Hydraulics & Waterways

Oral

394703 - A Mechanistic Model to Predict Gas Ebullition Rate in the Presence of NAPLs in Sediments

Wednesday, June 6
4:00 PM - 5:30 PM
Location: Greenway CD
Co-Authors: Karl Rockne, University of Illinois at Chicago – University of Illinois at Chicago

The main source of gas ebullition in polluted sediments is the biogenic production of methane gas by mixtures of eubacteria and Archaea. Previous work in our lab has verified that gas ebullition facilitates the transport of organic and inorganic pollutants to the overlying water column at levels greatly exceeding diffusive and advective release. We now focus on whether surface-active pollutants like non-aqueous phase liquids (NAPLs) impact the mechanical characteristics of the sediment, and thus its fracture formation rate during gas bubble formation. NAPLs are commonly found in polluted sediment and can be mobilized by gas ebullition events. What is less known, is whether they impact the rate of fracture formation in sediments when present in high concentrations.
We developed a multi-physics model using biological, chemical and geomechanics principles to understand the simultaneous impact of NAPLs and methanogenesis on gas ebullition. In the model, bubbles nucleate when methanogenesis exceeds solubility. Sediments behave like an elastic solid that resists bubble expansion, and thus the sediment strength has a significant impact on gas ebullition rate. We hypothesize that the presence of NAPLs in cohesive sediment impacts the sediment elasticity and stress intensity. We tested this hypothesis by measuring the mechanical properties of cohesive sediment mixtures with different concentrations of a model NAPL consisting of mineral oil. Measurements include the elastic modulus and stress intensity in a lab-scale model column system. Using these data, the methanogenic gas production rate has been predicted using the mechanistic model with sediment labile organic matter and temperature as input variables and the gas ebullition rate as output. These experimental and simulation results demonstrate the importance of NAPL concentration in cohesive sediments and consequently gas ebullition rate, and thus the ability of ebullition to mobilize sediment-bound pollutants by entrainment on rising gas bubbles in the sediment.

Morvarid Khazraee Zamanpour

PhD Student, Research Assistant
University of Illinois at Chicago

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394703 - A Mechanistic Model to Predict Gas Ebullition Rate in the Presence of NAPLs in Sediments



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