Representation of Multiaquifer Wells in MODFLOW

Multiaquifer wells, i.e. wells that are open across more than one aquifer, can have a profound effect on the hydraulics of the groundwater system. They change the physical system by acting as very permeable tubes that establish direct hydraulic links between nonadjacent strata. The inclusion of a multiaquifer well in a numerical model of groundwater flow also changes the governing equations. Several methods have been adopted to simulate multiaquifer wells in the context of comprehensive simulators. However, none of these methods have been “officially” implemented in the popular code MODFLOW, which has become the de-facto standard for hydrogeologic modeling. In this paper we review four different methods to represent multiaquifer wells. Our principal objective is to test a specialized code developed but never formally released by the United States Geological Survey (USGS), the Multiaquifer Well (MAW1) package. The MAW1 package can be used as an alternative to the standard Well (WEL) package, but as far as we are aware has never been documented and tested formally. We examine the performance of the MAW1 package in the context of a benchmarking study against the analytical solution of Papadopulos (1966). Our results demonstrate that the MAW1 package is capable of matching an exact solution for pumping and non-pumping conditions, with both coarse and refined grids. Other methods of representing multiaquifer wells yield correct results when the grid is refined to the dimensions of the extraction well.