Examining the Vertical Dispersion and Degradation of TPH in Freshwater and Saltwater Media

Abstract

The persistent contamination of aquatic systems by crude oil poses significant environmental and ecological risks, particularly in stagnant water bodies where natural dispersion is limited. This study investigates the vertical transport and degradation of total petroleum hydrocarbons (TPH) in freshwater and saltwater systems using laboratory-scale batch reactors. Two cylindrical reactors were prepared—one containing freshwater and the other saltwater—and each was spiked with 250 cm³ of crude oil. Water samples were collected from six evenly spaced points at biweekly intervals over three months. Key physicochemical parameters, including pH, total dissolved solids, conductivity, chloride, sulphate, temperature, and others, were analyzed to monitor environmental conditions. The degradation of TPH was modeled using a two-dimensional advection-diffusion transport framework combined with first-order kinetics and Monod equations. Horizontal transport was assumed to occur via diffusion, while vertical transport incorporated both diffusion and gravity-driven convection. Rate constants for both models were derived from experimental data. Results indicated that predicted TPH concentrations decreased with increasing depth, consistent with experimental observations. The first-order kinetic model consistently provided a closer approximation of measured TPH values across all sampling points and depths compared to the Monod model. In freshwater, TPH decreased from 5,955.91 ppm at 0.25 m depth to 959.69 ppm at 1.25 m by Day 28, while in saltwater, values declined from 4,253.44 ppm to 757.90 ppm. Error analysis confirmed the superior predictive performance of first-order kinetics over the Monod approach. These findings demonstrate the applicability of first-order kinetic modeling in simulating the vertical dispersion and degradation of crude oil in stagnant water environments and provide a basis for risk assessment and remediation planning in oil-impacted aquatic systems.