# Climate-driven hydrological extremes increase subsurface mobility of petroleum hydrocarbons: an experimental study under semi-continental climate conditions

Amélie Cavelan, Fabrice Golfier, Catherine Lorgeoux, Stéfan Colombano, Anne-Julie Tinet, Constantin Oltean, Dorian Davarzani, Jacques Deparis, Noële Enjelvin, Pierre Faure

► To cite this version:

Amélie Cavelan, Fabrice Golfier, Catherine Lorgeoux, Stéfan Colombano, Anne-Julie Tinet, et al. Climate-driven hydrological extremes increase subsurface mobility of petroleum hydrocarbons: an experimental study under semi-continental climate conditions. Science of the Total Environment, 2025, 1008, pp.181070. 10.1016/j.scitotenv.2025.181070. hal-05382787

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# Climate-driven hydrological extremes increase subsurface mobility of petroleum hydrocarbons: an experimental study under semi-continental climate conditions

Amélie Cavelan<sup>a,b,*</sup>, Fabrice Golfier<sup>c</sup>, Catherine Lorgeoux<sup>c</sup>, Stéfan Colombano<sup>b</sup>, Anne-Julie Tinet<sup>c</sup>, Constantin Oltean<sup>c</sup>, Dorian Davarzani<sup>b</sup>, Jacques Deparis<sup>b</sup>, Noële Enjelvin<sup>d</sup>, Pierre Faure<sup>a</sup>

## HIGHLIGHTS

* Extreme precipitation and water table shifts increased LNAPL mobility.

* LNAPL smear zone thickness increased by 50 % under extreme climate conditions.

* BTEX volatilisation rises 2.8× after 120 days of the extreme climatic scenario.

* Dissolved LNAPL concentration increases by 25 %.

## GRAPHICAL ABSTRACT

<figure><img src="image_4.png" /></figure>

## ARTICLE INFO

**Keywords:**

LNAPL

Soil

Groundwater

Climate change

Attenuation

Semi-continental climate

## ABSTRACT

While the influence of groundwater table fluctuations on the fate of Light Non-Aqueous Phase Liquids (LNAPLs) in soils is recognised, the impact of these fluctuations' intensity was never quantified and remains poorly understood. Yet, in the climate change context, these factors may alter their long-term environmental risks and challenge existing management strategies. To address this gap, a lysimetric column (2 m³) was developed to measure in situ the soil moisture, soil weight, pH, and redox potential and investigate the fate of a LNAPL-contaminated sandy soil exposed to controlled extreme rainfall and water table level fluctuations over a 120-day period. This scenario simulated the most extreme effects of climate change predicted by the IPCC for 2100 in the Grand Est Region, France. Molecular analyses of the free, gaseous and dissolved LNAPL phases (GC-

<footnote><sup>a</sup> Université de Lorraine, CNRS, LIEC, F-54000, Nancy, France</footnote>

<footnote><sup>b</sup> BRGM, F-45000, Orléans, France</footnote>

<footnote><sup>c</sup> Université de Lorraine, CNRS, GeoRessources, F-54000, Nancy, France</footnote>

<footnote><sup>d</sup> Université de Lorraine, INRAE, LSE, GISFI, F-54000, Nancy, France</footnote>

<footnote>* Corresponding author at: Université de Lorraine, CNRS, LIEC, F-54000, Nancy, France.</footnote>

<footnote>E-mail address: a.cavelan@brgm.fr (A. Cavelan).</footnote>
TQD, micro-GC) revealed that, compared to current local climatic conditions (low water table fluctuation intensity), the extreme scenario led to a 50 % increase in the smear zone thickness, 30 % greater depletion of the free LNAPL phase, and a 2.8-fold rise in BTEX volatilisation. Additionally, dissolved LNAPL concentrations increased by 25 % and the natural attenuation rate reached 37.5 g/m²/day, 50 % higher than under current hydrological conditions. This work provides the first quantification of the effect of groundwater level fluctuation intensity on the multi-phase redistribution of LNAPL in sandy soils. This study demonstrates that the intensity of groundwater level fluctuations is a key driver of LNAPL saturation redistribution, enhancing their short-term releases but reducing their persistence.

# 1. Introduction

Many studies have explored the mechanisms governing Light Non-Aqueous Phase Liquids (LNAPL) behaviour in soil and their remobilisation towards the gaseous and aqueous phases (Cavelan et al., 2022, 2024; Dobson et al., 2007; Garg et al., 2017; Guo et al., 2019; Gupta et al., 2019; Jiale et al., 2023; Lenhard et al., 1993, 2019; Mineo, 2023; Qi et al., 2020; Smith et al., 2022; Soucy and Mumford, 2017; Tomlinson et al., 2017; Van De Ven et al., 2021). LNAPLs such as diesel fuel may contain hundreds of hydrocarbons. When released in sufficient volumes, LNAPLs can infiltrate the soil under the influence of gravity and capillary forces, accumulating and spreading at the top of the water table as a free continuous liquid phase (Lenhard et al., 1993). A portion of LNAPL can remain residually trapped in the soil pore space within the unsaturated zone (USZ) during infiltration and can be leached by rainwater infiltration. The dissolution of some LNAPL compounds from both the free and residual LNAPL phases can lead to the formation of a dissolved LNAPL plume in the groundwater, that may threaten drinking water resources for decades (Garg et al., 2017; Mineo, 2023; Smith et al., 2022). Meanwhile, the Volatile Organic Compounds (VOCs) emitted from the dissolved, free, and residual LNAPL phases can accumulate in the soil porosity (Guo et al., 2019) and generate gaseous surface emissions. In addition, during the multiphase redistribution, all LNAPL phases are subject to biodegradation processes, contributing to their natural attenuation, with attenuation rates commonly ranging from 0.7 to 13.51 kg/m²/an (Garg et al., 2017; Smith et al., 2022).

Previous works have shown that natural or pump-induced groundwater level fluctuations may exacerbate the LNAPL redistribution within this intricate air/water/LNAPL multiphase system (Cavelan et al., 2022, 2024; Dobson et al., 2007; Guo et al., 2019; Qi et al., 2020; Van De Ven et al., 2021). The succession of drainage and imbibition cycles promotes the trapping of LNAPL droplets in the saturated zone (SZ) (entrapped phase) and, more generally, the LNAPL spreading across the smear zone (defined as the thickness of the SZ and USZ of the soil affected by the presence of LNAPL). The resulting increase in the air/water/LNAPL surface contact and the alternating redox conditions may favour the transfer of the pollutants towards the gaseous and aqueous phases, enhancing biodegradation processes (Cavelan et al., 2022, 2024; Dobson et al., 2007; Guleria et al., 2023; Gupta et al., 2019; Rezanezhad et al., 2014; Van De Ven et al., 2021; Zhou et al., 2015). Recent experimental and numerical studies have demonstrated that drainage/imbibition cycles and alternating transport regimes (advective or diffusive) can nonlinearly modify fluid interfaces, pore geometry, and pore-scale transport pathways through hysteresis effects, thereby impacting solute dispersion in unsaturated porous media (Saeibehrouzi et al., 2024, 2025). These effects tend, nevertheless, to decrease after repeated regular cycles. This study suggests that variable intensities of groundwater level fluctuations may modify LNAPL multi-phase transport, alternating stages of high dispersion with stages of trapping and reduced migration processes. Recently, Yang et al. (2024) demonstrated that soil moisture variations, whether tied to groundwater levels, irrigation or rainfall infiltration, control soil NAPL redistribution. Changes in soil moisture affect NAPLs' viscosity and penetration depth, thereby promoting their vertical migration in soil or favouring their retention depending on the type of soil and its initial water saturation. The rise in water table levels

can also push the VOCs accumulating in the USZ towards the surface through a “piston-like effect,” thereby influencing the fluxes of VOC (Guo et al., 2019; Qi et al., 2020). These results underscore the need to better consider groundwater level dynamics when assessing NAPL transport processes and risks, since these processes complicate the understanding of the fate of such multiphase systems and challenge remediation treatments.

Future climate predictions from the IPCC suggest that climate change will strongly impact rainfall and groundwater recharge patterns (IPCC, 2021, 2023), creating more frequent and intense variations in groundwater levels due to increased and more frequent extreme events (i.e. droughts, heavy rains) (Fallahi et al., 2023; Goderniaux et al., 2009, 2011; Green, 2016; Green et al., 2011; Lu et al., 2025; Smerdon, 2017). These phenomena could impact LNAPL remobilisation and attenuation processes by modifying soil moisture content and multiphase transport conditions more frequently (Cavelan et al., 2022; Mineo, 2023). Indeed, recent studies suggest that various groundwater level fluctuation dynamics may differently affect LNAPL redistribution in soils (Alazaiza et al., 2020; Yang et al., 2024). Although the influence of groundwater level variations on the LNAPL redistribution is well recognised, the impact of the intensity of these variations (e.g., rapid drainage/imbibition cycles, larger amplitudes of water level variations) on the multiphase behaviour of petroleum pollutants was never plainly quantified and remains, however, largely unexplored and unclear. These knowledge gaps limit our ability to predict and model the long-term environmental fate and risks associated with LNAPLs and design effective remediation strategies, particularly in the context of climate change. This gap also partly explains why LNAPL remediation often remains incomplete, long and unsatisfactory over the long term.

To address this gap, an instrumented lysimetric column (2 m high, 1.152 m in diameter) was filled with sandy soil, contaminated with 50 L of LNAPL (diesel fuel), and submitted to a 120-day controlled scenario of rainfalls and water table fluctuations. This experimental setup allows for continuous in-situ monitoring of the soil's physico-chemical properties (e.g., soil moisture, pH, and redox), mass budget calculation, and molecular characterisation of the free, gaseous, and dissolved LNAPL phases (Gas Chromatography (GC)-Triple/Tandem Quadrupole Mass Spectrometry (TQD), field micro-GC) to investigate how the climatic scenarios impact the fate of a multi-phase LNAPL contamination over a hydrological year. The applied scenario was specifically designed to reproduce the most extreme effects of climate change predicted by the IPCC for 2100 (emission scenario SSP5-8.5 2081-2100 simulated by Coupled Model Intercomparison Project Phase 6-CMIP6) in the region where the experiment was carried out (Grand Est Region, France) (Carvalho et al., 2021; IPCC, 2021). The local climate conditions are representative of a typical semi-continental climate, which exists in many regions of the world. The impacts on transfer, spreading, volatilisation, dissolution, and global attenuation of LNAPL were characterised and compared with a reference experiment conducted previously (Cavelan et al., 2024) with an identical device, soil, and LNAPL but under the current climatic conditions of the experimental site (i.e., more frequent and less intense precipitation regimes and less pronounced groundwater level variations). The objectives of this experiment are multiple:
(i) To elucidate and quantify for the first time the relationship between the intensity of the groundwater level fluctuations on the fate of LNAPL spills, by integrating the complex multiphase behaviour of the contamination.

(ii) Propose an original and reproducible method based on a large-scale experimental system that allows a better approximation of natural conditions than small-scale laboratory experiments and pore-scale numerical models.

(iii) Propose a comparison of realistic climate scenarios, enabling these results to be projected onto numerous sites subject to similar intensification of groundwater level fluctuations.

Some authors suggested that the expected rise in temperature due to climate change could enhance LNAPL biodegradation and remobilisation towards the dissolved and gaseous phases (Cavelan et al., 2022; McAlexander and Sihota, 2019; Yadav et al., 2012; Zeman et al., 2014). However, as the temperature effects on LNAPL remobilisation appear to be limited, given the projections (averaging between +1.5 °C and + 4.8 °C, IPCC, 2023), temperature will not be considered in this study.

# 2. Materials and methods

## 2.1. Experimental setup

The experimental setup used for this study was described in Cavelan et al. (2024) and summarised in Table 1. The lysimeter was equipped with in situ sensors to monitor the evolution of soil physico-chemical

<table><caption>**Table 1**
Summary of the experimental setup and hydrological scenario.</caption><thead><tr><th></th><th>Current scenario (Cavelan et al., 2024)</th><th>Extreme scenario (this study)</th></tr></thead><tbody><tr><td>Simulated hydrological scenario</td><td>Current climatic conditions in the Grand Est Region, France. Based on the 10-year average rainfall intensities and frequencies measured by the weather station in the experimental site and from the ADES database (https://ades.eaufrance.fr/).</td><td>Emission scenario SSP5-8.5 for 2081 to 2100, simulated by Coupled Model Intercomparison Project Phase 6-CMIP6, combining the IPSL-CM6A-LR and CNRM-CM6-1 models for France, Grand Est Region.</td></tr><tr><td>Rainfalls</td><td>Annual cumulated rainfall: 654 mm<br>Rainfall intensity: 10 L/h<br>Rainfall duration:<br>- Wet season: 12 min<br>- Dry season: 20 min<br>Rainfall frequency:<br>- Wet season: 1 rain /24 h<br>- Dry season: 1 rain /48 h</td><td>Annual cumulated rainfall: 630 mm<br>Rainfall intensity: 10 L/h<br>Rainfall duration:<br>- Wet season: 30 min<br>- Dry season: 2 h<br>Rainfall frequency:<br>- Wet season: 1 rain /48 h<br>- Dry season: 1 rain /20 days</td></tr><tr><td>Water table level</td><td>Amplitude of groundwater level variations:<br>- Intraseasonal variations: + or - 25 cm<br>- Interseasonal variations: + 50 cm<br>Frequency of groundwater level variations:<br>- Intraseasonal variations: every 20 days<br>- Interseasonal variations: every 60 days</td><td>Amplitude of groundwater level variations:<br>- Intraseasonal variations: + or - 50 cm<br>- Interseasonal variations: + 100 cm<br>Frequency of groundwater level variations:<br>- Intraseasonal variations: every 20 days<br>- Interseasonal variations: every 60 days</td></tr><tr><td>Column dimension</td><td>PEHD column (surface: 1 m², high: 2 m)</td><td></td></tr><tr><td>Soil type</td><td>Homogeneous sandy soil (see supplementary Table S1)</td><td></td></tr><tr><td>LNAPL</td><td>50 L of fresh commercial diesel fuel (from TotalEnergies®)</td><td></td></tr><tr><td>Parameters monitored in situ</td><td>pH, redox potential, column weight, LNAPL thickness in monitoring well, Water table level</td><td></td></tr><tr><td>Sampling (analyses)</td><td>Free LNAPL phase (GC-TQD)<br>Dissolved LNAPL phase at 10 cm of elevation (GC-TQD)<br>Surface BTEX emission (micro-GC)</td><td></td></tr></tbody></table>

and hydrogeological properties during rainfall and water level variation events (Fig. 1A, B). It has been shown that this device can accurately reproduce and monitor the processes undergone by LNAPL-contaminated soil under controlled climate conditions (Cavelan et al., 2024). The elevation of pH and Redox electrodes was modified to adapt to the climatic scenario. A scale measured the lysimeter's weight with an accuracy of ±0.2 kg every minute to calculate the mass budget. More information on the protocol, mass budget calculation, the properties of the sandy soil mixture used in the lysimeter, and the sensor calibration procedure is available in Cavelan et al. (2024) and in the Supplementary Information (Table S1).

Once the column was filled, the water table was stabilised at 50 cm of elevation, and 50 L of LNAPL (TotalEnergies®) was versed onto the lysimeter under the same conditions as described in Cavelan et al. (2024) to simulate an accidental multi-component LNAPL spill (hydraulic properties in Supplementary information, Table S1). This accounts for 38.5 kg of LNAPL (Density LNAPL = 0.78 g/cm³). The applied water table level variations and rainfall scenarios differ from the previous study in that they reproduce the most extreme effect of climate change predicted by the IPCC for 2100 in the region where the experiment was carried out (Grand Est Region, France). The local climate conditions are representative of a semi-continental climate, as observed in many world regions. This chosen emission scenario is SSP5-8.5 for 2081 to 2100, simulated by the Coupled Model Intercomparison Project Phase 6-CMIP6, combining the IPSL-CM6A-LR and CNRM-CM6-1 models for France. This climate trajectory, marked by pronounced changes in hydrological regimes, is widely adopted as a benchmark for climate impact assessments (Reinecke et al., 2021). Therefore, this choice provides a robust and realistic framework for evaluating how extreme hydrological dynamics and intensified groundwater level fluctuations impact LNAPL mobilisation. This choice enhances the relevance of the results to predict future risks and guide adaptive LNAPL management strategies. Under this emission scenario, the total annual water balance in the region is expected to decline by approximately 24 mm/year by 2100 (IPCC, 2021). Rainfall frequency is supposed to decrease during the wet and dry seasons (with more drought periods), while extreme rainfall events will become 25% more frequent. The intensity of groundwater level fluctuations is also expected to increase (Christensen and Christensen, 2007; IPCC, 2021; Nygren et al., 2020). Accordingly, the scenario previously used was modified as follows (Fig. 1B, Supplementary information): each intensity of water level variation was multiplied by two; rainfalls are less frequent and more intense in the wet season (1 rain of 30 min at 10 L/h every 48 h) and dry season (rain at 10 L/h during 2 h every 20 days). During the experiment, the column is buried for passive temperature control. This experiment and those performed by Cavelan et al. (2024) were carried out simultaneously at the same location to be subjected to similar temperatures.

The apparent free-floating LNAPL thickness was regularly measured in the monitoring well using an oil/water interface Solinsts® probe (accuracy: ±1 mm). The real floating LNAPL thickness and fluid saturation profiles were determined using the methods previously described by Lenhard et al. (2017). The saturations of each LNAPL fraction (free (floating), entrapped (trapped in the SZ), and residual (trapped in the USZ)) were integrated throughout the depth to calculate their respective total volumes in the columns at each monitoring stage using the soil porosity and the column's cross-sectional area.

Water, free LNAPL, and gaseous surface BTEX emissions were sampled and analysed using the protocol previously described (Cavelan et al., 2024). Surface BTEX fluxes were quantified using an Agilent micro-Gas Chromatograph (micro-GC)-490 connected to a gas collection chamber placed on the column (Fig. 1A). The 500 mL of water regularly sampled by the suction probe at 10 cm of elevation (Fig. 1A) was extracted by Solid Phase Extraction (SPE) with an AutoTrace™ 280 (Dionex™) and separated into Aliphatic (ALI), Aromatic (ARO), and Polar fractions using an automated Gilson GX-274 ASPEC® system. An Agilent GC 7890B coupled with an Agilent 7010B mass spectrometer
<figure><img src="image_5.png" /><figcaption>**Fig. 1.** A) Experimental system. B) Scenarios of rainfalls and groundwater level variations applied for this study (solid lines) compared to the scenario previously applied (Cavelan et al., 2024) (dotted lines).</figcaption></figure>

performed the ALI and ARO fractions analyses to characterise the evo-
lution of the free and dissolved LNAPL composition. The 25 µg of free
LNAPL sampled in the monitoring well after 0, 60, and 120 days were
extracted and separated into ALI, ARO, and polar fractions using a GX-
274 ASPEC® before being analysed with an Agilent GC 7890B gas
chromatography. The main ALI and ARO compounds in the free LNAPL
and water samples were quantified using an internal calibration method:
prior to analysis, 20 µL of a standard deuterated n-alkane mixture and a
standard deuterated PAH mixture (see Appendix 1) were added to the
ALI and ARO extracts, respectively. Calibration was done using 25 n-
alkanes (C₁₀-C₃₅) and 19 PAHs at various concentrations (0.025, 0.05,
0.10, 0.25, and 0.50 µg/mL) to which were added 3.6 µg/mL of ALI and

2 µg/mL of the ARO quantification standard mixture. The calibration
was controlled for every ten samples using standard analysis at 0.025
and 0.25 µg/mL. The analysis of blanks between each sample prevented
external contamination. The compounds were identified by comparing
the results with the spectra in the WILEY and NIST libraries. Triplicate
analyses were performed and exhibited an uncertainty of ±3.4 %.

**3. Results and discussion**

3.1. LNAPL saturation redistribution and global attenuation rate

Fig. 2 demonstrates that, regardless of the water table fluctuation
<figure><img src="image_6.png" /><figcaption>**Fig. 2.** Evolution of the water, residual (LNAPL trapped in the unsaturated zone), free (liquid mobile LNAPL phase accumulate at the top the water table), entrapped LNAPL (LANPL trapped in the saturated zone as droplets) volumes (V), and soil saturations (S) with depth in the soil of the lysimeter calculated using the model of Lenhard et al. (2017) from the current and historical free LNAPL and water levels measured in the monitoring well at different times of the current (after Cavelan et al., 2024) and the extreme scenario (this study). The column on the right of each figure labelled “MW” represents the corresponding elevations of each fluid (same elevation scale in cm) in the monitoring well of the lysimeters at the different times of the scenario.</figcaption></figure>

scenario, a rising water table leads to a significant vertical spreading and partial entrapment of LNAPL within the SZ (several litres at each step), as indicated by the increase in entrapped LNAPL volumes and the corresponding decrease in free and residual LNAPL volumes. As a result, the smear zone thickness increases. In contrast, during water table recession, part of the LNAPL previously entrapped in the SZ is remobilised and reconnected to the free LNAPL phase (inducing a rise in the free LNAPL volume), while another fraction becomes trapped in the USZ, leading to an increase in the residual LNAPL volume. At the end of both scenarios, LNAPL is redistributed throughout the entire zone affected by water level fluctuations (Fig. 2). In the extreme scenario, the more pronounced rises and falls in the water table lead to greater free LNAPL phase redistribution into trapped and residual phases, as evidenced by more marked fluctuations in the volumes of the different LNAPL phases. Thus, after 120 days, the smear zone reaches 145 cm for the extreme scenario, compared to 98 cm for the current scenario. Because of this wider vertical redistribution, the extreme scenario exhibits the lowest free LNAPL volume after 120 days. Alazaiza et al., 2020, Guleria et al., 2023 and Van De Ven et al., 2021 demonstrated that variations in groundwater level strongly influence the LNAPL saturation distribution, alternating phases of increase mobility and entrapment, which is consistent with the increase in smear zone thickness (+50 %) and the continuous redistribution observed between the free, residual, and entrapped phases volumes during this experiment. This phenomenon is well known and has already been documented (Cavelan et al., 2022,

2024; Gupta et al., 2019; Lenhard et al., 1993, 2017; Van De Ven et al., 2021). However, this experiment provides, for the first time, a quantitative assessment. Indeed, the results show that the volumes of LNAPL thus remobilised are significant: between 18 % (under lower intra-seasonal water table level variations) and 50 % (under more intense seasonal water level variations) of the total LNAPL volume for the current scenario and between 44 % and 96 % for the extreme scenario; identifying this phenomenon as a key control parameter of LNAPL transfer and subsurface behaviour. However, contrary to what is suggested in the literature (Dobson et al., 2007; Gupta et al., 2019; Saei-behrouzi et al., 2025; Van De Ven et al., 2021), these redistribution processes do not appear to diminish with successive drainage/imbibition cycles and remain significant even at the end of the experiment. Therefore, under the studied conditions, the experiment's duration was insufficient to achieve system stabilisation, indicating that these phenomena may persist over a longer period.

The total LNAPL volume decreases during the two experiments, but more strongly in the extreme scenario, where a loss of 6 ± 1 L of LNAPL was observed after 120 days (Fig. 2) compared to 4 ± 1 L for the current scenario. This corresponds to a 4.7 ± 0.8 kg loss for the extreme scenario, which concords with the mass loss of 4.5 kg ±0.2 kg recorded on this lysimeter (Fig. 3A). As discussed previously for the current scenario (Cavelan et al., 2024), this loss of LNAPL mass can be attributed to three primary remobilisation mechanisms:
<figure><img src="image_7.png" /><figcaption>Fig. 3. Evolution of A) the average mass lost in the lysimeter, B) the floating free LNAPL phase composition, C) the dissolved concentration of LNAPL in the water table at 10 cm of elevation, and D) the surface gaseous BTEX fluxes with time for the current scenario (in yellow, (Cavelan et al., 2024)) and the extreme scenario (in blue, this study).</figcaption></figure>

* Volatilisation, resulting in a release of LNAPL into the atmosphere in the gaseous form;

* Dissolution, whereby contaminants are solubilised in the ground-water and infiltrated rainwater. As the water table level recedes, the excess water is drained from the column with the dissolved LNAPL constituents it contains;

* Biodegradation, which affects the most biodegradable fractions of the contamination and contributes to natural attenuation.

This loss of LNAPL mass corresponds to an average and progressive natural attenuation rate of 37.5 g/m²/day, 50 % more than the average natural attenuation rate estimated for the current scenario (25 g/m²/day). Hence, increased water table fluctuations and more extreme rainfalls favoured at least one of the above phenomena, increasing the contamination's natural attenuation rate. These effects well-corroborate previous observations (Cavelan et al., 2022; Dobson et al., 2007; Rain-water et al., 1993; Rezanezhad et al., 2014; Suthersan et al., 2015; Van De Ven et al., 2021; Zhou et al., 2015). However, this study demon-strates that a 50 % increase in smear zone thickness resulted in a similar increase in the LNAPL natural attenuation rate, indicating that these two parameters are strongly correlated under the experimental conditions (permeable and homogeneous sandy soil).

## 3.2. Chemical evolution of the free LNAPL phase and remobilisation processes

### 3.2.1. Evolution of the free LNAPL phase composition

As in the first scenario, the LNAPL mass loss during the most extreme scenario with time corresponds to a relatively constant decrease in the concentration in the free LNAPL phase of n-alkanes (−58 %), methyl-alkanes (−36 %), indenes (−35 %), cycloalkanes (−33 %), benzenes (−29 %), and naphtalenes (−28 % after 120 days) (Fig. 3B, Table S1, Appendix 1). Among these compound families, the most depleted sub-stances are those with the lowest molar mass, i.e., the most volatile, soluble, and biodegradable compounds (e.g., n-alkanes < C21, Fig. S2, Appendix 1). This depletion resulted in a mass loss of 1.57 kg after 120 days, 30 % more than in the current scenario (−1.23 kg, Cavelan et al., 2024). Among all the substances present in the free LNAPL phase, only 69.5 % of the substances in the ARO and ALI fractions were quantified (the total peak area of the quantified substances represents 69.5 % of the total peak area of the ALI and ARO chromatograms). This quantification excludes substances that are too light or too heavy to be detected by GC-TQD, explaining why the actual total mass loss likely exceeds 1.75 kg. This greater LNAPL depletion observed under the extreme scenario confirmed that intensified water table fluctuations and higher rainfall intensity — particularly during extreme events — promoted deeper infiltration through soil pores. This, in turn, led to enhanced leaching and weathering of the free LNAPL phase and the preferential
remobilisation of the most mobile compounds. These findings support the close positive correlation reported in the literature between LNAPL redistribution among pure, entrapped, and residual phases, exposure to the leaching processes and soil gas phases, and the mobility and bioavailability of the most LNAPL mobile compounds suggested in the literature (Cavelan et al., 2022; Guleria et al., 2023; Rezanezhad et al., 2014; Suthersan et al., 2015).

### 3.2.2. Volatilisation

As in the current scenario, some of the mobilisable substances in the extreme scenario were dissolved in the pore water and/or volatilised, as shown by the amounts of aromatic compounds in the dissolved (Fig. 3C) and vapour (Fig. 3D) phases. In the vapour phase, C₆ to C₁₂ n-alkanes, cycloalkanes, and methylalkanes were also identified but not quantified (see Appendix 1, Fig. S3). The observed trends in gaseous surface BTEX fluxes during the extreme scenario are the same as in the current scenario (i.e., a gradual decline with time, Fig. 3D), but remain systematically higher than those measured for the current scenario. The corresponding mass loss after 120 days is 7.46 g (calculated using the method described by Cavelan et al., 2024, Appendix 1, Table S2). This represents a 2.8-fold increase compared to the current scenario. The initial BTEX content in the LNAPL was previously estimated at 14.25 g (Cavelan et al., 2024), indicating that after 120 days, 52 % of the BTEX volatilised. This volatilisation rate is substantially higher than that observed under the current climate condition scenario.

As previously hypothesised regarding the influence of groundwater level fluctuations on LNAPL contamination (Cavelan et al., 2022; Guo et al., 2019; McCarthy and Johnson, 1993; Qi et al., 2020; Soucy and Mumford, 2017), this study confirms that intensified groundwater level fluctuations significantly enhance volatilisation processes and surface emissions of volatile organic carbon (VOC). Specifically, when the water table drops, the soil is partially drained from free LNAPL and water, exposing the residual phase to the air in the USZ (Fig. 2). This exposure favours the transfer of contaminants into the vapour phase and facilitates VOC accumulation in soil pore space (Ismail et al., 2023). Conversely, when the water table level rises, air and LNAPL ganglia become entrapped beneath the SZ (Fig. 2), forming a water/air/LNAPL three-phase system that is well known to enhance the volatilisation of both dissolved and entrapped contaminants (Boyle and Witherington, 2007; McCarthy and Johnson, 1993; Qi et al., 2020; Soucy and Mumford, 2017). These gases accumulated within the soil matrix were subsequently expelled towards the surface by piston-like effects induced by the upward movement of the water table. The enhanced vertical redistribution of LNAPL phases through soil porosity driven by the more intense water table fluctuations observed under the extreme scenario (Fig. 2) appears to facilitate these volatilisation processes. However, even if the mass of BTEX emitted at the surface is almost three times higher in the extreme scenario compared to the current one, it is still negligible compared to the total LNAPL mass lost during the experiment. Therefore, volatilisation alone cannot explain the additional depletion of LNAPL observed in the extreme scenario.

### 3.2.3. Dissolution and biodegradation processes

Groundwater fluctuations are known to facilitate the renewal and diffusion of nutrients and oxygen in the SZ and USZ and the alternation of oxidising and reducing soil conditions, favouring the functional diversity of microbial communities (Fry et al., 1997; Suthersan et al., 2015; Zhou et al., 2015) and LNAPL biodegradation (Cavelan et al., 2022; Dobson et al., 2007; Gupta et al., 2019). In the present study, greater variations in redox potential (ranging from 0.4 to -0.3 V at 90 cm of elevation, Fig. 4) were observed under the most extreme scenario due to the broader vertical extent of the water table fluctuation zone. Furthermore, the enhanced redistribution of the contamination among the free, residual, and entrapped LNAPL phases during this scenario have led to an increase in the smear zone thickness and surface area occupied by low LNAPL saturation zones (Fig. 2). These processes are

<figure><img src="image_8.png" /><figcaption>**Fig. 4.** Evolution of the Redox potential as a function of pH at different elevations for the current (Cavelan et al., 2024) and the extreme scenario (this study).</figcaption></figure>

both favorable for microbial activity and degradation processes (Cavelan et al., 2022; Dobson et al., 2007; Gupta et al., 2019; Rainwater et al., 1993; Rezanezhad et al., 2014; Suthersan et al., 2015; Van De Ven et al., 2021; Zhou et al., 2015). Therefore, enhanced biodegradation is expected in all LNAPL phases for the extreme scenario, which may explain the observed faster LNAPL attenuation rate (Fig. 3A) marked by a more pronounced depletion in the concentrations of lighter saturated and aromatic compounds (Fig. 3B). However, under the conditions of the experiment, fresh water is regularly injected at the base of the lysimeter to control the water table level. This explains why high oxygen content and redox potential are maintained at the base of the column. This phenomenon may have mitigated the effects of groundwater table variations on redox conditions, compared to natural environments.

Changes in dissolved LNAPL compound concentrations follow the same trends in both scenarios:

(i) Dissolved concentrations reflect only the residual, non-degraded compounds (Fig. 3C), explaining the relatively high phenanthrene concentration compared to more labile compounds such as benzene (Das and Chandran, 2011; Varjani, 2017).

(ii) From 0 to 60 days (end of the dry season), concentrations increase exponentially due to successive rainwater leaching events and water table fluctuations that mobilise the LNAPL residual and free phases (Fig. 3C).

(iii) Around day 60, marking the transition to the wet season, a sharp decrease in dissolved concentrations occurs as the water level rises.

(iv) Between 60 and 120 days (end of the wet season), concentrations increase again due to continued rainwater infiltration and dynamic water table fluctuations that remobilise residual and free LNAPL phases.

(v) Dissolved concentrations never reach the effective solubility values of the LNAPL compounds (Table 2) during the experiments, further indicating the absence of equilibrium between the LNAPL source and the dissolved phase.

These trends indicate that other processes impact the variation of the dissolved phase concentrations than dissolution alone, notably:
<caption>**Table 2**</caption>

Evolution of concentrations in the pure LNAPL phase, effective water solubility and measured dissolved concentrations of some major LNAPL compounds with time during the extreme scenario.

<table><thead><tr><th rowspan="2"></th><th colspan="2">Concentration in the pure phase (10<sup>-2</sup> mg/g)</th><th colspan="2">Effective solubility (mg/L)<sup>a</sup></th><th colspan="2">Measured dissolved concentration (mg/L)</th></tr><tr><th>90 days</th><th>120 days</th><th>90 days</th><th>120 days</th><th>90 days</th><th>120 days</th></tr></thead><tbody><tr><td>Naphthalene</td><td>48.70</td><td>44.60</td><td>8.09E-02</td><td>7.41E-02</td><td>1.75E-04</td><td>2.75E-03</td></tr><tr><td>1-Methylnaphtalene</td><td>42.10</td><td>41.90</td><td>1.80E-01</td><td>1.79E-01</td><td>2.10E-04</td><td>1.24E-03</td></tr><tr><td>1,2-Dimethylnaphtalene</td><td>7.59</td><td>7.87</td><td>9.62E-04</td><td>9.97E-04</td><td>3.19E-05</td><td>9.66E-05</td></tr><tr><td>1,6,7-Trimethylnaphtalene</td><td>9.15</td><td>9.13</td><td>3.85E-04</td><td>3.85E-04</td><td>8.75E-06</td><td>9.81E-06</td></tr><tr><td>Phenanthrene</td><td>12.30</td><td>12.20</td><td>7.01E-05</td><td>6.95E-05</td><td>3.17E-05</td><td>1.55E-05</td></tr><tr><td>2-Methylphenanthrene</td><td>7.24</td><td>6.84</td><td>1.49E-05</td><td>1.40E-05</td><td>1.71E-06</td><td>4.92E-06</td></tr><tr><td>3,5-Dimethylbiphenyl</td><td>13.70</td><td>12.50</td><td>4.35E-04</td><td>3.97E-04</td><td>1.03E-05</td><td>4.86E-06</td></tr><tr><td>1-Methylindene</td><td>47.00</td><td>31.10</td><td>1.31E-02</td><td>8.67E-03</td><td>0.00E+00</td><td>2.45E-04</td></tr><tr><td>1,3-Dimethyl-5-ethylbenzene</td><td>32.90</td><td>32.30</td><td>4.23E-03</td><td>4.15E-03</td><td>0.00E+00</td><td>4.73E-04</td></tr></tbody></table>

a Calculated from the water solubility at 25 °C and the molar fraction of each compound in the pure LNAPL phase according to the Raoult's law: Effective solubility i = C<sub>i</sub> × x<sub>i</sub>; with x<sub>i</sub> the molar fraction of the compounds i in the pure LNAPL phase, C<sub>i</sub> the solubility of i at the corresponding temperature (25 °C), and C<sub>eff</sub> the effective solubility of i in the aqueous phase.

(i) The biodegradation of the most biodegradable compounds permanently depleted the dissolved LNAPL plume into saturated and light aromatic compounds and prevented any equilibrium from being reached.

(ii) The dilution of the dissolved plume in the water table resulted in a decrease in the total measured dissolved concentration at 10 cm of elevation each time the water table level rose (as at 60 days).

After 120 days, dissolved concentrations are 25 % higher under the extreme scenario (Fig. 3C). This measurement was taken at the base of the lysimeter, which is farther from the source of contamination (floating-free phase) due to a higher water table level in the extreme scenario. Hence, this increase in dissolved concentrations occurs despite two mitigating factors: (i) potentially more intense biodegradation and (ii) greater dilution of the LNAPL dissolved plume at each water table rise, driven by the larger volume of water injected into the column in the extreme scenario to increase the water level. These results confirmed that the presence of water table fluctuations enhances the contact between LNAPL and water, accelerating dissolution of mobile LNAPL compounds, as previously suggested (Cavelan et al., 2024; Guleria et al., 2023; Gupta et al., 2019; Van De Ven et al., 2021). However, this work also demonstrated that doubling the intensity of groundwater fluctuations does not result in a 2-fold increase in dissolved phase concentrations, as other mitigating processes, such as biodegradation or dilution, are also favoured. As previously mentioned (Cavelan et al., 2022, 2024; Rezanezhad et al., 2014), this process contributes to the faster overall attenuation of contamination observed under the extreme scenario, as evidenced by the greater mass of LNAPL lost after 120 days (Fig. 3A).

### 3.2.4. Limits and perspectives

The use of a large instrumented lysimeter (2 m³) enables a more realistic representation of multiphase processes under controlled climate conditions than small-scale laboratory studies or modelling alone. It also provides, for the first time, quantification and demonstration of the key impact of increased groundwater level fluctuation intensity on the multi-phase redistribution and attenuation of LNAPLs.

Although these findings are constrained by the experimental scale (2 m³ lysimetric columns) and duration (120 days) and do not account for soil heterogeneity, microbiological community dynamics, or vegetation cover - which may influence LNAPL behaviour in natural settings - they clearly demonstrate that even modest changes in precipitation patterns and groundwater level fluctuation can strongly affect LNAPL behaviour. In addition, these results extend previous observations (e.g., Alazaiza et al., 2020; Cavelan et al., 2022, 2024; Gupta et al., 2019; Van De Ven et al., 2021) by demonstrating that not only the presence but also the intensity of groundwater fluctuations significantly alter LNAPL transfer, with cumulative effects on volatilisation and dissolution processes. They also reveal that seasonal groundwater fluctuations, particularly when

intense, create a more favorable environment for natural attenuation, through a more pronounced alternation of oxidising and reducing conditions and the greater redistribution of LNAPL saturation that promotes microbial activity and LNAPL availability.

While these results suggest that climate change may increase the mobility of LNAPLs, their environmental impacts, and short-term risks in many regions where similar hydrological extremes are expected, these findings can also have concrete implications for the management of contaminated sites. Thus, targeting periods of high groundwater level fluctuations for site intervention (through pumping, bioventing, or biological stimulation) could improve treatment efficiency by maximising the pollutant's availability in source zone areas (on provided that the side effects: increased transfer risks, risk of migration, etc. are properly controlled). This strategy could even be considered proactively: indeed, Guleria et al. (2023) have shown that artificially controlled groundwater fluctuations in sandy tanks may significantly increase the recovery of dissolved compounds, such as toluene. Hence, integrating groundwater level dynamics into LNAPL management strategies can be a key lever for optimising treatments, particularly in sandy or relatively permeable and homogeneous aquifers.

## 4. Summary and conclusions

To quantify how climate-driven groundwater level variations' intensity affects LNAPL behaviour in soil, this study compared the fate of an LNAPL-contaminated sandy soil exposed in 2 m³ instrumented lysimetric columns for 120 days under two controlled climatic conditions: one simulated current regional climate records (Grand Est, France), the other the most extreme IPCC scenario (SSP5-8.5 for 2081-2100) characterised by amplified water-table fluctuations. After 120 days, the extreme scenario led to:

* A 50 % increase in the smear zone thickness;

* A 30 % increase in the depletion of the free LNAPL phase in mobilisable compounds;

* A factor 2.8 increase in the mass of BTEX lost by volatilisation;

* A 25 % increase in the total LNAPL dissolved concentration;

* Higher natural attenuation rate (37.5 g/m²/days);

* A reduced contamination lifetime and long-term threat.

These results underscore, for the first time, how climate-driven intensification of groundwater-level dynamics may induce indirect unforeseen environmental consequences by exacerbating the impacts of certain pollutants on soil, air, or water quality. This methodology and findings are transferable to other regions with homogeneous sandy aquifers experiencing similar hydrological stress. However, future investigations are needed to explore the long-term effects of groundwater fluctuations in heterogeneous soils under varying temperature regimes
and to couple these experiments with reactive transport modelling to
improve predictive capabilities.

CRediT authorship contribution statement

**Amélie Cavelan:** Writing – review & editing, Writing – original draft, Methodology, Investigation, Formal analysis, Conceptualization.
**Fabrice Golfier:** Writing – review & editing, Supervision, Project administration, Methodology, Funding acquisition. **Catherine Lorgeoux:** Writing – review & editing, Investigation, Formal analysis.
**Stéfan Colombano:** Writing – review & editing, Supervision, Project administration, Methodology, Funding acquisition. **Anne-Julie Tinet:** Writing – review & editing, Validation, Supervision, Methodology, Investigation.
**Constantin Oltean:** Writing – review & editing, Validation, Supervision, Methodology.
**Dorian Davarzani:** Writing – review & editing, Supervision, Methodology, Conceptualization.
**Jacques Deparis:** Writing – review & editing, Validation, Investigation.
**Noële Enjelvin:** Writing – review & editing, Supervision, Methodology, Investigation.
**Pierre Faure:** Writing – review & editing, Supervision, Project administration, Funding acquisition, Conceptualization.

Declaration of competing interest

The authors declare no competing interests.

Acknowledgements

This work was funded by the French Projet d'Investissement d'Avenir (PIA) "Lorraine Université d'Excellence," reference ANR-15-IDEX-04-LUE (DEEPSUR project), the project ANR BATNAP 2024 reference ANR-24-CE04-4319, the project ADEME BATMOBIL Reference 2272D0067 and the BRGM. This work is included in the scientific program of the GISFI research consortium (Groupement d'Intérêt Scientifique: requalification des territoires dégradés - Interdisciplinarité et Innovation- https://gisfi.univ-lorraine.fr). The authors thank the Lorraine Earth and Environment Observatory (OTELo, https://otelo.univ-lorraine.fr/) and the GISFI for their support in developing the experiment. They also thank the GISFI for providing field site devices at the experimental station in Homécourt and the technical staff (Cyrielle Bonne and Fabien Domptail). They also thank Maxime Cochennec and Lazzat Amangaliyeva for their help during the experiments.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.scitotenv.2025.181070.

Data availability

Data related to this project are available at the following link:
https://doi.org/10.24396/ORDAR-175 shared in the manuscript.

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