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MULTIVARIATE ANALYSIS APPLIED TO THE STUDY OF ACID MINE DRAINAGES AND SURROUNDINGS SUPERFICIAL WATERS (ERVEDOSA MINE, NORTHERN PORTUGAL)

Paulo J.C. Favas

Abstract

The old Ervedosa mine (Northern Portugal) was intermittently exploited for tin by underground and open pit mining till 1969. The tailings were deposited on the ground. The mine was abandoned without any environmental remediation. Mineralization occur in quartz veins, and consists of cassiterite as the main ore mineral, and several sulphides (arsenopyrite, pyrite, sphalerite, pyrrhotite, chalcopyrite and stannite), matildite, bismuthinite, native bismuth and other minerals. The present study aims to characterize the acid mine drainages (AMD) and superficial waters at the Ervedosa mine area, based on a bimonthly monitoring scheme for a year. Portable instruments were used to measure water pH, Eh, temperature and electric conductivity (EC) in situ. HCO3 was also measured in situ using the titration method. Laboratory analyses were performed using current analytical methods: atomic absorption spectrometry (AAS) for Ca, Mg, Na and K; coupled graphite furnace AAS for Fe, Mn, Cu, Zn, Cd, Co, Ni, Pb and As; the spectrophotometric method for Cl; SO4 was analyzed by gravimetry. Principal component analysis (PCA) was applied to the study of the AMD of Ervedosa mine and surroundings superficial waters. The data matrix used in the analysis consisted of 69 water samples and 17 hydrochemical variables (pH, HCO3, SO4, Ca, Mg, Na, K, Cl, Fe, Mn, Zn, Cu, Pb, As, Cd, Co e Ni). The first three principal components explain 87.9% of the total variance in the data, and are related with the more important processes of mineralization and contamination of these waters: (1) the SO4-Fe-Mn-Cu-Zn-Cd-Pb-As association, related with the sulphides oxidation and their impact on the chemical composition of waters from mining galleries; and (2) the HCO3-Ca-K-Cl-Na-Mg association, found in waters derived from tailings drainage, related with the silicates hydrolysis. In this way, PCA shows a clear distinction between AMD from galleries and AMD from tailings, standing out the higher level of contamination of the AMD from tailings, with evident consequences on the quality of the surface waters under its influence.

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DOI resolver: doi.org/10.5593/sgem2018/5.1/s20.073

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