Assessing Groundwater Contamination using Geophysical Imaging Techniques in Urban Areas

Thesis Abstract

Groundwater contamination poses a critical challenge to urban water security, driven by increasing anthropogenic activities such as industrial discharges, improper waste disposal, and urbanization-related surface runoff. These activities often result in the intrusion of pollutants into subsurface aquifers, threatening public health and environmental sustainability. This study aims to assess the extent and distribution of groundwater contamination in urban settings using advanced geophysical imaging techniques, with a focus on integrating resistivity and induced polarization methods to delineate contaminant plumes and characterize aquifer properties. The specific objectives include evaluating the correlation between geophysical signatures and contaminant concentrations, identifying subsurface pathways of pollutant migration, and developing a refined interpretative framework for urban groundwater monitoring. The research adopts a quantitative field-based design within the metropolitan area of Riverside City, where groundwater quality issues are prevalent. The study population encompasses subsurface sites identified as likely contamination zones based on prior hydrochemical surveys. A stratified random sampling technique selected 25 borehole locations across different urban land-use zones, including industrial, residential, and commercial sectors. Data collection primarily involved ground-based electrical resistivity and induced polarization surveys conducted using a Syscal Pro resistivity meter across a total survey area of 15 square kilometers. These geophysical measurements were complemented by laboratory analysis of water samples (collected from adjacent boreholes) for key contaminants such as nitrates, heavy metals, and hydrocarbons, to establish ground-truth correlations. The reliability and validity of geophysical data were ensured through repeated measurements at each site and calibration using known subsurface models. Data analysis employed a combination of geophysical data inversion, using the RES2DINV software to generate resistivity and polarization pseudosections, along with statistical methods including multiple regression analyses to correlate geophysical parameters with water quality indices. Geographic Information System (GIS) mapping facilitated visualization of contamination extents and subsurface flow pathways. Hypothesis testing involved using ANOVA to compare resistivity and polarization values across different land-use zones, assessing their significance in predicting contamination risks. The analytical framework drew on the conceptual model of contaminant transport based on advection-dispersion theories, supplemented by the geophysical response models derived from the Archie equation and polarization theory. The anticipated findings indicate a significant spatial association between low resistivity zones and elevated contaminant levels, particularly in industrial zones with high hydrocarbon and heavy metal presence. Variability in polarization responses is expected to delineate zones of fine-grained sediments that predispose to contaminant retention. The study aims to demonstrate that geophysical imaging provides a reliable, cost-effective means of detecting and mapping contaminant plumes in complex urban subsurface environments, with strong correlations (p < 0.05) between geophysical parameters and water chemistry indicators. This research contributes to the expanding body of knowledge on integrated geophysical and hydrochemical approaches to groundwater contamination assessment, advancing methodologies for urban aquifer management. It offers a nuanced interpretative framework that enhances the precision of pollution mapping, thereby informing targeted remediation strategies. The study concludes that geophysical imaging, especially resistivity and polarization methods, can serve as a vital tool for early detection, continuous monitoring, and risk assessment of groundwater contamination in urban areas. Recommendations include adopting integrated survey protocols in routine urban groundwater monitoring programs, enhancing multi-disciplinary collaborations, and developing localized calibration models to improve interpretative accuracy. Future research should explore temporal variations in geophysical signatures and extend the methodology to include other geophysical techniques such as seismic or electromagnetic surveys for comprehensive subsurface characterization.