Medical radioisotopes in the environment - following the pathway from patient to river sediment.

Motivated by the detection of (131)I in river sediment in routine long-term surveillance samples, a systematic short-term study of the wastewater treatment chain was planned and conducted. Inflow, effluent and primary sludge were collected on a daily basis during two weeks at a regional wastewater treatment plant. Samples were investigated by gamma spectroscopy. Four medically used isotopes could be identified ((131)I and (99m)Tc regularly, (153)Sm and (123)I sporadically). The concentration levels coincide well with literature data for (131)I, and with our own long-term data for (131)I and (99m)Tc for the same plant. Cosmogenic (7)Be activity in primary sludge correlated well with rainfall intensity. Surface sediment was sampled at low tide at both shores of the river, up- and downstream of the plant. (131)I was identified in all samples, with a sharp maximum (about 100 Bq kg(-1)d.m.) at the discharge point of the plant and lower levels elsewhere, decreasing monotonically in downstream direction. (7)Be and (137)Cs showed the same behaviour, but no peak at the discharge point. Predictions from simple equilibrium models for the transport and sedimentation of (131)I show good agreement with the experimental data and suggest that the wastewater treatment plant is the main source for this isotope.