Study region: Island of Curaçao, Caribbean Sea Study focus: Groundwater is a critical resource for many Caribbean islands. This study assessed the groundwater quality (main and trace elements, E. coli) of Curaçao during 4 subsequent wet seasons (2020–2024). New hydrological insights for the region: Three main hydrochemical processes were identified with Principal Component Analysis (PCA): 1) seawater influence (median EC = 1900 µS/cm), 2) nutrient contamination, and 3) silicate weathering from altered basalts. Elevated median nitrate concentrations (all 206 samples = 35 mg/L) are highest in urban areas (102 mg/L) suggesting wastewater leakage from cesspits. Lower Cl/B mass ratios (550) in urban areas compared to natural vegetated (1000) areas, indicate a wastewater source derived from reverse osmosis drinking water relatively high in Boron. Enrichment of δ¹⁵N-NO₃ and δ¹⁸O-NO₃ versus wastewater signatures indicates considerable occurrence of denitrification. Phosphate concentrations remain low (∼0.1 mg/L), except for sites irrigated with large volumes of treated wastewater (>1 mg/L). Cl-B endmember mixing model showed 36 % wastewater contribution to groundwater, making it an important additional freshwater source given the semi-arid conditions. The findings underscore the value of comprehensive groundwater monitoring and the need for sanitation improvements. Addressing these challenges will benefit water management practices on Curaçao and similar islands. Additionally, it will improve groundwater quality, and groundwater-affected ecosystems, such as nearshore coral reefs.

Past hydrogeological processes and human impacts may exert substantial memory effects on
today’s groundwater systems. Thorough characterization of such long-term processes is
required for scientists and policymakers to predict the hydrogeological impacts of land
management options. Especially in data-scarce areas, historical data are essential to unravel
long-term hydrogeological processes, which could not be identified by short-term fieldwork or model simulations alone. However, historical data are often overlooked or only used as
background information in most hydrogeological studies. We show that the combination of
historical reports and quantitative data yields major insights in the hydrogeological system of
Curaçao, a small semi-arid Caribbean island. Reconstructing the island’s groundwater
conditions over the past 500 years revealed that deforestation and excessive abstraction has had a detrimental effect on the island’s groundwater reserves. Historical notes and data revealed major signs of seawater intrusion, especially during abstraction peaks in the island’s industrial era. Intrusion effects are still observed locally on the island today, but additional groundwater recharge by waste water has caused freshening elsewhere. We hypothesize that the observed aquifer replenishment locally enhances submarine groundwater discharge, flushing accumulated nutrients and pollutants towards Curaçao’s fringing coral reefs. We expect that this study’s insights motivate more hydrogeologists to use historical reports and data in future studies.