Sulfur Hexafluoride and Potassium Bromide as Groundwater Tracers for Managed Aquifer Recharge

Sulfur hexafluoride (SF₆) is an established tracer for use in managed aquifer recharge projects. SF₆ exsolves from groundwater when it encounters trapped air according to Henry's law. This results in its retardation relative to groundwater flow, which can help determine porous media saturation and flow dynamics. SF₆ and the conservative, nonpartitioning tracer, bromide (Br⁻ added as KBr), were introduced to recharge water infiltrated into stacked glacial aquifers in Thurston County, Washington, providing the opportunity to observe SF₆ partitioning. Br⁻, which is assumed to travel at the same velocity as the groundwater, precedes SF₆ at most monitoring wells (MWs). Average groundwater velocity in the unconfined aquifer in the study area ranges from 3.9 to 40 m/d, except in the southwestern corner where it is slower. SF₆ in the shallow aquifer exhibits an average retardation factor of 2.5 ± 3.8, suggesting an air-to-water ratio on the order of 10⁻³ to 10⁻² in the pore space. Notable differences in tracer arrival times at adjacent wells indicate very heterogeneous conductivity. One MW exhibits double peaks in concentrations of both tracers with different degrees of retardation for the first and second peaks. This suggests multiple flowpaths to the well with variable saturation. The confining layer between the upper two aquifers appears to allow intermittent connection between aquifers but serves as an aquitard in most areas. This study demonstrates the utility of SF₆ partitioning for evaluating hydrologic conditions at prospective recharge sites.     

Quantifying uncertainties in projections of extremes��a perturbed land surface parameter experiment

Uncertainties in the climate response to a doubling of atmospheric CO₂ concentrations are quantified in a perturbed land surface parameter experiment. The ensemble of 108 members is constructed by systematically perturbing five poorly constrained land surface parameters of global climate model individually and in all possible combinations. The land surface parameters induce small uncertainties at global scale, substantial uncertainties at regional and seasonal scale and very large uncertainties in the tails of the distribution, the climate extremes. Climate sensitivity varies across the ensemble mainly due to the perturbation of the snow albedo parameterization, which controls the snow albedo feedback strength. The uncertainty range in the global response is small relative to perturbed physics experiments focusing on atmospheric parameters. However, land surface parameters are revealed to control the response not only of the mean but also of the variability of temperature. Major uncertainties are identified in the response of climate extremes to a doubling of CO₂. During winter the response both of temperature mean and daily variability relates to fractional snow cover. Cold extremes over high latitudes warm disproportionately in ensemble members with strong snow albedo feedback and large snow cover reduction. Reduced snow cover leads to more winter warming and stronger variability decrease. As a result uncertainties in mean and variability response line up, with some members showing weak and others very strong warming of the cold tail of the distribution, depending on the snow albedo parametrization. The uncertainty across the ensemble regionally exceeds the CMIP3 multi-model range. Regarding summer hot extremes, the uncertainties are larger than for mean summer warming but smaller than in multi-model experiments. The summer precipitation response to a doubling of CO₂ is not robust over many regions. Land surface parameter perturbations and natural variability alter the sign of the response even over subtropical regions.     

Thermal microstructural changes of grain-supported limestones

The microstructure of grainstone, grain-supported limestone, samples representing different geographic and stratigraphic intervals in Egypt were examined before and after calcination at 950°C for 0.25, 0.5, 1 and 2?h. The examination utilized are X-ray diffraction (XRD), X-ray fluorescence (XRF), transmitted light microscopy (TLM), scanning electron microscopy (SEM) and X-ray micro tomography (??-CT). Both the free lime content and reactivity of the produced quicklime were measured. The quicklime produced at 950°C for 0.25 and 0.5?h is un-reactive (R_DIN < 10) due to its low content of free lime. However, quicklime produced at 1 and 2?h conditions is highly reactive (R_DIN > 30) with variable reactivity due to the differences in the lime microfabric. The grainstones enriched in nummulites resulted in quicklime of higher reactivity than those dominated by foraminifera, peloids and miliolids. This is mainly attributed to the preservation of the intraparticle pores of the original limestone and the development of pinhole and fracture micropores. The shape of pores and not their amount is the main controlling variable in the quicklime reactivity. The grainstones enriched in fracture pores showed more reactive lime, whereas those with compact structures are less reactive post calcination.     

Mechanical design of the solar telescope GREGOR

The mechanical structure of the GREGOR telescope was installed at the Observatorio del Teide, Tenerife, in 2004. New concepts for mounting and cooling of the 1.5‐meter primary mirror were introduced. GREGOR is an open telescope, therefore the dome is completely open during observations to allow for air flushing through the open, but stiff telescope structure. Backside cooling system of the primary mirror keeps the mirror surface close to ambient temperature to prevent mirror seeing. The large collecting area of the primary mirror results in high energy density at the field stop at the prime focus of the primary which needs to be removed. The optical elements are supported by precision alignment systems and should provide a stable solar image at the optical lab. The coudé train can be evacuated and serves as a natural barrier between the outer environmental conditions and the air‐conditioned optical laboratory with its sensitive scientific instrumentation. The telescope was successfully commissioned and will start its nominal operation during 2013 (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A retrospective of the GREGOR solar telescope in scientific literature

In this review, we look back upon the literature, which had the GREGOR solar telescope project as its subject including science cases, telescope subsystems, and post‐focus instruments. The articles date back to the year 2000, when the initial concepts for a new solar telescope on Tenerife were first presented at scientific meetings. This comprehensive bibliography contains literature until the year 2012, i.e., the final stages of commissioning and science verification. Taking stock of the various publications in peer‐reviewed journals and conference proceedings also provides the “historical” context for the reference articles in this special issue of Astronomische Nachrichten/Astronomical Notes (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Numerical analysis of the groundwater regime in the western Dead Sea escarpment, Israel + West Bank

Water is scarce in the semi-arid to arid regions around the Dead Sea, where water supply mostly relies on restricted groundwater resources. Due to increasing population in this region, the regional aquifer system is exposed to additional stress. This results in the continuous decrease in water level of the adjacent Dead Sea. The interaction of an increasing demand for water due to population growth and the decrease of groundwater resources will intensify in the near future. Thus, the water supply situation could worsen significantly unless sustainable water resource management is conducted. In this study, we develop a regional groundwater flow model of the eastern and southern Judea Group Aquifer to investigate the groundwater regime in the western Dead Sea drainage basin of Israel and the West Bank. An extensive geological database was developed and consequently a high-resolution structural model was derived. This structural model was the basis for various groundwater flow scenarios. The objective was to capture the spatial heterogeneity of the aquifer system and to apply these results to the southern part of the study area, which has not been studied in detail until now. As a result we analyzed quantitatively the flow regime, the groundwater mass balance and the hydraulic characteristics (hydraulic conductivity and hydraulic head) of the cretaceous aquifer system and calibrated them with PEST. The calibrated groundwater flow model can be used for integrated groundwater water management purposes in the Dead Sea area, especially within the framework of the SUMAR-Project.     

Reactivity of sandstone and siltstone samples from the Ketzin pilot CO2 storage site-Laboratory experiments and reactive geochemical modeling

To evaluate mineralogical-geochemical changes within the reservoir of the Ketzin pilot CO₂ storage site in Brandenburg, Germany, two sets of laboratory experiments on sandstone and siltstone samples from the Stuttgart Formation have been performed. Samples were exposed to synthetic brine and pure CO₂ at experimental conditions and run durations of 5.5 MPa/40 °C/40 months for sandstone and 7.5 MPa/40 °C/6 months for siltstone samples, respectively. Mineralogical changes in both sets of experiments are generally minor making it difficult to differentiate natural variability of the whole rock samples from CO₂-induced alterations. Results of sandstone experiments suggest dissolution of the anorthite component of plagioclase, anhydrite, K-feldspar, analcime, hematite and chlorite + biotite. Dissolution of the anorthite component of plagioclase, anhydrite and K-feldspars is also observed in siltstone experiments. In an inverse modeling approach, an extensive set of equilibrium simulations was set up in order to reproduce the experimental observations of the sandstone experiments. Simulations generally show fairly good matches with the experimental observations. Best matches with measured brine data are obtained from mineral combinations of albite, analcime, anhydrite, dolomite, hematite, illite, and kaolinite. The major discrepancies during equilibrium modeling, however, are reactions involving Fe²⁺ and Al³⁺. The best matching subsets of the equilibrium models were finally run including kinetic rate laws. These simulations reveal that experimentally determined brine data was well matched, but reactions involving K⁺ and Fe²⁺ are not fully covered. The modeling results identified key primary minerals as well as key chemical processes, but also showed that the models are not capable of covering all possible contingencies.     

Slope failures and erosion rates on a glacierized high‐mountain face under climatic changes

In this study, rapid topographic changes and increased erosion rates caused by massive slope failures in a glacierized and permafrost‐affected high‐mountain face were investigated with respect to the current climatic change. The study was conducted at one of the highest periglacial rock faces in the European Alps, the east face of Monte Rosa, Italy. Pronounced changes in ice cover and repeated rock and ice avalanche events have been documented in this rock wall since around 1990. The performed multi‐temporal comparison of high‐resolution digital terrain models (DTMs) complemented by detailed analyses of repeat photography represents a unique assessment of topographic changes and slope failures over half a century and reveals a total volume loss in bedrock and steep glaciers in the central part of the face of around 25 × 10⁶ m³ between 1988 and 2007. The high rock and ice avalanche activity translates into an increase in erosion rates of about one order of magnitude during recent decades. The study indicates that changes in atmospheric temperatures and connected changes in ice cover can induce slope destabilization in high‐mountain faces. Analyses of temperature data show that the start of the intense mass movement activity coincided with increased mean annual temperatures in the region around 1990. However, once triggered, mass movement activity seems to be able to proceed in a self‐reinforcing cycle, whereby single mass movement events might be strongly influenced by short‐term extreme temperature events. The investigations suggest a strong stability coupling between steep glaciers and underlying bedrock, as most bedrock instabilities are located in areas where surface ice has disappeared recently and the failure zones are frequently spatially correlated and often develop from lower altitudes progressively upwards. Copyright © 2013 John Wiley & Sons, Ltd.     

Robust flood statistics: comparison of peak over threshold approaches based on monthly maxima and TL-moments

ABSTRACT

Flood quantile estimation based on partial duration series (peak over threshold, POT) represents a noteworthy alternative to the classical annual maximum approach since it enlarges the available information spectrum. Here the POT approach is discussed with reference to its benefits in increasing the robustness of flood quantile estimations. The classical POT approach is based on a Poisson distribution for the annual number of exceedences, although this can be questionable in some cases. Therefore, the Poisson distribution is compared with two other distributions (binomial and Gumbel-Schelling). The results show that only rarely is there a difference from the Poisson distribution. In the second part we investigate the robustness of flood quantiles derived from different approaches in the sense of their temporal stability against the occurrence of extreme events. Besides the classical approach using annual maxima series (AMS) with the generalized extreme value distribution and different parameter estimation methods, two different applications of POT are tested. Both are based on monthly maxima above a threshold, but one also uses trimmed L-moments (TL-moments). It is shown how quantile estimations based on this “robust” POT approach (rPOT) become more robust than AMS-based methods, even in the case of occasional extraordinary extreme events.

Editor M.C. Acreman Associate editor A. Viglione     

Accuracy of the master-event and double-difference locations: synthetic tests and application to seismicity in West Bohemia, Czech Republic

The relative locations of earthquake hypocentres determined with the master-event (ME) or the double-difference (DD) methods are more accurate and less dispersive compared to the absolute locations. In this paper, we conduct synthetic tests to assess the accuracy of the ME and DD location methods, to study the effects of the control parameters on the locations and possible distortions of the foci geometry. The results indicate that the DD locations are, in general, more accurate than the ME locations and perform significantly better for large earthquake clusters due to their independence of the master event position. The location precision, however, strongly depends on the control parameters used. If the control parameters are optimally chosen, the location errors can be considerably reduced. Moreover, it is proved that no distortion such as artificial clustering of foci is introduced if relative locations are used. Finally, the efficiency of both location methods is exemplified on locations of swarm micro-earthquakes that occurred in the West Bohemia region, Czech Republic, in order to reveal a detailed geometry of the active fault zone.