Environmental Impact of Abandoned Mine Waste: a review

2.6. Biological Implications

There is a general, although not simple, relationship between the heavy metal content of soils and plants growing on those soils (Davies, 1987; Baker and Brooks, 1989; Adriano, 2001). Uptake through the roots is influenced by soil parameters such as acidity or redox potential, and different plant species absorb metals to different extents. But in general, the higher the heavy metal concentration of the soil, the higher will be the concentration in plants (Baker, 1981).

An absorption sequence Zn>Co>Cu>Ni>Fe>Cr, consistent with leaching tests, was found by Dinelli and Lombini (1996) in wild plants growing on mine soils, suggesting plant uptake to be controlled by soil solution composition. Fontana et al. (2010) report that in wild plants of mine soils the less mobile among the trace elements considered is Pb (average TF = 0.37), which tends to remain blocked in the roots, because it is not essential for plant nutrition, thereby suggesting some exclusion strategy by plants. Chromium, Cu, Zn, Fe, are present in similar concentrations in leaves and roots (TF ≈ 1), while Mn appears to be the most translocated among the elements considered (average TF = 2.33). metal translocation is probably influenced by the bioavailability of the metal and by the species of plant considered, which are two determining key factors in the evaluation of the absorption of pollutants by the plant compartment (Kabata-Pendias, 2004). Similar results were obtained by Bini et al (2000) for chromium in wild plants growing on Cr-contaminated soils.

Among heavy metals, cadmium and lead are toxic to animals and people, and the accumulation of these metals in foodstuffs raises the question whether human health might be impaired by ingesting small amounts of toxic metals. Thornton (1996) reported lead concentrations in garden vegetables in mining areas of Derbyshire to be 2-4 times higher than those of urban soils; accidently ingested soil resulted to be the major source of Pb intake. In an early paper, Davies and Roberts (1975) found that radish plants (Raphanus sativus L.) from some gardens close to mine sites in Wales, contained more than the British legal limit for lead in food (2mg/kg fresh weight), and cadmium concentrations were high enough to cause concern. Similarly, mining at a site in SW England resulted in extremely high Cd (and Zn) concentration in soils and leafy vegetables, including cabbage, spinach and lettuce (Thornton, 1996).

Exposure to heavy metals may affect severely human population by metabolic and neurological disorders, psychomotor retardation, intoxication, respiratory diseases, liver and kidney damage, skin and internal cancer (Jarup, 2003). Acute toxicity by lead inhalation or ingestion, both direct and indirect, via the food chain (Abrahams, 2002), brings out malfunctioning to the reproductive system, kidney insufficiency, damage to neurological system and brain. Food ingestion has been found by Zheng et al. (2007) to be the most common way (>90%) of human exposure to metal contamination, in comparison to other exposure ways such as inhalation and dermal contact.


Chapter 3

Consequences of Mining Operations on Environmental Transformations and Mine Soil Evolution

Numerous processes, both physical and chemical, contribute to environmental transformations consequent to former mining activities and subsequent restoration of the exploited areas.

In the initial stage of rock alteration, physical processes prevail. These are particularly effective in areas with steep morphology, where most mining districts are located. Loose and coarse grained material forms as a consequence of rock fragmentation. Rock fragments migration on instable slopes, erosion of fine particles by wind and runoff, all these processes contribute to land modelling. Meanwhile, chemical processes begin to act, further contributing to environmental transformations by oxidation (Eh>250mV), acidification (pH<7), hydrolysis, metal leaching, precipitation of oxyhydroxides and sulphates, argillogenesis. All these processes may be conditioned by water availability and temperature (i.e. the climate conditions), that enhance mine waste reactivity.

As a consequence of the above processes, a set of physical and chemical features characterize the soils developed from mine spoils. Once the parent material is finely subdivided and weathered, the formation of a biologically active substrate may occur, thereby permitting pioneer vegetation (lichens, mosses) development (Burykin, 1985). Litter accumulation (OL horizon) is the process that characterizes the early stage of soil formation. Subsequently, organic matter decomposition (OF horizon), humus formation (OH horizon), mineralisation (A horizon) constitute a first pedogenetic phase. According to Jabiol et al. (2007), this phase may bring to the differentiation of several types of humus as a function of litter composition, microflora and microfauna activity, pH and climate conditions.

A second pedogenetic phase is determined by in situ mineral transformations (e.g. acid hydrolysis), oxyhydroxides and clay formation (stage of cambic horizon formation). In this phase, colour varies from very dark brown (10YR 3/3) to dark brown (7,5YR 3/3), reddish brown (2,5YR 3/2), dark yellowish brown (10YR 4/5), or blackish (5YR 2,5/5), in relation to the nature of the bedrock, and /or to the amount of mine waste.

A third pedogenetic phase is consistent with solute leaching and particles migration towards bottom (stage of argillic horizon formation); precipitation of new minerals (e.g. carbonate, sulphate) is likely to occur. However, this third phase is difficult to assess in mine waste materials, since the time elapsed from mining operations generally is not sufficient for Bt formation, if we consider that the landscape morphology is generally undulated, with slopes ranging from 15% and 45%, and therefore erosion is a prominent process. Yet, soils developed from waste dumps are generally shallow (20-100 cm), skeletal, coarse-textured (sandy loam to loamy sand), little developed, with limited horizonation.

Table 2. Selected properties of the Spolic Xerorthent illustrated in Figure 8.

Soil Horizon	Depth	Particle size (USDA) %			pH	Total Carbonates	Organic Carbon	Organic Matter	CEC
	cm	silt	clay	sand		g/kg	g/kg	g/kg	cmol(+)/ kg
A1	0-10	25	14.8	60.2	7.8	0	16	28	20
A2	10-30	26	27.4	46.6	7.6	0	14	24	22.5
2C	30-70	32.7	37.5	29.8	7.5	0	5	9	14.5