Abstract:
Dhaka city has been experiencing rapid growth of urban population over the past 50 years and it is expected to continue in the near future. Solid waste management in the city is poor and there are many formal and informal waste dump sites in the city. Groundwater contamination from leachates underneath these dump sites is a major concern. The primary objective of this study was to assess the probable groundwater contamination using Electrical Resistivity Tomography (ERT), which is a non-destructive test. Three different locations at waste dump sites in Hazaribagh, Dhaka were selected as the study area.
Three ERT tests were carried out in Hazaribagh, where 32 electrodes were used in each test employing the “Wenner– Schlumberger array”; measurements were carried out with electrodes positioned on the surface. The ERT test data were processed through a 2-D inverse method, using the program RES2DINV. The data were processed by creating a pseudo-section of the apparent resistivity values using RES2DINV software. Computer iterations were then carried out and two dimensional (2-D) resistivity contour maps were created. The resistivity model was used to identify possible groundwater pollution through comparison with reference resistivity values of materials. Subsurface soil profiles have been assessed by both ERT and Microtremor measurement. Microtremor data were processed by using GeoSIG Soft software. The resistivity model of a test site was also compared with electrical conductivity (EC) of ground water samples collected from the site, in order to assess the validity of the ERT method for identification of ground water contamination.
From the analyzed resistivity models, it has been found that the subsurface below the dumpsites is characterized by low resistivity, possibly due to leaching of salts and contaminants from the dumps. For example, the maximum depth penetrated by the current during the acquisition of data at one test site (ERT-1) was 14m, and for this test site the resistivity values of subsurface layers were found to be mostly below 5 Ωm, except few cases of high resistivity in the top layer and isolated high resistivity at depth. The resistivity of water contaminated with high concentration of dissolved solids is usually less 10 Ωm. The resistivity-depth model at ERT-1 site suggests infiltration of leachate containing high concentration of dissolved solids to the subsurface environment. The contamination scenarios of two other test sites (ERT-2 and ERT-3) have been found to be similar to ERT-1. These results suggest that the groundwater in and around the dump sites at Hazaribagh has become contaminated up to a depth of about 15 m with leachate containing high concentration of dissolved salts. The soil profiles developed by the resistivity model and Microtremor analysis have been found to be similar, suggesting applicability of the ERT method. Good agreement of resistivity model and the measured EC values of ground water at a dump site also supports the applicability of the non-destructive ERT method for identification of ground water contamination. Results from this study suggest that the non-destructive Electrical Resistivity Tomography (ERT) could be a useful tool for quick and easy identification of shallow groundwater contamination at waste dump sites.
