Geochemistry of Estuarine Sediments of Ghana: Provenance, Tracemetal Accumulation Trends and Ecotoxicological Risks
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Date
2014-07
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Publisher
University Of Ghana
Abstract
The sedimentary record, as revealed in sediment cores, can be used to reconstruct the past history of contaminant input into the aquatic environment. This is because contaminant inputs equilibrate rapidly with sediment supply, and the sediment column represents a continuous sequence of sediments that are associated with contaminant accumulation. With radiometric techniques, it is possible to date recent sediments over a period corresponding to about five half-lives (approximately 100-200 years using 210Pb). To understand the geochemistry of estuarine sediments in relation to their provenance, trace metal accumulation trends and potential ecotoxicological risks, this study investigated sediment cores and surficial sediments from the Ankobra, Pra, Amisa, Densu, Sakumo II and Volta estuaries of Ghana. Geochemical and petrographical techniques were used to reveal the source of sediment supply into the estuarine environments. In addition, Gamma Spectroscopy and Inductively Coupled Plasma Mass Spectrometric (ICP-MS) techniques were used for deriving geochronologies, sedimentation rates, spatial and temporal accumulation of Mo, Cd, Pb, V, Cr, Cu, Zn, As and Hg in the study sites. The pollution status of the estuarine sediments was investigated using Aluminum-normalized enrichment factors (EFs). Cluster and correlation analysis differentiated natural from anthropogenic sources of trace metal in the estuaries. The excess fluxes of trace metals, estimated using sedimentation rates helped in quantifying the extent of anthropogenic contamination of metals in each estuary. AQUARISK software was used to assess any potential ecotoxicological risks associated with trace metal enrichments. Results showed that sediments from the Ankobra, Pra, Densu and Volta estuaries comprised quartzose sedimentary provenance while those of Amisa and Sakumo II estuaries comprised mafic igneous provenance. The sedimentation rates estimated from the constant rate of supply model were 0.24±0.10 gcm-2y-1, 0.47±0.20 gcm-2y-1, 0.43±0.02 gcm-2y-1, 0.31±0.13 gcm-2y-1, 0.20±0.01gcm-2y-1 and 0.54±03 gcm-2y-1 for the Ankobra, Pra, Amisa, Densu, Sakumo II and Volta estuaries respectively. The sediment core from the Ankobra estuary was significantly enriched with Hg (5.0<EF≤8.4) and extremely enriched with As (69.3<EF≤114.3) over time. Similarly, the sediment core from the Pra estuary showed significant enrichment with Hg (5.0<EF≤10.2) between 1980 AD and 1995 AD and very high enrichment with As (EFs around 20.5) since 1875 AD to present. Likewise, the sediment core from the Amisa estuary showed significant enrichment with Hg (EF=6.3) around 1940 AD and a significant enrichment with As (5.0<EF≤6.5) from 1960 AD to present. On the contrary, the sediment core from the Densu estuary was not enrichment with trace metals although a sharp peak correlating to a moderate EF of 4 was observed around 1965 AD for Hg. The sediment core from the Sakumo II estuary was significantly enriched with Hg (5.0<EF≤6.3) since 2005 AD to present, with other trace metals showing potentials for enrichment over the last 20 years. Extremely high enrichments of Hg (EF=375.2, 16.01 and 111.3) were observed around 2003 AD, 1977 AD and 1965 AD respectively in the Volta estuary. In addition, significant enrichments of As were seen around 1995 AD and 1965 AD in this estuary. Results of ecotoxicological risk assessment showed that As and Hg posed potential ecotoxicological risks to the biota in the Ankobra estuary while Cd, Cu, Pb and Hg posed potential risk to the biota in the Sakumo II estuary. None of the metals analyzed posed ecotoxicological risks to the biota in the Volta estuary. To achieve at least 75% species protection in the Ankobra estuary, a substantial percent reduction of about 97.8% and 12.7% in the current levels of As and Hg are required respectively. In the same way, a substantial percent reduction of about 99.8%, 99.9%, 73.8% and 100% in the current levels of Cd, Pb, Hg and Cu respectively are currently required to achieve at least a 75% species protection in the Sakumo II estuary.
Description
PHD.
Keywords
Geochemistry, Estuarine Sediments, Ecotoxicological Risks, Aquatic Environment, Ghana