Seasonal analysis of water and sediment along the Umgeni River, South Africa.

Abstract

Increased pollution caused by industrialization, urbanization, afforestation and agriculture has resulted in severe reductions to the health of rivers in South Africa. Numerous studies carried out on South African rivers show that one of the biggest pollutant threats to the river health are heavy metals (e.g. Cr, Pb, Cd and Zn). Typically, metal pollutants exceed river quality limits set by the South African Bureau of Standards (SABS) and international standards including Environmental Protection Agency (EPA). The Umgeni River is the major river in the province of KwaZulu- Natal. It suffers from heavy pollution due to effects of industrialization and increasing urban population. Research was carried out to investigate the impact of these industries on the ecosystem by measuring the total and bioavailable concentrations of Cd, Cr, Cu, Pb and Zn in water and sediments over the four seasons. Seasonal sampling of water and sediment was done at nine sites. The total and bioavailable metal concentrations (Pb, Zn, Cr, Cd and Cu) were measured using ICP-OES. To ensure the validity of the data, certified reference materials were used; NIST 2702 for total metals and BCR 701 for the bioavailability study. Both CRM’s showed high recoveries. Metal concentrations in water ranged from 1.0 – 6.0 μg L⁻¹ Cd, 1.0 – 11.0 μg L⁻¹ Cu, 0.3 – 82.7 μg L⁻¹ Cr, 2.7 – 65 μg L⁻¹ Zn and 0.3 – 16 μg L⁻¹ Pb. The total metal content of sediments ranged from 0.07 – 264.5 mg kg⁻¹ Cd, 11.9 – 168.5 mg kg⁻¹ Cu, 28.6 – 135.1 mg kg⁻¹ Cr, 29.5 – 602.1 mg kg⁻¹ Zn and 12.1 – 601.7 mg kg⁻¹ Pb. Analysis of Variance (ANOVA) was used for statistical evaluation. This data showed that there was a significant difference between the dry (autumn/winter) and the wet seasons (spring/summer). Considering the total metal concentration the river is typically in poor health as defined by the SABS and EPA. All the metals, except Cd, were above the stipulated limited in both the sediment and water. The sequential extractions showed all metals primarily in the residual fraction (unavailable). However, very little Cd and Cr has potential of mobility. Cu has high affinity to partition in the Fe/Mn oxide and organic fraction indicating that the presence of reducing agents and microorganisms would avail the metal pollutant. Zn was associated with carbonates, and is highly mobile and bioavailable. This bioavailability data shows that any change in the environment condition may result in the release of the metal pollutants and would adversely affect the environment.