Accumulation of Toxic and Essential Elements in Clams and Sediments from the Volta Estuary

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The clams (Galatea paradoxa) and sediments were collected from the Volta Estuary in Ghana for six months’ period at Ada and Aveglo. They were analysed for six different elements (Cd, Cr, Cu, Hg, Ni, and Se) using Flame Atomic Absorption Spectrometer (AAS) and Automatic mercury analyzer Model HG 5000. A total of one hundred and eighty (180) clams were analysed over the period. The mean elements concentrations in the tissues of the G. paradoxa from Ada sampling site ranged from: Cd: 0.09 -0.17 mg/kg; Cr: 3.42 – 20.51 mg/kg; Cu: 0.55 – 3.10 mg/kg; Hg: 0.05 – 0.12 mg/kg; Ni: 5.49 – 27.96 mg/kg and Se: 0.34 – 0.49 mg/kg. That of o sediments were: Cd: 0.09 mg/kg; Cr: 15.03 mg/kg; Cu: 89.94 mg/kg; Hg: 0.06 mg/kg; Ni: 47.16 mg/kg and Se: 0.41 mg/kg. For Aveglo the mean metal concentrations in clam tissues were: Cd: 0.07 - 0.14 mg/kg; Cr: 2.01 –24.10 mg/kg; Cu: 0.79 – 3.65 mg/kg; Hg: 0.04 – 0.09 mg/kg; Ni: 10.25 – 28.14 mg/kg and Se: 0.13 – 0.29 mg/kg and the sediment samples recorded the mean metal concentrations as: Cd: 0.07 mg/kg; Cr: 16.28 mg/kg; Cu: 75.66 mg/kg; Hg: 0.04 mg/kg; Ni: 51.25 mg/kg and Se: 0.38 mg/kg. The analysis of the elements concentrations in the tissues of the different clams in relation to body size for the Ada sampling site shows significant difference (p < 0.05) for Cd and Cu in the various clam sizes. The mean concentrations of Cr, Hg, Ni and Se exhibited significant differences (p>0.05) in concentration between some clam sizes at Ada sampling site. The test for difference in mean concentrations of Cd and Cr showed significant differences (p < 0.05) between the various clam size classes at the Aveglo sampling site. Nevertheless, there was no significant differences (p > 0.05) showed between the mean concentrations of Cu, Hg, Ni and Se for some clam sizes. The evaluation of the risk associated with consumption of clams from the Volta estuary using WHO/FAO Standards for Bivalves, the Tolerable daily Intake (TDI), the rate of shellfish consumption (RSC), Risk Quotients (RQs) and the levels of concerns (LOCs) suggest that the normal consumption rates should be safe with regards to Cd, Cu, Hg, Ni and Se in the clam tissues except for Cr in medium to large clam sizes whose concentrations exceeded the WHO/FAO Standards for Bivalves legal limits The mean elements concentration of Cu in sediments for both Ada and Aveglo exceeded the NOAA Effects Range – Low (ERL) value of 20.9 mg/kg and CSQG Threshold Effects Level (TEL) value of 52.3 mg/kg whilst Ni at both sampling sites exceeded the NOAA Effects Range – Low (ERL) of 20.9 mg/kg. Sediment metal concentrations for Cd, Cr, Hg and Se were below the various effect ranges at all sites. The geoaccumulation index indicates practically unpolluted sediments as far as elements Cd, Cr, Hg, Ni and Se are concerned except for Cu.  
A Thesis Submitted to the Department of Chemistry, Kwame Nkrumah University of Science and Technology in partial fulfilment of the requirements for the Award of the Degree of Master of Philosophy (Mphil) In Analytical Chemistry, June-2013