Evaluation of Seasonal Factors on Petroleum Hydrocarbon Vapor Biodegradation and Intrusion Potential in a Cold Climate

Abstract

A detailed seasonal study of soil vapor intrusion at a cold climate site with average yearly temperature of 1.9 °C was conducted at a house with a crawlspace that overlay a shallow dissolved‐phase petroleum hydrocarbon (gasoline) plume in North Battleford, Saskatchewan, Canada. This research was conducted primarily to assess if winter conditions, including snow/frost cover, and cold soil temperatures, influence aerobic biodegradation of petroleum vapors in soil and the potential for vapor intrusion. Continuous time‐series data for oxygen, pressure differentials, soil temperature, soil moisture, and weather conditions were collected from a high‐resolution monitoring network. Seasonal monitoring of groundwater, soil vapor, crawlspace air, and indoor air was also undertaken. Petroleum hydrocarbon vapor attenuation and biodegradation rates were not significantly reduced during low temperature winter months and there was no evidence for a significant capping effect of snow or frost cover that would limit oxygen ingress from the atmosphere. In the residual light nonaqueous phase liquid (LNAPL) source area adjacent to the house, evidence for biodegradation included rapid attenuation of hydrocarbon vapor concentrations over a vertical interval of approximately 0.9 m, and a corresponding decrease in oxygen to less than 1.5% v/v. In comparison, hydrocarbon vapor concentrations above the dissolved plume and below the house were much lower and decreased sharply within a few tens of centimeters above the groundwater source. Corresponding oxygen concentrations in soil gas were at least 10% v/v. A reactive transport model (MIN3P‐DUSTY) was initially calibrated to data from vertical profiles at the site to obtain biodegradation rates, and then used to simulate the observed soil vapor distribution. The calibrated model indicated that soil vapor transport was dominated by diffusion and aerobic biodegradation, and that crawlspace pressures and soil gas advection had little influence on soil vapor concentrations.

Role of the source to building lateral separation distance in petroleum vapor intrusion

“…Based on the evidences provided by the above-mentioned empirical studies, U.S.EPA (2015a) suggested that additional investigation may be not necessary when the source to building vertical separation distance is greater than 1.8 m (6 ft) for dissolved contamination or 4.6 m (15 ft) for light non-aqueous phase liquid (LNAPL). Furthermore, results from numerical (Hers et al, 2000;Abreu and Johnson, 2006;Abreu et al, 2009;Knight and Davis, 2013;Hers et al, 2014) and analytical models (DeVaull, 2007;Yao et al, 2014;Verginelli and Baciocchi, 2014;Yao et al, 2016) were consistent with the empirical exclusion distance values reported above, showing that, in nearly all cases, a source to building vertical separation distance greater than 2 m or 5 m is sufficient to attenuate to acceptable risk-based levels petroleum hydrocarbon vapors from dissolved-phase or LNAPL sources, respectively.…”

Estimating the oxygenated zone beneath building foundations for petroleum vapor intrusion assessment

“…It is well known that microorganisms can oxidize petroleum hydrocarbons to carbon dioxide while utilizing electron acceptors such as molecular oxygen [8, 9]. Unlike recalcitrant compounds such as chlorinated solvents, a large number of petroleum hydrocarbon are susceptible to aerobic biodegradation at rates that are quite rapid with respect to rates of physical transport by diffusion and advection [21–22], leading to vapors attenuation by several orders of magnitude within a few meters [2, 10–18]. Based on this, U.S. EPA and ITRC guidances [6,7] proposed vertical screening distances (i.e.…”

Comparison of bacterial and archaeal communities in depth-resolved zones in an LNAPL body

“…The water table was found to be between 2.5 and 2.9 m bgs [1547.2-1546.8 m elevation] for the contaminated core, and taking into account water table fluctuations and the capillary fringe, zone III is largely either saturated or characterized by high moisture content (e.g., in the capillary fringe). High moisture content generally results in reduced oxygen levels (Hers et al 2014). These factors may explain why observed microbial communities in zone III were distinct from those observed in zone II.…”