Iodine-131 levels in sludge and treated municipal wastewaters near a large medical complex

Iodine-131 was found to dominate the gamma spectra of dried sludge and concentrated liquid effluent samples from a sewage treatment plant serving the area containing the Texas Medical Center in 1975. The concentration of Iodine-131 varied considerably on a time scale of weeks but was fairly stable in the course of a single day. The partition of Iodine-131 between the dried sludge and the liquid effluent was found to be reasonably constant during the sampling period. Knowledge of the concentration in these phases and the daily output of sludge and effluent led to the estimate of an annual discharge of over one Curie of Iodine-131. Circumstantial evidence linking this discharge to the practice of nuclear medicine in the area is considered strong, but an attempt to correlate variation in the effluent activity with actual nuclide use was inconclusive due, among other reasons, to an insufficient clinical data base. An analysis of possible exposure pathways indicated that negligible human exposure resulted from the Iodine-131 released primarily because the receiving body of water is a saline industrial waterway. A more extensive analysis would be in order for a similar plant discharging treated wastewaters into an inland water system.