Irina Engelhardt

Thermal-hydraulic experiments with bentonite/crushed rock mixtures and estimation of effective parameters by inverse modeling

Abstract The permeability and thermal conductivity of bentonite/crushed rock mixtures used as backfill for a nuclear waste repository have an important impact on the maximum radioactive load of the waste canister that can be embedded. Our research pursues useful methods for estimating the permeability, thermal conductivity, and specific heat of various bentonite/crushed rock mixtures for the conditions expected to prevail at the Aspo Hard Rock Laboratory (ASPO HRL). We conducted laboratory experiments and employed inverse modeling techniques to estimate effective thermal and hydraulic parameters suitable for predictive modeling of non-isothermal flow and transport from a nuclear waste repository. Thermal parameters are often calculated based on empirical relationships developed for homogeneous clays, i.e., they are not necessarily valid for mixtures. The applicability of these methods to model thermal-hydraulic processes within the bentonite/crushed rock mixtures in a deep repository needs to be assessed. All experiments were conducted with mixtures containing sodium- (SPV Volclay) or calcium-bentonite (Calcigel) and we used water from Aspo. Hydraulic column experiments were carried out with a specially designed permeameter and Darcys law was applied to determine the hydraulic conductivity, which followed a lognormal distribution with mean values of 1.64×10 −11 and 4.93×10 −9 m/s for the two bentonite/crushed rock mixtures studied. The thermal laboratory experiments were analyzed using inverse modeling techniques. The simulated temperature distribution matched the measured data very well at all locations along the column and for all times. The inversely estimated thermal conductivity ranged from 1.6 to 2.2 W/mK, and the specific heat from 810 to 1020 J/kg K, both consistent with the predictions of the empirical relationships. However, the calculation of the effective parameters was very sensitive to heat loss through the insulation. The newly developed experimental setup in combination with inverse modeling allows the identification of key parameters governing the hydraulic and thermal processes of bentonite/crushed rock mixtures under repository conditions.

Soil column experiments to quantify vadose zone water fluxes in arid settings

For the determination of groundwater recharge processes in arid environments, vadose zone water fluxes and water storage should be considered. To better understand and quantify vadose zone processes influencing groundwater recharge, a soil column experimental setup has been developed that mimics arid atmospheric conditions and measures water and temperature fluxes in high temporal and spatial resolution. The focus of the experiment was on the determination of water infiltration, redistribution, evaporation and percolation under non-isothermal conditions. TDR rod sensors and a specific TDR “Taupe” cable sensor were used for water content measurements and allowed the infiltration fronts to be traced over the whole column length. Applying single irrigations of different amount and intensity showed the applicability of the experimental setup for the measurement of water movement in the unsaturated soil column.

Fate of the antibiotic sulfadiazine in natural soils: Experimental and numerical investigations

Based on small-scale laboratory and field-scale lysimeter experiments, the sorption and biodegradation of sulfonamide sulfadiazine (SDZ) were investigated in unsaturated sandy and silty-clay soils. Sorption and biodegradation were low in the laboratory, while the highest leaching rates were observed when SDZ was mixed with manure. The leaching rate decreased when SDZ was mixed with pure water, and was smallest with the highest SDZ concentrations. In the laboratory, three transformation products (TPs) developed after an initial lag phase. However, the amount of TPs was different for different mixing-scenarios. The TP 2-aminopyrimidine was not observed in the laboratory, but was the most prevalent TP at the field scale. Sorption was within the same range at the laboratory and field scales. However, distinctive differences occurred with respect to biodegradation, which was higher in the field lysimeters than at the laboratory scale. While the silty-clay soil favored sorption of SDZ, the sandy, and thus highly permeable, soil was characterized by short half-lives and thus a quick biodegradation of SDZ. For 2-aminopyrimidine, half-lives of only a few days were observed. Increased field-scale biodegradation in the sandy soil resulted from a higher water and air permeability that enhanced oxygen transport and limited oxygen depletion. Furthermore, low pH was more important than the organic matter and clay content for increasing the biodegradation of SDZ. A numerical analysis of breakthrough curves of bromide, SDZ, and its TPs showed that preferential flow pathways strongly affected the solute transport within shallow parts of the soil profile at the field scale. However, this effect was reduced in deeper parts of the soil profile. Due to high field-scale biodegradation in several layers of both soils, neither SDZ nor 2-aminopyrimidine was detected in the discharge of the lysimeter at a depth of 1m. Synthetic 50 year long simulations, which considered the application of manure with SDZ for general agricultural practices in Germany and humid climate conditions, showed that the concentration of SDZ decreased below 0.1 μg/L in both soils below the depth of 50 cm.

Complexity vs. Simplicity: Groundwater Model Ranking Using Information Criteria

A groundwater model characterized by a lack of field data about hydraulic model parameters and boundary conditions combined with many observation data sets for calibration purpose was investigated concerning model uncertainty. Seven different conceptual models with a stepwise increase from 0 to 30 adjustable parameters were calibrated using PEST. Residuals, sensitivities, the Akaike information criterion (AIC and AICc), Bayesian information criterion (BIC), and Kashyaps information criterion (KIC) were calculated for a set of seven inverse calibrated models with increasing complexity. Finally, the likelihood of each model was computed. Comparing only residuals of the different conceptual models leads to an overparameterization and certainty loss in the conceptual model approach. The model employing only uncalibrated hydraulic parameters, estimated from sedimentological information, obtained the worst AIC, BIC, and KIC values. Using only sedimentological data to derive hydraulic parameters introduces a systematic error into the simulation results and cannot be recommended for generating a valuable model. For numerical investigations with high numbers of calibration data the BIC and KIC select as optimal a simpler model than the AIC. The model with 15 adjusted parameters was evaluated by AIC as the best option and obtained a likelihood of 98%. The AIC disregards the potential model structure error and the selection of the KIC is, therefore, more appropriate. Sensitivities to piezometric heads were highest for the model with only five adjustable parameters and sensitivity coefficients were directly influenced by the changes in extracted groundwater volumes.

Quantification of long-term wastewater fluxes at the surface water/groundwater-interface: An integrative model perspective using stable isotopes and acesulfame

The suitability of acesulfame to trace wastewater-related surface water fluxes from streams into the hyporheic and riparian zones over long-term periods was investigated. The transport behavior of acesulfame was compared with the transport of water stable isotopes (δ(18)O or δ(2)H). A calibrated model based on a joint inversion of temperature, acesulfame, and piezometric pressure heads was employed in a model validation using data sets of acesulfame and water stable isotopes collected over 5months in a stream and groundwater. The spatial distribution of fresh water within the groundwater resulting from surface water infiltration was estimated by computing groundwater ages and compared with the predicted acesulfame plume obtained after 153day simulation time. Both, surface water ratios calculated with a mixing equation from water stable isotopes and simulated acesulfame mass fluxes, were investigated for their ability to estimate the contribution of wastewater-related surface water inflow within groundwater. The results of this study point to limitations for the application of acesulfame to trace surface water-groundwater interactions properly. Acesulfame completely missed the wastewater-related surface water volumes that still remained in the hyporheic zone under stream-gaining conditions. In contrast, under stream-losing conditions, which developed after periods of stagnating hydraulic exchange, acesulfame based predictions lead to an overestimation of the surface water volume of up to 25% in the riparian zone. If slow seepage velocities prevail a proportion of acesulfame might be stored in smaller pores, while when released under fast flowing water conditions it will travel further downstream with the groundwater flow direction. Therefore, under such conditions acesulfame can be a less-ideal tracer in the hyporheic and riparian zones and additional monitoring with other environmental tracers such as water stable isotopes is highly recommended.

Development of a regional geothermal resource atlas

Increased interest in geothermal energy has led to the need for more sophisticated analyses of available geothermal resources for heating and power production. The evaluation methods have traditionally used 1D models, but progress in computing power allows us now to perform joint interpretations that include large-scale effects of geological, topographical and hydrogeological structures. This paper describes the steps taken in such an integrated assessment. The evaluation of geothermal potential is actually finalized for one of the most populated areas of Switzerland. The methodology accounts for individual utilization scenarios and is based on various temperature data that have been systematically collected over many years. The state-of-the-art compilation involves comprehensive 3D regional geological and thermal models. An excellent data fit with >20 boreholes in northern Switzerland is achieved using the calculation scheme presented here. Zones of significant convective flow are identified and the flow velocity is quantified. In contrast to the results obtained from earlier geothermal resource assessments, the existing geological, hydrogeological and petrophysical data are included in the full 3D numerical evaluation. The results obtained for a regional scale of a well-documented subsurface area in northern Switzerland are displayed in terms of geothermal productivity and energy. The calculations identify the topmost crystalline basement as a most promising structure for geothermal exploitation with predicted maximum doublet productivities of >100 MWt. The annually extractable energy of 13 PJ km−3 easily covers a broad range of Swiss energy needs. The atlas can be further expanded to cover utilization schemes not treated here, such as ground-coupled heat pump systems.

Reactive Transport of Iomeprol during Stream-Groundwater Interactions

The transport and biochemical transformations of the iodinated X-ray contrast medium (ICM) iomeprol were studied at the stream/groundwater interface. During a one-month field experiment piezometric pressure heads, temperatures, and concentrations of redox-sensitive species, iomeprol and 15 of its transformation products (TPs) were collected in stream- and groundwater. The data set was analyzed and transformation processes and rates identified by comparing conservative and reactive transport simulations. ICM and TP transformations were simulated as a cometabolic process during organic carbon degradation. Using iomeprol/TPs ratios as calibration constrain mitigated the uncertainties associated with the high variability of the ICM wastewater discharge into the investigated stream. The study provides evidence that biodegradation of ICM occurs at the field-scale also for predominantly denitrifying conditions. Under these anaerobically dominated field conditions shortest simulated half-life (21 days) was in the same range as previously reported laboratory-determined half-lives for aerobic conditions.

Experimental and Numerical Investigations of Silver Nanoparticle Transport under Variable Flow and Ionic Strength in Soil

Unsaturated column experiments were conducted with an undisturbed loamy sand soil to investigate the influence of flow interruption (FI) and ionic strength (IS) on the transport and retention of surfactant-stabilized silver nanoparticles (AgNP) and the results were compared to those obtained under continuous flow conditions. AgNP concentrations for breakthrough curves (BTCs) and retention profiles (RPs) were analyzed by ICP-MS. Experimental results were simulated by the numerical code HP1 (Hydrus-PhreeqC) with the DLVO theory, extended colloid filtration theory and colloid release model. BTCs of AgNP showed a dramatic drop after FI compared to continuous flow conditions. Evaporation increased due to FI, resulting in increased electrical conductivity of the soil solution, which led to a totally reduced mobility of AgNP. A reduction of IS after FI enhanced AgNP mobility slightly. Here the strongly increased Al and Fe concentration in the effluent suggested that soil colloids facilitated the release of AgNP (cotransport). The numerical model reproduced the measured AgNP BTCs and indicated that attachment to the air-water interface (AWI) occurring during FI was the key process for AgNP retention.

Impact of manure-related DOM on sulfonamide transport in arable soils

Field application of livestock manure introduces colloids and veterinary antibiotics, e.g. sulfonamides (SAs), into farmland. The presence of manure colloids may potentially intensify the SAs-pollution to soils and groundwater by colloid-facilitated transport. Transport of three SAs, sulfadiazine (SDZ), sulfamethoxypyridazine (SMPD), and sulfamoxole (SMOX), was investigated in saturated soil columns with and without manure colloids from sows and farrows, weaners, and fattening pigs. Experimental results showed that colloid-facilitated transport of SMOX was significant in the presence of manure colloids from fattening pigs with low C/N ratio, high SUVA280nm and protein C, while manure colloids from sows and farrows and weaners had little effect on SMOX transport. In contrast, only retardation was observed for SDZ and SMPD when manure colloids were present. Breakthrough curves (BTCs) of colloids and SAs were replicated well by a newly developed numerical model that considers colloid-filtration theory, competitive kinetic sorption, and co-transport processes. Model results demonstrate that mobile colloids act as carriers for SMOX, while immobile colloids block SMOX from sorbing onto the soil. The low affinity of SMOX to sorb on immobile colloids prevents aggregation and also promotes SMOXs colloid-facilitated transport. Conversely, the high affinity of SDZ and SMPD to sorb on all types of immobile colloids retarded their transport. Thus, manure properties play a fundamental role in increasing the leaching risk of hydrophobic sulfonamides.

New Tools for Coherent Information Base for IWRM in Arid Regions: The Upper Mega Aquifer System on the Arabian Peninsula

In arid regions like the Arabian Peninsula, available water resources are essentially restricted to groundwater, requiring a detailed understanding of the local and regional hydrogeological conditions and water budgets. In the framework of the IWAS initiative, the 1.8 × 106 km2 large sedimentary Upper Mega Aquifer of the Arabian Peninsula was chosen as a model region to develop concepts and methodologies to quantify water fluxes in such an arid environment. Field and laboratory studies were conducted to analyse (i) precipitation patterns, (ii) groundwater recharge, (iii) the hydrochemical evolution of groundwater and (iv) evaporation particularly from Sabkhas in detail. Results were used as input parameters for a 3D groundwater model for the central part of the Peninsula, which was later extended to the full dimension of the Upper Mega Aquifer. It could be shown that in such a region different components of the water cycle have to be quantified with great care and several methods should be applied to reduce data uncertainty. It was not possible to make use out of satellite products to receive reliable actual precipitation patterns for the peninsula. It was observable; recharge estimations based on average annual precipitation are not applicable but should be based on singular precipitation events. A threshold of 6 mm/event was derived, below of which no recharge in sand seas occurs. The loss of water from UMA, due to sabkha evaporation reaches about 40 mm/a under the given recent climatic conditions.