Hydrogeochemical signatures and human impact: A comprehensive analysis of groundwater quality on the semi-arid island of Curaçao

Island of Curaçao, Caribbean Sea 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). 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 δ 15 N-NO 3 and δ 18 O-NO 3 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. showing relevant processes that determine the groundwater availability and quality on Curaçao. • PCA revealed mixing, nutrient pollution, and silicate weathering are dominant. • Groundwater NO 3 below Willemstad is linked to cesspits and wastewater reuse. • δ 15 N-NO 3 and δ 18 O-NO 3 show a sewage source and denitrification in groundwater. • Low Cl/B ratios indicate sewage leaks from RO water, useful tracer on islands. • Cl-B mixing model estimate 36 % wastewater contribution to groundwater.