Aquaculture as Source of Environmental Contaminants: A Study of Earthen Pond versus Plastic Pond

: In Sub-Sahara Africa, small-scale aquaculture systems (earthen ponds and plastic basin ponds) have become popular as means of fish cultivation, both for subsistence and as a source of income. Their successful practice, however, involves the use of organic and inorganic chemical substances which may accumulate in pond water and sediments, ultimately finding their ways into the larger environment when the ponds are eventually emptied. This may have significant impact on the environment, since individual communities have thousands of such small-scale systems which are largely unregulated. In this study, we investigated the physico-chemical characteristics and heavy metal composition of typical earthen and plastic ponds. Result of physicochemical analyses revealed the possibility of release of carbonaceous organic materials which may deplete the oxygen levels in natural waters for their decomposition. Such organic materials would come mainly from plastic ponds, as a result of leaching of the plastic monomers and additives into the pond water. The study indicated that both earthen and plastic ponds could contribute to the contamination of the environment by heavy metals, especially Copper (Cu), manganese (Mn) and Zinc (Zn). Possible contributions to Cu, Mn and Znlevels are 3.92 mg, 2.68 mg and 2.63 mgrespectively, per litre of earthen pond water; and 2.57 mg, 1.94 mg and 2.08 mgrespectively, per litre of plastic pond water discharged into the environment. Sediment/sludge of both earthen and plastic ponds have the potential to spike Cu levels in the environment by 5.27 mg and 3.61 mg per kg of sediment discharged, respectively. This study constitutes an initial investigation into the environmental impacts of earthen and plastic pond aquaculture (with respect to chemical contaminants) and could assist authorities in tropical Africa to formulate policies to ensure that the sustainability and quality of the natural environments are not compromised by the proliferation of small-scale effect of heavy metals on plant species, on fauna such as arthropods and earthworms, on the development of microbiological processes, the tendency of the metals to go into solution, and the possibility to enter ground and surface waters. Thus, values above the MPC suggest that the particular metals in the sediment or soil may constitute a hazard to the environment. For metals that the MPC are specified for, the results show that the concentration of metals measured in the earthen and plastic pond sediments are mostly below the MPC, implying that the levels of metals in sediment/sludge resulting from single operation of earthen or plastic pond do not constitute a source of environmental concern. The only exception is Cu, in which the MPC is exceeded in both sediment types, even after taking out the background Cu concentration in the soil for the earthen pond. As for the pond waters in both cases, this study reveals that Cu concentration in the sediments/sludge of both pond types constitute a source of contamination of the environment with this metal.