Evaluation of Crude Oil Contaminated Soils in Parts of Bayelsa State, Nigeria

Abstract

The study evaluated crude oil contaminated soils in both central and eastern parts of Bayelsa State, Nigeria. Soil samples were collected by adopting a rectangular grid and drilling with the aid of a hand auger at the center (highest point of concentration) of the rectangle for proper representation of samples. Soil samples were collected at varying depths of top/bottom, 1m, 2m, and 3m, and were analyzed at Anal Concept Laboratory, Port Harcourt using standard methods as applicable. Results show acidity trend in all locations including control site with pH ranging from 3.6 to 4.9, except at ETE where a pH of 8.0 was obtained. Even this figure is slightly above the stipulated range of 6.0 to 7.0 pH for healthy plants growth.  Mean values of EC range from 11us/cm to 59us/cm, whilst EC measurement at control location recorded very high value of 759us/cm showing negative influence of spill on soil nutrients. Seasonal variation was also noticed as mean EC levels dropped tremendously from 5.9 to 41.0us/cm during the wet season. Although HCO₃^2- and CO₃^- are not very important nutrients for soil because they react with soil calcium to form Ca(HCO3)2 or CaCO3., they act as organic matter stabilizers in calcareous soils.  The result showed that both parameters had reduced concentrations for dry and wet seasons in all locations compared with control results of (4.1 and 2.0 mg/l) and (2.7 and 1.5 mg/l) for dry and wet seasons. SO₄^2- (mg/kg) values ranged from 2.0 to 14.0 mg/l in dry period and reduced further to 1.0 – 3.0mg/l in the wet season. Measurement in the control obtained a high level of 200 and 129mg/l for dry and wet season respectively. Chloride values ranged from 4.0 – 24mg/l for dry period and 2.0 – 9.0mg/l for wet period.  Whilst control station recorded 31.0 and 62.0mg/l for wet and dry seasons respectively, depicting a drastic reduction in spill impacted areas. For organics, PAH recorded peak mean figure of 5.87mg/l at IKM top soil and a minimum mean value of 0.01mg/l at OTU top soil. PAH increased in concentration at different depths in all locations depicting a redistribution and infiltration of petroleum in the soil. On the contrary, OTU and IKM had very low values of PAH indicating low PAH concentration. Similarly, TPH peak for dry season was obtained at IkM T/B (539mg/l) followed by Orm 1-3 (318.26mg/l). But low levels were observed at ETE 1m and Oru 1-3m. The wet season had huge increase in concentration levels as maximum value of 2094.4mg/l was obtained at IKM T/B depicting crude redistribution aided by flood impact. High trend continued almost in all locations except for ETE 1 and ORU 1-3m. Cation such as Calcium showed increased concentration in the wet season at all locations, while, Na+, Mg2+ and K+ experienced drastic reduction during the wet season depicting dissolution on contact with water. For heavy metals, Arsenic recorded very low value of <0.01mg/l across the entire study area, Nickel had not much seasonal variations except for ETE 1 and ORM 1-3m where slight concentration levels were observed. Lead and Vanadium also behaved in similar manner as Ni2+ because increased levels were only encountered at ETE 1, ORM 1-3m and OTU 0-0.3m. On the contrary, Cadmium increased in levels at all locations in both seasons except for in ETE 1. Results from the control station also showed high values of 8.43mg/l and 4.57mg/l for Nickel and Lead respectively indicating natural existence of heavy metals in the earth which can be enhanced by anthropogenic activities. Deductions from the soil profile show predominance of sandstone across the study area indicative of high infiltration most likely the reason for the high level of crude oil observed in spill impacted samples.

Keywords: soils, contamination, crude oil, impacted area

DOI: 10.7176/JEES/13-5-05

Publication date:July 31^st 2023