Recognition of the pollution zone related to acid mine drainage using three-dimensional modeling of geoelectrical data at Alborz-e- Sharghi coal washing plant area, Semnan Province, Iran

Document Type : Research Article

Authors

Abstract

Nowadays, geophysics, and three-dimensional (3D) ground geoelectrical methods in particular are successfully associated with environmental investigations. The waste produced by coal washing operations often contains sulfide materials specifically pyrite. The appropriate atmospheric conditions and moisture favor rapid pyrite oxidation and subsequent acid mine drainage (AMD) formation. This AMD that contains high concentrations of iron, sulfate, low pH and variable concentrations of toxic metals is a major cause of long-term environmental problems. 
Since, the pollutants produced by pyrite oxidation processes in the groundwater flow system may change considerably the conductivity of the polluted zone, the electric and electromagnetic (EM) geophysical methods could effectively be used to map these zones. Resistivity and very low frequency electromagnetic (VLF-EM) are commonly used for this purpose. 
This paper discusses the results of a geophysical survey incorporating two different methods comprising VLF-EM and 3D electrical resistivity and attempts to detect the pollution emanated from the wastes produced by Alborz Sharghi Coal Washing Plant. This plant, which is located at 380 km northeast of Tehran and 57 km northwest of Shahrood City in Semnan Province, has being working for 30 years. The input feed of the processing plant is 500,000 ton per year. The coal recovery in the plant is 50%. The rest of the input feed is dumped as wastes around the plant. Depending on the method used for coal processing, two kinds of the waste are produced and dumped in the distinct places. The first kind is produced by a jig machine while the second is produced by a flotation process. It is expected that the amount of the coal waste to be about 3 million tons in the study area.
A geophysical survey using the VLF-EM method was first performed with a measuring spacing of 5 meters on 4 parallel profiles of 30-m distance in the downstream of the waste dump in order to investigate the likely polluted zones. The VLF measurements were carried out using a portable WADI-VLF digital instrument of ABEM Co. The measured data was then processed using RAMAG computer software. To simplify the data interpretation, the VLF raw data were filtered using the Karous-Hjelt technique and a set of vertical current density pseudo-sections were provided. The results of interpretation detected two polluted zones in the downstream of the waste dump. These polluted zones can be easily recognized between profiles 1 and 3 with high values of current density. The VLF survey was found to be good in identifying the path ways for pollution movement downstream the coal washing dump, but limited in its ability to exactly distinguish the depth of polluted zones. Due to this problem, the area located between profiles 1 and 3 was then selected for a 3D geoelectrical survey to better investigate the pollutant leaching process. The electrical resistivity method produces an image and/or an approximate model of the subsurface resistivity. Various arrays including pole-pole, pole-dipole, and dipole-dipole arrays are normally used for 3D resistivity surveys. In the present research, a 3D geoelectrical survey using a pole-dipole array was carried out on a 6 6 rectangular grid with different electrode spacing of 15 m and 30 m in x- and y directions, respectively by a portable SAS 1000 instrument from ABEM Co. As the pole-dipole array is an asymmetrical array, measurements were made with the forward and reverse arrangements of the electrodes on each profile in x- and y directions. Two more profiles were also considered in the direction of diameters of the rectangular grid. 
A computer software called RES3DINV which incorporates a smoothness-constrained least-squares approach was then used to perform an inverse modeling on the measured apparent resistivity data. To perform a 3D inverse modeling, the subsurface of the survey area was divided into several layers and each layer was further subdivided into a number of rectangular blocks with unknown resistivities. The interior blocks within each layer had the same size. The main objective of the inversion process was to determine the resistivity of each block in a manner that the model response fitted well the measured apparent resistivity data. The RES3DINV program utilizes least squares Gauss–Newton and quasi-Newton optimization methods for modeling process. This model can apply both numerical finite difference (FD) and finite element (FE) methods for calculation purposes. 
The results of inversion have been provided as sets of horizontal and vertical resistivity sections and also a 3D resistivity model was finally presented using Slicer\Dicer software. This 3D geoelectrical model illustrated a polluted zone with a thickness of about 30 meters at the depths between 30 and 60 meters. 
Nowadays, geophysics, and three-dimensional (3D) ground geoelectrical methods in particular are successfully associated with environmental investigations. The waste produced by coal washing operations often contains sulfide materials specifically pyrite. The appropriate atmospheric conditions and moisture favor rapid pyrite oxidation and subsequent acid mine drainage (AMD) formation. This AMD that contains high concentrations of iron, sulfate, low pH and variable concentrations of toxic metals is a major cause of long-term environmental problems.
Since, the pollutants produced by pyrite oxidation processes in the groundwater flow system may change considerably the conductivity of the polluted zone, the electric and electromagnetic (EM) geophysical methods could effectively be used to map these zones. Resistivity and very low frequency electromagnetic (VLF-EM) are commonly used for this purpose.
This paper discusses the results of a geophysical survey incorporating two different methods comprising VLF-EM and 3D electrical resistivity and attempts to detect the pollution emanated from the wastes produced by Alborz Sharghi Coal Washing Plant. This plant, which is located at 380 km northeast of Tehran and 57 km northwest of Shahrood City in Semnan Province, has being working for 30 years. The input feed of the processing plant is 500,000 ton per year. The coal recovery in the plant is 50%. The rest of the input feed is dumped as wastes around the plant. Depending on the method used for coal processing, two kinds of the waste are produced and dumped in the distinct places. The first kind is produced by a jig machine while the second is produced by a flotation process. It is expected that the amount of the coal waste to be about 3 million tons in the study area.
A geophysical survey using the VLF-EM method was first performed with a measuring spacing of 5 meters on 4 parallel profiles of 30-m distance in the downstream of the waste dump in order to investigate the likely polluted zones. The VLF measurements were carried out using a portable WADI-VLF digital instrument of ABEM Co. The measured data was then processed using RAMAG computer software. To simplify the data interpretation, the VLF raw data were filtered using the Karous-Hjelt technique and a set of vertical current density pseudo-sections were provided. The results of interpretation detected two polluted zones in the downstream of the waste dump. These polluted zones can be easily recognized between profiles 1 and 3 with high values of current density. The VLF survey was found to be good in identifying the path ways for pollution movement downstream the coal washing dump, but limited in its ability to exactly distinguish the depth of polluted zones. Due to this problem, the area located between profiles 1 and 3 was then selected for a 3D geoelectrical survey to better investigate the pollutant leaching process. The electrical resistivity method produces an image and/or an approximate model of the subsurface resistivity. Various arrays including pole-pole, pole-dipole, and dipole-dipole arrays are normally used for 3D resistivity surveys. In the present research, a 3D geoelectrical survey using a pole-dipole array was carried out on a 66rectangular grid with different electrode spacing of 15 m and 30 m in x- and y directions, respectively by a portable SAS 1000 instrument from ABEM Co. As the pole-dipole array is an asymmetrical array, measurements were made with the forward and reverse arrangements of the electrodes on each profile in x- and y directions. Two more profiles were also considered in the direction of diameters of the rectangular grid.
A computer software called RES3DINV which incorporates a smoothness-constrained least-squares approach was then used to perform an inverse modeling on the measured apparent resistivity data. To perform a 3D inverse modeling, the subsurface of the survey area was divided into several layers and each layer was further subdivided into a number of rectangular blocks with unknown resistivities. The interior blocks within each layer had the same size. The main objective of the inversion process was to determine the resistivity of each block in a manner that the model response fitted well the measured apparent resistivity data. The RES3DINV program utilizes least squares Gauss–Newton and quasi-Newton optimization methods for modeling process. This model can apply both numerical finite difference (FD) and finite element (FE) methods for calculation purposes.
The results of inversion have been provided as sets of horizontal and vertical resistivity sections and also a 3D resistivity model was finally presented using Slicer\Dicer software. This 3D geoelectrical model illustrated a polluted zone with a thickness of about 30 meters at the depths between 30 and 60 meters.
 
Key words: 3D geoelectrical survey, VLF method, Alborz Sharghi, coal washing wastes, 3D modeling, AMD

Keywords

References

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Iranian Journal of Geophysics
Volume 6, Issue 2
September 2012
Pages 95-111

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