Comparative Chemical Characterisation of Produced Water from Onshore and Offshore Oil Fields in the Niger Delta, Nigeria

Abstract

Background: Produced water is the largest waste stream generated during petroleum exploration and production and may contain chemical constituents that can cause adverse environmental impacts if inadequately managed. Aim: This study comparatively characterised the chemical composition of produced water from selected onshore and offshore oil fields in the Niger Delta, Nigeria, and evaluated compliance with environmental discharge standards. Materials and Methods: Twenty-four produced water samples were collected from eight oil fields, comprising four onshore and four offshore production facilities. Physicochemical parameters, major ions, heavy metals, total petroleum hydrocarbons (TPH), oil and grease, and polycyclic aromatic hydrocarbons (PAHs) were determined using standard analytical methods. Water Quality Index (WQI), Pollution Index (PI), Pearson correlation analysis, Principal Component Analysis (PCA), and Hierarchical Cluster Analysis (HCA) were employed to evaluate produced water quality, contamination patterns, and relationships among the measured parameters. Comparative statistical analyses were performed to determine differences between onshore and offshore produced water. Results: The pH ranged from 6.66 to 7.34, while electrical conductivity and total dissolved solids ranged from 34,628 to 47,426 µS/cm and from 22,874 to 33,184 mg/L, respectively. Sodium (7,964–11,248 mg/L) and chloride (12,246–17,462 mg/L) were the dominant dissolved ions. Iron (3.86–8.42 mg/L) exhibited the highest heavy metal concentration, whereas cadmium ranged from 0.01 to 0.06 mg/L. Total petroleum hydrocarbons varied between 46.82 and 138.46 mg/L, oil and grease ranged from 18.64 to 62.84 mg/L, and total PAHs ranged from 16.50 to 58.62 mg/L, with consistently higher concentrations recorded in offshore produced water. Water Quality Index values ranged from 381.8 to 706.4, classifying all samples as unsuitable for direct environmental discharge. Strong positive correlations were observed between electrical conductivity and total dissolved solids (r = 0.996) and between oil and grease and TPH (r = 0.981). Principal Component Analysis explained 88.64% of the total variance, while Hierarchical Cluster Analysis grouped the oil fields into three distinct chemical clusters. Conclusion: Offshore produced water exhibited significantly higher contaminant concentrations than onshore produced water, with several parameters exceeding regulatory discharge limits. These findings underscore the need for improved produced water treatment technologies, continuous environmental monitoring and stricter compliance with petroleum wastewater discharge regulations in the Niger Delta.