Eine Echtzeit-Online-Methode zur Beurteilung der künstlichen Grundwasseranreicherung und Grundwasserentnahme

Grundwasser - Eine nachhaltige Wasserversorgung in urbanen Gebieten stellt aufgrund des Nutzungsdrucks sowie vielfältiger anthropogener Einträge eine große Herausforderung dar....     

Einsatz von Direct Push im Rahmen von Sanierungsuntersuchungen – Fallbeispiel Elsterwerda (Brandenburg)

Monitoring of ground water for remediation projects is usually done using conventional ground water sampling. However, vertical variations in hydrochemistry cannot be detected well with this approach and, as a result, can create serious problems. Such problems were observed at the Elsterwerda site in Brandenburg where a spill of about 470 t of unleaded gasoline contaminated the Quaternary aquifer in 1997. Despite the installation of an increasing number of monitoring wells during site remediation, evaluation of the spatial development of the plume was not possible. Direct Push ground water sampling was therefore used to obtain depth-dependent hydrochemistry information for the design of additional monitoring wells. A mobile gas chromatograph enabled an adaptive site investigation that allowed on-site determination of the appropriate depths and lengths of the screened intervals for small-diameter multichannel monitoring wells. The installation of these multichannel wells provided reproducible, depth dependent information on site hydrochemistry. The investigations at the Elsterwerda site demonstrated the power of direct push technology and showed that simple cleaning of the Direct Push rods and the use of a new sampling tube for each profile eliminates potential cross contamination.     

Passive Probennahme mit Keramikdosimetern: M?glichkeiten und Grenzen der Einsetzbarkeit

As a result of a railway-accident, the aquifer in Elsterwerda (southern Brandenburg, Germany) has become contaminated with unleaded fuel. To monitor groundwater quality at this site, several ceramic dosimeters were installed in selected observation wells. The adsorber material in the dosimeters was conditioned to sample benzene, toluene, ethylbenzene and xylene. The dosimeters were installed between two regularly scheduled groundwater sampling events. Active and passive sampling results showed considerable differences in concentration in some cases. In addition to the diameter of the observation well, the main reason for the difference was a high dependency of passive sampling on the hydraulic connection between the observation wells and the aquifer. As a result of the comparison of active and passive sampling as well as considering additional site investigation data related to the spatial spreading of the contamination, it was possible to determine the hydraulic connection between the well and aquifer for the investigated wells. Hence, the application of ceramic dosimeters at Elsterwerda was not only suitable to acquire depth-dependent concentrations, but also to obtain information on the physical and hydraulic condition of the observation wells. However, rather limited additional information can be gained from the application of ceramic dosimeters at sites where there is no a priori information on the hydraulic connection of a well or on the depth-dependent distribution of the contamination.     

Self-organizing nanodot structures on InP surfaces evolving under low-energy ion irradiation: analysis of morphology and composition

Surfaces of InP were bombarded by 1.9 keV Ar⁺ ions under normal incidence. The total accumulated ion fluence Φ the samples were exposed to was varied from 1 × 10¹⁷ cm⁻² to 3 × 10¹⁸ cm⁻², and ion fluxes f of (0.4 − 2) × 10¹⁴ cm⁻² s⁻¹ were used. The surface morphology resulting from these ion irradiations was examined by atomic force microscopy (AFM). Generally, nanodot structures are formed on the surface; their dimensions (diameter, height and separation), however, were found to depend critically on the specific bombardment conditions. As a function of ion fluence, the mean radius r, height h, and spacing l of the dots can be fitted by power-law dependences: r ∝ Φ^0.40, h ∝ Φ^0.48, and l ∝ Φ^0.19. In terms of ion flux, there appears to exist a distinct threshold: below f ~ (1.3 ± 0.2) × 10¹⁴ cm⁻² s⁻¹, no ordering of the dots exists and their size is comparatively small; above that value of f, the height and radius of the dots becomes substantially larger (h ~ 40 nm and r ~ 50 nm). This finding possibly indicates that surface diffusion processes could be important. In order to determine possible local compositional changes in these nanostructures induced by ion impact, selected samples were prepared for atom probe tomography (APT). The results indicate that APT can provide analytical information on the composition of individual InP nanodots. By means of 3D APT data, the surface region of such nanodots evolving under ion bombardment could be examined with atomic spatial resolution. At the InP surface, the values of the In/P concentration ratio are distinctly higher over a distance of approximately 1 nm and amount to 1.3 to 1.7.     

Hydrogen chemisorption on the Si(1 1 1)√3 × √3R30 °-Al, -Ga, -B surfaces: An ab initio HF/DFT molecular orbital modelling using atomic clusters

A comprehensive study of atomic hydrogen chemisorption on the Si(1 1 1) √3 × √3R30 ° -Al, -Ga and -B cluster modelled surfaces is presented using Hartree-Fock/density functional theory methods. Extrapolation of the results to the extended (1 1 1) silicon surface is also discussed. It is found that the chemisorption of hydrogen on the Al and Ga terminated surfaces induces a transition from the √3 × √3 structure to a local 1 × 1: H-like reconstruction with a stable SiAl (or SiGa) sites. The subsurface boron induced √3 × √3 reconstruction is also lifted by hydrogen chemisorption but, in this case, boron adatoms are likely to be segregated on the surface, predominantly as BH or/and BH₂.