Ir2dinv: A finite-difference model for inverse analysis of two-dimensionallinear or radial groundwater flow

We have developed a program for inverse analysis of two-dimensional linear or radial groundwater flow problems. The program, lr2dinv, uses standard fi nite difference techniques to solve the groundwater flow equation for a hor izontal or vertical plane with heterogeneous properties. In radial mode, th e program simulates flow to a well in a vertical plane, transforming the ra dial flow equation into an equivalent problem in Cartesian coordinates. The physical parameters in the model are horizontal or x-direction hydraulic c onductivity, anisotropy ratio (vertical to horizontal conductivity in a ver tical model, y-direction to x-direction in a horizontal model), and specifi c storage. The program allows the user to specify arbitrary and independent zonations of these three parameters and also to specify which zonal parame ter values are known and which are unknown. The Levenberg-Marquardt algorit hm is used to estimate parameters from observed head values. Particularly p owerful features of the program are the ability to perform simultaneous ana lysis of heads from different tests and the inclusion of the wellbore in th e radial mode. These capabilities allow the program to be used for analysis of suites of well tests, such as multilevel slug tests or pumping tests in a tomographic format. The combination of information from tests stressing different vertical levels in an aquifer provides the means for accurately e stimating vertical variations in conductivity, a factor profoundly influenc ing contaminant transport in the subsurface. (C) 2001 Elsevier Science Ltd. All rights reserved.