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Multi-phase multi-component flow and transport models are a key instrument for analyzing and predicting gas transport inside the vadose zone. The soil moisture distribution within the vadose zone varies in time and space. Thus, accurate gas transport prediction relies on the precise knowledge of the saturation-dependency of the transport parameters such as the effective gas diffusion coefficient Deff. Although recent advances from typical small scale experiments (diffusion apparatus with typical soil core size of 100 cm3) show that Deff-saturation(S)-relationships are not only dependent on general soil characteristics such as air-filled porosity and total porosity, but can also be derived from pore network characteristics, such as pore connectivity and geometry, most model frameworks rely on simple empirical formulations such as Millington & Quirk (1961), which find wide acceptance, but have been found to not be universally applicable.The current state of research lacks extensive performance tests for the application of Deff-S-relationships beyond the small scale and especially for realistic natural conditions, where soil moisture changes with depth and gas transport processes may be more complex than in standard laboratory setups used for the experimental determination of Deff, which leads to unknown errors in the numerical prediction of sub-surface gas transport processes.We test different Deff-S-functions within a multi-phase-multi-component flow and transport model by reproducing a laboratory gas transport experiment, where a tracer gas is injected into a quasi-2D Darcy-scale sand tank with a soil moisture distribution that covers the full range from wet to dry and comparing simulated and measured gas concentrations at several locations within the tank over time. The systematic evaluation of different functions leads to the conclusion that the saturation-dependency of Deff in the tested sand follows power law-scaling at low gas phase saturation and linear scaling above, in line with the physically based concepts of percolation theory and effective medium theory and with recent experimental results (Ghanbarian et al., 2018). Other approaches such as (Buckingham, 1904; Penman, 1940; Millington & Quirk, 1961; Moldrup et al., 2000) lead to large errors between numerical and experimental results. We demonstrate that the use of an inaccurate Deff-S-function can lead to a misrepresentation of diffusion coefficients by a factor of up to 105, which underlines the need for a correct representation of the saturation-dependency of Deff in numerical modeling of sub-surface gas transport.

Environmental remediation of polluted sites in developing communities often faces difficulties due to the misalignment of project objectives with the needs and interests of local communities, establishment of unrealistic expectations for the outcome of the project, and failure to account for the available resources in the community itself. Remediation decisions favor technical data and technical solutions as the main means of determining remediation options, requiring significant investment and local capacity that are often not found in developing communities. Often these systems fail to account for the social aspects in environmental management and remediation. In contrast, new approaches in local knowledge-based solutions focus on pre-existing environmental management initiatives to establish objectives that are meaningful to the community itself. “Positive deviance” – or the idea that there are solutions already in communities that are successful at addressing challenges despite the barriers that a community faces – are often overlooked during project design and implementation. Identifying examples of positive deviance and working with community members who are well versed in local challenges, available resources, and local capacity presents opportunities for community members to establish project objectives and provides the foundation for future environmental management projects in the area. We present a case study of artisanal and small-scale gold mining (ASGM) in the Andes municipality, Antioquia, Colombia and local initiatives to overcome community-identified environmental pollutants. Through a series of semi-structured interviews, surveys, and site visits, we identified community perception of environmental pollutants, management techniques for mining waste, and grassroots initiatives that were locally developed to address them. Through key communication and interaction between innovators with identified positive deviance projects, technical experts, and other miners facing similar challenges, innovative projects that account for the social, political, and economic realities of a developing community can become widespread, leading to improved environmental and social conditions.

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...

Methane (CH4) leakage from natural gas (NG) pipelines poses an environmental, safety, and economic threat to the public. While previous leak detection and quantification studies focus on the aboveground infrastructure, the analysis of underground NG pipeline leak scenarios is scarce. Furthermore, no data from controlled release experiments have been published on the accuracy of methods used to (1) quantify emissions from an area source and (2) use these emissions to quantify the size of a subsurface leak. This proof-of-concept work uses CH4 mole fraction, as measured by a single gas sensor, as an input to a simple dispersion-based model (WindTrax) under ideal conditions (i.e., in a field) and compares the calculated emissions to the known controlled NG release rates. The aboveground and surface CH4 mole fractions were measured for 5 days at a field testbed using controlled underground release rates ranging from 0.08 to 0.52 kg hr–1 (3.83–24.94 ft3 hr–1). Results confirmed that the mean normalized CH4 mole fraction increases as the atmosphere transitions from the Pasquill–Gifford (PG) stability class A (extremely unstable) to G (extremely stable). The estimated surface CH4 emissions showed large temporal variability, and for the emission rates tested, at least 6 h of data are needed to have a representative estimate from subsurface pipeline leaks (±27% of the controlled release rate on average). The probability that the emission estimate is within ±50% of the controlled release rate (P±50%) is approximately 50% when 1 h of data is collected; the probability approaches 100% with 3–4 h of data. Findings demonstrate the importance of providing enough data over time for accurate estimation of belowground leak scenarios. By adopting the estimation method described in this study, operators can better estimate leakage rates and identify and repair the largest leaks, thereby optimizing annual greenhouse gas emissions reductions and improving public safety.

Underground natural gas (NG) pipeline leakage can result in methane (CH4) buildup and migration through the soil. What is not well understood in such scenarios is how the soil conditions affect the gas migration behavior, particularly in regard to the relative contributions of specific soil properties such as soil moisture content. The objective of this study was to investigate the effects of soil properties on CH4 concentration and migration from leaking underground NG pipelines. Site characteristics such as surface cover and spatial dimensions, soil samples, and gas concentration data were collected from over 70 gas leakage sites across the United States using a standardized sampling method. Soil samples were collected from excavation sites that were 1.5′–5′ in depth. The collected soil samples were analyzed in the laboratory to measure the soil texture, permeability, and moisture. Statistical analysis was performed to evaluate the effects of soil properties on CH4 migration distance and concentration. Soil texture was consistent across geographic locations due to standardized pipeline backfill protocols, allowing for the analysis of gas concentration and transport data with respect to soil conditions. Soil moisture content was the dominant influence on the gas concentration and spreading distance. High soil moisture content was associated with reduced lateral diffusion and elevated concentrations near the leak point, whereas dry conditions were associated with reduced concentrations and greater spreading distance. Increasing soil moisture content reduced the lateral diffusion of CH4 into the soil due to water-induced tortuosity, resulting in elevated concentrations close to the leak point. Lateral migration of CH4 was suspected to be by diffusion, starting at 5 m from the leaks, while transport within the immediate vicinity of the leak was controlled by advection. These findings demonstrate a pronounced effect of soil moisture content and permeability on CH4 migration distance and concentration, providing key insight into the effects of soil conditions on NG migration and how to account for such effects in leak detection surveys.

Using a creative capacity building (CCB) framework, co-design workshops among artisanal and small-scale gold miners, government officials, NGO representatives, and academics can provide an effective mechanism to identify pressing needs and desires of miners and operators towards formalization. As recently implemented in Peru, one of these workshops identified the need for a single forum (association) to allow artisanal and small-scale miners to represent their voice in front of government and large-scale mining operations and the need for a streamlined simple process for formalization that included mining, environment, health, safety, and employment considerations. A first step towards the single forum was taken with the creation of a social media outlet that allows participants to voice their concerns and share information. Future workshops are needed to cement the creation of the forum and provide specific recommendations for the streamlining of the formalization process.

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...

The 2015 Paris agreement aims to cut greenhouse gas emissions and keep global temperature rise below 2 °C above pre-industrial levels. Reducing CH

With the increased use of natural gas, safety and environmental concerns from underground leaking natural gas pipelines are becoming more widespread. What is not well understood in leakage incidents is how the soil conditions affect gas migration behavior, making it difficult to estimate the gas distribution. To shed light on these concerns, an increased understanding of subsurface methane migration after gas release is required to support efficient leak response and effective use of available technologies. In this study, three field-scale experiments were performed at the Methane Emission Technology Evaluation Center in Colorado State University to investigate the effect of soil textural heterogeneity, soil moisture, and leak rate (0.5 and 0.85 kg/h) on methane migration caused by leaking pipelines. Subsurface methane concentrations, in addition to soil moisture and meteorological data, were collected over time. A previously validated numerical model was modified and used to understand the observed methane distribution behavior. Results of this study illustrate that the influence of soil texture, leak rate, and moisture on subsurface methane distribution is determined by the relative contribution of advection and diffusion and closely related to the distance to the leak source. Advection dominates gas transport within 1–1.5 m of the leak source, driving the migration of high concentration contours. Beyond this distance, diffusion dominates migration of lower concentration contours to the far-field. Although large leak rates initially result in faster and further gas migration, the leak rate has little influence on the diffusion dominated migration farther from the leak source. Soil moisture and texture complicate gas behavior with texture variations and elevated soil moisture conditions playing a dominant role in locally increasing methane concentrations. Scenarios highlight the importance of understanding the effects of soil moisture, texture, and leak rate on gas migration behavior in an attempt to unravel their contribution to the gas concentration within the soil environment.

Author(s): Shanujah, M; Deepagoda, TKKC; Karunarathna, AK; Nasvi, MCM; Shreedharan, V; Babu, GLS; Smits, KM; Oldenburg, CM | Abstract: Open dumps constitute a major source of greenhouse gases (GHGs), predominantly methane and carbon dioxide, in developing countries. In an aged dump, typical waste composition is dominated by the “soil-like” fraction of which physical, hydraulic and gas transport characteristics markedly affect GHG emissions. This study characterized soil-gas diffusivity (Dp/D0), soil-water characteristics (SWC), and particle size distribution in “soil-like” fractions of aged solid waste retrieved at 2.5–5nm depth from an old open dumpsite situated at Kurunegala, Sri Lanka. The “soil-like” fraction was proportioned into three groups based on particle size (0–4.75, 4.75–9.5, and 9.5–25nmm) to investigate the particle size effect on Dp/D0 and SWC. The simulated methane concentration profiles in different size groups were also examined using the transport simulator TOUGH2-EOS7CA based on the multiphase flow of multicomponent gas mixture (methane, water vapour and air) under dry and half-saturation conditions across a predefined temperature gradient. The results highlighted distinct two-region characteristics (i.e., inter-aggregate and intra-aggregate pore regions) in all three size fractions which could be adequately parameterized with existing and modified bimodal functions. We proposed a useful practical tool for estimating Dp/D0 for known mean particle size and volumetric water content in the absence of direct measurements. The results further revealed that Dp/D0 is particle-size dependent; however, Dp/D0 remained invariant across all size fractions at the volumetric water content of ∼0.22–0.25 cm3 cm−3. Numerical results further showed a pronounced effect of particle size and soil moisture on gas transport properties.

Rapid response to underground natural gas leaks could mitigate methane emissions and reduce risks to the environment, human health and safety. Identification of large, potentially hazardous leaks could have environmental and safety benefits, including improved prioritization of response efforts and enhanced understanding of relative climate impacts of emission point sources. However, quantitative estimation of underground leakage rates remains challenging, considering the complex nature of methane transport processes. We demonstrate a novel method for estimating underground leak rates based on controlled underground natural gas release experiments at the field scale. The proposed method is based on incorporation of easily measurable field parameters into a dimensionless concentration number, e , which considers soil and fluid characteristics. A series of field experiments was conducted to evaluate the relationship between the underground leakage rate and surface methane concentration data over varying soil and pipeline conditions. Peak surface methane concentrations increased with leakage rate, while surface concentrations consistently decreased exponentially with distance from the source. Deviations between the estimated and actual leakage rates ranged from 9% to 33%. A numerical modeling study was carried out by the TOUGH3 simulator to further evaluate how leak rate and subsurface methane transport processes affect the resulting methane surface profile. These findings show that the proposed leak rate estimation method may be useful for prioritizing leak repair, and warrant broader field-scale method validation studies. A method was developed to estimate fugitive emission rates from underground natural gas pipeline leaks. The method could be applied across a range of soil and surface covering conditions.

Transport of gas components in the unsaturated zone and across the soil surface plays a role for transport of volatile contaminants, gases from pipe leaks or greenhouse gases. When estimating flow rates from the soil into the atmosphere, a good understanding of the transport processes is important. In general, component transport in the gas phase is considered to be mainly due to diffusion. However, the wind field above the soil surface can induce flow into the subsurface and influence transport and mass fluxes.We present a study on gas component transport through dry and partially saturated soil into a free air flow above the soil surface, considering gas components of different density. Laboratory experiments in a quasi-2d sand tank were carried out. The tank was placed underneath a wind tunnel, and different wind velocities were used. Gases with different densities were injected with constant rate at an inlet port. Concentration distributions were measured continuously with sensors that were installed inside of the tank. After establishing a steady state concentration distribution, the gas injection was stopped and the decrease of gas concentrations inside the tank was monitored.The experiments show that the concentration profiles under steady state gas injection depend on gas density and the different diffusion coefficients. They depend only slightly on the velocity of the overlaying wind field and the influence is mainly seen very close to the soil surface. The transient gas transport out of the soil, however, did not only depend on the different diffusion coefficients, but was clearly influenced by the wind field. The transient 2d concentration distribution fields illustrate that the wind field induced a flow field inside the tank that depends on the wind velocity and the component density and influences the gas component transport. The influence increases under partly saturated conditions.To reproduce the transport correctly, it is necessary to capture the coupling between free flow and porous medium flow and the transport in the coupled flow. To do so, we use a fully coupled flow and transport model implemented into the environment DuMux ((Dune for Multi-(Phase,Component, Scale, ...) flow and transport in porous media). It can be shown that including the coupling concept, the main features of the concentration distributions can be reproduced for both the steady state and the transient case. With the model it is also demonstrated, that although advective fluxes inside the porous medium introduced by the wind field (horizontal and lateral) are relatively small in comparison to the diffusive fluxes, they cause relevant changes in the concentration distribution and thus indirectly influence the mass fluxes inside the porous medium and across the soil-atmosphere interface.

Technical advancements in mine tailings recycling have led to the production of construction products such as bricks, cement, road pavers, and mortars in artisanal and small-scale mining (ASM) communities. Although such efforts show promise, the environmental, health, and social implications of such practices are often overlooked by stakeholders including researchers, miners, and community members. This paper summarizes and evaluates the environmental and health implications of mine tailings recycling activities (MTRAs) in ASM. A comprehensive review was conducted using both academic and gray literature to analyze the current state of practice linked with environmental and health considerations. From the fifty-three analyzed MTRAs, twenty-three articles considered aspects of environmental concerns, and three studied aspects of health in mine tailings recycling processes. When included, environmental and health considerations mainly focused on toxicity leaching protocols or analyzing health risks and exposure routes of facilities constructed out of mine tailings construction materials. Results suggest that studies mostly focus on lab-scale experiments evaluating the material properties of the MTRAs products and have limited consideration of field application and social concerns. No study considered environmental, health and social implications in unison. To comprehensively address these concerns, a Comprehensive Approach to Recycling Tailings (CART) framework is proposed that can assist communities and researchers to better understand the possible environmental and health implications of MTRAs and to incorporate such understanding into practice.

International audience

Soil water evaporation plays a critical role in mass and energy exchanges across the land–atmosphere interface. Although much is known about this process, there is no agreement on the best modeling approaches to determine soil water evaporation due to the complexity of the numerical modeling scenarios and lack of experimental data available to validate such models. Existing studies show numerical and experimental discrepancies in the evaporation behavior and soil water distribution in soils at various scales, driving us to revisit the key process representation in subsurface soil. Therefore, the goal of this work is to test different mathematical formulations used to estimate evaporation from bare soils to critically evaluate the model formulations, assumptions and surface boundary conditions. This comparison required the development of three numerical models at the REV scale that vary in their complexity in characterizing water flow and evaporation, using the same modeling platform. The performance of the models was evaluated by comparing with experimental data generated from a soil tank/boundary layer wind tunnel experimental apparatus equipped with a sensor network to continuously monitor water–temperature–humidity variables. A series of experiments were performed in which the soil tank was packed with different soil types. Results demonstrate that the approaches vary in their ability to capture different stages of evaporation and no one approach can be deemed most appropriate for every scenario. When a proper top boundary condition and space discretization are defined, the Richards equation-based models (Richards model and Richards vapor model) can generally capture the evaporation behaviors across the entire range of soil saturations, comparing well with the experimental data. The simulation results of the non-equilibrium two-component two-phase model which considers vapor transport as an independent process generally agree well with the observations in terms of evaporation behavior and soil water dynamics. Certain differences in simulation results can be observed between equilibrium and non-equilibrium approaches. Comparisons of the models and the boundary layer formulations highlight the need to revisit key assumptions that influence evaporation behavior, highlighting the need to further understand water and vapor transport processes in soil to improve model accuracy.

Pollution remediation decisions in developing communities are often made with limited technical data and stakeholder engagement. Local knowledge of contamination is oftentimes neglected, resulting in efforts that fail to align with community objectives. In this work, we propose a new approach for incorporating local knowledge on contamination into remediation efforts, resulting in a community-informed conceptual site model framework for pollution remediation. This framework uses qualitative and quantitative social science methods to develop a holistic view of contamination as defined by the community, pairing social knowledge with existing technical information about a site. An example of the application of this framework is provided from a field study conducted in a mercury-contaminated artisanal and small-scale gold mining community in Andes, Colombia. We conducted qualitative unstructured and semi-structured interviews and surveys to elicit local knowledge on mercury contamination. We combined this stakeholder input with limited scientific published literature on mercury contamination in artisanal mining communities to develop different conceptual site models that vary based on the level and type of stakeholder input. Our findings demonstrate how local knowledge on contamination varies due to contextual factors within the community, revealed through the engagement. This work also demonstrates how the new framework can be utilized by environmental practitioners to define the needs, wants, and priorities of stakeholder groups throughout a remediation project, ensuring maximum participation in solution development.

Leaks in natural gas distribution networks are often initially detected by odor and then localized during operators' walking surveys. The size of the enhancement detected and the leak's proximity to infrastructure are used to determine leak severity of the leak and how quickly it is repaired. Methane (CH4) enhancements on the surface above the leak change with atmospheric conditions, but currently the leak severity assessment is made without considering the environmental conditions and could result in misdiagnosis of the leak. This study has developed the “ESCAPE” model, a tool that can be used to estimate CH4 enhancements on the surface above a leak using meteorological and leak data as input. Surface CH4 concentrations calculated using the ESCAPE model agrees well with measurements made during controlled experiments (m = 0.95 and R2 = 0.95). This study highlights the effect of micrometeorology on gas transport from the surface to the atmosphere, where the surface and the atmospheric conditions have the largest effect on the flux. Here, recommendations are made to improve industry best practices, including recording the meteorological conditions at the time of the leak detection, avoiding walking surveys on days where there are strong winds or strong solar irradiance and that known leak locations should be revisited and measured in different conditions. Mitigating CH4 emissions from the gas distribution network is a cost-effective and economically realistic target. Distribution operators can be directed by the output of ESCAPE model to correctly identify and repair the largest distribution leaks and could reduce annual CH4 emissions by 0.69 Tg, this may go some way to reducing energy sector emissions in accordance with the Paris Agreement.

Mitigation of atmospheric emission of methane from leaky underground infrastructure is important for controlling the global anthropogenic greenhouse gas burden. Overexposure to methane may also cause occupational health problems in indoor/outdoor environments at the local scale. Subsurface soil conditions (e.g. soil heterogeneity) affect methane migration in soils while near†surface atmospheric boundary conditions (e.g. wind and temperature) affect off†site emissions across the soil†atmosphere interface. This study investigated the above†surface methane concentration boundary†layer development under different soil conditions (homogenous and layered) and atmospheric boundary controls (wind and temperature). A series of controlled bench†scale experiments was conducted using an open†loop boundary†layer wind tunnel interfaced with a porous media tank inside which a simulated methane point source was embedded to mimic a buried leaky pipeline. Results revealed pronounced effects of wind and, to a lesser extent, temperature on above†surface methane boundary†layer development. High atmospheric temperature contributed to the concentration boundary†layer build†up whereas high wind velocity caused erosion of the boundary layer. The boundary†layer methane concentration profiles were adequately simulated within an advection†diffusion modeling framework combined with a Navier†Stokes free flow domain by coupling free air flow, heat flow and flow of a multicomponent gas mixture. Simulations further implied that the Fickian diffusion approach may have limited applications when pronounced non†isothermal conditions prevail within the system. © 2017 Society of Chemical Industry and John Wiley & Sons, Ltd.

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.

An understanding of soil thermal conductivity after a wildfire or controlled burn is important to land management and post-fire recovery efforts. Although soil thermal conductivity has been well studied for non-fire heated soils, comprehensive data that evaluate the long-term effect of extreme heating from a fire on the soil thermal conductivity are limited. The purpose of this study was to evaluate the long-term impact of fire on the effective thermal conductivity of soils by directly comparing fire-heated and no-fire control soils through a series of laboratory studies. The thermal conductivity was measured for ten soil samples from two sites within the Manitou Experimental Forest, Colorado, USA, for a range of water contents from saturation to the residual degree of saturation. The thermal conductivity measured was compared with independent estimates made using three empirical models from literature, including the Campbell et al. (1994), Cote and Konrad (2005), and Massman et al. (2008) models. Results demonstrate that for the test soils studied, the thermal conductivity of the fire-heated soils was slightly lower than that of the control soils for all observed water contents. Modeling results show that the Campbell et al. (1994) model gave the best agreement over the full range of water contents when proper fitting parameters were employed. Further studies are needed to evaluate the significance of including the influence of fire burn on the thermal properties of soils in modeling studies.

A soil’s grain-size distribution affects its physical and hydraulic properties; however, little is known about its effect on soil thermal properties. To better understand how grain-size distribution affects soil thermal properties, specifically the effective thermal conductivity, a set of laboratory experiments was performed using binary mixtures of two uniform sands tightly packed with seven different mixing fractions over the full range of saturation. For each binary mixture, the effective thermal conductivity, λ, capillary pressure, hc, and volumetric water content, θ, were measured. Results demonstrated that the λ–θ relationship exhibited distinct characteristics based on the percentage of fine- and coarse-grained sands. We further compared measured λ–θ properties with independent estimates from two semi-empirical models (Campbell Model and Lu and Dong Model) to evaluate the models’ applicability in relation to physically based parameters associated with changes in soil mixing (e.g., porosity and grain size). Both models were able to fit experimental data but to varying degrees based on the number of physically based parameters used. In general, model improvements are needed to capture the λ–θ relationship solely on physically based parameters.

The interactions between turbulent free flow and flow in a porous medium are of key interest in different fields, e.g., meteorology, agriculture, building physics, and aerospace engineering. Properly understanding the strongly coupled exchange processes between the two domains is crucial to describing these interactions. In (Mosthaf et al. in Water Resour Res 47(10):W10522, 2011. doi: 10.1029/2011WR010685 ), a concept for coupling laminar compositional single-phase free flow to compositional two-phase porous-medium flow under non-isothermal conditions was presented. In this study, the existing coupling concept is first extended to turbulent free-flow conditions. This includes the interface conditions between a Reynolds-averaged Navier–Stokes free flow using algebraic turbulence models and a Darcy porous-medium flow. Second, eddy viscosity and boundary layer models for a rough interface are integrated into this model concept. Results from laboratory evaporation experiments are used for comparison of the developed model concept. A sensitivity analysis of the evaporation rate and porous-medium quantities on different model setups, boundary conditions, Beavers–Joseph coefficients, and roughness lengths is performed. Results demonstrate how including turbulence, either with eddy viscosity or boundary layer models, affects the evaporation rate. The model concept is able to predict early stage-I and intermediate to later stage-II evaporation rates. Sand-grain roughness concepts are successfully included into the model and show the desired qualitative effect.

Grazed pastures rich in nitrogen (N) from ruminant urine and fertilizer inputs are significant sources of nitrous oxide (N2O), a highly potent greenhouse gas. Diffusion-controlled emission of N2O from pasture systems can be described by soil-gas diffusivity (Dp/Do), and its dependency on soil physical properties and soil moisture dynamics. But studies linking soil aggregation, soil moisture variation, Dp/Do and N2O emissions are lacking. Using coarse (2–4 mm) and fine (

Environmental risk mitigation strategies employed in developing communities to address mercury pollution from artisanal and small-scale gold mining (ASGM) have largely failed to meet community needs, resulting in their abandonment. In contrast, the water and wastewater treatment sector has gained more traction, introducing sustainable and community supported water treatment strategies. This paper discusses why initiatives in water and wastewater treatment in developing communities are succeeding while initiatives in ASGM environmental risk mitigation are falling short. Selected case studies illustrate that water and wastewater treatment project success is contingent on the inclusion of a fully comprehensive approach that considers the socio-economic and ecological context of communities and individuals along with behavior change interventions. These elements of successful WASH projects were then applied to past ASGM environmental projects to determine their applicability to the sector. By adopting the core traits of successful water and wastewater treatment projects in developing communities, a framework can be developed for ASGM projects that, when applied correctly, could lead to measurable impact and long-term sustainability.

Effective thermal conductivity of soils (λeff) is a critical parameter for agriculture, environment science, and engineering. Functions to estimate λeff from readily available soil properties, known as soil thermal conductivity (STC) schemes, are needed by land-surface models (LSMs), hydrological models, and soil-vegetation-atmosphere transfer (SVAT) models to study the land surface energy balance, heat flux, and soil thermal regime under various climates and geographic regions. The selection of a STC scheme can result in large differences in temperature estimates in LSMs, sometimes masking the effects of climate change. Therefore, accurate selection of a STC scheme is critically important to LSM estimates. Although a number of STC schemes have been incorporated in various LSMs, no study has systematically evaluated their performance. Therefore, the objectives of this study were to review and evaluate STC schemes employed by LSMs by comparing (1) predicted and measured STCs and (2) modelled land surface temperature (LST) using the Community Land Model at three selected sites and the corresponding LST data from the moderate resolution imaging spectrometer (MODIS). In total, 24 STC schemes were collated from 38 mainstream LSMs, SVAT, and hydrological models. They were divided into three categories based on model types: one physically-based scheme, eight linear/non-linear regression schemes, and 13 normalized schemes. We also include two schemes that express STC as a function of matric potential (ψ, hereafter referred as λeff (ψ) schemes). The first three types of STC schemes were evaluated with a large compiled dataset consisting of 439 unfrozen and frozen measurements of λeff from 16 soils. The λeff (ψ) schemes were evaluated with simultaneously measured λeff (ψ) from eight soils from two separate or independent studies. Results showed that none of the STC schemes could be used to accurately predict λeff for all soil types. STC scheme performance largely depended on the size (number of samples) and characteristics (e.g., soil types) of the data used for comparison. Some STC schemes work well on certain types of soils, but care should be taken for larger scale applications. LSM simulated LST for 24 STC schemes varied when compared with MODIS LST. In general, the STC schemes performed better in medium- and coarse-textured soils than in fine-textured soils. However, large discrepancies were observed on the estimated LST using different STC schemes for medium and coarse-textured soils. We recommend that LSM modelers be mindful of the inherent bias in STC schemes on the surface temperature estimates and hence overall model predictions. Orchestrated efforts are urgently needed on the part of the soil science, hydrology, and climatology communities to develop a more extensive and systematic λeff database for development and evaluation of improved STC schemes for wider and more accurate applications.

Characterization of differently textured porous materials, as well as different volumetric porous media mixtures, in relation to mass and heat transport is vital for many engineering and research applications. Functional relations describing physical (e.g., grain-size distribution, total porosity), thermal, and gas diffusion properties of porous media and mixtures are necessary to optimize the design of porous systems that involve heat and gas transport processes. However, only a limited number of studies provide characterization of soil physical, thermal, and gas diffusion properties and the functional relationships of these properties under varying soil water contents, especially for soil mixtures, complicating optimization efforts. To better understand how mixing controls the physical, thermal, and gas diffusion properties of porous media, a set of laboratory experiments was performed using five volumetric mixtures of coarse- and fine-grained sand particles. For each mixture, the grain-size distribution (GSD), thermal conductivity, and gas diffusivity were obtained and parameterized using existing and suggested parametric models. Results show that the extended, two-region Rosin–Rammler particle-size distribution model proposed in this study could successfully describe the bimodal behavior of the GSD of binary mixtures. Further, the modified Côté and Konrad thermal conductivity model adequately described the thermal conductivity–water saturation relations observed in different mixtures. The proposed simple soil-gas diffusivity descriptive model parameterized the upper limit, average, and lower limit behavior in gas diffusivity–air content relations in apparently texture-invariant gas diffusivity data. Results further show a close analogy between gas diffusivity and thermal conductivity and their variation with saturation across different binary mixtures. Overall, the results of the study provide useful numerical insight into the physical, thermal, and gas transport characteristics of binary mixtures, with wide implications for future engineering and research applications that involve multicomponent porous systems.

Although siting of thermal energy storage systems in the vadose zone may be beneficial due to the low thermal conductivity of unsaturated soils, water phase change and vapor diffusion in so...

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 mixing of different sized soil particles is known to affect soil properties such as density, porosity, hydraulic conductivity, and soil water retention. To better understand how the mixing controls soil water retention characteristics, a set of laboratory experiments was performed using binary mixtures of coarse and fine particles with seven different mixing fractions. For each mixture, the water retention characteristics, porosity, and saturated hydraulic conductivity were obtained. To enhance the effects of mixing, coarse and fine sands with a relatively large particle size contrast were used. Results showed that at equal to or above the fine fraction of ∼0.3, the fine particles controlled the behavior. Starting from the fine-only sample, introducing more coarse particles to the fine particles led to rather gradual changes in the water retention characteristics. Wall effects at the coarse–fine particle interfaces were presumed to be responsible for the changes in the water retention curves. The water retention curves for the mixtures with a fine fraction lower than 0.3 were controlled by the coarse particles yielding the displacement pressure similar to that of the coarse sand. Furthermore, it was inferred that different mixing fractions led to different spectra of pore sizes, suggesting that a parametric model with sufficient flexibility is needed to describe the water retention characteristics of binary mixtures.

Oscillating cycles of dewatering (termed drying) and rewetting during managed aquifer recharge (MAR) are used to maintain infiltration rates and could also exert an influence on subsurface microbial structure and respiratory processes. Despite this practice, little knowledge is available about changes to microbial community structure and trace organic chemical biodegradation potential in MAR systems under these conditions. A biologically active two-dimensional (2D) synthetic MAR system equipped with automated sensors (temperature, water pressure, conductivity, soil moisture, oxidation-reduction potential) and embedded water and soil sampling ports was used to test and model these important subsurface processes at the meso-scale. The fate and transport of the antiepileptic drug carbamazepine, the antibiotics sulfamethoxazole and trimethoprim, and the flame retardant tris (2-chloroethyl) phosphate were simulated using the finite element analysis model, FEFLOW. All of these compounds exhibit moderate to poor biodegradability in MAR systems. Within the operational MAR scenario tested, three episodic drying cycles spanning between 18 and 24 days were conducted over a period of 184 days. Notably, cessation of flow and partial dewatering of the 2D synthetic aquifer during dry cycles caused no measurable decrease in soil moisture content beyond the near-surface layer. The episodic flow introduction and dewatering cycles in turn had little impact on overall trace organic chemical biotransformation behavior and soil microbial community structure. However, spatial differences in oxidation-reduction potential and soil moisture were both identified as significant environmental predictors for microbial community structure in the 2D synthetic aquifer.

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.

Improvements in buried mine detection using remote sensing technology rest on understanding the effects on sensor response of spatial and temporal variability created by soil and environmental conditions. However, research efforts on mine detection have generally emphasized sensor development, while less effort has been made to evaluate the effects of the environmental conditions in which the mines are placed. If the processes governing moisture and temperature distribution near the ground surface can be captured, sensor development and deployment can be more realistically tailored to particular operational scenarios and technologies. The objective of this study is to investigate the effects of the soil environment on landmine detection by studying the influence of the thermal boundary conditions at the land-atmosphere interface and the buried objects themselves on the spatial and temporal distribution of soil moisture around shallow-buried objects. Two separate large tank experiments were performed with buried objects with different thermal properties. Experimental results were compared to results from a fully coupled heat and mass transfer numerical model. Comparison of experimental and numerical results suggests that the vapor enhancement factor used to adjust the vapor diffusive flux described based on Fick's law is not necessary under dry soil conditions. Data and simulations from this study show that the thermal signature of a buried object depends on the complex interaction among a soil's water content and its thermal and hydraulic properties. Simulated thermal and saturation contrasts were generally very different for a buried landmine than for other buried objects.