We performed a comprehensive groundwater monitoring in areas where organic fertilizers (e.g. animal manure) are used as soil amendment in crops. We selected sites where groundwater is used as a drinking water source. Results showed the occurrence of antibiotics widely used in veterinary medicine at low concentrations (ng L^-1), and in some cases, they even reached remarkable levels (µg L^-1). Genes conferring resistance to these antibiotics were also found (values from 4×10² to 5×10⁶ copy numbers/L water).

DOM analysis revealed that most molecular formulas were CHO compounds. However, a remarkable percentage contained nitrogen and sulphur (~15-20%, respectively) as well. The use of van Krevelen diagrams (plots of H/C versus O/C atomic ratios) showed that all our samples were dominated by molecules falling in the lignin-like region. Finally, we could also establish relationships between antibiotics and DOM composition.

The analysis of samples within different seasons showed large variability, which hinders the assessment of groundwater quality and the implementation of efficient monitoring strategies.