Abteilung Wasserressourcen und Trinkwasser

In the lowlands of Nepal, as well as in several other South Asian countries, the level of arsenic in groundwater extracted from Quaternary alluvial sediments frequently exceeds the World Health Organization (WHO) drinking water guideline of 10 μg/L. The widely accepted explanation refers to the reductive dissolution of Fe-bearing minerals releasing As-oxyanions from the soil minerals. However, this hypothesis is only to some extent applicable. Given that arsenic and iron in the groundwater are weakly correlated or decoupled and iron(III) (hydr)oxides are scarcely present, a substantial portion of As and Fe has to be retained in clay minerals. The low concentration of iron in groundwater can be explained by the origin of As and Fe in the rocks (leucogranites); in the High Himalayas, they are low in Fe but considerably enriched in Li, B, As, Se, Br, Sr, Mo, Cd, P, and U. This unique geochemical setting has a major impact on the performance of the Kanchan Arsenic Filters (KAFs) used to eliminate As from groundwater in Nepal. Lack of sufficient Fe in groundwater, combined with limited release of iron by corrosion of iron nails, call for an adapted version of these filters. The first results from 20 modified filters show a clear increase in As removal efficiency, achieved mainly by the replacement of old nails, an added upper sand layer over the nails, and elongation of the outlet tube to avoid wet–dry cycles and to keep the nail bed immersed. Proper instructions to the users on the operation and maintenance of filters, by replacing the reactive materials (iron nails) and the lower sand bed regularly, are imperative.

We study the upscaling of pore-scale solute transport in partially saturated porous media at different saturation degrees. The interaction between structural heterogeneity, phases distribution and small-scale flow dynamics induces complex flow patterns and broad probability distributions of flow, which control key aspects of transport, such as residence and arrival times, dispersion, and spatial solute distributions, as well as chemical reactions. A continuous-time random walk (CTRW) framework that integrates the processes of advection, diffusion, and trapping in immobile zones is used to upscale and evaluate the transport of diluted solutes. Results of this model were compared to direct numerical simulations solving the advection-diffusion equation in experimental saturation patterns. The comparison between simulations results, with different Péclet numbers (Pe), and the physics-based upscaled CTRW approach allows for a quantitative analysis of the governing factors of transport in partially saturated porous media. This analysis shows that the fluid phase saturation decreases the advective tortuosity, the media's characteristic length, the fraction of the immobile region, and the mean trapping time. At the same time, for a given saturation degree, the normalized mean trapping time is proportional to the Pe. This suggests that the characteristic trapping length is proportional to the media's characteristic (correlation) length. Moreover, the trapping frequency decreases with increasing Pe.