An Assessment of Heavy Metal Contamination of Sediments and Tissues of the Clam Galatea Paradoxa (Born 1778) In the Volta Estuary, Ghana

The concentrations of four heavy metals, Mn, Zn, Fe (essential heavy metals) and Hg (non-essential heavy metal) were determined in sediments and in whole soft tissue of the clam Galatea paradoxa (Born 1778) from two clam fishing locations, Ada and Aveglo at the Volta Estuary in Ghana over an 18-month period. Thirty (30) clams were obtained from each sampling location monthly and grouped into three size classes of 10 individuals each based on shell lengths as follows: small (25 -40mm), medium (41-55mm), and large (above 55mm). The groupings were chosen based on the three dominant size groups in the natural population to give a broad and representative range of metal concentrations in the clams. All the results were expressed as total concentrations (µg/g dry weight (dw). Mean concentrations of analyzed metals in the tissue of the clams from the Ada sampling station were: Mn: 152.9 µg/g; Fe: 174.9 µg/g, Zn: 34.6 µg/g and Hg: 0.043 µg/g. The mean metal concentrations in the Aveglo clams were: Mn: 130.0 µg/g, Fe: 187.0 µg/g, Zn: 37.1 µg/g and Hg: 0.046 µg/g. Mean metal concentrations in the sediments were Mn: 186.0 µg/g, Fe: 1770.4 µg/g, Zn: 3.2 µg/g and Hg: 0.0086 µg/g for the Ada sampling station. The Aveglo sediments had mean metal concentrations as follows: Mn: 171.9 µg/g, Fe: 1758.5 µg/g, Zn: 3.7 µg/g and Hg: 0.0115 µg/g. Metal concentrations in the tissues of the different clam size-classes (small vs. small, medium vs. medium and large vs. large) from the two sampling stations were almost identical and did not vary significantly (p>0.05). A comparative evaluation of the metal concentrations in the clams and sediments from the two stations, however revealed significant variations in concentrations for Zn Fe and, Hg. Concentration of Fe in the Ada sediment samples for June was as much as 18 times higher than the concentration in the clams and Hg concentrations were approximately five (5) times higher in the clam tissues than in the sediments during the study period. On the basis of calculated BSAFs the metal enrichment in the tissues of the clams rank in the following order Zn>Hg>Mn>Fe. The BSAFs indicated a significant accumulation of Zn and Hg in the clam tissues relative to the concentrations of these metals in the sediments although no clear relationships were established between the concentrations of the studied heavy metals in the clam tissues and sediments. There were no significant differences (p >0.05) in Mn, Fe and Zn concentrations among the different size classes except for Hg concentration in clams from Ada, indicating a similar bioavailability of Mn, Fe, Zn at both locations and, possibly, an efficient metabolism to keep the concentrations of Mn, Fe and Zn relatively similar in the tissues of the different clam sizes. Spatial variations in metal concentrations in the clams (i.e., Ada small vs. Aveglo small, Ada medium vs. Aveglo medium, and Ada large vs. Aveglo large) were not significant for all four studied metals in the compared size classes. Results of the statistical test for spatial variations in the sediment samples from the two stations also revealed no significant differences (p>0.05) in the concentrations of Mn, Zn, Fe and THg during the study period. To understand the relationships between metal concentration in the sediments and accumulation in the tissues of the three clam size classes as far as Mn, Zn, Fe and Hg were concerned, the monthly concentrations of the studied metals were graphed to observe distinct metal accumulation patterns The graphs revealed no simple linear relationships between the concentrations of heavy metals in the clam tissues and the sediments at the two sampling stations although some distinct accumulation trends were observed as far as Mn was concerned. Analysis of risks levels associated with the consumption of clams by humans revealed that the concentration of the Mn, Zn, Fe and Hg found in the clam tissues were within permissible limits using reference guides such as the WHO Safety Reference Standards for Bivalves and various indicators as the Tolerable daily Intake (TDI), rate of shellfish consumption (RSC), Risk Quotients (RQs) and levels of concerns (LOCs) Based on geoaccumulation calculations, the sediments from the two sampling stations are unpolluted as far as the heavy metals, Manganese, Zinc, Iron and Mercury are concerned and the samples are similar to those observed in areas under low pollution impact.

Source: KNUST