Influence of Eco-hydrological Changes on Flow Velocities in a Shallow Alluvial Aquifer - Findings From Tracer Tests at the Merdingen Test Site, Germany.

Since 1979, the Merdingen test site, located in the alluvial deposits of the Upper Rhine Graben, has been used for tracer tests using fluorescent dyes, heavy metals, microorganisms, microspheres and other tracers. In 1999, the storm Lothar badly affected the test site and significantly changed the vegetation cover. The previously forested area is now predominantly covered by bushes and a few remaining trees. 13 wells can be used for tracer experiments over a distance of up to 200 meters. Before the storm (1988), uranine gave a maximum flow velocity (vmax) of 1.08 m/h and a peak flow velocity (vpeak) of 0.07 m/h over a distance of 25 meters. Vmax was not influenced by rainfall, whereas the peak might have been accelerated by rainfall events. In 2005, uranine gave a vmax of 2.55 m/h and a vpeak of 0.07 m/h, during a period of falling water table and no rainfalls. This means that vmax increased by a factor of 2.3, whereas vpeak remained stable. The vmax for 1 μm microspheres increased by a factor of 4.6 between 1988 and 2005, the vmax for the Serratia marcescens bacterium increased by the same factor, while vpeak remained stable. This behavior can, at least partly, be explained by the change in vegetation caused by the storm, particularly by the impact of uprooted trees and intensively rooting bushes on the shallow aquifer, which obviously created additional preferential flowpaths that allow for higher maximum velocities. The even higher observed increase of the maximum velocities for bacteria and microspheres can be explained by the fact that particle transport is known to be even more influenced by preferential flowpaths than solute transport (pore exclusion). Our findings suggest that vegetation changes, particularly trees being overthrown and uprooted, can actually influence groundwater flow velocities in shallow alluvial aquifers, which can result in higher maximum transport velocities of microorganisms along preferential flowpaths.