Seasonal and spatial distribution of heavy metals in surface water and sediments of Msunduzi river, KwaZulu-Natal, South Africa.

Abstract

Some physicochemical parameters and heavy metal concentrations of surface water as well as sediments in the Msunduzi River were investigated to provide information on the spatial and seasonal characteristic patterns of surface water quality, concentrations, chemical speciation and environmental risks of heavy metals in surface sediments. The physicochemical properties of surface water were determined following standard analytical procedures as described by the American Public Health Association (APHA). The levels of total metals in water and sediments were quantified by inductively coupled plasma-mass spectrometry (ICP-MS) after digestion with a mixture of acids, while the chemical speciation of heavy metals in the sediment was evaluated following the Community Bureau of Reference (BCR) sequential chemical extraction method. The physicochemical, concentrations and chemical speciation patterns of heavy metals in the investigated samples showed significant (p<0.05) spatial and seasonal variations. The value ranges for the measured physicochemical properties of water were: temperature 12.9 - 28.7 oC, pH 5.08 - 9.02, EC 84.0 - 1165 μS cm-1, TDS 60.9 - 952 mg L-1, DO 3.08 - 6.18 mg L-1, Cl- 8.73 - 102 mg L-1, NO3- nd - 7.96 mg L-1 and SO4- 0.27 - 103 mg L-1. Total metal concentrations in surface water for all seasons ranged from 0.22 - 5.44, 0.00 - 1.35, 0.80 - 50.2, 0.16 - 21.9 and 0.03 - 658 μg L-1 for As, Cd, Cr, Pb and Zn, respectively while those of the sediment varied from 1.5 - 2.79, 42.26 - 246, 8.3 - 63.7 and 36.9 - 402 mg kg-1 for Cd, Cr, Pb and Zn, respectively.The water quality index (WQI) for surface water based on physicochemical and heavy metal characteristics were 50, 85, 134 and 155 for autumn, winter, spring and summer, respectively. The WQI values for this river system fell within the “very poor quality” to “unsuitable for drinking” water categories. The ecological risk index indicated that there is low ecotoxicological risk associated with exposure to heavy metals in these sediments. The speciation patterns indicated that Cd, Pb and Zn were found mostly in the residual and Fe-Mn oxide phases while Cr existed mainly in the residual and oxidisable phases.