USING REACTIVE TRANSPORT MODELING TO EVALUATE THE ROLE OF GAS BUBBLE FORMATION AND ENTRAPMENT ON NATURAL ATTENUATION OF PETROLEUM HYDROCARBONS

Passive remediation systems and monitored natural attenuation often rely on redox processes for contaminant removal. Many redox reactions, e.g. aerobic degradation, denitrification and methanogenesis, involve the production or consumption of gases making them important chemical constituents in these systems. In the saturated zone, gas bubbles may form due to biogenic gas production or may be entrapped due to water table fluctuations, providing significant sources and sinks for various gases. In addition, the presence of gas bubbles can affect the hydraulic properties of an aquifer, which can be important in passive systems where groundwater flows under natural gradients. In the unsaturated zone, redox processes alter the gas concentrations, affecting the composition of recharge water or entrapped bubbles, and making interpretation of field data more complex. As such, it is vital to understand gas transport and bubble formation mechanisms to asses their impact on the performance and longevity of passive systems.

Near Bemidji, MN a crude oil spill site has undergone natural attenuation since 1979, and has developed a large methanogenic region in the saturated and unsaturated zones. Previous work conducted at the site has shown that the biogenic formation of gas bubbles, and the entrapment of gas bubbles due to water table fluctuations are potentially important factors in the observed attenuation.

This study presents the results of reactive transport simulations including both saturated and unsaturated zone processes, focusing on the role of gases in this system. The modeling includes transport of dissolved gases across the water table, gas bubble formation due to methanogenesis, which provides a sink for dissolved CH4, gas bubble entrapment near the water table, which tends to enhance O2 transport into the saturated zone, and relative permeability changes in the saturated zone due to the presence of gas bubbles. The simulations are used to assess contaminant degradation rates and the potential for source removal at the site. Although the results presented are site-specific and focus on natural attenuation, the model is flexible and generally applicable to passive remediation systems that involve significant production of gases.