Geochemistry of metals in grain size fractionated sediments from estuarine and coastal regions along Mumbai: A baseline information

Abstract

Differential metal accumulation in fractionated sediments within highly urbanized land-sea ecosystems is significant, often due to an imbalance between sluggish hydrodynamics and over dumping of contaminants. Here, an initial constraint on differential metal enrichment in fractionated sediments is highlighted along the estuary, creek, and nearshore regions of one of India's highly urbanized regions, Mumbai. Sediments were segregated into four fractions (A: < 25 µm; fine, B: 25 – 63 µm; moderate, C > 63 µm; coarse, and D: bulk) using the wet-sieving technique and were analysed for 23 metals. The results indicated that bulk sediment (D) constituted a moderate enrichment of transition metals Cr, Co, Ni, Cu, Hg, and Tl. The Distribution Factor (DF) indicated that metals such as Cr, Co, Ni, Cu, and Zn frequently showed DF > 1 in fraction C. The reason for higher concentration of these metals in fraction C is indicative to weak tidal influence, that resist the flow of the higher density fraction and increases its residence time in the anthropogenically stressed region. Grain size showed a notable dominance of silt (> 80 %), mainly transported from the mudflats. Turbulence caused by activities such as periodic deepening, high-frequency vessel movement, and other modernization activities provoked the transport of fine particles. Considering that weak dynamic sediment transportation is mainly conducive to man-made changes, the association-dissociation of metals in size-specific sediment particles will significantly impact their further transport and cycling.