Case story: Map Acid Mine Drainage
Summary: The generation and transport of AMD (Acid Mine Drainage) was effectively outlined with a combination of electrical resistivity tomography (ERT) and induced polarization (IP) measurements. The geophysical investigations were combined with hydro–chemical data for a better insight into the hydrogeological dynamics in the investigation area.
Project
Method: Resistivity and IP.
Solution: The ABEM Terrameter LS resistivity meter (81 electrodes).
Measurement: 5 meter electrode spacing. ERT using Wenner-Schlumberger array.
Software for processing and interpretation: 2D: Res2Dinv, Seequent. 3D visualisation: Oasis Montaj, Geosoft.
Challenge
Acid mine drainage (AMD) is produced due to chemical weathering of either a natural ore deposit or mining wastes. The AMD can mix with groundwater and leach high concentrations of dissolved metals as well as other components from rocks and waste materials, and by that effect valuable groundwater resources.
This type of hazardous drainage was suspected to be present in both the sulphide rich tailings piles and in the underlying rocks at a large uranium mine in Brazil.
Solution
To gain a better understanding of the subsurface condition’s groundwater sampling and hydro-chemical data was collected. And as direct current (DC) resistivity and induced polarization (IP) methods can be used to recognize inorganic contaminants in both soils and rocks and have great spatial coverage, these methods were used to complement the hydro-chemical data gained from ground water sampling.
The geophysical investigation was carried out as ERT (Electrical Resistivity Tomography) measurements along seven transects spaced 30 meters apart. The ERT was carried out with the Wenner-Schlumberger electrode protocol. Non-polarizable porous-pot electrodes in a Cu-CuSO4 solution were used to avoid local polarizations that would interfere with chargeability measurements during the IP survey.
Top) The geophysical survey lines. A) Non-polarizable porous-pot electrodes. B) ABEM Terrameter LS resistivity meter for simultaneous measurement of resistivity and chargeability.
Results
The resistivity results allow for the interpretation of the 3D distribution of key geological and hydrogeological units and inference of fluid retention and migration. IP highlights areas of probable AMT generation as spatially coinciding with an extensive saturated zone. Inferred hydraulic connectivity in combination with the aquifer and suspected AMT zone provides a mechanism for groundwater infiltration by pollutants.
2D resistivity inversion models (3 of the 7 profiles), where zones with low resistivity values are in blue shades and zones with high resistivity values are showed yellow and reddish shades. The black dashed lines outline saturated zones.
Visualization levels from 2, 12, 22 and 32 m depth with data from the resistivity and chargeability 3D models. The black arrows indicate flow direction. The numbered anomalies in the IP result represent areas of interpreted high sulfide concentrations and potential AMT generation areas.
More to Read
Acknowledgement
We would like to thank the Geosciences and Exact Sciences Institute (IGCE) at Sao Paulo State University, in Brazil (https://www2.unesp.br) for sharing the information above.
The full paper (Targa, D.A., Moreira, C.A. & Casagrande, M.F.S. Hydrogeological Analysis of Sulfide Tailings at a Uranium Mine Using Geophysical and Hydrochemical Methods. Mine Water Environ 40, 671–689 (2021) is found here: https://doi.org/10.1007/s10230-021-00791-1
