Environmental Impact of Abandoned Mine Waste: a review

Case Studies in Italy

Besides early investigations of the great Italian geologist Bernardino Lotti (1847-1933), previous studies carried out by several working groups on mine sites in Italy ( Zucchetti, 1958; Burtet Fabris and Omenetto, 1971; Corsini et al., 1975; Zuffardi, 1977, 1990; Gianelli and Puxeddu, 1978; Lattanzi and Tanelli, 1981; Cipriani and Tanelli, 1983; Deschamps et al., 1983) pointed primarily at understanding the complex genesis of ore deposits and the possibility of mineral exploitation (Figure 11). More recently, after the closure of mines, attention was focused on the environmental impact of mining operations (Leita and De Nobili, 1988; Benvenuti et al., 1999; 2000; Caboi et al., 1999, Mascaro et al., 2001b; Bini and Gaballo, 2006; Cidu et al., 2009; Fontana et al., 2010), and possible land restoration (Dinelli e Lombini, 1996; Zerbi and Marchiol, 2004; Marchiol et al., 2010; Bini et al, 2010; 2011). In fact, once ore deposits were exploited, environmental problems connected to the discharge and spreading of mine waste on conterminous land and streams became a concern, constituting elsewhere a waste area on the modern landscape. The tailings surface remained uncovered for a long time, until weathering and pedogenic processes enhanced soil formation and revegetation in the mine spoil, producing significant environmental impacts in the whole mining area (Benvenuti et al., 1999).

A soil survey of the abandoned mine areas in Italy has been on-going since the 1990s at various Universities (Cagliari, Florence, Milan, Siena, Udine, Venice) and Research Centres, in the frame of a national research project. The survey was preceded by mapping the distribution of mine spoils discharged at the surface. The rationale for soil sampling was focused on soils developed from mine spoils of different age and in the conterminous areas with soil unaffected by spoil. Several soil pits were opened and profiles (totally more than 200 pit soils) described and sampled in the following type of sites: spoil areas, no spoil proximal areas (spoil <0.5 km), and distal areas (spoil> 0.5 km). At some sites, roastings, flotation tailings and overbank sediments were sampled. The results are summarised in the case studies hereafter.

Figure 11. Regions of Italy. Numbers are abandoned mine sites cited in the study: 1 = Elba Island; 2 = Metalliferous Hills; 3 = Temperino mine; 4 = Bottino, Apuane Alps; 5 = Sardinia, Sulcis-Iglesiente district; 6 = Imperina Creek Valley, Dolomites.

4.1 Tuscany

There is a long history of mining activity for mixed sulphides (mainly Cu, Fe, Pb, Zn) at several sites in Tuscany. Mining dates back at least to Etruscan times (7^th cent. B.C.), flourished under the Romans (1^th cent. B.C.), during the Middle Age-Renaissance (10^th-16^th cent. A.D.), and in the 19^th - 20^th century (Tanelli, 1985).

The southern Tuscany metallogenic province (including the Elba island) is of primary importance due to the occurrence of several ore deposits associated with volcano-sedimentary, magmatic, metamorphic and geothermal environments (Lattanzi et al, 1994; Costagliola et al., 2008). The mining district is characterized by deposits of pyrite and mixed sulphides (Fe, Cu-Pb-Zn-Ag, As, Sb, Hg, Sn and Au).

Among the many types of ore deposits occurring in southern Tuscany, the Fe oxide deposits of Elba island and the pyrite and other base-metal sulphide deposits of the Metalliferous Hills district have been extensively exploited since the 1^st millennium BC under the Etruscans, although metal mining and smelting dates back to the late Bronze age (Cipriani and Tanelli, 1983; Corretti and Benvenuti, 2001).

4.1.1. Elba Island District

Elba Island was one of the most important Italian mining sites, dating back to Roman age, as it is demonstrated by metallurgic findings of Roman period (Costagliola et al., 2008). Iron exploitation in the island ceased in the 1980s (Servida et al., 2009), and a part of the ancient mining area is presently used as an open site, where tourists may search and collect minerals (see Figure 3). More than one hundred mineralogical species have been recorded in the various ore deposits of the island. The main ore bodies of Elba island are located in a narrow belt along the eastern coast of the island, where they occur in variable settings (lodes, veins, pods), differently related to the host rocks (Costagliola, 2008). The primary mineralogy of the ore deposits is composed mainly of Fe-oxide (hematite, limonite and magnetite) in the northern part, whereas iron and base-metal sulphides (pyrite, chalcopyrite ±As, Bi, Pb, Sb, Zn) are more common in the southern part. More than 4.46x10⁶ m³ of material was removed (Servida et al., 2009), and most minerals exported and smelted at smelting centres in Southern Tuscany (Costagliola et al., 2008), particularly during Etruscan and Roman periods.

The accumulation and the exposure to the atmospheric agents of the sulphide-bearing earth materials without adequate management initiated the AMD processes The consequent environmental hazard depends on the mobility of metals, the climatic conditions, the porosity of earth material, etc. (Servida et al., 2009).

The average content of selected heavy metals, pH and texture of earthy material in the vicinity of mine sites at Elba island is reported in Table 5. Bulk elemental composition of mine waste is similar to silicatic bedrock and related soils. Metal amounts are higher in the close vicinity of mines, and decrease with distance from the mine waste. Correspondently, the pH shows an opposite trend, increasing with distance, while texture decreases. It is likely that a geochemical halo forms around the metal hotspot, and a dilution occurs with distance from the mine waste. Iron is the most abundant metal, as expected from the geology of ore deposits, and decreases sharply in distal soils; basic metals, instead, are rather persistent in the examined soils.

Table 5. Average trace elements concentrations, mean values of pH and texture at different sites in the Elba mine area

	Fe	Mn	Cu	Pb	Zn	pH	texture
Mine waste	22	0.2	730	336	919	n.d.	gravel
Mine soil (1995)	18	0.09	230	146	634	5.3	Gravel, sand
Proximal soil (1995)	12	0.07	120	164	451	6.3	Sandy loam
Distal soil (1995)	8	0.07	62	131	401	6.6	Loamy sand

Source: Corsini et al., 1980; Bini et al., 1995; Servida et al., 2009).

Fe, Mn are expressed as %; Cu, Pb, Zn as mgkg^-1.

4.1.2. The Metalliferous Hills District (Massa Marittima)

A number of polymetallic Cu-Pb-Zn-Ag vein deposits, controlled by late Apenninic horst-and -graben structures, have been mined for several millennia in the area around the medieval town of Massa Marittima (Benvenuti et al., 1999). Yet, the Massa Marittima area hosts mineral deposits of variable extent and importance, located at different sites (Costagliola et al., 2008). In the past century exploitation focused mainly on the Cu-Pb-Zn-Ag deposits of Fenice Capanne, and the pyrite deposits of Niccioletta, Boccheggiano and Gavorrano. Minor mineral occurrence was located at Monte Arsenti, where Cu-Zn-Pb-Ag sulphides and sulphosalts were exploited discontinuously in the past 3000 years (Cipriani and Tanelli, 1983).

Mine ores, including those brought out from the Elba island, were smelted at different metallurgical centres in the close vicinity of Massa Marittima, where stream water, wood, and refractory material (clay and sandstone) were easily available (Costagliola et al., 2008).

The techniques for separating ore from rock remained broadly the same throughout a long mining period. At first, smelting took place near the mines but diminishing supplies of wood for burning and roasting led to the development of specialised smelting centres (Davies, 1987). Massa Marittima was one of the most important mining and metallurgical centre where Pb, Cu, (Ag) production was carried out mainly during the Middle Age, in the XII-XIV century (Mascaro et al., 2001), and under the Medicean dynasty (Cipriani and Tanelli, 1983), leaving about 500.000metric tonnes of Fe slags, dismantled and re-used in the last century for reclamation of surrounding land (Costagliola et al., 2008).

4.1.3. Fenice Capanne

The Fenice Capanne sulphide deposit is mainly constituted of two polymetallic vein bodies, linked to the principal tectonic dislocation in the area (Benvenuti et al., 1999). The primary mineral association is characterised by the presence of predominant chalcopyrite (Cu 2.5-11% d.w.) with Zn and Pb sulphides (Zn 4.5-16%; Pb 1-3.5%). The ore body originated from hydrothermal processes associated with the late orogenetic magmatism, which were responsible also for the alteration and replacement of host rocks, with the formation of skarn silicates (mainly pyroxenes, epidotes, ilvaite), pyrite, chalcedony, kaolinite and alunite (Mascaro et al., 2001).

At Fenice Capanne, exploitation and processing of the polymetallic (Zn, Cu, Pb, Fe, Ag) sulphide deposits, initiated during Etruscan times (6^th-7^th century B.C.), closed in the 1980’s, being the last operative mine in Southern Tuscany. The main exploitation occurred in the medieval age and during the 19-20^th centuries. Total production is estimated in the order of some thousand tonnes of Cu, Pb, Zn and some tonnes of Ag (Mascaro et al., 2001a).

The morphology of the area is gently undulating, with maximum elevation around 450 m a.s.l.; vegetation is represented by Mediterranean maquis. Climate is mesothermic subhumid with marked summer deficit (maT = 15°C, maP = 850 mma^-1 ). The river Bruna (20km length) is the main waterway draining the whole area (approx 35 km² ).

The mine waste produced by metal exploitation includes huge dumps of roasting products and excavation waste, mainly dating back to the 19^th century, and flotation tailings produced during the period 1950-1984. The oldest waste were dispersed on the surrounding land, while roasting dumps occupy about 250ha, partially forested.

The roasting piles are produced by the roasting of low grade ore (<4% d.w. Cu). Most excavation and roasting dumps are 15-20 m high and partially reforested. The flotation tailings are mainly discharged into four artificial impoundments filled in the period 1957-1984. Their total capacity is about 850,000m³. The oldest flotation tailings lie over an unconfined area, spatially associated with piles of roastings.

The particle size of waste material is variable: roasting products are composed of coarse sand and gravels; the pH is acidic, in the range 3.3-3.8. Unsaturated water conditions, and the coarse texture of roastings, enhanced sulphide oxidation; therefore, secondary minerals (jarosite, barite, gypsum) formation occurred.

The flotation tailings, on the contrary, are fine-grained (silty-clayey), with slightly acidic to neutral pH (range 6.2- 7.8). Secondary mineralogical phases are Fe-oxyhydroxides, jarosite, gypsum, illite, kaolinite. Water draining flotation tailings 1km downward present acidic pH and high metal and sulphate contents.

The mineralogy of mine waste has been recently studied in detail by Mascaro et al. (2001a). Quartz and hematite, partly derived from roasting processes, are the most abundant primary mineralogical phases (see Jambor, 1994, for definition). Minor contents of feldspars, epidotes and phyllosilicates (muscovite and clay minerals) are present, together with pyrite, traces of chalcopyrite, barite and galena. The secondary phases are mainly Fe oxyhydroxides, jarosite and gypsum. The flotation tailings consist mainly of quartz and feldspar, with minor hedenbergite, phyllosilicates, hematite and pyrite; sphalerite and chalcopyrite are present only in traces, whereas carbonates (dolomite, siderite, calcite) are always nearly absent. The main secondary phases are still Fe oxyhydroxides, jarosite, gypsum and clay minerals. Sulphides, siderite and dolomite are replaced, partially or completely, by Fe oxyhydroxides. Microscopic observations (Mascaro et al., 2001a) show that feldspars are mainly orthoclase and rare adularia, with minor contents of plagioclase; commonly, silica is represented by chalcedony.

All types of mine waste materials show in the fine grained fraction high contents of illite and kaolinite (about 70-100% of total clay minerals). Other clay minerals are chlorite, montmorillonite and illite-montmorillonite interstratified. Kaolinite probably occurs both as primary hydrothermal phase and secondary phase: SEM-EDS observations show the formation of secondary kaolinite from the alteration of primary silicates (in particular K-felspar) in more acidified mine tailings.

Most of the samples have high and variable contents of toxic elements, in particular Pb, Zn and Cu (Table 6). The metal content commonly decreases with decreasing age of waste; proximal and distal soils show nearly the same concentrations, suggesting limited dispersion to occur with distance. Zinc constitutes an exception in this frame. The near neutral pH suggests that the processes of sulphide oxidation and of acid solution buffering balance each other. The acidification and sulphide oxidation observed in the roastings is probably caused by the lack of carbonates and by the large grain size and long residence time.

Soils proximal to mine waste and roastings are scarcely deep and present limited development, with simple A-C horizonation. Texture is sandy skeletal, reaction acidic (pH=3.3-3.8); the ratio C/N=14 suggests humification to be effective in surface horizon; bulk density is low (1.2gcm^-3).

Table 6. Average concentrations of heavy metals, pH and grain size at Fenice Capanne

Sample (age)	Fe %	As	Bi	Cu	Mn	Pb	Zn	pH	Texture
Roastings (ca.1890)	17	927	320	3,450	418	825	2,480	3.5	Gravel, pebble
Tailings (ca.1950)	3.6	515	22	853	424	2,050	2,490	4.2	Sand, silt
Tailings1 (ca.1960)	7.7	230	84	3,400	4,320	1,310	5,470	7.1	Sand, silt
Tailings2 (1970)	5.1	116	30	508	3,520	214	1,560	7.4	Silt, clay
Proximal soil (1999)	5.8	123	28	595	3,700	353	1,720	6.4	Coarse loamy
Distal soil (1999)	6.3	119	27	445	2,900	413	522	6.7	Coarse loamy

Adapted from Mascaro et al., 2001a.
Distal soils too present limited differentiation, but have silty-clayey texture, neutral pH (6.2 – 7.8), C/N ratio = 15, bulk density a little higher (1.4gcm^-3). The mineralogy of the fine fraction is composed of illite, kaolinite, jarosite, gypsum, Fe-Mn oxyhydroxides.

4.1.4. Boccheggiano

The Boccheggiano district was one of the most important mine areas in Italy, and has been widely explored since the early ‘900 (D’Achiardi, 1927).

Mining activity in the Boccheggiano area of southern Tuscany has been documented back to the 16^th century A.D., but likely dates to at least Etruscan times (Benvenuti et al., 1997; 1999). Since than, a number of base metals (Hg, Sb, Fe oxides, pyrite, chalcopyrite) have been intensely exploited in the district, yielding about 1.5x10⁶ tonnes of ore at 4-8% Cu in the last century. Up until about 1910, the main focus was on base-metal (Cu(Pb-Zn-Ag)) ores, but then, from 1906 to 1994, some tens of millions of tonnes of pyrite were produced from several deposits in the area (Tanelli, 1985; Lattanzi et al., 1994).

The geology of the area is composed of Palaeozoic metamorphic basement in strict contact with the Mesozoic Tuscan nappe and Cretaceous-Eocene flysch; the morphology is undulated, with elevation up to 700m a.s.l. and 35-40% slope. Climate is warm Mediterranean (maT=16°C, maP=1020mm); the land use is partly a mixed forest with prevailing oak, and partly arable land; part of the area was afforested with pine (Pinus pinea L.) in the ‘50s.

The Merse river drains the region with mean flow of 7m³sec^-1 and frequent flooding episodes.

Such an extensive and protracted mining activity has left behind many abandoned mines and mine wastes, and huge masses of slags and roastings. The dumps of copper-pyrite excavation waste are 10-15 m high, and extend for about 1.5 km along the Merse river.

Three main types of mine waste have been identified in the study area: waste-rock dumps, a flotation tailings impoundment and roasting-smelting waste (Benvenuti et al., 1997; 1999). Waste rock material of mining activities represents a primary source of pollution for drainage waters, sediments and soils, because of the generally high metal concentrations. Benvenuti et al. (1997, 1999) studied the mineralogical and chemical features of mine tailings sediments, soils and drainage waters, with special focus on the exogenous minerals considered mineral traps for the toxic elements (Cu, Pb, Zn, As, Cd, Bi), and on the dispersion mechanisms and halos of these elements.

Following Jambor suggestions (1994), in the dumps the Authors distinguished three classes of minerals: 1) primary (ore, gangue, and pyrometallurgical phases: sphalerite, galena, pyrite, chalcopyrite, iron oxides, quartz, calcite, micas, chlorite); 2) secondary (minerals formed in situ within the waste disposal area): Fe, Al and Cu sulphates (jarosite, copiapite, alunite, chalcantite), gypsum, Cu carbonates (malachite), Fe oxyhydroxides, and 3) ternary and quaternary minerals, not formed in situ, but developed after sampling and oven-drying at >60°: trace amounts of siderotil, bassanite, metaluminite.

Table 7. Average and range of trace elements concentrations and mean values of pH and texture at different sites in the Boccheggiano mine area

Sample (age)	Fe	As	Bi	Cu	Pb	Zn	pH	Grain size
Mine waste (ca.1889)	12.8 (1.6-19.3)	188 (13-264)	153 (37-339)	589 (529.642)	590 (277-1110)	292 (234-327)	3.1	Gravel, pebble
Mine waste (ca.1910)	13.6 (4.7-24.5)	241 (88-429)	26 (4-80)	754 (36-1790)	3160 (34-28000)	758 (77-1970)	4.2	Gravel, pebble
Mine waste (ca.1950)	10.4 (9.4-11.3)	737 (55-1000)	29 (14-40)	196 (29-468)	304 (106-956)	392 (40-1300)	4.3	Pebble, sand
Tailings (1970)	9.4 (5.2-15.2)	233 (74-887)	13 (3-44)	203 (71-377)	2145 (449-4920)	4980 (154-9900)	5.5	Silt, clay
Proximal soil (1995)	10.1 (1.6-15.9)	640 (50-900)	17 (2-29)	81 (35-800)	402 (110-956)	312 (97-446)	4.8	Coarse loamy
Distal soil (1995)	4.2 (1.6-5.8)	55 (40-350)	10 (1-14)	59 (29-206)	81(50-890)	176 (40-331)	6.7	Coarse loamy

Fe is expressed as %; As, Bi, Cu, Pb, Zn as mgkg^-1.

Adapted after Benvenuti et al., 1999.

Waste samples show extremely variable amounts of metals, and this feature may be ascribed to metal incorporation by solid solution or adsorption mechanisms into “mineral traps” (Jambor, 1994) as primary minerals (e.g. clay minerals), organic materials (amorphous colloids) and/or secondary minerals (e.g. jarosite, iron oxyhydroxides). The highest metal concentrations occur close to the wastes and rapidly decrease moving downstream some hundred of meters. The bulk analyses of the waste samples revealed high concentrations of heavy metals and typically low pH (Table 7). As and Pb are dominant in waste rock dumps (range: As = 55-1000 ppm; Pb = 30-27600 ppm) ; Zn and Pb in the flotation tailings (range: Zn = 150-9900 ppm; Pb = 450-4920); Cu and Bi in the roasting waste (range: Cu = 250-2280 ppm; Bi = 10-885 ppm). The investigated waste materials appear alterate (metal-depleted, acidified), and this could be ascribed to several factors: 1) primary mineralogical composition, and particularly the amount of metal sulphides and of pH-buffering phases (carbonates, chlorites and micas), 2) the age of mine wastes; 3) waste bodies morphology and grain size; 4) hydrological and chemical features of drainage waters.

The flotation impoundment contains tailings from the processing of pyrite ore during the period from 1957 to 1972. The impoundment is about 100x300m wide, with a maximum depth of about 10m, and dips gently to the north. There is a gradation from silt and sand in the south of the basin to mud and clay in the north. The northern zone is often flooded in the wet season and alteration is less pronounced in the north than in the south, presumably because oxidation of the waste is impeded by flooding (Benvenuti et al., 1997). Samples were taken at several sites, from the surface to depths of 40-70 cm. Sulphides and aluminosilicates are more abundant than at other localities, and the alteration sequence of the sulphides is apparently:

pyrrhotite>chalcopyrite=galena>arsenopyrite=sphalerite>pyrite.
The high susceptibility of chalcopyrite and galena to weathering is somewhat surprising. Jambor (1994) points out that the apparent resistance of chalcopyrite in several tailings impoundments may be due to its occurrence as locked inclusions in silicates and quartz. SEM/EDS analyses show that galena is usually corroded, with rare replacement by anglesite, and the scarcity of anglesite coatings may enhance the process of galena dissolution (Tanelli and Lattanzi, 1986). The comparative resistance of sphalerite to weathering may be due to a low Fe content; SEM/EDS analyses of sphalerite from tailings are consistent with microprobe analyses that report 1-9 mol% FeS (Tanelli and Lattanzi, 1986).

The uppermost sediments in the southern zone, down to about 20 cm depth, are sandy, with quite low pH (up to 5). Therefore, alteration of sulphides and alumosilicates is very advanced, and the main phases are quartz, iron-hoxyhydroxides, gypsum and minor jarosite.

Soils

The anomalous elemental concentrations in the solid fraction is not restricted to the dump proximity. High contents of polluting elements (As, Bi, Cd, Cu, Ni, Pb, Z,n) in soils collected in the Merse river alluvial plain were recorded as far as 30 km moving downstream from waste areas.

Soils developed on, or proximal to, waste piles (“waste soils”) were compared to natural (“distal”) soils in the area. Generally, the soil evolution from waste is very limited, owing to the nature of parent material, the coarse grain size and the hydrological system. Waste soils show a shallow A-C profile (<60 cm), dark brown (7,5YR 3/2), subangular blocky structure; texture is coarse loamy. Natural soils are more deep (up to 100cm) and developed, with a marked discontinuity between topsoil and subsoil (A-2Bt-2C profile), where an illuvial (Bt horizon) formed. Colour ranges from dark brown (7,5YR 3/4) in the top, and dark reddish brown (5YR 3/4) in the bottom; structure varies from crumby to blocky, texture from loamy sand to clay loam; pH ranges from subacidic to subalkaline in the bottom, owing to the presence of calcareous fragments from the original limestone bedrock.

Compared with natural soils, waste-proximal soils (<50m) are very acidic, having a pH (2.5-4.8) lower than the former (3.5-7.5) and may include minerals from the ore bodies (pyrite, chalcopyrite), from ore processing (hematite), and from the weathering of these minerals (goethite, jarosite, alunite, copiapite, melanterite, anglesite and others). Distal soils (>50m from waste) are characterized by primary phases such as quartz, muscovite, chlorites, calcite, dolomite with minor amounts of kaolinite, illite, smectite rutile, ilmenite, zircon, monazite, derived from local bedrock weathering (phyllites, quartzites, limestone and flysch formation). Accordingly with the different mineralogy, the heavy-element content (especially Pb, Bi, As) in waste soils is appreciably higher than in natural soils (see Table 7), their average being higher than the concentrations considered dangerous or toxic for plants (Kabata-Pendias, 2004). Natural soils, either in close proximity to waste or downslope from them, are contaminated and acidified, almost in the topsoil. The contamination is probably caused by mechanical transport of primary and secondary minerals, including pyrite, hematite, sphalerite and jarosite, from the waste. Since only a part of the soil metal content is available for plants (Adriano, 2001), in order to evaluate the environmental (vegetation) hazard of the investigated mine sites, bio-available (EDTA-extractable) elements were determined in soil samples, assuming extractable metal concentrations to be directly correlated with the amount of metals taken-up by plants. The amounts of EDTA-extractable metals are rather high (Cu = 0.3-22 ppm; Pb = 0.7-380 ppm; Zn = 0.1-860; Mn = 0.2-230; Fe = 4-1300 ppm), and exceed the limits usually considered toxic for plants (Kabata-Pendias and Pendias, 2001). In contrast, natural soils far from waste (>50m) contain only minerals such as quartz, muscovite, chlorite, calcite and dolomite, derived from the main lithologies outcropping in the area. Their metal content (total and EDTA-extracted) are below the limits of pollution. The soil metal contents commonly decrease with increasing depth, probably due to intense leaching and a longer residence time, and is likely controlled by morphology and grain size, that enhance water circulation. Yet, waste soils are typically water-unsaturated, and have a relatively high hydraulic conductivity. Aeolian transport and gravitational runoff, instead, are limited to the immediate vicinity of the mine sources.

In summary, several processes occur at the investigated sites in the Boccheggiano mine district:

• 
  Sulphide oxidation and acid mine drainage production;
  
• 
  Decreasing acidification with distance from the mined area;
  
• 
  Metal content decrease with distance (dispersion halo);
  

• 
  Argillogenesis and clay migration;
  
• 
  Sulfatation and secondary minerals formation;
  
• 
  Runoff and leaching to adjacent streams, with consequent dilution.
  

4.1.5. Campiglia Marittima - Temperino Mine District

The Campiglia Marittima ore district has long been known for Cu-Pb-Zn skarn deposits enclosed within white marbles derived from contact metamorphism of Mesozoic limestone (Bertolani, 1958). These deposits lie 1-2 km E and NE of the Botro ai Marmi granitic stock (K/Ar age 5.7Ma), in strict spatial association with nearly coeval (4-5My) porphyry dikes (Corsini et al., 1980). Mining activity in the area dates back at least to Etruscan times (VII century BC), and flourished under the Romans (I century B.C.), in the Middle Age-Renaissance (X-XVI century A.D.), and in the XIX-XX centuries, until final closure in 1976. The mining district is characterized by deposits of pyrite and mixed sulphides (Fe, Cu-Pb-Zn-Ag, Sb, Hg, Sn) and Au (Cipriani and Tanelli, 1983; Tanelli, 1985; Lattanzi et al., 1994 and references therein). In particular, two main styles of pyrite and polymetallic deposits have been identified (Tanelli, 1985): massive conformable bodies related to Palaeozoic-Triassic siliciclastic lithologies, and structurally controlled deposits associated with tectonic features of the Tertiary Apenninic orogeny or with Miocene-Pliocene magmatic rocks.

The morphology of the mined area is gently undulated, with elevation ranging from 150m to 450m a.s.l., warm Mediterranean climate (maT=16°C, maP=700mm). The vegetation climax is the Quercetum ilicis, and the present vegetation cover is a dense, deciduous forest (Mediterranean maquis) dominated by holm-oak. At some places, corresponding to mine spoil outcrops or to more exposed slopes, the forest is substituted by a shrubby formation (the so called garigue) dominated by rock-rose, or by discontinuous coverage with native grasses, especially fescue. Vegetation cover in the mineralized area is discontinuous, and, besides crusting lichens, hosts metal tolerant/accumulator plant species (Baker, 1981), as Cistus salvifolius, Inula viscosa, Silene paradoxa, Silene armeria, Festuca inops.

Waste rocks resulting from surface and underground mine working in the last two centuries were discharged in close proximity to the mine, and presently constitute a waste dump covering an area of about 0.1 km² (Corsini et al., 1980; Baldini et al., 2001). The potential of the abandoned waste dumps to pollute the environment at these localities is enhanced to various degrees by the high topographic relief, the lack of vegetation cover and the proximity of the waste to streams.

The prevailing mineralogical phases in waste samples are quartz, ilvaite, hedembergite and pyrite, with smaller amounts of carbonates, Fe-Cu oxyhydroxides and chrysocolla. Until now, pH conditions (average value 6.4, range 5.7-7.0) have slowed the alteration processes of minerals, and favoured the absorption phenomena of leached metals onto oxyhydroxides surface.

A prolonged and continuous disposal of metals over time, however, may have important consequences for the environment and the plants life (Bini and Gaballo, 2006). For this reason, the mineralogy and geochemistry of both the waste material and the soils developed from, were investigated (Bini and Gaballo, 2006) in order to determine the extent of heavy metal dispersion in the conterminous land, and the related environmental hazard.

Three kinds of materials were sampled and analysed at selected sites in abandoned mine areas:

• 
  Waste-rock dumps
  
• 
  Soils (50 profiles, both in mineralized areas and outside)
  
• 
  Vegetation (selected plants, in spring and autumn)
  

Most dumps consist of coarse-grained waste rock from excavation; tailings from mineral processing also occur. The soils in the mineralized area, because of the generally steep morphology, are not very thick (mostly up to 40-50 cm, or up to 1m in terraced areas), neither very developed. They usually show coarse-grained textures with abundant lithic fragments, and are characterized by high permeability. Pedogenesis of waste dumps is normally minor, and mostly confined at the peripheral portions. Vegetation is herbaceous on the most recent mine wastes, while shrubby and arboreal plants colonize the older ones and the conterminous areas.

Waste-rock Dumps

The primary mineralogy is composed of pyrite and chalcopyrite, ilvaite, hedembergite, sphalerite, Fe-oxides, quartz, muscovite and chlorite, with minor and variable amounts of secondary minerals (especially carbonates, clay minerals, jarosite and Fe-oxyhydroxides). Pyrite is commonly rimmed by Fe-oxyhydroxides, whereas dissolution features prevail in chalcopyrite. The particle size of the sulphides is variable (up to a few millimetres).

The prevailing mineralogical phases in waste samples are quartz, ilvaite, hedenbergite and pyrite, with smaller amounts of carbonates (calcite, dolomite and smithsonite), Fe-Cu oxyhydroxides and chrysocolla.

Waste dump materials contain relatively high amounts of toxic metals (average values, in wt%: Cu= 1.3, Pb= 0.2, Zn=1, As=0.01, Bi=0.02) exceeding the maximum permitted limits according to Italian legislation (D.M. 152/2006). A relevant part of these metals may be transferred to conterminous soils by chemical alteration, runoff and wind transport, thus determining a potential concern to the environment.

Soils

The soils of the mineralized areas show a noteworthy spatial variability, as evidenced by a different degree of evolution. Entisols (Lithic, Typic, Spolic Xerorthents) are common on recent mine dumps (<100y), while Inceptisols (Haploxerepts and Dystroxerepts) and Alfisols (Haploxeralfs and Rhodoxeralfs) are frequent on more ancient dumps or in the conterminous areas, where a lithological discontinuity occurs between the “primary” parent material (limestone, shale, metamorphic or magmatic rocks) and the “secondary” mine waste (polycyclic soils, chronosequences). Immature Entisols are characterized by a thin solum (<30cm), little organic matter accumulation (mean 14g/kg organic carbon, range 1-33 g/kg), dark brown (10YR3/3) to reddish (5YR4/6) colour, coarse texture (sandy loam to loamy), and subalkaline pH (mean 7.4, range 6.9-7.8). An environmental consequence of the high content of toxic heavy metals (see Table 8 and Figure 8), in combination with reduced soil thickness, is a discontinuous vegetation coverage.

The soils developed from old mine dumps, or in the proximal parts (<0.5 km) of the dumps, are characterized by a solum >50 cm thick, sandy loam to loam texture, blocky structure, slightly acidic pH (mean value 6.3, range 4.9-7.7), humus accumulation (up to 14% organic matter in the A horizon), moderate to low cation exchange capacity (mean 20 cmol(+)/kg), with significant desaturation (base saturation <60%). Generally, they have distinct A-B-C horizonation and a well formed cambic horizon. Therefore, they are Inceptisols (Spolic Haploxerepts or Spolic Dystroxerepts, see Table 8 and Figure 9). Data (not reported) indicate relevant differences and a remarkable polycyclic evolution, owing to the superposition of mine spoil over the normal soil. Colour, texture, reaction, and cation exchange capacity are the most prominent features that present major differences. Soil horizons show dark brown (7.5YR3/2) to dark reddish brown (5YR 3/3) colour, well individualized structure, from crumby to fine blocky peds. Texture is coarse (sandy loam to loam) in surface horizons sampled on mine spoils and loamy to clayey underneath. Soil reaction is slightly acidic (pH 6.3) at the surface, subalkaline (pH 7.4) and base-saturated at the bottom. Cation exchange capacity increases with depth, from 15 to 25 cmol(+)/kg.

Soils described and sampled at major distance (>0.5km) from the mine dumps present little evidence of mine spoil in the profile. Sulphide minerals are found especially at the surface, as revealed by mineralogical and chemical composition (see Table 8 and Figure 10). An abrupt textural change indicates a marked discontinuity between the upper and lower part of the soil profile. The upper part (A and E horizons) has dark brown (10YR2/2) to yellowish brown (7.5YR3/4) colours, loam to sandy loam texture, crumby structure, high organic carbon content (mean 35 g/kg, range 21-57), and subalkaline pH. The lower part (Bt horizon) presents reddish colours (5YR3/4 to 2.5YR3/4), clay content increase, with clay loam to clay texture, organic carbon and pH decrease (mean 6.4, range 5.1-7.9), and carbonate is absent. These features are consistent with soil development from limestone in the Mediterranean environment (the so called “Terra rossa”). Therefore, they are classified as Alfisols (Spolic Rhodoxeralfs or Spolic Haploxeralfs) or, alternatively, as Spolic Xerorthent over Typic Rhodoxeralf (or Haploxeralf).

Soil Mineralogy and Geochemistry

Waste soils are characterized by high contents of primary and secondary metalliferous phases (sulphides, skarn-silicates, oxyhydroxides, carbonates, sulphates, Fe-Cu oxyhydroxides). Phyllosilicates are present in limited amounts in pedogenic horizons. Chlorite and mica may form from skarn silicates and mine spoil as well, while illite is likely to be inherited from the terra rossa. Moreover, aeolian dust could have contributed to soil mineralogy, especially quartz and phyllosilicates, as reported by Bini et al. (2006) for similar soils.

The bulk chemical composition of waste soils indicates (Table 8) high amounts (up to 73%) of silica and alumina (15%) in A horizons of Entisols. Total Fe, Mg and Mn, instead, present higher amounts in C horizon (14.70%, 3.36%, and 4.36%, respectively) in comparison to the A horizons. Sodium and K decrease with depth, while Ca increases (6.47%). Titanium concentration is rather constant, as expected for a very stable element. Heavy metal concentrations are higher in the C horizon than at the surface, being related to the original composition of the spoil material.

Lesser amounts of Si, Al, Mg in surface horizons, in comparison to subsurface horizons, are recorded in Inceptisol (see Table 8). Instead, Fe, Mn, and Ca occur in higher concentrations at the surface than at the bottom. Titanium is rather constant, as in the previous Entisol. Extremely high concentrations of heavy metals, contributed by spoil material, are recorded in the whole profile; their distribution with depth corresponds to soil discontinuities.

Mine soils over Alfisols present (see Table 8) a general increase in Si and Al, and a decrease in Ca and Mg, with depth, while Fe, Mn, K, and Ti are quite constant. Heavy metal concentrations are high, and are distributed irregularly, as a consequence of differential contribution from spoil parent material.

As already mentioned, most soil pH values fall within one unity from neutrality (range 4.9-7.4, average value 6.4). Until now, these pH conditions have slowed the alteration processes of minerals (as confirmed by minor alteration of sulphidic phases), and favoured the adsorption phenomena of leached metals onto oxyhydroxides surfaces. Therefore, the solid phase of waste soils is strongly enriched in metals, whose levels overcome the target values of current Italian legislation (D.M.152/2006). Their concentration, however, depends on the distance from mining areas and on the age of waste.

The results of leaching tests (Baldini et al., 2001) indicate a low degree of exchangeability (i.e., bioavailability) of all the metals, with Zn>Fe>Cu=Mn. (average values, in % of total metal contents: Zn=3; Fe=0.5; Cu=Mn=0.3). Since the bioavailable metal fraction is quite low, phytotoxicity is quite unlikely; yet, it is noteworthy to consider that the absolute metal content of soils cannot lead to progressive metal enrichment in plants. Nevertheless, a prolonged and continuous disposal of metals on the land may have important consequences for plants’ life as well.

4.3. Apuane Alps

A set of papers (Mascaro et al., 1999; 2000; Benvenuti et al., 1999; 2000; 2001) has been addressed to evaluate the environmental impact determined by past mine activity and metal production at the Bottino mine, Apuane Alps. The evaluation need is corroborated by the fact that the investigated area is located within the Natural Park of Apuane Alps, a protected area with high natural interest.

Table 8. Bulk chemical composition of selected soil profiles (<2 mm fraction; major element concentrations are expressed in weight %, trace elements in mg/kg)

Soil Profile (SSS, 1999)	Soil Horizon	Na2O	MgO	Al2O3	SiO2	P2O5	K2O	CaO	TiO2	MnO	Fe2O3	Cu	Pb	Zn
Spolic	A1	1.10	1.45	12.53	72.64	0.06	2.07	0.63	0.63	0.19	6.66	170	78	260
Xerorthent	A2	0.90	1.66	15.23	66.35	0.06	2.39	0.62	0.69	0.24	8.15	260	24	120
Waste soil	2C	0.43	3.36	12.70	41.54	0.40	1.70	6.47	0.73	4.36	14.70	1000	840	2000
Spolic	A1	0.49	2.04	6.60	43.57	0.05	1.23	3.32	0.21	3.91	33.81	9665	9200	15000
Dystroxerept	A2	0.18	1.39	3.87	47.35	0.04	0.69	4.55	0.12	3.10	44.46	14100	4100	5840
	Bw	0.62	2.99	10.52	37.53	0.07	1.76	1.73	0.41	3.16	23.78	3790	11900	19800
Proximal soil	2Bwb	0.19	1.46	13.72	37.17	0.03	0.66	6.22	0.21	3.92	41.33	14900	3770	6240
	2Bwb’	0.21	3.09	13.75	64.41	0.08	2.54	4.73	0.54	1.30	10.61	7310	1100	2640
	3BCb	0.18	3.73	16.62	65.94	0.04	2.68	0.50	0.56	0.89	6.38	3700	340	2400
	3Cb	0.18	7.35	22.16	49.14	0.05	3.04	0.69	0.70	2.83	9.61	9230	320	4300
Spolic	A	0.02	4.26	15.15	45.95	0.16	3.05	4.75	0.79	1.50	10.38	420	420	920
Xerorthent	E	0.21	4.55	15.72	60.29	0.06	2.84	0.89	0.73	1.03	9.83	280	460	620
Over	EB	0.22	3.86	16.42	58.84	0.07	2.95	0.90	0.71	1.18	10.04	590	1500	1260
Rhodoxeralf	2Bt1	0.35	2.69	18.47	60.35	0.06	3.34	0.65	0.81	0.95	9.21	390	470	780
Distal soil	2Bt2	0.31	2.45	24.33	52.82	0.05	3.21	0.64	0.77	1.04	10.91	140	330	510
	3C	0.05	11.52	22.67	45.02	0.04	3.72	0.89	1.00	0.57	9.98	16	65	540

Table 9. Average and range of trace elements concentrations, mean values of pH and texture at different sites in the Bottino mine area. (Fe, Mn, S are expressed as %; As, Cd, Cu, Pb, Sb, Zn as mgkg^-1).

	Fe	Mn	Pb	Zn	S	As	Cd	Cu	Sb	pH	texture
Mine waste (1593)	9.8 (6.9-12.5)	0.6 (0.3-0.9)	0.5 (0.05-1	0.3 (0.3-0.4)	0.7 (0.5-0.8)	86 (3-174)	30 (24-38)	442 (272-568)	110 (3-170)	6.8 (6.5-7.3)	Gravel, pebble
Mine waste (1840)	7.6 (4.1-10.4)	0.4 (0.05-0.7)	0.6 (0.06-3.9)	0.6 (0.01-3.2)	1.0 0.2-4.9)	75 (3-150)	75 (5-485)	177 (40-430)	82 (18-260)	6.8 (3.5-7.9)	Gravel, pebble
Mine waste (1929)	7.8 (6.8-9.5)	0.3 (0.1-0.6)	0.8 (0.2-2.0)	0.7 (0.1-2.5)	1.0 (0.4-2.7)	266 (95-576)	45 (5-140)	384 (133-983)	224 (41-680)	6.2 (4.7-6.6)	Gravel, pebble
Mine soil (1995)	5.7 (7.2-8.7)	0.3 (0.3-0.4)	0.7 (0.2-2.4)	0.9 (0.3-1.3)	1.5 (0.4-3.1)	470 (100-760)	60 (24-90)	188 (70-320)	700 (110-1750)	5.7 (5.3-6.1)	Sandy skeletal
Proximal soil (1995)	5.6 (5.0-6.6)	0.1 (0.07-0.1)	0.05 (0.02-0.1)	0.2 (0.1-0.2	0.04 (0.01-0.2)	112 (90-130)	6 (5-9)	52 (35-70)	16 (7-28)	5.1 (4.9-5.3)	Sandy loam
Distal soil (1995)	4.9 (3.0-5.8)	0.02 (0.01-0.03)	0.03 (0.02-0.03)	0.2 (0.2-0.3)	0.03 (0.01-0.03)	70 (50-80)	7 (5-11)	23 (6-33)	7 (5-8)	4.6 (4.0-5.0)	Loamy sand

Adapted after Mascaro et al. (2000; 2001b).

The ore deposits in the Apuane Alps district, northern Tuscany, has been exploited more recently than in Southern Tuscany. The Pb(Zn)-Ag vein deposit at Bottino was mined in the 16^th and 19^th centuries, with recorded production totalling 4000 t Pb, 600 t Zn and 22 t Ag. The quartz-carbonate-sulphide veins are hosted by metamorphosed phyllites and acidic volcanites of the Palaeozoic basement (Benvenuti et al., 1999). The ore bodies lie typically next to the Bottino creek, over steep slopes (about 40°) at high elevation (500-700m asl). The vegetation cover is discontinuous and is composed of mixed forest (prevalently oak); terraced slopes cultivated with chestnut occur at distal sites. Climate is humid temperate (maT=14°C, maP=1800mm). Soil development over the dumps is moderate (<40cm), and mostly confined at the borders. Both proximal and distal soils present more developed horizonation, with total thickness >100cm.

Waste piles in the area include excavation waste and tailings from hand-picking and jigging. Five dumps were investigated; they are elongated heaps that extend some tens of metres downslope, with widths of about 10 m and a maximum thickness of 2-4 m. The material is coarse grained (pebbles and sand).

The moderate thickness, the coarse-particle size and the generally steep slopes suggest that most, if not all, waste bodies are within the vadose zone (Benvenuti et al., 2000). As very little action was undertaken to minimize the environmental impact during and after exploitation, many waste piles are left scattered over the land, and drainage from mining tunnels freely runs into the stream network. Waste bodies cover an area of approximately 5300 square meters, with thickness of 2-5 m. The total volume of waste material is estimated between 20.000 and 50.000 cubic meters.

The mineralogy and chemistry of the analysed surface material are highly heterogeneous, and heavy-metal concentrations are typically high. Weathering processes are more pronounced in both the older and finer material than in the more recent and coarser one.

Primary minerals (i.e. minerals originally present in the mineralization and/or in the country rocks, irrespective of their origin; see Jambor, 1994 for definition) are much more abundant than secondary minerals. Among primary mineralogical phases, quartz and white mica are ubiquitous, and quite abundant. Other common silicates include chlorite, albite, and tourmaline. Carbonates are mainly represented by terms of the siderite-magnesite and dolomite-ankerite solid solutions; calcite is comparatively rare. The most abundant sulphides are sphalerite, galena, and pyrite; chalcopyrite, pyrrhotite, arsenopyrite, Ag-bearing tetrahedrite and boulangerite also occur. Clay minerals occur as primary (illite and chlorite) and as secondary phases (kaolinte, montmorillonite, vermiculite).

The main secondary minerals (i.e. secondary phases that developed in situ by supergene processes) are goethite, lepidocrocite, pyrolusite and cerussite. Yellow rusty crusts are present in some samples. Other phases occurring in small amounts that preclude conclusive identification, include poorly crystalline or amorphous Fe-Mn-Al-hydroxides, containing in places appreciable amounts of other metals (up to 20%Zn, 25% Pb, 7% Cu, 2% Ni, !% Co). As suggested by Benvenuti et al. (2000, 2001) and by Mascaro et al. (2000), these may result from:
a) submicroscopic admixtures of separate phases;

b) isomorphogenous substitution in the lattice;

c) surface adsorption;

d) any combination of the three possibilities.

Minor minerals are ferrihydrite, pyrochroite, cuprite, malachite, Fe-sulfates, unidentified Fe and Pb sulfarsenate, amorphous or cryptocrystalline material, and native gold. The gold is closely associated with Fe and Mn-oxyhydroxides and its texture suggests that it is the result of secondary redistribution (Porto and Hale, 1995). A comparison of mineralogical data and the pH measured in the waste material indicates that the alteration occurs under nearly neutral pH conditions. Yet, most pH values fall within one unity from neutrality (see Table 9).

The relative alterability of sulphides is consistent with what is commonly observed in sulphide mine waste elsewhere (Jambor, 1994), and follows the sequence:

pyrrhotite > galena-sphalerite>chalcopyrite-arsenopyrite-pyrite.
Oxidation of pyrrhotite generates Fe-oxyhydroxides, and galena forms cerussite, while sphalerite presents more pronounced dissolution, accompanied by rims of Fe(Mn) oxyhydroxides (Benvenuti et al., 2000). The main reactions controlling the generation of acidity, which in turn promotes further metal release from primary sulphides, are oxidation of pyrite and dissolution of carbonates.

The potential threat to the environment represented by the high metal contents of this mine waste is mitigated by their entrapment in comparatively stable phases such as goethite, lepidocrocite, pyrolusite, and cerussite, whereas entrapment in metastable or easily soluble phases such as ferrihydrite and Fe-sulphates is obviously ephemeral.

In the Bottino mining district, the abandoned waste piles contain variable amounts of heavy metals that are of potential concern for the environment. The observed neutral pH values favour a relatively efficient fixation of heavy metals in stable phases (e.g. Fe-Mn oxyhydroxides and cerussite), in such conditions. Supergene alteration of primary minerals in the dumps may occur in two ways:
in situ pseudomorphic replacement of secondary phases (e.g. galena/cerussite):

PbS + CO₃²⁺ + 2O₂ → PbCO₃↓ + SO₄^2-, and/or

leaching and dissolution. The elements that are released may eventually reprecipitate as secondary phases (e.g. galena/anglesite):

PbS + 2O₂ → Pb²⁺ + SO₄^2- = PbSO₄↓

The most relevant results of the mineral chemistry studied by SEM/EDS semiquantitative analysis (Benvenuti et al., 2000) are:
Fe and Mn-oxyhydroxides contain appreciable amounts of Co, Cu, Ni, Sb and S;

Cerussite may contain traces of Zn;

Some Fe-sulphates contain variable amounts of Ag, Cu, Pb, Sn, and Zn;

Fe-Pb sulfarsenates do not contain other metals;

The amorphous and cryptocrystalline substances contain appreciable amounts of Cu, Pb, Sb, and Zn.
The presence of large amounts of secondary minerals suggests that the waste materials are slowly approaching a mature stage of supergene alteration. However, given the low amounts of both sulphides and calcite in the area, and the nearly-neutral pH, acidification processes are quite limited, and therefore the alteration phenomena occurred, until now, with no significant environmental pollution.

The metal contents of soils developed from waste material, and those proximal and distal from the mine dumps, have been investigated by Mascaro et al. (2000; 2001b), together with wild plants growing at the same sites. The results concerning waste and soils are summarized in Table 9.

Mine waste typically are enriched in metals with respect to waste soils. Metal contents generally decrease with the age of the mine spoil, and with distance from the metal source, as observed also by Bini and Gaballo (2006). Yet, soil evolution with time, and the distance from the mine spoil, greatly contributes to the dilution effect: pedogenic processes such as acidification, humification, leaching, mobilize metals in different forms (soluble, chelate, or adsorbed), so that they may be leached away from the soil system to groundwaters.

Waste soils (Lithic Udorthents in the USDA soil taxonomy, 1999) show little thickness (<50cm), coarse-grained texture and abundant lithic fragments, with high permeability; profile evolution is limited, with A-C horizonation, and their characteristics reflect those of the parent material. Proximal and distal soils, instead, are more developed, with clear ABwC horizonation (i.e. they are Inceptisols: Dystric Eutrudepts or Umbric Dystrudepts in the USDA soil taxonomy, 1999). Colour change, from gray in entisols to reddish brown in inceptisols, is the most evident character; texture is finer in inceptisols than in entisols; organic matter content is quite low (<1 % O.C.), and scarcely humified; the most common humus feature is the moder with subacid reaction. The acidic pH could be the main cause of metal release from the solid phase. However, the metal content in soils is likely related mainly to mechanical transport (runoff, aeolian, gravitative) of metal-bearing material from mine waste, than to in situ transformation of primary phases.

Heavy metal and sulphur average contents in both waste and proximal soils usually exceed the maximum permitted values for farming soils of the Italian legislation (DM 152/2006); distal soils, instead, present metal contents below such limits, except for As. The almost acidic nature of soils is likely due to the parent material and to the presence of organic acids produced by the forest ecosystem, and the scarcity of acidity buffers such as calcite and dolomite. Extraction tests show that the metal contents of exchangeable fraction do not exceed 1-3% of the total concentrations (Mascaro et al., 2001b); on the contrary, in agreement with mineralogical data, metals bound to sulphide (± organic matter) fraction is comparatively high, ranging from 20% (Mn) to 80% (Pb). The carbonate-bound fraction is commonly 10-20% of total, except for Mn (54%), probably “trapped” as carbonate. The percentage of Mn, Fe and Pb in the reducible fraction (i.e. that due to the dissolution of Fe-Mn-(Pb) oxyhydroxides) are nearly 10% of the total metal concentrations; lower percentages have been calculated for Cu and Zn (1-7%). The relative high percentages of Fe, Zn and Cu in the residual fraction (25-35%) may be explained by the occurrence of chlorite and sulphides encapsulated within quartz grains.

The occurrence of high metal contents in some plants can be accounted for by continuous, although limited, supply of metals from the contaminated material. The reduced thickness, coarse particle size and high metal content of mine waste hinders the growth of arboreal vegetation at the top of waste piles. The same factors are also responsible for the reduced pedogenesis of the waste material.