1 Hydrology and Geo-Environmental Sciences, Vrije Universiteit Amsterdam, De Boelelaan 1085, NL-1081 HV Amsterdam, The Netherlands

2 Department of Biological and Agricultural Engineering, University of Georgia, Athens, Georgia 30602, USA

Abstract By analysing three-dimensional flow patterns in 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". Experiments include aquifers in which all layers have a laterally hetero-geneous 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 2-D interpretation of the main characteristics of complex whirls patterns. Clockwise and counter-clockwise 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. 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, which may have a significant impact on contaminant spreading.

Keywords anisotropy; analytical model; groundwater whirls; heterogeneity; layered aquifers; mechanical dispersion; saturated groundwater; stream function; streamlines