Capture Zone Delineations on Island Aquifer Systems

Abstract

AbstractThe objective of this study is to assess the effects of variable density ground water flow on capture zones for pumping wells on islands and the ability of numerical and analytical methods to account for these effects. Mathematical modeling is used to simulate variable density ground water flow for an island aquifer system. Coupled fresh water‐salt water transport equations derived using sharp interface theory are solved using the finite‐element method. Reverse particle tracking is used to compute the capture zone for a well. Sensitivity studies are completed using a quasi‐three‐dimensional model to identify the influence of variable density flow on the computed zone of contribution to a pumping well. Quantitative results indicate that the size and shape of the capture zone are sensitive to the natural geometry of the fresh water lens and the effects of fresh water‐salt water upconing. This is especially true for pumping wells located close to the coastline. For the conditions considered in this study, accounting for variable density ground water flow resulted in capture zones approximately 21% larger than those predicted assuming a uniform fresh water aquifer thickness and constant hydraulic gradient. This effect of variable density flow on the predicted capture zones diminished away from the coastline. The study results serve to caution model users of the limitations inherent in using analytical methods to compute capture zone delineations for island aquifers.