Environmental Impact of Abandoned Mine Waste: a review

3.1. Mine Soils

Mining activity, although discontinuous in space and time, determined the dispersal pattern of mine spoils. These anthropogenic deposits were irregularly spread over the land surface, giving origin to a new (soil) parent material, completely different from the one the original soils had developed from. Immature Entisols (Lithic Spolic Xerorthents and Spolic Xerorthents as proposed by USDA Soil Taxonomy,1999, and ICOMANTH; Figure 8) are formed in mine spoil <100 y of age. These soils 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). A consequence of the high content of toxic heavy metals, in combination with reduced soil thickness, leads to discontinuous vegetation coverage that is composed mainly of crust lichens, mosses and fescue. Detailed descriptions of soil properties of a selected profile on mine waste are given in Table 2.

Figure 8. Soil profile of a Spolic Xerorthent developed from mine spoils at Temperino, Tuscany. (Photo Bini).

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 3 and Figure 9). Frequently, a discontinuity occurs between the upper part and the lower part of the profile, which developed from the underlying bedrock. 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.

Table 3. Selected properties of the Spolic Dystroxerept illustrated in Figure 9

Soil Horizon	Depth	Particle size (USDA) %			pH	Total Carbonates	Organic Carbon	Organic Matter	CEC	Exch. Acid.	Base Saturation
	cm	silt	clay	sand		g/kg	g/kg	g/kg	cmol (+)/ kg	cmol (+)/ kg	%
A1	0-47	34.7	15.3	50	6.4	0	27	46	25.5	15.6	62
A2	47-70	33.3	15.1	51.6	6.3	0	21	36	15.4	17.4	47
Bw	70-90	30.8	11.1	58.1	6.2	0	8	13	13.0	9.3	58

Figure 9. Soil profile of a Spolic Dystroxerept developed from mine spoils at Temperino, Tuscany. (Photo Bini).

Figure 10. Soil profile of a Spolic Haploxeralf developed from mine spoils at Temperino, Tuscany. (Photo Bini).

Table 4. Selected properties of the Spolic Haploxeralf
illustrated in Figure 10

hori-zon	depth	Particle size USDA			pH	Carbonate	O. C.	Organic Matter	CEC
	cm	silt	clay	sand		g/kg	g/kg	g/kg	cmol(+)/ Kg
A1	0-3	23,9	9,5	66,6	7,6	0	23	40	29,5
A2	3-15	29,8	13,7	56,5	7,7	0	19	33	37
E	15-40	35,7	9,9	54,4	7,9	0	6	11	33
Bt1	40-110	40,7	40,3	19	8,0	4,1	7	12	19
Bt2	110-120	39,9	30	30,7	8,0	58	11	19	23

Shrubby vegetation with shallow trees (holm-oak, strawberry tree, heath, etc.) is the typical vegetation cover at these sites, where rock-rose is the dominant plant.

Soils described and sampled at major distance (>0.5km) from the mine dumps present little evidence of mine spoil in the profile. Sulphidic minerals are found especially at the surface, as revealed by mineralogical and chemical composition (Baldini et al., 2001). An abrupt textural change (Table 4) 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 21 g/kg), and subalkaline pH. The lower part (Bt horizon) presents reddish colours (5YR3/4 to 2.5YR3/4), a strong clay content increase (clay loam to clayey texture), organic carbon decreases (as expected), pH is subalkaline with traces of carbonate. These features are consistent with soil development from limestone in the Mediterranean environment. Therefore, they are classified as Alfisols in the USDA Soil Taxonomy (1999). Since there is evidence for mine waste in the profile, these soils should be classified as Spolic Rhodoxeralfs or Spolic Haploxeralfs (Figure 10). However, considering the net discontinuity already mentioned, these soils could be classified as Spolic Xerorthent over Typic Rhodoxeralf (or Haploxeralf).

Chapter 4