Heterogeneous Subsurface Environments Research Articles

Integrated Geophysical Models for Interpretations of Heterogeneous Subsurface Environments

Near-surface characterization can be vulnerable to misinterpretation due to limited resolution and methodological limitations. Validation from different parameters is necessary to substantiate the results and reduce errors during interpretations. This research aims to develop an alternative integrated method of a 2-D cross-plot model to enhance subsurface interpretations based on the model's criteria, resulting in better geological interpretation. Geophysical methods such as electrical resistivity and seismic refraction are utilized in a test model and a case study area to observe the capabilities of the integrated method approach. The goal of this study was to cluster two or more different parameters into a single model for a direct presentation of the subsurface model. This includes the characterization of subsurface properties, data integration using cross-plot analysis, and the development of a 2-D cross-plot model. This method visually represents the relationship between two or more attributes, allowing for the identification of anomalies. In the test model, the lithology consists of sandy silt derived from granitic residual soil, whereas in the case study model, it was dominated by weathered granitic residual soil with the presence of saturated zones. The 2-D cross-plot model provides a comprehensive interpretation where Quadrant, Q1 shows the vulnerable zones for sliding mass bodies. The plane weakness was successfully identified based on the cross-plot model as the weathered zone, and subsurface features were determined. The estimated volume of mass movement was successfully calculated for the case study area based on the determination of the sliding plane. The integrated method of cross-plot analysis along with the development of 2-D cross-plot models proves to be an informative approach for subsurface characterization and enhances subsurface imaging.

Towards Understanding Factors Affecting Arsenic, Chromium, and Vanadium Mobility in the Subsurface.

Arsenic (As), chromium (Cr), and vanadium (V) are naturally occurring, redox-active elements that can become human health hazards when they are released from aquifer substrates into groundwater that may be used as domestic or irrigation source. As such, there is a need to develop incisive conceptual and quantitative models of the geochemistry and transport of potentially hazardous elements to assess risk and facilitate interventions. However, understanding the complexity and heterogeneous subsurface environment requires knowledge of solid-phase minerals, hydrologic movement, aerobic and anaerobic environments, microbial interactions, and complicated chemical kinetics. Here, we examine the relevant geochemical and hydrological information about the release and transport of potentially hazardous geogenic contaminants, specifically As, Cr, and V, as well as the potential challenges in developing a robust understanding of their behavior in the subsurface. We explore the development of geochemical models, illustrate how they can be utilized, and describe the gaps in knowledge that exist in translating subsurface conditions into numerical models, as well as provide an outlook on future research needs and developments.

Enhanced biostimulation coupled with a dynamic groundwater recirculation system for Cr(VI) removal from groundwater: A field-scale study

A large gap exists between laboratory findings and successful implementation of bioremediation technologies for the treatment of chromium (Cr)-contaminated sites. This work conducted the enhanced bioremediation of Cr(VI) in situ via the addition of organic carbon (ethanol) coupled with a dynamic groundwater recirculation (DGR)-based system in a field-scale study. The DGR system was applied to successfully (1) remove Cr(VI) from groundwater via enhanced flushing by the recirculation system and (2) deliver the biostimulant to the heterogeneous subsurface environment, including a sand/cobble aquifer and a fractured bedrock aquifer. The results showed that the combined extraction and bioreduction of Cr(VI) were able to reduce Cr(VI) concentrations from 1000 to 2000 mg/L to below the clean-up goal of 0.1 mg/L within the operation period of 52 days. The effectiveness of Cr(VI) bioremediation and the relationship between microbial communities and geochemical parameters were evaluated. Multiple-line of evidence demonstrated that the introduction of ethanol significantly stimulated a variety of bacteria, including those responsible for denitrification, sulfate reduction and reduction of Cr(VI), which contributed to the establishment of reducing conditions in both aquifers. Cr(VI) was removed from groundwater via combined mechanisms of physical removal through the DGR system and the bioreduction of Cr(VI) followed by precipitation. In particular, it was found competitive growth among Cr(VI)-reducing bacteria (such as the enrichment of Geobacter, along with the reduced relative abundance of Acinetobacter and Pseudomonas) was induced by ethanol injection. Furthermore, Cr(VI), total organic carbon, NO2−, and SO42− played important roles in shaping the composition of the microbial community and its functions.

Subsurface object detection and characterization using Ground Penetrating Radar

Subsurface characterization and information about buried utility infrastructure is an important issue affecting the public safety and progress of development projects. A heterogeneous subsurface environment is often insufficiently characterized by the data collected through various direct and indirect means, particularly in dense urban areas. The present study aims to detect the subsurface objects and map the stratigraphic environment in a city region using a non-invasive geophysical technique called Ground Penetrating Radar (GPR). In this study, antennas of central frequency 200 and 80 MHz have been used to identify the underground utilities and subsurface layer information, respectively. A methodology based on a geometrical approach using Support Vector Machines (SVM) is developed for computing the depth and radius of buried pipes. Also, the electrical discontinuities in the GPR profiles are identified through various processing techniques to extract the subsurface layer information. The results indicate that the 200-MHz antenna and SVM-based methodology estimate the buried pipe parameters with reasonable accuracy at various site combinations. It is found that the bistatic low-frequency 80-MHz antenna suitably characterizes the subsurface layers, which are in close agreement with the borehole data. The processed data illustrate a strong correlation between the radar signals and the characteristics of the strata resolving the uncertainty. The study highlights the capability of GPR in extracting the subsurface data and recommends a multi-frequency approach to map and interpret the complete subsurface environment at a specific site.

Characterizing the Self‐Potential Response to Concentration Gradients in Heterogeneous Subsurface Environments

AbstractSelf‐potential (SP) measurements can be used to characterize and monitor, in real‐time, fluid movement and behavior in the subsurface. The electrochemical exclusion‐diffusion (EED) potential, one component of SP, arises when concentration gradients exist in porous media. Such concentration gradients are of concern in coastal and contaminated aquifers and oil and gas reservoirs. It is essential that estimates of EED potential are made prior to conducting SP investigations in complex environments with heterogeneous geology and salinity contrasts, such as the UK Chalk coastal aquifer. Here we report repeatable laboratory estimates of the EED potential of chalk and marls using natural groundwater (GW), seawater (SW), deionized (DI) water, and 5 M NaCl. In all cases, the EED potential of chalk was positive (using a GW/SW concentration gradient the EED potential was ca. 14 to 22 mV), with an increased deviation from the diffusion limit at the higher salinity contrast. Despite the relatively small pore size of chalk (ca. 1 μm), it is dominated by the diffusion potential and has a low exclusion efficiency, even at large salinity contrasts. Marl samples have a higher exclusion efficiency which is of sufficient magnitude to reverse the polarity of the EED potential (using a GW/SW concentration gradient the EED potential was ca. −7 to −12 mV) with respect to the chalk samples. Despite the complexity of the natural samples used, the method produced repeatable results. We also show that first order estimates of the exclusion efficiency can be made using SP logs, supporting the parameterization of the model reported in Graham et al. (2018, https://doi.org/10.1029/2018WR022972), and that derived values for marls are consistent with the laboratory experiments, while values derived for hardgrounds based on field data indicate a similarly high exclusion efficiency. While this method shows promise in the absence of laboratory measurements, more rigorous estimates should be made where possible and can be conducted following the experimental methodology reported here.

Landslide activation behaviour illuminated by electrical resistance monitoring

AbstractA common factor in landslide activation (or reactivation) is subsurface moisture and associated pore pressure variations linked to rainfall. Monitoring of these subsurface hydrogeological processes is necessary to improve our understanding of water‐induced landslide activation. Geophysical approaches, electrical methods in particular, are increasingly being applied to landslide monitoring because they provide non‐invasive spatial information in heterogeneous subsurface environments that can be difficult to characterise using surface observations or intrusive sampling alone. Electrical techniques are sensitive to changing subsurface moisture conditions, and have proven to be a useful tool for investigating the hydrogeology of natural and engineered slopes.The objectives of this investigation were to further develop electrical resistance monitoring for slope stability assessment, and to validate the approach at an intermittently‐active UK landslide system to advance the understanding of complex landslide activation mechanisms. A long‐term transfer resistance dataset was collected from a grid of electrodes to allow spatial monitoring of the landslide. These data were interpreted using a synthesis of rainfall, temperature, GPS and piezometric records. The resistance data were corrected for seasonal temperature variations and electrode movements were monitored, as these processes were shown to mask moisture related changes. Results reveal that resistance monitoring is sensitive to soil moisture accumulation, including changes in piezometric levels, and can be used to study the principal activation mechanism of slow‐moving shallow earthflows. Spatial monitoring using resistance maps was shown to be particularly valuable as it revealed the evolution of subsurface moisture distribution, in the lead up to landslide activation.Key benefits of this approach are that it provides a simple, rapid and non‐invasive means of spatially monitoring subsurface moisture dynamics linked to landslide activation at high‐temporal resolution. Crucially, it provides a means of monitoring subsurface hydraulic changes in the build‐up to slope failure, thereby contributing to early warning of landslide events. Copyright © 2018 John Wiley & Sons, Ltd.

Improving the Sweeping Efficiency of Amendment by Using Polymer

Effective delivery of remedial amendment to the subsurface contamination zone is a challenge when the source zone is either in an aquifer or in a vadose zone. Polymer solutions are shear thinning fluids which can be used as delivery media to improve the distribution of remedial amendments injected into heterogeneous subsurface environments by increasing the viscosity and decreasing the velocity of the flooding solution. In this study, combinations of polymer (xanthan) and common remediation agent (potassium permanganate) were used for sweep efficiency-enhanced. This study utilized 2-D flow-tank experiments to examine the SE used by xanthan with KMnO4. Two different experimental models (Multi-layer and Surrounding models) were studied. They simulated the different flushing mode. The results showed that SE improved 13.5% and 21.3% for model 1 and model 2, respectively. Moreover, the lower concentration (500 mg/L) is the best choice in our experiment. Our work found that some advantages such as high viscosity retention, moderate remediation agent demand make xanthan/remediation agent become the most compatible for polymer-enhanced sweep efficiency. This will be used further experiments.

Using geochemical indicators to distinguish high biogeochemical activity in floodplain soils and sediments

A better understanding of how microbial communities interact with their surroundings in physically and chemically heterogeneous subsurface environments will lead to improved quantification of biogeochemical reactions and associated nutrient cycling. This study develops a methodology to predict potential elevated rates of biogeochemical activity (microbial “hotspots”) in subsurface environments by correlating microbial DNA and aspects of the community structure with the spatial distribution of geochemical indicators in subsurface sediments. Multiple linear regression models of simulated precipitation leachate, HCl and hydroxylamine extractable iron and manganese, total organic carbon (TOC), and microbial community structure were used to identify sample characteristics indicative of biogeochemical hotspots within fluvially-derived aquifer sediments and overlying soils. The method has been applied to (a) alluvial materials collected at a former uranium mill site near Rifle, Colorado and (b) relatively undisturbed floodplain deposits (soils and sediments) collected along the East River near Crested Butte, Colorado. At Rifle, 16 alluvial samples were taken from 8 sediment cores, and at the East River, 46 soil/sediment samples were collected across and perpendicular to 3 active meanders and an oxbow meander. Regression models using TOC and TOC combined with extractable iron and manganese results were determined to be the best fitting statistical models of microbial DNA (via 16S rRNA gene analysis). Fitting these models to observations in both contaminated and natural floodplain deposits, and their associated alluvial aquifers, demonstrates the broad applicability of the geochemical indicator based approach.

Long-term in situ oxidation of biogenic uraninite in an alluvial aquifer: impact of dissolved oxygen and calcium.

Oxidative dissolution controls uranium release to (sub)oxic pore waters from biogenic uraninite produced by natural or engineered processes, such as bioremediation. Laboratory studies show that uraninite dissolution is profoundly influenced by dissolved oxygen (DO), carbonate, and solutes such as Ca(2+). In complex and heterogeneous subsurface environments, the concentrations of these solutes vary in time and space. Knowledge of dissolution processes and kinetics occurring over the long-term under such conditions is needed to predict subsurface uranium behavior and optimize the selection and performance of uraninite-based remediation technologies over multiyear periods. We have assessed dissolution of biogenic uraninite deployed in wells at the Rifle, CO, DOE research site over a 22 month period. Uraninite loss rates were highly sensitive to DO, with near-complete loss at >0.6 mg/L over this period but no measurable loss at lower DO. We conclude that uraninite can be stable over decadal time scales in aquifers under low DO conditions. U(VI) solid products were absent over a wide range of DO values, suggesting that dissolution proceeded through complexation and removal of oxidized surface uranium atoms by carbonate. Moreover, under the groundwater conditions present, Ca(2+) binds strongly to uraninite surfaces at structural uranium sites, impacting uranium fate.

Efficient chemical oxidation of high levels of soil-sorbed phenanthrene by ultrasound induced, thermally activated persulfate

The sorption of hydrophobic organic contaminants (HOCs) and limited mass transfer in heterogeneous subsurface environment are major hurdles to efficient remediation of highly contaminated sites by traditional activated persulfate. In this work, ultrasound was coupled to persulfate oxidation to enhance the remediation. The mechanisms of the coupling process were investigated, using soil spiked with high levels of phenanthrene as a model compound for PAHs and other HOCs. The results indicate that, when ultrasound is applied to the soil slurry, the resultant heat, due to the ultrasonic energy absorbed by the soil particles, activates persulfate to generate sulfate radicals to oxidize the dissolved phenanthrene, and the dissolution and desorption of the phenanthrene are also enhanced by elevated temperature, ultrasound itself and the oxidation by persulfate, thereby creating a positive-feedback loop that induces rapid and highly efficient oxidation of the contaminants. Furthermore, the effects of ultrasound intensity, contaminant concentration, oxidant dosage, soil to water ratio, and addition of external activators on the oxidation of phenanthrene were also studied. For potential field applications, coupling downhole ultrasound technologies with traditional persulfate process was proposed to facilitate the remediation of HOCs contaminated sites.

Stochastic contaminant transport monitoring in heterogeneous sand and gravel aquifers of the United Arab Emirates

Stochastic numerical simulations were performed to investigate the evolution of plumes from pulse sources that can result from accidental leaks. The stochastic advection–dispersion equation was solved for hydraulic conductivities typical to the heterogeneous sandy and gravel aquifers encountered in the United Arab Emirates. Dispersivities were similar to those found in field studies at sandy aquifers, such as those conducted at Borden and Cape Cod, and at the Vejen, Denmark tracer tests. Our work showed that the detection probability, Pd, of a monitoring network was affected strongly by the medium’s dispersivity with a large number of wells (larger than 12) required, even in relatively simple geological environments, in order to detect contaminants with confidence. Monitoring systems following minimum regulatory requirements in terms of the number of wells were able to detect contamination at best in only one out of five cases. The frequency of sampling did not appear to be critical when the dispersivity was low and bi-annual sampling appeared to be satisfactory. In highly dispersive media monthly sampling was needed in order to increase detection. Increase of a medium’s dispersivity in relative homogeneous aquifers reduced the performance of large well-systems to less than 50 %. Strongly heterogeneous and dispersive subsurface environments led all monitoring systems to fail in detection, irrespectively of frequency of sampling. Finally, large contaminant quantities did not improve the detection capabilities of low density well-systems with detection enhancements restricted to high density ones.

Toxicity and bioremediation of pesticides in agricultural soil

Pesticides are one of the persistent organic pollutants which are of concern due to their occurrence in various ecosystems. In nature, the pesticide residues are subjected to physical, chemical and biochemical degradation process, but because of its high stability and water solubility, the pesticide residues persist in the environment. Moreover, the prevailing environmental conditions like the soil characteristics also contribute for their persistence. Bioremediation is one of the options for the removal of pesticides from environment. One important uncertainty associated with the implementation of bioremediation is the low bioavailability of some of the pesticides in the heterogeneous subsurface environment. Bioavailability of a compound depends on numerous factors within the cells of microorganism like the transportation of susbstrate across cell membrane, enzymatic reactions, biosurfactant production etc. as well as environment conditions such as pH, temperature, availability of electron acceptor etc. Pesticides like dichlorodiphenyltrichloroethane (DDT), hexachlorocyclohexane (HCH), Endosulfan, benzene hexa chloride (BHC), Atrazine etc. are such ubiquitous compounds which persist in soil and sediments due to less bioavailability. The half life of such less bioavailable pesticides ranges from 100 to 200 days. Most of these residues get adsorbed to soil particles and thereby becomes unavailable to microbes. In this review, an attempt has been made to present a brief idea on ‘major limitations in pesticide biodegradation in soil’ highlighting a few studies.

A quantitative framework for understanding complex interactions between competing interfacial processes and in situ biodegradation

In situ bioremediation of contaminated groundwater is made technologically challenging by the physically, chemically, and biologically heterogeneous subsurface environment. Subsurface heterogeneities are important because of influences on interfacial mass transfer processes that impact the availability of substrates to microorganisms. The goal of this study was to perform a "proof-of-concept" evaluation of the utility of a quantitative framework based on a set of dimensionless coefficients for evaluating the effects of competing physicochemical interfacial and biokinetic processes at the field scale. First, three numerical modeling experiments were completed, demonstrating how the framework can be used to identify the rate-limiting process for the overall bioremediation rate, and to predict what engineered enhancements will alleviate the rate-limiting process. Baseline conditions for each scenario were established to examine intrinsic biodegradation with a given rate-limiting process (either dispersion, biokinetics, or sorption). Then different engineering treatments were examined. In each case, the treatment predicted to be appropriate for addressing the overall rate-limiting process based on the quantitative framework alleviated the limitation more successfully, and enhanced the in situ biodegradation rate more than the alternative enhancements. Second, the quantitative framework was applied to a series of large-scale laboratory and field-scale experiments, using reported parameter estimates to calculate the relevant dimensionless coefficients and predict the rate-limiting process(es). Observations from the studies were then used to evaluate those predictions.

Rheological behavior of xanthan gum solution related to shear thinning fluid delivery for subsurface remediation

Xanthan gum solutions are shear thinning fluids which can be used as delivery media to improve the distribution of remedial amendments injected into heterogeneous subsurface environments. The rheological behavior of the shear thinning solution needs to be known to develop an appropriate design for field injection. In this study, the rheological properties of xanthan gum solutions were obtained under various chemical and environmental conditions relevant to delivery of remedial amendments to groundwater. Higher xanthan concentration raised the absolute solution viscosity and increased the degree of shear thinning. Addition of remedial amendments (e.g., phosphate, sodium lactate, ethyl lactate) caused the dynamic viscosity of xanthan solutions to decrease, but they maintained shear-thinning properties. Use of mono- and divalent salts (e.g., Na+, Ca2+) to increase the solution ionic strength also decreased the dynamic viscosity of xanthan and the degree of shear thinning, although the effect reversed at high xanthan concentrations. A power law analysis showed that the consistency index is a linear function of the xanthan concentration. The degree of shear thinning, however, is best described using a logarithmic function. Mechanisms to describe the observed empiricism have been discussed. In the absence of sediments, xanthan solutions maintained their viscosity for months. However, the solutions lost their viscosity over a period of days to weeks when in contact with site sediment. Loss of viscosity is attributed to physical and biodegradation processes.

Mycelia Promote Active Transport and Spatial Dispersion of Polycyclic Aromatic Hydrocarbons

To cope with heterogeneous subsurface environments mycelial microorganisms have developed a unique ramified growth form. By extending hyphae, they can obtain nutrients from remote places and transport them even through air gaps and in small pore spaces, repectively. To date, studies have been focusing on the role that networks play in the distribution of nutrients. Here, we investigated the role of mycelia for the translocation of nonessential substances, using polycyclic aromatic hydrocarbons (PAHs) as model compounds. We show that the hyphae of the mycelial soil oomycete Pythium ultimum function as active translocation vectors for a wide range of PAHs. Visualization by two-photon excitation microscopy (TPEM) demonstrated the uptake and accumulation of phenanthrene (PHE) in lipid vesicles and its active transport by cytoplasmic streaming of the hyphae ('hyphal pipelines'). In mycelial networks, contaminants were translocated over larger distances than by diffusion. Given their transport capacity and ubiquity, hyphae may substantially distribute remote hydrophobic contaminants in soil, thereby improving their bioavailability to bacterial degradation. Hyphal contaminant dispersal may provide an untapped potential for future bioremediation approaches.

Sampling Frequency of Groundwater Monitoring and Remediation Delay at Contaminated Sites

This study examines the frequency of sampling at contaminated sites located in heterogeneous subsurface environments. The impact of delays in remedial response is also investigated in terms of the growth that such delays incur on contaminated areas and remediation costs. Our work utilizes high-resolution numerical Monte Carlo realizations to simulate contaminant movement in heterogeneous, two-dimensional aquifers and to calculate the probabilities of detection Pd attained by various monitoring well arrangements. For all types of soils Pd was seen to decrease as sampling became less frequent, with 8 wells sampled daily, or 20 wells sampled monthly required in order to maintain detection probabilities that are higher than failure probabilities. Irrespective of the density of a monitoring network at highly dispersive subsurface environments a very rigorous sampling schedule must be maintained in order to retain the detection performance of the network. Highly heterogeneous soils through the presence of low permeability zones appeared to impede the spread of the contaminants and hence, ameliorate the effects of dispersion. Analysis of the time lag, between the time that contaminants first appeared at monitoring locations and the time they were observed, as well as of the enlargement of the plume area that resulted from this time lag led to the conclusion that monthly sampling is required for a wide range of hydrogeologic environments. Finally, in highly dispersive environments the remediation response must be of the order of a few months if one does not wish the contaminated areas and remediation costs to grow significantly.

Geochemical Processes Governing the Fate and Transport of Chromium(III) and Chromium(VI) in Soils

Chromium has served as an exceptional and necessary elemental component of many industrial processes and consumer products. Its prevalence in the global environment as both a dissolved and wind‐borne constituent has prompted concern during the last several decades due to the large migration potential and biological toxicity of various Cr chemical species. The objective of this study was to develop an improved understanding and predictive capability of the rates and mechanisms of competing geochemical redox and sorption reactions that govern the fate and transport of Cr(III) and C(VI) in heterogeneous subsurface environments. Batch and miscible displacement experiments, coupled with solid‐phase spectroscopy methods, were utilized to quantify the interaction of Cr with subsurface materials acquired from three geographically distinct locations within the continental United States that represented soils from different Department of Energy facilities known to have issues regarding Cr contamination. Soil chemical and mineralogical properties were found to be important factors controlling the mechanisms of Cr–solid phase interactions, with many of the reactive processes being time dependent. Both sorption and redox reactions impacted Cr(III)– and Cr(VI)–solid phase interactions and were modeled as nonlinear, nonequilibrium or equilibrium, reversible or nonreversible reactive processes. The research investigations within this study highlight the environmental significance of Cr speciation and solid‐phase reactivity in heterogeneous subsurface soil systems with contrasting geochemical and mineralogical properties.

Impact of In Situ Chemical Oxidation on Contaminant Mass Discharge: Linking Source-Zone and Plume-Scale Characterizations of Remediation Performance

A large-scale permanganate-based in situ chemical oxidation (ISCO) effort has been conducted over the past ten years at a federal Superfund site in Tucson, AZ, for which trichloroethene (TCE) is the primary contaminant of concern. Remediation performance was assessed by examining the impact of treatment on contaminant mass discharge, an approach that has been used for only a very few prior ISCO projects. Contaminant mass discharge tests were conducted before and after permanganate injection to measure the impact at the source-zone scale. The results indicate that ISCO caused a significant reduction in mass discharge (approximately 75%). The standard approach of characterizing discharge at the source-zone scale was supplemented with additional characterization at the plume scale, which was evaluated by examining the change in contaminant mass discharge associated with the pump-and-treat system. The integrated contaminant mass discharge decreased by approximately 70%, consistent with the source-zone-scale measurements. The integrated mass discharge rebounded from 0.1 to 0.2 kg/d within one year after cessation of permanganate injections, after which it has been stable for several years. Collection of the integrated contaminant mass discharge data throughout the ISCO treatment period provided a high-resolution, real-time analysis of the site-wide impact of ISCO, thereby linking source-zone remediation to impacts on overall risk. The results indicate that ISCO was successful in reducing contaminant mass discharge at this site, which comprises a highly heterogeneous subsurface environment. Analysis of TCE sediment concentration data for core material collected before and after ISCO supports the hypothesis that the remaining mass discharge is associated in part with poorly accessible contaminant mass residing within lower-permeability zones.

Integrated Sensor and Media Modeling Environment Developed and Applied to Ground-Penetrating Radar Investigation of Bridge Decks

Integrated sensor and media modeling environment has been developed to simulate subsurface sensing systems and environmental parameters relevant to the subsurface sensing modalities. The modeling environment is designed to represent complexity in subsurface features, sensor models, and the integration of the sensors with the subsurface environment. The ability to model complex subsurface environments and any potential random distribution of subsurface properties allows for realistic modeling of heterogeneous subsurface environments such as bridged deck/pavement systems. Many applications can benefit from the modeling, simulation, and interpretation capabilities in the new modeling environment that supports improved understanding of system behavior through simulations to evaluate the ability of a particular modality to detect defects. While numerous modeling packages exist to simulate different wave-based modalities, the integrated sensor and media modeling environment is developed to, in a straightforward manner, physically represent the complex subsurface civil infrastructure environment. Physical modeling capabilities enable the object-oriented programming environment facility portability to other application domains as a generic volume serves as the boundaries for internal elements modeled to represent realistic changes in material properties and buried objects. Model development is demonstrated on a realistic bridge deck example.

Uncertainty quantification in modeling flow and transport in porous media

This special issue of Stochastic Environmental Research and Risk Assessment in honor of Shlomo P. Neuman presents recent developments in uncertainty quantification in predictions of flow and transport in heterogeneous porous media. It covers subjects as diverse as stochastic hydrogeology, aquifer characterization, effective (average) descriptions of subsurface processes, inverse modeling, and scaling theory, all of which benefited from the insightful contributions of Shlomo P. Neuman. We hope that this volume will become a reference point for students learning these subject, scientists pursuing innovative research, and professionals and water resources planners looking to quantify predicting uncertainty arising from the intrinsic complexity of the subsurface environment. S. P. Neuman is one of the pioneers in the use of stochastic modeling as a means of dealing with uncertainty in hydraulic and transport parameters of heterogeneous subsurface environments. This special issue contains a number of contributions in this field. Riva et al. introduce a stochastic approach to interpret field-scale tracer tests conducted in heterogeneous aquifers. By using Monte Carlo simulations, the authors demonstrate that directly linking porosity and permeability distribution to the spatial variability of soil particle sizes renders the best prediction of the observed heavy tailing of measured breakthrough curves. Zhang et al. compare the performance of Monte Carlo and stochastic collocation methods to evaluate the probability of the concentration of a conservative solute attaining a given threshold. Their analysis shows that Latin Hypercube sampling and Quasi Monte Carlo outperform standard Monte Carlo simulations, and that a sparse-grid collocation and a probabilistic collocation method provide an accurate and efficient alternative to Monte Carlo simulations. Broyda et al. develop partial differential equations satisfied by the probability density function (PDF) of the concentration of solutes undergoing heterogeneous reactions in porous media with uncertain chemical properties. The authors obtain a semi-analytical solution for the concentration PDF and demonstrate that its shape and evolution are significantly affected by both spatial variability of advective velocity and the Damkohler number. Tartakovsky presents a novel Lagrangian particle model based on smoothed particle hydrodynamics (SPH). His analysis reveals the advantages of Lagrangian methods for the stochastic analysis of miscible density-driven fluid flows in the presence of RayleighTaylor instability. When used in the context of Monte Carlo simulations, the method can provide reliable estimated of the probability of occurrence of rare events. Two papers are concerned with the estimation of hydrogeological parameters governing groundwater flow. Liu et al. present an approach to efficient and accurate parameter estimation for nonlinear flow and transport problems in porous media. They discuss the relative merits and weaknesses of different schemes, including Maximum a posteriori (MAP) and Monte Carlo based methods, used to parameterize a DNAPL dissolution/transport model. Methods to diagnose Markov Chain Monte Carlo (MCMC) samples are introduced, compared and discussed in the context of computationally demanding parameter estimation A. Guadagnini (&) Dipartimento di Ingeneria Idraulca, Ambientale, Infrastrutture Viarie e Rilevamento, Politecnico di Milano, Piazza L. Da Vinci, 32, 20133 Milan, Italy e-mail: alberto.guadagnini@polimi.it