Environmental Impact of Abandoned Mine Waste: a review

Discussion

The present review of some of the many sites of former mining activity in Italy shows that the abandoned waste dumps contain significant amounts of heavy metals that are potentially harmful to the environment.

As reported by Benvenuti et al. (1999) and Mascaro et al. (2001a, b), the main factors controlling the release of toxic elements are:

• 
  The original content and composition of the metal-bearing rocks and the buffering phases in the waste. In particular, where pyrite is abundant, as at Boccheggiano and Fenice Capanne, the acidity produced by its oxidation rapidly uses up the buffering phases such as carbonates and aluminosilicates, whereas where galena and sphalerite are dominant, as at Bottino, alteration occurs under near-neutral conditions.
  
• 
  The morphology and particle size of the mine waste. Waste dumps are typically unsaturated with respect to water, and have relatively high hydraulic conductivity that favours oxidation. and influences the alteration processes.
  
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  Unlikely, in the tailings impoundments infiltration of oxygenated waters is limited by the low effective porosity of the fine sandy or silty-clayey material, and by the presence of a shallow water table in the wet season. For example, at Fenice Capanne, flotation tailings deposited in unconfined dumps show more advanced alteration than similar material in the impoundments. The particle size of material in the waste dumps influences the alteration processes: all other factors being equal (mineralogy, age, etc.), coarser grained tailings (pebbles and gravel) show less advanced alteration than sandy-silty materials.
  
• 
  The age of waste pile. This factor seems to be of less importance than the preceding two, but it may become significant in the long term (decades or centuries), as it appears from soils developed from chronologically different waste dumps at the Temperino mine (Bini and Gaballo, 2006). At Bottino, for example, the alteration processes in Renaissance-age waste dumps are significantly more advanced than in dumps from the past century.
  

The results available for the Boccheggiano area (Benvenuti et al., 1999) suggest that pollutant transport over long distances occurs as suspended particulate matter in streams, and that gravitational runoff, aeolian transport, and transport in solution are limited to the immediate proximity of the sources. There is evidence that a portion of the metals is fixed in relatively stable solid phases, either in specific primary or secondary mineral species, or as minor elements in solid solution; a portion is also adsorbed onto clay minerals and iron-oxyhydroxides. However, metal fixation in the easy soluble or poorly crystalline minerals is ephemeral, and the metals may be released because of geochemical changes related to meteoric events, bacterial activity, or photochemically induced redox reactions (McKnight et al, 1988). Unlikely, in the Imperina Valley district aeolian transport and suspended particulate matter in streams seem to have an important function in disseminating metal pollution in areas conterminous to the mine district (Bini et al., 2011).

Soils influenced by mine spoils present little morphological evidence of profile evolution. Soil parent material may be transformed into immature soils in a relatively short time, as it was found by Néel et al. (2003) in 35-year-old sulphide mine tailings. However, data presented indicate relevant differences in soil development at various sites, where parent material is the main soil forming factor. Evidence of the spatial variability of soils is given by the uneven distribution of vegetation coverage. This is related to the spreading of mine tailings rich in phytotoxic heavy metals (Cu, Pb, Zn) on the land surface: the shorter is the time passed, the slower is natural land revegetation. The processes of soil formation have been driven by the nature and properties of the spoil parent material, such as mineralogy and chemistry, grain size, porosity, etc. Specific processes consist of weathering, oxidation, leaching, and humus addition. As a consequence, a new kind of soil started to form, first as a discontinuous soil cover with A-C profile, and subsequently with a well-developed surface A horizon enriched in organic matter, characteristic of Entisols in Soil Taxonomy (Soil Survey Staff, 1999). Similar Entisols with up to 10-cm thick A-C sequence are known to develop in less than 100 years from natural unconsolidated deposits (Forth and Turk, 1972; Bini et al., 2004), and from mine spoils (Néel et al., 2003). At the sites investigated, the time of exposure to weathering varies from 2700 years to the present, and most soils investigated show little development. Soil thickness increases with time gradually, while plant roots explore thicker layers with increasing density; a thick (up to 100 cm) A-B-C sequence with a cambic horizon may form, giving origin to Inceptisols. However, according to the new proposed classification (ICOMANTH, 2005), all these soils originated from anthropogenic material, and therefore may be called Anthrosols (Technosols in the new ICOMANTH circular letter, 2007).

Combining these results with historical and archaeological data, soil characteristics, and literature on soil genesis in Mediterranean areas (Mirabella et al., 1992; Bech et al., 1997; Bini et al., 2006), the following tentative scheme of soil evolution (Table 13), corresponding to a provisional chronosequence (although based on relative age), may be suggested for Anthrosols containing mine spoil as a parent material in the profile, and for the corresponding “normal” soils:

Table 13. Tentative scheme of soil evolution, and correlation between Anthrosols and “normal soils” in the study area

Estimated age	Anthrosols	“Normal” soils
<100y	(LITHIC) SPOLIC XERORTHENT	TYPIC XERORTHENT
<1000y	SPOLIC DYSTROXEREPT SPOLIC HAPLOXEREPT	TYPIC HAPLOXEREPT
> 1000 y	SPOLIC RHODOXERALF	TYPIC RHODOXERALF
	SPOLIC XERORTHENT over TYPIC RHODOXERALF

Anthrosol taxonomy from ICOMANTH (2005).
As stated by Bini and Zilocchi (2004), the occurrence of soils at different stages of development (chronosequences) enables the establishment of chronofunctions related to a given time interval (2700 years in the present study). The soil-time function (or chronofunction) is defined by the following equation (Jenny, 1941, 1980):

in which s=soil, t=time, cl=climate, o=organisms, r=relief, p=parent material.

Several soil properties have been utilized, and numerous examples of chronofunctions have been reported in the literature (e.g. Harden, 1982; Bockheim, 1980, 1990; Schaetzl et al., 1994; Rabenhorst, 1997). In a recent study, in order to evaluate the effect of time on soil formation from mine dumps, Bini and Gaballo (2006) tried to find out different soil properties (SOC, colour, A horizon thickness, total thickness, pH, particle size) to relate to soil age, and to determine a possible chronofunction for such soils. Among the investigated soil properties, the most effective in defining an actual chronofunction proved the A horizon thickness. We have measured the thickness of the A horizon of the examined profiles, and we have plotted it against the relative (archaeological) age of mine spoil. The A horizon thickness increases linearly with increasing soil age, as expressed by the equation:

A horizon thickness (cm) = 0.011(soil age in yrs).
The correlation coefficient (R²) is 0.9766, indicating that nearly all variation in soil thickness is a function of soil age. This suggests that organic matter addition and humification are key processes in the first steps of soil development, and, therefore, vegetation has a key role as initial soil forming factor. Moreover, although it is generally difficult to quantify the effect of time on pedogenesis (Bockheim, 1980; Yaalon, 1997), a single soil property (e.g., A horizon thickness, organic carbon content) can be effective to build up a simple chronofunction, at least over short time intervals.

In conclusion, the comparative study of Anthrosols and “normal” soils (unaffected by mine spoil) allowed reconstruction of the natural and anthropogenic stages of soil development, as well as the identification of the main governing factors.

Mine tailings may be considered colluvial deposits whose discharge at surface caused different trends in pedogenesis and noteworthy spatial variability. Current soil development is governed by the composition and properties of mine spoil. The age of spoil, combined with inherited effects of the spoil are responsible for different pedogenetic stages; tailings are likely to generate different types of soils (Anthrosols) over short distances (<0.5km) from mine waste.

The original, “normal,” soil (paleosol) is truncated and/or buried by potentially phytotoxic spoil. The anthropic intervention in the mining area and in the conterminous land impacted heavily the landscape and the natural environment. About 1/6 of the territory examined has been impacted by mine spoil, with effects such as changed surface topography, immature infertile soils, and poorly structured, herbaceous and shrubby vegetation coverage replacing the original Mediterranean maquis.

According to this, and compared with results obtained in other contexts (Néel et al., 2003; Bini and Zilocchi, 2004), the recorded trend of soil evolution contributes to a better knowledge of areas affected by similar waste material, and may be utilized in remediation of abandoned mine areas.


Chapter 6