Geochemical Characterization of Soil in the Bitumen Environment of Ondo State, Southwestern Nigeria

Abstract

Surface (0-30 cm) and subsurface (60-90 cm) soil samples from the bitumen environment of Ondo state were characterized for total and phytoavailable element concentration, mineralogy, geochemistry and polycyclic aromatic hydrocarbons (PAHs). Physical properties that were known to influence element mobility as well as those that determine plant nutrients accessibility were determined using standard procedures. Total multi element concentrations were determined by digesting the soil samples with mixtures of 10 ml HF + 5 ml HClO3 + 2.5 ml HCl + 2.5 ml HNO3 and then sequentially fractionated operationally into a 6 fractions thus: soluble + exchangeable (F1); bound to organic matter (F2); precipitation of carbonates (F3); bound to Fe and Mn oxides (F4); precipitation of sulphides (F5). The difference between the sum F1-F5 and total is the residue (F6). Single stage leaching was investigated i. reversed osmosis purified water and ii. 1M NH4NO3-0.025M Na2EDTA (pH 7). Elements concentration was determined in the respective solutions by inductively coupled plasma optical emission spectrometry (ICP-OES). Major oxide geochemistry was obtained using X-Ray Fluorescence analysis (XRF) while qualitative and quantitative mineralogical analysis of the soil samples was done by applying respectively EVA software and TOPAS software with Rietveld refinement method on the XRD diffractogram. Polycyclic aromatic hydrocarbon distribution in the area was obtained using Microwave Assisted Extraction-Gas Chromatography-Mass Spectrometry (MAE-GC-MS) method. The soil in the study area is relatively acidic with low cation exchange capacity, low organic matter content, low available nitrogen and low available phosphorus. The surface soil is observed to be undergoing acidification process despite the relative acidity of subsurface soil; this can be due to natural acidification process by carbonic and organic acid dissolution. There can be additional anthropogenic influence due to dry/wet deposition of SO2 and/or NO2 with possible nitrogenous fertilization. Generally, all the elements investigated were found in all samples analyzed. Available elements extractable with water (usually referred to as soluble) were negligible with majority of elements analyzed accounting generally less than 2% while there is appreciable solubility in 1M NH4NO3-0.025M Na2EDTA (pH 7). The following elements (Pb, Cu, Mn, Ba, Ni, Zn and Cr) showed appreciable phytoavailability in surface soil samples while in subsurface soil samples the following elements (Pb, Zn, Mn, Ba, Cu, V, Co and Cr) are more phytoavailable. All the elements show different preference for soil phases with the highest concentration of elements residing in the residue. Using risk assessment code (RAC) (MoF1, MoF2 and MoF3) based on the strength of the bond between the element and the different geochemical fractions in soils and the ability of the elements to be released and enter into the food chain, all the elements shows some level of risk; however, the consistency of Cd, Cu, Pb, Se and Zn with medium risk at both surface and subsurface environment and also that of high risk Cd, Co and Cu at subsurface environment are noteworthy; firstly for direct effect on plants and animal health including human being and secondly, infiltration of underground water aquifer. Concern is however shown for Cd and Cu in this environment because of potential hazard that are known to be associated with their availability in soil. The extent of anthropogenic contamination is expressed as transfer factor (Tf) using Ti as normalizer. Al, As, Cr, Fe, Mo, Se and Zn are not anthropogenically enriched i.e. they are essentially lithogenic whereas Ba, Cd, Co, Cu, Mn, Ni, Pb, Sr and V are having minimal enrichment anthropogenically. On the basis of relative abundance of the total geochemical components, the soil samples can be grouped into 3 for purposes of their genesis (formation); the most abundant group are Si, Ti, Al and Fe, the second group is moderate Mg, Ca and K and the third group which is least are Mn, P, Cr and Na. The observed high quartz, low kaolinite, low feldspar and low gibbsite is an indication of moderate to high degree of weathering which resulted in low levels of basic oxides, phosphorus and possibly boron. The primary minerals (quartz, feldspars) and secondary mineral (kaolinite) are the main mineral components in the soil of bitumen environment of Ondo state. The Rieltveld refinement gave generally quartz value as 83.76-93.76%, kaolinite group as 4.38-10.60%, feldspars as 1.86-3.82%, albite about 1.82% with traces of other minerals that could not be identified during phase identification. The spatial concentration of polycyclic aromatic hydrocarbons (PAHs) in the study area did not show appreciable variation as most data obtained were almost constant for all the PAHs in all the soil samples analyzed; diagnostic source apportionment shows that the surface soil PAHs are derived principally from pyrogenic sources which atypical bush burning which is predominant in the study area; the subsurface soil could either be pyrogenic or petrogenic. In this study generally, all the environmental indicators have low values but there are still concern for some of them either in the surface or subsurface environment and it can however be concluded that the bitumen environment of Ondo state is still very pristine in all the parameter characterised, human influence on the environment are still very minimal.