Soil risk assessment of heavy metal

Table 2. Semivariogram model and parameters of heavy metals

Heavy metals

Fitted model

Nugget (C ₀ )

Sill (C + C ₀ )

Range (A) (m)

Nugget/Sill

Cr

Circular

0.411

1.981

1240

0.207

Cd

Circular

0.950

1.156

1022

0.821

Pb

Gaussian

0.175

0.737

1930

0.237

Ni

Gaussian

0.276

0.597

2321

0.462

concentration of Pb was high. A higher concentration

transformation was performed for Cd and Ni because

of lead has also been found in soil impacted with

their skewness was greater than one.

petroleum exploration and production activities

(Asia et al., 2007; Owamah, 2013). The greatest and

Semivariogram analysis of heavy metals

the smallest standard deviation were observed in the

Semivariogram analysis was used to characterize

Ni (44.1) and pH (0.47), respectively. Organic carbon,

and quantify spatial variability and RMSE was used

Cd and Ni exhibit a high variation (>50%) according

for different theoretical semivariogram models to

toguidelinesprovidedbyWarrick(1998).Skewnessis

fit the experimental semivariogram values for each

the most common form of departure from normality.

micronutrient. Analysis of the isotropic variogram

If a variable has positive skewness, the confidence

indicated that the Cr and Cd semivariograms were

limits on the variogram are wider than they would

well described with the circular model, with the

otherwise be and consequently, the variances are less

distance of spatial dependence being 1240 and 1022

reliable. A logarithmic transformation is considered

m, respectively, while the Pb and Ni semivariogram

where the coefficient of skewness is greater than one

was well described with the Gaussain model, with

(Webster and Oliver, 2001). Therefore, a logarithmic

the distance of spatial dependence being 1930 and

2321 m, respectively (Table 2).

Figure 1. Location and grid map of the study area

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