BIOREMEDIATION OF SUBSURFACE OIL IN A TIDALLY-INFLUENCED SAND BEACH: IMPACT OF HYDRAULICS AND INTERACTION WITH PORE WATER CHEMISTRY

The bioremediation of tidally influenced beaches contaminated with oil was investigated numerically in this work using realistic beach and tide conditions from the Gulf of Mexico. Bioremediation was conducted through the application of a solution containing nutrient at concentrations larger than the background, and oxygen concentration in equilibrium with the atmosphere to enhance the biodegradation of low solubility hydrocarbons. The application was assumed to occur onto the upper intertidal zone of the beach. The numerical model BIOMARUN was used to simulate the biodegradation of hydrocarbons and transport processes of associated solute species (i.e., oxygen and nitrogen) in a tidally influenced beach environment. It was found that, compared to natural attenuation, bioremediation increased the concentration of nutrient and oxygen in the oiled zone thereby promoting subsurface oil removal efficiency. While the increase in nutrient concentration is expected, the increase in oxygen concentration was found necessary for efficient oil biodegradation. This means that increasing the nutrient concentration in the applied solution might not enhance oil biodegradation as the oxygen might become the limiting compound. Oil biodegradation was more notable in the upper and mid- intertidal zone of the beach. The slow biodegradation in the lower intertidal zone was due to reduced nutrient and oxygen replenishment as very little of the amendment solution reached that zone. Biochemical retention time map (BRTM) showed that extending the solution application period would increase the residence time of high nutrient solution in the beach, and would increase the subsurface space occupied by the a high nutrient plume. Simulation results revealed a complex interaction between the applied solutions and the tidally driven seawater-groundwater dynamics within the beach. Therefore, approaches that ignore the hydrodynamics due to tide are unlikely to provide the optimal bioremediation solutions.