**2.5 Permissible limits of heavy metals in soil and water**

As a guide, levels of heavy metals from the study will be compared with local and international standards of specification of heavy metals in soil and water bodies, as depicted in **Table 1**. Ghana's EPA utilizes the standards Romania set for heavy metal levels in the soil. This and Ghana EPA permissible limits for heavy metals were used as national standards to compare the heavy metals investigated in the samples. The WHO/ FAO standards, available at https://www.researchgate.net/publication/345903051 and http://www.fao.org/3/t0234e/T0234E06.htm#ch5.5, were used as heavy metal limits in soil and water. These standards have also been used by other authors elsewhere.

### **2.6 Indices for determination of soil pollution**

Three pollution indices were employed to evaluate how much the four heavy metals had polluted the scrapyard and its environment. The geoaccumulation index *Heavy Metal Pollution Resulting from Informal E-Waste Recycling in the Greater Accra Region… DOI: http://dx.doi.org/10.5772/intechopen.112397*


**Table 1.**

*Ghana EPA and WHO/FAO permissible levels of heavy metals in soil and water.*

(Igeo) determines the contamination of heavy metals by assessing their concentrations in sampled soils relative to background concentrations during pre-industrial periods [22, 23]. Igeo is computed using the mathematical formula:

$$\mathbf{I}\_{\rm geo} = \log\_2 \left( \mathbf{C}\_{\rm n} / \mathbf{1}.\mathbf{SB}\_{\rm n} \right) \tag{1}$$

Cn measures the heavy metal levels in the sediment under investigation, while Bn represents the geochemical background level [24]. The constant of 1.5 is to lessen the possible variations in the background data. Based on the results obtained, soils can be categorized into seven quality grades of pollution, as follows: practically unpolluted, where Igeo < 0; unpolluted to moderately polluted, where Ige,o = 0–1; moderately polluted, where Igeo = 1–2; moderately to strongly polluted, where Igeo = 2–3; strongly polluted, where Igeo = 3–4; strongly to extremely polluted, where Igeo = 4–5 and extremely polluted, where Igeo > 5 [25].

The contamination factor (CF) evaluates quantities of an element in a sample normalized over the pre-industrial baseline value of the component [26]. Mathematically, CF is expressed as

$$\mathbf{CF} = \mathbf{C}\_{\mathbf{e}} / \mathbf{C}\_{\mathbf{i}} \tag{2}$$

Where Ce and Ci are, respectively, the heavy metal concentration levels in the sample of interest and the background value of the heavy metal of interest, based on values obtained, soil or sediments can be classified as no or low contamination, where CF < 1; moderate contamination, where 1 < CF < 3; considerable contamination, where 3 < CF < 6; very high contamination, where CF > 6 [27].

The pollution load index (PLI) [28] examines the mutual contribution of groups of metals to the pollution of a site. Mathematically,

$$\text{PLI} = \{\text{CF}\_1 \times \text{CF}\_2 \times \text{CF}\_3 \times \text{CF}\_4 \times \text{CF}\_5 \times ... \text{CF}\_n\}^{1/n} \tag{3}$$

where CF represents the contamination factor of each heavy metal element in a sampled soil and n is the number of heavy metals under consideration. The PLI indicates whether the site under consideration is lightly polluted, where PLI ≤ 1; moderately polluted; where 1 < PLI ≤3; highly polluted, where PLI > 3 [27, 28].

For instance, the metallic pollution levels in a study conducted by Fosu-Mensah et al. [28] were assessed using the WHO/FAO standards, whose links have been provided already.

### **2.7 Statistical and data analysis**

Descriptive statistical variables, such as the mean, maximum, minimum, and standard deviation of heavy metal concentrations computed using Microsoft Excel software 2016 version. Pearson correlation, t-test, and coefficient of variation (CV) of the heavy metal concentrations were calculated by SPSS, version 21.0.
