Investigation of possible leakage from wastewater dam walls of the Miduk copper mine using geophysical methods

During wet mineral processes in mines, a large amount of wastewater is produced and stored in tailing dams. In Miduk copper mine, Shahr-e-Babak, Kerman, Iran, there are tailing dams and wastewater dams as a result of mineral processing. The wastewater dam normally is constructed in order to separate and to reserve the water content of tailings, and finally, to reuse it for mineral processing. In this study, we have used the geophysical methods (electrical resistivity tomography (ERT) and self-potential (SP)) to investigate the seepage from the wastewater dam of the Miduk copper mine. The ERT surveys show a low electrical resistivity zone (less than 20 Ωm). It can be as a result of a clayey alteration zone or fractured zone. In addition, SP measurements show a positive anomaly in the same area due to the seepage from the dam walls. It can be due to an electrokinetic source origin. The observed natural source in the same zone, confirmed the geological, hydrogeological and geophysical interpretations. These results show the ability of ERT and SP methods to investigate the seepage zones related to wastewater dam walls.

Mining like other industrial activities can affect the environment negatively. Tailing dams have long been associated with mining activities and have formed a major negative impact. Excessive seepage in the foundation of the dam threatens the integrity of the structure. Geophysics has been used intensively in environmental problems, in particular in the study of mine tailings problems. In fact, geophysical surveys especially electrical resistivity tomography (ERT) and self-potential (SP) methods constitute a comprehensive methodology for assessment of anomalous seepage conditions by detecting, mapping and monitoring the zones.

The field survey to detect the seepage in Miduk wastewater dam was composed of a geophysical survey and a hydrogeological study. The hydrological study was performed in order to identify water table, surface water and groundwater quality, and to understand the relation between water collected in dam and water resources. Such a study is very common in environmental geochemical investigations. The ERT survey was carried out along 2 survey lines, called P1 and P2, using the dipole–dipole array with an electrode spacing of 20 m in the survey line P1 that has a total length of 400 m, however, the survey along the survey line P2 has been carried out using an electrode spacing of 5 m. The survey line P2 is 100 m long. A maximum n value of 11 was used for both P1 and P2 ERT profiles. All data were acquired by a WDDS-2 resistivity meter using multi-electrodes cable. The SP survey along SP1 and SP2 survey lines was carried out on the northeast abutment of the dam. The SP measurements were taken using two non-polarizing electrodes (Cu-CuSO4), one electrode was located at the base station as a stationary electrode and the other moving along the desired line at pre-fixed stations. The base-station was chosen at a point convenient for operation but away from the expected anomaly. Electrodes polarization was controlled between measurements using two constant points in the site. In order to identify the water table, in a part of the hydrological study, water depth in 6 observation wells, located downstream of the water retention dam, was measured. In the second part of the hydrological study, hydrochemical investigation was carried out by taking 17 water samples collected from 15 stations. The hydrogeochemical characteristics of water were obtained through physicochemical analysis of the water samples.

The ERT survey shows a low electrical resistivity zone (less than 20 Ωm). It can be the result of a clayey alteration zone or fractured zone. The SP measurements show a positive anomaly in the same area due to the seepage from the dam wall. According to the hydrochemical study, the water type of this area is bicarbonate-chlorate while the water type of one borehole and the dam water sample are both sulfate-chlorate. The obtained information from the survey area confirms the geological, the hydrogeological and the geophysical interpretations