The effect of water table fluctuation and hydrogeochemical properties on NAPLs natural attenuation

Abstract

Human activities have recently resulted in the degradation and contamination of groundwater. Petroleum spills, such as crude oil, gasoline, and diesel fuel, are a major source of groundwater contamination, especially along coastlines. Crude oil contamination causes major social, economic, and environmental problems around the world because it progressively changes the makeup of soil and groundwater. Predicting the fate and transport of contaminants through the soil, as well as managing water resources, irrigation, and fertilizer all rely on accurate predictions of water movement in the saturated and unsaturated zones. In this thesis, three experiments were performed to understand the effect of water table fluctuations on the distribution and natural attenuation of BTEX and the hydrogeochemical properties of the soil. Each one of the experiments were performed to understand the effect of the following parameters: salinity effect, soil layering, temperature, and the effect of aerobic and anaerobic conditions. Different pore water analyses were performed during the experiment which included ORP, EC, pH, cations and anions, and GC analysis for aqueous and soil samples. The experiments were performed in the laboratory using natural soil samples from the beach of simaisma in the eastern side of Qatar. The first study investigating the impact of two salinities, brackish water and seawater, observed spatial and temporal variations in redox conditions within the soil columns. Moreover, ORP values showed consistent patterns in the stable column and bottom of the fluctuating column but varied in the middle of the fluctuating columns due to organic presence and changing water table levels, while no significant differences were observed between high and low salinity columns. EC values decreased initially in the fluctuating columns, followed by an increase, while stable columns showed a consistent decline. The pH range was narrower in stable columns and became more alkaline over time in fluctuating columns. Sulfate was abundant as a major electron acceptor, while iron, manganese, nitrogen, and nitrate were present in low concentrations. Benzene and toluene concentrations decreased faster in fluctuating columns, resulting in a shorter lifespan for the source zone. The consumption of sulfate indicated improved natural attenuation in fluctuating columns. The study suggested the potential of water table fluctuations to expedite LNAPL remediation in contaminated aquifers, considering site-specific factors. In the second study, soil layering lowers the enhancement of reducing conditions as BTEX are present in low permeability layer, same as the slower fluctuation’s intensity, while temperature increase resulted in ORP reduction. Statistical tests showed significant differences in middle and bottom ORP values. EC values were greatly influenced by fluctuation and temperature, with statistical tests confirming significant differences. Statistical tests confirmed significant differences in sulfate concentrations. The positive correlation with ORP and negative correlation with DIC, combined with the association between BTEX and sulfate concentrations, indicated an increase in biodegradation in the columns.