*5.1.1.4. Uranium mines*

The old U mine of Sevilha (Central Portugal) is one of several small mines exploited by ENU (Portuguese Uranium Company). After the removal of the main ore body, the site was filled with the mine wastes and a reclamation process was initiated. This action was somewhat unsuccessful because the selected allochthonous plant species (*Lupinus* sp.) did not survive.

Current U soil contamination on the Sevilha mine ranges from 8 to 560 mg/kg [99]. Species of Compositae and Ericaceae (among the most abundant families of terrestrial plants) accumu‐ lated highest U concentrations (Figure 17). Among Compositae members, an average of 4.91 mg/kg DW and a maximum of 13.1 mg/kg DW was found in *Helichrysum stoechas* and an average of 4.07 mg/kg DW and a maximum of 10.5 mg/kg DW was recorded in *Hypochaeris radicata* (Figure 17). In *Erica umbellata* an average of 1.70 mg/kg DW and a maximum of 7.50 mg/kg DW were obtained (Figure 17). Even though the concentrations obtained in the latter are not high, it is particularly interesting because it has a high bio-productivity. This accumu‐ lation potential might be intensified if uptake enhancement strategies, such as addition of citric acid, are adopted. A restoration program can be applied to the soils of Sevilha mine by adopting revegetation with endemic species allied to a process of continuous phytoremediation that avoids dispersion of U into the streamlets.

Even though the soils in this mine are not highly contaminated, the lixiviation of refilling materials has been responsible for the dispersion of U into ground and superficial water bodies. These waters are being used for subsistence agriculture and, therefore, the risk of contamina‐ tion spreading to humans can be acute, due to food chain accumulation [99]. The plant survey revealed that some of the native plant species are well adapted to U contamination in soils, therefore, they are metallotolerants. Their phytoremediation potential has to be evaluated. Dispersion of U into the streamlets can be minimized by a strategic combination of terrestrial and aquatic plant phyto-systems. Revegetation with *Helichrysum stoechas*, *Hypochaeris radica‐ ta* and *Erica umbellata* will allow fixation of U in the plants and a consequent reduction in its dispersion. This site can be an excellent prototype for the restoration of other mines in Portugal where levels of contamination are a matter of concern.

#### *5.1.1.5. Chromium mines (Serpentine soils)*

The abandoned mining area of Pingarela in North-east Portugal has serpentine soils and associated flora. These soils are disproportionately rich in trace elements like Ni, Cr, Co and poor in Ca. Serpentine outcrops have been referred to as barrens because they are often sparsely vegetated and extremely poor in essential nutrients, hence not of much agricultural value. Serpentine ecosystems can generally be distinguished by their grey-green or reddish rocky soils (soils are very thin), and shrubby or stunted vegetation with plants having small leathery leaves.

Plant species found on serpentine soils can be divided into two groups: (a) serpentine-tolerant or serpentine-facultative plants, which are able to survive on serpentine but grow better elsewhere; (b) serpentinicolous, serpentine-endemic or serpentine-obligate plants, which grow exclusively on serpentine soils and are not found on other substrates [83,100]. Both these

**Figure 17.** Accumulation of U (mg/kg DW) in plant species of the Sevilha mining area.

*5.1.1.4. Uranium mines*

504 Environmental Risk Assessment of Soil Contamination

avoids dispersion of U into the streamlets.

where levels of contamination are a matter of concern.

*5.1.1.5. Chromium mines (Serpentine soils)*

leaves.

The old U mine of Sevilha (Central Portugal) is one of several small mines exploited by ENU (Portuguese Uranium Company). After the removal of the main ore body, the site was filled with the mine wastes and a reclamation process was initiated. This action was somewhat unsuccessful because the selected allochthonous plant species (*Lupinus* sp.) did not survive.

Current U soil contamination on the Sevilha mine ranges from 8 to 560 mg/kg [99]. Species of Compositae and Ericaceae (among the most abundant families of terrestrial plants) accumu‐ lated highest U concentrations (Figure 17). Among Compositae members, an average of 4.91 mg/kg DW and a maximum of 13.1 mg/kg DW was found in *Helichrysum stoechas* and an average of 4.07 mg/kg DW and a maximum of 10.5 mg/kg DW was recorded in *Hypochaeris radicata* (Figure 17). In *Erica umbellata* an average of 1.70 mg/kg DW and a maximum of 7.50 mg/kg DW were obtained (Figure 17). Even though the concentrations obtained in the latter are not high, it is particularly interesting because it has a high bio-productivity. This accumu‐ lation potential might be intensified if uptake enhancement strategies, such as addition of citric acid, are adopted. A restoration program can be applied to the soils of Sevilha mine by adopting revegetation with endemic species allied to a process of continuous phytoremediation that

Even though the soils in this mine are not highly contaminated, the lixiviation of refilling materials has been responsible for the dispersion of U into ground and superficial water bodies. These waters are being used for subsistence agriculture and, therefore, the risk of contamina‐ tion spreading to humans can be acute, due to food chain accumulation [99]. The plant survey revealed that some of the native plant species are well adapted to U contamination in soils, therefore, they are metallotolerants. Their phytoremediation potential has to be evaluated. Dispersion of U into the streamlets can be minimized by a strategic combination of terrestrial and aquatic plant phyto-systems. Revegetation with *Helichrysum stoechas*, *Hypochaeris radica‐ ta* and *Erica umbellata* will allow fixation of U in the plants and a consequent reduction in its dispersion. This site can be an excellent prototype for the restoration of other mines in Portugal

The abandoned mining area of Pingarela in North-east Portugal has serpentine soils and associated flora. These soils are disproportionately rich in trace elements like Ni, Cr, Co and poor in Ca. Serpentine outcrops have been referred to as barrens because they are often sparsely vegetated and extremely poor in essential nutrients, hence not of much agricultural value. Serpentine ecosystems can generally be distinguished by their grey-green or reddish rocky soils (soils are very thin), and shrubby or stunted vegetation with plants having small leathery

Plant species found on serpentine soils can be divided into two groups: (a) serpentine-tolerant or serpentine-facultative plants, which are able to survive on serpentine but grow better elsewhere; (b) serpentinicolous, serpentine-endemic or serpentine-obligate plants, which grow exclusively on serpentine soils and are not found on other substrates [83,100]. Both these groups include species with different efficiencies to uptake or exclude a variety of metals. Serpentinophytes often experience drought, nutrient stress and excessive exposure to heavy metal and high light intensity. This means there is less substrate in which nutrients and water can be held and made available to plants.

An area of ~8,000 ha in north-east Portugal is serpentinized with characteristic geology and flora. The serpentine plant community and respective soils were analyzed [83] to examine the trace metal budget in different tissues of the plants exhibiting resistance to trace metals. 135 plant species belonging to 39 families and respective soils were analyzed for total Co, Cr, Cu, Fe, Mn, Ni, Pb and Zn. Heavy metal concentrations recorded in sampled soils are shown in Table 6. The high contents of Ni and Cr obtained were to be expected, considering the geochemistry of the sampling site. However, the high variability of Cr, Ni, Fe and Mn in the soils is noteworthy.


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

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 (Figure 18).

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

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

**Figure 19.** Accumulation of Cr (mg/kg DW) in serpentine plant species of the Pingarela mining area.

Although leaves of serpentine plants show 10 times higher Co levels than those of plants growing on non-serpentine soils, the absolute concentrations are only about 10 mg/kg DW in most species. In this study highest accumulation of Co was found in the aboveground tissues of *A. serpyllifolium* with 145 mg Co/kg DW and *L. spartea* with 63.2 mg Co/kg DW [83].
