**3. Results and discussion**

#### **3.1. Soil pH**

Soil pH is one of the most important physico-chemical properties which control many other soil physical, chemical, and biological properties. The pH of a soil is affected by the concen‐ tration of CO2 in the soil air, salt concentration (salt effect), and the presence of colloidal particles (suspension effect). The higher the CO2 concentration in the soil solution, the lower the pH, and the pH of a neutral or calcareous soil is very sensitive to small changes in CO2 concentration. pH greatly affects the solubility of minerals in soils. Most minerals are more soluble in acid soils than in neutral or slightly basic solutions [81]. The pH of a soil is related to the bioavailability of metals in the soil to plants. At low pH, metals are easily bioavailable because there is increase in the solubility of metals. However, at high pH i.e. slightly alkaline to highly alkaline conditions, metals are not easily mobilized, because they are not easily soluble at high pH, hence not easily bio available [82]. The pH of the composite soils collected at the five different points (SA1-SA5) opposite the market at a distance of about 100 m to the market was slightly acidic with the pH of the topsoil (0-15cm) slightly lower than that of the subsoil (15-30cm) except at the downstream of the river (sampling area C) (Table 2).

#### **3.2. Soil Organic Matter**

Soil organic matter is any material produced originally by living organisms (plant or animal) that is returned to the soil and goes through the decomposition process. At any given time, it


**Table 2.** Summary of Soil physicochemical parameters

The concentration of each analyte was determined by calculating the amount of analyte or hydrocarbon range injected from the peak response in area ratio. The contribution from the solvent front and the surrogate compound were excluded from the total area of the sample.

Soil pH is one of the most important physico-chemical properties which control many other soil physical, chemical, and biological properties. The pH of a soil is affected by the concen‐ tration of CO2 in the soil air, salt concentration (salt effect), and the presence of colloidal particles (suspension effect). The higher the CO2 concentration in the soil solution, the lower the pH, and the pH of a neutral or calcareous soil is very sensitive to small changes in CO2 concentration. pH greatly affects the solubility of minerals in soils. Most minerals are more soluble in acid soils than in neutral or slightly basic solutions [81]. The pH of a soil is related to the bioavailability of metals in the soil to plants. At low pH, metals are easily bioavailable because there is increase in the solubility of metals. However, at high pH i.e. slightly alkaline to highly alkaline conditions, metals are not easily mobilized, because they are not easily soluble at high pH, hence not easily bio available [82]. The pH of the composite soils collected at the five different points (SA1-SA5) opposite the market at a distance of about 100 m to the market was slightly acidic with the pH of the topsoil (0-15cm) slightly lower than that of the

subsoil (15-30cm) except at the downstream of the river (sampling area C) (Table 2).

Soil organic matter is any material produced originally by living organisms (plant or animal) that is returned to the soil and goes through the decomposition process. At any given time, it

*Cf* =

Cf

Wi

Vf

Df

Rf

Where,

*A*(*p*) *x Rf x Vf x Df x* 1000 *Wi*

240 Environmental Risk Assessment of Soil Contamination

= Final Sample concentration (µg/L)

= Initial weight extracted (g dry weight)

= Dilution factor of sample or extract if diluted.

Concentration (p) = Total concentration of range

= Response factor from the calibration standard calculation

A(p) = Measured area of peak (peaks)

= Final extract volume (ml).

**3. Results and discussion**

**3.2. Soil Organic Matter**

*Rf* <sup>=</sup> *Concentration* (*P*) *Area* (*P*)

**3.1. Soil pH**

consists of a range of materials from the intact original tissues of plants and animals to the substantially decomposed mixture of materials known as humus. Most soil organic matter originates from plant tissue. Plant residues contain 60-90 percent moisture. The remaining dry matter consists of carbon (C), oxygen, hydrogen (H) and small amounts of sulphur (S), nitrogen (N), phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg). Although present in small amounts, these nutrients are very important from the viewpoint of soil fertility man‐ agement [83]. Soil organic matter (SOM) serves as a soil conditioner, nutrient reservoir, substrate for microbial activity, preserver of the environment, and major determinant for sustaining and increasing agricultural productivity. The OM content of soils ranges from less than1% in desert soils to close to 100% in organic soils. A typical agricultural soil may contain between 1 and 5% OM in the top 15 cm [84]. The percentage organic matter of the soil samples collected around the site is shown in Table 2. Generally, the subsoil has more organic matter content than their respective topsoil. The top soil contains less than 1% OM while the subsoil contains higher values. Soil organic matter performs very important functions in the soil such as: acts as a binding agent for mineral particles, this is responsible for producing friable (easily crumbled) surface soils; increases the amount of water that a soil may hold; and provides food for organisms that inhabit the soil. Humus is an integral component of organic matter because it is fairly stable and resistant to further decomposition. Humus is brown or black and gives soils its dark colour. Like clay particles, humus is an important source of plant nutrients.

#### **3.3. Particle size distribution (soil texture)**

Particle size is a fundamental property of any sediment, soil or dust deposit that can provide important clues to nature and provenance and influences a variety of other properties [85]. Soil particle size distribution is of greta importance to soil water movement, soil erosion and soil solute migration [86]. It is the determination of the proportion of each soil fraction in the samples and includes the % clay, % silt and % sand. Generally, in all the soil samples collected, sand has the highest percentage. The topsoil of the site has the highest percentage of sand in all the sampling areas. Figure 2 shows the variation in the physico-chemical parameters in the soil.

#### **3.4. Concentration of heavy metals in soil**

The results of heavy metals analyzed in the soil samples are shown in Table 3 while Figure 3 shows the distribution around the study site. There is variation in the distribution of the metals around the market with some of the metals such as lead (23.8 mg/kg) and copper (10.4 mg/kg) having the highest concentrations in the subsoil (15-30 cm). The concentrations of all the metals around the market is the highest when compared to the downstream and upstream of the river. This may be attributed to the anthropogenic activities in this area. In most cases, the subsoil has higher concentration of the metals than their respective topsoil, except Zn, Cd, and Ni. However, Cadmium compounds being of very low solubility in water will have little down‐ ward movement and accumulate in the topsoil [87]. This may be due to strong adsorption of Cd by the surface soils, which have a higher organic matter content and higher pH; factors which are known to increase Cd adsorption in soils [88]. Also, it was reported that Cd accumulated in the soil surface layers and had low vertical movement when mobility of heavy metals contained in the sludge and the wastewater used for the irrigation was studied in the soil samples collected from the soil profiles from the surface to 100 cm depth of farmland [89]. The mean concentrations and standard deviation of Pb, Zn, Cu, Cr, Cd and Ni in the topsoil (0 – 15cm) and subsoil (15 – 30cm) around the river basin at Isheri are 14.6 ± 4.5 mg/kg and 18.1 ± 8.1 mg/kg; 38.0 ± 27 mg/kg and 44.6 ± 28 mg/kg; 6.41 ± 3.3 mg/kg and 7.77 ± 3.7 mg/kg; 7.35 ± 0.1 mg/kg and 10.5 ± 0.4 mg/kg; 0.74 ± 0.3 mg/kg and 0.63 ± 0.03 mg/kg ; and 3.81 ± 0.5 mg/kg and 5.70 ± 0.3 mg/kg, respectively.

Comparing the results of heavy metal concentrations obtained in this study with the control sample collected at a rural community, Lalupon in Ibadan and some typical values in rural and urban soil around the world (Table 4). The average metal concentrations in both the top and subsoil were higher than the values obtained from the control sample. There are no background and baseline data on toxic metals concentrations in Nigeria. Metal concentrations in the study site when compared to the control sample suggested that anthropogenic inputs have occurred over the years. The concentrations of all the metals were below the values reported [90] except Cd which appeared higher in both the top and the subsoil of the study area.


**Table 3.** Heavy metal concentrations (mg/kg) in the soil around the cattle market in Isheri, Nigeria

There is positive and significant correlation between Zn, Pb and Cu and Ni with Cr. Atmos‐ pheric fallout of the metals such as lead from the motor vehicle exhaust from the nearby busy Lagos-Ibadan express road as well as movement of goods around the market may have contributed to the high concentrations of heavy metals in the soil. Also, there is positive and significant correlation between organic matter and Ni, Pb, and Cu. than the values obtained from the control sample. There are no background and baseline data on toxic metal concentration in Nigeria. Metal concentrations in the study site when compared to the control sample suggested that anthropogenic inputs have occurred over the years. The concentrations of all the metals were below the values reported [90] except Cd which appeared higher in both the top and the subsoil of the study area. There is positive and significant correlation between Zn, Pb and Cu and Ni with Cr. Atmospheric fallout of the metals such as lead from the motor vehicle exhaust from the nearby busy Lagos-Ibadan

mg/kg, respectively.

organic matter and Ni, Pb, and Cu.

Metals

of heavy metals contained in the sludge and the wastewater used for the irrigation was studied in the soil samples collected from the soil profiles from the surface to 100 cm depth of farmland [89]. The mean concentrations and standard deviation of Pb, Zn, Cu, Cr, Cd and Ni in the topsoil (0 – 15cm) and subsoil (15 – 30cm) around the river basin at Isheri are 14.6 ± 4.5 mg/kg and 18.1 ± 8.1 mg/kg; 38.0 ± 27 mg/kg and 44.6 ± 28 mg/kg; 6.41 ± 3.3 mg/kg and 7.77 ± 3.7 mg/kg; 7.35 ± 0.1 mg/kg and 10.5 ± 0.4 mg/kg; 0.74 ± 0.3 mg/kg and 0.63 ± 0.03 mg/kg ; and 3.81 ± 0.5 mg/kg and 5.70 ± 0.3

around the world (Table 4). The average metal concentrations in both the top and subsoil were higher

express road as well as movement of goods around the market may have contributed to the high concentrations of heavy metals in the soil. Also, there is positive and significant correlation between

**3.4. Concentration of heavy metals in soil**

242 Environmental Risk Assessment of Soil Contamination

mg/kg and 5.70 ± 0.3 mg/kg, respectively.

area.

The results of heavy metals analyzed in the soil samples are shown in Table 3 while Figure 3 shows the distribution around the study site. There is variation in the distribution of the metals around the market with some of the metals such as lead (23.8 mg/kg) and copper (10.4 mg/kg) having the highest concentrations in the subsoil (15-30 cm). The concentrations of all the metals around the market is the highest when compared to the downstream and upstream of the river. This may be attributed to the anthropogenic activities in this area. In most cases, the subsoil has higher concentration of the metals than their respective topsoil, except Zn, Cd, and Ni. However, Cadmium compounds being of very low solubility in water will have little down‐ ward movement and accumulate in the topsoil [87]. This may be due to strong adsorption of Cd by the surface soils, which have a higher organic matter content and higher pH; factors which are known to increase Cd adsorption in soils [88]. Also, it was reported that Cd accumulated in the soil surface layers and had low vertical movement when mobility of heavy metals contained in the sludge and the wastewater used for the irrigation was studied in the soil samples collected from the soil profiles from the surface to 100 cm depth of farmland [89]. The mean concentrations and standard deviation of Pb, Zn, Cu, Cr, Cd and Ni in the topsoil (0 – 15cm) and subsoil (15 – 30cm) around the river basin at Isheri are 14.6 ± 4.5 mg/kg and 18.1 ± 8.1 mg/kg; 38.0 ± 27 mg/kg and 44.6 ± 28 mg/kg; 6.41 ± 3.3 mg/kg and 7.77 ± 3.7 mg/kg; 7.35 ± 0.1 mg/kg and 10.5 ± 0.4 mg/kg; 0.74 ± 0.3 mg/kg and 0.63 ± 0.03 mg/kg ; and 3.81 ± 0.5

Comparing the results of heavy metal concentrations obtained in this study with the control sample collected at a rural community, Lalupon in Ibadan and some typical values in rural and urban soil around the world (Table 4). The average metal concentrations in both the top and subsoil were higher than the values obtained from the control sample. There are no background and baseline data on toxic metals concentrations in Nigeria. Metal concentrations in the study site when compared to the control sample suggested that anthropogenic inputs have occurred over the years. The concentrations of all the metals were below the values reported [90] except Cd which appeared higher in both the top and the subsoil of the study

**Metals Sampling Area A Sampling Area B Sampling Area C Mean Conc.**

**Table 3.** Heavy metal concentrations (mg/kg) in the soil around the cattle market in Isheri, Nigeria

**A1S1 A2S2 B1S1 B2S2 C1S1 C2S2 Topsoil Subsoil** (0-15cm) (15-30cm) (0-15cm) (15-30cm) (0-15cm) (15-30cm) (mg/kg) (mg/kg) Pb 12.9 13.9 17.7 23.8 11.4 12.4 14.6±4.5 18.1±8.1 Zn 20.8 20.7 57.0 64.7 19.0 24.5 38.0±4.5 44.6±28 Cu 6.12 6.19 8.73 10.4 4.09 5.14 6.41±3.3 7.77±3.7 Cr 10.4 10.4 7.3 10.7 7.4 10.2 7.35±0.1 10.5±0.4 Cd 0.73 0.55 0.93 0.61 0.55 0.65 0.74±0.3 0.63±0.03 Ni 5.27 4.72 4.19 5.91 3.43 5.48 3.81±0.5 5.70±0.3

Nigeria **Figure 2.** Variation in the physicochemical parameters in the soil around the cattle market in Isheri, Nigeria


Figure 1. Variation in the physicochemical parameters in the soil around the cattle market in Isheri,

**Table 4.** Comparison of mean concentrations (mg/kg) of metals in this study with the control sample and typical soil values around the world

and typical soil values around the world

Table 4. Comparison of mean concentrations (mg/kg) of metals in this study with the control sample

Figure 3. Heavy metal distribution in the soil around the cattle market in Isheri, Nigeria **Figure 3.** Heavy metal distribution in the soil around the cattle market in Isheri, Nigeria

#### **3.5. Concentration of Polynuclear Aromatic Hydrocarbons (PAHs) 3.5. Concentration of Polynuclear Aromatic Hydrocarbons (PAHs)**

The results of the PAHs in the soil samples collected along the River Ogun basin at Isheri, along Lagos-Ibadan Express road are shown in the Table 5. Regular burning of all kinds of waste, tyres etc. was observed in this area and this might be responsible for the presence of PAHs around the study site. Some of the PAHs such as naphthalene, phenanthrene, fluoranthene, chrysene benzo(a)anthracene and perylene, were found at significantly higher concentrations around the area. Out of the seven species regarded as potential carcinogenic compounds, namely: chrysene, benzo(a)anthracene, benzo(k)flouranthene, benzo(g,h,i)perylene, dibenzo(a,h)anthracene, indeno(1,2,3-cd)pyrene, and benzo(a)pyrene, chrysene has the highest concentration in the samples collected at all the sampling points. Its concentration was relatively higher in the subsoil (15-30cm) than the topsoil (0-15cm) except at sampling point C (downstream), where there is little variation in the concentrations in both layers. Figure 3 shows the variation in the concentration of PAHs with depth. The results of the PAHs in the soil samples collected along the River Ogun basin at Isheri, along Lagos-Ibadan Express road are shown in the Table 5. Regular burning of all kinds of waste, tyres etc. was observed in this area and this might be responsible for the presence of PAHs around the study site. Some of the PAHs such as naphthalene, phenanthrene, fluoranthene, chrysene benzo(a)anthracene and perylene, were found at significantly higher concentrations around the area. Out of the seven species regarded as potential carcinogenic compounds, namely: chrysene, benzo(a)anthracene, benzo(k)flouranthene, benzo(g,h,i)perylene, diben‐ zo(a,h)anthracene, indeno(1,2,3-cd)pyrene, and benzo(a)pyrene, chrysene has the highest concentration in the samples collected at all the sampling points. Its concentration was relatively higher in the subsoil (15-30cm) than the topsoil (0-15cm) except at sampling point C (downstream), where there is little variation in the concentrations in both layers. Figure 4 shows the variation in the concentration of PAHs with depth.

21


**Table 5.** PAHs concentration (µg/kg) in the soil around the cattle market in Isheri, Nigeria (µg/kg) Table 5. PAHs concentration (μg/kg) in the soil around the cattle market in Isheri, Nigeria

**Figure 4.** Variation in the concentration of PAHs with respect to depth in the soil around the cattle market in Isheri, Nigeria

22

21

both layers. Figure 3 shows the variation in the concentration of PAHs with depth.

shows the variation in the concentration of PAHs with depth.

Figure 3. Heavy metal distribution in the soil around the cattle market in Isheri, Nigeria

**Figure 3.** Heavy metal distribution in the soil around the cattle market in Isheri, Nigeria

**3.5. Concentration of Polynuclear Aromatic Hydrocarbons (PAHs)**

Pb Zn Cu Cr Cd Ni **Metals**

**3.5. Concentration of Polynuclear Aromatic Hydrocarbons (PAHs)** 

The results of the PAHs in the soil samples collected along the River Ogun basin at Isheri, along Lagos-Ibadan Express road are shown in the Table 5. Regular burning of all kinds of waste, tyres etc. was observed in this area and this might be responsible for the presence of PAHs around the study site. Some of the PAHs such as naphthalene, phenanthrene, fluoranthene, chrysene benzo(a)anthracene and perylene, were found at significantly higher concentrations around the area. Out of the seven species regarded as potential carcinogenic compounds, namely: chrysene, benzo(a)anthracene, benzo(k)flouranthene, benzo(g,h,i)perylene, dibenzo(a,h)anthracene, indeno(1,2,3-cd)pyrene, and benzo(a)pyrene, chrysene has the highest concentration in the samples collected at all the sampling points. Its concentration was relatively higher in the subsoil (15-30cm) than the topsoil (0-15cm) except at sampling point C (downstream), where there is little variation in the concentrations in

The results of the PAHs in the soil samples collected along the River Ogun basin at Isheri, along Lagos-Ibadan Express road are shown in the Table 5. Regular burning of all kinds of waste, tyres etc. was observed in this area and this might be responsible for the presence of PAHs around the study site. Some of the PAHs such as naphthalene, phenanthrene, fluoranthene, chrysene benzo(a)anthracene and perylene, were found at significantly higher concentrations around the area. Out of the seven species regarded as potential carcinogenic compounds, namely: chrysene, benzo(a)anthracene, benzo(k)flouranthene, benzo(g,h,i)perylene, diben‐ zo(a,h)anthracene, indeno(1,2,3-cd)pyrene, and benzo(a)pyrene, chrysene has the highest concentration in the samples collected at all the sampling points. Its concentration was relatively higher in the subsoil (15-30cm) than the topsoil (0-15cm) except at sampling point C (downstream), where there is little variation in the concentrations in both layers. Figure 4

Table 4. Comparison of mean concentrations (mg/kg) of metals in this study with the control sample

Dow nstream (15-30cm) Dow nstream (0-15cm) Around the Market (15-30cm) Around the Market (0-15cm) Upstream (15-30cm) Upstream (0-15cm)

and typical soil values around the world

244 Environmental Risk Assessment of Soil Contamination

0

50

100

**Concentration(mg/kg)**

150

200

250

Comparing the values obtained in this study with background soil concentrations of PAHs (Table 5) [91], the concentrations obtained in this study was higher than the concentrations in rural and agricultural soils in most cases except benzo(g,h,i) perylene and indeno(1,2,3 cd)pyrene. There is a significant and positive correlation between phenanthrene versus flourene and perylene; flouranthene versus pyrene, benzo(k)flouranthene, dibenzo(a,h)an‐ thracene and benzo(g,h,i)perylene; and benzo(k) flouranthene versus benzo(g,h,i)perylene and dibenzo(a,h)anthracene.


Note: NE-Not established

**Table 6.** Background soil concentrations of PAHs

#### **3.6. Organochlorine pesticides**

There is variation in the concentrations of the organochlorine pesticides (OCP) in the soil samples collected around the cattle market (Table 6). The major OCP residue identified in the area with their percentage concentrations were p,p'-DDT, 344 µg/kg (24.5%), δ-BHC, 290 µg/kg (20.7%), Endosulfan sulphate, 186 µg/kg (13.2%), lindane (γ-BHC), 129 µg/kg (9.1%), Endrin, 110 µg/kg (7.8%), Aldrin, 93.9µg/kg (6.7%), Dieldrin, 93.8 µg/kg (6.7%), o,p-DDE, 45.0 µg/kg (3.2%), β-BHC, 42.1 µg/kg (3.0%), Endosulfan II, 34.5 µg/kg (2.5%), and Heptachlor, 22.8 µg/kg (1.6%). The variation in the concentrations of the organochlorine pesticides with respect to depth around the cattle market is presented in Figure 4. Dieldrin, a metabolite of aldrin is present at higher concentration in most of the sampling points when compared to the parent aldrin. The presence of o,p' DDE revealed that there is historical use as well as recent us of the parent DDT in the vicinity of the cattle market. Though, DDT has been banned for over 20 years, it's presence around the cattle market indicate that it is illegally imported into the country as it is not produced in Nigeria. Also, lindane, one of the nine new POPs was found at high concentration around the market. The presence of some of the banned chemical residue around the cattle market revealed that there is weak regulation and low enforcement on banned chemicals in the country.

Comparing the values obtained in this study with background soil concentrations of PAHs (Table 5) [91], the concentrations obtained in this study was higher than the concentrations in rural and agricultural soils in most cases except benzo(g,h,i) perylene and indeno(1,2,3 cd)pyrene. There is a significant and positive correlation between phenanthrene versus flourene and perylene; flouranthene versus pyrene, benzo(k)flouranthene, dibenzo(a,h)an‐ thracene and benzo(g,h,i)perylene; and benzo(k) flouranthene versus benzo(g,h,i)perylene

**Compounds Concentrations (x 10-3 mg/kg)**

Acenaphthene 1.7 6 NE Acenaphthylene NE 5 NE Anthracene NE 11-13 NE Benzo(a)anthracene 5-20 56-110 169-59,000 Benzo(a)pyrene 2-1,300 4.6-900 165-220 Benzo(b)fluoranthene 20-30 58-220 15,000-62,000 Benzo(c)perylene NE 53-130 60-14,000 Benzo(g,h,i)perylene 10-70 66 900-47,000 Benzo(k)fluoranthene 10-110 58-250 300-26,000 Chrysene 38.3 78-120 251-640 Fluoranthene 0.3-40 120-210 200-166,000 Fluorine NE 9.7 NE Indeno(1,2,3-cd)pyrene 10-15 63-100 8,000-61,000 Phenanthrene 30.0 48-140 NE Pyrene 1-19.7 99-150 145-147,000

There is variation in the concentrations of the organochlorine pesticides (OCP) in the soil samples collected around the cattle market (Table 6). The major OCP residue identified in the area with their percentage concentrations were p,p'-DDT, 344 µg/kg (24.5%), δ-BHC, 290 µg/kg (20.7%), Endosulfan sulphate, 186 µg/kg (13.2%), lindane (γ-BHC), 129 µg/kg (9.1%), Endrin, 110 µg/kg (7.8%), Aldrin, 93.9µg/kg (6.7%), Dieldrin, 93.8 µg/kg (6.7%), o,p-DDE, 45.0 µg/kg (3.2%), β-BHC, 42.1 µg/kg (3.0%), Endosulfan II, 34.5 µg/kg (2.5%), and Heptachlor, 22.8 µg/kg (1.6%). The variation in the concentrations of the organochlorine pesticides with respect to depth around the cattle market is presented in Figure 4. Dieldrin, a metabolite of aldrin is present at higher concentration in most of the sampling points when compared to the parent aldrin. The presence of o,p' DDE revealed that there is historical use as well as recent us of the parent DDT in the vicinity of the cattle market. Though, DDT has been banned for over 20

**Rural Soil Agricultural Soil Urban Soil**

and dibenzo(a,h)anthracene.

246 Environmental Risk Assessment of Soil Contamination

Note: NE-Not established

**Table 6.** Background soil concentrations of PAHs

**3.6. Organochlorine pesticides**

In most cases, the concentrations of organochlorine pesticides is highest at the sampling point C which could be attributed to the use of pesticides to control household pests and insects in the area; as the area is a slum with most houses built of planks, rust iron sheets and polypro‐ pylene material and a dumpsite at the bank of the river. There is very strong and significant correlation at the 0.01 levels (2-tailed) between DDT versus δ-BHC, heptachlor, aldrin and dieldrin; dieldrin versus δ-BHC, heptachlor and aldrin; aldrin versus heptachlor as well as endrin versus δ-BHC.


**Table 7.** Organochlorine pesticides concentrations (µg/kg) in the soil around the cattle market in Isheri, Nigeria

Figure 5. Variation in organochlorine pesticide residue concentrations with respect to depth in the soil around the cattle market in Isheri, Nigeria **Figure 5.** Variation in organochlorine pesticides residue concentrations with respect to depth in the soil around the cattle market in Isheri, Nigeria

In this study, the distribution, concentration and profiles of some ubiquitous environmental

## **4. Conclusion 4. Conclusion**

25 pollutants such as heavy metals (Pb, Ni, Cu, Cr, Zn and Cd) , polycyclic aromatic hydrocarbons (PAHs) and organochlorine pesticides in soil of a cattle market around River Ogun Basin, Isheri, Nigeria were assessed as well as some soil physic-chemical characteristics. The pH of the top soil was observed to be lower than the corresponding subsoil in most of the areas sampled except the areas downstream the market. The pH of the topsoil being lower than the subsoil may be responsible for the high metal concentrations in the subsoil. Low pH of top soil means more heavy metals will be dissolved in top soil and the texture of the soil being sandy and loose also might favour the migration/leaching of heavy metals to lower fractions. The solubilising heavy metals in the top soil may be migrating into the subsoil, except in cadmium, which was concentrated in the top soil. The soil of the entire sample area was sandy loamy soil with low concentration of organic matter. The cattle wastes released in the area might have relatively increased the organic matter content of the soil. Though, the topography of the area and the location of the cattle market, being sloppy and close to the river basin favoured the washing away of the top soil In this study, the distribution, concentration and profiles of some ubiquitous environmental pollutants such as heavy metals (Pb, Ni, Cu, Cr, Zn and Cd), polycyclic aromatic hydrocarbons (PAHs) and organochlorine pesticides in soil of a cattle market around River Ogun Basin, Isheri, Nigeria were assessed as well as some soil physico-chemical characteristics. The pH of the top soil was observed to be lower than the corresponding subsoil in most of the areas sampled except the areas downstream of the river. The pH of the topsoil being lower than the subsoil may be responsible for the high metal concentrations in the subsoil. Low pH of top soil means more heavy metals will be dissolved in top soil and the texture of the soil being sandy and loose also might favour the migration/leaching of heavy metals to lower fractions. The solubilising heavy metals in the top soil may be migrating into the subsoil, except in cadmium, which was concentrated in the top soil. The soil of the entire sample area was sandy loamy soil with low concentration of organic matter. The cattle wastes released in the area might have relatively increased the organic matter content of the soil. Though, the topography of the area and the location of the cattle market, being sloppy and close to the river basin favoured the washing away of the top soil by run-off into the river and this might be responsible for the high organic matter in the subsoil. The pH of the soil of the area was slightly acidic to neutral. The soil sample with high organic matter content has high metal concentrations in most cases except Cd, Pb, Cu and Cr. Generally, there is positive and significant correlation between the organic matter content versus Pb, Zn, Cu, Ni and Cr. There is variation in the concentrations of PAHs and organochlorine pesticides residues with depth around the river basin. The detection of banned organochlorine pesticides in the soil of the area could be attributed to both the historical use as well as recent use of these chemicals in the area. Thus, there is need for stricter regulations on banned chemicals to safe our environment from ubiquitous and persistent environmental pollutants.
