Measuring and modeling biocide entry from facades in urban swale sediment

Urban swales collect stormwater runoff containing micropollutants such as biocides washed off from facades during wind driven rain. Although swales retain contaminants, they might eventually reach groundwater through soil. However, there is little data available of biocide occurrence in urban swale sediment. In this study, we measured the biocide entry to an urban swale and its distribution in sediment. The selected swale in south-west Germany collects runoff from a 3 ha residential area with 46 houses. Two pipes lead into the swale, one collecting runoff from a 1 ha area (“East”) and one from a 2 ha area (“West”). We took sediment samples after dry and wet periods close to both pipes and additional water grab samples during wet periods. In total, we collected 19 stormwater samples during 7 events and 116 sediment samples during 8 days over a period of two years. Water samples were analyzed for three biocides (diuron, terbutryn, octylisothiazoline= OIT) and four transformation products (= TPs, diuron-desmethyl, terbuthylazine-2-hydroxy, terbutryn-desethyl, terbumeton) using LC-MS/MS. Sediment samples were analyzed for terbutryn, OIT and 3 TPs of terbutryn. Finally, we linked a water balance model to a leaching model and simulated longterm biocide input into the swale. This we compared with biocide concentration in the swale sediments using a mass balance approach. We found all biocides and all TPs in water samples at both pipes confirming biocide input to the swale. In the sediment, terbutryn concentrations were generally below 1 ng/g. Of three measured TPs of terbutryn we detected only one, terbutryn-desethyl (<1 ng/g dry weight). There were no significant differences of terbutryn concentration in the sediment before and after storm events. This suggests continuous input and long presence of biocides. Concentrations at pipe West were higher than at pipe East. This pointed to differences in the connected areas such as differences in paints used at individual houses. At the outlet of pipe West, we found maximum concentrations of terbutryn (26 ng/g) in saturated sediments below standing water, which suggests more sorption and less degradation at this location. We thus assume that dry condition in swales during dry weather promote degradation. Overall, our findings emphasize that not only the quantity of urban runoff should guide the design of urban swale systems but also potential biocide entry.