Uranium
(updated 2022)
Guideline
Based on health considerations, the concentration of uranium in drinking water should not exceed the health-based guideline value of 0.02 mg/L. This health-based guideline value is based on chemical toxicity. The chemical toxicity of uranium is more restrictive than its radiological toxicity.
General description
Uranium may be present in the environment as a result of leaching from soils, rocks and natural deposits. It can be released from mining and mill tailings, from the combustion of coal and other fuels, and from the production or use of phosphate fertilizers (which can contain as much as 150 mg/kg uranium). Naturally occurring uranium is a mixture of three radionuclides: uranium-238, uranium-234 and uranium-235. Uranium-238 and uranium-234 decay predominantly by alpha particle emission, whereas uranium-235 emits both gamma rays and alpha particles. Natural uranium consists almost entirely of the uranium-238 isotope. The other two isotopes are less than 1% abundant.
Studies overseas have reported uranium concentrations in drinking water of generally less than 0.001 mg/L; however, concentrations as high as 0.7 mg/L have been reported in some private water supplies in Canada (Moss et al. 1983).
Food is the major source of uranium intake and the highest concentrations are typically found in shellfish (UNSCEAR 2000). Dietary intake of uranium through food is estimated between 0.001 and 0.004 mg/day (WHO 2004). Intake through drinking water is normally low; however, drinking water can contribute the majority of daily intake in circumstances where uranium is present at higher concentrations in drinking water (WHO 2004).
Typical values in Australian drinking water
In most Australian drinking water supplies uranium concentrations are well below 0.02 mg /L. However, concentrations up to 0.12 mg/L have been recorded in some groundwater supplies in remote areas.
Studies of Australian drinking water supplies have shown that generally uranium levels are below 50 mBq/L (0.004 mg/L). However, for some groundwater samples levels may be close to 1 Bq/L (0.08 mg/L) (ARPANSA 2008, Kleinschmidt et al. 2011, Walsh et al. 2014).
Treatment of drinking water
Conventional treatment processes are not effective in removing uranium from water supplies. Some laboratory or pilot scale studies have found that coagulation using ferric sulfate at optimal pH dosages can achieve 80–95% removal of uranium, whereas at least 99% removal can be achieved using lime softening, anion exchange resin or reverse osmosis processes (WHO 2004).
Measurement
The concentration of uranium in water can be determined using solid fluorimetry with laser excitation (Blanchard et al. 1985), or inductively coupled plasma mass spectrometry (Boomer et al. 1987). The limit of determination is about 0.0001 mg/L.
The isotopes of uranium can be determined by radiochemical techniques using high resolution alpha spectrometry to measure their activity (APHA 2017, ISO 2014). The limit of determination is approximately 0.5 mBq/L (equivalent to approximately 0.00004 mg/L uranium).
Health considerations
The toxicity of uranium has been reviewed by the World Health Organization (WHO 2004), the Swedish National Food Administration (Svensson et al. 2005), the United Kingdom Committee on Toxicity (COT 2006) and Health Canada (Health Canada 2001).
Average absorption of dietary uranium by the gastrointestinal tract is 1-2%, but may be as low as 0.1% or as high as 5-6% depending on the solubility of the uranium compound ingested. Uranium rapidly appears in the bloodstream and is primarily associated with red blood cells. Uranyl compounds readily combine with proteins and nucleotides to form stable complexes. Clearance of uranium from the blood is rapid but it accumulates in the kidney and bone, with little in the liver. Once equilibrium in the skeleton has occurred, uranium is excreted in the urine and faeces. The half-life of uranium in rat and rabbit kidney is of the order of 5–15 days, but in bone it is 100–300 days (Health Canada 2001).
In humans and experimental animals, the main toxic effect of short-term exposure to high concentrations of uranium is inflammation of the kidney. Little information is available on the effects of long-term exposure to low concentrations. Epidemiological studies report increases in urinary markers of possible kidney proximal tubule damage at drinking water concentrations between 0.1 and 1 mg/L, but not at lower concentrations (Moss et al. 1983, Mao et al. 1995, Zamora et al. 1998, Kurttio et al. 2002, Kurttio et al. 2006, Seldén et al. 2009, Magdo et al. 2007).
A tolerable daily intake (TDI) of 0.0006 mg per kg bodyweight (mg/kg bw) has been derived by WHO (2004) and Health Canada (2001). This is based on a lowest-observed-adverse-effect level (LOAEL) of between 0.06 (males) and 0.09 (females) mg/kg bw/day in a 91-day rat drinking water study (Gilman et al. 1998) and application of an uncertainty factor of 100 (10 for interspecies extrapolation and 10 for intraspecies variation). The critical effect was degenerative kidney lesions, noted by the authors as not being severe. Although these represented a clear adverse effect, they were not dose-related. In addition, because the effects were minimal, it is considered that the dose at which they occurred may be close to the no-observed-adverse-effect level (NOAEL) (WHO 2004, COT 2006, Health Canada 2001). Thus an uncertainty factor to extrapolate from a LOAEL to a NOAEL was not applied in the derivation of the TDI.
No data are available on chemically induced mutagenic effects in relation to uranium.
Studies have shown high specific activity uranium isotopes to be carcinogenic in animals, causing malignant tumours in mice and bone sarcomas in rats. Similar studies using natural uranium (uranium-238) have not shown similar effects, possibly due to the lower radiation doses involved. Epidemiological data are inadequate to show whether exposure to uranium in drinking water will lead to an increased risk of cancer.
Derivation of guideline
i) From chemical toxicity data:
The health-based guideline value for uranium in drinking water of 0.02 mg/L was set from chemical toxicity data as follows:
where:
0.0006 mg/kg bodyweight/day is the TDI (WHO 2004, Health Canada 2001).
70 kg is taken as the average weight of an adult.
0.8 is a proportionality factor based on a conservative assumption that 80% of total daily intake may be attributable to the consumption of water (WHO 2004).
2 L/day is the estimated maximum amount of water consumed by an adult.
The calculated value of 0.0168 mg/L is rounded to a final health-based guideline value of 0.02 mg/L as per the rounding conventions described in Chapter 6. The difference is not significant.
ii) From radiological data:
where:
0.1 mSv/year is one tenth of the reference level for dose of 1 mSv/year.
730 L/year is the estimated maximum amount of water consumed by an adult (2 L/day x 365 days).
4.5 x mSv/Bq is the committed effective dose received per unit intake of uranium-238 activity (Bq) (ICRP 2012).
iii) Comparing the chemical and radiological data:
The health-based guideline value of 0.02 mg/L is equivalent to an activity concentration of 0.2 Bq/L. This indicates that the health-based guideline value based on chemical toxicity is considerably more restrictive than one based on radiological data. If uranium needs to be considered with regard to radiological dose, refer to Chapter 7, Section 7.6.2 for information on the calculation of dose.
References
APHA, AWWA, WEF (American Public Health Association, American Water Works Association, Water Environment Federation) (2017). 7500-U C Isotopic Method.
ARPANSA (Australian Radiation Protection and Nuclear Safety Agency) (2008). The Radioactive Content of Some Australian Drinking Waters. Technical Report Series No 148.
Blanchard RL, Hahne RMA, Kahn B, McCurdy D, Mellor RA, Moore WS, Sedlet J, Whittaker E (1985). Radiological sampling and analytical methods for national primary drinking water regulations. Health Physics, 48:587–600.
Boomer DW, Powell MJ (1987). Determination of uranium in environmental samples using inductively coupled plasma mass spectrometry. Analytical Chemistry, 59:2810–2813.
COT (UK Committee on Toxicity of Chemicals in Food, Consumer Products and the Environment) (2006). COT Statement on uranium levels in water used to reconstitute infant formula. COT statement 2006/07, May 2006.
Gilman AP, Villeuve DC, Secours VE, Yagminas AP, Tracy BL, Quinn JM, Valli VE, Willes RJ, Moss MA (1998). Uranyl Nitrate: 28-Day and 91-Day Toxicity Studies in the Sprague-Dawley Rat. Toxicological Sciences, 41(1):117-128.
Health Canada (2001). Guidelines for Canadian Drinking Water Quality: Supporting Documentation. Uranium.
ICRP (International Commission on Radiation Protection) (2012). Compendium of Dose Coefficients based on ICRP Publication 60. ICRP Publication 119. Anna. ICRP 41 (Suppl.).
ISO (International Organization for Standardization) (2014). Water quality – Uranium Isotopes-Test method using alpha spectrometry. ISO, International Standard ISO 13166:2014, Geneva, Switzerland.
Kleinschmidt R, Black J, Akber R (2011). Mapping radioactivity in groundwater to identify elevated exposure in remote and rural communities. Journal of Environmental Radioactivity, 102:235-243.
Kurrtio P, Harmoinen A, Saha H, Salonen L, Karpas Z, Komulainen H, Auvinen A (2006). Kidney toxicity of ingested uranium from drinking water. American Journal of Kidney Diseases, 47(6): 972-982.
Kurttio P, Auvinen A, Salonen L, Saha H, Pekkanen J, Makelainen I, Vaisanen SB, Penttila IM, Komulainen H (2002). Renal effects of uranium in drinking water. Environmental Health Perspectives, 110(4): 337-342.
Magdo HS, Forman J, Graber N, Newman B, Klein K, Satlin L, Amler RW, Winston JA, Landrigan PJ (2007). Grand rounds: nephrotoxicity in a young child exposed to uranium from contaminated well water. Environmental Health Perspectives, 115:1237-1241.
Mao Y, Desmeules M, Schaubel D, Berube D, Dyck R, Brule D, Thomas B (1995). Inorganic components of drinking water and microalbuminuria. Environmental Research, 71(2): 135-140.
Moss MA, McCurdy RF, Dooley KC, Givner ML, Dymond LC, Slayter JM, Courneya MM (1983). Uranium in drinking water – report on clinical studies in Nova Scotia. In: Brown SS, Savory J (eds), Chemical Toxicology and Clinical Chemistry of Metals, Academic Press, London, pp 149-152.
Seldén AI, Lundholm C, Edlund B, Högdahl C, Ek B-M, Bergström BE, Ohlson C-G (2009). Nephrotoxicity of uranium in drinking water from private drilled wells. Environmental Research, 109(4): 486-494.
Svensson K, Darnerud PO, Skerfving S (2005). A risk assessment of uranium in drinking water, National Food Administration of Sweden, Sweden.
UNSCEAR (United Nations Scientific Committee on the Effects of Atomic Radiation) (2000). 2000 Report Vol. I. Sources, effects and risks of ionising radiation. Report to the General Assembly, with Scientific Annexes.
Walsh M, Wallner G, Jennings P (2014). Radioactivity in drinking water supplies in Western Australia. Journal of Environmental Radioactivity, 130:56-62.
WHO (World Health Organization) (2004). Uranium in drinking-water: Background document for development of WHO Guidelines for Drinking-water Quality. Geneva, Switzerland.
Zamora ML, Tracy BL, Zielinski JM, Meyerhof DP, Moss MA (1998). Chronic ingestion of uranium in drinking water: A study of kidney bioeffects in humans. Toxicological Sciences, 43(1): 68-77.
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