| Complex groundwater whirl systems |
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By analyzing three-dimensional flow patterns in finite-element models of layered anisotropic aquifers, we found that streamlines often have the shape of spirals. A bundle of such spiralling streamlines was termed a ‘groundwater whirl’. Several analytic solutions have been developed that confirm the existence of groundwater whirls in anisotropic layered aquifers. Further experiments include aquifers in which all layers have a laterally heterogeneous anisotropy. In box-shaped aquifers with horizontal layers and a uniform horizontal gradient along the boundaries, all whirls have their axes in the same flow direction. In such cases projected streamlines can be represented by stream function contours. It allows an easy 2D interpretation of the main characteristics of complex whirls patterns. Clockwise and counterclockwise whirl axes occur at the interfaces of adjacent layers with different anisotropic hydraulic conductivities. One or more clockwise whirls may occur within one counter-clockwise whirl, and vice versa. Where contours of different whirls meet, saddle points are found, both within the model and on its boundaries. There is a fixed relation between the number of whirls and the number of saddle points. As a consequence of groundwater whirls, the exchange of water between aquifer layers is intensified. The spreading of dissolved substances increases as a result of variations in the directions and magnitude of horizontal anisotropy, and may form an important component of the mechanical dispersion.
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