Crude oil spill research has been largely discussed because of the environmental problems caused as a result of interest from many oil producing nations in this natural endowment. Pedogenesis, the process of soil formation as regulated by the effects of place, environment and history, takes its own perspectives on the problem.

The earth comprises two thirds water and one third soil. Oil spills affect both. Worldwide, crude oil is extracted at the rate of over 65 million barrels per day (Giovanni and Simon, 1998) and meets the bulk of world energy requirements. In Nigeria, crude oil spills account for about 35,000 barrels of crude oil on the Niger-Delta marine body (Obi, 2012). Researchers (Nwilo and Badejo, 2001; Bronwen, 1999; UNDP Report, 2006; Giovanni and Simon, 1998) have concentrated efforts in making inferences about its effect in time and in space with other resources of our natural environment including water bodies, vegetation and even the air we breathe. More often than not, cases of oil spillage have always been reported in southern parts of the Nigerian state.

Soils take time to form in space and time. It is believed that processes which affect soil formation and development are: additions, losses, transformations and translocations (Brady and Weil, 1999; Esu 2010). When crude oil spills on land, a process of addition has been initiated, which goes a long way in affecting soil formation when the “time factor” of Hans Jenny (1994) is put into consideration. Oil spillage has been known to affect both soil and water. Its effect on soil includes its contribution on pedogenesis and the resulting changes to soil fertility/nutrition.

Because of the influence of chronosequence on soil formation that take a long time to manifest, most researchers have concentrated their research efforts on the influence of oil spillage on the fertility status of the soils as it transcends to immediate human consumption. Pedogenesis will always consider the influence of the soil additions on the environmental history, soil development and its influence on the pedogenic functions of the soil, which include its function as a ground water filter, sink of the global carbon resource and as a support for all the biota.

Present research has concentrated efforts in studying the pedogenesis of soils where a crude oil spill has occurred. This study, therefore, discussed how the oil spillage in the Bodo city of the Niger Delta in the Southern part of Nigeria has affected some processes of additions which in turn have a cascade effect on losses, translocations and transformation processes on the soil and its materials. For this purpose pedons, or soil samples, in the affected areas were extensively studied for a proper understanding of these processes. Further research may seek to understand the difference between these soils and an uncontaminated soil.

Description of the study area

The study was conduced at Bodo city in Gokana Local Government Area, Rivers State, which lies between Latitude 4° 36’ and 4° 40’ N and Longitude 7° 15’ and 7° 30’ E within a humid tropical climate, with the rainy reason extending more than the dry season. The mean annual rainfall is about 3,000mm while the mean annual temperature is about 29°C. The vegetation is a mixture of swamp and rainforest with the latter dominating in upland soils of the study area. The Atlantic Ocean and the River Niger Delta are major hydrological entities influencing soils. Socioeconomic activities range from capture fishery, small-scale agriculture, gathering from the wild, and oil prospecting.

Soil sampling and laboratory analysis

Four geomorphic surfaces of the crude oil spill area were identified: terrace, marsh, levee and mudflat. A soil profile pit was dug in each of the geomorphic surfaces using the guidelines of FAO (2006). Soil samples were collected from the bottom-most layer to the top-most to avoid contamination of the soils from horizons. In preparation for analysis of some important properties for characterising the pedogenesis of the soils, samples were air dried using the 2mm sieve. Soil colour (using the Munsell colour system) and other macromorphological features, were determined in situ.

Particle size analysis was determined by hydrometer method (Gee and Or, 2002). Bulk density was determined using the core method as described by Blake and Hartge (1986). Moisture was obtained by gravimetric method, while total porosity was computed based on the relationship between bulk density and particle density as follows:

Total Porosity =100 –( BD⁄PD )×100 (Foth, 1984).

Macroporosity was calculated as volume of water drained at 60 cm tension divided by volume of bulk soil. Microporosity was estimated as the difference between total porosity and macroporosity. Mean values of attributed soil properties were calculated using SAS (1999).

Most of the soils of the terrace geomorphic units had a very dark grey (VDG) colouration in their mineral horizons (Table 1). This was as a result of the formation of pseudogleys that has formed up to the mineral soils from capillary fringes. Gleization is known to occur as a result of change in the oxidation state (Esu, 2010) of the iron mineral from sesquioxides that were the major composition of the coastal plain soils materials. Slow drainage (Table 2, microporosity ≥ 37.5 ≤ 49.5) that occurred in these mineral soils resulted in slow decomposition of the organic materials in the soil surface. Observations from most of the soil solum showed that there was evidence of illuviation as a result of eluviations of most of the decomposed organic materials. This had occurred more in the marsh geomorphic surface, which had dark greyish black to olive colouration in the solum. Oil spill could have resulted in the slow decomposition of the surface litter in most of the geomorphic units, as this effected the microporosity of the soils resulting in the reduction of the soil air that aids the survival of the aerobic microbes. Soil structure differed at the epipedons as soils of the terrace and levee geomorphic units had moderate granular structure, while the marsh and mudflat had platy structures.

Table 1: Macromorphological properties

  • Structure: 1 = weak, 2 = moderate, 3 = strong, vf = very fine, f = fine, c = coarse, vc = very coarse, gr = granular, pl = platy, sbk = subangular, col = columnar.
  • Consistence: mfr = moist friable, mfi = moist firm, ml = moist loose, wss = wet slightly sticky, wvs = wet very sticky.
  • Root Abundance: m = many, f = few, abt = abundance
  • Boundary Distinctiveness: a = abrupt, g = gradual, c = clear, d = diffuse
  • Boundary Topography: w = wavy, s = smooth, i = irregular, b = broken structures

Results on soil structure indicate structural instability in soils of marsh and mudflat geomorphic units. It is postulated that oxidation and deposition of iron around channels increased aggregate stability (Obi, 2003) which is expected more in soils of terrace and levee unless disturbed by cultivation. The natural arrangement of the soil matrix could not have changed as a result of oil spillage, although mechanical pressure on the soil surface could alter the structural arrangement of the peds. The soil’s consistency ranged from moist friable to moist loose in the epipedons of the terrace and levee geomorphic units.

Table 2: Selected Soil Physical Properties

  • FS = fine sand
  • CS = coarse sand
  • SCR = silt-clay ratio
  • TC = textural class
  • MC = moisture content
  • BD = bulk density
  • S = sand
  • LS = loamy sand
  • SL = sandy loam

The consistency coincided with the gravimetric moisture in these horizons, which ranged between 19 g/kg to 49 g/kg (Table 2). Analysis of deliberately spilled area by Uzoije and Agunwamba (2011) showed that most of the spilled crude oil was absorbed by the soil matrix and as well were mixed up with the soil moisture. This could have contributed to the near constant moist and wet consistency of all the horizons. Most of the geomorphic units had few roots below the solum. The anaerobic condition and higher bulk densities (Table 2) of the epipedons could have resulted in the reduced root penetration ability. Most of the horizon boundaries were diffuse, except that of the terrace and levee, which were abrupt and clear respectively. The distinctiveness of the horizon could be as a result of the redoximorphic reaction in the horizons, resulting in gleization and ferrolysis of most of the horizons. Ferrolysis had occurred mostly in the horizons of the marsh and levee geomorphic units. All these were majorly as a result of changes in the redoximorphic features of the soil as a result of oil spillage and the coastal nature (wetland) of the spilled areas.

“analysis of deliberately spilled area by Uzoije and Agunwamba (2011) showed that most of the spilled crude oil was absorbed by the soil matrix”

Table 3 shows distribution of particle sizes, moisture content and bulk density of studied soils. Sandy textures dominated the oil spilled (wetland soils) with sand ranging from 780 to 880 g/kg (Terrace), 810 to 950 g/kg (Marsh), 890 to 950 g/kg (Levee) and 630 to 870 g/kg (Mudflat). Clay content of soils ranged from 100 to 190 g/kg (Terrace), 40 to 170 g/kg (Marsh), 40 to 100 g/kg (Levee) and 30 to 180 g/kg (Mudflat). Silt ranged from 20 to 50 g/kg (Terrace), 10 to 20 g/kg (Marsh), 09 to 10 g/kg (Levee) and 100 to 260 g/kg (Mudflat). Coarse sand (220 to 940 g/kg) predominated over fine sand (10 to 410 g/kg) in all the geomorphic units. The silt-clay ratio indicated that soils are of advanced weathering except soils of the mudflat that are relatively young. It can be deduced that soils of the geomorphic units are detached old soils from upland areas that moved to the present location by coastal erosion processes. Soils of the mudflat, however, tend to be reforming, with clay content decreasing with depth.

Table 3: Bulk Density and Porosity Characteristics of Soil

  • BD = Bulk Density

With the exception of soils of terrace, clay decreased with depth in all soil units. Results of moisture content showed that soils of the marsh had highest moisture content (63 – 480 g/kg), followed by soils of mudflat (48 – 70 g/kg), then soils of terrace (35 – 49 g/kg), and least value was recorded on soils of the levee (19 – 24 g/kg). One would have expected that soils of the mudflat with least sandiness should be having highest volume of moisture but results proved the contrary, implying that other un-investigated factors influenced moisture content of soils. Soils of the mudflat had least value of bulk density (1.19 mg/m3) followed by soils of the levee (1.23 mg/m3), while surface soils of terrace and marsh had 1.29 mg/m3 and 1.38 mg/m3, respectively. Bulk density increased with depth in all geomorphic soil units. Ocean overflow tends to influence soils of the marsh as water pond on the soils more than other geomorphic units hence higher bulk density compared to other geomorphic surfaces. However, tillage activities on the soils of the terrace followed by occasional flood could be responsible for high bulk density of soils of the terrace.

“gleization of the epipedons were due to capillary fringes from the lower horizons that had an endosaturation”

Soils of most of the epipedons and endopedons had mostly grey colouration showing an evidence of gleization in the horizons. The soils were mostly in a reduced condition than oxidised condition because of the coastal nature of the spilled area. It was obvious that gleization of the epipedons was due to capillary fringes from the lower horizons that had an endosaturation. Some of the horizons had evidence of ferrolysis, which could be due to the lower occurrences of oxidiation of iron mineral from the sesquioxides. The soil structures were mostly unaltered having moderate granular and platy structures. Drainage of the soils would be recommended as this would go a long way in cleaning up the spilled crude oil in the sites. It is good advice to also apply lime or basic fertilisers when planting on the soils after drainage, as acidity would be generated by not only the hydrogen ion reduced soil condition, but also the hydrocarbon and micronutrient contents of the spilled crude oil.

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