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Context Occupancy and persistence of amphibian populations in patchy wetland habitats is influenced by landscape heterogeneity, species traits, and hydroclimatic variability. Such information is helpful for understanding the key drivers for reported world-wide declines in amphibian populations over past decades. Objectives The overarching goal of this study is to investigate how the combination of dynamic patch habitat attributes, as influenced by stochastic hydroclimatic forcing and landscape heterogeneity, and species traits drive long-term spatiotemporal patterns of wetland patch occupancy for amphibian metapopulations. Methods We used a data-model synthesis approach, integrating a long-term record of monitoring for Rana pipiens with simulations using a dynamic stochastic patch occupancy model, which links parsimonious representations of ecohydrological dynamics. Analyzed data were collected over a 20-yr period at the Cottonwood Lake Study Area in the Prairie Pothole Region in North Dakota, USA. Results The stability of the mean hydroclimatic forcing during the two decades of amphibian monitoring, and access to dense and diverse wetlands, contributed to persistence of the R. pipiens, despite seasonal spatiotemporal habitat dynamics. The amphibian occupancy pattern simulated for a larger domain surrounding the study area showed that the increased number of wetland habitats dampens variability in patch occupancy, contributing to persistence in the R. pipiens metapopulation. Conclusions The proposed framework is useful for understanding how spatial heterogeneity in habitat attributes and temporal variability in hydroclimatic forcing could affect metapopulation persistence in dynamic wetlandscapes. This integrated perspective can then be used to guide monitoring and management strategies based on statistically representative areas of heterogeneous and dynamic wetlandscapes.

Cities are the drivers of socioeconomic innovation and are also forced to address the accelerating risk of failure in providing essential services such as water supply today and in the future. Here, we investigate the resilience of urban water supply security, which is defined in terms of the services that citizens receive. The resilience of services is determined by the availability and robustness of critical system elements or “capitals” (water resources, infrastructure, finances, management efficacy, and community adaptation). We translate quantitative information about this portfolio of capitals from seven contrasting cities on four continents into parameters of a coupled system dynamics model. Water services are disrupted by recurring stochastic shocks, and we simulate the dynamics of impact and recovery cycles. Resilience emerges under various constraints, expressed in terms of each city's capital portfolio. Systematic assessment of the parameter space produces the urban water resilience landscape, and we determine the position of each city along a continuous gradient from water insecure and nonresilient to secure and resilient systems. In several cities stochastic disturbance regimes challenge steady‐state conditions and drive system collapse. While water insecure and nonresilient cities risk being pushed into a poverty trap, cities which have developed excess capitals risk being trapped in rigidity and crossing a tipping point from high to low services and collapse. Where public services are insufficient, community adaptation improves water security and resilience to varying degrees. Our results highlight the need for resilience thinking in the governance of urban water systems under global change pressures.

At any given time, a tropical cyclone (TC) vortex has multiple intensity pathways that are possible. We conceptualize this problem as a scenario where each of the TC's intensity pathways is a distinct attractor basin, and a combination of several external and internal factors across multiple scales dictates as to which of the many pathways the TC vortex actually takes. As with any complex system, it is difficult to know the details of the multiscale processes that cause or initiate the tipping of the TC vortex into an attractor basin. A stochastic shock arising from any of the various scales within a TC vortex and the subsequent cross‐scale energy transactions may rapidly increase the probability of the vortex intensifying or weakening. To address this problem and apply our conceptual framework to actual TC case studies, we formulate a novel scale‐specific stochastic model that examines the multiscale energetics at and across individual wave numbers within the TC vortex. The stochastic term is modeled in a realistic manner in that the lower and higher wave numbers are treated differently. High‐resolution Hurricane Weather and Research Forecast model outputs of two Bay of Bengal TCs, Phailin (intensifying) and Lehar (weakening), are used as case studies. An ensemble of intensity pathways is generated, and the nonstationary probability distributions of the intensity transitions at each time are examined. Our approach is another step toward an improved understanding of the stochastic dynamics of multiscale transitions of a TC vortex.

Spatio-temporal dynamics in habitat suitability and connectivity among mosaics of heterogeneous wetlands are critical for biological diversity and species persistence in aquatic patchy landscapes. Despite the recognized importance of stochastic hydroclimatic forcing in driving wetlandscape hydrological dynamics, linking such effects to emergent dynamics of metapopulation poses significant challenges. To fill this gap, we propose here a dynamic stochastic patch occupancy model (SPOM), which links parsimonious hydrological and ecological models to simulate spatio-temporal patterns in species occupancy in wetlandscapes. Our work aims to place ecological studies of patchy habitats into a proper hydrologic and climatic framework to improve the knowledge about metapopulation shifts in response to climate-driven changes in wetlandscapes. We applied the dynamic version of the SPOM (D-SPOM) framework in two wetlandscapes in the US with contrasting landscape and climate properties. Our results illustrate that explicit consideration of the temporal dimension proposed in the D-SPOM is important to interpret local- and landscape-scale patterns of habitat suitability and metapopulation occupancy. Our analyses show that spatio-temporal dynamics of patch suitability and accessibility, driven by the stochasticity in hydroclimatic forcing, influence metapopulation occupancy and the topological metrics of the emergent wetlandscape dispersal network. D-SPOM simulations also reveal that the extinction risk in dynamic wetlandscapes is exacerbated by extended dry periods when suitable habitat decreases, hence limiting successful patch colonization and exacerbating metapopulation extinction risks. The proposed framework is not restricted only to wetland studies but could also be applied to examine metapopulation dynamics in other types of patchy habitats subjected to stochastic external disturbances.

Subsurface contamination due to excessive nutrient surpluses is a persistent and widespread problem in agricultural areas across Europe. The vulnerability of a particular location to pollution from reactive solutes, such as nitrate, is determined by the interplay between hydrologic transport and biogeochemical transformations. Current studies on the controls of subsurface vulnerability do not consider the transient behaviour of transport dynamics in the root zone. Here, using state-of-the-art hydrologic simulations driven by observed hydroclimatic forcing, we demonstrate the strong spatiotemporal heterogeneity of hydrologic transport dynamics and reveal that these dynamics are primarily controlled by the hydroclimatic gradient of the aridity index across Europe. Contrasting the space-time dynamics of transport times with reactive timescales of denitrification in soil indicate that ~75% of the cultivated areas across Europe are potentially vulnerable to nitrate leaching for at least one-third of the year. We find that neglecting the transient nature of transport and reaction timescale results in a great underestimation of the extent of vulnerable regions by almost 50%. Therefore, future vulnerability and risk assessment studies must account for the transient behaviour of transport and biogeochemical transformation processes., Excess fertilizer use causes subsurface contamination. Here, the authors conduct an assessment of water quality vulnerability across Europe, finding that 75% of agricultural regions are susceptible to nitrate contamination for least one-third of the year, two times more than using standard estimation procedure.

The connectivity among distributed wetlands is critical for aquatic habitat integrity and to maintain metapopulation biodiversity. Here, we investigated the spatiotemporal fluctuations of wetlandscape connectivity driven by stochastic hydroclimatic forcing, conceptualizing wetlands as dynamic habitat nodes in dispersal networks. We hypothesized that spatiotemporal hydrologic variability influences the heterogeneity in wetland attributes (e.g., size and shape distributions) and wetland spatial organization (e.g., gap distances), in turn altering the variance of the dispersal network topology and the patterns of ecological connectivity. We tested our hypotheses by employing a DEM-based, depth-censoring approach to assess the eco-hydrological dynamics in a synthetically generated landscape and three representative wetlandscapes in the United States. Network topology was examined for two end-member connectivity measures: centroid-to-centroid (C2C), and perimeter-to-perimeter (P2P), representing the full range of within-patch habitat preferences. Exponentially tempered Pareto node-degree distributions well described the observed structural connectivity of both types of networks. High wetland clustering and attribute heterogeneity exacerbated the differences between C2C and P2P networks, with Pareto node-degree distributions emerging only for a limited range of P2P configuration. Wetlandscape network topology and dispersal strategies condition species survival and biodiversity.

Resilience of engineered systems is measured by the ability to anticipate, prepare for, recover, learn, and improve from an external disturbance regime that comprises of a series of chronic low-intensity and infrequent acute shocks, which disrupt functionality. Here, we present a new systems-level model for coupled technological systems, which provide functionality, and social systems in charge of management. Each system is characterized by a single, aggregated, dynamic state variable, namely (1) critical service deficit, representing services/functionality not provided by the technological system to match demands, and (2) adaptive capacity, representing total resources available to the managing/social institutions to maintain and repair critical services. These coupled systems are subjected to an external stochastic disturbance regime (Poisson shocks), and temporal perturbations in the two state variables are simulated. We use this “toy” model to simulate four hypothetical scenarios to illustrate likely coupled system temporal trajectories and shifts between a desirable (full service) and an undesirable (limited service) regime or complete system collapse (no service, no adaptive capacity). We also present several quantitative approaches to assess time series data and examine coupled systems dynamics. Resilience of the coupled systems for coping with and recovering from service losses is a dynamic property, contingent on system parameters that define the initial conditions before the shocks and recovery, and the frequency and magnitude of shocks.

We employed the well-established Horton-Strahler, hierarchical, stream-order (ω) scheme to investigate scaling of nutrient loads (P and N) from ~845 wastewater treatment plants (WWTPs) distributed along the river network in urbanized Weser River, the largest national basin in Germany (~46K km2; ~8.4 million population). We estimated hydrologic and water quality impacts at the reach- and basin-scales, at two steady river discharge conditions (median flow, QR50; low-flow, QR90). Of the five WWTPs class-sizes (1 ≤ k ≤ 5), ~68% discharge to small low-order streams (ω

Mass depletion–mass flux relationships usually applied to a groundwater plume were established at field scale for groundwater pumped from within the source zone of a dense non-aqueous phase liquid (DNAPL). These were used as part of multiple lines of evidence in establishing the DNAPL source mass and architecture. Simplified source mass-dissolved concentration models including those described by exponential, power, and error functions as well as a rational mass equation based on the equilibrium stream tube approach were fitted to data from 285 days of source zone pumping (SZP) from a single well which removed 152 kg of dissolved organics from a multi-component, reactive brominated solvent DNAPL. The total molar concentration of the source compound, tetrabromoethane and its daughter products was used as a single measure of contaminant concentration to relate to source mass. A partitioning inter-well tracer test (PITT) conducted prior to the SZP provided estimates of groundwater travel times, enabling parameterisation of the models. After accounting for capture of the down-gradient dissolved plume, all models provided a good fit to the observed data. It was shown that differentiation between models would only emerge after appreciably more pumping from the source zone. The model fits were not particularly sensitive to the exponent parameters and variance of groundwater travel time. In addition, the multi-component nature of the DNAPL did not seem to affect the utility of the models for the period examined. Estimates of the DNAPL mass prior to the start of SZP from the models were greatest where the log of the variance of travel time was used explicitly in the source depletion models (mean 295 kg) compared to where the associated power exponent and variance was fitted freely (mean 258 kg). The estimates of source mass were close to that of 220 kg determined from the PITT. In addition to the PITT, multi-level groundwater sampling from within the source zone provided important supporting information for developing the conceptual model of the source zone. It is concluded that SZP may be an effective and relatively simple means for characterising DNAPL source zones.

Recent natural and man-made catastrophes, such as the Fukushima nuclear power plant, flooding caused by Hurricane Katrina, the Deepwater Horizon oil spill, the Haiti earthquake, and the mortgage derivatives crisis, have renewed interest in the concept of resilience, especially as it relates to complex systems vulnerable to multiple or cascading failures. Although the meaning of resilience is contested in different contexts, in general resilience is understood to mean the capacity to adapt to changing conditions without catastrophic loss of form or function. In the context of engineering systems, this has sometimes been interpreted as the probability that system conditions might exceed an irrevocable tipping point. However, we argue that this approach improperly conflates resilience and risk perspectives by expressing resilience exclusively in risk terms. In contrast, we describe resilience as an emergent property of what an engineering system does, rather than a static property the system has. Therefore, resilience cannot be measured at the systems scale solely from examination of component parts. Instead, resilience is better understood as the outcome of a recursive process that includes: sensing, anticipation, learning, and adaptation. In this approach, resilience analysis can be understood as differentiable from, but complementary to, risk analysis, with important implications for the adaptive management of complex, coupled engineering systems. Management of the 2011 flooding in the Mississippi River Basin is discussed as an example of the successes and challenges of resilience-based management of complex natural systems that have been extensively altered by engineered structures.

There have been several calls made for hydrologic synthesis research: namely activities which unify diverse data sources across sites, scales and disciplines to uncover new connections and to promote a holistic understanding of water science. This paper draws on the NSF-funded Hydrological Synthesis Project (HSP) run by the University of Illinois at Urbana-Champaign to elucidate mechanisms, benefits and challenges of implementing hydrologic synthesis research from the perspectives of participants in a pilot research study. Two broadly different mechanisms of implementing synthesis were adopted in the HSP: 6-week Summer Institutes in which Ph.D. students conducted team-based research under the guidance of faculty mentors, and focused workshops which disseminated knowledge and shared experiences between scientists at many different career levels. The Summer Institutes were a test bed in which new ideas could be explored, assisted students in developing a wide range of skills, and were highly productive, but posed challenges for mentors and students because the ‘new’ research topics initiated during the Institutes' programmes needed to be completed in competition with students' ongoing Ph.D. research or mentor's existing research programs. The workshop-based model circumvented this conflict and was also highly productive, but did not offer the same opportunity to experiment with new ideas as part of the synthesis research. Leadership, trust, flexibility and long gestation times were all important to bringing synthesis research to a positive resolution. Funding models that embrace the exploratory aspects of synthesis and provide adequate support to mentors and students over these long timescales would facilitate future hydrologic synthesis research. Copyright © 2011 John Wiley & Sons, Ltd.

Surfactant-enhanced air sparging (SEAS) was evaluated in this laboratory-scale study to assess: (i) the removal efficiency of volatile contaminant from an aquifer model contrasted to conventional air sparging; and (ii) the effect of mass removal of dense non-aqueous phase liquid (DNAPL) during air sparging on the changes in aqueous flux of dissolved DNAPL. We conducted sparging experiments to remove perchloroethene (PCE) sources from laboratory flow chambers packed with sand. PCE was emplaced in rectangular zones at three locations within the flow chamber. The resident water was supplemented with the anionic surfactant, sodium dodecylbenzene sulfonate (SDBS), to reduce the surface tension of water, and then sparged with nitrogen gas at a constant flow rate of 0.12 L/min. It was found that SEAS was significantly more efficient than conventional air sparging for removing PCE. For SEAS, about 78% and 75% of total PCE mass was depleted from the flow chamber at a surface tension of 52.2 dynes/cm (350 mg/L SDBS) and 63.1 dynes/cm (150 mg/L SDBS), respectively, whereas only 38% was removed at 72.5 dyne/cm (no SDBS added). Before and after sparging, PCE mass flux in the aqueous phase during steady water flow through the chamber was measured in the flow chambers. Post-SEAS PCE fluxes were reduced, but not in direct proportion to the reduction in PCE mass.

We investigated, using model simulations, the changes occurring in the distribution of dense non-aqueous phase liquid (DNAPL) mass (Sn) within the source zone during depletion through dissolution, and the resulting changes in the contaminant flux distribution (J) at the source control plane (CP). Two numerical codes (ISCO3D and T2VOC) were used to simulate selected scenarios of DNAPL dissolution and transport in three-dimensional, heterogeneous, spatially correlated, random permeability fields with emplaced sources. Data from the model simulations were interpreted based on population statistics (mean, standard deviation, coefficient of variation) and spatial statistics (centroid, second moments, variograms). The mean and standard deviation of the Sn and J distributions decreased with source mass depletion by dissolution. The decrease in mean and standard deviation was proportional for the J distribution resulting in a constant coefficient of variation (CV), while for the Sn distribution, the mean decreased faster than the standard deviation. The spatial distributions exhibited similar behavior as the population distribution, i.e., the CP flux distribution was more stable (defined by temporally constant second moments and range of variograms) than the Sn distribution. These observations appeared to be independent of the heterogeneity of the permeability (k) field (variance of the log permeability field=1 and 2.45), correlation structure (positive vs. negative correlation between the k and Sn domains) and the DNAPL dissolution model (equilibrium vs. rate-limited), for the cases studied. Analysis of data from a flux monitoring field study (Hill Air Force Base, Utah) at a DNAPL source CP before and after source remediation also revealed temporal invariance of the contaminant flux distribution. These modeling and field observations suggest that the temporal evolution of the contaminant flux distribution can be estimated if the initial distribution is known. However, the findings are preliminary and broader implications to sampling strategies for remediation performance assessment need to be evaluated in additional modeling and experimental studies.

A new configuration of the passive fluxmeter (PFM) is presented that provides for simultaneous measurements of both the magnitude and the direction of ambient groundwater specific discharge qo and Cr(VI) mass flux J(Cr). The PFM is configured as a cylindrical unit with an interior divided into a center section and three outer sectors, each packed with a granular anion exchange resin having high sorption capacity for the Cr(VI) oxyanions CrO4(2-) and HCrO4-. The sorbent in the center section is preloaded with benzoate as the "resident" tracer. Laboratory experiments were conducted in which PFMs were placed in porous packed bed columns, through which was passed a measured volume of synthetic groundwater containing Cr(VI). During the deployment period, some of the resident tracer is depleted while the Cr(VI) is sorbed. The resin was then removed from the four sectors separately and extracted to determine the "captured" mass of Cr(VI) and the residual mass of the resident tracer in each. Cumulative specific discharge, q0t, values were assessed using the residual mass of benzoate retained in the center section. The direction of this discharge theta was ascertained from the mass distribution of benzoate intercepted and retained in the outer three sections of the PFM. Cumulative chromium fluxes, J(Cr)t, were quantified using the total Cr(VI) mass intercepted and retained on the PFM. Experiments produced an average measurement error for direction theta of 3 degrees +/- 14 degrees, while the average measurement errors for q0 and J(Cr) were, respectively, -8% +/- 15% and -12% +/- 23%. Results demonstrate the potential utility of the new PFM configuration for characterizing groundwater and contaminant fluxes.

The transient heat transfer for laminar flow inside a circular microtube, embedded in a rectangular substrate, during power startup has been numerically investigated using the finite element method. Silicon and silicon carbide were the substrates used, and water and FC-72 were the working fluids. Equations governing the conservation of mass, momentum, and energy were solved in the fluid region. Within the solid wafer, the heat-conduction equation was solved. A thorough investigation for velocity and temperature distributions for different substrates and coolants was performed by varying geometrical dimensions. The Prandtl number, thermal conductivity ratio, and diameter ranges were 6.78-12.68, 27-2658, and 300-1000 pm, respectively

The methods presented in this work provide a potential tool for characterizing contaminant source zones in terms of mass flux. The problem was conceptualized by considering contaminant transport through a vertical “flux plane” located between a source zone and a downgradient region where contaminant concentrations were measured. The goal was to develop a robust method capable of providing a statement of the magnitude and uncertainty associated with estimated contaminant mass flux values. In order to estimate the magnitude and transverse spatial distribution of mass flux through a plane, the problem was considered in an optimization framework. Two numerical optimization techniques were applied, simulated annealing (SA) and minimum relative entropy (MRE). The capabilities of the flux plane model and the numerical solution techniques were evaluated using data from a numerically generated test problem and a nonreactive tracer experiment performed in a three-dimensional aquifer model. Results demonstrate that SA is more robust and converges more quickly than MRE. However, SA is not capable of providing an estimate of the uncertainty associated with the simulated flux values. In contrast, MRE is not as robust as SA, but once in the neighborhood of the optimal solution, it is quite effective as a tool for inferring mass flux probability density functions, expected flux values, and confidence limits. A hybrid (SA-MRE) solution technique was developed in order to take advantage of the robust solution capabilities of SA and the uncertainty estimation capabilities of MRE. The coupled technique provided probability density functions and confidence intervals that would not have been available from an independent SA algorithm and they were obtained more efficiently than if provided by an independent MRE algorithm.

Determining representative infiltration rate parameters for use in modeling field-scale flow and transport processes is difficult because of the spatial variability of soil properties. To determine how steady-state infiltration rate variability is affected by support scale, steady-state infiltration rates (I,) were measured at three spatial scales (local, hillslope, and landscape) along a 710-m transect on a swell-swale landscape in Indiana. Spatial variability at the local scale was studied using measurements in a 1 × 1 m 2 array of 100 ring infiltrometers (7.2-cm diam.) for three soils at three horizons each. Studies were conducted at the hillslope and landscape scales using three nested infiltrometers of sizes 20 × 20, 60 × 60, and 100 × 100 cm 2 . Geostatistical analyses show a decrease in the sample variance of the I s values and an increase in spatial correlation of I s with depth. They also suggest that an area >10, 7.2-cm diam. rings (i.e., approximately >400 cm 2 ) is needed to provide a representative measurement area (RMA; i.e., area needed to filter out smaller-scale heterogeneities) at the local scale. Hillslope- and landscape-scale tests indicate that I s measurements with infiltrometers require an infiltrometer with a support area greater than the local-scale RMA to show the spatial correlation of the larger scales. In addition, these infiltrometer measurements may not provide appropriate effective I s estimates at these greater scales unless they are averaged over a domain that extends across the landscape's range of variability, estimated from the computed semivariograms to be 120 to 200 m for this study.

Remediation of sites contaminated by chlorinated organic compounds is a significant priority in the environmental field. Subsequently, the addition of cosolvent solutions for in situ flushing of contaminated source zones has been successfully field tested. However, the treatment of effluent fluids in such cleanup efforts is an often overlooked component of this technology implementation. The purpose of this research was to evaluate the effectiveness of zero-valent iron (Fe 0 ) in treating perchloroethylene (PCE) in an aqueous solution, and how the presence of a cosolvent (ethanol) and modification of the iron surface altered dechlorination. The modified iron surfaces included in this study were nickel-plated iron, acid-treated iron, and untreated iron surfaces. PCE dechlorination in the presence of each of the iron surfaces displayed pseudo first-order kinetics. The highest degradation rate of PCE occurred on the nickel-plated iron surface, 5.83×10 −3 h −1 , followed by the acid-treated iron, 4.92×10 −3 h −1 , and the untreated iron, 3.34×10 −3 h −1 . Dechlorination on each of the surfaces decreased with increasing cosolvent fractions. It was shown that as cosolvent fractions increased, PCE adsorption decreased and resulted in a concomitant decrease in PCE degradation rates.

The transport of nonreactive solutes through a coupled, two-dimensional, randomly heterogeneous vadose-saturated zone system subject to temporally random rainfall is predicted by Monte Carlo simulation and compared with previously published analytic results for three different rainfall patterns. The relative contributions of the uncertain inputs (i.e., rainfall and saturated conductivity) to the prediction uncertainty of solute transport are quantified in terms of the statistical moments of the pore water velocity, the plume spatial moments, and solute flux breakthrough curves at downstream control planes. Results show that the mean and variance of the saturated zone pore water velocity were approximately equivalent for the cases of uniform and random rainfall and were well predicted by the analytical relationships developed by Rubin and Bellin [1994]. As a result, the mean plume displacement, estimated by the trajectory of the mean plume center of mass, was found to be nearly identical for these cases. In the temporally random rainfall case, the saturated zone mean plume experienced more spread in the direction of mean flow at early times. However, the asymptotic rates of spatial spreading of the mean solute plumes were found to be approximately equivalent for the uniform and random rainfall cases and well predicted by the approximate expressions for longitudinal macrodispersivity in nonuniform flow proposed by Destouni and Graham [1995]. Random rainfall and random soil properties increased prediction uncertainty of the solute plume behavior in the vadose zone by an order of magnitude when compared with the uniform rain and random soil case. This effect was reduced considerably when the solute entered the saturated zone, where random rainfall produced only slightly larger prediction uncertainty than the uniform rainfall case. The analytic model developed by Destouni and Graham [1995] accurately predicted the temporal breakthrough of the mean solute plume at saturated zone control planes for all cases, if transport through the unsaturated zone accounted for the effects of temporally random rainfall using the methodology developed by Foussereau et al. [2000a, 2000b]. Results of this work indicate that for the humid climates studied here, uncertain rainfall patterns dominate transport prediction uncertainty in the shallow unsaturated zone, while uncertain solute breakthrough to the saturated zone and uncertain hydraulic conductivity dominate prediction uncertainty in the saturated zone.

Remediation of source zones at sites contaminated with dense non-aqueous-phase liquids using aggressive in situ flushing technologies, such as the addition of chemical additives known as cosolvents, have been implemented and successfully demonstrated in recent field tests. However, treatment of the waste fluids generated from such cleanup technologies has not received much attention. The purpose of this laboratory research was to evaluate the feasibility of using sonication as a method for treating waste fluids produced during in situ alcohol flushing at a site contaminated with a dense non-aqueous-phase liquid. Experimental results showed that sonochemical destruction of perchloroethylene (PCE) followed pseudo first-order kinetics and that increasing ethanol cosolvent percentages in the aqueous effluent resulted in decreasing degradation rates of PCE in solution. The energy efficiency (G = degradation/energy input) of the sonication treatment also decreased as the ethanol volume fraction increased. Overa...

A pilot-scale field test of in-situ alcohol flushing for enhanced solubilization and extraction of a dense nonaqueous phase liquid (DNAPL) source zone was conducted at a former dry cleaner site located in Jacksonville, Florida. This study was conducted to evaluate the feasibility of in-situ flushing for remediation of DNAPL sites in Florida. Groundwater at this site was contaminated with tetrachloroethylene (PCE) that had migrated below the water table, located at 3 m below ground surface (bgs), and collected at high saturations in thin, discontinuous layers in the 7.9 m to 9.4 m bgs depth interval. An oblong source zone (7.3 m × 2.7 m) was delineated using direct-push technologies and further characterized using soil coring and partitioning tracer techniques. Tracer tests and in-situ alcohol flushing were conducted using three injection wells that ap proximately bisected the source zone and six recovery wells located on the outer perimeter of the source zone. Over-extraction through the recovery wells en...

Estimates of spatially variable residual NAPL saturations SN are obtained in heterogeneous porous media using first temporal moments of breakthrough curves (BTCs) obtained from multilevel samplers during in situ partitioning tracer tests. An approach is adopted in which the distribution of the log NAPL/water volumetric ratio (Y = ln [SN/(1 − SN∥]) and log hydraulic conductivity (F = ln K) are treated as spatially correlated random fields. A nonlinear Gauss-Newton search technique is used to identify the spatial distribution of Y that minimizes the weighted sum of the deviation of the temporal moment predictions from their measured values and the deviation of the estimate of Y from its prior estimate obtained from the temporal moments of extraction well BTCs. Sensitivities required for the algorithm are obtained using a coupled flow and transport adjoint sensitivity method. In addition to obtaining optimal estimates for the spatial distribution of Y, the method also provides the estimation error covariance. The estimation error covariance can be used to evaluate the information that may be obtained from alternate pumping and monitoring configurations for tracer tests designed to detect NAPL in the subsurface. To this end, we tested the method using two different NAPL distributions (one with a random spatially correlated field and a second that was a block of NAPL) and three different pumping configurations (a double five-spot pattern, an inverted double five-spot pattern, and a line-drive pattern). The results show that measured temporal moments are more sensitive to Y in the double five-spot and inverted double five-spot patterns, and estimates produced in these configurations are slightly superior to those produced in the line-drive pattern.

Approximate analytic solutions are developed to predict the transport of inert solutes in variably saturated, randomly heterogeneous porous media subject to a random boundary flux condition. The mean and variance of solute mass flux past a series of vadose zone control planes are determined using a Lagrangian approach. The method requires the specification of an advective travel time probability density function (pdf) between the soil surface and the control planes and an impulse response function which quantifies the specific flow and transport processes that affect the solute along an individual streamline in the soil. In this study an advective-dispersive impulse response function for inert solute is assumed, and an advective travel time pdf is derived from the statistics of the boundary flux and soil properties using a one-dimensional transient vadose zone flow model [Foussereau et al., this issue]. Monte Carlo simulations are conducted using random boundary flux sequences and spatially correlated random hydraulic conductivity fields in a multidimensional variably saturated flow and transport code to test the analytic model assumptions. The analytic model accurately predicts the ensemble mean solute flux breakthrough curves for a variety of soil-climate combinations. The general behavior of the coefficient of variation of solute flux is also accurately reproduced; however, the minimum coefficient of variation at the mean center of mass was slightly underpredicted in most cases. Results show that for shallow soils in humid climates, boundary flux variability dominates soil variability in determining the uncertainty of solute transport predictions in the vadose zone.

A stochastic unsaturated water flow model is developed for heterogeneous soils subject to a transient flow regime. Equations are developed for a fully three-dimensional soil profile, and results are presented for an example one-dimensional problem. The model predicts the mean and covariance of the soil water content, Darcy's flux, and pore water velocity as a function of the boundary flux and saturated hydraulic conductivity statistics. The statistics of the pore water velocity can be used to predict solute transport in soils, as shown by Foussereau et al. [this issue]. Approximate flow-related moment equations are solved analytically in the Laplace domain. Then, the analytical results are numerically inverted using a modified fast Fourier transform algorithm. The model predictions are compared to results obtained from Monte Carlo simulations for two different boundary flux patterns characteristic of humid climates and two different soil types (a fine sand and a sandy loam). Comparing the approximate solutions of the statistical moments to the outputs of the Monte Carlo simulations shows (1) the dominance of the boundary flux variability over that of the saturated conductivity on the overall prediction uncertainty, particularly at shallow depths, and (2) the good performance of the stochastic unsaturated flow model, particularly for fine-textured soils subject to boundary fluxes with coefficients of variation up to ∼1.5. As the boundary flux coefficient of variation increases and the soil becomes coarser, the model performance deteriorates because the flow system becomes significantly more nonlinear.

In situ flushing groundwater remediation technologies, such as cosolvent flushing, rely on the stability of the interface between the resident and displacing fluids for efficient removal of contaminants. Contrasts in density and viscosity between the resident and displacing fluids can adversely affect the stability of the displacement front. Petroleum engineers have developed techniques to describe these types of processes; however, their findings do not necessarily translate directly to aquifer remediation. The purpose of this laboratory study was to investigate how density and viscosity contrasts affected cosolvent displacements in unconfined porous media characterized by the presence of a capillary fringe. Two-dimensional flow laboratory experiments, which were partially scaled to a cosolvent flushing field experiment, were conducted to determine potential implications of flow instabilities in homogeneous sand packs. Numerical simulations were also conducted to investigate the differential impact of fluid property contrasts in unconfined and confined systems. The results from these experiments and simulations indicated that the presence of a capillary fringe was an important factor in the displacement efficiency. Buoyant forces can act to carry a lighter-than-water cosolvent preferentially into the capillary fringe during displacement of the resident groundwater. During subsequent water flooding, buoyancy forces can act to effectively trap the cosolvent in the capillary fringe, contributing to the inefficient removal of cosolvent from the aquifer.

The difficult task of locating and quantifying nonaqueous phase liquids (NAPLs) present in the vadose and saturated zones has prompted the development of innovative, nondestructive characterization techniques. The use of the interwell partitioning tracer`s (IWPT) test, in which tracers that partition into the NAPL phase are displaced through the aquifer, is an attractive alternative to traditional coring and analysis. The first field test of IWPT was conducted in a hydraulically isolated test cell to quantify the total amount of a complex NAPL (a mixture of JP-4 jet fuel and chlorinated solvents) trapped within a 1.5-m smear zone in a shallow, unconfined sand and gravel aquifer at Hill Air Force Base (AFB), Utah. Tracer breakthrough curves (BTCs) were measured in three extraction wells (EWs) following a tracer pulse introduction through four injection wells (IWs). The measured retardation of the partitioning tracer (2,2-dimethyl-3-pentanol) relative to the nonreactive tracer (bromide) was used to quantify the NAPL present. The EW data were used to estimate an average NAPL saturation of 4.6--5.4% within the test cell. NAPL saturations estimated by using measured concentrations in soil cores of two significant compounds present in the NAPL were 3.0 and 4.6%.

The use of surface-active tracers for measuring the interfacial area between nonaqueous phase liquids (NAPLs) and water was evaluated in a hydraulically isolated test cell installed in a surficial aquifer located at Hill Air Force Base (AFB), Utah. Interfacial tracers were developed at the University of Florida for quantifying immiscible fluid-fluid interfaces (air-water or NAPL-water) in porous media. Sodium dodecyl benzene sulfonate (SDBS) was used as the interfacial tracer to measure the effective NAPL-water interfacial area (a{sub Nw}), while 2,2-dimethyl-3-pentanol (DMP) was used as the partitioning tracer to estimate the NAPL saturation (S{sub N}). The observed retardation of SDBS and DMP when compared to a nonreactive tracer (bromide or methanol) at eight multi-level sampling locations and one extraction well, was used to quantify the a{sub Nw} and S{sub N} values averaged over the interval between the injection wells and each monitoring point. The NAPL morphology index, defined here as H{sub N} = a{sub Nw}/{psi}S{sub N} ({psi} = porosity), varied significantly within the test cell. In locations where the magnitude of H{sub N} was large, the NAPL was assumed to be more or less uniformly spread, providing good contact with the mobile fluid. In contrast, regions with low H{sub N} valuesmore » were assumed to have NAPL that was more heterogeneously distributed as isolated patches providing poor contact with the mobile fluid. The index H{sub N}, provided by the combined use of interfacial and partitioning tracers, has important implications for NAPL source region remediation employing in situ flushing technologies.« less

A Winsor Type I surfactant/alcohol mixture was used as an in situ flushing agent to solubilize a multicomponent nonaqueous phase liquid (NAPL) as a single-phase microemulsion (SPME) in a hydraulically isolated test cell at Hill Air Force Base (AFB), Utah. The surfactant (polyoxyethylene(10) oleyl ether) and alcohol (1-pentanol) together comprised 5.5 wt % of the flushing solution. The NAPL was extremely complex, containing more than 200 constituents and a ‘pitch' fraction that was not solvent-extractable. The NAPL removal effectiveness of the SPME flood was evaluated using information from soil cores, partitioning tracer tests, and NAPL constituent breakthrough curves (BTCs) measured at three extraction wells. Soil core data indicated that approximately 90−95% of the most prevalent NAPL constituents were removed from the cell by the SPME flood. A comparison of pre- and postflushing partitioning tracer data indicated that about 72% of the measured NAPL volume was removed by the SPME flood. Integration of N...

Stochastic methods are applied to the analysis of partitioning and nonpartitioning tracer breakthrough data to obtain optimal estimates of the spatial distribution of subsurface residual non–aqueous phase liquid (NAPL). Uncertainty in the transport of the partitioning tracer is assumed to result from small-scale spatial variations in a steady state velocity field as well as spatial variations in NAPL saturation. In contrast, uncertainty in the transport of the nonpartitioning tracer is assumed to be due solely to the velocity variations. Partial differential equations for the covariances and cross cpvariances between the partitioning tracer temporal moments, nonpartitioning tracer temporal moments, residual NAPL saturation, pore water velocity, and hydraulic conductivity fields are derived assuming steady flow in an infinite domain [Gelhar, 1993] and the advection-dispersion equation for temporal moment transport [Harvey and Gorelick, 1995]. These equations are solved using a finite difference technique. The resulting covariance matrices are incorporated into a conditioning algorithm which provides optimal estimates of the tracer temporal moments, residual NAPL saturation, pore water velocity, and hydraulic conductivity fields given available measurements of any of these random fields. The algorithm was tested on a synthetically generated data set, patterned after the partitioning tracer test conducted at Hill AFB by Annable et al. [1997]. Results show that the algorithm successfully estimates major features of the random NAPL distribution. The performance of the algorithm, as indicated by analysis of the “true” estimation errors, is consistent with the theoretical estimation errors predicted by the conditioning algorithm.

At hazardous waste sites, removal of contaminant nonaqueous phase liquids (NAPLs) by mobilization using surfactant and cosolvent flushing is an attractive remediation option. Study of the changes in NAPL saturations (S n ) and specific NAPL-water interfacial areas (a nw ) before, during, and after the remediation process is essential in the characterization of waste sites, design, and evaluation of remediation strategies and risk assessment. Empirical indices, such as the capillary number (N Ca ) and, recently, the total trapping number (N T ), were presented in the literature to describe mobilization. Methods available for the measurement of S n allowed an adequate characterization and modeling of the S n -N Ca relationship. In contrast, no methods were available to measure a nw , and hence, no attempts to estimate or model the a nw -N Ca were reported. In the current work, the a nw -N T relationship was experimentally evaluated, using the recently reported interfacial tracers (IFTr) technique to estimate a nw and surfactant-flushing as a means of mobilization. It was found that, following mobilization of residual decane using AOT-100 surfactant, a nw initially remained insensitive to increase in N T , decreased passing through a minimum, and finally increased a with further increase in N T . This final increase is possible due to droplet dispersion or emulsification. A simple analytical model to describe the same relationship also is presented. The methods and results presented can be used as a basis for future investigations of changes in a nw following chemical flooding.

Management and closure of contaminated sites is increasingly being proposed on the basis of mass flux of dissolved contaminants in groundwater. Better understanding of the links between source mass removal and contaminant mass fluxes in groundwater would allow greater acceptance of this metric in dealing with contaminated sites. Our objectives here were to show how measurements of the distribution of contaminant mass flux and the overall mass discharge emanating from the source under undisturbed groundwater conditions could be related to the processes and extent of source mass depletion. In addition, these estimates of mass discharge were sought in the application of agreed remediation targets set in terms of pumped groundwater quality from offsite wells. Results are reported from field studies conducted over a 5-year period at a brominated DNAPL (tetrabromoethane, TBA; and tribromoethene, TriBE) site located in suburban Perth, Western Australia. Groundwater fluxes (qw; L3/L2/T) and mass fluxes (Jc; M/L2/T) of dissolved brominated compounds were simultaneously estimated by deploying Passive Flux Meters (PFMs) in wells in a heterogeneous layered aquifer. PFMs were deployed in control plane (CP) wells immediately down-gradient of the source zone, before (2006) and after (2011) 69–85% of the source mass was removed, mainly by groundwater pumping from the source zone. The high-resolution (26-cm depth interval) measures of qw and Jc along the source CP allowed investigation of the DNAPL source-zone architecture and impacts of source mass removal. Comparable estimates of total mass discharge (MD; M/T) across the source zone CP reduced from 104 g day− 1 to 24–31 g day− 1 (70–77% reductions). Importantly, this mass discharge reduction was consistent with the estimated proportion of source mass remaining at the site (15–31%). That is, a linear relationship between mass discharge and source mass is suggested. The spatial detail of groundwater and mass flux distributions also provided further evidence of the source zone architecture and DNAPL mass depletion processes. This was especially apparent in different mass-depletion rates from distinct parts of the CP. High mass fluxes and groundwater fluxes located near the base of the aquifer dominated in terms of the dissolved mass flux in the profile, although not in terms of concentrations. Reductions observed in Jc and MD were used to better target future remedial efforts. Integration of the observations from the PFM deployments and the source mass depletion provided a basis for establishing flux-based management criteria for the site.

The construction of a gas purge system for measurement of sorption kinetics is described in detail. The system features continuous monitoring of a gas-phase organic compounds using a photoionization detector, both open and closed modes of operation, and a convenient flask-switching design. Detector response was found to be adversely affected by small fluctuations in gas flow rate and by electrical interference from the temperature control system. Modification of the initial design incorporated flow control within the detector loop and uses a proportional temperature controller. Dynamic measurements of Henry`s Law constants in the gas purge system are reproducible and agree with literature values for most of the organic compounds tested. 22 refs., 4 figs., 2 tabs.

Increased inputs of nutrients into the waters of Lake Okeechobee has raised concern that the lake is becoming hypereutrophic. One aspect in understanding the overall cycling and dynamics of the nutrients in the system is the effect of benthic organisms on solute transport. Various diffusional models have been used to approximate the effect of benthic organisms on solute transport within sediments using diffusion coefficient values which represent the pooled contributions of molecular diffusion (D s ) and enhanced solute mixing due to macrobenthos activity (D i ). The objective of this study was to investigate the impact of benthic activity on solute transport by measuringD s (i.e., no benthic activity) and an apparent-dispersion or mixing coefficientD m (i.e., with benthic activity) for the four major sediment types of Lake Okeechobee, Florida using a reservoir technique. This method involved monitoring the depletion of a conservative tracer (tritiated water) from the overlying water (reservoir) resulting from transport into sediments using disturbed sediments repacked in cores (3.2 cm diam.) and undisturbed cores (3.2 to 12 cm diam.) obtained from the lake. Additional estimates ofD m andD s were also obtained by measuring tracer concentration profiles in the sediment cores at the end of a specified diffusion period. Molecular diffusion coefficients (D s ) measured forrepacked cores of sand, littoral, mud and peat sediments ranged from 0.90 to 1.29 cm2 d−1, and estimates ofD s were slightly higher in undisturbed cores without benthic organisms.D m values for undisturbed cores of mud, sand and littoral sediments having macrobenthic populations ranged from 2.09 to 24.78 cm2 d−1; values that were 1.6 to 15 times higher than those in sediments without benthic activity. Undisturbed cores of varying diameter from mud sediments had similar estimates ofD m for tritium; however, the undisturbed cores with larger diameters from littoral sediments yielded larger estimates ofD m , reflecting the heterogeneity of benthic population densities and activity. Therefore,D s estimates may not adequately represent transport processes for mud, sand and littoral sediments of Lake Okeechobee; hence careful consideration should be given to the role of benthic organisms in the overall transport of solutes across the sediment-water interface.

Recent natural and man-made catastrophes, such as the Fukushima nuclear power plant, flooding caused by Hurricane Katrina, the Deepwater Horizon oil spill, the Haiti earthquake, and the mortgage derivatives crisis, have renewed interest in the concept of resilience, especially as it relates to complex systems vulnerable to multiple or cascading failures. Although the meaning of resilience is contested in different contexts, in general resilience is understood to mean the capacity to adapt to changing conditions without catastrophic loss of form or function. In the context of engineering systems, this has sometimes been interpreted as the probability that system conditions might exceed an irrevocable tipping point. However, we argue that this approach improperly conflates resilience and risk perspectives by expressing resilience exclusively in risk terms. In contrast, we describe resilience as an emergent property of what an engineering system does, rather than a static property the system has. Therefore, resilience cannot be measured at the systems scale solely from examination of component parts. Instead, resilience is better understood as the outcome of a recursive process that includes: sensing, anticipation, learning, and adaptation. In this approach, resilience analysis can be understood as differentiable from, but complementary to, risk analysis, with important implications for the adaptive management of complex, coupled engineering systems. Management of the 2011 flooding in the Mississippi River Basin is discussed as an example of the successes and challenges of resilience-based management of complex natural systems that have been extensively altered by engineered structures.