Enrichment and distribution of metals in surface sediments of the Thanh Hoa coastal area, Viet Nam
Graphical abstract
Introduction
The sedimentary environment is crucial for the organisms that live and grow on it. These organisms will be affected if the sediments become polluted; metals are common pollutants, and excess concentrations thereof are among the most significant problems affecting coastal areas today.
Major and trace elements occur naturally in the Earth’s crust, but their over-enrichment may result from both natural mechanisms and human activities that cause them to accumulate over time, affecting all environmental matrices (sediments, waters, and biota). Natural mechanisms include the weathering of parent rocks followed by deposition in waterbodies (Matys Grygar and Popelka, 2016). However, the main causes of over-enrichment are rooted in human activities that affect the environment and cause metals to accumulate in sediments, water, and organisms (Qian et al., 2015).
In the coastal Thanh Hoa Province, the National Marine Environment Monitoring Station (NMEMS) has been in place since 1999 to monitor the area from 6 to 10 m depths; its findings report high concentrations of metals in the sediment, with increasing levels of Cu, Pb, and Zn, some years in excess of benchmarks from the interim marine sediment quality guidelines (ISQGs). Data have shown that metals increasingly come from growing industrial zones in the north of Viet Nam and that, of the wastewater from industrial zones, only a total of 25% was treated while 75% went untreated. Moreover, 25% of the treated wastewater did not reach Viet Nam’s required standard (Dang et al., 2010). In the same area, Tran et al. (2018) measured concentrations of Cu, Pb, and Zn in mangrove sediment cores that were higher than the ISQG levels; these concentrations increased over time and showed higher values than those reported by NMEMS from 1999 to 2009 (Tran et al., 2018).
Earlier studies in small coastal areas (Dang et al., 2010, Tran et al., 2018) have demonstrated that the impact of human activities on coastal areas increases over time. In this study, we expanded the dimension of the study area to include the entire Thanh Hoa coastal area, collecting 32 surface sediment samples taken from the coast to a depth of 30 m and analyzing major and trace elements and grain sizes (sand, mud). This information may improve our understanding of Thanh Hoa’s coastal area.
Section snippets
Regional setting
Thanh Hoa Province is one of the larger provinces in Viet Nam. The terrain may be divided into three types: mountains and hills, the delta plain, and the coastal area. Mountains and hills account for three-quarters of the total provincial area. Nevertheless, the population is mainly concentrated on the plains. Here, the coastal area, which extends across six districts and through which five rivers flow (Len, Truong, Ma, Lach Ghep, Lach Bang), is home to numerous industrial areas (Fig. 1).
Sampling
Thirty-two surface sediment samples were collected during August 2011 in the Thanh Hoa coastal area by Petersen grab, as illustrated in Fig. 1. In the field, sediments were stored in plastic bags and kept at 4 °C until transported to the laboratory.
Grain size analysis
At the laboratory of the Institute of Marine Environment and Resources (Marine Geo-Environment Laboratory, IMER, Viet Nam), grain size separation was performed using a 63-m sieve after the removal of organic matter and salt using hydrogen peroxide
Grain sizes
The sand fraction ranged from 0.5 to 82.7%, averaging 33.7%, while the mud fraction ranged from 99.5 to 17.3%, averaging 66.3% (Table 2). Sediments with high mud content were distributed in the north of the study area, while those with lower mud content were distributed in the southern part of the study area. The mud fraction was distributed close to large rivers, including the Ma, the Truong, and the Day (Fig. 2a). Sediments with high sand fraction were distributed in the southern part of the
Comparisons of metal levels in sediments of the Thanh Hoa and other coastal areas
Table 4 shows a comparison of metal concentrations with other areas of Viet Nam and other areas in southeast Asia. The concentrations at the Thanh Hoa coastal area were higher than some areas (Table 4).
In particular, the Cua Ong and Tien Yen coastal areas had concentrations that were two to three times lower than the Thanh Hoa coastal area with respect to Fe, Cu, Pb, Co, Ni, Mn, and V (Ho et al., 2010, Tran et al., 2012). At the same time, the Hai Phong coastal area showed higher concentrations
Conclusion
In the Thanh Hoa coastal area, the content of the mud fraction was twice as high as the sand fraction. The continental impact was more prevalent in the north, while the south was commonly influenced by the sea. The concentration of elements was high; in particular, Cu, Pb, Zn, As, and Cr were in excess of ISQG levels.
Assessment indexes of metals with Igeo showed moderate pollution for Pb and As; no to moderate pollution for Cu, Zn, Cd, and Li; and no pollution for Fe, Co, Ni, Sn, Cr, Mo, V, and
CRediT authorship contribution statement
Dang Hoai Nhon: Conception and design of study, Acquisition of data, Analysis and/or interpretation of data, Writing - original draft, Writing - review & editing. Nguyen Van Thao: Writing - original draft. Tran Đinh Lan: Writing - original draft. Nguyen Manh Ha: Acquisition of data. Duong Thanh Nghi: Acquisition of data. Tran Manh Ha: Acquisition of data. Do Manh Hao: Acquisition of data. Nguyen Van Chien: Writing - original draft. Tran Duc Thanh: Writing - original draft, Writing - review &
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.
Acknowledgment
The authors thank the Vietnam Academy of Science and Technology (VAST)for providing financial support (VAST06.04/20-21, NVCC23.01/19-19, NVCC23.01/20-20). All authors approved the version of the manuscript to be published.
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