Managed aquifer recharge in island aquifer under thermal influences on the fresh-saline water interface

Abstract

Storing and utilizing freshwater in coastal aquifers with managed aquifer recharge (MAR) presents significant challenges, particularly in small island settings where seawater intrusion is a major source of groundwater salinization. Existing studies usually evaluate density-dependent flow under isothermal condition, which neglects the potential impact of temperature on fluid density. To fill the knowledge gap, we performed both laboratory experiments and numerical modeling to investigate the impact of warmer water injection on freshwater storage and extracted water quality for island aquifer. It is found that warmer water (70 °C) injection compromise the “barrier effect” achieved by normal water injection, resulting in a reduced ability to prevent coastal seawater intrusion by, creating an upward hydraulic gradient beneath the injection well. In addition, warm water injection result in a 5 % decrease in freshwater storage capacity for island aquifer. However, by employing a combination of receding tide cycles and intermittent injection-extraction operations, the duration of lowered saline concentration within the extracted water can be extended by approximately 10 % with an 11 % reduction in the overall extracted saline concentration. This study provides valuable insights into the influence of warmer water injection on coastal aquifer quality. The findings from both the laboratory experiments and numerical simulations enhance the understanding of thermal density-dependent dynamics of coastal aquifers, offering valuable guidance for effective freshwater storage and usage in island settings.