Description of the study area

Robertkiri, Idama and Jokka are located in Akuku-Toru Local Government Area in  River  State,  Nigeria.  Akuku- Toru  is  an  oil-producing area in river state and occupies a landmass of 1,443 km² and a population of 156, 600 (UNDP, 2016). The major occupation of the inhabitants includes farming, milling of palm oil, making of local gin, fishing and trading. The map of the study area is presented in Figure 1.

Geology of the study area

The Niger delta basin is an intrinsic part of the sedimentary basins of southern Nigeria. The dominant sedimentology characteristics of the soils in this region is influenced by depositional sedimentary pile divided into three diachronous lithostratigraphic formation, the Akata, Agbada and Benin formation from Eocene to Recent  (Raijers, 2011).

Soil sample collection and analysis

The surface and sub-surface soil samples were collected randomly from each location at 0 -15 cm and 15 - 30 cm depth at each sampling point. Samples were air-dried at ambient temperature, ground and sieved with a 2 mm sieve and characterized for soil physical, chemical and microbiological characteristics of microflora present in the soil sample.  Particle size was determined by Bouyoucos hydrometer (Gee and Or, 2002). Available Phosphorous (P) was determined by Bray P-1 method (Anderson and Ingram, 1993). Total Nitrogen (N) was determined by macro-kjedhal method (Brookes et al., 1985). Soil pH was determined in a 1:2 soil to water suspension using a pH meter (Thomas, 1996). Exchangeable bases were extracted using NH₄OAC buffered at pH 7.0 (Thomas, 1982). While Potassium (K) and Sodium (Na) were read from a flame photometer, Exchangeable Calcium (Ca) and Magnesium (Mg) were determined using atomic absorption spectrophotometer with cathode lamp at different wavelength of absorption.

Determination of heavy metals in soil samples

The soil was digested through the wet digestion methods as described by Anderson (1974). 10 g of soil was weighed into a clean 300 ml calibrated This was then filtered through Whatman No. 42 filter papers digestion tube and 5 ml of concentrated sulphuric acid (H₂SO₄) was added in the fume hood, swirled gently. 5 ml of HCl  was added and then heated to 240°C for  one hour  and stored in pre-cleaned polyethylene bottles for further analysis. Atomic Absorption Spectrophotometer (AAS) was set for: Iron, Zinc, Copper, Chromium. Cadmium, Nickel and Lead. Thereafter, the heavy metal contents of soil were determined using standard methods (APHA, 1995; Ademoroti, 1996).

Microbiological analysis

The microbial analysis of samples was determined by the numeration of total heterotrophic bacteria (THB) and fungi and hydrocarbon utilizing bacteria and fungi on plate count Agar at 37° C and the count recorded after 4 h using standard methods (ISO 4833-1, 2013). Indirect cell count was carried out to determine the total viable microbial populations. The test methods used was the ASTM D5465 – 93 to determining Microbial Colony Counts from soil Analysis by Plating Methods, and APHA 907: Standard PlateCount. Total microbial colonies were calculated as follows:

Plate Count (cfu/ml) = No. of Colonies on Plate × Dilution factor

Statistical analysis

Statistical analysis was carried out for the basic descriptive statistics to determine the significance of difference between the controls and sampling point at a probability level of 5% using the Data Analysis package of Microsoft Excel 2016.  

The selected soil physical and chemical properties in the study area are presented in Table 1. The sand content ranged from 74.8 to 86.3% and 80.6 to 86.3% for surface and subsurface soils in all the locations. The clay content  of the soils ranged between 8.0 to 13.2% and show a decreasing trend with depth across the locations relative to the control. The observed decrease in the clay content conformed with the work of Raji et al. (2001) in Northern Nigeria, who reported that clay content also decreased with depth. No  trend  was  observed  on  the  silt  content  at 0-15 and 15 – 30 cm depth in Robertiki, Idama and Jokka (Table 1). The pH values in the three locations ranged between 4.7 and 6.2. This result is contrary to the results of Kentucky (2001) who reported that large variety of aquatic species is found within the pH range of 6.5 to 8.0. The textural classes of the soils ranged between loamy sand to clay loam at depth. Total N content ranged between 0.029 to 0.092 mg/kg and 0.113 to 0.316 mg/kg across depth. However, N content was high in the control than other locations. Similar trend was observed on P content in the three locations relative to the control. No trend was observed in K  content in all the locations evaluated. Na²⁺, Ca²⁺ and Mg²⁺ content of the soils decreased with depth across the locations. The concentration of heavy metals in the soil is presented in Table 2. There is a spatial variability in the concentration of individual heavy metals among the locations. The result showed that Fe concentration was highest in the contaminated locations at 15-30 cm depth with values 1362.01, 894.01 and 674.11 mg/kg for Robertkiri, Idama and Jokka compared to 0-15 cm depth and the control.  

Similar trend was observed at 15-30 cm depth for Zn 33.61, 41.20 and 33.98 mg/kg within the study locations. Whereas Cu and Cr had lowest concentration of 0.49 and 0.78 mg/kg in Robertkiri, 0.74 and 1.17 mg/kg in Idama and 0.76 and 1.87 mg/kg in Jokka at 15-30 cm depth (Table 2) The THB and total heterotrophic fungi (THF) count was relatively low and ranged between 0.59 to 2.85 cfu at 0-15 and 15-30 cm depth in Robertkiri and Idam. While at jokka, THB and THF count ranged between 0.23 to 1.94 cfu at 0-15 and 15-30 cm depth compared to control (Table 3). The variability of the heavy metals in the study area may be due to the anthropogenic activities and geological composition of the soil (Ebong et al., 2020). The high concentration of heavy metals found in the contaminated locations than the control may be adduced to the industrial waste/gas   flaring   released  from  oil  exploration activities of oil companies which contribute to the increase of the heavy metal load in the soil. This observation is in line with Kausar et al. (2019), who reported that the major causes of heavy metals in the soil could be attributed to discharge of industrial and domestic waste, sewage sludge and effluent. The concentration of heavy metals in the soil of the study area is in the order of Fe > Zn > Cr > Cu > Cd > Pb.  The microbiological characteristics of soils in the study location are presented in Table 3. The THB and THF count was relatively low and ranged between 0.59 to 2.85 cfu/mg × 10³ at 0-15 and 15-30 cm depth in Robertkiri and Idam. While at jokka, THB and THF count ranged between 0.23 to 1.94 cfu × 10³ at 0-15 and 15-30 cm depth compared to control. Likewise, the hydrocarbon utilizing bacteria (HUB) and hydrocarbon utilizing fungi (HUF) count was found to  be  low  and  ranged  between  0.19  and 0.71 cfu/mg × 10³ and 0.06 to 0.19 cfu/mg × 10² and increased across depth in all the three locations relative to the control (Table 3). The low count of soil microflora in the study locations may be due to the high levels of heavy metals in the contaminated locations. This observation is consistent with earlier findings by Dian (2018), who reported that increase in heavy metals in soil and environment would interfere with key biological processes and reduction in microbial diversity.

The study revealed that there is a gradual accumulation of heavy metals and the concentration was high in the contaminated than the   controlled  locations.  The  low  count  of  soil  microflora in the study locations may be due to the high levels of heavy metals in the contaminated locations.

The   authors  have  not  declared  any  conflict  of interests.

The authors appreciate the benevolence of Dukoria International Limited for their logistics support and laboratory analysis.