Heavy Metal Pollution in Gas Flare-Impacted Soils: An Assessment Using Pollution Indices in Obunagha, Bayelsa State, Nigeria

Abstract

Gas flaring is the controlled burning of natural gas which occurs as a result of the activities of oil exploration and exploitation using flare stacks. In Nigeria, gas flaring activities is high and it mainly takes place in the Niger Delta region. The aim of this study was to investigate the bioavailability and concentration of heavy metals pollution of gas flare-impacted soils using pollution indices such as contamination factor, geo-accumulation index and pollution load index. Soil samples were collected with soil auger at depths of 0-15 cm from distances of 200 m, 500 m, 1000 m, 2000 m, and 3000 m away from the flaring point. Flame atomic absorption spectrometry was used to assess the presence of the following heavy metals in the soils: cadmium, chromium, nickel, zinc, copper, and manganese. The findings revealed that Fe (90% bioavailability) had the highest bioavailability among the metals tested in the soils, while Cr (45%) had the lowest. The metals bioavailability decreased in the order of Fe>Mn>Cu>Cd>Ni>Zn>Cr. The geo-accumulation index revealed that the soils were practically uncontaminated by Zn and Cu at all distances, moderately polluted by Cd and Ni at 1000 m and 2000 m and strongly polluted with Cd at a distance of 200 m. The Contamination factor results were consistent with the geo-accumulation index results. The pollutant load index (PLI) was found to be high but decreasing (1.37-0.49) as the distance from the flare point increased, indicating that the research region was polluted. In conclusion, the study's overall findings suggested that the soils were contaminated with heavy metals (particularly Cd) as a result of nearby gas flaring. As a result, the area must be appropriately monitored and managed to prevent future soil contamination by heavy metals to a level that is hazardous to human health.