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Abstract: This paper presents the results of numerical groundwater flow and contaminant transport simulations assuming different flow conditions of a river influenced aquifer. The purpose of the study was to compare the simulated dissolved plume movement in steady-state and in transient flow fields showing the effects of the river for transport phenomenon. The differences in advective and dispersive transport were examined by computing the flow velocity fields, flow path lengths and dispersion in space and time. In transient simulation the flow velocities and flow directions change through time periods, and the effects of river floods can be incorporated. While in the steady-state model a constant, average flow field is assumed. Significant differences were detected in the flow paths between the two modelling approach. The flow path lengths across perpendicular sections showed that water particles - including dissolved contaminants - take longer paths approaching the river in steady-state simulation. The calculated dispersion was also different for the steady-state and for the transient simulations. During flood periods the steady-state model underestimates the dispersion of contaminants. The overall impacts of these differences, that the dissolved plume moves slower toward the river in the transient case compared to the steady-state simulation.
Keywords: numerical simulation, transient flow field, plume movement, river influenced transport
1. Introduction
Rivers can drain or feed aquifers and depending on the strength of connection they can influence the groundwater flow conditions. Due to changes in river stages, the groundwater level can be constantly changing in a dynamic way. The hydraulic impact of a river is complex, and not only the narrow riparian zone is influenced. The river affects the groundwater flow velocity, the flow gradient and flow direction even over long distances and even by a short flood period. As a result, transport phenomena in the aquifer can also be influenced by the connected surface water and can be dependent on the actual state and time variability of the groundwater flow system. Generally, in the relationship between surface and groundwater there may be two conditions: the surface water body either drains or recharges the groundwater. The difference in hydraulic potential between the two elements determines the flow condition. If the groundwater level is higher than the river stage, groundwater flows into the river. If the...