Assessment of sediment quality using total and bioavailable concentration of metals in intertidal mudflats of a tropical (Vashishti) estuary, west coast of India

Abstract

The mudflat sediment cores collected from lower (VS-1) and middle (VS-2) regions of the Vashishti estuary were sub-sampled at 2-cm intervals and analyzed to determine grain size, organic carbon, and total metal (Al, Fe, Mn, Ni, Co, Cu, Zn, and Cr) concentration. To distinguish between anthropogenic and natural input of trace metals, enrichment factor (EF), geo-accumulation index (Igeo), and contamination factor (CF) were computed. Further, speciation of metals in selected sub-samples was carried out to understand the bioavailability of metals, and the toxicity of metals to associated biota was assessed using the sediment quality values. The total metal analyses showed higher concentration of Fe, Mn, Ni, Co, Cu, Zn, and Cr in the middle estuarine region which was attributed to metal input from the Lote Parshuram industrial estate located in proximity and adsorption of metals, viz., Ni and Cr onto iron oxide-coated sand particles. EF, Igeo, and CF supported anthropogenic input of metals in the Vashishti estuary. The speciation of metals revealed an increase in Mn concentration in Fe-Mn oxide and exchangeable fractions from bottom to the surface of cores VS-1 and VS-2 respectively, while Co in the exchangeable fraction, and to a lesser extent, Zn in the Fe-Mn oxide fraction showed an increase towards the surface of the core VS-2. These elements exhibited diagenetic remobilization and showed an increase in bioavailability near the surface. Further, comparison of total and bioavailable concentration of metals with sediment quality values indicated risk of Mn and Co to sediment-associated biota.