The Concawe LNAPL Toolbox is one of the first completely web-based tools for managing and remediating light non-aqueous phase liquids (LNAPL) affected sites. An off-line downloadable version is also available for users who do not want to access it via the internet. LNAPLs are typically historical hydrocarbons released into soils and groundwater, such as liquid fuels, crude oil and condensates. The toolbox was developed by Concawe, the Scientific Division of European Fuel Manufacturers Association, with the support of the engineering and environmental science-consulting agency GSI Environmental. The toolbox, written on the R-Shiny platform, was released in 2022 and includes over 20 different tools, such as, for example, infographics, nomographs, calculators, mobility models, videos and checklists. These tools are organized into three tiers, with Tier 1 offering basic graphics and essential information, Tier 2 providing simple quantitative models and calculators and Tier 3 featuring access and explanations of more complex tools. The toolbox is designed to address six key questions that are important for environmental consultants and regulators managing LNAPL sites, including determining the amount of LNAPL, estimating LNAPL migration and persistence, assessing LNAPL risk over time, determining the effectiveness of LNAPL recovery and estimating Natural Source Zone Depletion (NSZD). The Concawe LNAPL Toolbox is accessible free of charge via a webpage on an internet browser or by downloading the toolbox for use on a personal computer.

Ever since the foundation of Concawe in 1963, the association has studied the impacts of the refining sector on the environment and ways to mitigate these impacts. Light non-aqueous phase liquids (LNAPLs) are non-dissolved hydrocarbons (e.g. crude oils, gasoline and diesel) that exist as a separate, undissolved phase in the subsurface at some sites with legacy releases of fuels (Sale et al. 2018). They are referred to as ‘light’ because most petroleum hydrocarbons are less dense than water (Tomlinson et al. 2017). The behaviour of LNAPLs is complex, making it important to understand factors such as the amount of LNAPL present at a site, its potential for migration, the possibility of recovery and remediation, changes in composition over time, persistence and the rate of natural attenuation (Suthersan et al. 2015). Understanding the behaviour of LNAPL in the subsurface is crucial for making appropriate site management decisions, particularly in the context of environmental remediation and risk assessment (ITRC 2018).

To address this complexity and to help make better remediation decisions, Concawe has developed the Concawe LNAPL Toolbox (Newell et al. 2021; Strasert et al. 2021) with support from GSI Environmental. The toolkit is a comprehensive yet user-friendly web-based tool that provides essential information to the LNAPL remediation community (Fig. 1). The toolbox is intended to be a clear, transparent tool that regulators can use to validate site information and make informed decisions using sound science. Further, practitioners can utilize the toolbox to enhance their understanding of their LNAPL site and improve their site's conceptual site model (CSM).

The Toolbox was written using the R-Shiny platform (https://shiny.rstudio.com), which gave the developers access to the R statistical programming language, graphing and mapping tools, with some detailed tools written in Python. The Concawe LNAPL Toolbox is designed to be accessed via a webpage on an internet browser (http://lnapltoolbox.concawe.eu/lnapl_toolbox/) or by downloading the toolbox for use on a personal computer.

The tool is structured around six key questions that are often asked at LNAPL sites.

  1. How much LNAPL is present?

  2. How far will LNAPL migrate?

  3. How long will LNAPL persist?

  4. How will LNAPL risk change over time?

  5. Will LNAPL recovery be effective?

  6. How can one estimate NSZD?

Each question is addressed using a three-tier approach. At the highest level, information that can be easily accessed and quickly viewed is presented in Tier 1. Tier 2 offers a variety of quantitative models, tools and calculators for further analysis. Lastly, Tier 3 serves as a gateway to access more advanced, established models that are available elsewhere. First, users decide which LNAPL management question they would like to address, then determine which tier they would like to apply, as shown in Table 1.

Content summary: how much LNAPL is present?

The Tier 1 answer to this question first provides a summary of the concept of LNAPL specific volume. This metric represents the volume of LNAPL per unit area of the formation. It can be thought of as the thickness of LNAPL that would remain in an LNAPL zone if the soil and water were removed in a hypothetical scenario. Tier 2 provides a new tool for determining the volume of subsurface LNAPL that has been developed for the Concawe LNAPL Toolbox, an extension of the LNAPL Distribution and Recovery Model (LDRM), which was developed for the American Petroleum Institute (API) by R. Charbeneau of the University of Texas (Charbeneau 2007). This new tool offers several enhancements such as the ability to accommodate multiple soil layers, multiple locations and a highly accurate integration method with automatic interpolation. The LDRM is widely used to determine the subsurface LNAPL specific volume (Do) and transmissivity (Tn) at a single location based on user input for up to three soil layers. However, a limitation of the software is that a separate input file is needed to calculate LNAPL Do and Tn at each location where LNAPL apparent thickness has been measured. This can make the process time-consuming and expensive when many measurements are needed.

To overcome these limitations, the Multi-Site LNAPL Volume and Extent Model was developed for the Toolbox to calculate Do and Tn. The simultaneous determinations of Do and Tn at many locations can save a significant amount of time when many locations must be analysed. A total LNAPL volume is estimated as an area-weighted average of the calculated thicknesses at each well location.

The Tier 3 module for the ‘How much LNAPL is present’ question compares the Concawe LNAPL Toolbox and the older, API LDRM Model. Table 2 shows the differences between these two tools.

Content summary: how far will the LNAPL migrate?

The Tier 1 module for the answer to this question provides a general conceptual model for LNAPL migration in the subsurface.

•  LNAPL experts typically call the LNAPL mass an ‘LNAPL Body’ to prevent any confusion with a dissolved hydrocarbon plume that may be generated by the LNAPL. The phrase ‘LNAPL plume’ should be avoided.

•  LNAPL bodies need energy (pressure) to force the LNAPL at the leading edge of the LNAPL body into the pore space of the unaffected soils.

•  The required pressure can be significant, and once the release of LNAPL to the surface is stopped, the LNAPL body will stabilize at some point on its own accord because the pressure becomes insufficient to drive LNAPL into additional pore spaces.

•  Recent advances in Natural Source Zone Depletion (NSZD) show that NSZD is also an important process for limiting LNAPL migration and for stabilizing and even shrinking LNAPL bodies.

(Newell et al. 2021)

For the Tier 2 module, two quantitative tools were provided: (1) the Kirkman LNAPL Body Additional Migration Tool (Newell et al. 2021); (2) an LNAPL migration model developed by Mahler et al. (2012). The Kirkman LNAPL Body Additional Migration Tool, developed specifically for the LNAPL Toolbox, is used to estimate the additional distance that the leading edge of an existing LNAPL body is expected to migrate until it stabilizes in the presence of NSZD. To use the model, three inputs are required: (1) a representative LNAPL transmissivity from bail-down tests or from transmissivities calculated using the Tier 2 LNAPL Volume and Extent Model; (2) the measured LNAPL body gradient; (3) the current LNAPL body radius. The model assumes that the LNAPL body is circular for simplification purposes (Fig. 2). The Mahler tool is based on a peer reviewed equation (Mahler et al. 2012) that assumes a constant source of LNAPL release to groundwater that is attenuated by NSZD processes and provides an estimate of the ultimate extent of LNAPL migration based on simplified source scenario.

The Tier 3 module introduces users to two historical LNAPL models, the US Environmental Protection Agency's Hydrocarbon Spill Screening Model (HSSM) (Charbeneau et al. 1995) and University of Texas chemical flood simulator (UTCHEM); University of Texas 2000). The Tier 3 elements consist of a short video, general flowcharts for running these more complex models, required input data and example output data.

Content summary: how long will the LNAPL persist?

The Tier 1 module for the answer to this question provides a graph of the decrease in median concentration of BTEX (benzene, toluene, ethylbenzene and xylenes) at underground storage tank sites in California over time (McHugh et al. 2013) updated for the Concawe LNAPL Toolbox. Between the years 2004 and 2017, a significant reduction in the median concentration of benzene in groundwater was observed at the highest concentration well at each of 1174 sites. Specifically, the median concentration was reduced by approximately 90%, from an initial level of around 4000 μg l–1 to a final level of around 500 μg l–1.

The Tier 2 module provides a box model of an LNAPL source zone, where the mass of the LNAPL present in the source zone is used along with the removal rate of LNAPL from NSZD to estimate a range of potential source lifetimes. Two different model expressions are used: (1) a zero-order model where the NSZD rate does not change over time; (2) a first-order model where the NSZD rate drops over time in proportion to the remaining LNAPL mass over time. In the Tier 3 module, the US Environmental Protection Agency's REMFuel model (Falta et al. 2012) and API's LNAST Model (Huntley and Beckett 2002) are highlighted and explained via text, checklists, screenshots and videos.

Content summary: how will LNAPL risk change over time?

The Tier 1 module explains that the potential risk at many LNAPL sites is driven by the presence of benzene and describes how benzene removal over time from natural processes results in a decreasing risk associated with LNAPL for several common pathways at many sites. For the Tier 2 Module, an LNAPL dissolution calculator is provided so that users can see how the chemical composition of LNAPL in groundwater might change over time. This dissolution calculation is based on Raoult's Law for partitioning of specific hydrocarbons between the LNAPL and aqueous phase over time as the LNAPL composition changes. The Tier 3 module highlights the API LNAPL Dissolution and Transport Screening Tool (LNAST) model using a video, checklist and example output.

Content summary: will LNAPL recovery be effective?

A simple graphic tool developed by the Texas Risk Reduction Program (TRRP) is used to illustrate the prospect of LNAPL recovery in the Tier 1 tool (TRRP 2013) (Fig. 3). It requires that users know the specific volume of LNAPL at a particular location and the hydraulic conductivity of the water-bearing unit to plot a point on the graph. Within the graph are curves for four common LNAPLs (gasoline, diesel, fuel oil #2 and fuel oil #4). If the plotted point is to the left of the appropriate curve, then LNAPL is not likely to be recoverable via pumping.

The Tier 2 module describes how the Multi-Site LNAPL Volume and Extent Model described above can also be used to estimate LNAPL recoverability by providing an estimate of the LNAPL transmissivity at a particular location if one knows the apparent thickness of LNAPL in a monitoring well and the soil characteristics of the water-bearing unit. According to guidance provided by the ITRC (2018), a key threshold for determining the feasibility of LNAPL recovery is the transmissivity value. Values above 0.0093–0.074 m2 day–1 indicate that LNAPL hydraulic recovery is likely to be cost-effective and efficient. However, if the calculated or measured LNAPL transmissivity falls below the lower value in this range, the chances of successful recovery through hydraulic methods decrease significantly. Wells with transmissivity values within this range are probably dominated by residual LNAPL. These values are based on considering various soil and LNAPL types according to the ITRC (2018). The Tier 3 module provides key resources, including four short videos, for accessing more complicated computer models and for obtaining LNAPL transmissivity data from field measurements.

Content summary: how can one estimate NSZD?

The Tier 1 module emphasizes how NSZD is becoming an important factor in the CSM for LNAPL sites and how it can be used to manage LNAPL at affected sites. Tier 1 also summarizes the key processes underlying NSZD at LNAPL sites and shows reported ranges from Garg et al. (2017) where the middle 50% of site-wide NSZD rates falls between 6600 and 26 000 l of LNAPL being directly biodegraded per hectare per year. The Tier 2 Module provides calculators to convert between the myriad different types of NSZD rates (e.g. from gallons per acre per year in US units to μmol per m2 per second for some research papers to litres of LNAPL biodegraded per hectare per year). Tier 3 provides access to a wide range of tools on how to measure and interpret NSZD at LNAPL-affected sites, including videos on key NSZD measurement technologies.

Option 1. Run the Toolbox by accessing the webpage on an internet browser using the URL https://lnapltoolbox.concawe.eu/lnapl_toolbox/. The recommended browsers are Google Chrome, Mozilla Firefox and Safari.

Option 2. Download the Toolbox at https://github.com/concawe/LNAPL-Toolbox- for use on your own computer or server. The required software is R (version > 4.0.2) or Python (version > 3.8). Since the Toolbox was issued in April 2022, user feedback has mostly been positive, with most issues related to the specific format of the layering input data for the Tool 1 Volume Calculator. Based on these comments, changes were made to the instructions and input file architecture for the input data.

A detailed web-based toolbox, written on the R-Shiny platform, was developed and as of April 2022 is freely available to help environmental consultants, regulators and site owners better manage LNAPL-affected sites. Key decision-making support is provided to help estimate the volume of LNAPL in the subsurface, determine if LNAPL is likely to migrate further, understand how long the LNAPL might persist, evaluate if any risk associated with the LNAPL will change over time and if LNAPL recovery systems will probably be effective, and recognize if Natural Source Zone Depletion is a key factor at their site. The Concawe LNAPL Toolbox is one of the first web-based LNAPL software tools designed specifically to help environmental professionals understand and manage LNAPL sites.

The authors would also like to acknowledge the contributions of P. Kulkarni of GSI Environmental and B. Strasert and H. Podzorski while at GSI Environmental. In addition, the authors would like to acknowledge the Concawe STF-33 Task Force for conceiving and leading this project: M. Hjort, E. Vaiopoulou, P. Eyraud, R. Gill, T. Greaves, T. Grosjean, W. Jones, A. Medve, J. Smith.

CJN: conceptualization (lead), investigation (lead), methodology (lead), validation (equal), writing – original draft (lead); PB: conceptualization (supporting), methodology (supporting), validation (supporting), writing – review & editing (supporting); KW: data curation (lead), methodology (supporting), software (lead), writing – review & editing (supporting); BS: data curation (supporting), software (supporting), writing – review & editing (supporting); MH: conceptualization (supporting), project administration (lead), supervision (supporting), validation (supporting), writing – review & editing (supporting); EV: project administration (supporting), supervision (lead).

This work was funded by Concawe (the Scientific Division of the European Fuel Manufacturers Association).

This project was funded by Concawe, an organization primarily funded by the fuel manufacturing industry, and Mr Markus Hjort and Dr Eleni Vaiopoulou are affiliated to it.

The datasets generated during and/or analysed during the current study are available in the GitHub repository, https://github.com/concawe/LNAPL-Toolbox-