## 3 Results and Discussion
### Investigation of the Selected Pesticides in River Water and Sea Water  
Figure S2 provides the area and position of the sampling points, excluding those covered by a confidentiality agreement. Table S4 lists the primary physicochemical properties  
Figure 2: Matrix effect (%) of the studied compounds in water (compound with strong %ME in red). \begin{table}
\begin{tabular}{l c c c c c c} \hline  & **River Water** & & & **Sea Water** & & \\  & **10 ng L\({}^{-1}\) ** & **50 ng L\({}^{-1}\)** & **200 ng L\({}^{-1}\)** & **10 ng L\({}^{-1}\)** & **50 ng L\({}^{-1}\)** & **200 ng L\({}^{-1}\)** \\ \hline Acetamipid & 8.2 & 10.0 & 42.2 & 42.6 & 4.5 & 37.6 \\ Boscalid & - & 7.7 & 31.9 & - & 3.5 & 41.1 \\ Chlorantraniliprole & 7.3 & 39.7 & 38.2 & 7.9 & 45.4 & 33.1 \\ Chlorpyrifos & - & 48.1 & **16.3** & - & **13.4** & **5.4** \\ Dimethoate & 12.3 & 45.8 & 3.5 & **4.6** & **9.6** & **5.8** \\ Diuron & 8.9 & **5.7** & 42.7 & 6.8 & **9.3** & 46.0 \\ Fenpyroximate & 48.4 & 12.0 & 47.4 & 37.2 & 3.5 & 43.9 \\ Fluazifop-p-butyl & 28.2 & 38.5 & 25.7 & 25.6 & 34.6 & 24.4 \\ Fluometuron & - & 9.9 & 39.1 & - & **6.2** & 37.0 \\ Imazalil & 8.4 & 16.8 & 41.9 & **9.4** & 17.3 & 38.3 \\ Linuron & - & 28.1 & 41.3 & - & 27.0 & 34.7 \\ Metalaxyl & 36.5 & **2.6** & 46.1 & 32.2 & 47.6 & 40.1 \\ Myelobutanil & - & **13.3** & **9.2** & - & **6.8** & 44.7 \\ Prometryn & 47.7 & 46.0 & 29.1 & **5.6** & 47.8 & 32.8 \\ S-metolachlor & **9.4** & **4.6** & 42.1 & **1.4** & **4.8** & 38.5 \\ Tebupirimfos & - & 37.9 & 45.7 & - & 38.8 & 44.3 \\ Thiacloprid & **10.8** & **9.3** & 48.8 & **7.2** & **5.8** & 38.7 \\ Thiamethoxam & 47.5 & **5.4** & **12.6** & **10.1** & **9.6** & 48.5 \\ \hline \end{tabular}  
* values in bold refer to the cases where recovery correction was applied; ** compounds with MQL higher than 10 ng L\({}^{-1}\) are noted with (-)  
\end{table} 
Table 3: Expanded measurement uncertainty (%MU) * for three spiking levels in river and sea water. of the samples. The measurements for the water samples were conducted promptly after sampling, at the location of the collection. Out of the 18 pesticides, 9 (50%) were found in 92% of the water samples (11 out of 12). Only two compounds were found at a concentration level higher than the quantification limit: imazalil and fenpyroximate. The compounds diuron, prometryn, S-metolachlor, tebupirimfos, metalaxyl, myelobutanil, and acetamipid demonstrated occasionally some weak peaks (<MQL). However, dimethoate, thiazolprid, fluometuron, linuuron, chlorpyrifos, and fluazifop-p-butyl were not detected in any of the samples. The most frequently detected compounds at all sampling points were fempyroximate (58%), myelobutanil (50%), and S-metolachlor (33%). Figure 3a displays positive detections in a descending order of rank. In terms of the categories of pesticide compounds, fungicides were the most detected pesticides (41%), followed by acaricides (25.8%), herbicides (24%) and insecticides (7%) (Figure 3b). Regarding the presence of multiple pesticides in a water sample, Figure 4 demonstrates that at least one compound was detected in 91.6% of the samples. More specifically, in 16.7%  
Figure 3: (**a**) Detection frequency (%) of pesticides in water samples. (**b**) Frequency of detection of pesticides in water by category. of the samples, only one pesticide was detected. Additionally, in 75.0% of the samples, multiple compounds were identified (more than one), whereas 16.7% of the samples tested positive for four compounds, signifying the sampling locations with the highest pollution levels (R2 and R4). Notably, only 8.3% of the samples did not contain pesticides at all. Among the detected pesticides, the highest quantified concentration levels were reported for fenpyroximate at 77.0 ng/L \(\pm\) 9.3 mg/kg (k = 2; 95%), as shown in Table S5. Table S5 provides the cases of positive detections for each water system, specifically for the compounds that were detected, while Figure 4 depicts the contribution of pesticide compounds to the pollution load of each sampling point.