Medically-derived I-131 as a tracer in aquatic environments

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Rose, Paula S.
The Graduate School, Stony Brook University: Stony Brook, NY.
&#182;Medically-derived <super>131</super>I (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 <super>131</super>I 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 &#177; 0.3 to 227 &#177; 2 Bq L<super>-1</super> in sewage effluent and 61 &#177; 12 to 2801 &#177; 32 Bq g<super>-1</super> in suspended solids > 0.7 &#924;m in the effluent. The primary source of <super>131</super>I is excreta from thyroid cancer inpatients treated at the Stony Brook University Medical Center (SBUMC). Time series measurements following known inputs indicated that <super>131</super>I 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 <super>131</super>I 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 <super>131</super>I are fairly continuous. &#182;The behavior of medically-derived <super>131</super>I 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 10<super>9</super> L d<super>-1</super> and has a maximum capacity of > 4 x 10<super>9</super> L d<super>-1</super>. Sewage effluent concentrations detected in samples collected from Blue Plains on different days ranged from 0.9 &#177; 0.1 to 8.1 &#177; 0.2 Bq L<super>-1</super>. Concentrations of <super>131</super>I detected in sewage effluent and in the river suggest a continuous discharge of the isotope from Blue Plains. Surface water <super>131</super>I ranged from 0.076 &#177; 0.006 to 6.07 &#177; 0.07 Bq L<super>-1</super>. Partitioning in sewage effluent and river water suggests that <super>131</super>I 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 &#177; 0.8 and 117 &#177; 2 Bq kg<super>-1</super> dry weight. The behavior of <super>131</super>I 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. &#182;Dissolved <super>131</super>I showed a strong, positive correlation with &#916;<super>15</super>N values of nitrate in the river. Surface water &#916;<super>15</super>NO<sub>3</sub> values ranged from 8.7 &#177; 0.3 to 33.4 &#177; 7.3&#8240; with dissolved inorganic nitrogen (NO<sub>3</sub> + NO<sub>2</sub>) concentrations between 0.38 &#177; 0.02 and 2.79 &#177; 0.13 mg N L<super>-1</super> (26 &#177; 1 and 186 &#177; 9 &#924;M). &#916;<super>15</super>N in sediments ranged from 4.7 &#177; 0.1 to 9.3 &#177; 0.1&#8240;. Sediment profiles of particulate <super>131</super>I and &#916;<super>15</super>N 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 &#177; 0.3 to 80.0 &#177; 0.3 Bq kg<super>-1</super> dry weight. Values of &#916;<super>15</super>N in sediments ranged from 4.7 &#177; 0.1&#8240; to 9.3 &#177; 0.1&#8240;. This work introduces <super>131</super>I 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. &#182;The presence of medically-derived <super>131</super>I has been documented in several aquatic environments. Continuous discharges of this radioisotope in sewage effluent are likely to be widespread. Further study of <super>131</super>I in receiving waters can provide valuable insight into the fate and transport of this radioisotope in the context of large scale accidental releases.