Assessment of contamination by metals and selected organic compounds in coastal sediments and waters of Mombasa, Kenya

Abstract

In 1995 a coastal-zone pollution monitoring programme for developing countries, the Land-Ocean Contamination Study (LOCS), was initiated by the British Geological Survey (UGS) under funding from the UK Overseas Development Administration (ODA) Natural Resources Division (NRD). The central objectives of LOCS arc (i) the provision of data regarding the sources, transport pathways and fates of contaminant metals and selected organic compounds along urbaniscd coastal margins, and (ii) promotion of the use of such data in integrated coastal-zone management (CZM). A systematic geochemical and hydrochemical survey of the inshore waters of Mornbasa, Kenya was carried out under the ODA-LOCS programme in liaison with the Kenya Marine and Fisheries Research Institute (KMFRl) during the period September 1995 to February 1996. '111e survey included an assessment of heavy metal, alkane, polycyclic aromatic hydrocarbon (PAH) and organochlorine concentrations in water, suspended particulate matter (SPM) and sediment at 48 localities within the inshore lagoonal waters, and the reef-fronted coastline extending to Mtwapa Creek some 13 km to the north. Oceanographic and sedimentological data of relevance to the interpretation of contaminant distributions and transport pathways were also collected and form the subject of an independent report (Rees et al., 1996). The fundamental hydrochemical properties (e.g. pH, salinity, dissolved oxygen) of individual creek and back-reef water bodies are readily differentiable, rei1ccting variations of residence-time, evaporative influence and freshwater influx, All are charactcrised by concentrations of Cu, Cd and Cr within the global background range for marine waters. The concentration of Pb is, however, systematically elevated. Anomalous concentrations of Zn and Cd prevail along the reef-front between Nyali and Mtwapa, although precise source has not been ascertained. Concentrations of several potentially toxic trace elements (e.g. Cr, As, Ni, Cu & V) are enriched in suspended particulate matter (SPM) in Tudor Creek, relative to SPM in other inshore and reef-front waters. A spatial correlation between these metals and Mn is evident. Such trends are inconsistent with an anthropogenic control, and are almost certainly attributable to (he dominance of mangrove-derived particulate matter in the overall SPM assemblage in Tudor Creek. Several heavy metals attain high concentrations in surficial sediments around Mornbasa. With the exception of Pb, Zn and Cu, the signatures are almost entirely related to lithology. Localiscd enrichment of Pb, Zn and Cu is evident in close proximity to several known point-sources including sewage outfalls to thc cast of Mombasa Island, Likoni and Kilindini docks. Temporal flux variations, typically involving increased trace metal deposition towards the sediment-water interface, arc apparent from downcore concentration profiles through the sediments of Makupa Creek, Port Kilindini and Tudor Creek. Following normalisation against Al203 or Ti02, however, no clear anthropogenic control can be identified. Sedimentary partitioning data for sediments from Makupa Creek indicate that labile geochemical fractions (e.g. reducible oxides) an: significant as carriers of Mn, Pb, ClI and Co. Detrital silicates and sulphides form the principal carriers of Fe, AI, V, Co, Cr and Ni. The available panitioning data and complementary data for intcrsitial pore-waters suggest that the post-depositional alteration of labile phases predominantly results in the imrnobilisation of metals as sulphides. Under such circumstances, the sediment reservoir can be considered to constitute a relatively long-term contaminant sink. The concentrations of n-alkanes, PAHs and organochlorines reconlcd in Mornbasa sediments arc very low (often falling below analytical detection limits). Alkanes and PAHs frequently remain as residues at sites of crude oil spillage. Their low concentrations in the sediments of Port Reitz and Makupa Creek, both known to have been impacted by spills during the last decade, signify high rates of biodegradation facilitated by conducive climatic and biochemical factors. The geochemical, oceanographic and sedimentological data collated under he LOCS programme for Mombasa provide a valuable baseline against which to evaluate the effects of future urban and industrial development. In an attempt to maximise the utility of the survey outputs in practical CZM and planning, a GIS has been developed allowing interrogation of pollution data in conjunction with pre-existing information concerning land- and marine resource use.