Henning Prommer

Tide‐induced recirculation across the aquifer‐ocean interface

A parametric analysis is conducted to examine the influence of tides, inland hydraulic conditions, and aquifer properties on the rate of tide-induced seawater recirculation through the nearshore aquifer. Understanding such influence is crucial for accurate prediction of subsurface chemical fluxes to coastal waters via groundwater discharge. The analysis is based on numerical simulations of density-dependent groundwater flow in a coastal aquifer subject to tidal oscillations across a sloping beach face. The results reveal that the amplitude of tidal oscillations and the inland hydraulic gradient are the primary parameters controlling the tide-induced recirculation rates. Significant tidal exchange is expected when the ratio of tidal to inland forcing is large. The horizontal tidal shoreline excursion and aquifer depth both display asymptotic behavior, influencing recirculation rates for only small values where the exchange process is limited by the potential for infiltration and shallowness of the aquifer, respectively. The analysis also indicates that tidal effects increase density-driven recirculation rates due to enhanced convective flow within the saltwater wedge.

Biogeochemical and isotopic gradients in a BTEX/PAH contaminant plume: model-based interpretation of a high-resolution field data set.

A high spatial resolution data set documenting carbon and sulfur isotope fractionation at a tar oil-contaminated, sulfate-reducing field site was analyzed with a reactive transport model. Within a comprehensive numerical model, the study links the distinctive observed isotope depth profiles with the degradation of various monoaromatic and polycyclic aromatic hydrocarbon compounds (BTEX/PAHs) under sulfate-reducing conditions. In the numerical model, microbial dynamics were simulated explicitly and isotope fractionation was directly linked to the differential microbial uptake of lighter and heavier carbon isotopes during microbial growth. Measured depth profiles from a multilevel sampling well with high spatial resolution served as key constraints for the parametrization of the model simulations. The results of the numerical simulations illustrate particularly well the evolution of the isotope signature of toluene, which is the most rapidly degrading compound and the most important reductant at the site. The resulting depth profiles at the observation well show distinct differences between the small isotopic enrichment in the contaminant plume core and the much stronger enrichment of up to 3.3 per thousand at the plume fringes.

Process-Based Reactive Transport Model To Quantify Arsenic Mobility during Aquifer Storage and Recovery of Potable Water

Aquifer storage and recovery (ASR) is an aquifer recharge technique in which water is injected in an aquifer during periods of surplus and withdrawn from the same well during periods of deficit. It is a critical component of the long-term water supply plan in various regions, including Florida, USA. Here, the viability of ASR as a safe and cost-effective water resource is currently being tested at a number of sites due to elevated arsenic concentrations detected during groundwater recovery. In this study, we developed a process-based reactive transport model of the coupled physical and geochemical mechanisms controlling the fate of arsenic during ASR. We analyzed multicycle hydrochemical data from a well-documented affected southwest Floridan site and evaluated a conceptual/numerical model in which (i) arsenic is initially released during pyrite oxidation triggered by the injection of oxygenated water (ii) then largely complexes to neo-formed hydrous ferric oxides before (iii) being remobilized during recovery as a result of both dissolution of hydrous ferric oxides and displacement from sorption sites by competing anions.

Evaluation of Conceptual and Numerical Models for Arsenic Mobilization and Attenuation during Managed Aquifer Recharge

Managed Aquifer Recharge (MAR) is promoted as an attractive technique to meet growing water demands. An impediment to MAR applications, where oxygenated water is recharged into anoxic aquifers, is the potential mobilization of trace metals (e.g., arsenic). While conceptual models for arsenic transport under such circumstances exist, they are generally not rigorously evaluated through numerical modeling, especially at field-scale. In this work, geochemical data from an injection experiment in The Netherlands, where the introduction of oxygenated water into an anoxic aquifer mobilized arsenic, was used to develop and evaluate conceptual and numerical models of arsenic release and attenuation under field-scale conditions. Initially, a groundwater flow and nonreactive transport model was developed. Subsequent reactive transport simulations focused on the description of the temporal and spatial evolution of the redox zonation. The calibrated model was then used to study and quantify the transport of arsenic. In the model that best reproduced field observations, the fate of arsenic was simulated by (i) release via codissolution of arsenopyrite, stoichiometrically linked to pyrite oxidation, (ii) kinetically controlled oxidation of dissolved As(III) to As(V), and (iii) As adsorption via surface complexation on neo-precipitated iron oxides.

Colloid release and clogging in porous media: Effects of solution ionic strength and flow velocity

The release and retention of in-situ colloids in aquifers play an important role in the sustainable operation of managed aquifer recharge (MAR) schemes. The processes of colloid release, retention, and associated permeability changes in consolidated aquifer sediments were studied by displacing native groundwater with reverse osmosis-treated (RO) water at various flow velocities. Significant amounts of colloid release occurred when: (i) the native groundwater was displaced by RO-water with a low ionic strength (IS), and (ii) the flow velocity was increased in a stepwise manner. The amount of colloid release and associated permeability reduction upon RO-water injection depended on the initial clay content of the core. The concentration of released colloids was relatively low and the permeability reduction was negligible for the core sample with a low clay content of about 1.3%. In contrast, core samples with about 6 and 7.5% clay content exhibited: (i) close to two orders of magnitude increase in effluent colloid concentration and (ii) more than 65% permeability reduction. Incremental improvement in the core permeability was achieved when the flow velocity increased, whereas a short flow interruption provided a considerable increase in the core permeability. This dependence of colloid release and permeability changes on flow velocity and colloid concentration was consistent with colloid retention and release at pore constrictions due to the mechanism of hydrodynamic bridging. A mathematical model was formulated to describe the processes of colloid release, transport, retention at pore constrictions, and subsequent permeability changes. Our experimental and modeling results indicated that only a small fraction of the in-situ colloids was released for any given change in the IS or flow velocity. Comparison of the fitted and experimentally measured effluent colloid concentrations and associated changes in the core permeability showed good agreement, indicating that the essential physics were accurately captured by the model.

Modelling of geochemical and isotopic changes in a column experiment for degradation of TCE by zero-valent iron

Zero-valent iron (ZVI) permeable-reactive barriers have become an increasingly used remediation option for the in situ removal of various organic and inorganic chemicals from contaminated groundwater. In the present study a process-based numerical model for the transport and reactions of chlorinated hydrocarbon in the presence of ZVI has been developed and applied to analyse a comprehensive data set from laboratory-scale flow-through experiments. The model formulation includes a reaction network for the individual sequential and/or parallel transformation of chlorinated hydrocarbons by ZVI, for the resulting geochemical changes such as mineral precipitation, and for the carbon isotope fractionation that occurs during each of the transformation reactions of the organic compounds. The isotopic fractionation was modelled by formulating separate reaction networks for lighter ((12)C) and heavier ((13)C) isotopes. The simulation of a column experiment involving the parallel degradation of TCE by hydrogenolysis and beta-elimination can conclusively reproduce the observed concentration profiles of all collected organic and inorganic data as well as the observed carbon isotope ratios of TCE and its daughter products.

Electrokinetic in situ oxidation remediation: Assessment of parameter sensitivities and the influence of aquifer heterogeneity on remediation efficiency

A newly developed groundwater and electrokinetic (EK) flow and reactive transport numerical model was applied to simulate electrokinetic in situ chemical oxidation (EK-ISCO) remediation. Scenario simulations that considered the oxidation of a typical organic contaminant (tetrachloroethene) by permanganate were used to gain a better understanding of the key processes and parameters that control remediation efficiency. In a first step a sensitivity analysis was carried out to investigate a range of EK, hydraulic and engineering parameters on the performance of EK-ISCO. While all investigated parameters affected the remediation process to some extent, the duration and energy required for remediation were shown to be most dependent upon the applied voltage gradient, the natural oxidant demand and the concentration of the injected oxidant. Secondly, the efficacy of EK-induced oxidant transport was further examined for a heterogeneous aquifer system with random permeability fields. Oxidant migration under EK was slower in low-permeability media due to the increased oxidant consumption of competing reductants. Instead of injecting oxidant only at the cathode, locating injection wells between the electrodes greatly increased the contaminant degradation by decreasing the distance the amendment had to migrate before reaching the contaminant.

Simulating MODFLOW-based reactive transport under radially symmetric flow conditions.

Radially symmetric flow and solute transport around point sources and sinks is an important specialized topic of groundwater hydraulics. Analysis of radial flow fields is routinely used to determine heads and flows in the vicinity of point sources or sinks. Increasingly, studies also consider solute transport, biogeochemical processes, and thermal changes that occur in the vicinity of point sources/sinks. Commonly, the analysis of hydraulic processes involves numerical or (semi-) analytical modeling methods. For the description of solute transport, analytical solutions are only available for the most basic transport phenomena. Solving advanced transport problems numerically is often associated with a significant computational burden. However, where axis-symmetry applies, computational cost can be decreased substantially in comparison with full three-dimensional (3D) solutions. In this study, we explore several techniques of simulating conservative and reactive transport within radial flow fields using MODFLOW as the flow simulator, based on its widespread use and ability to be coupled with multiple solute and reactive transport codes of different complexity. The selected transport simulators are MT3DMS and PHT3D. Computational efficiency and accuracy of the approaches are evaluated through comparisons with full 2D/3D model simulations, analytical solutions, and benchmark problems. We demonstrate that radial transport models are capable of accurately reproducing a wide variety of conservative and reactive transport problems provided that an adequate spatial discretization and advection scheme is selected. For the investigated test problems, the computational load was substantially reduced, with the improvement varying, depending on the complexity of the considered reaction network.

Kinetic reaction modeling framework for identifying and quantifying reductant reactivity in heterogeneous aquifer sediments.

Water-sediment interactions triggered by the injection of oxidized aqueous solutions into anoxic groundwater systems usually modify both the aquifer matrix and control the final aqueous composition. The identification and quantification of these reactions in complex heterogeneous systems remains a challenge for the analysis and prediction of water quality changes. Driven by the proposed injection of large quantities of oxic water into a deep anoxic heterogeneous pyritic aquifer; this study was undertaken to quantify the reactivity of aquifer sediments with respect to oxidant consumption and to characterize the variability of the reaction rates across different lithological units. A total of 53 samples were incubated for periods of 14, 37, and 50 days, during which the gas-phase was continuously monitored and the aqueous composition analyzed. A geochemical modeling framework was developed that incorporated a mixed set of equilibrium and kinetic reactions and supported the interpretation and quantification of the geochemical controls. The good agreement between simulated and experimental results of O2 consumption, CO2 production, pH, major ions, and trace metals suggests that the framework was able to successfully quantify reaction rates of competing redox and buffering reactions for the different lithological aquifer material.

Numerical Modeling of Arsenic Mobility during Reductive Iron-Mineral Transformations.

Millions of individuals worldwide are chronically exposed to hazardous concentrations of arsenic from contaminated drinking water. Despite massive efforts toward understanding the extent and underlying geochemical processes of the problem, numerical modeling and reliable predictions of future arsenic behavior remain a significant challenge. One of the key knowledge gaps concerns a refined understanding of the mechanisms that underlie arsenic mobilization, particularly under the onset of anaerobic conditions, and the quantification of the factors that affect this process. In this study, we focus on the development and testing of appropriate conceptual and numerical model approaches to represent and quantify the reductive dissolution of iron oxides, the concomitant release of sorbed arsenic, and the role of iron-mineral transformations. The initial model development in this study was guided by data and hypothesized processes from a previously reported,1 well-controlled column experiment in which arsenic desorption from ferrihydrite coated sands by variable loads of organic carbon was investigated. Using the measured data as constraints, we provide a quantitative interpretation of the processes controlling arsenic mobility during the microbial reductive transformation of iron oxides. Our analysis suggests that the observed arsenic behavior is primarily controlled by a combination of reductive dissolution of ferrihydrite, arsenic incorporation into or co-precipitation with freshly transformed iron minerals, and partial arsenic redox transformations.