The passive sampling method overestimated C6 and C8-BAC (i.e., underestimated sorption by kaolinite), which is related to the influence of kaolinite on PA sorption as discussed above. For the other substances, passive sampling also tended to give similar or slightly higher Cw than direct sampling. Since any suspended kaolinite particles remaining in water after centrifugation would increase the directly measured Cw, the current results suggest that centrifugation effectively removed kaolinite and that direct measurement of water was not significantly affected by residual kaolinite.