Speciation of metals and their distribution in tropical estuarine mudflat sediments, southwest coast of India

Abstract

Two sediment cores collected from a mudflat sedimentary environment of Swarna estuary (S3) and Gurpur estuary (MF6), representing the middle estuarine region, Karnataka, India, were investigated to understand bioavailability of metals and their toxicity. The subsamples were analyzed for sand, silt, clay, organic carbon and total metal concentration of Aluminum (Al), Iron (Fe), Manganese (Mn), Nickel (Ni), Zinc (Zn), Copper (Cu), Cobalt (Co) and Chromium (Cr) at 2 cm intervals. Sediments (average) are relatively coarser in Gurpur estuary whereas silt, clay, organic carbon along with the studied metals (except Mn and Cu) is noted to be higher in the Swarna estuary. Significant correlations were observed of Al, Fe with finer sediments and most of the trace metals in both the cores indicating a mainly lithogenic source. Further, metal speciation analysis carried out for Fe, Mn, Ni, Zn, Cu, Co and Cr on selected samples of both the cores indicated that Fe was largely associated with the residual fraction with 93 plus or minus 0.5 percent in Gurpur and 84 plus or minus 6 percent in Swarna estuary. The concentration of studied metals in the residual fraction in the sediments of Gurpur estuary was in the order Mn(90 plus or minus 5 percent) greater than Cr(85 plus or minus 1 percent) greater than Ni(72 plus or minus 5 percent) greater than Zn(69 plus or minus 3 percent) greater than Cu(57 plus or minus 5 percent) greater than Co(55 plus or minus 2 percent) and; Cr(80 plus or minus 7 percent) greater than Mn(77 plus or minus 10 percent) greater than Ni(76 plus or minus 7 percent) greater than Zn(67 plus or minus 10 percent) greater than Cu(67 plus or minus 10 percent) greater than Co(50 plus or minus 7 percent) in Swarna estuary. When the total (bulk) metals were compared with the Sediment Quality Values (SQV) following Screening Quick Reference Table (SQUIRT), Co values of both the cores fell above Apparent Effect Threshold (AET) values. When the sum of the average bioavailable fractions in sediments was considered, Co values exceeded the AET in core S3 (Swarna estuary). When the variations are viewed with depth, bioavailability of Mn, Ni, Cu and Co in Gurpur estuary indicated anthropogenic addition in recent years whereas in Swarna estuary most of the studied metals showed diagenetic remobilization and diffusion to the water column from surface sediments. Metal speciation study indicated no harm to the aquatic life, except of Co, in Swarna estuary wherein it posed a high risk of toxicity to organisms associated with the sediments.