Geochemical assessment and fractionation of trace metals in estuarine sedimentary sub-environments, in Mumbai, India

Highlights

• Sediment cores studied from mudflat and mangrove regions of an estuary.

• Higher sediment contamination seen mudflat core as compared to mangrove core.

• Metals contributed from Geogenic as well as Anthropogenic sources.

• Metal fractionation showed most of the metals in residual fraction except Mn and Zn.

• Potential risk to the estuarine system if the sediment gets disturbed.

Abstract

The distribution and concentrations of selected metals (Fe, Mn, Cu, Pb, Co, Zn and Cr) in intertidal regions (mudflat and mangrove) of Ulhas estuary, located in Mumbai, India were studied. The sediment metal contamination was assessed by applying Enrichment Factor (EF), Anthropogenic factor (AF), Contamination factor (CF) and Pollution load Index (PLI). Also, Sediment Quality Guidelines (SQGs) by Environmental Protection Agency was used to determine the risk of increased metal levels to biota. Amongst the two sub-environments studied, the mudflat region exhibited higher metal contamination than the mangrove region necessitating fractionation analysis of the core. The geochemical fractionation analysis showed higher metal percentage in the residual fraction, except for Mn and Zn. The fractionation results, in general, indicated that contamination due to the metals- Mn and Zn might pose a potential risk to the estuarine system if the sediment gets disturbed with time.

Introduction

Metals are indestructible with most of them having toxic effects on organisms. Unpolluted soils/sediments exhibit low concentrations of metals mostly derived from rock and soil weathering (Reza and Singh, 2010, Varol and Sen, 2012), while in urban-rural areas, anthropogenic inputs, such as industry and agricultural emissions, may become predominant metal sources (Li and Davis, 2008, Tang et al., 2014). The pollution of the aquatic environment with metals has become a worldwide problem during recent years (Guo et al., 2012, Ndimele and Kumolu-Johnson, 2012). Sediments can act as a sink for various pollutants such as pesticides and trace metals and also as a source by playing a significant role in the remobilization of contaminants (Yao and Gao, 2007). The contamination of aquatic systems by heavy metals, especially in sediments, has become one of the most challenging pollution issues owing to the toxicity, abundance, persistence and subsequent bio-accumulation of these materials (Gielar et al., 2012, Varol and Sen, 2012). Numerous studies, have demonstrated that marine sediments from industrialised coastal areas are greatly contaminated by heavy metals (Li et al., 2013, Singh et al., 2013, Fernandes et al., 2011). Elevated levels of heavy metals in aquatic sediments, may pose a risk to human health due to their transfer in aquatic media and uptake by living organisms, thereby entering the food chain (Satarug et al., 2010, Hussein et al., 2012, Tang et al., 2014). The knowledge of the total concentration of an element in specific environmental compartment is often inadequate to explain its roles and properties. Mobility and toxicity of metals associated with sediments are generally affected by metal speciation and sediment composition. Sequential extraction analysis is a technique developed for the overall assessment of the speciation and potential mobility of metals in sediments (Calmano et al., 1996, Hass and Fine, 2010). There are many such schemes (Forstner, 1982, Tessier et al., 1979) amongst which the most widely used is the one proposed by Tessier et al. (1979).

Continuous industrial growth and rapid economic development of Mumbai, in India, has resulted in dramatic increase in percentage of population. Consequently, the environmental quality of the soil is becoming more and more important with regards to human health. Rathod et al. (2002) reported decline in fin fish and shell fish fauna along Mumbai due to industrial and domestic activities. In Ulhas estuary, pollution adversely affected the distribution and abundance of fish species such as Bhing and Pala (BMRDA, 1983). Relative high concentration of Cr reported in the tissue of fish from the Ulhas estuary reflected anthropogenically added Cr in the estuary (Srinivasan and Mahajan, 1987). Singh et al. (2007) reported presence of heavy metals (Cd, Fe, Pb, Zn and Cu) in water, sediment and body tissues of Red worms (Tubifex spp.) collected from natural habitats around Mumbai. Therefore, there is a need to determine the chemical form of metals in the soil to know its toxicity, mobilization capacity and behaviour in the environment. There is not much information available in the published literatures about the geochemical assessment and fractionation of heavy metals in the Mumbai area especially the estuarine region. Knowledge of the chemical forms of heavy metals as well as their complexes will help to better evaluate the environmental impact of contaminated sediments. Therefore, the aim of this study was to investigate the metal sources, distributions, concentrations and contamination status of selected elements (Fe, Mn, Cu, Pb, Co, Zn and Cr) in the sediments and compare it with standards for sediment quality as well as determination of mobility of the metals in the sediments.