Risk based characterization and modeling of soils contaminated with Volatile Organic Compounds (VOCs) and (benzene, toluene, ethylbenzene, and xylene (BTEX)) in parts of Bonny Island, Rivers State, Nigeria

ABSTRACT

The investigation of the environmental impact of a recent oil spill incident in Bonny terminal using soil media was studied. The study aimed at establishing the presence of contaminants of concern (CoC), determine, quantify and model spilled volume and ascertain potential health risk associated with the spill incident. The CoC are the BTEX compounds – benzene, toluene, ethylbenzene, and xylene. Soils and groundwater were sampled in the vicinity of the spill incidents and further away into the surrounding communities. Soils were sampled at depths of (0.1m, 0.5m,1.0m, 1.5m), and the results of sieve analysis revealed the area is predominantly silty sand in composition. The following parameters also exceeded the DPR intervention limits; Benzene with DPR intervention value of 30 µg/L), Ethylbenzene (150 µg/L), m,p-xylene (70 µg/L) and o-xylene (70 µg/L). Only Toluene had a value within the DPR intervention limit of 1000 µg/L. At all other locations, BTEX compounds were below the detectable limit of the instrument (<0.01 µg/L). This result therefore showed that BW-3 had the most deteriorated water quality. This is not surprising, because BW-3 is situated at the centre of the spill site (Bonny oil terminal). Apart from BW-1, BW-2, BW-3 and BW-4, all other sampled borehole lies within the residential areas. Geochemical analysis of soils revealed that BTEX range from 0.01 to 1.36 mg/kg. A 3-D grid generated for the spill area showed a total volume of 1,198,500 m³. The 3-D grid populated with the various modeled CoC, cross-sections and depth slices extracted from the model revealed that the CoC concentrations generally decreased with depth, with the highest concentrations centered towards the south-eastern part of the study area. Only 3% of the area exceeded the target value of 0.4 mg/kg for BTEX. Three soil zones were delineated from this study based on CoC concentration; Zone 1: highly contaminated soils (BS-8, BS-9, BS-10), Zone 2: moderately contaminated soils (BS-1, BS-2, BS-6, BS-7), Zone 3: low contamination soils (BS-3, BS-4, BS-5). This study therefore showed the effectiveness of the use of a model-based approach in quantifying hydrocarbon contamination volumes.

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WSN 185 (2023) 125-147

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