Assessment of vulnerability to groundwater contamination in the Santa Rosa River basin, Ecuador

Groundwater is a vital resource that supplies nearly half of the world’s population and sustains ecosystem integrity. In the Santa Rosa River basin (SW Ecuador), groundwater plays an essential role due to the variable quality of surface water resources. This study assessed groundwater contamination risk based on the premise that floodplain sectors with shallow water tables and high permeability are more susceptible to contamination, with the aim of supporting environmental management priorities. Cartographic and secondary data were integrated with field investigations. Intrinsic vulnerability was assessed using the GOD and DRASTIC methods, whereas contamination hazards were evaluated using the POSH approach, supported by in-situ measurements of water-table levels and physicochemical parameters. The GOD method classified the basin into negligible (371.18 km²; 49.31%), low (263.39 km²; 34.99%), and moderate (118.13 km²; 15.70%) vulnerability classes. DRASTIC identified low vulnerability as the dominant class (58.18%), mainly across the central, southern, and eastern sectors of the basin, whereas high vulnerability represented the second-largest class (21.75%) and was concentrated primarily in the western–northwestern sector, with localized patches along the northern margin. This spatial pattern is related to shallow groundwater conditions, permeable alluvial deposits, and low-relief terrain. The main pollution sources included agrochemicals, urban discharges, livestock excreta, artisanal mining, and solid waste. The integrated risk map identified moderate to high contamination risk across 56.0% of the basin, particularly in the western-northwestern sector and along major river and road corridors. These findings provide a regional-scale basis for prioritizing groundwater monitoring and contamination management in vulnerable sectors of the basin, supporting groundwater-quality protection and safer water-resource planning in the priority areas identified in this study.