FLC), Lutong Beach (LB), Piasau Boat Club Beach (PBC) and, Miri River estuary (MRE) fall under the NE
Conclusion
The modified BCR sequential extraction method was adopted for the first time in the study area to analyze the potential mobility of trace metals and their seasonal variations in the sediment samples collected from the different beaches of the Miri coast. The sediments are predominantly composed of fine sand (SW segment) and are followed by silt (NE segment). The association of trace metals in the exchangeable fraction (the most mobile and bioavailable) can be ranked as
Credit author statement
A.Anandkumar: Conceptualization (equal), methodology, investigation (lead), writing-original draft (lead). R. Nagarajan – Funding acquisition, conceptualization (equal), supervision, validation (lead), data curation, formal analysis (equal), writing-review and editing (lead). Eswaramoorthi Sellappa Gounder – Formal analysis (equal), validation (supporting), writing-review and editing (supporting). K. Prabakaran – Investigation (supporting), writing-original draft (supporting), visualization.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The first author wishes to express his gratefulness to Curtin Sarawak Research Institute Academic Grant (CSRI 1011: Ramasamy Nagarajan) during his doctoral program at Curtin University Malaysia. The valuable help by Dr. Vijith Hamza in this study is highly acknowledged. We are grateful to Mr. David Patenaude for his valuable comments and linguistic corrections. The authors extend their sincere thanks to the Editor and two anonymous reviewers for their valuable comments that improved the quality
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