Rapid Consumption of Dihydrogen Injected into a Shallow Aquifer by Ecophysiologically Different Microbes.

The envisaged future dihydrogen (H2) economy requires a H2 gas grid as well as large deep underground stores. However, the consequences of an unintended spread of H2 through leaky pipes, wells, or subterranean gas migrations on groundwater resources and their ecosystems are poorly understood. Therefore, we emulated a short-term leakage incident by injecting gaseous H2 into a shallow aquifer at the TestUM test site and monitored the subsequent biogeochemical processes in the groundwater system. At elevated H2 concentrations, an increase in acetate concentrations and a decrease in microbial α-diversity with a concomitant change in microbial ß-diversity were observed. Additionally, microbial H2 oxidation was indicated by temporally higher abundances of taxa known for aerobic or anaerobic H2 oxidation. After H2 concentrations diminished below the detection limit, α- and ß-diversity approached baseline values. In summary, the emulated H2 leakage resulted in a temporally limited change of the groundwater microbiome and associated geochemical conditions due to the intermediate growth of H2 consumers. The results confirm the general assumption that H2, being an excellent energy and electron source for many microorganisms, is quickly microbiologically consumed in the environment after a leakage.

Temperature Effects on PCE Degradation on ZVI in Column Experiments with Deionized Water.

The effects of rising groundwater temperatures on zerovalent iron (ZVI)-based remediation techniques will be critical in accelerating chlorinated hydrocarbon (CHC) degradation and side reactions. Therefore, tetrachloroethylene (PCE) degradation with three ZVIs widely used in permeable reactive barriers (Gotthart-Maier cast iron [GM], Peerless cast iron [PL], and ISPAT sponge iron [IS]) was evaluated at 10-70 °C in deionized water. From 10 to 70 °C, PCE degradation half-lives decreased from 25 ± 2 to 0.9 ± 0.1 h (PL), 24 ± 3 to 0.7 ± 0.1 h (GM), and 2.5 ± 0.01 to 0.3 ± 0.005 h (IS). Trichloroethylene (TCE) degradation half-lives at PL and GM decreased from 14.3 ± 3 to 0.2 ± 0.1 h (PL) and 7.6 ± 2 to 0.4 ± 0.1 h (GM). This acceleration of CHC degradation and the stronger shift toward reductive ß-elimination reduced the concentration of potentially harmful metabolites with increasing temperatures. PCE and TCE degradation yields an activation energy of 28 (IS), 58 and 40 kJ mol-1 (GM), and 62 and 53 kJ mol-1 (PL). Hydrogen gas production by ZVI corrosion increased by 3 orders of magnitude from 10 to 70 °C, and an increased chance of gas clogging was observed at high temperatures.

Anaerobic Corrosion of Zero-Valent Iron at Elevated Temperatures.

Increasing groundwater temperatures caused by global warming, subsurface infrastructure, or heat storage projects may interfere with groundwater remediation techniques using zero-valent iron (ZVI) technology by accelerating anaerobic corrosion. The corrosion behavior of three ZVIs widely used in permeable reactive barriers (PRBs), Peerless cast iron (PL), Gotthart-Maier cast iron (GM), and an ISPAT iron sponge (IS), was investigated at temperatures between 25 and 70 °C in half-open batch reactors by measuring the volume of hydrogen gas generated. Initially, the corrosion rates of all tested ZVIs increased with temperature; at temperatures ≤40 °C, a material-specific steady state is reached, and at temperatures >40 °C, passivation causes a decrease in long-term corrosion rates. The observed corrosion behavior was therefore assumed to be superimposed by accelerating and inhibiting effects, caused by surface precipitates where the fitting of measured corrosion rates by a modeling approach, using the corroded amount of Fe0 to account for passivating minerals, yields intrinsic activation energies (Ea, ZVI) of 81, 90, and 107 kJ mol-1 for IS, GM, and PL, respectively. An increase in H2 production might not be directly transferable to an increase in general ZVI reactivity; however, the results suggest that an increase in chlorinated hydrocarbon degradation rates can be expected for ZVI-PRBs in the immediate vicinity of low-temperature underground thermal energy storages (UTESs) or in the impact areas of high-temperature UTES with temperatures of ≤40 °C.

Geochemical Effects of Millimolar Hydrogen Concentrations in Groundwater: An Experimental Study in the Context of Subsurface Hydrogen Storage.

Hydrogen storage in geological formations is one of the most promising technologies for balancing major fluctuations between energy supply from renewable energy plants and energy demand of customers. If hydrogen gas is stored in a porous medium or if it leaks into a shallow aquifer, redox reactions can oxidize hydrogen and reduce electron acceptors such as nitrate, FeIII and MnIV (hydro)oxides, sulfate, and carbonate. These reactions are of key significance, because they can cause unintentional losses in hydrogen stored in porous media and they also can cause unwanted changes in the composition of protected potable groundwater. To represent an aquifer environment enclosing a hydrogen plume, laboratory experiments using sediment-filled columns were constructed and percolated by groundwater in equilibrium with high (2-15 bar) hydrogen partial pressures. Here, we show that hydrogen is consumed rapidly in these experiments via sulfate reduction (18 ± 5 µM h-1) and acetate production (0.030 ± 0.006 h-1), while no methanogenesis took place. The observed reaction rates were independent from the partial pressure of hydrogen and hydrogen consumption only stopped in supplemental microcosm experiments where salinity was increased above 35 g L-1. The outcomes presented here are implemented for planning the sustainable use of the subsurface space within the ANGUS+ project.

Significance of temperature as a key driver in ZVI PRB applications for PCE degradation.

We report on the potential of elevated groundwater temperatures and zero-valent iron permeable reactive barriers (ZVI PRBs), for example, through a combination with underground thermal energy storage (UTES), to achieve enhanced remediation of chlorinated hydrocarbon (CHC) contaminated groundwater. Building on earlier findings concerning deionized solutions, we created a database for mineralized groundwater based on temperature dependence of tetrachloroethylene (PCE) degradation using two popular ZVIs (i.e., Gotthart-Maier cast iron [GM] and ISPAT sponge iron [IS]) in column experiments at 25 °C-70 °C to establish a temperature-dependent ZVI PRB dimensioning approach. Scenario analysis revealed that a heated ZVI PRB system in a moderate temperature range up to 40 °C showed the greatest efficiency, with potential material savings of ~55% to 75%, compared to 10 °C, considering manageability and longevity. With a 25 °C-70 °C temperature increase, rate coefficients of PCE degradation increased from 0.4 ± 0.0 h-1 to 2.9 ± 2.2 h-1 (GM) and 0.1 ± 0.1 h-1 to 1.8 ± 0.0 h-1 (IS), while TCE rate coefficients increased from 0.6 ± 0.1 h-1 to 5.1 ± 3.9 h-1 at GM. Activation energies for PCE degradation yielded 32 kJ mol-1 (GM) and 56 kJ mol-1 (IS). Temperature-dependent anaerobic iron corrosion was key in regulating mineral precipitation and passivation of the iron surface as well as porosity reduction due to gas production.

Monitoring of a simulated CO(2) leakage in a shallow aquifer using stable carbon isotopes.

Artificial carbon dioxide leakage into a shallow aquifer was monitored using stable carbon isotope measurements at a field site near the town of Wittstock, Brandenburg, Germany. Approximately 400 000 L of CO(2) were injected into a shallow aquifer at 18 m depth over 10 days. The (13)C/ (12)C ratios of the CO(2) were measured in both groundwater and soil gas samples to monitor the distribution of the injected CO(2) plume and to evaluate the feasibility and reliability of this approach to detect potential CO(2) leakage, for example from carbon capture and storage (CCS) sites. The isotopic composition of the injected CO(2) (δ(13)C -30.5 ‰) was differentiable from the background CO(2) (δ(13)C -21.9 ‰) and the artificial CO(2) plume was monitored over a period spanning more than 204 days. The results demonstrate that this stable isotope monitoring approach can be used to identify CO(2) sources and detect potential CO(2) migration from CCS sites into overlying shallow aquifers or even into the upper subsurface. A significant difference between the isotope ratios of the natural background and the injected CO(2) is required for this monitoring approach to be effective.

Temperature influence on the NAPL-water interfacial area between 10 °C and 60 °C for trichloroethylene.

Underground thermal energy storage (UTES) can contribute to renewable energy usability, especially in urban areas with the most demand and available infrastructure. But UTES may interact in those areas with non-aqueous phase liquids (NAPL) by increasing the temperature in storage formations. To determine temperature effects on NAPL dissolution rates into groundwater, the effective specific interfacial area (anw) between trichloroethylene (TCE) and water, as a function of temperature and TCE pore saturations, was calculated. The interfacial tension between the flushing solution and the NAPL, the adsorption coefficient and the retardation of a reactive tracer were determined by the drop weight method and interfacial tracer tests at 10 °C, 30 °C and 60 °C. From 10 to 60 °C anw increased by a factor of six to eight. Based on the results, a function to describe the anw between TCE and water was developed, which could improve numerical models on NAPL dissolution rates. The main mechanisms for the increase in anw are suggested to be NAPL blob migration on pore scale, and thermal-induced changes in wettability and in blob shape correlating with a temperature-induced decline in effective porosity of up to 32%. These results contribute to the understanding and predictability of UTES in contaminated aquifers, the general response of NAPL behavior on artificial increasing aquifer temperatures and the improvement of thermal and other groundwater remediation techniques.

Henry's law constants of volatile organic compounds between 0 and 95 °C - Data compilation and complementation in context of urban temperature increases of the subsurface.

In urban areas with frequently occurring contamination by volatile organic hydrocarbons (VOC) possible uncontrolled contaminant mobilization e.g. by volatilization is feared in case of subsurface temperature increases induced by high temperature underground thermal energy storage (HT-UTES) or due to urban heat islands (UHI). However, volatilization of VOC is the main process utilized by thermal remediation approaches, wherefore a combination of contaminant remediation with UTES is increasingly discussed. To predict VOC volatilization under changing temperature conditions, which is substantially characterized by Henry's law constants (H), temperature dependency of H must be known substance-specifically in the total concerned temperature range. To provide an overview of available H and to evaluate their quality, published data of 41 pollutants were compiled, revealing the need for further measurements above 40 °C for at least 24 compounds (as e.g. TCE/PCE, BTEX). Additionally, the temperature dependence of H was calculated based on the compiled data using an established temperature regression function. Furthermore, H was complementarily measured for 10 relevant VOCs between 10 and 90 °C using the EPICS-method (Equilibrium Partitioning In Closed Systems). The measurements quantified the expected strong increase in H with rising temperature while providing high data quality (R2 = 0.84-0.99, mostly low standard deviations), thus improving the general data availability of H for VOCs and extending the scope of the regression function. The newly measured data and enhanced evaluation of compiled data contribute to a more reliable assessment of the pollutant behaviour in terms of volatilization at elevated temperatures caused e.g. by UTES or UHI.

Predictability of initial hydrogeochemical effects induced by short-term infiltration of ∼75 °C hot water into a shallow glaciogenic aquifer.

Despite their potential in heating supply systems, thus far high-temperature aquifer thermal energy storages (HT-ATES) currently lack widespread application. Reducing the potential risks by improving the predictability of hydrogeochemical processes accelerated or initiated at elevated temperatures might promote the development of this technology. Therefore, we report the results of a short-term hot water infiltration field test with subsurface temperatures above 70 °C, along with associated laboratory batch tests at 10, 40 and 70 °C for 28 sediment samples to determine their usability for geochemical prediction. Most groundwater components had lower maximal concentrations and smaller concentration ranges in field samples compared to the batch tests. This indicates that the strongest geochemical effects observed in laboratory tests with sufficient site-specific sediment samples will likely be attenuated at the field scale. A comparison of field measurements with predicted concentration ranges, based on temperature induced relative concentration changes from the batch tests, revealed that the predictive power was greatest, where the hot infiltrated water had cooled least and the strongest geochemical effects occurred. The batch test-based predictions showed the best accordance with field data for components, with significant temperature-induced concentration changes related to ion exchange and (de)sorption processes. However, accurate prediction of concentration changes based on other processes, e.g. mineral dissolution, and downstream reversals in concentrations, requires further investigation. The here presented procedure enables the prediction of maximal expectable temperature-dependant concentration changes for most environmentally relevant ancillary groundwater components, e.g. As, with limited effort.

Monitoring of the effects of a temporally limited heat stress on microbial communities in a shallow aquifer.

Aquifer thermal energy storage (ATES) is a key concept for the use of renewable energy resources. Interest in ATES performed at high temperature (HT-ATES; > 60 °C) is increasing due to higher energetic efficiencies. HT-ATES induces temperature fluctuations that exceed the natural variability in shallow aquifers, which could lead to adverse effects in subsurface ecosystems by altering the groundwater chemistry, biodiversity, and microbial metabolic activity, resulting in changes of the groundwater quality, biogeochemical processes, and ecosystem functions. The aim of this study was to emulate the initial operating phase of a HT-ATES system with a short-term infiltration of warm water into Pleistocene sandur sediment and, consequently, to monitor the thermal effects on the groundwater microbiome inhabiting an imitated affected space of an HT-ATES system. Therefore, local groundwater was withdrawn, heated up to 75 °C, and re-infiltrated into a shallow aquifer located near Wittstock/Dosse (Brandenburg, Germany) for around five days. Groundwater samples taken regularly before and after the infiltration were analyzed by 16S rRNA gene amplicon sequencing for microbial diversity analyses as well as total cell counting. During the infiltration, a thermal plume with groundwater temperatures increasing from 9 ± 2 to up to ~65 °C was recorded. The highest temperature at which groundwater samples were taken was 34.9 °C, a temperature typically arising in the affected space of an HT-ATES system. The microbial communities in the groundwater were mainly composed of Gammaproteobacteria, Alphaproteobacteria, Bacteroidia, and Actinobacteria, and the total cell numbers ranged from 3.2 * 104 to 3.1 * 106 cells ml-1. Neither the compositions of the microbial communities nor the total number of cells in groundwater were significantly changed upon moderate temperature increase, indicating that the diverse groundwater microbiome was resilient to the temporally limited heat stress.

Temperature influence on mobilisation and (re)fixation of trace elements and heavy metals in column tests with aquifer sediments from 10 to 70 °C.

The operation of seasonal underground thermal energy storages (UTES) as part of renewed heat supply systems can cause amplified temperature variations in the urban subsurface. Therefore, long-term environmental consequences for water extractions by trace elements and heavy metals (TEHMs) are a key point of concern regarding temperature effects on aquifer hydrogeochemistry. To address this issue, we report the results of flow-through and circular-flow column tests conducted with 4 anoxic northern German aquifer sediments, tempered to 10, 25, 40 and 70 °C and analysed for 20 TEHMs. Increased temperatures in column tests caused increasing Li, As, Mo, Sb and Ba concentrations and decreasing Ni concentrations in all of the sediments with a sediment-specific extent, whereas effects on V, Mn, Co, Tl and U concentrations varied sediment-specifically. Apart from Ba, all these components were released as a pulse in the initial heating phase, indicating a temperature dependent, finite, elutable pool. Re-cooling of the previously heated circular-flow column tests to 10 °C caused reversals of concentration changes by 30-95%. This indicates a return to initial hydrochemical conditions after termination of heat storage operation and downstream from heat storages during the operations. The latter was approximated for As with a simplified analytical 1-dimensional approach, presuming transferability from a laboratory to a field scale. This reversal in concentration changes enables active cooling as a countermeasure in cases of unexpected, adverse TEHM progression. From the perspective of our findings, TEHM concentration changes appear to be temporally and spatially limited.

The aqueous solubility of common organic groundwater contaminants as a function of temperature between 5 and 70 °C.

High-temperature thermal energy storage in shallow aquifers can potentially increase ambient groundwater temperatures up to 70 °C or even more. Since an increase in temperature is expected to influence contaminant mass flux into groundwater monitoring the spreading of organic contaminants located in the subsurface is crucial. In numerous former studies, the NAPL solubility, one major parameter controlling mass flux on field scale, was measured at temperatures up to 70 °C for a broad spectrum of organic substances. However, quantitative calculations of solubilities as a function of temperature considering a compiled database are largely missing. Aiming to examine the reliability of existing solubility-temperature relationships, to describe them functionally and further to identify knowledge gaps, previously published data on solubilities of 42 different organic groundwater contaminants were evaluated in this study. By using a common temperature regression function, the calculated solubility curves from compiled solubility data for 5-70 °C show relative changes between a few percent (CHCs and BTEX) and up to 2000% (PAHs). As published temperature-dependent solubilities for chlorinated ethylenes are contradictory in parts, solubilities of tetrachloroethylene, trichloroethylene, 1,2-cis-dichloroethylene and 1,2-trans-dichloroethylene were additionally investigated in more detail using batch experiments between 5 and 70 °C. The results show distinctive solubility minima at medium temperatures (20-40 °C) with concentrations decreasing from 5 °C to the minimum by 10-20%. The measured and calculated temperature-dependent solubilities enable a more reliable assessment of thermal energy storage at contaminated sites, of existing thermal remediation approaches and of combinations of underground heat storage with groundwater remediation.

Effect of injection velocity and particle concentration on transport of nanoscale zero-valent iron and hydraulic conductivity in saturated porous media.

Successful groundwater remediation by injecting nanoscale zero-valent iron (NZVI) particles requires efficient particle transportation and distribution in the subsurface. This study focused on the influence of injection velocity and particle concentration on the spatial NZVI particle distribution, the deposition processes and on quantifying the induced decrease in hydraulic conductivity (K) as a result of particle retention by lab tests and numerical simulations. Horizontal column tests of 2m length were performed with initial Darcy injection velocities (q0) of 0.5, 1.5, and 4.1m/h and elemental iron input concentrations (Fe(0)in) of 0.6, 10, and 17g/L. Concentrations of Fe(0) in the sand were determined by magnetic susceptibility scans, which provide detailed Fe(0) distribution profiles along the column. NZVI particles were transported farther at higher injection velocity and higher input concentrations. K decreased by one order of magnitude during injection in all experiments, with a stronger decrease after reaching Fe(0) concentrations of about 14-18g/kg(sand). To simulate the observed nanoparticle transport behavior the existing finite-element code OGS has been successfully extended and parameterized for the investigated experiments using blocking, ripening, and straining as governing deposition processes. Considering parameter relationships deduced from single simulations for each experiment (e.g. deposition rate constants as a function of flow velocity) one mean parameter set has been generated reproducing the observations in an adequate way for most cases of the investigated realistic injection conditions. An assessment of the deposition processes related to clogging effects showed that the percentage of retention due to straining and ripening increased during experimental run time resulting in an ongoing reduction of K. Clogging is mainly evoked by straining which dominates particle deposition at higher flow velocities, while blocking and ripening play a significant role for attachment, mainly at lower injection velocities. Since the injection of fluids at real sites leads to descending flow velocities with increasing radial distance from the injection point, the simulation of particle transport requires accounting for all deposition processes mentioned above. Thus, the derived mean parameter set can be used as a basis for quantitative and predictive simulations of particle distributions and clogging effects at both lab and field scale. Since decreases in K can change the flow system, which may have positive as well as negative implications for the in situ remediation technology at a contaminated site, a reliable simulation is thus of great importance for NZVI injection and prediction.

Influence of permeability on nanoscale zero-valent iron particle transport in saturated homogeneous and heterogeneous porous media.

Nanoscale zero-valent iron (NZVI) particles can be used for in situ groundwater remediation. The spatial particle distribution plays a very important role in successful and efficient remediation, especially in heterogeneous systems. Initial sand permeability (k 0) influences on spatial particle distributions were investigated and quantified in homogeneous and heterogeneous systems within the presented study. Four homogeneously filled column experiments and a heterogeneously filled tank experiment, using different median sand grain diameters (d 50), were performed to determine if NZVI particles were transported into finer sand where contaminants could be trapped. More NZVI particle retention, less particle transport, and faster decrease in k were observed in the column studies using finer sands than in those using coarser sands, reflecting a function of k 0. In heterogeneous media, NZVI particles were initially transported and deposited in coarse sand areas. Increasing the retained NZVI mass (decreasing k in particle deposition areas) caused NZVI particles to also be transported into finer sand areas, forming an area with a relatively homogeneous particle distribution and converged k values despite the different grain sizes present. The deposited-particle surface area contribution to the increasing of the matrix surface area (θ) was one to two orders of magnitude higher for finer than coarser sand. The dependency of θ on d 50 presumably affects simulated k changes and NZVI distributions in numerical simulations of NZVI injections into heterogeneous aquifers. The results implied that NZVI can in principle also penetrate finer layers.

Competing TCE and cis-DCE degradation kinetics by zero-valent iron-experimental results and numerical simulation.

The successful dechlorination of mixtures of chlorinated hydrocarbons with zero-valent metals requires information concerning the kinetics of simultaneous degradation of different contaminants. This includes intraspecies competitive effects (loading of the reactive iron surface by a single contaminant) as well as interspecies competition of several contaminants for the reactive sites available. In columns packed with zero-valent iron, the degradation behaviour of trichloroethylene (TCE), cis-dichloroethylene (DCE) and mixtures of both was measured in order to investigate interspecies competition. Although a decreasing rate of dechlorination is to be expected, when several degradable substances compete for the reactive sites on the iron surface, TCE degradation is nearly unaffected by the presence of cis-DCE. In contrast, cis-DCE degradation rates decrease significantly when TCE is added. A new modelling approach is developed in order to identify and quantify the observed competitive effects. The numerical model TBC (Transport, Biochemistry and Chemistry, Schäfer et al., 1998a) is used to describe adsorption, desorption and dechlorination in a mechanistic way. Adsorption and degradation of a contaminant based on a limited number of reactive sites leads to a combined zero- and first-order degradation kinetics for high and low concentrations, respectively. The adsorption of several contaminants with different sorption parameters to a limited reactive surface causes interspecies competition. The reaction scheme and the parameters required are successfully transferred from Arnold and Roberts (2000b) to the model TBC. The degradation behaviour of the mixed contamination observed in the column experiments can be related to the adsorption properties of TCE and cis-DCE. By predicting the degradation of the single substances TCE and cis-DCE as well as mixtures of both, the calibrated model is used to investigate the effects of interspecies competition on the design of permeable reactive iron barriers. Even if TCE is present in only small concentrations (>3% of molar cis-DCE concentration) it is the contaminant limiting the residence time and the required thickness of the iron barrier.

Evaluation of combined direct-push methods used for aquifer model generation.

Most established methods to characterize aquifer structure and hydraulic conductivities of hydrostratigraphical units are not capable of delivering sufficient information in the spatial resolution that is desired for sophisticated numerical contaminant transport modeling and adapted remediation design. With hydraulic investigation methods based on the direct-push (DP) technology such as DP slug tests, DP injection logging, and the hydraulic profiling tool, it is possible to rapidly delineate hydrogeological structures and estimate their hydraulic conductivity in shallow unconsolidated aquifers without the need for wells. A combined application of these tools was used for the investigation of a contaminated German refinery site and for the setup of hydraulic aquifer models. The quality of DP investigation and the models was evaluated by comparisons of tracer transport simulations using these models and measured breakthroughs of two natural gradient tracer tests. Model scenarios considering the information of all tools together showed good reproduction of the measured breakthroughs, indicating the suitability of the approach and a minor impact of potential technical limitations. Using the DP slug tests alone yielded significantly higher deviations for the determined hydraulic conductivities compared to considering two or three of the tools. Realistic aquifer models developed on basis of such combined DP investigation approaches can help optimize remediation concepts or identify flow regimes for aquifers with a complex structure.

Long-term effects of dissolved carbonate species on the degradation of trichloroethylene by zerovalent iron.

The effect of different concentrations of total inorganic carbon (TIC) and flow rates on the reactivity of iron metal with trichloroethylene (TCE) was studied in column experiments to verify whether concentration or mass flux of TIC is the major key parameter for barrier performance. First-order rate coefficients (kobs) for TCE degradation vary initially between 0.15 and 0.32 h-' and are positively related to TIC influent concentration. Maximal kobs were reached after 164 and 591 PV, varied between 0.55 and 1.1 h(-1), and were positively correlated to the TIC mass flux, followed by a decrease resulting in values similar to the reference system at the end of the experiments. Enhancement of iron corrosion (0.7 to 3.5 mmol kgFe(-1) d(-1) and formation of gas bubbles during the initial experimental phase were observed and were also positively correlated to TIC mass flux. The higher gas bubble formation probably has a more significant effect on porosity than mineral precipitations in Fe0-systems. The results suggest that higher TIC mass fluxes cause a more pronounced acceleration in CHC degradation, but also a faster inhibition in the longer-term. This faster inhibition has serious implication for the design of funnel and gate systems.