Modelling antibiotics in a densely populated reservoir catchment: insights from Cirata Reservoir, Indonesia

Understanding the sources and fate of antibiotics from livestock, humans, and aquaculture, as well as their transport via rivers to reservoirs, is crucial to design effective management strategies to mitigate associated risks. This study investigates the sources, fate, and transport of antibiotic residues in the river-reservoir system to inform strategies for mitigating their ecological impacts on water-related ecosystem. The MARINA-antibiotics (China-1.0) model (Model to Assess River Inputs of Pollutants to Seas for Antibiotics in freshwater) was adjusted for the Cirata Reservoir catchment in Indonesia and potential antibiotic pollution from aquaculture was incorporated into the model. Using this adjusted model, the inputs of 14 antibiotics from human use, livestock farming, and aquaculture were quantified. Seasonal trends revealed higher antibiotic loads during the wet season, predominantly from human sources, livestock, and aquaculture, with point sources as the largest sources. The primary contributors were livestock farming, particularly sheep, cattle, and broiler farming. Antibiotic residues were detected at higher concentrations in the river than in the reservoir, highlighting the river's critical role in transporting antibiotics from subbasins to the reservoir. Relevant fate processes for some antibiotic classes included, degradation in soil, and for others minimal sedimentation before they entered the reservoir and continued degradation afterwards. Antibiotic degradation varied by antibiotic class and hydrological conditions. Model validation showed that 80 % of antibiotic residues concentration in the reservoir fell within the observed range. Sensitivity analysis indicated that the model was sensitive to human and livestock populations, antibiotic consumption rates, manure management practices, and hydrological characteristics. This study provides the first assessment of antibiotic sources and dynamics in an Indonesian river-reservoir system, offering key insights to support strategies for reducing the impact of antibiotic residues on water-related ecosystems.