Climate variability and changes in shallow groundwater quality on Indonesia’s small tropical island

Generally, the water resources in small islands depend on limited shallow groundwater influenced by rainwater. So, understanding the climate variability affecting shallow groundwater is needed for the sustainability of water resources management on small islands. This study aims to determine the climate variability and other factors influencing the change in shallow groundwater quality on Pari Island, a small island in Indonesia. The study used Standardized Precipitation Evapotranspiration Index (SPEI) analysis to assess anomalous climate conditions. Traditional hydrochemistry analysis methods (Piper, Durov, Scholler, and Gibbs diagrams), ion ratios, and correlation were applied to determine the change in water quality and the factors that influence it. The results showed that high-intensity precipitation events are becoming more frequent. The correlation between electrical conductivity and short-term timescale SPEI was very strong, indicating groundwater quality's sensitivity to precipitation variations. The results of groundwater hydro-chemical analysis showed that differences in shallow groundwater quality in the dry and wet seasons were due to reverse ion exchange, rock weathering, and evaporation. Although the characteristics of freshwater were more substantial in the wet season than in the dry season, it was difficult to find true freshwater on Pari Island. Geographically, the influence of climate variability on Pari Island weakened from west to east, corresponding to patterns related to the direction of the monsoon winds of tropical Indonesia. The total organic carbon value also shows the same due to tidal seawater intrusions or domestic activities that still require further study to determine the source of organic matter.