Environmental Impact of Abandoned Mine Waste: a review

4.4. Sardinia

Sardinia, the biggest Italian island, for millennia has been the main metalliferous region of Italy. Historical mining was carried out for long time in various mining districts, in absence of effective regulations and controls on the environmental impact, and was characterized by disregard of environmental issues, with consequent diffuse contamination (Da Pelo et al., 2009). Only in the last decades, Regional Authorities decided to counteract the environmental impact of abandoned mines, and to restore metal-contaminated areas. Since that time, numerous natural areas and mine-archaeological parks have been realized in the island, where the cessation of mining activity left large quantities of mine wastes on dumps and flotation tailings, estimated at about 45 Millions m³ for the whole mining district (Cidu et al., 2009). Several studies, therefore, have been carried out in the last decade on these abandoned mine areas (Caboi et al., 1999; Cidu and Fanfani, 2002; Frau and Ardau, 2003; Musu et al., 2007; Trois et al., 2007; Cidu et al., 2009; Da Pelo et al., 2009; Frau et al., 2009), with the aim of understanding the environmental impact of mining operations.

A century of exploitation of huge galena-sphalerite deposits hosted in the Iglesiente-Sulcis mining district, for Pb and As production (Frau et al., 2009), has caused remarkable environmental impact, mainly due to discharging tailings from the flotation basins directly into streams that drain the mined areas. This has caused diffuse contamination in the whole catchment, and the dispersion of highly contaminated materials over a distance of about 10 km downstream from the mine. The geology of the district consists of ore bodies embedded in Cambrian limestones and dolostones with gently undulating morphology. Climax vegetation is the Mediterranean maquis, here in a degraded stage, with a discontinuous cover composed mainly of herbaceous and shrubby vegetation. Climate is typically Mediterranean, with warm and dry summer and mild humid winter (maT = 16°C, maP = 650mm). Soils are scarcely developed Entisols (USDA soil taxonomy) mixed with bedrock outcrops. Some red soils too outcrop in the conterminous areas not affected by mine waste.

An inventory of the mineral resources of the Southern Sardinia mine district has been made recently by Cidu et al. (2009) and Frau et al. (2009), who collected samples from waste-rock dumps, flotation tailings and stream sediments.

The main mineralogical composition of the waste dumps consists of quartz, K-feldspar, chlorite, muscovite and biotite, derived from the bedrock. In mine dumps, secondary phases are the abundant presence of anglesite as an oxidation product of galena, cerussite (linked to the carbonatic fraction) and resistant minerals such as Pb-Fe sulphate (plumbojarosite). Tailings samples contain minor amounts of montmorillonite and gypsum, and traces of oxyhydroxides.

According to Frau et al. (2009), surface waters, with the exception of two acidic samples, are neutral or slightly alkaline (pH 7-8) and oxidizing (Eh 0.4-0.6 V). These chemical-physical conditions enhance dissolution of sulphates deriving directly (anglesite), or indirectly (gypsum), from sulphides oxidation; this determines metal release at sites close to the flotation basin, and a metal concentration decrease about 1.5 km upstream. Metal attenuation in surface waters does not depend on mixing, but rather on a removal process, following the sequence Pb>Cu>Zn>Cd suggested by Caboi et al. (1999) for another mine area. Another possible removal mechanism might be represented by precipitation of secondary plumbojarosite as small grains dispersed in a ferrihydrite mass or as coatings. Indeed, most of the plumbojarosite contained in stream sediments/tailings was formed by direct precipitation from the flotation basin. SEM/EDX observations carried out by Frau et al. (2009) on waste-rock dumps actually show that galena crystals have an alteration rim composed of anglesite, while ferrihydrite and Pb-jarosite coatings formed on quartz grains in stream-bed sediments and flotation tailings.

In the recently open gold deposit at Furtei, exploitation was preceded and accompanied by studies on the environmental impact. Da Pelo et al. (2009) collected solid samples, including mineralized rocks and related proximal soils, weakly mineralized rock (and distal soils), and tailings from impoundment. Moreover, water collected immediately after a heavy rain event was assumed to represent the natural leaching of exposed materials.

The total concentration of selected elements in the solid phases is reported in Table 10. Iron is mostly abundant in mine-waste (average 5.9%, range 3.2-7.8), decreasing down to 1.7% in tailings and to 0.3% in distal soils, as well as Mn. Consistently, also trace elements (As, Cd, Cu, Pb, Zn) present higher concentrations in waste-rocks than in tailings and soils, as expected as a consequence of lower pH, minor grain size and a dilution effect determined by distance from the mineral source.

Table 10. Average trace elements concentrations, mean values of pH and texture at different sites in the South Sardinia mine area

	Fe	Mn	As	Cd	Cu	Pb	Zn	pH	texture
Mine waste	5.9	1.20	225	0.6	982	68	107	3.3	Gravel, pebble
Tailings	1.7	0.75	280	<0.5	500	94	41	6.7	Coarse Loamy
Distal soils	0.4	0.33	32	<0.5	35	87	56	5.9	Sandy loam

Fe, Mn are expressed as %; As, Cd, Cu, Pb, Zn as mgkg^-1. Data from Trois et al., 2007 ; Da Pelo et al., 2009; Frau et al., 2009.
Leaching experiments show (Frau et al., 2009) intermediate to low concentrations of minor components, with toxic elements mostly immobile, being related to the residual fraction, in a near-neutral pH. Exception to this general behaviour is given by As, whose relatively high, albeit variable, percentage of extractability is linked to the large availability of pyrite. Yet, pyrite is abundant in mineralized rocks, and little in distal samples. Tailings contain minor amounts of montmorillonite and gypsum, along with traces of Fe-oxyhydroxides and pyrite.

High amounts of S, Fe (from pyrite) and metals (especially Cu, As and Ba) are present in dumps.

Pyrite oxidation in acidic environment (average pH 3.3; range 2.6-7.6) promotes the oxidation of other sulphides, the release of metals and the formation of soluble secondary minerals. The majority of base metal sulphides are solubilised in an abiotic manner in acidic conditions by sulphuric acid or by an oxidising agent such as ferric ion. The solubilisation kinetics may be increased by microorganisms such as bacteria to such an extent (10 ⁵ times) that they can be regarded as producers of sulphuric acid. Microbiological viability tests (Trois et al., 2007) were aimed at recognizing the presence of acidophilic chemolithoautotrophs in the dump layers. The oxidation and solubilisation of mineral metal sulphides, catalyzed by chemolithoautotrophic acidophiles of the genus Acidithiobacillus ferrooxidans, are the main causes of acidic rock drainage. It was found that they were absent from the upper layers, whereas in the underlying layers they are pronouncedly present, due to an increase in moisture content. Yet, there is evidence of a strong dependence between the presence of a acidophilic chemolithoautotroph microflora inside the dump and the quality of effluents (Trois et al., 2007). Moreover, the presence of bacteria and/or of dissolved ferric iron from pyrite oxidation speeds up mineral dissolution, with consequent release of As and Cu to the water. A comparatively slow reaction rate can still result in the release of a harmful amount of contaminants.

Typical features of the process are:

the pronounced acidity of the effluents after irrigation;

the repetitive presence of metal ions concentration peaks on resumption of each irrigation, regardless the extent of dry periods;

the almost complete and lasting disappearance of metal ions from the effluent after neutralization of the inflowing water, coupled with a drastic reduction of the viable microflora.
The highest potential threat for the environment is mostly represented by mineralized rocks exposed in waste dumps and open pits. The waste dumps associated with dismissed mining activities produce, during rainfall events, acidic solutions containing potentially toxic elements (As, Cd, Co, Cu, Ni, Pb, Zn) in concentrations that exceed the discharge limits. After a sufficiently long rainy period, the effluents are low in toxic metals, and this may generate the impression that the contamination potential of dumps can be reduced by sufficiently long irrigation strategies (Trois et al., 2007). Moreover, mechanical removal (wind blown or gravitational) and/or rain leaching may contaminate conterminous farming soils and groundwaters (Cidu et al., 2009). Therefore, mitigation actions must be addressed to land reclamation. A natural attenuation of acidity and metal load just occurs upon interaction between minerals and waters in the vadose zone, as ascertained by ongoing monitoring programs (Da Pelo et al., 2009). Tailings confinement to a restricted site should minimize their environmental impact. The remediation of the environmental impact posed by dumps of rocks containing base metals has been successful in a number of mine sites, taking into consideration the rock geochemistry, the dumps geotechnical properties and the climatic conditions. Revegetation of exposed rocks is ongoing at some sites, based on the presence of an active microflora (Trois et al., 2007).