Tracing the occurrence and spatial drivers of organic contaminants in urban ponds along urbanization gradients.

In cities, numerous synthetic chemicals, such as pesticides, pharmaceuticals, and industrial chemicals can enter urban ponds through different input pathways, including surface runoff, drainage systems, wastewater-impacted streams, or improper waste disposal. Nevertheless, knowledge on the occurrence and sources of trace organic contaminants (TrOCs) and the impact of urbanization on the chemical water quality in urban ponds is scarce. We sampled 43 urban ponds across the city of Berlin, Germany, and analyzed 37 dissolved TrOCs, general water quality parameters, and stable water isotopes to investigate the occurrence and sources of TrOCs. All ponds contained mixtures of TrOCs, with rubber-associated chemicals being present in all ponds with sum concentrations up to 9.3 µg L-1. 1,3-Diphenylguanidine was most frequently detected in 42 ponds with the highest median concentration of 0.4 μg L-1 among all TrOCs. Linking TrOC data and urbanization parameters by distance-based redundancy analysis (dbRDA) revealed that high concentrations of rubber-associated chemicals in ponds correlated positively with stormwater discharge. Elevated pharmaceutical concentrations in urban ponds were driven by their proximity to wastewater-impacted streams within a 50 m radius. Information on groundwater levels and pond depths in combination with stable water isotope data indicated groundwater-surface water interactions for certain ponds with high TrOC concentrations, posing a potential risk to groundwater quality. Our transferrable multi-tracer approach, combined with characterization of the urban environment, provides unique insights into the pollution and water sources of urban ponds, thereby supporting measures for improved urban water management, water treatment, and protection of ecosystem functioning.