This article describes a system dynamics model to simulate recharge and flow mechanisms in a fractured bedrock aquifer in northwestern Vermont. The model was constructed to aid in the interpretation of data collected as part of a stable and radiogenic isotope study of groundwater. Use of the model guided data collection and analysis throughout the course of the study, leading to the derivation of equations describing temporal and spatial changes in recharge and flow mechanisms in the study area. Copyright (C)1999 John Wiley & Sons, Ltd.