## 3 Results and Discussion
### Source Distribution of Anticancer Drugs  
Based on the data compiled in this article, the major load source distribution of anticancer drugs in the Kanzaki-Ai River basin could be estimated. The amount of the individual anticancer drugs flowing from the upstream region (S3, R6) to the farthest downstream sampling site (R5) was determined.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Compound**} & \multicolumn{2}{c}{**Hirakata Bridge**} & \multicolumn{2}{c}{**Suita Bridge**} \\ \cline{2-5}  & **log\(K_{\text{d}}\) (L/kg)** & \(r^{2}\) & log\(K_{\text{d}}\) (L/kg)** & \(r^{2}\) \\ \hline Bicalutamide (BLT) & 0.4 & 0.98 & 1.4 & 0.85 \\ Tamoxifen (TAM) & 2.1 & 0.99 & 1.6 & 0.99 \\ \hline Cyclophosphamide (CP) & 0.8 & 0.99 & 0.6 & 0.99 \\ \hline Capecitabine (CAP) & \(-0.4\) & 0.99 & 0.1 & 0.99 \\ Doxifuridine (DFUR) & 0.9 & 0.99 & 1.5 & 0.97 \\ Tegafur (TGF) & 1.3 & 0.99 & 0.4 & 0.86 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Sorption distribution coefficients (log\(K_{\text{d}}\)) of anticancer drugs for river sediments in sorption experiments (reproduced from [31]).  
Figure 2: Distribution of the target anticancer drugs in the river sediments and suspended solids (abbreviation of each anticancer drug is shown in Table 1) (reproduced from [31]).  
The contribution of each load source to the total mass flux was then calculated as mass flux (%) (Figure 3) [30].  
For all anticancer drugs except TAM, the contribution of STP effluents as pollutant loading sources was large, amounting about 50-92% of the total load, while the contribution of tributaries was as low as in the level of 0.3-2.0%. A typical example of these mass flow transitions by using the case of BLT as a representative anticancer drug is shown in Figure 4 [30]. Consequently, STP effluent was strengthened to be a major contributor to the pollution of river waters by the target anticancer compounds in the surveyed area.  
These results also showed that the contribution of STP effluents to the total load of TAM was low (19%; Figure 3). In the case of this anticancer drug, the contributions of the main stream and tributaries were higher than the other anticancer drugs. Recently, a screening assessment of TAM has been published [71], along with a review of its ecotoxicological effects [21; 22; 72]. However, because of a lack of enough information about the fate of this drug after its discharge into environmental waters, more extended investigations are needed to explain the findings mentioned above. In addition, the bulk of the contributions remained as unassigned in the total mass flux of the target anticancer drugs came  
Figure 4: Transition of mass flows (g/day) from upstream to downstream regions in the Kanzaki–Ai River basin for bicalutamide (BLT). N.A.: not available (reproduced from [30]).  
Figure 3: Source distribution of anticancer drugs in the Kanzaki–Ai River basin (abbreviation of each anticancer drug is shown in Table 1) (reproduced from [30]).  
from the upstream points surveyed in the main stream [73]. In the present article, pollution loads might have come from areas beyond the sample collection points, because the Kanzaki River originates from a branch on the right bank of the Yodo River [74]. For this reason, advanced water-processing techniques should be introduced not only at STPs located in the middle and downstream reaches, as well as the upstream reaches, to decrease entire pollution loads and improve the water quality of the Kanzaki-Ai River system.