Biocides used in paints and renders prevent algae and fungi growth but can wash off during wind-driven rain and enter urban environments. Retention ponds represent part of stormwater management that retain water and partly pollutants. However, it is poorly understood which percentage of biocides leached from facades reaches a pond and how efficiently biocides are retained inside ponds although biocides can have harmful environmental effects. Here, we combined measurements and modeling to address diffuse biocide loss and a pond's retention capacity in a delimited residential area of 3 ha, with detached houses connected to an ephemeral retention pond. Six stormwater events were sampled within 2 years and confirmed biocidal residuals at pond inflow. Model results revealed that during the sampled events only 11% of terbutryn leachate arrived at the pond while the major part of this biocide was diffusely lost in the residential area. Measured low terbutryn concentrations in the sediment (mean 2.6 ng g−1) confirmed this result. Model results suggested that approximately 50% of terbutryn reaching the pond were retained and degraded. Our results are site-specific but suggest that biocide retention in ponds is limited, environmental entry pathways are diverse and that biocide use should be limited at its source.