Experimental investigations for trapping oxygen gas in saturated porous media for in situ bioremediation

Abstract

Abstract. Oxygen is often the rate-limiting factor in aerobic in situ bioremediation. This paper investigates the degree to which air or oxygen gas can be emplaced into the pore space of saturated porous media and provide a significant mass of oxygen. Column experiments were performed to test three emplacement methods: direct gas injection, injection of water supersaturated with gas, and injection of a hydrogen peroxide solution. The direct gas injection method fills 14-17% of the pore space with trapped gas. Water supersaturated with gas fills 18-27% of the pore space with a trapped gas phase, and hydrogen peroxide solution injections emplaces trapped gas in 17-55% of the pore space. In addition to supplying oxygen, gas entrapment causes a decrease in hydraulic conductivity which could be an advantage by decreasing the flow of contaminants offsite. The relative hydraulic conductivity of porous media with a trapped gas volume of 14-55% was 0.62-0.05. IntroductionAlternative methods are needed for introducing oxygen into groundwater for aerobic in situ bioremediation of contaminants. In situ bioremediation is often limited by the amount of oxygen available to the microorganisms in the subsurface. To degrade a simple hydrocarbon (e.g., benzene), approximately 3.1 times more oxygen than contaminant in mass/volume is necessary to meet the stoichiometric requirements. Since the solubility of oxygen in water is low, it is difficult to significantly increase the oxygen mass in a contaminated aquifer by dissolving the oxygen in water first before transferring it to the aquifer. Twenty-eight times more oxygen per volume can be stored in the gas phase than can be dissolved in water, assuming equilibrium based on Henry's law at 15øC.We are investigating whether a wall or zone of trapped gas bubbles can be emplaced into an otherwise-saturated porous medium and provide a substantial source of oxygen for bioremediation of contaminated groundwater (Figure 1). If 15% of the pore space can be filled with oxygen gas, 20 times more oxygen will be emplaced compared to the case in which oxygen is dissolved in the pore water in equilibrium with air. Once a wall or zone of oxygen bubbles is emplaced into an aquifer, the oxygen will be dissolved by the water and be potentially available for use by microorganisms in biodegradation. When the oxygen is used up, it can be emplaced again. This cycle may be repeated until the contaminant is degraded to levels that meet the regulatory requirement. Because trapped gas emplacement does not require continuous injection, a bubble wall or zone may be constructed using modified sampling equipment (e.g., a Hydropunch TM or GeoprobeTM), which will allow for much closer spacing of injection points than injection through a well casing. Here we refer to oxygen because it is the gas that is most widely needed for bioremediation of contaminated groundwater, but other gases, such as hydrogen and methane, which have been shown to be effective in remediation of contaminated groundwater, could also be introdu...

Effects of air injection on flow through porous media: Observations and analyses of laboratory‐scale processes

“…[4] Motivated mainly by the aforementioned applications, the behavior of air in saturated porous media has also been the focus of various studies [e.g., Hunt and Manga, 2003;Roosevelt and Corapcioglu, 1998;Fry et al, 1997;Ronen et al, 1989;Faybishenko, 1995]. Such studies represent a distinct subset of the extensive literature dealing with flow through the unsaturated zone in general and conductivitysaturation relationships in particular [e.g., Maidment, 1993].…”

“…Here ''barrier'' refers to a region of greatly reduced hydraulic conductivity that serves to impede groundwater flow. Perhaps the closest experimental work is the investigation of intentional air trapping to form a ''bubble wall'' for use in bioremediation [Fry et al, 1997].…”

“…Here we explore the physical and chemical effects of air injection into sand-filled laboratory flow cells as a preliminary assessment of air-injection barriers used to reduce groundwater flow. Although not a new concept [e.g., Fry et al, 1997], air injection to reduce hydraulic conductivity and slow solute migration has been mentioned in the ''gray'' literature [Boger, 2001;P. Witherspoon, personal communication, 2001] where no experimental data are presented.…”

Monitoring microbe-induced physical property changes using high-frequency acoustic waveform data: Toward the development of a microbial megascope

“…As the effect of such gas was what was to be measured, the goal was to ensure that all of pore spaces were fluid-filled before starting. The methods used for ensuring near total saturation have been described in Faybeshenko [1995] and Fry et al [1997].…”

“…The inoculated sand was then transferred to four specially designed columns and packed in the manner described in Fry et al [1997]. One column was used for gas sampling, the second was used to measure Κ s , the third was used to make the cross-column acoustic measurements, and the fourth was used as the control column.…”