Feb 21, 1995 - AECL Research, Whiteshell Laboratories, Pinawa, Manitoba, ROE 1L0, ... AECL Research is studying options for the storage and eventual.
Arch. Environ. Contam. Toxicol. 29, 344-350 (1995)
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Environmental Contamination a n d Toxicology
Toxicity of Iodine, Iodide, and Iodate to Daphnia magna and Rainbow Trout
(Oncorhynchus mykiss) M. J. Laverock, M. Stephenson, C. R. Macdonald AECL Research, Whiteshell Laboratories, Pinawa, Manitoba, ROE 1L0, Canada Received: 29 September 1994/Revised:21 February 1995 Abstract. The acute toxicity (96-h LC5o) of aqueous stable iodine species (I-, IO3, I 2) to rainbow trout and Daphnia magna were measured at three individual concentrations of hardness, total organic carbon, and chloride. Rainbow trout were most sensitive to I2 (LCso >t 0.53 mg/L), and much less sensitive to I O - 3 (LC5o I> 220 mg/L) or I - (LCso/> 860 rag/ L). Daphnia magna were equally sensitive to I 2 (LCso/> 0.16 mg/L) and I - (LCso/> 0.17 rag/L), but were less sensitive to IO3 (LC5o/> 10.3 rag/L). The external and internal radiological dose imparted by equivalent molar quantities of radioactive 125I, 129I, and 131I were calculated for both the Daphnia and trout using the LC5o values obtained from a standard water treatment. As expected, the dose from 125I and 131I would exceed the expected lethal dose rate long before a chemically toxic level is reached. In contrast, a molar concentration of 129I likely to cause death by chemical toxicity would impart a radiological dose less than that expected to be lethal. Thus, for short-lived aquatic organisms, risks due to chemical toxicity of 129I may exceed risks due to its radioactive emissions.
AECL Research is studying options for the storage and eventual safe disposal of nuclear fuel waste. The Canadian concept for the disposal of nuclear fuel waste involves a vault, constructed at a depth of 500 to 1000 m in plutonic rock on the Canadian Precambrian Shield, to safely isolate hazardous radionuclides from the biosphere. Over a very long timespan, radionuclides may become dissolved in groundwater and transported by advective processes to the surface (Wuschke et al. 1981). The primary risk to humans and the environment resulting from radionuclides escaping from a vault may be attributable to 1291 (Davis et al. 1993). Iodine-129 is produced in irradiated fuel as a product of the fission of uranium (NCRP 1983). Iodine-129 could be released rapidly during the reprocessing of fuel (which is not under consideration in Canada), or over a much longer period of time from direct disposal of used fuel in a vault. With a half-life of
Correspondence to: M. J. Laverock
1.57 × 107 years, and a correspondingly low radioactive emission rate, it is not presently clear whether the risk from 1291to short-lived organisms, or those with rapid life cycles, would be attributable more to chemical than to radiological toxicity. For this reason, it is important to establish the chemical toxicity of stable iodine species to representative organisms to facilitate comparisons with the radiological dose that would be imparted by comparable molar quantities of 1291. Information is sparse regarding the acute or chronic toxicity of iodine to freshwater biota. The present study was designed to determine the toxicity, as the median lethal concentration or LCso, of elemental iodine (I2) , iodide (I-), and iodate (IO-~) to two representative aquatic organisms: rainbow trout (Oncorhynchus mykiss) and the cladoceran Daphnia magna. The toxicity of halides may be influenced by many factors, including the size and health of the test organism, temperature, and the chemistry of the water being tested (Pimentel and Bulkley 1983). In this study, we modified water hardness, chloride, and total organic carbon (TOC) concentrations to explore whether they increased or decreased the toxicity of the iodine species to the early life stages of rainbow trout (fry) and Daphnia magna (neonate).
Materials and Methods
Rainbow Trout Rainbow trout fry were obtained from certified disease-free commercial hatcheries, transported to our facility, and maintained at 6°C until needed. The fish were fed a maintenance ration of a commercial pelleted trout food (Martin Feed Mills Limited, Ontario). The water supply for the continuous flow-through holding tanks was obtained from the Winnipeg River (in eastern Manitoba, Canada), purified by sand filtration and ultraviolet light sterilization, and distributed through polyvinylchloride pipes. Randomly selected fish were acclimated at I5°C (-2°C) for a period of >/5 days prior to testing. Feeding was suspended 24-h prior to and during the test period. The 96-h static tests were aerated and conducted at 15°C (±2°C) in a temperature- and photoperiod- (16-h light, 8-h dark) controlled facility. All fish holding and testing protocols followed the Ontario Ministry of the Environment guidelines (Craig et al. 1983). A summary of the chemistry of the water
Toxicity of Iodine Species to Trout and D. magna
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Table 1. Summary of seasonal chemistries for culture waters a Milner Ridge Spring Na K Ca Mg Fe C1 NO 3 SO 4 Alkalinity (as CaCO3) Hardness (as CaCO3) pH Dissolved oxygen Conductivity (p~S/cm) Total inorganic carbon Total organic carbon
(Daphnia magna)
Winnipeg River (Rainbow Trout)
4.87 + 0.27 2.49-+ 0.25 32.7 ± 2.0 16.8 ± 0.4
