Speciation and chemical mobility of heavy metals were assessed with a view to provide information on bioavailability of metals in a sediment matrix in order to account for the vulnerability of sediment dwelling fauna within the Middleton river ecosystem. Sediment was collected in triplicate from seven locations (five reflecting activities of oil installations in the study area and two controls). Sediments were collected in the months of March and August 2017 to reflect the dry and wet seasons respectively. Concentration of metals in each fraction was determined using a GBC Avanta Atomic Absorption Spectrophotometer. The residual fraction was the most important for Cr with average levels of (49.4%) and (41.1%) for the dry and wet seasons respectively. Also, the most prevalent fractions of Zn were the residual fractions which revealed (31.7%) and (42.6%) during the dry and wet seasons respectively. The metalloid (As) had (14.7%) and (13.8%) of its total concentration bound to the residual and crystalline Fe-oxide fractions during the dry and wet seasons respectively. The oxidation-reduction potential (ORP) was measured electrometrically using a redox potential meter; ORP ranged from (+288.67 to +351.33 mV) and (+331.00 to +465.67 mV) for the dry and wet seasons respectively, hence, depicting a fairly oxidizing sediment environment. Mobility factor of metals depicted (As > Cu > Cr) and (As > Cr > Cu) for the dry and wet seasons respectively. The effect of oxidation-reduction potential on metal mobility revealed the transportability of chromium with increasing redox potential while arsenic was barely mobile with increasing oxidation level. Also, zinc mobility diminished strongly with increasing oxidation-reduction potential.

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