An approach for quantification of the heterogeneity of DNAPL source zone geometries.

Many studies have investigated the migration and entrapment processes of source zones from dense non-aqueous phase liquid (DNAPL) contamination under different conditions. However, the characterization of occupying area by source zone (or source shape) in water-saturated aquifers is still rudimentarily considered. In this study, we demonstrated this issue (1) by providing a brief review of existing approaches for source shape consideration, (2) by proposing an approach with simple shape parameters based on the non-uniformity of source widths, and (3) by providing exemplary applications of our proposed approach on shapes already published in previous research works. Our literature review suggested that the source zone in mathematical approaches is generally characterized as simple geometrical shapes (arbitrary lines or rectangles) or system-defined parameters that contrast to complex and discontinuous shapes observed in the real world. But the characterization of such complex shapes is still not possible with acceptable efforts. Therefore, we proposed an approach to parameterize the source shape by considering the variation of width and midpoints over the depth of the entire source zone and formulate four parameters based on population statistics (mean, standard deviation). To illustrate the suitability of our approach, we applied it to the results of lab experiments, and by analyzing these complex shapes, we highlighted the potential for improving the characterization techniques of non-uniformity of the source zones.

An Approach for Selecting a Model for the Assessment of Potentially Contaminated Sites.

Assessment of potentially contaminated sites (PCS) can be expensive; hence, simple and less demanding methods and models are required. This work attempts to provide an approach that can aid in selecting the most appropriate model for the PCS. The developed method uses over 100 field site data to evaluate four test models (analytical/empirical) that provide the maximum plume length (Lmax ), which is used as a principal model ranking quantity in this work. Analysis of site data shows that field plume length (Lf ) follows a log-normal distribution. Subsequently, Lmax is delineated with respect to Lf using a threshold probability as underestimating, overestimating, and overly-overestimating. Akaike information criterion (AIC) and analytical hierarchy process (AHP) are considered to support the threshold approach results. The classical AIC is modified (to AICmod ) to fit the term represented by the difference between Lf and Lmax . Additionally, the threshold factors as a product of subjective weights are added to the AICmod . Using Lf and Lmax , the AICmod provides a distinct ranking of the test models. For the AHP approach, the goodness of fit, underestimation, overly overestimation, and model complexity are the four chosen criteria. Similar to AICmod , the AHP approach provides a distinct ranking of the test models. The final decision on the best fitting model can be made on user criteria following the scheme developed in this work.

Determining the impact of flow velocities on reactive processes associated with Enterococcus faecalis JH2-2.

This study focuses on the impact of infiltration rates on colloidal transport and reactive processes associated with Enterococcus faecalis JH2-2 using water-saturated sediment columns. The infiltration rates influence the physical transport of bacteria by controlling the mean flow velocity. This, in turn, impacts biological processes in pore water owing to the higher or lower residence time of the bacteria in the column. In the present study, continuous injection of E. faecalis (suspended in saline water with varying conditions of dissolved oxygen and nutrient concentrations) into a lab-scale sediment column was performed at flow velocities of 0.02 cm min-1 and 0.078 cm min-1, i.e., at residence times of 1-5 hours. The impact of residence times on reactive processes is significant for field scale setups. A process-based model with a first-order rate coefficient for each biological process was fitted for each obtained condition-specific dataset from the experimental observations (breakthrough curves). The coefficients were converted to a dimensionless form to facilitate the comparison of biological processes. These results indicate that the processes of attachment and growth were flow-dependent. The growth process in the absence of dissolved oxygen was the most dominant process, with a Damkoehler number of approximately 48.

Contamination Assessment and Site-Management Tool (CAST): A Browser-Based Tool for Site Assessment.

Groundwater dependency is increasing globally, while millions of potentially contaminated sites are yet to be characterized for contamination levels. In particular, groundwater contamination due to light nonaqueous phase liquids (LNAPLs) continues to be a global challenge. Mathematical approaches (i.e., analytical, semi-analytical, empirical, numerical) are preferred for an initial site assessment to circumvent the high characterization costs and limited site data availability. However, the site-specific nature of contamination restricts the generalization of any single approach. Hence, the requirement is for an easy-to-use computing interface that provides site-specific data management, the selection and use of multiple-model interfaces for computing, and site characterization, with extension for the latest models as they become available. This work provides one such interface called CAST or Contamination Assessment and Site-management Tool. CAST is an open-source browser-based (online/offline) tool that provides an interface for six different analytical models (e.g., BIOSCREEN-AT), a MODFLOW based numerical model, and two empirical models (including a hybrid numerical-analytical model). Additionally, CAST includes interfaces for site data management, their evaluation, and scenario-based modeling. CAST's development is in a modular format, which simplifies the addition of new computing or data interfaces. Furthermore, the entire code-base of CAST is based on open-source (dominantly Python programming) libraries and standards. This further simplifies the modification or extension of this tool. This paper introduces CAST, its different computing, and data management interfaces and provides examples of the tool's functionality primarily for the initial evaluation of contaminated sites.

Biological and Physical Clogging in Infiltration Wells: Effects of Well Diameter and Gravel Pack.

Gravity-driven infiltration into the shallow subsurface via small-diameter wells (SDWs), i.e., wells with an inner diameter smaller than 7.5 cm (3 inches) and no gravel pack) has proven to be a cost-efficient and flexible tool for managed aquifer recharge (MAR), as it provides relatively high recharge rates with minimal construction effort. SDWs have a significantly smaller open filter area than larger diameter wells with gravel pack, making the infiltration of low-quality waters through these wells more at risk clogging. To investigate their susceptibility for biological and physical clogging, 24 physical models with different well setups were evaluated by infiltrating either nutrient-poor but turbid water or nutrient-rich but clear water. The experiments showed that smaller diameters and the lack of a gravel pack increase the well's susceptibility to both kinds of clogging. However, this effect was observed to be much more pronounced for physical than for biological clogging. Our conclusion is that SDWs show severe disadvantages with respect to the infiltration of highly turbid waters in comparison to large diameter wells with a gravel pack. Nevertheless, this disadvantage is much less severe when it comes to the infiltration of clear but nutrient-rich waters (e.g., treated wastewater). Depending on the economic and geological circumstances of a MAR-project, this disadvantage could be outweighed by the significantly lower construction costs of SDWs.

Reactive-transport modelling of Enterococcus faecalis JH2-2 passage through water saturated sediment columns.

The reuse of treated wastewater (e.g. for irrigation) is a common practice to combat water scarcity problems world-wide. However, the potential spread of opportunistic pathogens and fecal contaminants like Enterococci within the subsoil could pose serious health hazards. Additional sources (e.g., leaky sewer systems, livestock farming) aggravate this situation. This study contributes to an understanding of pathogen spread in the environment, using a combined modelling and experimental approach. The impact of quartz sediment and certain wastewater characteristics on the dissemination of Enterococcus faecalis JH2-2 is investigated. The transport processes of advection-dispersion and straining were studied by injecting conservative saline tracer and fluorescent microspheres through sediment packed columns, and evaluating resulting breakthrough curves using models. Similarly, simultaneously occurring reactive processes of microbial attachment, decay, respiration and growth were studied by injecting Enterococcus faecalis JH2-2 suspended in water with or without dissolved oxygen (DO) and nutrients through sediment, and evaluating resulting inlet and outlet concentration curves. The processes of straining, microbial decay and growth, were important when DO was absent. Irreversible attachment was important when DO was present. Sensitivity analysis of each parameter was conducted, and field scale behavior of the processes was predicted, to facilitate future work.

Influence of recharge rates on steady-state plume lengths.

A large number of potentially contaminated sites reported worldwide require cost- and time-effective assessment of the extent of contamination and the threats posed to the water resources. A significant risk assessment metric for these sites can be the determination of the maximum (i.e., steady-state) contaminant plume length (Lmax). Analytical approaches in the literature provide an option for such an assessment, but they include a certain degree of uncertainty. Often, the causes of such uncertainties are the simplifications in the analytical models, e.g., not considering the influence of hydrogeological stresses such as recharge, which impact the plume development significantly. This may lead to an over- or underestimation of Lmax. This work includes the influence of the recharge for the effective estimation of Lmax. For that, several two-dimensional (2D) numerical simulations have been performed by considering different aquifer thicknesses (1 m- 4 m) and recharge rates (ranging from 0 to 3.6 mm/day). From the numerical results of this work, it has been deduced that 1) the application of the recharge shortens Lmax, and the recharge entering the aquifer top causes the plume to tilt, 2) the reduction percentage in Lmax depends on the recharge rate applied and the aquifer thickness, and 3) the reduction percentage varies in a non-linear manner with respect to the recharge rate for a fixed aquifer thickness. Based on these results, a hybrid analytical-empirical solution has been developed for the estimation of Lmax with the inclusion of the recharge rate. The proposed hybrid analytical-empirical solution superimposes an empirically obtained correction factor onto an analytical solution. Although extensive confirmation steps of the developed model are required for including the effect of the recharge on aquifer hydraulics, the proposed expression improves the estimation of the Lmax significantly. The hybrid analytical-empirical solution has also been confirmed with a selection of limited field contamination sites data. The hybrid model result (Lhyb) provides a significant improvement in the estimation, i.e., an order of magnitude lower mean relative error compared to the analytical model.

Hydrogeochemical and mixing processes controlling groundwater chemistry in a wastewater irrigated agricultural system of India.

In many parts of the world, wastewater irrigation has become a common practice because of freshwater scarcity and to increase resource reuse efficiency. Wastewater irrigation has positive impacts on livelihoods and at the same time, it has adverse impacts related to environmental pollution. Hydrochemical processes and groundwater behaviour need to be analyzed for a thorough understanding of the geochemical evolution in the wastewater irrigated systems. The current study focuses on a micro-watershed in the peri-urban Hyderabad of India, where farmers practice intensive wastewater irrigation. To evaluate the major factors that control groundwater geochemical processes, we analyzed the chemical composition of the wastewater used for irrigation and groundwater samples on a monthly basis for one hydrological year. The groundwater samples were collected in three settings of the watershed: wastewater irrigated area, groundwater irrigated area and upstream peri-urban area. The collected groundwater and wastewater samples were analyzed for major anions, cations and nutrients. We systematically investigated the anthropogenic influences and hydrogeochemical processes such as cation exchange, precipitation and dissolution of minerals using saturated indices, and freshwater-wastewater mixtures at the aquifer interface. Saturation indices of halite, gypsum and fluorite are exhibiting mineral dissolution and calcite and dolomite display mineral precipitation. Overall, the results suggest that the groundwater geochemistry of the watershed is largely controlled by long-term wastewater irrigation, local rainfall patterns and water-rock interactions. The study results can provide the basis for local decision-makers to develop sustainable groundwater management strategies and to control the aquifer pollution influenced by wastewater irrigation.

An Approximation of Inner Boundary Conditions for Wells Intersecting Highly Conductive Structures.

Inner boundary conditions describe the interaction of groundwater wells with the surrounding aquifer during pumping and are associated with well-skin damage that limits water production and water derived from wellbore storage. Pumping test evaluations of wells during immediate and early time flow require assignment of inner boundary conditions. Originally, these concepts were developed for vertical well screens, and later transferred to wellbores intersecting highly conductive structures, such as preferential flow zones in fractured and karstic systems. Conceptual models for pumping test analysis in complex bedrock geology are often simplified. Classic analytical solutions generally lump or ignore conditions that limit or enhance well productivity along the well screen at the onset of pumping. Numerical solutions can represent well drawdowns in complex geological settings, such as karst systems, more precisely than many analytical solutions by accounting for additional physical processes and avoiding assumptions and simplifications. Suitable numerical tools for flow simulations in karst are discrete pipe-continuum models that account for various physical processes such as the transient hydraulics of wellbores intersecting highly conductive structures during pumping.

Collected Rain Water as Cost-Efficient Source for Aquifer Tracer Testing.

Locally collected precipitation water can be actively used as a groundwater tracer solution based on four inherent tracer signals: electrical conductivity, stable isotopic signatures of deuterium [δ2 H], oxygen-18 [δ18 O], and heat, which all may strongly differ from the corresponding background values in the tested groundwater. In hydrogeological practice, a tracer test is one of the most important methods for determining subsurface connections or field parameters, such as porosity, dispersivity, diffusion coefficient, groundwater flow velocity, or flow direction. A common problem is the choice of tracer and the corresponding permission by the appropriate authorities. This problem intensifies where tracer tests are conducted in vulnerable conservation or water protection areas (e.g., around drinking water wells). The use of (if required treated) precipitation as an elemental groundwater tracer is a practical solution for this problem, as it does not introduce foreign matters into the aquifer system, which may contribute positively to the permission delivery. Before tracer application, the natural variations of the participating end members' tracer signals have to be evaluated locally. To obtain a sufficient volume of tracer solution, precipitation can be collected as rain using a detached, large-scale rain collector, which will be independent from possibly existing surfaces like roofs or drained areas. The collected precipitation is then stored prior to a tracer experiment.

The fate of DNAPL contaminants in non-consolidated subsurface systems - Discussion on the relevance of effective source zone geometries for plume propagation.

Dense non-aqueous phase liquids, i.e., DNAPLs and the evolving contaminant plumes in aquifers provide significant potential to pose hazards affecting both environment and human health. Therefore, a proper assessment of contaminant spreading within the subsurface is critical. This includes a sufficient characterization of governing parameters describing both the subsurface and the contaminant itself. Thereby, knowledge on the contaminant source zone and especially the source zone geometry, i.e., SZG is critically required, yet very uncertain. This study identifies current limitations and open research questions in the formation and shape determination of source zone geometry, as well as its relevance for contaminant plumes. Our literature review reveals that existing characterization methods are subject to data interpretation uncertainties, while the application of these methods on field scale is limited by technical demands and accompanied efforts. In a next step, methods to implement increased source zone information into calculation methods are discussed. By means of an exemplary application of selected assessment tools, i.e., plume response models, results clearly proof the relevance of SZGs for site assessment. However, existing plume response models consider over-simplified geometries that may compromise their suitability. Our findings identify the demand for improved characterization of complex SZGs and the need to better evaluate the dependency of DNAPL migration on system properties and external influences. With emphasized knowledge on the most relevant SZG features, the delineation of "effective" SZGs allowing for straightforward implementation into plume response models and an adaption of the latter to incorporate more information on SZGs should be possible.

Spatio-temporal distribution and chemical characterization of groundwater quality of a wastewater irrigated system: A case study.

Wastewater irrigation is a common livelihood practice in many parts of the developing world. With the continuous irrigation supply, groundwater systems in these regions perceive adverse impacts due to inadequate infrastructure to treat the wastewater. The current study area, Musi River irrigation system, is one such case study located in the peri-urban Hyderabad of South India. The Musi River water, which is used for irrigation, is composed of untreated and secondary treated wastewater from Hyderabad city. Kachiwani Singaram micro-watershed in the peri-urban Hyderabad is practicing wastewater irrigation for the last 40 years. The current quality of (untreated) wastewater used for irrigation is expected to have adverse impacts on the local aquifers, but detailed investigations are lacking. To elucidate the groundwater quality dynamics and seasonality of the wastewater irrigation impacts on the peri-urban agricultural system, we analyzed the groundwater quality on a monthly basis for one hydrological year in the wastewater and groundwater irrigated areas, which exist next to each other. The spatio-temporal variability of groundwater quality in the watershed was analyzed with respect to wastewater irrigation and seasonality using multivariate statistical analysis, multi-way modeling and self-organizing maps. This study indicates the significance of combining various statistical techniques for detailed evaluation of the groundwater processes in a wastewater irrigated agricultural system. The results suggest that concentrations of the major ionic substances increase after the monsoon season, especially in wastewater irrigated areas. Multi-way modeling identified the major polluted groundwaters to come from the wastewater irrigated parts of the watershed. Clusters of chemical variables identified by using self-organizing maps indicate that groundwater pollution is highly impacted by mineral interactions and long-term wastewater irrigation. The study recommends regular monitoring of water resources and development of sustainable management strategies to mitigate the aquifer pollution in wastewater irrigation systems.

Description and verification of a novel flow and transport model for silicate-gel emplacement.

We present a novel approach for the numerical simulation of the gelation of silicate solutions under density-dependent flow conditions. The method utilizes an auxiliary, not density-dependent solute that is subject to a linear decay function to provide temporal information that is used to describe the viscosity change of the fluid. By comparing the modeling results to experimental data, we are able to simulate the behavior and the gelation process of the injected solute for three different compositions, including long-term stability of the gelated area, and non-gelation of low concentrations due to hydro-dynamic dispersion. This approach can also be used for other types of solutes with this gelling property and is useful in a variety of applications in geological, civil and environmental engineering.

Model-based prediction of long-term leaching of contaminants from secondary materials in road constructions and noise protection dams.

In this study, contaminant leaching from three different secondary materials (demolition waste, municipal solid waste incineration ash, and blast furnace slag) to groundwater is assessed by numerical modeling. Reactive transport simulations for a noise protection dam and a road dam (a typical German autobahn), in which secondary materials are reused as base layers, were performed to predict the breakthrough of a conservative tracer (i.e., a salt) and sorbing contaminants (e.g., PAHs like naphthalene and phenanthrene or heavy metals) at the groundwater table. The dam constructions have a composite architecture with soil covers in inclined layers and distinct contrasts in the unsaturated hydraulic properties of the used materials. Capillary barrier effects result in strong spatial variabilities of flow and transport velocities. Contaminant breakthrough curves at the groundwater table show significant tailing due to slow sorption kinetics and a wide distribution of travel times. While conservative tracer breakthrough depends primarily on subsoil hydraulic properties, equilibrium distribution coefficients and sorption kinetics represent additional controlling factors for contaminant spreading. Hence, the three secondary materials show pronounced differences in the temporal development of leached contaminant concentrations with consequences for breakthrough times and peak concentrations at the groundwater table. Significant concentration reductions due to dispersion occur only if the source concentrations decrease significantly prior to the arrival of the contaminant at the groundwater table. Biodegradation causes significant reduction of breakthrough concentrations only if flow velocities are low.

Multitracer test approach to characterize reactive transport in karst aquifers.

A method to estimate reactive transport parameters as well as geometric conduit parameters from a multitracer test in a karst aquifer is provided. For this purpose, a calibration strategy was developed applying the two-region nonequilibrium model CXTFIT. The ambiguity of the model calibration was reduced by first calibrating the model with respect to conservative tracer breakthrough and later transferring conservative transport parameters to the reactive model calibration. The reactive transport parameters were only allowed to be within a defined sensible range to get reasonable calibration values. This calibration strategy was applied to breakthrough curves obtained from a large-scale multitracer test, which was performed in a karst aquifer of the Swabian Alb, Germany. The multitracer test was conducted by the simultaneous injection of uranine, sulforhodamine G, and tinopal CBS-X. The model succeeds to represent the tracer breakthrough curves (TBCs) of uranine and sulforhodamine G and verifies that tracer-rock interactions preferably occur in the immobile fluid region, although the fraction of this region amounts to only 3.5% of the total water. However, the model failed to account for the long tailing observed in the TBC of tinopal CBS-X. Sensitivity analyses reveal that model results for the conservative tracer transport are most sensitive to average velocity and volume fraction of the mobile fluid region, while dispersion and mass transfer coefficients are least influential. Consequently, reactive tracer calibration allows the determination of sorption sites in the mobile and immobile fluid region at small retardation coefficients.

Karst spring responses examined by process-based modeling.

Ground water in karst terrains is highly vulnerable to contamination due to the rapid transport of contaminants through the highly conductive conduit system. For contamination risk assessment purposes, information about hydraulic and geometric characteristics of the conduits and their hydraulic interaction with the fissured porous rock is an important prerequisite. The relationship between aquifer characteristics and short-term responses to recharge events of both spring discharge and physicochemical parameters of the discharged water was examined using a process-based flow and transport model. In the respective software, a pipe-network model, representing fast conduit flow, is coupled to MODFLOW, which simulates flow in the fissured porous rock. This hybrid flow model was extended to include modules simulating heat and reactive solute transport in conduits. The application of this modeling tool demonstrates that variations of physicochemical parameters, such as solute concentration and water temperature, depend to a large extent on the intensity and duration of recharge events and provide information about the structure and geometry of the conduit system as well as about the interaction between conduits and fissured porous rock. Moreover, the responses of solute concentration and temperature of spring discharge appear to reflect different processes, thus complementing each other in the aquifer characterization.

Process-based interpretation of tracer tests in carbonate aquifers.

A tracer test in a carbonate aquifer is analyzed using the method of moments and two analytical advection-dispersion models (ADMs) as well as a numerical model. The numerical model is a coupled continuum-pipe flow and transport model that accounts for two different flow components in karstified carbonate aquifers, i.e., rapid and often turbulent conduit flow and Darcian flow in the fissured porous rock. All techniques employed provide reasonable fits to the tracer breakthrough curve (TBC) measured at a spring. The resulting parameter estimates are compared to investigate how each conceptual model of flow and transport processes that forms the basis of the analyses affects the interpretation of the tracer test. Numerical modeling results suggest that the method of moments and the analytical ADMs tend to overestimate the conduit volume because part of the water discharged at the spring is wrongly attributed to the conduit system if flow in the fissured porous rock is ignored. In addition, numerical modeling suggests that mixing of the two flow components accounts for part of the dispersion apparent in the measured TBC, while the remaining part can be attributed to Taylor dispersion. These processes, however, cannot reasonably explain the tail of the TBC. Instead, retention in immobile-fluid regions as included in a nonequilibrium ADM provides a possible explanation.

Modelling of diffusion-limited retardation of contaminants in hydraulically and lithologically nonuniform media.

A new reactive transport modelling approach and examples of its application are presented, dealing with the impact of sorption/desorption kinetics on the spreading of solutes, e.g. organic contaminants, in groundwater. Slow sorption/desorption is known from the literature to be strongly responsible for the retardation of organic contaminants. The modelling concept applied in this paper quantifies sorption/desorption kinetics by an intra-particle diffusion approach. According to this idea, solute uptake by or release from the aquifer material is modelled at small scale by a "slow" diffusion process where the diffusion coefficient is reduced as compared to the aqueous diffusion coefficient due to (i) the size and shape of intra-particle pores and (ii) retarded transport of solutes within intra-particle pores governed by a nonlinear sorption isotherm. This process-based concept has the advantage of requiring only measurable model parameters, thus avoiding fitting parameters like first-order rate coefficients. In addition, the approach presented here allows for modelling of slow sorption/desorption in lithologically nonuniform media. Therefore, it accounts for well-known experimental findings indicating that sorptive properties depend on (i) the grain size distribution of the aquifer material and (ii) the lithological composition (e.g. percentage of quartz, sandstone, limestone, etc.) of each grain size fraction. The small-scale physico-chemical model describing sorption/desorption is coupled to a large-scale model of groundwater flow and solute transport. Consequently, hydraulic heterogeneities may also be considered by the overall model. This coupling is regarded as an essential prerequisite for simulating field-scale scenarios which will be addressed by a forthcoming publication. This paper focuses on mathematical model formulation, implementation of the numerical code and lab-scale model applications highlighting the sorption and desorption behavior of an organic contaminant (Phenanthrene) with regard to three lithocomponents exhibiting different sorptive properties. In particular, it is shown that breakthrough curves (BTCs) for lithologically nonuniform media cannot be obtained via simple arithmetic averaging of breakthrough curves for lithologically uniform media. In addition, as no analytical solutions are available for model validation purposes, simulation results are compared to measurements from lab-scale column experiments. The model results indicate that the new code can be regarded as a valuable tool for predicting long-term contaminant uptake or release, which may last for several hundreds of years for some lithocomponents. In particular, breakthrough curves simulated by pure forward modelling reproduce experimental data much better than a calibrated standard first-order kinetics reactive transport model, thus indicating that the new approach is of high quality and may be advantageously used for supporting the design of remediation strategies at contaminated sites where some lithocomponents and/or grain size classes may provide a long-term pollutant source.