Determination of background values and assessment of pollution and ecological risk of heavy metals in urban agricultural soils of Yaoundé, Cameroon

Presence of heavy metals in urban agriculture is considered a major ecological risk. The aim of this research is to determine concentrations and background values of heavy metals in urban agricultural soils in Yaounde (Cameroon), in order to assess the degree of soil pollution and to evaluate ecological risk. Forty-five composite soil samples were taken from three market garden sites. Heavy metals (Cd, Cr, Cu, Mn, Ni, Pb and Zn) were determined by inductively coupled plasma-optical emission spectrometry (ICP-OES). Geochemical background threshold values (GBTV) were determined using the robust statistical method of median ± median absolute deviation (MAD). Single (geoaccumulation index and enrichment factor) and complex (pollution load index and Nemerow pollution index) indices of pollution and of ecological risk (potential ecological risk index) were calculated. Significant difference between the three market garden sites was evaluated using the nonparametric test of Kruskal–Wallis. Median concentrations varied considerably, ranging from the highest to the lowest levels (mg/kg) as Mn 259.0 > Cu 111.2 > Zn 81.9 > Cr 54.6 > Pb 19.3 > Ni 14.9 > Cd 0.1. All median values were below their corresponding geochemical backgrounds, except for Pb. Single pollution indices showed that the study site was contaminated with Pb. Nemerow pollution index showed that 60% of soil samples were in the very high level of pollution. Based on potential ecological risk index, 16% of soil samples were at high risk. The three market garden sites differed significantly regarding their heavy metal contents, except Cd, as well as their pollution and ecological risk indices with the Nkolondom site having, in general, the highest levels. More than half of the study area is polluted, mainly with Pb, whereas ecological risk is limited to a small part of it. This study can be used as a baseline to develop future long-term risk assessment strategies on the potential migration of heavy metals in urban agricultural soils to vegetables, animals and human beings.