**5. Phytoremediation potential of native flora of contaminated soils**

It is possible to find a wide variety of plant species that can colonize areas highly pollut‐ ed with heavy metals and metalloids, such as mine tailings or soils degraded and contami‐ nated by mining/industrial activities. These are referred to as metallophyte and pseudometallophyte species.

Metallophytes are endemic plant species of natural mineralized soils and, therefore, have developed physiological mechanisms of resistance and tolerance to survive on substrates with high metal levels [61,62]. Since metallophytes, in general, and hyperaccumulators, in particular, are relatively rare and usually produce reduced biomass, the study of pseudome‐ tallophytes, indigenous species of contaminated soils, is of great value. Pseudometallo‐ phyte species (or facultative metallophytes) aren't specialized in metalliferous soils and have a more extensive distribution, but, due to selective pressure, are capable to survive in metalliferous soils [63-65]. Thus, the high pressure of metalliferous soils (natural or contaminated by human action) allows the selection of populations of common species, with higher tolerance than other populations of the same species. Therefore, their capaci‐ ty of adaptation to these environments and, eventually, of accumulation of metals and metalloids, can be very interesting with a view to their use, for example, in ecological restoration, phytoremediation and bioindication actions.

In recent decades many studies have been conducted in contaminated mining and industrial areas and in natural metalliferous soils [35,51,58,66-78] in order to inventory and screen the indigenous species and evaluate their potential for phytoremediation of contaminated soils.

#### **5.1. Phytoremediation potencial of native flora of contaminated soils in Portugal**

Several studies to survey the indigenous plant species of diverse contaminated areas and evaluate their potential for phytoremediation have been performed in Portugal [79-93]. The authors have undertaken studies to evaluate the phytotechnological potential (phytoreme‐ diation, phytomining, bioindication, biogeochemical prospecting) of native flora of soils enriched with metals and metalloids, in distinct abandoned mining areas of tin/tungsten (Sn/W), copper (Cu), lead (Pb), uranium (U), and chromium (Cr) and the results are presented in this chapter.

#### *5.1.1. Native flora of old mining areas*

*TF* <sup>=</sup> *Cshoot Croot*

then, the remediation process may take from 15 to 20 years [3].

492 Environmental Risk Assessment of Soil Contamination

The commercial efficiency of phytoextraction can be estimated by the rate of metal accumu‐ lation and biomass production. Multiplying the rate of accumulation (metal (g)/plant tissue (kg)) by the growth rate (plant tissue (kg)/hectare/year), gives the metal removal value (g/kg of metal per hectare and per year) [3,19,54,57]. This rate of removal or extraction should reach several hundred, or at least 1 kg/ha/year, for the species to be commercially useful, and even

Some soils are so heavily contaminated that removal of metals using plants would take an unrealistic amount of time. The normal practice is to choose drought-resistant fast-growing

In contrast to phytoextraction, phytostabilization aims at reducing the mobility of contami‐ nants in the soil. In this technique, contaminated soil is covered by vegetation tolerant to high concentrations of toxic elements, limiting the soil erosion and leaching of contaminants in to groundwater. Mobility of contaminants can be reduced by surface adsorption/accumulation in roots as well as their precipitation in rhizosphere by induced changes in pH or by oxidation of the root environment [3,12,58]. For example, the immobilization of arsenic in iron plates in the rhizosphere of salt marsh plants [58,59]. Phytostabilization can also be promoted by plant species with the capacity to exude high amounts of chelating substances. These substances lead to immobilization of contaminants by preventing their absorption, while simultaneously reducing their mobility in soil. Thus, plants with phytostabilization potential can be of great

crops or fodder which can grow in metal-contaminated and nutrient-deficient soils.

value for the revegetation of mine tailings and contaminated areas [58,60].

pseudometallophyte species.

**5. Phytoremediation potential of native flora of contaminated soils**

It is possible to find a wide variety of plant species that can colonize areas highly pollut‐ ed with heavy metals and metalloids, such as mine tailings or soils degraded and contami‐ nated by mining/industrial activities. These are referred to as metallophyte and

Metallophytes are endemic plant species of natural mineralized soils and, therefore, have developed physiological mechanisms of resistance and tolerance to survive on substrates with high metal levels [61,62]. Since metallophytes, in general, and hyperaccumulators, in particular, are relatively rare and usually produce reduced biomass, the study of pseudome‐ tallophytes, indigenous species of contaminated soils, is of great value. Pseudometallo‐ phyte species (or facultative metallophytes) aren't specialized in metalliferous soils and have a more extensive distribution, but, due to selective pressure, are capable to survive in metalliferous soils [63-65]. Thus, the high pressure of metalliferous soils (natural or contaminated by human action) allows the selection of populations of common species, with higher tolerance than other populations of the same species. Therefore, their capaci‐ ty of adaptation to these environments and, eventually, of accumulation of metals and

(2)

In the old mining areas studied, several line transects were made in mineralized and nonmineralized zones as well as tailings. Soils and plants were collected at 20 m intervals along the line transects (0, 20, 40 m, etc.) in circle of ≅2 m radius. At each location four random partial soil samples weighing 0.5 kg each were collected from 0 to 20 cm depth and mixed to obtain one composite sample to save time and costs. These were oven-dried at a constant temperature, manually homogenized and quartered. Two equivalent fractions were obtained from each quartered sample. One was used for the determination of pH, and the other for chemical analysis. The samples for chemical analysis were sieved using a 2 mm mesh sieve to remove plant matter and subsequently screened to pass through a 250 µm screen. Samples were also obtained from all species of plants whenever found growing within the 2 m radius of each sampling point. The plant sample focused on the aerial parts, taking into consideration similar maturity of the plants and the proportionality of the different types of tissues, or the separation of different types of tissues (leaves and stems) in some species. In the laboratory, the vegetal material was washed thoroughly, first in running water followed by distilled water, and then dried in a glasshouse. When dry, the material was milled into a homogenous powder. Soil pH was determined in water extract (1:2.5 v/v). The soil and plant samples were acid-digested for elemental analysis. Analytical methods included colorimetry for W, atomic absorption spectrophotometry (AAS, Perkin-Elmer, 2380) for Ag, Co, Cr, Cu, Fe, Mn, Ni, Pb and Zn and hydride generation system (HGS) for As and Sb. Fluorometry (Fluorat-02-2M analyzer, Lumex) was the methodology that was adopted for the determination of the U content in the plant and soil samples. Data quality control was performed by inserting triplicate samples into each batch. Certified references materials were also used.

### *5.1.1.1. Tin/tungsten mines*

The studied areas included several abandoned Sn/W mines (Sarzedas mine, Fragas do Cavalo mine, Tarouca mine, Vale das Gatas mine, Adoria mine, Ervedosa mine, Regoufe mine, and Rio de Frades mine). Results obtained from Sarzedas (Central Portugal) and Vale das Gatas mines (Northern Portugal) are presented.

A summary of trace element data in soil from the Sarzedas mine is shown in Table 2. Among the elements present in the soils, Ag, As, Pb, Sb and W show the most relevant anomalies. Soil pH was negatively correlated to mineralization. Low pH values observed near the mineralized area can be explained by the presence of sulfides in the mineralization [84]. High levels of sulfides, in particular pyrite and arsenopyrite that are easily weathered, favors the dissolution of toxic elements, allowing higher dispersion and bioavailability.


**Table 2.** Trace elements content (mg/kg) and pH of soil samples (N=24, Sarzedas mine).

In the flora of Sarzedas mine area, As was accumulated in aerial tissues of *Pinus pinaster* and *Digitalis purpurea*. Therefore, these species are suited for recognizing the anomaly. High accumulation of As was present in leaves (Figure 4), and it increased in the older tissues. This translocation is a common mechanism in plants to avoid toxicity in young leaves as their metabolic activity is higher [84]. *Digitalis purpurea* also accumulated substantial amount of Sb (Figure 5), indicating its tolerance to this element, although the assimilation occured at low concentrations in the soil. Species that are capable of accumulating W are *D. purpurea*, *Cistus ladanifer*, *P. pinaster*, *Calluna vulgaris* and *Helichrysum stoechas* (Figure 6).

*5.1.1.1. Tin/tungsten mines*

mines (Northern Portugal) are presented.

494 Environmental Risk Assessment of Soil Contamination

of toxic elements, allowing higher dispersion and bioavailability.

**Table 2.** Trace elements content (mg/kg) and pH of soil samples (N=24, Sarzedas mine).

In the flora of Sarzedas mine area, As was accumulated in aerial tissues of *Pinus pinaster* and *Digitalis purpurea*. Therefore, these species are suited for recognizing the anomaly. High accumulation of As was present in leaves (Figure 4), and it increased in the older tissues. This translocation is a common mechanism in plants to avoid toxicity in young leaves as their metabolic activity is higher [84]. *Digitalis purpurea* also accumulated substantial amount of Sb (Figure 5), indicating its tolerance to this element, although the assimilation occured at low

The studied areas included several abandoned Sn/W mines (Sarzedas mine, Fragas do Cavalo mine, Tarouca mine, Vale das Gatas mine, Adoria mine, Ervedosa mine, Regoufe mine, and Rio de Frades mine). Results obtained from Sarzedas (Central Portugal) and Vale das Gatas

A summary of trace element data in soil from the Sarzedas mine is shown in Table 2. Among the elements present in the soils, Ag, As, Pb, Sb and W show the most relevant anomalies. Soil pH was negatively correlated to mineralization. Low pH values observed near the mineralized area can be explained by the presence of sulfides in the mineralization [84]. High levels of sulfides, in particular pyrite and arsenopyrite that are easily weathered, favors the dissolution

pH 3.3 – 5.2 4.7 4.8 0.5 Ag 0.69 – 1.91 0.98 0.92 0.32 As 11.1 – 651 76.3 19.9 181 Co 5.40 – 14.9 8.80 8.41 2.60 Cr 50.7 – 129 96.5 100 26.3 Cu 15.5 – 78.2 40.7 35.1 21.1 Fe 21,881 – 58,644 39,981 37,356 12,883 Mn 22.0 – 92.0 50.0 47.0 22.0 Ni 11.2 – 52.5 21.8 19.7 10.2 Pb 35.7 – 417 85.4 53.9 106 Sb 30.5 – 5,986 663 87.8 1,689 W 0.80 – 684 663 2.90 52.3 Zn 29.0 – 127 58.8 53.3 24.3

**Range Mean Median Standard deviation**

**Figure 4.** Accumulation of As (mg/kg DW) in plant species of the Sarzedas mining area.

**Figure 5.** Accumulation of Sb (mg/kg DW) in plant species of the Sarzedas mining area.

**Figure 6.** Accumulation of W (mg/kg DW) in plant species of the Sarzedas mining area.

It was concluded that the species and organs best suited for biogeochemical prospecting and/ or with potential for mine restoration in the Sarzedas mine area are by order of importance: 1) As: old needles of *P. pinaster*, aerial tissues of *C. vulgaris*, *Chamaespartium tridentatum*, leaves of *C. ladanifer*, *Erica umbellata* and *Quercus ilex* subsp. *ballota*; 2) Sb: *D. purpurea*, *E. umbellata*, stems of *C. ladanifer*, *C. vulgaris*, *C. tridentatum* and stems of *P. pinaster*; 3) W: *D. purpurea*, *C. triden‐ tatum*, old stems and needles of *P. pinaster*, stem and leaves of *C. ladanifer*, *E. umbellata* and stems and leaves of *Q. ilex* [84].


**Table 3.** Trace elements content (mg/kg) and pH of soil samples (N=69, V. Gatas mine).

Very high maximum values for Pb (6,299 mg/kg), As (5,770 mg/kg) and W (636 mg/kg) were observed at the Vale das Gatas mine (Table 3). The Cu-Mn-W-As-Pb-Zn association, which reflects the presence of mineralised veins in the area, is inversely correlated with pH [93]. In general, the content variations in plant materials were strongly related to the content variations in soils. It has also been verified that in contaminated locations or tailings, the concentration of metals in plant tissues is high due to the high metal concentrations in the soil.

The leaves of *Agrostis castellana* and *Holcus lanatus* reflect the Cu, Pb and Ni pedogeochemical anomalies. The aerial parts of *Pteridium aquilinum* and *Juncus effusus* seem to be indicative of Zn anomalies in the soil [94]. *Holcus lanatus* and *A. castellana* were the main accumulators of As (Figure 7), Cu (Figure 8), Fe (Figure 9) and Pb (Figure 10) and good accumulators of Zn (Figure 11). *Pteridium aquilinum* was a good accumulator of As, Pb and Zn (Figures 7, 10, 11). *Juncus effusus* appeared to be a Zn accumulator (Figure 11).

**Figure 7.** Accumulation of As (mg/kg DW) in plant species of the V. Gatas mining area.

**Figure 6.** Accumulation of W (mg/kg DW) in plant species of the Sarzedas mining area.

stems and leaves of *Q. ilex* [84].

496 Environmental Risk Assessment of Soil Contamination

It was concluded that the species and organs best suited for biogeochemical prospecting and/ or with potential for mine restoration in the Sarzedas mine area are by order of importance: 1) As: old needles of *P. pinaster*, aerial tissues of *C. vulgaris*, *Chamaespartium tridentatum*, leaves of *C. ladanifer*, *Erica umbellata* and *Quercus ilex* subsp. *ballota*; 2) Sb: *D. purpurea*, *E. umbellata*, stems of *C. ladanifer*, *C. vulgaris*, *C. tridentatum* and stems of *P. pinaster*; 3) W: *D. purpurea*, *C. triden‐ tatum*, old stems and needles of *P. pinaster*, stem and leaves of *C. ladanifer*, *E. umbellata* and

pH 3.5 – 6.3 5.0 5.0 0.8 As 26.7 – 5,770 446 56.7 1,178 Cu 11.7 – 352 88.0 29.0 101 Fe 18,482 – 60,100 33,039 29,443 12,463 Mn 103 – 898 336 167 248 Ni 11.6 – 61.2 30.6 23.6 15.1 Pb 55.4 – 6,299 499 102 1,285 Zn 63.1 – 469 180 125 112 W 2.00 – 636 73.8 10.6 162

**Table 3.** Trace elements content (mg/kg) and pH of soil samples (N=69, V. Gatas mine).

**Range Mean Median Standard deviation**

**Figure 8.** Accumulation of Cu (mg/kg DW) in plant species of the V. Gatas mining area.

**Figure 9.** Accumulation of Fe (mg/kg DW) in plant species of the V. Gatas mining area.

**Figure 10.** Accumulation of Pb (mg/kg DW) in plant species of the V. Gatas mining area.

**Figure 11.** Accumulation of Zn (mg/kg DW) in plant species of the V. Gatas mining area.

The *P. pinaster* trees growing on the tailings and contaminated soils of Vale das Gatas mine accumulated the studied elements in quantities greater than observed in plants of the areas representative of the local geochemical background. These values were also higher than those typically observed in this species.

In the *P. pinaster* samples from tailings and contaminated soil locations, the older needles (2 and 3-years-old) show a tendency to accumulate higher concentrations of As, Fe, Zn, Pb and W while Ni and Cu were preferentially accumulated in young needles and stems (1-year-old) [93]. This allowed the authors to conclude that the metal/metalloid concentrations of elements in plants depend as much on the plant organ as on its age and in biogeochemical studies, it is important not to mix foliar and woody material in the same sample. The species showed a great variability in the accumulation behaviour of As, Fe, Mn, Cu, Zn, Pb, Ni, and W with the age of the organ. Thus, the 1-year-old needles and stems accumulated higher levels of Cu (Figure 8) and Ni (Figure 12). While the older needles accumulated higher levels of As, Fe, Pb, Zn and W (Figures 7, 9, 10, 11 and 13). The 2-years-old stems may also be appropriate samples to detect higher levels of Fe, Zn and Pb.

**Figure 12.** Accumulation of Ni (mg/kg DW) in plant species of the V. Gatas mining area.

**Figure 9.** Accumulation of Fe (mg/kg DW) in plant species of the V. Gatas mining area.

498 Environmental Risk Assessment of Soil Contamination

**Figure 10.** Accumulation of Pb (mg/kg DW) in plant species of the V. Gatas mining area.

**Figure 11.** Accumulation of Zn (mg/kg DW) in plant species of the V. Gatas mining area.

**Figure 13.** Accumulation of W (mg/kg DW) in plant species of the V. Gatas mining area.

### *5.1.1.2. Copper mines*

The São Domingos mine (abandoned in 1966) located in south-east Portugal is also included in this study. This is one of the historical mining centres, known for its activity since pre-Roman times, with extraction of gold, silver and copper [95] though copper production was the highlight.

A summary of soil trace element data is presented in Table 4. High levels of As, Cu, Pb and Zn were recorded in the soils. Copper concentration in soils reached up to 1,829 mg/kg as a result of the former activities at the site (copper smelter). Maximum concentration of As in soils was very high, reaching 1,291 mg/kg. The concentration of Pb in the soil was also very high, 2,694 mg/kg as the average value registered. The average Zn concentration in soils was of 218 mg/kg but it could reach 714 mg/kg, a level that can be extremely toxic for plants. Cobalt and Cr concentrations in soils were normally low, ranging from 20.1 to 54.3 mg/kg for Co and 5.1 to 84.6 mg/kg for Cr. Nickel and Ag were also low, varying from 27.2–52.9 mg/kg and 2.5– 16.6 mg/kg, respectively.


**Table 4.** Trace elements content (mg/kg) and pH of soil samples (N=21, S. Domingos mine).

In plants, Pb concentration was rather high for some species, varying from 2.9 to 84.9 mg/kg dry weight (DW) (Figure 14). Semi-aquatic species sampled in the mining area, *Juncus conglomeratus* and *Scirpus holoschoenus*, showed high accumulation of Pb in plant tissues. Lead above 20 mg/kg DW was found in leaves of two species of *Cistus*, typical Mediterranean shrubs known for their tolerance to drought and low nutrients availability. Arsenic concentration in plant tissues ranged from 0.3 to 23.5 mg/kg DW. Maximum As was recorded in *J. conglomera‐ tus*, *Thymus mastichina*, *J. effusus* and *S. holoschoenus* [82]. Semi-aquatic species from the Juncaceae family showed the highest content of both metals. Copper concentration in plant tissues ranged from 3.60 to 28.9 mg/kg DW (Figure 15). These Cu values are within the range considered normal for plants [96]. The species *Cistus monspeliensis* and *Daphne gnidium* showed the highest Zn concentrations [82]. A few trees, *Eucalyptus*, *Quercus* and *Pinus* species, were found in the contaminated area showing accumulation of different metals in the aboveground tissues. Due to their high biomass, they can be very effective for metals phytoextraction and phytostabilization especially when established in the less contaminated soils on the peripheral zone of the study area [82].

*5.1.1.2. Copper mines*

500 Environmental Risk Assessment of Soil Contamination

16.6 mg/kg, respectively.

highlight.

The São Domingos mine (abandoned in 1966) located in south-east Portugal is also included in this study. This is one of the historical mining centres, known for its activity since pre-Roman times, with extraction of gold, silver and copper [95] though copper production was the

A summary of soil trace element data is presented in Table 4. High levels of As, Cu, Pb and Zn were recorded in the soils. Copper concentration in soils reached up to 1,829 mg/kg as a result of the former activities at the site (copper smelter). Maximum concentration of As in soils was very high, reaching 1,291 mg/kg. The concentration of Pb in the soil was also very high, 2,694 mg/kg as the average value registered. The average Zn concentration in soils was of 218 mg/kg but it could reach 714 mg/kg, a level that can be extremely toxic for plants. Cobalt and Cr concentrations in soils were normally low, ranging from 20.1 to 54.3 mg/kg for Co and 5.1 to 84.6 mg/kg for Cr. Nickel and Ag were also low, varying from 27.2–52.9 mg/kg and 2.5–

pH 4.0 – 6.7 5.1 5.1 0.6 Ag 2.50 – 16.6 7.50 7.00 3.60 As 37.2 – 1291 393 353 324 Co 20.1 – 54.3 31.0 29.4 8.40 Cr 5.10 – 84.6 26.5 8.30 31.7 Cu 87.3 – 1,829 553 444 443 Ni 27.2 – 52.9 42.2 43.9 6.60 Pb 234 – 12,218 2,694 2,355 2,345 Zn 104 – 714 218 163 145

**Table 4.** Trace elements content (mg/kg) and pH of soil samples (N=21, S. Domingos mine).

In plants, Pb concentration was rather high for some species, varying from 2.9 to 84.9 mg/kg dry weight (DW) (Figure 14). Semi-aquatic species sampled in the mining area, *Juncus conglomeratus* and *Scirpus holoschoenus*, showed high accumulation of Pb in plant tissues. Lead above 20 mg/kg DW was found in leaves of two species of *Cistus*, typical Mediterranean shrubs known for their tolerance to drought and low nutrients availability. Arsenic concentration in plant tissues ranged from 0.3 to 23.5 mg/kg DW. Maximum As was recorded in *J. conglomera‐ tus*, *Thymus mastichina*, *J. effusus* and *S. holoschoenus* [82]. Semi-aquatic species from the Juncaceae family showed the highest content of both metals. Copper concentration in plant tissues ranged from 3.60 to 28.9 mg/kg DW (Figure 15). These Cu values are within the range considered normal for plants [96]. The species *Cistus monspeliensis* and *Daphne gnidium* showed the highest Zn concentrations [82]. A few trees, *Eucalyptus*, *Quercus* and *Pinus* species, were found in the contaminated area showing accumulation of different metals in the aboveground tissues. Due to their high biomass, they can be very effective for metals phytoextraction and

**Range Mean Median Standard deviation**

**Figure 14.** Accumulation of Pb (mg/kg DW) in plant species of the S. Domingos mining area.

**Figure 15.** Accumulation of Cu (mg/kg DW) in plant species of the S. Domingos mining area.

### *5.1.1.3. Lead mines*

The Barbadalhos mine is an abandoned Pb mine in Central Portugal. It was exploited for Pb by underground mining from 1887 till the 1940s. The concentrated ore was smelted on site. As per the usual practice at the time, tailings were deposited on the ground.

Metal concentrations in soil are shown in Table 5. Lead concentration in soils reached 9,331 mg/kg while the average value was 928 mg/kg; obviously due to mining of galena at the site. In soils from mineralized zone, the mean Pb concentration (2,380 mg/kg) was nearly 9 times the threshold for industrial soils suggested by Canadian Environmental Quality Guidelines [97].


**Table 5.** Trace elements content (mg/kg) and pH of soil samples (N=45, Barbadalhos mine).

Samples from 49 species of the native flora were investigated at this site. Individual elements and species displayed different trends of accumulation. All plants collected along mineralized zone accumulated eight metals (Ag, Co, Cr, Cu, Fe, Ni, Pb, and Zn) but many plants from nonmineralized zone accumulated only five metals (Ag, Cu, Fe, Pb, and Zn). A few however did accumulate the remaining three (Co, Cr, and Ni); bringing the count of metals accumulated at par with those of mineralized zone [92].

Most plants were seen to be tolerant of soil Pb concentrations. In mineralized zone, Pb concentrations in plants ranged from 1.11 to 548 mg/kg DW. This is far above the 100 – 400 mg Pb/kg content considered toxic for most plants [98]. Significant accumulation of Pb was seen in *Cistus salvifolius* (548 mg/kg), *Lonicera periclymenum* (318 mg/kg), *Anarrhinum bellidifolium*, *Phytolacca americana*, *Digitalis purpurea*, *Mentha suaveolens* (255 – 217 mg/kg) [listed in decreas‐ ing order] (Figure 16). Pteridophytes like *Polystichum setiferum*, *Pteridium aquilinum*, and *Asplenium onopteris* also showed 117 – 251 mg/kg Pb in aerial parts. In plants from nonmineralized zone, Pb content was not significant ranging from 0.94 to 11.6 mg/kg.

*5.1.1.3. Lead mines*

502 Environmental Risk Assessment of Soil Contamination

Guidelines [97].

The Barbadalhos mine is an abandoned Pb mine in Central Portugal. It was exploited for Pb by underground mining from 1887 till the 1940s. The concentrated ore was smelted on site. As

Metal concentrations in soil are shown in Table 5. Lead concentration in soils reached 9,331 mg/kg while the average value was 928 mg/kg; obviously due to mining of galena at the site. In soils from mineralized zone, the mean Pb concentration (2,380 mg/kg) was nearly 9 times the threshold for industrial soils suggested by Canadian Environmental Quality

pH 3,6 – 6.4 4.7 4.6 0.5 Ag 0.71 – 13.0 1.71 1.06 2.03 As 2.77 – 208 16.9 8.07 31.7 Co 3.74 – 50.5 20.1 16.7 12.4 Cr 61.3 – 196 89.0 85.7 22.1 Cu 21.4 – 193 41.7 34.5 27.9 Fe 24,145 – 98,510 40,283 38,497 13,751 Mn 44.4 – 2,224 596 381 588 Ni 7.68 – 87.0 30.5 28.1 12.2 Pb 24.4 – 9,331 928 68.8 2,119 Zn 30.4 – 517 134 90.3 109

**Table 5.** Trace elements content (mg/kg) and pH of soil samples (N=45, Barbadalhos mine).

par with those of mineralized zone [92].

Samples from 49 species of the native flora were investigated at this site. Individual elements and species displayed different trends of accumulation. All plants collected along mineralized zone accumulated eight metals (Ag, Co, Cr, Cu, Fe, Ni, Pb, and Zn) but many plants from nonmineralized zone accumulated only five metals (Ag, Cu, Fe, Pb, and Zn). A few however did accumulate the remaining three (Co, Cr, and Ni); bringing the count of metals accumulated at

Most plants were seen to be tolerant of soil Pb concentrations. In mineralized zone, Pb concentrations in plants ranged from 1.11 to 548 mg/kg DW. This is far above the 100 – 400 mg Pb/kg content considered toxic for most plants [98]. Significant accumulation of Pb was seen in *Cistus salvifolius* (548 mg/kg), *Lonicera periclymenum* (318 mg/kg), *Anarrhinum bellidifolium*, *Phytolacca americana*, *Digitalis purpurea*, *Mentha suaveolens* (255 – 217 mg/kg) [listed in decreas‐ ing order] (Figure 16). Pteridophytes like *Polystichum setiferum*, *Pteridium aquilinum*, and

**Range Mean Median Standard deviation**

per the usual practice at the time, tailings were deposited on the ground.

Though at first glance maximum Pb content observed in trees like *Acacia dealbata* (84 mg/kg: leaves), *Olea europaea* (62 mg/kg: twigs), and *Quercus suber* (58 mg/kg: twigs) from mineralized zone is not very impressive compared to that of smaller plants mentioned above, nevertheless these trees can be very effective due to their higher biomass. When combined with the hardy nature, biomass and abundance of this species, the moderate accumulation indicates immense potential for phytoextraction of Pb in the area [92].

In mineralized zone, Zn concentrations in plants reached 1,020 mg/kg in *D. purpurea*. And ranged from 262 to 887 mg/kg in *L. periclymenum*, *P. americana*, *Solanum nigrum*, *P. setiferum*, *M. suaveolens*, *Viola riviniana*, and *A. bellidifolium* [listed in decreasing order] [92].

**Figure 16.** Accumulation of Pb (mg/kg DW) in plant species of the Barbadalhos mining area.
