In Situ Bioremediation of a Gasoline‐Contaminated Vadose Zone: Implications from Direct Observations

Abstract

In situ bioremediation of a contaminated vadose zone requires implementing hydraulic and chemical conditions that stimulate the development of indigenous bacteria capable of degrading contaminants in the subsurface. We investigated enhanced biostimulation of a gasoline-contaminated deep vadose zone through nutrient-and O 2 -amended water infiltration. A vadose zone monitoring system (VMS) provided real-time observations of the treatment process's effect on hydrocarbon attenuation. The VMS data included continuous measurements of variations in water content, concentrations and isotopic compositions of methyl tert-butyl ether and benzene, toluene, ethylbenzene, and xylene in pore-water and gas phases, and concentrations of O 2 and CO 2 in the vadose zone gas phase. Real-time observations from the unsaturated zone enabled interactive adjustment of the remediation strategy and improved biostimulation conditions for biodegradation of the target compounds. In the course of three infiltration events that included infiltration of an O 2 -and nutrient-enriched water solution, a significant reduction in contaminant mass was observed across the unsaturated zone.Abbreviations: BTEX, benzene, toluene, ethylbenzene, and xylene; CSIA, compound-specific isotope analysis; MTBE, methyl tert-butyl ether; NAPL, nonaqueous-phase liquid; VMS, vadose zone monitoring system; VOC, volatile organic compound.Vadose zone pollution by contaminants originating from gasoline is a widespread problem due to frequently occurring surface spills and leaks from underground gasoline storage tanks. When released into the subsurface, nonaqueous-phase liquids (NAPLs) migrate downward through the unsaturated zone, sometimes reaching the groundwater (Moran et al., 2005). Volatile organic compound (VOC) fractions of the gasoline, such as benzene, toluene, ethylbenzene, and xylene (BTEX) and methyl tert-butyl ether (MTBE), are potentially toxic or carcinogenic and can seriously affect water quality (Varjani et al., 2017).Demands to remediate polluted sites highlight the need to develop a variety of treatment techniques with minimal cost and without causing further environmental disturbance (Jørgensen, 2007). Bioremediation has been found to be a more reliable, lower cost, and less intrusive method than other conventional treatments (Yadav and Hassanizadeh, 2011), especially for the unsaturated zone (Das and Chandran, 2011). Enhanced bioremediation can accelerate the natural processes by which bacteria either degrade the gasoline constituents as a carbon source into less complex byproducts or complete oxidation or mineralization to CO 2 and H 2 O by providing electron acceptors and nutrients and increasing moisture (Wellman et al., 2012). Typically, for BTEX, aerobic degradation is the most rapid and complete process and is hence preferred in engineered bioremediation (Yadav and Hassanizadeh, 2011). Bioventing, infiltration, and O 2 -releasing compounds are the most common ways to deliver O 2 to the unsaturated zone (Machackova et al., 2012;Suthersan et al....

Vadose Zone Monitoring as a Key to Groundwater Protection

“…Although the presence of volatile compounds in the unsaturated zone's gas phase is considered a direct indicator for liquid non-aqueous phase liquid (NAPL) pollution, such as hydrocarbons, it is very limited in its ability to depict the transport processes of liquid pollution. While gravitational forces dictate the downwards migration of the polluted liquid across the unsaturated zone, the gas phase is subject to significant lateral diffusion and dispersion (Moshkovich et al, 2017). Daily variation of atmospheric pressure results in significant transport of gases in a mechanism called barometric pump (Neeper, 2001;Rimon et al, 2011b).…”

“…In addition, contaminant spreading in the aquifer often results in very low concentrations on the edges of the contaminant plume, while contaminant concentration in the unsaturated zone often preserves the source's high concentrations, with only minimal dilution. Accordingly, pollution identification capability in the unsaturated zone is higher compare to the saturated zone (Gal et al, 2009;Dahan et al, 2014;Turkeltaub et al, 2016;Avishai et al, 2017;Moshkovich et al, 2017).…”

“…The potential for in-situ monitoring of the unsaturated zone has been demonstrated through several studies where the VMS was used to measure flow velocities, solute transport, and chemical transformation of contaminants across the unsaturated zone. It has been implemented in a variety of studies on water flow and contaminant transport including: (a) rainwater infiltration and groundwater recharge (Rimon et al, 2007(Rimon et al, , 2011a, (b) floodwater infiltration from stream channels and reservoirs (Shani, 2006;Dahan et al, 2007Dahan et al, , 2008Dahan et al, , 2009, (c) impact of agricultural practices on groundwater quality (Baram et al, 2012;Dahan et al, 2014;Turkeltaub et al, 2015bTurkeltaub et al, , 2016Weissman et al, 2020), (d) leachate generation and migration in landfills (Aharoni et al, 2017(Aharoni et al, , 2020, and (e) interactive remediation of contaminated sites (Dahan et al, 2017;Moshkovich et al, 2017;Levakov et al, 2019). Scientific and environmental implications from a number of these studies are presented here to demonstrate the capability to continuously monitor water flow and contaminant migration in the unsaturated zone.…”

“…In-situ bioremediation of a contaminated unsaturated zone depends mainly on the ability to control the hydrological, physical, and chemical conditions in the subsurface. Application of vadose zone monitoring technology during a remediation operation enables optimizing the remediation strategy on the basis of real-time data on the unsaturated zone's chemical composition, saturation degree, and flow characteristics (Avishai et al, 2017;Dahan et al, 2017;Moshkovich et al, 2017;Levakov et al, 2019). Direct information from the unsaturated zone was used for real-time adjustment of water application cycles, nutrient augmentation, and supply of electron donors/acceptors to the treated soil.…”

Volatilization process of toluene from contaminated soil under consideration of co-existence of pore water

“…For example, diesel oil with high carbon content volatilizes more slowly than those with low carbon content (Ma et al 2014). The concentrations of VOCs are often dozens or even hundreds of times higher than controlled values according to the national standard when the accidients of underground storage tanks and gas stations cause leakages (Alharbi et al 2018, Makarenko &Budak 2017, Moshkovich et al 2018). The change of contamination degree signi cantly in uences the volatilization and migration process of VOCs.…”