Tripob Bhongsuwan and Darunee Bhongsuwan

pp. 89 - 106

Abstract

The study aimed to analyse the heavy metals Mn, Fe, Ni, Pb, Cr and Cd in 20 bottom sediment samples from 0 - 9 cm depth collected in 4 sites in the Outer Songkhla Lake. These sediment samples were deposited during the year B.E. 2520-2538 according to the Cs-137 dating. Total acid digestion was used to totally dissolve the samples using aqua regia and hydrofluoric acids. A quality control was performed by digesting the certified reference material IAEA SL-1 (lake sediment). Percent recoveries of selected heavy metals were between 79.96 (Ni) and 99.26 (Cr). Concentrations of Mn, Fe, Ni, Pb and Cr in the sediment samples were in ranges 368-, 21014-6683, 1.1-16.7, 14.5-59.8 and 30.1-6.5 mg/kg, respectively. Cadmium was non-detectable but calculated to be less than 2.5 mg/kg in all samples. Result showed that Mn has significantly increased between the depth 9-0 cm (the deposition years B.E. 2520-2538), while Fe, Ni, Pb and Cr had not changed significantly. It was concluded that Mn, Fe, Ni and Cr were still in low concentration comparable to those of sediments in several remote lakes worldwide both in Europe and America. Only Pb concentration was found to be higher than that in remote lakes worldwide. Iron was found to be the best conservative element in the outer Songkhla Lake. Regional anomaly criteria showed that Mn, Ni, Pb and Cr in most samples were within the natural geochemical population except for Ni in one sample of core U, which was considered to be a regional anomaly. Geoaccumulation index calculated in comparison with heavy metal concentration in average shale showed that sediment samples of the outer Songkhla Lake were not polluted by Mn, Fe, Ni, Crat any site, and were unpolluted/moderately polluted for Pb at site T. Analysis of the magnetic susceptibility and the heavy metal contents in sediments showed no clear relation between them.