Water flow and solute transport under dynamic water table conditions in different porous media

Franziska Rühle1, Christine Stumpp1
1 Helmholtz Zentrum München, Institut für Grundwasserökologie

O 3.3 in From atmosphere to groundwater – hydrology of the vadose zone

29.05.2014, 10:40-11:00, H19, NW II

Water fluxes near the groundwater table are highly variable over time and depth leading to changes in the water table level and the formation of a variably saturated transition zone between the unsaturated and saturated zone. In order to understand transport and fate of solutes and contaminants into groundwater, improved understanding about hydrological processes at this dynamic interface between the unsaturated and saturated zone is needed. The objective of this study was to investigate the impact of different water table dynamics (increase, decrease, periodic cycling) on vertical flow and solute transport in different porous media (different average grain size and grain size distribution). Therefore, flow-through columns (L=50cm, Ø=9 or 14cm) were filled with uniform glass beads of different grain sizes or natural sediment and constantly irrigated from the top. Several multi-tracer experiments were conducted with a statically fixed water table and compared to experiments where the water table level was dynamic. Identifying the transport processes, mathematical modeling was performed with the Richards equation and the advection-dispersion equation in HYDRUS-1D. The results showed that under certain hydrological conditions water table dynamics led to increased solute spreading depending on the type of water table movement and the characteristics of the porous medium. In fine uniform porous media a falling water table caused increased spreading since the water table decline rate was faster than the water flux resulting in a more extensive solute distribution over depth. In coarse uniform media, it was observed that a rising water table caused higher tracer spreading due to diffusive solute exchange with immobile water zones. In homogeneous natural sediment with greater grain size distribution but smaller average grain size and negligible immobile water compared to the glass beads, periodic cycling of the water table did not alter solute spreading. It was assumed that variations in water saturation and flow velocities were less pronounced due to high capillary forces. However, it cannot be excluded that differences in transport parameters at decreasing and increasing water table were attenuated, which is not detectable when observing tracer concentrations in the outflow only. In conclusion, the findings highlight the importance of taking into account variable hydrological fluxes near dynamic water tables to understand and correctly simulate transport processes in the subsurface.

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last modified 2013-10-31