Laser-induced fluorescence logging as a high-resolution characterisation tool to assess LNAPL mobility

Contaminant mass discharge estimation of a sulfonamide plume by use of hydraulic profiling tool (HPT) and fluorescence techniques

The contaminant mass discharge is a relevant metric to evaluate the risk that a groundwater plume poses to water resources. However, this assessment is often vitiated by a high uncertainty inherent to the assessment method and often limited number of measurement points to carry out the assessment. Direct-Push techniques in combination with profiling tools and dedicated sampling can be an interesting alternative to increase the measurement point density and hence reduce the mass discharge uncertainty. The main objective of our study was to assess if DP logging and sampling could be employed to get a reasonable estimate of contaminant mass discharge in a large sulfonamide contaminant plume (> 1500 m wide), compared to a more traditional approach based on monitoring wells. To do so, an Hydraulic Profiling Tool (HPT) logging with a dedicated site calibration was used to estimate the hydraulic conductivity field. The sulfonamide concentrations were inferred from the compound fluorescence properties measured by laboratory spectrofluorometry (λEx / λEm = 255/340 nm) and a dedicated log-log linear regression model. Our results show that HPT-derived hydraulic conductivity values are in good agreement with the monitoring well results, and within the order of magnitude reported in similar studies or indirect geophysical techniques. Fluorescence appears as a powerful proxy for the sulfonamide concentration levels. Ultimately, the contaminant mass discharge estimate from HPT and fluorescence techniques lies within a factor 2 from the estimate by monitoring wells, with 549 [274-668] and 776 [695-879] kg/yr respectively. Overall, this study highlights that DP logging tools combined with indirect methods (correlation with fluorescence) could provide a relevant contaminant mass discharge estimate for some optically active substances, given that a proper calibration phase is carried out.

Mapping and monitoring dense non-aqueous phase liquid source zone by fused surface and cross-borehole electrical resistivity tomography

Effective characterization of dense non-aqueous phase liquid (DNAPL) source zones is crucial for remediating polluted sites. DNAPL often reside as residuals or pools within high-permeability lenses and above impermeable layers due to soil heterogeneity, gravity, and capillary barriers. Given the high cost of drilling, electrical resistivity tomography (ERT) techniques-including surface ERT and cross-borehole ERT, are commonly used for DNAPL source zone mapping and monitoring. However, the low spatial resolution of ERT increases uncertainty in source zone investigations. This study proposes a method for improving DNAPL mapping and monitoring by fusing surface and cross-borehole ERT data. Sandbox experiments were conducted to simulate a heterogeneous DNAPL source zone, employing both ERT methods for static mapping and dynamic monitoring. Reflective light imaging (RLM) was used to visualize DNAPL migration and provide saturation data, allowing for the quantification of ERT's effectiveness in characterizing DNAPL distribution. The results indicate that individual ERT methods face significant challenges in DNAPL source zone mapping due to background interference. Surface ERT alone tends to underestimate the extent of deeper DNAPL source zones. However, fusing surface and cross-borehole ERT results in a complementary enhancement of vertical spatial resolution, thereby improving the characterization of DNAPL source zones. The fusion of static and time-lapse ERT data substantially enhances DNAPL source zone mapping and monitoring capabilities. By calculating the ratio of the ERT-monitored area to the actual area using resistivity change contours (5 %, 10 %, 15 %), it was found that fusing surface and cross-borehole ERT data improved monitoring resolution by 50.48 % compared to surface ERT alone and by 22.95 % compared to cross-borehole ERT. Principal component analysis (PCA) was effective in fusing time-lapse data, while the weighted average method (WAM) outperformed PCA for static resistivity data fusion.

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.

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

Contamination by Light Non-Aqueous Phase Liquids (LNAPL) represents a challenge due to the difficulties encountered in its underground assessment and recovery. The major risks arising from subsoil LNAPL accumulation face human health and environment, gaining a social relevance also in the frame of a continuously changing climate. This paper reports on a literature review about the underground contamination by LNAPL, with the aims of providing a categorization of the aspects involved in this topic, analyzing the current state of the art, underlying potential lacks and future perspectives. The review was focused on papers published in the 2012-2022 time-interval, in journals indexed in Scopus and WoS databases, by querying "LNAPL" within article title, abstract and/or key words. 245 papers were collected and classified according to three "key approaches" -namely laboratory activity, field based-data studies and mathematical simulations- and subordinate "key themes", so to allow summarizing and commenting the main aspects based on the application setting, content and scope. Results show that there is a wide experience on plume dynamics and evolution, detection and monitoring through direct and indirect surveys, oil recovery and natural attenuation processes. Few cues of innovations were found regarding both the use of new materials and/or specific field configuration for remediation, and the application of new techniques for plume detection. Some limitations were found in the common oversimplification of the polluted media in laboratory or mathematical models, where the contamination is set within homogeneous porous environments, and in the low number of studies focused on rock masses, where the discontinuous hydraulic behavior complicates the address and modeling of the issue. This paper represents a reference for a quick update on the addressed topic, along with a starting point to develop new ideas and cues for the advance in one of the greatest environmental banes of the current century.

Advancing "Autonomous" sensing and prediction of the subsurface environment: a review and exploration of the challenges for soil and groundwater contamination

Can we hope for autonomous (self-contained in situ) sensing of subsurface soil and groundwater pollutants to satisfy relevant regulatory criteria? Global advances in sensors, communications, digital technologies, and computational capacity offer this potential. Here we review past efforts to advance subsurface investigation techniques and technologies, and computational efforts to create a digital twin (representation) of subsurface processes. In the context of the potential to link measurement and sensing to a digital twin computation platform, we outline five criteria that might make it possible. Significant advances in sensors based on passive measurement devices are proposed. As an example of what might be achievable, using the five criteria, we describe the deployment of online real-time sensors and simulations for a case study of a petroleum site where natural source zone depletion (NSZD) is underway as a potential biodegradation management option, and where a high-quality conceptual site model is available. Multiple sensors targeting parameters (major gases and temperature influenced by soil moisture) relevant to the subsurface NSZD biodegradation processes are shown to offer the potential to map subsurface processes spatially and temporally and provide continuous estimates of degradation rates for management decisions, constrained by a computational platform of the key processes. Current limitations and gaps in technologies and knowledge are highlighted specific to the case study. More generally, additional key advances required to achieve autonomous sensing of subsurface soil and groundwater pollutants are outlined.

Natural Source Zone Depletion (NSZD) Quantification Techniques: Innovations and Future Directions

Natural source zone depletion (NSZD) is an emerging technique for sustainable and cost-effective bioremediation of light non-aqueous phase liquid (LNAPL) in oil spill sites. Depending on regulatory objectives, NSZD has the potential to be used as either the primary or sole LNAPL management technique. To achieve this goal, NSZD rate (i.e., rate of bulk LNAPL mass depletion) should be quantified accurately and precisely. NSZD has certain characteristic features that have been used as surrogates to quantify the NSZD rates. This review highlights the most recent trends in technology development for NSZD data collection and rate estimation, with a focus on the operational and technical advantages and limitations of the associated techniques. So far, four principal techniques are developed, including concentration gradient (CG), dynamic closed chamber (DCC), CO2 trap and thermal monitoring. Discussions revolving around two techniques, “CO2 trap” and “thermal monitoring”, are expanded due to the particular attention to them in the current industry. The gaps of knowledge relevant to the NSZD monitoring techniques are identified and the issues which merit further research are outlined. It is hoped that this review can provide researchers and practitioners with sufficient information to opt the best practice for the research and application of NSZD for the management of LNAPL impacted sites.