Seasonal Variation in Hydrocarbon Contamination of Sediments and Biota in Bonny Estuary, Rivers State, Nigeria

Abstract

This study examined the seasonal variation in hydrocarbon contamination of sediments and biota within the Bonny Estuary, Rivers State, Nigeria. Samples of sediment, surface water, and fish were collected during the dry and wet seasons and analysed for total petroleum hydrocarbons (TPH) and polycyclic aromatic hydrocarbons (PAHs) using standard analytical methods and gas chromatography. Physico-chemical parameters were also measured to evaluate environmental factors influencing hydrocarbon distribution. The results showed significant differences in pH, dissolved oxygen, and total organic carbon between seasons, with higher organic content recorded during the wet season. TPH levels ranged from 15.72 ± 1.3 mg/kg in the dry season to 24.35 ± 2.1 mg/kg in the wet season. Similarly, total PAHs varied from 4.86 ± 0.5 mg/kg to 7.42 ± 0.7 mg/kg, both exceeding the permissible limits set by the National Environmental Standards and Regulations Enforcement Agency (NESREA). Hydrocarbon concentrations in fish tissues also exhibited seasonal variation, with levels of naphthalene, phenanthrene, and benzo[a]pyrene surpassing FAO/WHO limits during the wet season. Correlation analysis revealed a strong positive relationship (r = 0.88) between hydrocarbon levels in sediment and biota, indicating an effective transfer through sediment–water–biota interactions. Source diagnostic ratios showed a shift from mainly petrogenic sources in the dry season to pyrogenic sources in the wet season, pointing to the combined effects of oil discharges, gas flaring, and combustion emissions. The study concludes that hydrocarbon pollution in Bonny Estuary is persistent and varies with seasons, posing potential ecological and public health risks for communities dependent on the estuary for fishing and domestic water.