Impact of Spatially Variable Recharge on Fresh Groundwater Lens Development in Small Strip Islands

Abstract:

Groundwater lenses in island surficial aquifers may provide primary freshwater supplies for island residents. The lens shape, thickness and volume are sensitive to spatially variable recharge caused by topography, soil type, evapotranspiration and human activities. We derive an analytical solution to describe transient and steady-state lens profiles for spatially variable recharge rates in a small strip island. Numerical investigations show that the recharge at the center area has significant impact on the entire lens profile. A Green’s function is derived based on the analytical solution to quantify the temporal and spatial effect of impulse recharge. The impulse recharge exerts the strongest influence on the recharge point and the influence decreases linearly away from the impulse location. Importantly, such linearly decaying property is consistent for any site regardless of the site-specific parameters. Uncertainty analysis is implemented to investigate the lens behavior under stochastic spatially dependent recharge. Results show that the lens tends to have larger fluctuations for recharge rates with higher variances and longer correlation lengths. Additionally, thinner lenses are more sensitive and vulnerable to recharge changes.