Sediment-Water Exchange of Selected Heavy Metals at the Backwaters of Makupa Creek, Kenya

Abstract

In situ benthic studies using incubation chambers were conducted at the backwaters of Makupa Creek (a peripheral creek located next to a municipal dumpsite) to directly measure fluxes of dissolved Mn, Fe, Ni, Cd, Cu and Pb. Physicochemical variations of the water overlying the chambers were also monitored and sediment cores were collected to measure their pH and Eh profiles. The sediment pH decreased markedly just below the water column at about the 0-2 cm segment, the decrease attributed to oxidation of organic matter and other inorganic chemical species. Organic matter oxidation contributed to the mobilization of heavy metals, demonstrating the significant effect of atmospheric oxidation on heavy metals dynamics at the Makupa creek backwaters during low tide. Humic and fluvic acid mobilization did not have a significant effect in the mobilization of Fe (Pearson correlation coefficient Fe-pH r = -0.34). However, Cu, Ni, Mn and Pb were to some extent mobilized due to solubalization of humic and fluvic acids at high pH. Ni and Cu were mobilized the most (Ni-pH r = 0.40 and Cu-pH r = 0.42), with both benthic chambers producing a pH peak at the 6th hour, which corresponded with a peak in dissolved Cu and Ni. Directly measured benthic fluxes ranged from +1680 to +8000 moles cm-2 h-1 for Fe, - 599 to +370 moles cm-2 h-1 for Mn, -1.1 to -0.0045 moles cm-2 h-1 for Cd, -33 to +1.2 moles cm-2 h-1 for Pb, +174 to +432 moles cm-2 h-1 for Cu and +74 to +361 moles cm-2 h- 1 for Ni.