The Supelco solid phase extraction (SPE) C18 cartridges (Bellefonte,PA, USA) were first washed with 5ml of ethyl acetate and conditioned by 5 ml of methanol, then washed with 2 × 5 ml of ultra-pure water.The filtered groundwater samples (500 ml) were percolated through the cartridges with a flow rate of 5ml/min under vacuum pump.