Abstract

MALTA, Margarida; OLIVEIRA, João M.; SILVA, Lídia  and  CARVALHO, Fernando P.. Radioactivity from Lisboa urban wastewater discharges in the Tejo River Estuary. RGCI [online]. 2013, vol.13, n.4, pp.399-408. ISSN 1646-8872.  https://doi.org/10.5894/rgci339.

Sediments, water, mussels and fish were collected monthly during a 19 month period at eight sampling stations along the North bank of the Tejo estuary (Lisboa, Portugal). Fresh samples were immediately analysed for short lived gamma emitting radionuclides. Amongst these, ¹³¹I and ^99mTc were detected in most samples of mussels (Mytilus galloprovincialis) and mullets (Liza ramada). Spatial distribution of samples containing these radionuclides consistently indicated contamination at Alcântara and Praça do Comércio, and, on occasion, at Chelas, all sites inside the estuary. Round-the-year, generally there was no contamination at Algés, near the estuary mouth, and Beirolas, upstream by the Vasco da Gama Bridge. The estuarine areas found contaminated by radionuclides correspond to the areas receiving urban wastewater discharges from the city of Lisboa. The artificial radionuclides ¹³¹I and ^99mTc are used in nuclear medicine practices and originate from hospital liquid effluent discharges into the city sewage system. Their presence near Praça do Comércio was related to the nearby discharges of a main sewer discharging untreated urban wastewater. Near Alcântara, the sewer outlet discharged treated urban wastewater from the Alcântara wastewater treatment plant and the presence of short-lived radionuclides indicated that sewage treatment and the time delay introduced before discharge of the treated effluent were not sufficient to allow for radioactive decay of those radionuclides. The highest concentration values of ¹³¹I and ^99mTc determined in mussels and fish were 12±2 and 136±20 Bq Kg^-1 wet weight (w.w.), respectively. These concentrations did not exceed the concentrations of naturally-occurring radionuclides, such as ⁴⁰K that averaged 66±13 Bq Kg^-1 (w.w.) in mussels and 618±21 Bq Kg^-1 (w.w.) in fish. Nevertheless, artificial radionuclides accumulated in biota increase the radiation dose received by organisms from natural radiation sources and may also increase the risk of radionuclide transfer to humans. Reduction of radioactivity in effluent discharges towards improved estuarine water and sediment quality is debated as a requirement for sustainable use of the estuary.

Keywords : Technetium-99m; Iodine-131; Cesium-137; Natural radionuclides; Radioactive contamination; Urban wastewater.

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