After each period of exposure time (3, 4 and 5 days) three traysper substratum type were randomly selected, the 10 animals ofeach tray (3 trays per treatment and exposure time) were returnedto a glass vessel with 125 ml USEPA water to allow re-hydration.The number of dead animals was monitored in each vessel after 4and 7 days of re-hydration. Animals were considered to be dead ifno reaction (i.e. no operculum movement or no movement of softbody) was observed after touching the snail with forceps.