Conceptualization and assessment of groundwater–seawater interactions on bedrock islands

Abstract

Groundwater and seawater interactions (GSIs) in coastal areas are significantly influenced by climate change and anthropogenic activities. However, few studies have focused on GSIs on a bedrock island scale. Compiled literature data from global investigations shows that over 68 % of investigated bedrock islands and coasts worldwide have been seriously threatened by groundwater salinization and NO₃-N contamination. This study investigates the Changshan Islands in Shandong, China. Utilizing hydrogeological data, historical rainfall, long-term groundwater levels (2010–2021), and multi-period hydrochemical data (2015–2021) to analyze groundwater dynamics and hydrochemical processes. Results reveal severe NO₃-N contamination in groundwater, with concentrations peaking at 40 mg/L, primarily due to domestic wastewater discharge and tourism activities. Excessive groundwater extraction exacerbates NO₃-N migration into deeper aquifers. Seawater intrusion (SWI) and submarine groundwater discharge (SGD) rates were calculated using Darcy’s law, with Cl^- and NO₃-N fluxes estimated. SWI is more pronounced on the north island, with an average SWI rate of 0.3 m/d. The associated Cl^- and NO₃-N fluxes are on average 119187.6 and 69.6 mmol/(m²·d), respectively. The southern island, largely free of SWI, has an average SGD rate of 0.2 m/d, with Cl^- and NO₃-N fluxes averaging 3912.1 and 81.8 mmol/(m²·d), respectively. Key factors influencing GSIs are precipitation, heterogeneous structure, hydrogeological conditions, and groundwater extraction. Based on these findings, we proposed a conceptual model of GSIs for these islands. This study would be helpful to enhance the understanding of island-scale GSIs’ patterns and the scientific management of island groundwater resources.