Pore Water Characteristics Following a Release of Neat Ethanol onto Pre‐existing NAPL

Abstract

Neat ethanol (75.7 L) was released into the upper capillary zone in a continuous‐flow, sand‐packed aquifer tank (8.2 m3) with an average seepage velocity of 0.75 m/day. This model aquifer system contained a residual nonaqueous phase liquid (NAPL) that extended from the capillary zone to 10 cm below the water table. Maximum aqueous concentrations of ethanol were 20% v/v in the capillary zone and 0.08% in the saturated zone at 25 and 30 cm downgradient from the emplaced NAPL source, respectively. A bench‐scale release experiment was also conducted for a similar size spill (scaled to the plan area). The concentrations of ethanol in ground water for both the bench‐ and pilot‐scale experiments were consistent with advective–dispersive limited mass transfer from the capillary to the saturated zone. Concentrations of monoaromatic hydrocarbons and isooctane increased in the pore water of the capillary zone as a result of both redistribution of residual NAPL (confirmed by visualization) and enhanced hydrocarbon dissolution due to the cosolvent effect exerted by ethanol. In the tank experiment, higher hydrocarbon concentrations in ground water were also attributed to decreased hydrocarbon biodegradation activity caused by preferential microbial utilization of ethanol and the resulting depletion of oxygen. These results infer that spills of highly concentrated ethanol will be largely confined to the capillary zone due to its buoyancy, and ethanol concentrations in near‐source zone ground water will be controlled by mass transfer limitations and hydrologic conditions. Furthermore, highly concentrated ethanol releases onto pre‐existing NAPL will likely exacerbate impacts to ground water, due to NAPL mobilization and dissolution, and decreased bioattenuation of hydrocarbons.

Increased fermentation activity and persistent methanogenesis in a model aquifer system following source removal of an ethanol blend release

“…However, such a high concentration (>1% v:v) would likely occur for only high ethanol content fuels and would be confined to the capillary fringe near the source zone where a non-aqueous phase liquid (NAPL) phase exists Barker, 2011, 2013b;Stafford et al, 2009;Stafford and Rixey, 2011). Concentrations of dissolved ethanol in groundwater impacted by ethanol-blended fuels are usually lower than 10,000 mg/L (Corseuil et al, 2011;da Silva and Corseuil, 2012;Freitas and Barker, 2013a;Spalding et al, 2011).…”

Biodegradation of Soybean and Castor Oil Biodiesel: Implications on the Natural Attenuation of Monoaromatic Hydrocarbons in Groundwater

“…Oxygen intrusion into anaerobic contaminated plumes (e.g., due to natural recharge from surrounding groundwater and from the surface) is a common phenomenon, and BTEX catabolism initiated by oxygenases has previously been observed at very low levels of dissolved oxygen (0.1 mg/L) (Costura and Alvarez 2000). Ethanol is highly soluble in water and tends to quickly partition into groundwater and migrate away from the source zone (Heermann and Powers 1998;Cápiro et al 2007), even though a small fraction can be retained in the pore water resulting in some mass transfer from the capillary zone into the groundwater (Stafford et al 2009). similar mechanisms as those reported for the common fuel additive, ethanol.…”

Monitoring Lateral Transport of Ethanol and Dissolved Gasoline Compounds in the Capillary Fringe

“…This has been shown in laboratory tests using 2D boxes (Silliman et al 2002;Henry and Smith 2003;Berkowitz et al 2004). For example, Stafford et al (2009) found ethanol concentrations in the capillary fringe two orders of magnitude higher than just below the water table. For example, Stafford et al (2009) found ethanol concentrations in the capillary fringe two orders of magnitude higher than just below the water table.…”