Assessing Groundwater Contamination using Electrical Resistivity Imaging in Urban Areas

Thesis Abstract

Urban groundwater resources are increasingly threatened by contamination resulting from anthropogenic activities, subsurface infrastructural failures, and urban pollution sources, necessitating effective detection and monitoring strategies. This study aims to assess groundwater contamination in urban settings through the application of Electrical Resistivity Imaging (ERI), providing detailed subsurface stratigraphy and identifying contamination zones with high spatial resolution. The specific objectives include evaluating the spatial distribution of resistivity anomalies associated with probable contaminant plumes, correlating resistivity variations with hydrochemical data, and developing a comprehensive geological and hydrogeological model of the study area to inform sustainable groundwater management. The research adopts a pragmatic mixed-methods approach combining empirical field investigations with laboratory analyses. The study was conducted in a densely populated urban locale covering approximately 15 square kilometers, characterized by mixed land use with known underground infrastructure and informal waste disposal sites. A stratified random sampling approach was employed to select five key locations for geophysical surveys, with a total of 120 electrodes deployed across the survey lines to enhance spatial resolution. Data collection involved high-resolution ERI using a Wenner-Schlumberger array, with field measurements processed utilizing Res2DInv software for inversion and interpretation. Complementary hydrochemical sampling involved collecting groundwater samples from 30 boreholes within the surveyed area, analyzed through inductively coupled plasma mass spectrometry (ICP-MS) and ion chromatography to determine concentrations of major ions and potential contaminants such as nitrates, heavy metals, and organic pollutants. The validity and reliability of geophysical measurements were ensured through repeated profiling and calibration with known resistivity standards, while laboratory analyses adhered to standard quality control procedures. Data analysis comprised a combination of geophysical inversion results, hydrochemical parameter assessment, and statistical techniques, including multiple regression analysis and Principal Component Analysis (PCA), to establish relationships between resistivity anomalies and chemical indicators of contamination. The conceptual framework was underpinned by the Hydrogeological Theory of Contaminant Transport and the Electrical Resistivity Model of Subsurface Water Content, aligning geophysical observations with hydrochemical data to identify zones of potential contamination. The study also utilized GIS-based spatial interpolation to develop contamination risk maps, integrating geophysical and hydrochemical data. Expected findings suggest that ERI can reliably delineate contamination-prone zones within the urban subsurface, with significant correlations (p < 0.05) between low-resistivity anomalies and elevated concentrations of nitrates, heavy metals, and organic pollutants. The geophysical-hydrochemical integration is anticipated to reveal proximity of contaminant plumes to specific sources such as waste disposal sites and infrastructural leaks. These insights will contribute significantly to the understanding of urban groundwater vulnerability, especially in densely populated settings with complex subsurface conditions. The study’s contributions include refining the application of ERI in urban hydrogeological assessments and developing an integrated model that enhances predictive capabilities for groundwater contamination. Main conclusions affirm that ERI is a cost-effective and non-invasive tool capable of identifying and mapping groundwater contamination in urban areas. The study recommends targeted groundwater monitoring, infrastructural upgrades to prevent leaks, and urban waste management reforms to mitigate contamination risks. Further research should explore long-term monitoring using time-lapse ERI and integrate additional geophysical techniques such as seismic refraction to enhance subsurface characterization. Ultimately, this research advances the scientific understanding of urban groundwater contamination and provides a practical framework for authorities to adopt geophysical methods in environmental monitoring, thereby promoting sustainable groundwater utilization amid increasing urbanization pressures.