All test chambers containing the sediment/water systems were fortified with the test substance at the start of the test by applying approximately 100 íL of the radiolabeled dosing solution the water layer to achieve a nominal test concentration ranging from 0.04 to 0.5 mg/L. Test concentrations were targeted for the low íg/L range while maintaining sufficient radioactivity for analysis of biotransformation products at 10% parent in the water and sediment phases. The radiopurities of the radiolabel test materials for the 3 pharmaceuticals and the specific activities of each are shown in Table 1. Following the addition of the test material to each test chamber, the water layer was gently mixed, making sure to minimize disturbance of the underlying sediment.