OCP analysis in sediment samples was carried out with a Trace GC Ultra gas chromatograph (Thermo Fisher Scientific, San Jose, CA, USA), a CombiPAL autosampler (CTC Analytics AG, Zwingen, Switzerland) equipped with a SPME fiber assembly, and an ITQ 900 mass spectrometer (Thermo Fisher Scientific). For all measurements, a SPME fiber coated with PDMS (100 µm) was used (Sigma-Aldrich, St. Louis, MO, USA). SPME extraction of prepared samples started with a heat up phase in the agitator for 5 min to 80 °C, followed by headspace extraction at the same temperature for 30 min. After extraction, the fiber was thermally desorbed in splitless mode in the GC injector for 3 min, after which it switched back to a split flow of 30 mL•min -1 . At the start and end of each SPME sample cycle, the fiber was desorbed in a needle heater for 5 min at 270 °C. Chromatographic separation was conducted on a fused silica capillary column (TG-XLBMS: 60 m, 0.25 mm inner diameter, 0.25 μm coating thickness; Thermo Fisher Scientific). Helium (≥ 99.999%, Praxair, Danbury, CT, USA) was used as carrier gas at a constant flow of 1.0 mL•min -1 . The initial oven temperature was set at 90 °C and held for 3 min. The temperature was ramped to 150 °C at a rate of 15 °C•min -1 . Then, it was ramped to 280 °C at a rate of 5 °C•min -1 and held for 3 min. Quantification was done in selected ion monitoring (SIM) mode based on one target and one qualifier ion. A list of ions and retention times used, is available in the Online Resource 1 (Table ). The peak areas of analytes in sediment samples were corrected with their respective internal standard (see caption of Table ). The respective concentration was determined by interpolation of the relative peak areas for each pesticide to standard peak areas of the calibration curve. as it was not possible to discern a difference between them with sufficient certainty in the map data.