**5. Results**

km). Nigerian population is estimated to be152,217,341 with a growth rate: of 1.9%; Its neighbors are Benin, Niger, Cameroon, and Chad. The lower course of the Niger River flows south through the western part of the country into the Gulf of Guinea. Swamps and mangrove forests border the southern coast; while inland areas are hardwood forests.The vegetation of Nigeria northwards include derived savannah,guinea savannah,sudan savannah and sahel savannah. Seven major soil groups in Nigeria include Alfisols, Ultisols, Inceptisols, Entisols, Vertisols, Oxisols and Histosols. Arable farming is a major socioeconomic activity while crude oil prospecting is a chief source of national revenue. Oil and oil-related activities plus urban

Soil samples were collected by random sampling in polluted soils studied. Soil samples were collected from epipedons (0-15 cm depth) only. The soil samples were sieved using 2-mm sieve

Cation exchange capacity was measured by ammonium acetate method at a pH 7 (Soil Survey Staff, 2003). Soil pH was determined using a 1:1 soil :water ratio (Soil Survey Staff, 2003). Total carbon content of soils was estimated by loss on ignition using LECO equipment (Leco Corp, St. Joseph, M. I.). Exchangeable Ca and Mg in ammonium acetate extracted solution (leachate) were estimated with atomic absorption spectrophotometer (Perkin Elmer Model) while exchangeable K and Na were measured by flame photometry. Total nitrogen was estimated by microkjeldahl (Bremner, 1996). Available phosphorus was obtained using Bray P No.2

Soil heavy metals were extracted with 1 M ammonium acetate (NH4OAc) [1:4 ratio of soil / 1 M NH4OAc extraction solution : 60 minutes reaction time (Reed and Martens, 1996]. After extraction, the suspension was centrifuged at 7500 x g relative centrifuge force for 30 minutes and the supernatant was passed through a Whatman No.42 paper. The filter paper was allowed

wastes constitute significant mechanism of pollution.

350 Environmental Risk Assessment of Soil Contamination

**Figure 1.** Location map of the study area

method (Olsen and Sommers, 1982).

before they were subjected to various laboratory analyses.

Table 1 shows chemical composition of cassava sludge as opposed to compost derived from it. The result showed that cassava sludge had higher values of total carbon and total nitrogen than the compost manure formed from it.Resultantly, the C:N ratio was 17 when compared to 10 got from the compost. Values of exchangeable calcium and potassium were 9.8 and 18.3 cmol/kg, respectively in the compost manure as opposed to 9.3 and 7.4 cmol/kg in cassava sludge. Composting of cassava sludge reduced total values of heavy metals in it (Table 1). The pHKCl value of composted cassava sludge increased to 6.3 (Table 1).


**Table 1.** Typical chemical characteristics of the cassava sludge and the produced compost (dry weight basis)

Certain factors influence ability of microbes to cause degradation (Table 2).Soil moisture requirement for optimum degradation of oil is high to very high ( 30-90 %) while 25- 28 % soil


**Table 2.** Factors Influencing Microbial Activities

moisture is required since a good number of these organisms are aerobes. Again, optimal soil pH range for oil degradation is 6.5 to 8.0 while microorganisms require 5.5 to 8.8.


**Table 3.** Effect of composted sludge on removability of contaminants at room temperature with time (days)

There was significant reduction ( p=0.05) in the concentration of heavy metals and total petroleum hydrocarbon with time when treated with compost manure derived from cassava sludge (Table 3).However, removability rate varied among soil pollutants over time. Higher of values of Nickel were removed between 60 and 120 days while Vanadium was steadily degraded in the soil. Generally, more values of these soil toxicants were removed in cumulative terms from soils on a long-term. But, heavy metal concentration differed between rainy and dry seasons prevalent in the area as well as between land use types (Table 4). There were greater variations among heavy metals in rainy season in soils affected by automobile services when compared with values in dry season in the same land use. The variation was highest in cadmium (CV=79%), followed by Nickel (CV=48 %) and least in mercury (CV=0 %).Similar trend was observed in arable soils of the area (Table 4).


CV=coefficient of variation in percentage, DS=dry season, RS=rainy season, Cd=cadmium, Cr=chromium, Ni=nickel, Hg=mercury, Pb=lead

(Onweremadu *et al*., 2007).

moisture is required since a good number of these organisms are aerobes. Again, optimal soil

Contaminants Not too toxic Hydrocarbon 5–10% of dry weight of

**Condition required for microbial**

**Optimum value for an oil**

**degradation**

10–40%

soil

**activity**

Soil moisture 25–28% of water holding capacity 30–90% Soil pH 5.5–8.8 6.5–8.0

Temperature (°C) 15–45 20–30

Heavy metals Total content 2000 ppm 700 ppm Type of soil Low clay or silt content

Nutrient content N and P for microbial growth C:N:P = 100:10:1

Oxygen content Aerobic, minimum air-filled pore space of 10%

> ((mg/kg)) 30 60 90 120 180 LSD0.05 Chromium 15 25 35 40 50 1.25 Cadmium 35 60 65 75 90 0.92 Vanadium 20 25 30 35 40 1.08 Nickel 25 40 60 80 85 0.96 TPH 1.8 2.5 6.5 7.5 9.0 0.09

**Table 3.** Effect of composted sludge on removability of contaminants at room temperature with time (days)

There was significant reduction ( p=0.05) in the concentration of heavy metals and total petroleum hydrocarbon with time when treated with compost manure derived from cassava sludge (Table 3).However, removability rate varied among soil pollutants over time. Higher of values of Nickel were removed between 60 and 120 days while Vanadium was steadily degraded in the soil. Generally, more values of these soil toxicants were removed in cumulative terms from soils on a long-term. But, heavy metal concentration differed between rainy and

pH range for oil degradation is 6.5 to 8.0 while microorganisms require 5.5 to 8.8.

**Pollutant Time in Days**

TPH= total petroleum hydrocarbon

(Source: Onweremadu, 2008)

**Environmental conditions affecting**

352 Environmental Risk Assessment of Soil Contamination

Sources:Vidali,2001; Thapa *et al*.2012,

**Table 2.** Factors Influencing Microbial Activities

**degradation. Parameters**

**Table 4.** Seasonal variability (CV) of heavy metal concentration (mg/kg) in soils affected by
