**6. Conclusion**

**Range Mean Median Standard deviation**

pH 4.9 – 9.3 6.7 6.7 0.6 Co 56.0 – 151 82.3 81.1 18.5 Cr 200 – 6,822 1,622 1,410 1,064 Cu 30.8 – 221 101 99.3 34.7 Fe 95.0 – 110,418 82,950 84,711 14,502 Mn 1,007 – 1,835 1,389 1,363 175 Ni 102 – 2,295 918 883 464 Pb 18.5 – 46.6 29.2 29.1 5.78 Zn 63.0 – 242 110 112 24.8

**Table 6.** Trace element content (mg/kg) and pH of serpentine soil samples (N=74, Pingarela mine).

**Figure 18.** Accumulation of Ni (mg/kg DW) in serpentine plant species of the Pingarela mining area.

(Figure 18).

a content of 173 mg Cr/kg DW in the twigs.

506 Environmental Risk Assessment of Soil Contamination

The Ni hyperaccumulating endemic of this region is *Alyssum serpyllifolium* subsp. *lusitani‐ cum*, which concentrated 38,105 mg Ni/kg DW in the aboveground tissues (Figure 18). *Bromus hordeaceus* with 1,467 mg Ni/kg DW and *Linaria spartea* with 492 mg Ni/kg DW in the aerial parts, also showed high concentration of Ni. Four other taxa *viz.—Plantago radicata*, *Ulmus procera*, *Lavandula stoechas* and *Cistus salvifolius* showed more than 100 mg Ni/kg DW

Chromium has low solubility in the serpentine soil solution due to the relatively high pH values of these soils [83]. This is reflected in the low uptake of this element by plants, which in general did not exceed 40 mg/kg. However, concentrations of 707 mg Cr/kg DW were reported in the above ground parts of *L. spartea* (Figure 19). *Alyssum serpyllifolium* also presented high content of Cr, reaching a maximum of 130 mg Cr/kg DW. *Ulmus procera* showed

The physico-chemical properties of the metalliferous or metal-contaminated soils tend to inhibit soil-forming processes and plant growth. In addition to elevated metal/metalloid concentrations, other adverse factors included absence of topsoil, erosion, drought, compac‐ tion, wide temperature fluctuations, absence of soil-forming fine materials and shortage of essential nutrients [84,101]. Degraded soils of mines usually have low concentrations of important nutrients, like N, P and K [102]. Toxic metals can also adversely affect the number, diversity and activity of soil organisms, inhibiting soil organic matter decomposition and N mineralization processes. The chemical form of the potential toxic metal, the presence of other chemicals which may aggravate or attenuate metal toxicity, the prevailing pH and the poor nutrient status of contaminated soil affects the way in which plants respond to it. Substrate pH affects plant growth mainly through its effect on the solubility of chemicals, including toxic metals and nutrients.

Metal toxicity issues do not generally arise in the case of native flora, considering that native plants become adapted over time to the locally elevated metal levels [76,103]. Native plants may be better phytoremediators for contaminated lands than the known metal hyperaccumu‐ lators because these are generally slow growing with shallow root systems and low biomass. Plants tolerant to toxic metals and low nutrient status with a high rate of growth and biomass are the ideal species to remediate degraded soils and habitats like those around mines. The native flora displayed its ability to withstand high concentrations of heavy metals in the soil. Some species also displayed variable accumulation patterns for metals at different soil concentrations. This variation was also observed in different parts of the same plant suggesting that full consideration of plant–soil interactions should be taken into account when choosing plant species for developing and utilizing methods such as phytoremediation.

Indigenous plant species growing on tailings and contaminated soils show tolerance to imposed stress conditions (metal-contamination and nutrient deficiency) and can fulfill the objectives of stabilization, pollution attenuation and visual improvement. Besides, these species are drought-resistant and some even exhibit high biomass and bioproductivity. In fact, the constraints related to plant establishment and amendment of the physical–chemical properties of the metalliferous soils depends upon the choice of appropriate plant species. Hence, the plant community tolerant to toxic trace elements plays a major role in remediation of degraded mine soils.

The existing natural plant cover at abandoned mining sites can be increased manifold by widescale planting and maintenance of native species with higher metal accumulation potential for some years. Even dispersal of seeds obtained from plants on site is to be encouraged. Adding organic amendment is essential to facilitate the establishment and colonization of these "pioneer plants". They can eventually modify the man-made habitat and render it more suitable for subsequent plant communities. Allowing native species to remediate soils is an attractive proposition since native wild species do not require frequent irrigation, fertilization, and pesticide treatments, while simultaneously a plant community comparable to that existing in the vicinity can be established.

Therefore, mine restoration could benefit from a broader perspective including different groups of plant species as they can perform distinct functional roles in the remediation process. The use of leguminous plants, for example, may enrich the nutrient content and the combined used of perennials and annuals can provide substantial inputs in terms of organic matter and nutrient recycling, thus contributing in distinct ways to the development of the soil [82,104]. This approach requires more information about plant communities growing on metalcontaminated soils in order to accurately determine their potential for remediation of polluted soils at abandoned mines. Ideal phytoremedial candidates can be screened out from the native flora and after assessing their individual requirements, suitable conditions/amendments can be created to develop them as good competitors with enhanced growth and proliferation than their counterparts growing on the same metal contaminated nutrient depleted soils.

Significant accumulation of heavy metals and metalloids in both soils and native wild flora suggests that metal contamination is a matter of great concern in the studied mining areas. The native flora displayed its ability to withstand high concentrations of heavy metals/ metalloids in the soil. However, accumulation patterns of metals/metalloids in the plants tested differed. As metal concentrations in above ground parts were maintained at low levels, metal tolerance in most cases may mainly depend on their metal excluding ability. However, metal/metalloid concentrations higher than toxic level in some species like *Agrostis castellana* (for As, and Fe), *Cistus ladanifer* subsp. *ladanifer* (for Cr, and W), *Cistus* *salvifolius* (for Ni, and Pb), *Digitalis purpurea* subsp. *purpurea* (for Sb, W, and Zn), *Helichry‐ sum stoechas* and *Hypochaeris radicata* (for U), *Holcus lanatus* (for As, Cu, and Fe), *Lonicera periclymenum*, *Mentha suaveolens* and *Phytolacca americana* (for Pb, and Zn), *Pinus pinaster* (for As, W, and Zn), *Polystichum setiferum* and *Solanum nigrum* subsp *nigrum* (for Zn), *Pteridi‐ um aquilinum* (for As), as well as the serpentine plant species *Alyssum serpyllifolium* subsp. *lusitanicum*, *Lavandula stoechas* subsp. *sampaiana*, *Linaria spartea* subsp. *virgatula* and *Ulmus procera* (for Cr, and Ni) and *Bromus hordeaceus* and *Plantago radicata* subsp. *radicata* (for Ni) indicate that internal detoxification metal tolerance mechanisms might also exist; there‐ fore, their utility for phytoremediation is possible. Furthermore, the plants could grow and propagate in substrata with low nutrient conditions which would be a great advantage in the revegetation of mine tailings. It was also observed that despite lower accumulation, trees of the studied regions can be very effective due to their higher biomass.

Some of the studied species also showed variable accumulation patterns for metals at different soil concentrations. This difference was also noted between parts of the same plant suggesting that full consideration of plant–soil interactions should be taken into account when choosing plant species for developing and utilizing methods such as phytoremediation.
