Biocides in urban groundwater – modeling entry pathways at a district level

Biocides leached from facades can reach urban groundwater, where they can have adverse environmental impacts. They are used in film preservatives and enter groundwater via swale-trench systems or diffuse pathways. However, there is little information on occurring biocide loads along different pathways to urban groundwater. The aim of this study is to quantify the input, transport and degradation of biocides to groundwater via different pathways, namely infiltration (1) in swale-trench systems, (2) adjacent to facades via vegetated soils, and (3) through permeable pavements. The study area (38ha) is located in the city of Freiburg, south-west Germany. There are a number of groundwater monitoring wells due to a chlorinated hydrocarbon (CHC) contamination site. Three biocides (diuron, octylisothiazolinone, terbutryn) and various transformation products were detected in groundwater during nine events over a six-year sampling period. In addition, more than a decade of groundwater level data and biannual sampling data from the CHC plume are available. Biocide concentrations in groundwater are assessed through the combination of four models. First, biocide leaching from facades is quantified (COMLEAM). Second, a water balance model (RoGeR_WB_Urban) calculates water infiltration into the swale-trench system and in the remaining district. Third, the results are combined to calculate biocide leaching through the soil at a depth of 1m (FOCUS-PELMO). This leachate forms the input to a groundwater flow and transport model (MODFLOW with MT3D-USGS). Terbutryn, a commonly used biocide, is chosen as the model compound. Concentrations in groundwater are modeled in daily time steps covering a period of more than two years at a spatial resolution of 5x5m. CHC measurements are used to calibrate the groundwater model. Additional biocide measurements on the facades and in the swale water help to validate the model chain. A first scenario assumes that all biocides enter groundwater via the swale-trench systems but overestimates the measured biocide concentration in groundwater by a factor of 15. A second scenario also includes diffuse pathways via vegetated soils and permeable pavements and more realistically reproduces measured terbutryn concentrations in groundwater in the range of a few ng/l. These results suggest that the diffuse entry of biocides via vegetated soils and permeable pavements is important at the urban district scale.  Hence, end of pipe measures to prevent biocide leaching into groundwater have limited efficiency in swale infiltration systems, biocide use should rather be avoided at the source.