Contaminant transport and storage in the estuarine creek systems 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 under funding from the UK Overseas Development Administration (ODA) Natural Resources Division. The central objectives of LOCS are (i) the provision of data regarding the sources, transport pathways an I fares of contaminant metals and selected organic compounds along urbani sed 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 estuarine creek systems of Mornbaxa, Kenya was carried out under the LOCS programme in liaison with the Kenya Marine and Fisheries Research Institute between September 1995 and February 1996. The main objective of this was to assess the concentration of heavy metals, alkanes, polycyclic aromatic hydrocarbons and organochlorines in water, suspended particulate matter and sediment at forty eight localities within the creek systems and nearby reef-fronted waters. The survey also focused on the storage and transport of these contaminant') within the waters and sediments of the creek systems. Such physical aspects of the creek settings are addressed by this report, which complements that of Williams et a1. (19ge) on the geochemical and hydrochemical settings. The estuarine creek systems consist of inland lagoons connected to the ocean by laterally-confined channels. One-dimensional (segmented) box modelling shows tha t the average resilience time of contaminated water within the lagoons is likely to be less than a week, and less than two days in the confined-channels. Contaminants are likely to reside longer during neap tides and in the: wet season, although (hey will be most widely dispersed during spring tides and in the dry season. The distribution of sediments reflects the ebb dominance of the creek systems as fluvially derived sediments arc generally transported seawards and open marine sediments only occur towards the seaward end of the confined-channels. The periodicity of the, extremely sporadic, major bedload transport events is unknown. Although those parts of the lagoons that are removed from channels are likely to have the highest sedimentation rates, all parts of the systems may be regarded as potential contarninuted sediment sinks because of the degree of active bioturbation in all interand sub-tidal environments. The results of the survey illustrate the importance of understanding the hydrodynamic and sedimentological setting of estuarine environments in Older to understand the storage and flux of contaminants within them. Although this survey adopted those techniques that are most appropriate for analysis of the environment, and available dataxetx of Mornbaxa, the approach used will be equally applicable to similar settings. The geochemical, hydrodynamic ami sedimentological data collated during the LOCS survey provide a valuable baseline against which to evaluate the effects of future urban and industrial development at Mombasa. In an attempt (0 maxirnise the utility of the survey outputs in practical CZM and planning, a G IS has been developed (Williams et al. IlJlJ{)h) allo ving interrogation of pollution data in conjunction with pre-ex ixtiug information concerning land- and marine-resource use.