Bioaccumulation of metals in freshwater crabs (potamonautes perlatus) of the Lourens River, Western Cape, South Africa

Abstract

Urban rivers are the most utilised and yet degraded rivers worldwide. The urban rivers of the Western Cape are no different. The Lourens River flows through the agricultural and urban areas of Somerset West in the Western Cape and as a result is subjected to a variety of pollution sources. In the upper reaches this river flows through two large farms where metal containing pesticides are used. Further downstream it passes through an urban area where a variety of pollution sources could contribute to the contamination of the river. The extent to which the Lourens River, and the ecosystem it supports, is affected by metal pollutants is not known. The aim of this study was to determine the concentrations of metals in the Lourens River as well as the contribution of agricultural and urban activities to metal contamination of the river. Sediment and crab (Potamonautes perlatus) samples were collected over a period of one year from seven sites over the length of the river. Sediment samples were also collected from a sedimentation pond on the bank of the river where orchard run-off water is remediated. Preliminary analysis of samples was done for ten metals (AI, Cd, Co, Cu, Cr, Fe, Mn, Ni, Pb and Zn). Results from these analyses determined the selection of six metals (AI, Cr, Cu, Fe, Mn and Zn) for further investigation in this study. The concentrations of metals detected in collected samples varied significantly throughout the sampling period. This can be attributed to various factors such as rainfall patterns, the fact that pesticide application varies throughout the year and other urban activities. AI, Cr, Fe and Zn were found in significantly higher concentrations in the urban areas. These higher levels of contamination, relative to the upper parts of the river, can probably be attributed to various urban activities contributing to the contamination of run-off into the river. The sedimentation pond results revealed high concentrations of AI and Fe, while Cu, Cr, Mn, and Zn were found in lower concentrations. All six metals however followed the same pattern where the first four sampling occasions showed higher concentrations than the last three occasions. It can be concluded that agricultural and urban activities do contribute significantly to the metal contamination of the Lourens River.