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In developing countries, remediation projects are predominantly implemented in areas with imminent risks due to human exposure routes. Projects experience challenges due to funding constraints deri...

Underground natural gas (NG) leaks pose an urgent safety threat, motivating ongoing efforts to improve leak detection methods. The objectives of this study were to investigate how realistic environ...

International audience

Meaningful model-based predictions of water quality and quantity are imperative for the designed footprint of managed aquifer recharge installations. A two-dimensional (2D) synthetic MAR system equipped with automated sensors (temperature, water pressure, conductivity, soil moisture, oxidation-reduction potential) and embedded water sampling ports was used to test and model fundamental subsurface processes during surface spreading managed aquifer recharge operations under controlled flow and redox conditions at the meso-scale. The fate and transport of contaminants in the variably saturated synthetic aquifer were simulated using the finite element analysis model, FEFLOW. In general, the model concurred with travel times derived from contaminant breakthrough curves at individual sensor locations throughout the 2D tank. However, discrepancies between measured and simulated trace organic chemical concentrations (i.e., carbamazepine, sulfamethoxazole, tris (2-chloroethyl) phosphate, trimethoprim) were observed. While the FEFLOW simulation of breakthrough curves captured overall shapes of trace organic chemical concentrations well, the model struggled with matching individual data points, although compound-specific attenuation parameters were used. Interestingly, despite steady-state operation, oxidation-reduction potential measurements indicated temporal disturbances in hydraulic properties in the saturated zone of the 2D tank that affected water quality.

Aquifers show troubling signs of irreversible depletion as climate change, population growth, and urbanization lead to reduced natural recharge rates and overuse. One strategy to sustain the groundwater supply is to recharge aquifers artificially with reclaimed water or stormwater via managed aquifer recharge and recovery (MAR) systems. Unfortunately, MAR systems remain wrought with operational challenges related to the quality and quantity of recharged and recovered water stemming from a lack of data-driven, real-time control. This paper presents a laboratory scale proof-of-concept study that demonstrates the capability of a real-time, simulation-based control optimization algorithm to ease the operational challenges of MAR systems. Central to the algorithm is a model that simulates water flow and transport of dissolved chemical constituents in the aquifer. The algorithm compensates for model parameter uncertainty by continually collecting data from a network of sensors embedded within the aquifer. At regular intervals the sensor data is fed into an inversion algorithm, which calibrates the uncertain parameters and generates the initial conditions required to model the system behavior. The calibrated model is then incorporated into a genetic algorithm that executes simulations and determines the best management action, for example, the optimal pumping policy for current aquifer management goals. Experiments to calibrate and validate the simulation-optimization algorithm were conducted in a small two-dimensional synthetic aquifer under both homogeneous and heterogeneous packing configurations. Results from initial experiments validated the feasibility of the approach and suggested that our system could improve the operation of full-scale MAR facilities.

In-situ bioremediation, a widely applied treatment technology for source zones contaminated with dense non-aqueous phase liquids (DNAPLs), has proven economical and reasonably efficient for long-term management of contaminated sites. Successful application of this remedial technology, however, requires an understanding of the complex interaction of transport, mass transfer, and biotransformation processes. The bioenhancement factor, which represents the ratio of DNAPL mass transfer under microbially active conditions to that which would occur under abiotic conditions, is commonly used to quantify the effectiveness of a particular bioremediation remedy. To date, little research has been directed towards the development and validation of methods to predict bioenhancement factors under conditions representative of real sites. This work extends an existing, first-order, bioenhancement factor expression to systems with zero-order and Monod kinetics, representative of many source-zone scenarios. The utility of this model for predicting the bioenhancement factor for previously published laboratory and field experiments is evaluated. This evaluation demonstrates the applicability of these simple bioenhancement factors for preliminary experimental design and analysis, and for assessment of dissolution enhancement in ganglia-contaminated source zones. For ease of application, a set of nomographs is presented that graphically depicts the dependence of bioenhancement factor on physicochemical properties. Application of these nomographs is illustrated using data from a well-documented field site. Results suggest that this approach can successfully capture field-scale, as well as column-scale, behavior. Sensitivity analyses reveal that bioenhanced dissolution will critically depend on in-situ biomass concentrations.

Evaporation and condensation in bare soils govern water and energy fluxes between the land and atmosphere. Phase change between liquid water and water vapor is commonly evaluated in soil hydrology using an assumption of instantaneous phase change (i.e. chemical equilibrium). Past experimental studies have shown that finite volatilization and condensation times can be observed under certain environmental conditions, thereby questioning the validity of this assumption. A comparison between equilibrium and non-equilibrium phase change modeling approaches showed that the latter is able to provide better estimates of evaporation, justifying the need for more research on this topic. Several formulations based on irreversible thermodynamics, first order reaction kinetics, or the kinetic theory of gases have been employed to describe non-equilibrium phase change at the continuum scale. In this study, results from a fully coupled non-isothermal heat and mass transfer model applying four different non-equilibrium phase change formulations were compared with experimental data generated under different initial and boundary conditions. Results from a modified Hertz-Knudsen formulation based on kinetic theory of gases, proposed herein, were consistently in best agreement in terms of preserving both magnitude and trends of experimental data under all environmental conditions analyzed. Simulation results showed that temperature dependent formulations generally better predict evaporation than formulations independent of temperature. Analysis of vapor concentrations within the porous media showed that conditions were not at equilibrium under the experimental conditions tested. This article is protected by copyright. All rights reserved.

The generation of vapor-phase contaminant plumes within the vadose zone is of interest for contaminated site management. Therefore, it is important to understand vapor sources such as non-aqueous-phase liquids (NAPLs) and processes that govern their volatilization. The distribution of NAPL, gas, and water phases within a source zone is expected to influence the rate of volatilization. However, the effect of this distribution morphology on volatilization has not been thoroughly quantified. Because field quantification of NAPL volatilization is often infeasible, a controlled laboratory experiment was conducted in a two-dimensional tank (28 cm × 15.5 cm × 2.5 cm) with water-wet sandy media and an emplaced trichloroethylene (TCE) source. The source was emplaced in two configurations to represent morphologies encountered in field settings: (1) NAPL pools directly exposed to the air phase and (2) NAPLs trapped in water-saturated zones that were occluded from the air phase. Airflow was passed through the tank and effluent concentrations of TCE were quantified. Models were used to analyze results, which indicated that mass transfer from directly exposed NAPL was fast and controlled by advective-dispersive-diffusive transport in the gas phase. However, sources occluded by pore water showed strong rate limitations and slower effective mass transfer. This difference is explained by diffusional resistance within the aqueous phase. Results demonstrate that vapor generation rates from a NAPL source will be influenced by the soil water content distribution within the source. The implications of the NAPL morphology on volatilization in the context of a dynamic water table or climate are discussed.

In an accompanying paper, we presented an overview of a wide variety of modeling concepts, varying in complexity, used to describe evaporation from soil. Using theoretical analyses, we explained the simplifications and parameterizations in the different approaches. In this paper, we numerically evaluate the consequences of these simplifications and parameterizations. Two sets of simulations were performed. The first set investigates lateral variations in vertical fluxes, which emerge from both homogeneous and heterogeneous porous media, and their importance to capturing evaporation behavior. When evaporation decreases from parts of the heterogeneous soil surface, lateral flow and transport processes in the free flow and in the porous medium generate feedbacks that enhance evaporation from wet surface areas. In the second set of simulations, we assume that the vertical fluxes do not vary considerably in the simulation domain and represent the system using one-dimensional models which also consider dynamic forcing of the evaporation process, for example, due to diurnal variations in net radiation. Simulated evaporation fluxes subjected to dynamic forcing differed considerably between model concepts depending on how vapor transport in the air phase and the interaction at the interface between the free flow and porous medium were represented or parameterized. However, simulated cumulative evaporation losses from initially wet soil profiles were very similar between model concepts and mainly controlled by the desorptivity, Sevap, of the porous medium, which depends mainly on the liquid flow properties of the porous medium.

Evaporation is an important component of the soil water balance. It is composed of water flow and transport processes in a porous medium that are coupled with heat fluxes and free air flow. This work provides a comprehensive review of model concepts used in different research fields to describe evaporation. Concepts range from nonisothermal two-phase flow, two-component transport in the porous medium that is coupled with one-phase flow, two-component transport in the free air flow to isothermal liquid water flow in the porous medium with upper boundary conditions defined by a potential evaporation flux when available energy and transfer to the free airflow are limiting or by a critical threshold water pressure when soil water availability is limiting. The latter approach corresponds with the classical Richards equation with mixed boundary conditions. We compare the different approaches on a theoretical level by identifying the underlying simplifications that are made for the different compartments of the system: porous medium, free flow and their interface, and by discussing how processes not explicitly considered are parameterized. Simplifications can be grouped into three sets depending on whether lateral variations in vertical fluxes are considered, whether flow and transport in the air phase in the porous medium are considered, and depending on how the interaction at the interface between the free flow and the porous medium is represented. The consequences of the simplifications are illustrated by numerical simulations in an accompanying paper.

This paper presents a novel method to couple an environmental bioremediation system with a subsurface renewable energy storage system. This method involves treating unsaturated contaminated soil using in-situ thermally enhanced bioremediation, the thermal system is powered by renewable energy. After remediation goals are achieved, the thermal system can then be used to store renewable energy in the form of heat in the subsurface for later use. This method can be used for enhanced treatment of environmental pollutants for which temperature is considered a limiting factor. For instance, this system can be used at a wide variety of petroleum-related sites that are likely contaminated with hydrocarbons such as oil refineries and facilities with above- and underground storage tanks. In this paper, a case-study example was analyzed using a previously developed numerical model of heat transfer in unsaturated soil. Results demonstrate that coupling energy storage and thermally-enhanced bioremediation systems offer an efficient and sustainable way to achieve desired temperature&ndash, moisture distribution in soil that will ultimately enhance the microbial activity.

Implementation of injection-extraction treatment well pairs for in situ, in-well, or on-site remediation may be facilitated by development and application of modeling tools to aid in hydraulic design and remediation technology selection. In this study, complex potential theory was employed to derive a simple one-step design equation and related type curves that permit the calculation of the extraction well capture zone and the hydraulic recirculation between an injection and extraction well pair oriented perpendicular to regional flow. This equation may be used to aid in the design of traditional fully screened injection-extraction wells as well as innovative tandem recirculating wells when an adequate geologic barrier to vertical ground water flow exists. Simplified models describing in situ bioremediation, in-well vapor stripping, and in-well metal reactor treatment efficiency were adapted from the literature and coupled with the hydraulic design equation presented here. Equations and type curves that combine the remediation treatment efficiency with the hydraulic design equation are presented to simulate overall system treatment efficiency under various conditions. The combined model is applied to predict performance of in situ bioremediation and in-well palladium reactor designs that were previously described in the literature. This model is expected to aid practitioners in treatment system screening and evaluation.

International audience; In an effort to develop methods based on integrating the subsurface to the atmospheric boundary layer to estimate evaporation, we developed a model based on the coupling of Navier-Stokes free flow and Darcy flow in porous medium. The model was tested using experimental data to study the effect of wind speed on evaporation. The model consists of the coupled equations of mass conservation for two-phase flow in porous medium with single-phase flow in the free-flow domain under nonisothermal, nonequilibrium phase change conditions. In this model, the evaporation rate and soil surface temperature and relative humidity at the interface come directly from the integrated model output. To experimentally validate numerical results, we developed a unique test system consisting of a wind tunnel interfaced with a soil tank instrumented with a network of sensors to measure soil-water variables. Results demonstrated that, by using this coupling approach, it is possible to predict the different stages of the drying process with good accuracy. Increasing the wind speed increases the first stage evaporation rate and decreases the transition time between two evaporative stages (soil water flow to vapor diffusion controlled) at low velocity values; then, at high wind speeds the evaporation rate becomes less dependent on the wind speed. On the contrary, the impact of wind speed on second stage evaporation (diffusion-dominant stage) is not significant. We found that the thermal and solute dispersion in free-flow systems has a significant influence on drying processes from porous media and should be taken into account.

[1] Bare soil evaporation is a key process for water exchange between the land and the atmosphere and an important component of the water balance. However, there is no agreement on the best modeling methodology to determine evaporation under different atmospheric boundary conditions. Also, there is a lack of directly measured soil evaporation data for model validation to compare these methods to establish the validity of their mathematical formulations. Thus, a need exists to systematically compare evaporation estimates using existing methods to experimental observations. The goal of this work is to test different conceptual and mathematical formulations that are used to estimate evaporation from bare soils to critically investigate various formulations and surface boundary conditions. Such a comparison required the development of a numerical model that has the ability to incorporate these boundary conditions. For this model, we modified a previously developed theory that allows nonequilibrium liquid/gas phase change with gas phase vapor diffusion to better account for dry soil conditions. Precision data under well-controlled transient heat and wind boundary conditions were generated, and results from numerical simulations were compared with experimental data. Results demonstrate that the approaches based on different boundary conditions varied in their ability to capture different stages of evaporation. All approaches have benefits and limitations, and no one approach can be deemed most appropriate for every scenario. Comparisons of different formulations of the surface boundary condition validate the need for further research on heat and vapor transport processes in soil for better modeling accuracy.

[1] In the shallow subsurface immediately below the land-atmosphere interface, it is widely recognized that the movement of water vapor is closely coupled to thermal processes. However, their mutual interactions are rarely considered in most soil water modeling efforts or in practical applications where it becomes necessary to understand the spatial and temporal distribution of soil moisture. The validation of numerical models that are designed to capture these processes is difficult due to the scarcity of field or laboratory data with accurately known hydraulic and thermal parameters of soils, limiting the testing and refinement of heat and water transfer theories. The goal of this paper is to perform controlled experiments under transient conditions of soil moisture and temperature and use this data to test existing theories and develop appropriate numerical models. Water vapor flow under varying temperature gradients was implemented on the basis of a concept that allows nonequilibrium liquid/gas phase change with gas phase vapor diffusion. To validate this new approach, we developed a long column apparatus equipped with a network of sensors and generated data under well-controlled thermal boundary conditions at the soil surface. The nonequilibrium approach yielded good agreement with the experimental results, validating the hypothesis that transport in the gas phase is better suited to be modeled with nonequilibrium liquid/gas phase change for highly transient field conditions where the thermal conditions at the land-atmosphere interface are constantly changing.

[1] Recently improved ECH2O soil moisture sensors have received significant attention in many field and laboratory applications. Focusing on the EC-5 sensor, a simple and robust calibration method is proposed. The sensor-to-sensor variability in the readings (analog-to-digital converter (ADC) counts) among 30 EC-5 sensors was relatively small but not negligible. A large number of ADC counts were taken under various volumetric water contents (θ) using four test sands. The proposed two-point α-mixing model, as well as linear and quadratic models, was fitted to the ADC – θ data. Unlike for conventional TDR measurements, the effect of sensor characteristics is lumped into the empirical parameter α in the two-point α-mixing model. The value of α was fitted to be 2.5, yielding a nearly identical calibration curve to the quadratic model. Errors in θ associated with the sensor-to-sensor variability for the two-point α-mixing model were ±0.005 cm3 cm−3 for dry sand and ±0.028 cm3 cm−3 for saturated sand. In the validation experiments, the highest accuracy in water content estimation was achieved when sensor-specific ADCdry and ADCsat were used in the two-point α-mixing model. Assuming that α = 2.5 is valid for most mineral soils, the two-point α-mixing model only requires the measurement of two extreme ADC counts in dry and saturated soils. Sensor-specific ADCdry and ADCsat counts are readily measured in most cases. Therefore, the two-point α-mixing model (with α = 2.5) can be considered as a quick, easy, and robust method for calibrating the ECH2O EC-5 sensor. Although further investigation is needed, the two-point α-mixing model may also be applied to calibrating other sensors.