**7. Conclusion**

between root and shoot tissues) for the particular metal [43, 44]. Heavy metals in toxic concentrations within the plant have inhibitory effects on enzymatic activity, stomatal function, photosynthesis and nutrient uptake, which may be expressed visually as chlorosis, reduced/stunted growth and yield depression. Plants vary widely in their ability to tolerate high concentrations of metals in their tissues. This variation is usually natural and dependent on inherent genetic factors. The genetic disposition confers the ability to employ a range of avoidance/exclusion or detoxification mechanisms that enable the plants cope with high metal loads. These may include the binding of metals (e.g. Ni and Cr) with amino acids, peptides and organic acids to form low molecular weight compounds, formation of phytochelatins, by binding (e.g. Cu and Pb) with sulphur-rich proteins and cellular adaptations. Other strategies may involve roles for mychorrhizas, the cell wall, extra-cellular exudates, efflux pumping mechanisms in the plasma membrane and formation of stress proteins etc [3, 45, 46, 47].

Plants with BCF of metals >1.0, have been described as suitable for phytoextraction [37, 38]. Some of the plants observed in this study with this potential include; *Combretum mucronatum, Ipomoea ascarafolia, Gardenia rubescens*, *Senna singuena*, *Gymnema sylvestre*, *Capparis polymopha* (syn*. C. tomentosa*), *Guiera senegalensis*, *Englerina gracilinus*, *Senna siberiana*, *Combretum nigri‐ cans*, *Dicrostachys cinerea*, *Crateva religiosa*, *Stylosanthes arectalea* and *Terminalia mollis* for Cu; *Senna occidentalis*, *Ipomoea ascarafolia, Leptadenia hastata*, *Capparis polymopha* (syn*. C. tomentosa*), *Senna siberiana* and *Ziziphus abyssinica* for Cd; *Combretum nigricans* for Ni. The ability of these plants to concentrate high levels of these metals suggests that they may have a good potential

No hyper-accumulator was observed in this study. Hyper-accumulators are plants that can accumulate at least 0.1% wt of Cu, Cd, Cr, Pb, Ni and Co or 1% wt of Zn and Mn [39]. There are possibilities for genetic modification of plants to enhance their capacity for metal

There is a great need to establish environmentally safe limits for metals in plant and soils of the various eco-regions in Nigeria. This need is emphasized by the observed variations in published background values from one country to another and even within the same country. These background values are often dependent on the geological history of the area. A com‐ parison of observed field values with national recommended levels for heavy metals, devel‐ oped from the background values will give a more realistic assessment of the pollution status. Furthermore, the search for alternative green technology that can be employed in remediation of pollution events must necessarily be a continuous one, due to the relative low cost and environmental friendliness of this option as compared to others. In this regard, ruderal species rather than those with agricultural value must be the candidates of choice for avoidance of obvious conflicts. The species that have indicated potentials for phytoextraction of Cu, Cd and

for phytoremediation.

716 Environmental Risk Assessment of Soil Contamination

**6. Recommendations for further research**

Ni in this research may therefore be further evaluated.

tolerance [48].

The concentrations of Cr, Cu, Ni and Zn in soils around the Zobe dam catchment and the environs of Katsina Steel Rolling Mill were found to be above the acceptable limits. This presents health risks to humans and other animals as the metals contaminate both aquatic and terrestrial ecosystems.

Although no hyper accumulator plant species was encountered in this study, eighteen (18) plant species were identified to have high bioconcentration of metals, which indicated tolerance to excessive or phytotoxic metal concentrations. In addition, they generally produce high above ground biomass, due to rapid vegetative growth. These plants include: *Combre‐ tum mucronatum, Ipomoea ascarafolia, Gardenia rubescens*, *Senna singuena*, *Gymnema sylvestre*, *Capparis polymopha* (syn*. C. tomentos*a), Guiera senegalensis, Englerina gracilinus, Senna siberiana, Combretum nigricans, Dicrostachys cinerea, Crateva religiosa, Stylosanthes arectalea, Terminalia mollis, Senna occidentalis, Leptadenia hastata, and Ziziphus abyssinica. These present further possibilities for evaluating metal tolerance in relation to their potential use in phytoremediation programmes in metal polluted sites.
