Medically-derived iodine-131 as a tracer in aquatic environments.

摘要

Medically-derived 131I (half-life = 8.04 d) has been measured in aquatic environments receiving sewage effluent discharges, yet few published data exist for the radioisotope in sewage effluent; most work has focused on sewage sludge. This work presents 131I concentrations detected in sewage effluent from a small water pollution control plant (WPCP) serving a regional thyroid cancer treatment facility in Stony Brook, NY, USA. The concentrations detected in the Stony Brook WPCP ranged from 1.8 +/- 0.3 to 227 +/- 2 Bq L-1 in sewage effluent and 61 +/- 12 to 2801 +/- 32 Bq g-1 in suspended solids > 0.7 mum in the effluent. The primary source of 131I is excreta from thyroid cancer inpatients treated at the Stony Brook University Medical Center (SBUMC). Time series measurements following known inputs indicated that 131I is discharged for many days following an inpatient treatment. The sewage half-life, analogous to a radioactive half-life, describes the time it takes for half of a wastewater component to be removed from a WPCP. The sewage half-life of 131I in this plant is 2.0 d. Due to the frequency of patients treated at the SBUMC and retention in the plant, sewage effluent discharges of 131I are fairly continuous.;The behavior of medically-derived 131I was investigated in the tidal Potomac River in the vicinity of Blue Plains, the world's largest advanced wastewater treatment plant. This plant serves all of Washington, DC, treats an average of 1.4 x 109 L d-1 and has a maximum capacity of > 4 x 109 L d-1 . Sewage effluent concentrations detected in samples collected from Blue Plains on different days ranged from 0.9 +/- 0.1 to 8.1 +/- 0.2 Bq L-1. Concentrations of 131I detected in sewage effluent and in the river suggest a continuous discharge of the isotope from Blue Plains. Surface water 131I ranged from 0.076 +/- 0.006 to 6.07 +/- 0.07 Bq L--1. Partitioning in sewage effluent and river water suggests that 131I is associated with colloidal and particulate organic material. Iodine-131 was detected in sediments to depths of 5 cm with specific activities between 1.3 +/- 0.8 and 117 +/- 2 Bq kg--1 dry weight. The behavior of 131I in the Potomac River is consistent with the cycling of natural iodine in aquatic environments. It is discharged to the river via sewage effluent, incorporated into particulate matter and deposited in sediments where it is subject to diagenetic remineralization.;Dissolved 131I showed a strong, positive correlation with delta 15N values of nitrate in the river. Surface water delta 15NO3- values ranged from 8.7 +/- 0.3 to 33.4 +/- 7.3‰ with dissolved inorganic nitrogen (NO 3- + NO2-) concentrations between 0.38 +/- 0.02 and 2.79 +/- 0.13 mg N L--1 (26 +/- 1 and 186 +/- 9 muM). delta15N in sediments ranged from 4.7 +/- 0.1 to 9.3 +/- 0.1‰. Sediment profiles of particulate 131I and delta15N indicate rapid mixing or sedimentation and in many cases remineralization of a heavy nitrogen source consistent with wastewater nitrogen. Iodine-131 concentrations in sediments ranged from 2.8 +/- 0.3 to 80.0 +/- 0.3 Bq kg --1 dry weight. Values of delta15N in sediments ranged from 4.7 +/- 0.1‰ to 9.3 +/- 0.1‰. This work introduces 131I as a potentially valuable tool to study the short-term fate of wastewater nitrogen in this system, but its utility as a tracer is not limited to use in the Potomac River.;The presence of medically-derived 131I has been documented in several aquatic environments. Continuous discharges of this radioisotope in sewage effluent are likely to be widespread. Further study of 131I in receiving waters can provide valuable insight into the fate and transport of this radioisotope in the context of large scale accidental releases.