Acute toxicity of produced water on selected organisms in the aquatic environment of the niger delta

Abstract

The discharge of produced water (PW) and drill cuttings from oil operations may elicit varied toxicity in fresh, brackish and marine organisms when exposed. The Niger Delta region which is the oil and gas province of Nigeria have incessantly been recipients of produced water. The study was aimed at investigating the toxic effects of produced water on freshwater fish- Oreochromis niloticus and brackish water shrimp- Palaemonetes africanus in the Niger Delta region of Nigeria. Probit toxicity tests were conducted on the organisms through a 96 h bioassay using produced water (PW) obtained from Mobil, Qua Iboe Terminal (QIT) in Eket, Akwa Ibom State, Nigeria. The organisms were tested in separate tanks containing produced water of 0.0, 100, 200, 400, 800 and 1000 mL, mixed with 2 L of habitat water (fresh/ brackish water). The physicochemical properties of the mixtures were determined by multiparameter hand held probe and atomic absorption photometer (AAS). The physico-chemical parameters, particularly the pH, and conductivity of the PW at 26.5 °C were significantly higher (p < 0.05) than those of the freshwater and brackish water samples. O. niloticus at the end of the 96 h test were 35%, 45%, 60%, 70% and 85%; while P. africanus had percentage mortalities of 25%, 35%, 45%, 60% and 80% at produced water (PW) of 5%, 10%, 20%, 40% and 50% respectively. Mortality rates of both test organisms being directly proportional to the percentages of PW and zero mortality recorded in the controls suggest that PW inflicted significant acute toxicity on the tested species. Higher mortality rates recorded in O. niloticus than P.africanus could be attributed to the increase of salinity following the introduction of the PW in the fresh water test media. The concentrations of PW administered in the study which were below the LC50 were are 49,500 and 99,000 ppm, while 198,000, 396,000, and 495,000 ppm were above the LC50. Results showed that the PW collected from the Mobil QIT effluent point source was toxic to the aquatic organisms, particularly the freshwater Oreochromis niloticus. We therefore speculate that 99,000 ppm of PW may be considered as the “no observed effect concentration” (NOEC).