An analytic element approach for modeling polygonal inhomogeneities in multi-aquifer systems

An analytic element approach is presented for the modeling of steady groundwater flow through multi-aquifer systems with piecewise constant aquifer and leaky layer properties. Different properties may be specified for domains bounded by closed polygons, referred to as polygonal inhomogeneities. The boundary of these inhomogeneities is modeled with two types of high-order line elements. First, a string of single-aquifer line-doublets is used; these elements cut through all aquifers and are valid both inside and outside the inhomogeneity. Second, two strings of multi-aquifer line-sinks are used, one string that is valid inside the inhomogeneity and one string that is valid outside; the comprehensive extraction of these line-sinks is zero at any point along the string. The proposed approach results in a comprehensive flow field of which the component normal to the boundary of the inhomogeneity is continuous across the boundary at any point. Within each individual aquifer, continuity of head and the component of flow normal to the boundary are met approximately across the boundary; the accuracy increases when the order of the line elements is increased and/or when shorter line elements are used. The proposed analytic element approach produces results that are virtually identical to the exact solution for a cylindrical inhomogeneity, and a high-resolution MODFLOW2000 model of two rectangular inhomogeneities with a shared boundary. The practical application of the approach is demonstrated through the solution of a problem with an irregularly shaped inhomogeneity with rivers crossing the inhomogeneity boundary.