Vol. 32 – 2006 - Ecologia Mediterranea

A. herba-alba and S. tenacissima do not colonize
the tailings themselves which remain
thus as nude surfaces exposed to wind erosion
despite the presence of a few individuals of
Silybum marianum (L.) Gaertner. Concentrations
of Cu, Zn and Pb in A. herba-alba and
S. tenacissima roots and stems for four plant
samples collected at these distances (50 m,
200 m and 1000 m) from the tailings are given
in Table 2.
It has to be mentioned that for both species
and for the three considered elements, maximal
concentration in the plant tissues were not
recorded nearby the tailings but 50 to 200 m
away from them. This is especially the case
when considering the root concentration. No
individual belonging to S. tenacissima was
found 200 m away from the tailing and data
are thus given for A. herba-alba only. When
plant samples were collected 1 km away from
those tailings, they contained only small
amounts of heavy metals. Not only the
absolute concentration, but also the shoot/root
ratio of concentrations in studied plants
strongly varied as a function of the distance
to the tailings. This ratio remained very low
when the plants exhibited the highest concentration
(50 m from the tailings for S.
tenacissima and 200 m away from the tailing
for A. herba-alba).
We were thus unable to establish a clear correlation
between heavy metal concentrations
in roots and stems because of a high variability.
Discussion
The levels of contamination around the mining
site of Zaida appeared heterogeneous,
although the importance of heterogeneity
depends on the type of considered heavy
metal: maximal values for lead, zinc and copper
(Table 2) were recorded for the tailings.
Values recorded for Pb clearly exceeded the
threshold level of 400 mg Kg-1 considered as
the maximal values in soils; in most of the
European legislation, agronomic soils should
not contain more than 70 mg Kg-1 of total lead
(Treme-Schaub & Feix 2006).
To be able to interpret these higher contents
of MTE in soils, it’s important to distinguish
between potential contaminations and natural
weathering (Baize 1997). The fact that total
heavy metal concentration at the tailing level
ecologia mediterranea – Vol. 32 – 2006
Heavy Metals in Tailings,Soils and Vegetation of an Abandoned Lead Mine Land in Morocco
was high does not imply that all the quantified
metal is available for plant absorption.
This is especially the case for lead, which is
commonly considered as an element exhibiting
a low bioavailability (Alloway 1990). Soil
acidity may strongly increase lead availability
in contaminated areas: in general terms,
heavy metals are available to plants in small
amounts when the environmental conditions
are alkaline (Hesse 1971). In the present case,
however, soil remains neutral or slightly alkaline
(7.5 to 8.7) and acidity therefore should
not be a major problem increasing lead availability.
On the other hand, soil consists in a
sandy substrate typical of semi-arid steppe
and thus exhibits a very low level of organic
matter susceptible to retain lead.
In the present study, very high levels of Pb,
Zn and Cu have been observed in both tested
tissues. The data appeared quite surprising for
lead: although those plants were never
reported as lead hyperaccumulators, it is noteworthy
that the analyzed plants always accumulated
this element to higher concentration
than the surrounding substrate (Table 1). Only
few plants were reported to be able to accumulate
lead (Reeves & Baker 2000). Since the
presence of lead constitutes a characteristic of
numerous mining areas throughout the world,
the opportunity to use these two plant species
to help in soil decontamination should be considered
in the future. Lead contents of A.
herba-alba and S. tenacissima colonizing
areas near the tailings were higher than normal
(Kabata & Pendias 1992). This might
suggest some kind of tolerance mechanism of
plants as they grow normally and survive in
spite of higher lead concentrations in soils
(Steinborn & Breen 1999).
Copper concentrations levels in plants were
also higher as compared to soils and tailings.
This implies that both species display unusual
abilities to extract, accumulate and tolerate
these heavy metals, even if the registered concentration
remains lower than in classical
hyperaccumulating plants (Fleming & Parle
1977; Türkan et al. 1995).
It is also noteworthy that the highest heavy
metal concentration was found in plants growing
at 50-200 m of the tailings and not nearby
the tailings. It may be hypothesized that hot
spots of contaminations were also present, not
only on the tailing themselves, but also in the
surrounding area. Wind erosion may have
occurred and transport contaminated particle
over a given distance: nevertheless, the fact
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