Full Tensor Representation of Anisotropy in Hydraulic Conductivity: Effects of Simulating Discharge of Groundwater to Lakes

It is well known that anisotropy plays an important role in discharge of groundwater to lakes; groundwater flows almost horizontally to a lake then over a short distance changes direction to an almost vertical direction in the vicinity of or right below the lake. At this scale anisotropy becomes important. However, usually most simulation studies assume that the principal directions of the hydraulic conductivity tensor are horizontal and not aligned with the sloping lake beds and maybe not even with sloping sedimentary layers next to the lake. FEFLOW was used to study the effects of including a full tensor representation of the individual layers (sediments, lake bed) on flow patterns and discharge to characteristic lakes in Denmark. Sloping layers with different angles to the horizontal have been incorporated into a conceptual 2D cross-sectional model. Differences are found between simulations, where the principal directions follows the horizontal axis and simulations, where the principal directions are aligned with the sloping sediment layers and lake bed; (1) a more complicated flow pattern beneath the lake, sometimes with flow reversals towards land and (2) thus, also a different discharge pattern to the lake deviating from the often assumed classical exponential distribution. The results are sensitive to the thickness of the layers/lake beds, anisotropy, and slope (angle of principal direction) but with the biggest effect seen in the distribution of discharge to the lake, e.g. where 90% of the discharge now occurs of an ~ 50 m wider lake shore. The effect on the total discharge and near-shore discharge are minor. Currently we are conducting geophysical offshore to on-land investigations to reveal lake bed and layer sedimentation (and angles) and the model will be used to simulate discharge to these lakes and compare with observations.