Programa e Resumos - I Congresso Ibérico de Ciência do Solo 2004

I Congresso Ibérico da Ciência do Solo – 15 a 18 de Junho de 2004, Bragança, Portugal
Geochemistry of soils on the influence of some abandoned mines of the
Trás-os-Montes region (Portugal).
Paula Ávila 1) & J.M. Santos Oliveira
1) Instituto Geológico e Mineiro paula.avila@igm.pt ; santos.oliveira@igm.pt
Resumo
Comunicação: Oral
It is known that mining may cause negative impacts in the surrounding environment. A
comparative assessment of the geochemistry of soils collected in the vicinity of three
“abandoned” mines of the Trás-os-Montes region was intended: Jales (in the
municipality of Vila Pouca de Aguiar), Vale das Gatas (Sabrosa) and Argoselo
(Vimioso). Tailings (with inert minerals and sulphides) originated from mining
operations in the past are disposed at surface. In total, 94 tailing samples and 205 soil
samples collected in the vicinity of the three mine sites were submitted to multielement
chemical analysis in IGM Lab. Conductive Plasma Emission Spectrometry and X-Ray
Fluorescence were utilised following methods accredited by the Portuguese IPQ.
Electron microprobe was used for mineralogical and microchemical determinations.
Analytical determinations of selected metals in the tailings indicate that Pb, Zn, Cu, Cd,
Sn, (Sb), (Bi) and (Ag) show concentrations of significant magnitude (from hundreds to
tens of thousands of ppm). The Argoselo tailings seem to constitute the most harmful
situation. Results obtained for soils show evidences of significant chemical
contamination, which is assigned to the presence of the tailings disposed nearby. It also
can be seen that the chemical element associations and the total concentrations are
slightly different in each case, depending on the original mineral paragenesis of the
ores. Metal concentrations are usually high in soils nearby the mine sites but their
contents tend to decrease rapidly with the distance to the polluting source (tailing). The
geochemical aureoles extend, in general, for less than 1 km but anomalies in Jales
appear to occur beyond this distance due to the action of the wind, which transports the
fine materials (minerals) from the tailing surface. Arsenic, Pb, Zn, Cu, Cd and Ag are
the most critical elements due to the magnitude of contents determined and to the
extension and dispersion gradients in the soils, but also to their toxicological properties.
Most trace elements appear to be associated mostly with low chemical stability
mineralogical phases of soils. This trend was supported by the application of techniques
of sequential selective chemical extraction (SSCE). These results suggest that metals are
mobilised from the tailings to the soil mainly by mechanical erosion and aerial
transport. Rainwater percolating within the tailings is also a powerful metal dispersion
agent. Analyses carried out on some tailing percolating waters show characteristics of
acid sulphate-calcic waters with significant metal charge (in particular, Cu, Zn, Cd, Al,
Mn and less As and Pb).
As a general conclusion, it can be said that soils under the influence of mining areas
may occur contaminated in metals (As, Pb, Cu, Zn, Cd, Bi, Sb, Ag...) when compared
with reference values from literature. From an environmental point of view, contents
lying above reference values proposed by some authors for toxic limits of polluted soils,
or exceeding admissible concentrations for agricultural soils, were found in some places
around the Jales, Vale das Gatas and Argoselo mines.
The degree of soil and land use by the local populations (agriculture, pasturing),
particularly of those who live nearby the old mines, is appreciable. These conditions
versus the results obtained recommend that soils from mining areas should be carefully
surveyed.
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