Spatial Distribution of Acid Mine Drainage Indexes In Different Water Reservoirs
Antunes, Margarida (1); Valente, Teresa (1); Gomes, Patrícia (1); Costa, Maria do Rosário (2); Fonseca, Rita (3); Moreno, Filipa (1)
1: ICT, University of Minho, Portugal; 2: GEOBIOTEC, University of Trás-os-Montes e Alto Douro, Portugal; 3: ICT, University of Évora, Portugal
The Iberian Pyrite Belt (IPB) is one of the largest metallogenic province in the world, with more than 90 polymetallic massive sulphide deposits associated with the volcano-sedimentary complex of the Iberian Peninsula. The geological and mineralogical characteristics of this region have a reflection on the existence of Acidic Mine Drainage (AMD) producing wastes dispersed by the numerous mining complexes and discharged into the river network affecting the water quality.
A several number of surface and groundwaters in the Iberian Pyrite Belt (south of Portugal) was collected and analyzed for physico-chemical properties and trace elements. Water sample sites were categorized into three similar groups based on source classification: mine water environments (10 samples), river network (6 samples) and groundwater (5 samples).
Mine water is highly acid (pH 1.8-3.6) with medium - high to extreme metal contents, while surface (pH 6.6 – 7.7) and groundwater (pH 7.2-8.0) are neutral with low metal contents. However, all the waters are extremely mineralized with an electrical conductivity mean value of 8926, 1262 and 1136 µS/cm, in mine waters, surface water and groundwater, respectively. Mine waters are the most contaminated with a maximum EC value of 27570 µS/cm and SO42- (80691 ppm), As (141 ppm), Cu (1445 mg/L), Fe (41023 mg/L), Zn (841 mg/L) contents.
Numerical indices of the severity of acidic mine drainage (AMDI) have been calculated to the study area. The ADMI index uses seven parameters (qi – pH, SO42-, Fe, Zn, Al, Cu and Cd) and their respective weightings (wi), which were considered the most indicative of AMD contamination. The group of mine water environments had a mean AMDI of 20.0 (1.7 – 39.7) reflecting a little or no dilution of direct mine water drainage. River network could be indicated as an impacted river downstream mine drainage (AMDI 74.0-86.5), while groundwater (AMDI 92.2) is uncontaminated by AMD. The groundwater composition will be strongly controlled by natural geochemical processes, from the geology of the ore deposits and water rock interaction.
The ADMI index is a useful method to quantitatively assess the relative intensity of contamination and impact of acid mine drainage, over time and space, that must be considered in environmental risk assessment of active and abandoned mine sites, before and along remediation and monitorization processes.