The Urban Biocide Terbutryn: Field investigations to explore release and reactive transport under environmental conditions

Urban biocides like terbutryn are used in construction materials such as render and paints on façades to prevent the growth of algae and fungi. With wind-driven rain, those contaminants leach into the environment and pose a risk for groundwater contamination. In our study, we combined leaching experiments with artificial facades to quantify biocide release, lysimeter experiments to get insights into the reactive transport under environmental conditions, and field sampling in an urban catchment to follow-up biocide release from source (building facades) to sinks (stormwater retention systems/ groundwater). We use concentrations of terbutryn as well as its four major transformation products to establish mass balances. Furthermore, we use Compound-Specific Isotope Analysis (CSIA) as a concentration-independent tool to identify the dominant degradation processes in the environment by taking into account (i) isotopic enrichment of stable isotopes by dual-isotope plots and (ii) patterns of formed transformation products. Façade leaching experiments show that high quantities of terbutryn remain on the facades and leaching continues for a long period. Transformation products are already formed on the facades through photodegradation. Reactive transport on typical urban surfaces (gravel, paved stones with joints, soil) indicates high retention of terbutryn in the grass lysimeter and faster leaching in gravel and pavement lysimeters with low recoveries of terbutryn in the first 2 months (3 %, 1 % and <1 % in gravel, paved and grass lysimeter, respectively). In all lysimeters, transformation products were formed, exceeding the concentrations of leached terbutryn at some sampling points, indicating higher mobility through better solubility in water of the daughter compounds. The overall extend of (bio-)degradation in the lysimeters was low for terbutryn, as supported by CSIA data (changes in Δδ¹³C < 2‰). Grab samples in a stormwater retention pond and swale system in an urban catchment confirmed the leaching of terbutryn and transformation products in a catchment from 6-year-old buildings with concentrations of terbutryn between 3 and 70 ng/L, as well as TP terbutryn sulfoxide, exceeding 400 ng/L. Our study implies the importance of detailed monitoring of urban biocides to understand the reactive transport processes on their pathways into groundwater and underlines the importance of monitoring also transformation products in typical monitoring schemes.