Department Water Resources and Drinking Water

Im Hardwald bei Muttenz wird zur Trinkwasseraufbereitung Rheinwasser versickert. Während der Bodenpassage wird rund die Hälfte der vorhandenen Spurenstoffe im Rheinwasser entfernt. Ein Grossteil der restlichen Substanzen wird durch die Aktivkohlefiltration des angereicherten Grundwassers entfernt. Wie gut die Spurenstoffe im Aktivkohlefilter zurückgehalten werden und ob eine vor- oder nachgeschaltete Oxidation zu einer besseren Entfernung führen kann, wird in der nachfolgend vorgestellten Studie aufgezeigt. Die Studie wurde im Rahmen des Projekts "Regionale Wasserversorgung Basel-Landschaft 21" realisiert.

Iron(III)-precipitates formed by Fe(II) oxidation in aqueous solutions affect the cycling and impact of Fe and other co-precipitated elements in environmental systems. Fresh Fe(III)-precipitates are metastable and their transformation into more stable phases during aging may result in the release of initially co-precipitated ions. Phosphate, silicate, Mg and Ca play key roles in determining the structure and composition of fresh Fe(III)-precipitates. Here we examine how these ions affect the structure and phosphate retention of Fe(III)-precipitates formed by oxidation of 0.5 mM dissolved Fe(II) at pH 7.0 after aging for 30 days at 40 °C.
Iron K-edge X-ray absorption spectroscopy (XAS) shows that aged precipitates consist of the same structural units as fresh precipitates: Amorphous Fe(III)- or Ca-Fe(III)-phosphate, ferrihydrite, and poorly crystalline lepidocrocite. Mg, Ca, and dissolved phosphate stabilize (Ca-)Fe(III)-phosphate against transformation into ferrihydrite. Silicate further attenuates (Ca-)Fe(III)-phosphate transformation. The crystallinity of lepidocrocite formed in phosphate- and silicate-free solutions slightly increases during aging. The transformation of Fe(III)- and Ca-Fe(III)-phosphate into ferrihydrite and ongoing ferrihydrite crystallization during aging result in the release of co-precipitated phosphate. Dissolved Ca on the other hand limits phosphate concentrations to values consistent with solubility control by octacalciumphosphate. Owing to the combined effects of Ca and silicate, phosphate is most effectively retained by Fe(III)-precipitates formed and aged in Ca- and silicate-containing solutions. The results from this study contribute to an improved understanding of the formation and transformation of Fe(III)-precipitates and emphasize that the complexity of Fe(III)-precipitate dynamics in the presence of multiple interfering solutes must be considered when addressing their impact on major and trace elements in environmental systems.

Sensitivity and identifiability analyses are common diagnostic tools to address over-parametrization in complex environmental models, but a combined application of the two analyses is rarely conducted. In this study, we performed a temporal global sensitivity analysis using the variance-based method of Sobol’ and a temporal identifiability analysis of model parameters using the dynamic identifiability method (DYNIA). We discuss the relationship between the two analyses with a focus on parameter identification and output uncertainty reduction. The hydrological model HydroGeoSphere was used to simulate daily evapotranspiration, water content, and seepage at the lysimeter scale. We found that identifiability of a parameter does not necessarily reduce output uncertainty. It was also found that the information from the main and total effects (main Sobol' sensitivity indices) is required to allow uncertainty reduction in the model output. Overall, the study highlights the role of combined temporal diagnostic tools for improving our understanding of model behavior.