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