Distribution, remobilization and accumulation of organic contaminants by flood events in a meso-scaled catchment system

Abstract Background Extreme weather events and natural disasters such as floods can cause severe damage and impacts on catchment systems covering natural as well as urban, industrial or agricultural areas. Thus, floods often lead to the acute and unusual release of organic pollutants, as well as the remobilization of legacy contaminations or old burdens. Floodplains are then of major relevance for the accumulation of pollutants. Accordingly, various floodplains distributed throughout the course of the Rur River were sampled immediately after two flood events in January/February and July 2021. The main objective was to address the general lack of knowledge on indirect effects of flooding and the corresponding distribution and accumulation of organic pollutants regarding different dimensions and dynamics of flood events. Results Gas Chromatography/Mass Spectrometry (GC/MS) based non-target screenings revealed the presence of several lipophilic to moderate polar organic pollutant groups, including PAHs (polycyclic aromatic hydrocarbons), hopanes, PCBs (polychlorinated biphenyls), LABs (linear alkylbenzenes) and various other industrial substances. These substances are indicators of petrogenic pollution, historical and current industry in the catchment area, and of wastewater and urban pollution, respectively. In general, concentrations detected after the extreme summer flood were higher than in winter. This points to additional emission sources due to substantially higher discharges and consequently more severe flooding in July. The main tributaries also had a major influence on the input and concentrations of organic pollutants at the receiving Rur River. Further on, structural features such as dams and reservoirs, but also (re)naturalized areas were clearly recognizable in the flood-related dispersion of organic pollutants. Interestingly, LAB contamination was similar after both flood events regardless of the specific dimension. Conclusions Flood dimension and frequency are of great relevance for the distribution, remobilization and accumulation of organic contaminants. However, special attention should be given to the introduction of wastewater pollutants for any flood extent. Overall, organic indicators are therefore very useful to obtain information on specific distribution patterns and the influence of tributaries or structural measures, providing an important basis for the assessment of short- and long-term environmental risks and hazards.