Application of Electrical Resistivity Ground Imaging (ERGI) to aid the Acid Mine Drainage (AMD) evaluation in the Rio Marina mining district (Elba Island)

The Rio Marina mining district (Elba island) was one of the greatest Italian iron ore and was exploited till 1981. Nowadays it is used as an excursion site and it is famous all around the world for its hematite-pyrite mineralogical samples and for its multi-colour acid pools. These pools are related to Acid Mining Drainage (AMD) process that occur in the sulfide-mining area. The effect of open-pit mining and other mining-related activities is that sulfides, previously isolated from the atmosphere, are exposed to oxygen. The oxidation of sulfides acidifies the flowing waters that leach ore minerals and dissolve elements such as arsenic, copper, manganese, iron, lead and zinc. This process leads to environmental pollution. The Rio Marina mining district is currently object of a geo-environmental characterization to assess hazard and persistence of the AMD process (Servida et al. 2006). In addition to investigations on surface waters, terrains and topographic evolution of the mining site, a geo-environmental model aiming to AMD evaluation requires the determination of: 1) waste rock thickness, to evaluate the amount of sulfuric acid that could be produced and the content of heavy metals that could be leached from terrains; 2) distribution of sulfides in the waste deposits; 3) distribution of the water-rich zones in waste deposits. The aim of this work is to supply these data by means of 2-D Electrical Resistivity Ground Imaging (ERGI) techniques. The pilot-study was performed on the Antenna plains, which represent one of the waste disposals of Rio Marina district. This area was chosen since 3-D topographical modelling, field survey and bibliographic data show no certain information concerning the thickness of the waste deposits. The following procedure was adopted: 1)acquisition of five ERGI profiles over the Antenna plains and identification of bodies showing homogeneous distribution of electrical resistivity. ERGI profiles were acquired by means of Wenner-Schlumberger array with 44 electrodes, with electrodes-spacing ranging from 3.0 to 5.0 m (maximum depth of investigation = 40.0 m). Profiles were acquired on a square grid in order to investigate the 3-D features of the subsurface; 2) direct-calibration of electrical resistivity class-values of bedrock (60-150 Ohm*m) and hematite-pyrite mineralized ore bodies (10-30 Ohm*m) by acquisition of 1 ERGI profile (Wenner-Schlumberger array; 30 electrodes, electrode-spacing = 2,5 m) located on an exposed quarry bench in the proximity of the Antenna plains; 3) interpretation of electrical resistivity distribution obtained by ERGI profiles by the light of a) the model of subsurface from 1954-1979 topographic and mining maps, b) knowledge of ore-geological model of the mining area, c) electrical resistivity obtained from the direct-calibration made on the outcropping analogue materials, d) physical and chemical properties of surface waters and terrains; 4) 3-D interpolation by geostatistical analysis (kriging) of the boundary between the electrical resistivity bodies correlated to bedrock-ore bodies complex and the hypothesized waste deposits. The results show that: 1) Antenna plains are composed by waste deposits with a maximum depth of about 25.0 m. These data confirm that Antenna was an old open-pit used as a mine-waste dump; 2) waste deposits have highly-conductive homogeneous electrical properties (<10 Ohm*m) and, consequently, they could have an homogeneous water-content and chemical composition. The low resistivity value is due to the high conductivity of groundwater (average groundwater conductivity: 7.54 mS/cm = 1.3 Ohm*m) and the occurrence of clay minerals produced by in-situ weathering of bedrock and debris; 3) bedrock has highly-conductive anomalies (10-30 Ohm*m) that, from their geometry, position and electrical resistivity values, could be interpreted as hematite-pyrite mineralized ore bodies.