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.

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.

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.

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.

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.

Assessing degradation rates of chlorinated ethylenes in column experiments with commercial iron materials used in permeable reactive barriers.

Multiple column experiments were performed using two commercial iron materials to evaluate the necessity and usefulness of preliminary investigations in permeable reactive barrier (PRB) design for chlorinated organics. Experiments were performed with contaminated groundwater and involved fresh iron granules or altered iron material excavated from PRBs. The determination of first-order rate coefficients by global nonlinear least-squares fittings indicated a variability in rate coefficients on 1 or 2 orders of magnitude. Geometric mean values of surface area normalized rate coefficients (in 10(-5) L m(-2) h(-1)) for fresh gray cast iron and iron sponge, respectively, are: tetrachloroethene (4.5, 2.6), trichloroethene (8.1, 3.3), cis-1,2-dichloroethene (3.1, 2.9), trans-1,2-dichloroethene (9.5, 5.3), 1,1-dichloroethene (4.0, 4.4), and vinyl chloride (1.6, 6.1). The increasing rate coefficients with decreasing grade of chlorination, which characterize degradation at iron sponge are linearly related to diffusion coefficients in water, suggesting diffusion limitation in the degradation process for this particular material, possibly due to a high inner surface. The variability in rate coefficients seems to be too high to use mean rate coefficients from published studies in the design procedure of PRBs, and variabilities cannot be related to groundwater characteristics, waterflow through the reactive cells, or secondary corrosion reactions.

Compost-based permeable reactive barriers for the source treatment of arsenic contaminations in aquifers: column studies and solid-phase investigations.

The bulk of arsenic (As) at contaminated sites is frequently associated with iron (hydr)oxides. Various studies ascribe increasing dissolved As concentrations to the transformation of iron (hydr)oxides into iron sulfides, which is initiated by dissolved sulfide. We investigated whetherthis processes can be utilized as a source treatment approach using compost-based permeable reactive barriers (PRB), which promote microbial sulfate reduction. Arsenic-bearing aquifer sedimentfrom a contaminated industrial site showed a decrease in As content of <10% after 420 days of percolation with sulfide-free artificial groundwater. In contrast, water that had previously passed through organic matter and exhibited sulfide concentrations of 10-30 mg/L decreased As content in the sediment by 87% within 360 days. X-ray diffraction showed no arsenic sulfides, but XANES spectra (X-ray absorption near edge structure) and associated linear combinations revealed that adsorbed arsenate of the original sediment was in part reduced to arsenite and indicated the formation of minor amounts of a substance that contains As and sulfur. The speciation of dissolved As changed from initially As(V)-dominated to As(III)-dominated after sulfide flushing was started, which increases the mobility of As. Because sulfide can be supplied not only by compost-based PRBs but also by direct injection, sulfide flushing has a wide range of application for the source treatment of arsenic.

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.