Toluene biodegradation rates in unsaturated soil systems versus liquid batches and their relevance to field conditions

Abstract

Contaminant biodegradation in unsaturated soils may reduce the risks of vapor intrusion. However, the reported rates show large variability and are often derived from slurry experiments that are not representative of unsaturated conditions. Here, different laboratory setups are used to derive the biodegradation capacity of an unsaturated soil layer through which gaseous toluene migrates from the water table upwards. Experiments in static unsaturated soil microcosms at 6-30 % water-filled porosity (WFP) and unsaturated soil columns at 9, 14, and 27 % WFP were compared with liquid batches containing the same culture of Alicycliphilus denitrificans. The biodegradation rates for the liquid batches were orders of magnitude lower than for the other setups. Hence, liquid batches do not necessarily reflect optimal conditions for bacteria; either oxygen or toluene mass transfer at the cell scale or the absence of soil-water-air interfaces seemed to be limiting bacterial activity. For the column setup, the rates were limited by mass supply. The microcosm results could be described by apparent first-order biodegradation constants that increased with WFP or through a numerical model that included biodegradation as a first-order process taking place in the liquid phase only. The model liquid phase first-order rates varied between 6.25 and 20 h(-1) and were not related to the water content. Substrate availability was the primary factor limiting bioactivity, with evidence for physiological stress at the lowest water-filled porosity. The presented approach is useful to derive liquid phase biodegradation rates from experimental data and to include biodegradation in vapor intrusion models.

A critical review of the influence of groundwater level fluctuations and temperature on LNAPL contaminations in the context of climate change

“…Mechanical dispersion and diffusion combined with the advective groundwater flux then lead to the transverse migration of dissolved phase constituents, increasing the risks to receptors (e.g. drinking water supply) where groundwater is discharged (Picone et al, 2013;Yadav and Hassanizadeh, 2011). It remains complex and challenging to predict the evolution of these LNAPL contaminated environments over time, and the effectiveness of remediation techniques (Newell, 1995).…”

Assessment of LNAPL in Subsurface under Fluctuating Groundwater Table Using 2D Sand Tank Experiments

“…The remaining pool of LNAPL provides sufficient dissolving area to the underlying groundwater in smear zone which starts dissolving with flowing groundwater and create a dissolved phase plume (Lee and Chrysikopoulos, 1998;Kim and Chrysikopoulos, 1999;Nema and Gupta, 2003;Oostrom et al 2006;. Molecular diffusion and mechanical dispersion along with advective flux cause spreading of dissolved plume to downgradient receiving locations (Das, 2002;Yadav and Hassanizadeh, 2011;Picone et al 2013). On the other hand, native potential microbes play significant role to degrade the dissolved LNAPL from polluted zone (Yadav et al 2013;Basu et al 2015, Mustapha et al 2018).…”