A Comparison of Three Methods to Assess Natural Source Zone Depletion at Paved Fuel Retail Sites

Abstract

Natural source zone depletion (NSZD) encompasses all processes that result in petroleum hydrocarbon light nonaqueous phase liquid (LNAPL) mass loss. Vertical gas transport between the subsurface and atmosphere is a key component of NSZD. Gas exchange with the atmosphere may be restricted at sites with ground cover, which is typical for European fuel retail sites. This raises questions of whether, and to what extent, the generic NSZD conceptual model applies at these sites. Here, we present a study that evaluated how concrete and asphalt pavement affected NSZD processes and data interpretation for three NSZD assessment methods: soil gas concentration gradient, biogenic heat and carbon dioxide traps. All methods demonstrated that NSZD was occurring and NSZD rates were generally within the low end of values reported in the literature for unpaved sites. However, there was considerable variability in the rates, which highlights the need for careful examination of the conceptual site model and potential interferences for each method. The results demonstrate the viability of soil gas and temperature data collected from existing monitoring wells screened into the unsaturated zone without the need for additional, intrusive subsurface installations. The results also provide useful guidance for developing optimal long-term NSZDmonitoring approaches, where necessary. Received 13 January 2021; revised 26 April 2021; accepted 11 May 2021 Light non-aqueous phase liquids (LNAPL) are immiscible organic liquids that are less dense than water. LNAPL-forming substances include petrol (gasoline), diesel, heating oils and jet fuel (kerosene). The occurrence of LNAPL in the subsurface can be the result of various kinds of releases at locations where these products are manufactured, stored or sold. When a release occurs, LNAPL will percolate downward under the influence of gravity and may spread laterally owing to geological heterogeneity or the presence of other preferential migration pathways. If a sufficient volume is released, LNAPL will continue to migrate downward into the saturated zone where it can spread laterally, often forming LNAPL bodies that are partially above and below the water table, similar to an iceberg (Sale et al. 2018). As LNAPL spreads, an increasing fraction of the LNAPL is trapped as a discontinuous non-wetting phase by capillary forces (i.e. residual LNAPL). Thus, an increasing volume of the released LNAPL is present as an immobile, residual phase, resulting in an overall decreasing volume of mobile LNAPL distributed over a larger volume of the subsurface. Following a release, the vertical and lateral extent of LNAPL typically reach a stable condition on a timescale of weeks to years, depending on a number of parameters, including the release history, aquifer matrix characteristics, LNAPL physical properties and the rate at which LNAPL are depleted through natural and/or engineered processes (ASTM International 2014; CL:AIRE 2014; ITRC 2018). The distribution of LNAPL in the subsurface following a release is referred to as the ‘source zone’. The source zone comprises both residual and potentially mobile LNAPL that can act as a source of contamination for groundwater or soil gas (ITRC 2018). Natural source zone depletion (NSZD) encompasses all attenuation processes that result in LNAPL mass loss in the subsurface (Garg et al. 2017; ITRC 2018). These processes include physical mass transfer by dissolution and vaporization of chemical constituents to the aqueous (groundwater) and gaseous (soil gas) phases and biodegradation of LNAPL constituents through microbial-facilitated reactions. The efficacy of natural attenuation of petroleum hydrocarbons in groundwater has been well established since the early 1990s (NRC 1993; Rice et al. 1995). Although there has long been evidence that microbiological degradation processes responsible for natural attenuation in dissolved phase plumes were also occurring within LNAPL source zones to ‘weather’ or change the composition of LNAPL (e.g. Christensen and Larsen 1993), there was a common historical perception that biodegradation of the source material itself was limited (Lyman et al. 1992; Newell et al. 1995). More recent research on NSZD at petroleum LNAPL sites (e.g. Johnson et al. 2006; Garg et al. 2017; CRC CARE 2020a) has demonstrated that the rate of natural LNAPL depletion is often of the order of thousands to tens of thousands of litres of LNAPL per hectare per year (l ha−1 a−1). The observation of natural depletion rates of this magnitude has highlighted the significance of NSZD in LNAPL conceptual site model (LCSM) development (e.g. Mahler et al. © 2021 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution 4.0 License (http://creativecommons.org/ licenses/by/4.0/). Published by The Geological Society of London. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics Case study Quarterly Journal of Engineering Geology and Hydrogeology Published Online First https://doi.org/10.1144/qjegh2021-005 by guest on September 3, 2021 http://qjegh.lyellcollection.org/ Downloaded from