Groundwater and soil contamination by LNAPL: State of the art and future challenges

Enhanced treatment of multiphase extraction wastewater from contaminated sites with Cu-Ce modified GAC three-dimensional electrodes

A three-dimensional (3D) electrode system is widely recognized as an effective technology for enhancing electrocatalytic effect. In this study, Cu-Ce modified granular activated carbon (GAC) particle electrodes were prepared using the impregnation method and applied to handle multiphase extraction wastewater. Structural and electrochemical characterization revealed that while the specific surface area of Cu-Ce/GAC decreased by 13.94%, the active area was 2.6 times greater than that of GAC. In addition, the influences of distinct impregnation concentrations, calcination temperatures, and calcination times on the performance of Cu-Ce/GAC electrodes were investigated. The results suggested the optimal preparation conditions of 15 mmol/L, 500 °C and 2 h. Under these conditions, the Cu-Ce/GAC electrode achieved a 92.39% removal of chemical oxygen demand (COD) from a multi-extract of groundwater, with an energy consumption of 13.44 kWh/(kg∙COD). The degradation efficiency improved by 62% compared to the conventional 2D system, while energy consumption decreased by 60%. The main organic pollutants in the multiple extracts, including benzene, toluene, dichloromethane, trichloromethane, were removed at rates exceeding 90% after 60 min treatment. This study yields a methodological and engineering approach for treating multiple extracts wastewater from contaminated groundwater.

Groundwater electro-bioremediation via diffuse electro-conductive zones: A critical review

Microbial electrochemical technologies (MET) can remove a variety of organic and inorganic pollutants from contaminated groundwater. However, despite significant laboratory-scale successes over the past decade, field-scale applications remain limited. We hypothesize that enhancing the electrochemical conductivity of the soil surrounding electrodes could be a groundbreaking and cost-effective alternative to deploying numerous high-surface-area electrodes in short distances. This could be achieved by injecting environmentally safe iron- or carbon-based conductive (nano)particles into the aquifer. Upon transport and deposition onto soil grains, these particles create an electrically conductive zone that can be exploited to control and fine-tune the delivery of electron donors or acceptors over large distances, thereby driving the process more efficiently. Beyond extending the radius of influence of electrodes, these diffuse electro-conductive zones (DECZ) could also promote the development of syntrophic anaerobic communities that degrade contaminants via direct interspecies electron transfer (DIET). In this review, we present the state-of-the-art in applying conductive materials for MET and DIET-based applications. We also provide a comprehensive overview of the physicochemical properties of candidate electrochemically conductive materials and related injection strategies suitable for field-scale implementation. Finally, we illustrate and critically discuss current and prospective electrochemical and geophysical methods for measuring soil electronic conductivity-both in the laboratory and in the field-before and after injection practices, which are crucial for determining the extent of DECZ. This review article provides critical information for a robust design and in situ implementation of groundwater electro-bioremediation processes.

Influence of dissolved and non-aqueous phase toluene on spectral induced polarization signatures of soils

Fifty-two laboratory experiments are undertaken to analyze the sensitivity of spectral induced polarization (SIP) to the presence of toluene in soils. Among these experiments, four experiments are conducted to collect SIP responses of soils containing dissolved phase toluene within the pore water using columns. The results demonstrate that SIP is not sensitive to the presence of dissolved phase toluene in soils. The remaining forty-eight experiments are undertaken with four types of soils mixed with non-aqueous phase toluene. The experimental results prove that SIP is sensitive to toluene saturation under varying salinity conditions. These observations are well-explained by a published petrophysical model accounting for the effects of water saturation on complex conductivity. The water saturation exponent n and quadrature conductivity exponent p in this model are obtained by fitting complex conductivity data versus saturation at different saturation levels. The petrophysical model is tested where in-phase and quadrature conductivity responses are predicted from water saturation, soil cation exchange capacity (CEC), and pore water conductivity. The petrophysical model provides satisfactory predictions for non-aqueous phase toluene saturation. Overall, this study contributes to our understanding of SIP as a non-intrusive tool for characterizing toluene contamination in soils with applications to the field.

Assessing data variability in groundwater quality monitoring of contaminated sites through factor analysis and multiple linear regression models

Monitoring of long-term contaminant concentrations trends is essential to verify that attenuation processes are effectively occurring at a site. However, monitoring data are often affected by extreme variability which prevents the identification of clear concentration trends. The variability is higher in long-screened monitoring wells, which are currently used at many contaminated sites, although it has been known since the 1980s that monitoring data from long-screened wells can be biased. Understanding the factors that may influence the variability of monitoring data is pivotal. To this end, following hydrochemical conceptual modelling using a multi-method approach, the variability of hydrocarbon concentrations from fully screened monitoring wells was assessed over eleven years at a former oil refinery located in Northern Italy. The proposed methodology combined factor analysis with multiple linear regression models. Results pointed out a higher variability in hydrocarbon concentrations at the plume fringe and a lower variability at the plume source and core. 44-46 % of the total variability in measured hydrocarbon concentrations is due to "intrinsic plume heterogeneity", related to the three-dimensional structure of a contaminant plume, which becomes thinner at the edge, creating a vertical heterogeneity of redox conditions at the plume fringe. This variability, expressed as increasing concentrations of sulfate and decreasing concentrations of methane, represents a background variability that cannot be reduced by improving sampling procedures. The remaining 56-54 % of the total variability may be due to the non-standardization of some purging and sampling operations, such as pump intake position, purging and sampling time/flow rates and variations in the analytical methods. This finding suggests that monitoring improvements in fully screened wells by standardizing all purging/sampling operations or using sampling techniques that can reduce the actual screen length (e.g., packers or separation/dual pumping techniques) would reduce data variability by more than half.

Pilot-scale experimental study on the enhanced natural attenuation of complex organic contaminants based on the recharge of electron acceptors

Anaerobic biodegradation plays a crucial role in attenuating organic contaminants in natural aquifers, where the concentration and type of electron acceptors directly determine the stages and rates of degradation progress. In this study, nitrate depletion was monitored in a simulated pilot-scale aquifer contaminated with toluene and trichloroethylene, while sulfate became the new periodic electron acceptor, accompanied by a decrease in the contaminant attenuation rate. Consequently, nitrate was injected into the contaminated plume in stages, and the hydro- and biochemical impacts were further monitored. The results revealed that the active recharge of nitrate became a new electron donor involved in anaerobic reduction processes, with contaminants attenuation rates increased by 2.48-3.88 times compared with that before injection. Moreover, the active recharge of nitrate inhibited further consumption of sulfate and oxidized pre-existing sulfides, which reduced the biological toxicity of the aquatic environment and provided favorable conditions for the growth of nitrate-reducing microorganisms. This study evaluated the enhancement of natural attenuation by recharging nitrate as an electron acceptor, providing a theoretical basis for environmentally friendly and economic remediation of petroleum-contaminated aquifers.

Co-migration behavior of toluene coupled with trichloroethylene and the response of the pristine groundwater ecosystems - A mesoscale indoor experiment

Experimental scale and sampling precision are the main factors limiting the accuracy of migration and transformation assessments of complex petroleum-based contaminants in groundwater. In this study, a mesoscale indoor aquifer device with high environmental fidelity and monitoring accuracy was constructed, in which dissolved toluene and trichloroethylene were used as typical contaminants in a 1.5-year contaminant migration experiment. The process was divided into five stages, namely, pristine, injection, accumulation, decrease, and recovery, and characteristics such as differences in contaminant migration, the responsiveness of environmental factors, and changes in microbial communities were investigated. The results demonstrated that the mutual dissolution properties of the contaminants increased the spread of the plume and confirmed that toluene possessed greater mobility and natural attenuation than trichloroethylene. Attenuation of the contaminant plume proceeded through aerobic degradation, nitrate reduction, and sulfate reduction phases, accompanied by negative feedback from characteristic ion concentrations, dissolved oxygen content, the oxidation-reduction potential and microbial community structure of the groundwater. This research evaluated the migration and transformation characteristics of typical petroleum-based pollutants, revealed the response mechanism of the ecosystem to pollutant, provided a theoretical basis for predicting pollutant migration and formulating control strategies.

Advanced oxidation of polycyclic aromatic hydrocarbons in tropical soil: Self-catalytic utilization of natural iron contents in an oxygenation reactor supported with persulfate

The catalysts derived from natural iron minerals in the advanced oxidation process offer several advantages. However, their utilization in soil remediation is restricted due to the presence of soil impurities, which can inhibit the catalytic activity of these minerals. The soils in tropical regions exhibit lower organic matter content, limited cation exchange capacity, and are non-saline, this enhances the efficiency of utilizing natural iron minerals from tropical soil as a catalyst. In this regard, the catalytic potential of naturally iron-bearing tropical soil was investigated to eliminate phenanthrene (PHE), pyrene (PYR), and benzo[α]pyrene (B[α]P) using an oxygenated reactor supported with persulfate (PS). The system showed an efficient performance, and the removal efficiencies under the optimum conditions were 81 %, 73 %, and 86 % for PHE, PYR, and B[α]P, respectively. This indicated that the catalytic activity of iron was working efficiently. However, there were changes in the soil characteristics after the remediation process such as a significant reduction in iron and aluminum contents. The scavenging experiments demonstrated that HO• had a minor role in the oxidation process, SO4•- and O2•- emerged as the primary reactive species responsible for the effective degradation of the PAHs. Moreover, the by-products were monitored after soil remediation to evaluate their toxicity and to propose degradation pathways. The Mutagenicity test showed that two by-products from each PHE and B[α]P had positive results, while only one by-product of PYR showed positive. The toxicity tests of oral rat LD50 and developmental toxicity tests revealed that certain PAHs by-products could be more toxic from the parent pollutant itself. This study represents a notable progression in soil remediation by providing a step forward in the application of the advanced oxidation process (AOP) without requiring additional catalysts to activate oxidants and degrade pollutant PAHs from the soil.

Experimental research on the transport-transformation of organic contaminants under the influence of multi-field coupling at a site scale

Organic-contaminated shallow aquifers have become a global concern of groundwater contamination, yet little is known about the coupled effects of hydrodynamic-thermal-chemical-microbial (HTCM) multi-field on organic contaminant transport and transformation over a short time in aquifers. Therefore, this study proposed a quick and efficient field experimental method for the transport-transformation of contaminants under multi-field coupling to explore the relationship between organic contaminants (total petroleum hydrocarbon (TPH), polycyclic aromatic hydrocarbons (PAHs), benzene-toluene-ethylbenzene-xylene (BTEX) and phthalates acid esters (PAEs)) and multi-field factors. The results showed that hydrodynamics (affecting pH, p < 0.001) and temperature (affecting dissolved oxygen, pH and HCO3-, p < 0.05) mainly affected the organic contaminants indirectly by influencing the hydrochemistry to regulate redox conditions in the aquifer. The main degradation reactions of the petroleum hydrocarbons (TPH, PAHs and BTEX) and PAEs in the aquifer were sulfate reduction and nitrate reduction, respectively. Furthermore, the organic contamination was directly influenced by microbial communities, whose spatial patterns were shaped by the combined effects of the spatial pattern of hydrochemistry (induced by the organic contamination pressure) and other multi-field factors. Overall, our findings imply that the spatiotemporal patterns of organic contaminants are synergistically regulated by HTCM, with distinct mechanisms for petroleum hydrocarbons and PAEs.

Radon deficit technique applied to the study of the ageing of a spilled LNAPL in a shallow aquifer

A recent diesel spill (dated January 2019 ± 1 month) in a refilling station is investigated by the Radon deficit technique. The primary focus was on quantifying the LNAPL pore saturation as a function of duration of ageing, and on proposing a predictive model for on-site natural attenuation. A biennial monitoring of the local fluctuating shallow aquifer has involved the saturated zone nine times, and the vadose zone only once. Rn background generally measured in external and upstream wells is elaborated further due to the site characteristics, using drilling logs and phreatic oscillations. Notably, this study marks the first application of the Rn deficit method to produce a detailed Rn background mapping throughout the soil depth. Simultaneously, tests are performed on LNAPL surnatant samples to study diesel ageing. In particular, they are focused on temporal variations of LNAPL viscosity (from an initial 3.90 cP to 8.99 cP, measured at 25 °C, after 34 months), and Rn partition coefficient between the pollutant and water (from 47.7 to 80.2, measured at 25 °C, after 14 months). Rn diffusion is also measured in different fluids (0.092 cm2 s-1, 1.14 × 10-5 cm2 s-1, and 2.53 × 10-6 cm2 s-1 at 25 °C for air, water and LNAPL, respectively) directly. All parameters and equations utilized during this study are introduced, discussing their influence on Radon deficit technique from a theoretical point of view. Experimental findings are used to mitigate the effect of LNAPL ageing and of phreatic oscillations on determination of LNAPL saturation index (S.I.LNAPL). Finally, S.I.LNAPL dataset is discussed and elaborated to show the pollutant attenuation across subsurface over time, induced by natural processes primarily. The proposed predictive model for on-site natural attenuation suggests a half-removal time of one year and six months. The significance of such models lies in their capability to assess site-specific reactions to pollutants, thereby enhancing the effectiveness of remediation efforts over time. These experimental findings may offer a novel approach to application of Rn deficit technique and to environmental remediation of persistent organic compounds.

Emerging paradigms into bioremediation approaches for nuclear contaminant removal: From challenge to solution

The release of radionuclides, including Cesium-137 (137Cs), Strontium-90 (90Sr), Uranium-238 (238U), Plutonium-239 (239Pu), Iodine-131 (131I), etc., from nuclear contamination presents profound threats to both the environment and human health. Traditional remediation methods, reliant on physical and chemical interventions, often prove economically burdensome and logistically unfeasible for large-scale restoration efforts. In response to these challenges, bioremediation has emerged as a remarkably efficient, environmentally sustainable, and cost-effective solution. This innovative approach harnesses the power of microorganisms, plants, and biological agents to transmute radioactive materials into less hazardous forms. For instance, consider the remarkable capability demonstrated by Fontinalis antipyretica, a water moss, which can accumulate uranium at levels as high as 4979 mg/kg, significantly exceeding concentrations found in the surrounding water. This review takes an extensive dive into the world of bioremediation for nuclear contaminant removal, exploring sources of radionuclides, the ingenious resistance mechanisms employed by plants against these harmful elements, and the fascinating dynamics of biological adsorption efficiency. It also addresses limitations and challenges, emphasizing the need for further research and implementation to expedite restoration and mitigate nuclear pollution's adverse effects.

An experimental multi-method approach to better characterize the LNAPL fate in soil under fluctuating groundwater levels

Light-Non-Aqueous phase liquids (LNAPLs) are important soil contamination sources, and groundwater fluctuations may significantly affect their migration and release. However, the risk assessment remains complex due to the continuous three-phase fluid redistribution caused by water table level variations. Hence, monitoring methods must be improved to integrate better the LNAPL multi-compound and multi-phase aspects tied to the groundwater level dynamics. For this purpose, a lysimetric contaminated soil column (2 m3) combining in-situ monitoring (electrical permittivity, soil moisture, temperature, pH, Eh), direct water and gas sampling and analyses (GC/MS-TQD, μGC) in monitoring well, gas collection chambers, and suction probes) were developed. This experiment assesses in an integrated way how controlled rainfalls and water table fluctuation patterns may affect LNAPL vertical soil saturation distribution and release. Coupling these methods permitted the investigation of the effects of rainwater infiltration and water table level fluctuation on contaminated soil oxygen turnover, LNAPL contaminants' soil distribution and remobilization towards the dissolved and the gaseous phase, and the estimate of the LNAPL source attenuation rate. Hence, 7.5% of the contamination was remobilized towards the dissolved and gaseous phase after 120 days. During the experiment, groundwater level variations were responsible for the free LNAPL soil spreading and trapping, modifying dissolved LNAPL concentrations. Nevertheless, part of the dissolved contamination was rapidly biodegraded, leaving only the most bio-resistant components in water. This result highlights the importance of developing new experimental devices designed to assess the effect of climate-related parameters on LNAPL fate at contaminated sites.

On quantifying global carbon emission from oil contaminated lands over centuries

Petroleum releases into the subsurface contribute to global soil carbon emissions. Quantifying releases and changes in releases of carbon from soils over the lifetime of a spill is complex. Natural source zone depletion (NSZD) of light non-aqueous phase liquids (LNAPLs) embodies all key mechanisms for transformation to carbon gases and their release from soils including partitioning, transport and degradation of petroleum components. Quantification of the interconnected behaviours of the soil microbiome, fluid flow, multi-component transport, partitioning, and biodegradation is crucial for understanding NSZD. Volatilization from LNAPL, aerobic biodegradation, methanogenesis, and heat production all lead to release of greenhouse gases to the atmosphere. To estimate carbon emissions, using a validated computational platform, we modelled the long term NSZD of four petroleum hydrocarbon types; crude oil, diesel, jet fuel and gasoline, to span the major products used globally. For two soil types, we estimated 150 years of carbon emissions from annual minor and 25 mostly major petroleum hydrocarbon land release incidents since 1950 - with an estimated released mass of ~9 million tonnes across the circumstances considered. Up to 2100 the mass of carbon emitted to the atmosphere is estimated to range from 4 to 6 Teragrams, with nearly 60 % currently released. Nomographs generated help predict the fate of LNAPL plumes and carbon emissions due to NSZD, which is crucially important to management of soil and groundwater contamination. The method provides a basis to include additionally identified and future petroleum releases. It is noted that the petroleum mixture composition, degradation rates, volatilization, and subsurface characteristics all can influence carbon emission estimations.

LNAPL migration processes based on time-lapse electrical resistivity tomography

Contamination from light non-aqueous phase liquids (LNAPLs) and their derivatives, arising from exploration, production, and transportation, has become a prevalent pollution source. This poses direct threats to human health. However, conventional investigative methods face limitations when applied to studying the extent and migration process of LNAPL contamination, as well as the redistribution of LNAPL during groundwater level fluctuations. Conventional methods lack the ability to rapidly, efficiently, and in real-time acquire information about contaminated areas. Therefore, this study utilizes time-lapse electrical resistivity tomography to investigate the migration mechanism of LNAPL under unsaturated conditions, constant groundwater levels, and groundwater level reductions. A relationship between resistivity and water and oil contents was established and used for inverse calculation of LNAPL content via resistivity inversion. Time-lapse electrical resistivity tomography revealed LNAPL migration in a "concave" shape across three conditions. Groundwater presence notably slowed migration, hindering downward movement and leading to a floating oil band. A robust mathematical model was established to derive the relationship between resistivity and water and oil contents. Finally, LNAPL distribution under unsaturated conditions was inversely obtained from resistivity data, showing highest content at the top leak point, obstructed area, and bottom of soil column. Consequently, time-lapse electrical resistivity tomography demonstrates a notable capacity to characterize the LNAPL migration process. This technique constitutes an effective geophysical method for monitoring and describing the characteristics of LNAPL migration. Its significance lies in enhancing our understanding of remediation for LNAPL-induced groundwater and land contamination.

Using groundwater monitoring wells for rapid application of soil gas radon deficit technique to evaluate residual LNAPL

The application of the 222Radon (Rn) deficit technique using subsurface soil gas probes for the identification and quantification of light non-aqueous phase liquids (LNAPL) has provided positive outcomes in recent years. This study presents an alternative method for applying this technique in the headspace of groundwater monitoring wells. The developed protocol, designed for groundwater monitoring wells with a portion of their screen in the vadose zone, is based on the use of portable equipment that allows rapid measurement of the Rn soil gas activity in the vadose zone close to the water table (i.e., smear zone) where LNAPL is typically expected. The paper first describes the step-by-step procedure to be followed for the application of this method. Then, a preliminary assessment of the potential of the method was carried out at two Italian sites characterized by accidental gasoline and diesel spills into the subsurface from underground storage tanks. Although the number of tests conducted does not allow for definitive conclusions, the results obtained suggest that, from a qualitative point of view, Rn monitoring in the headspace of monitoring wells is a promising, fast, and minimally invasive screening method that could also potentially reduce the costs associated with field data acquisition. This method proves to be suitable for detecting the presence of LNAPL in both the mobile and residual phases with results consistent with the other lines of evidence available at the sites, such as groundwater and soil gas monitoring. Future efforts should be directed toward evaluating the accuracy of this method for a quantitative assessment of residual LNAPL saturations.