Page 5: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Radiological Parameters
Part II: Science and Technical Considerations
A brief summary of analytical methods is given below for the radionuclides most commonly encountered in Canadian drinking water supplies and for which MACs are established. U.S. EPA-approved methods are listed in this document. There are also a number of U.S. EPA-approved analytical methods that were not developed by the U.S. EPA but are listed on their website (U.S. EPA, 2008). Where relevant, methods used or developed by Health Canada are also noted, although these are generally used in a research context.
Analytical methods for 210Pb generally involve an initial purification of the lead by precipitation (Chiu and Dean, 1986). After allowing sufficient time for ingrowth of 5-day 210Bi, the bismuth is isolated by solvent extraction and subsequently precipitated (e.g., as bismuth oxychloride). The precipitate is collected on filter paper, and the high-energy beta particles are detected in a low-background gas-flow proportional counting system (Chiu and Dean, 1986). Detection limits of 0.005-0.02 Bq/L are routinely achievable for a 1-L water sample.
Since 210Pb is a decay product of 222Rn, the presence of dissolved radon in groundwater can alter the measurement of 210Pb, leading to an artificially high value. This is likely to occur if the sample has been allowed to sit in a sealed airtight container or where high concentrations of radon are present. When groundwater sources are being monitored for radionuclides, either the 210Pb should be extracted from freshly collected samples or dissolved radon should be removed before significant decay to 210Pb has occurred. According to the widely used Pylon technique (Pylon, 1989), detection of radon in drinking water is performed using a water degassing unit and Lucas scintillation chambers. Dissolved radon can be removed either by boiling the sample or by aerating the sample using a blender. Water that has been left to stand will have reduced radon activity.
No U.S. EPA-approved analytical method was identified for 210Pb.
The conventional method for detecting 226Ra is through co-precipitation with barium sulphate followed by alpha counting on a gas-flow proportional counter or alpha spectrometer (Chiu and Dean, 1986). A routinely achievable detection limit is 0.005 Bq/L.
A more rapid procedure has been developed at Health Canada that involves multiple ion exchange column separations followed by liquid scintillation counting of the ingrowth of 222Rn. This gives a detection limit of 0.002 Bq/L.
The U.S. EPA-approved analytical methods for 226Ra include EPA Methods 903.0 and Ra-03 using radiochemical methodology, and EPA Methods 903.1 and Ra-04 using the radon emanation technique. The U.S. EPA-approved analytical methods for 228Ra include EPA Methods 904.0 and Ra-05 using radiochemical methodology (U.S. EPA, 2008).
The traditional method for analysing total uranium in water involves fusion of the uranium with a sodium fluoride pellet. The uranium is then excited by ultraviolet light, and the resulting fluorescence is measured by a photomultiplier. A detection limit of 0.1 µg/L has been achieved by Health Canada laboratories using this method (Health Canada, 2000a). In an alternative procedure, uranium in water is complexed with phosphoric acid (Bring and Miller, 1992). The uranyl complex will then phosphoresce after being excited by laser light, and a measurement of the light output gives the concentration of uranium in the sample. The detection limit routinely achievable by this method is 0.5 µg/L.
If the isotopic composition of the uranium is required, then the methods of alpha spectrometry (EML, 1983) and inductively coupled plasma mass spectrometry (Igarashi et al., 1989) are available.
The U.S. EPA-approved analytical methods for uranium include EPA Method 908.0 using radiochemical methodology, EPA Method 908.1 using fluorometric methodology and EPA Method 00-07 using alpha spectrometry (U.S. EPA, 2008).
Tritium decays by the emission of a low-energy (18.6 keV) beta particle, with no associated gamma rays or X-rays. The best method to measure tritium in water is liquid scintillation counting, which avoids self-absorption losses of the beta radiation. A detection limit of about 10 Bq/L is routinely achievable at Health Canada laboratories (Health Canada, 2000a).
U.S. EPA-approved analytical methods for tritium include EPA Method 906.0 using liquid scintillation (U.S. EPA, 2008).
Strontium-90 decays by pure beta emission with a maximum energy of 546 keV. Since there is no associated gamma ray for easy identification, it is usually necessary to chemically separate the strontium before measuring the radiation with a beta detector. The conventional method employed by Health Canada (Department of National Health and Welfare, 1977) involves carbonate precipitation of the alkaline earth group, followed by dissolution in nitric acid, removal of the barium and radium by chromate precipitation, and final precipitation of the strontium as a carbonate. The strontium beta particles are counted with a gas-flow proportional counter. A typical detection limit is 0.001 Bq/L.
Recently, a more rapid method has been developed at Health Canada to allow processing of large numbers of samples during a nuclear emergency (Larivière et al., 2009). This procedure involves separation of strontium from multiple water samples on a cation exchange manifold followed by cleanup with high-pressure ion exchange chromatography. The strontium beta particles are counted either immediately by liquid scintillation or later with a Cerenkov counter. The liquid scintillation method is more rapid but gives a detection limit of only 0.2 Bq in a 1-L water sample. The off-line method must allow for the ingrowth of 2.7-day 90Y and gives a detection limit of 0.02 Bq/L.
The U.S. EPA-approved analytical methods for both 89Sr and 90Sr include EPA Method 905.0 using radiochemical methodology (U.S. EPA, 2008).
Radioactive isotopes of iodine from 131 through 135 are easily detected and measured by a gamma spectrometry system. Detection limits are comparable to those achievable for radiocesium.
The U.S. EPA-approved analytical methods for radioiodine include EPA Method 902.0 using radiochemical methodology and EPA Method 901.1 using gamma ray spectrometry (U.S. EPA, 2008).
Cesium-137 can be readily detected by a gamma spectrometry system through its characteristic 661.6-keV gamma ray (ASTM, 2006). A detection limit of 0.001 Bq/L can be achieved at Health Canada by evaporating down 60 L of water before counting.
The U.S. EPA-approved analytical methods for cesium are EPA Method 901.0 using radiochemical methodology and EPA Method 901.1 using gamma ray spectrometry (U.S. EPA, 2008).
Two methods are approved and one alternative method is recommended by the U.S. EPA (1999) for the measurement of radon in drinking water, all of which use scintillation counting. The approved methods are Standard Method 7500-Rn B - Liquid Scintillation (APHA et al., 2005) and the EPA de-emanation method (U.S. EPA, 1987). The alternative method recommended is ASTM Method D-5072-92 (ASTM, 1998). All methods require careful sampling because of the rapid loss of radon from the water when it is agitated and opened to the atmosphere.
In Standard Method 7500-Rn B, the water is injected directly into a scintillation solution and counted in an automated liquid scintillation device; a minimum detectable concentration of 18 pCi (0.67 Bq/L) is listed (APHA et al., 2005). In the de-emanation method, radon is degassed from the water and transferred into a Lucas scintillation cell, with a detection limit of approximately 0.05 Bq/L (Crawford-Brown, 1989). Radon in drinking water can be measured at concentrations above 0.04 Bq/L by ASTM Method D-5072-92 (ASTM, 1998).
Analysing drinking water for gross alpha and gross beta activities (excluding radon) may be done by evaporating a known volume of the sample until dry and then measuring the activity of the residue. Since alpha radiation is easily absorbed within a thin layer of solid material, the reliability and sensitivity of the method for alpha determination may be reduced in samples with a high total dissolved solids (TDS) content.
Where possible, standardized methods should be used to determine concentrations of gross alpha and gross beta activities. Standardized methods for evaporation include ISO 9696:2007 for gross alpha determination (ISO, 2007) and ISO 9697:2008 for gross beta determination (ISO, 2008). Determining gross beta activity using the evaporation method must take into account the contribution from 40K. An additional analysis of total potassium should be done if the gross beta screening value is exceeded. The evaporation method is used for groundwater with a TDS content greater than 0.1 g/L. The detection limit for this method ranges from 0.02 to 0.1 Bq/L.
Another standardized method, the co-precipitation technique (APHA et al., 1998), excludes the contribution from 40K and therefore does not require the determination of total potassium. This method cannot be used in assessing water samples containing certain fission products, such as 137Cs; however, concentrations of fission products in drinking-water supplies generally are extremely low. TDS is not a concern with this method. The detection limit of this method is 0.02 Bq/L (APHA et al., 1998).
Although screening for gross alpha and gross beta activities reduces the number of costly analyses for specific radionuclides, it is a measurement tool that has a number of drawbacks. Some of these drawbacks include false-positive detections, particularly in the case of gross alpha measurements when dissolved radon is present. Concentrations of tens of becquerels per litre are not uncommon; however, in most of these cases, detailed analyses show no radionuclides to be in excess of their MACs. False negatives may also occur if there is a large amount of TDS in the water sample. When the sample is evaporated to dryness, self-absorption of the particles may lead to a significant loss in count rate. Laboratories that carry out gross measurements routinely report wide fluctuations in count rates, even for samples taken from the same source. If detectors used for gross measurements are operated in the alpha-and-beta mode to allow simultaneous detection, this can lead to crosstalk or spillover between alpha and beta channels and increase the analytical errors in an unpredictable manner. Despite these drawbacks, gross alpha and gross beta screening is useful in detecting changes in a drinking water supply whose composition has been well characterized by previous radionuclide measurements. In order to reduce costly and repetitive analyses for specific radionuclides, an agency may wish to define its own operational screening levels based on gross radioactivity measurements.
If a drinking water sample exceeds the screening value for gross alpha activity (0.5 Bq/L) or gross beta activity (1 Bq/L), it is recommended that the analysis be repeated to check the validity of the result. If the initial result is confirmed, then the sample should be analysed for specific radionuclides whose presence might be suspected, based on the type and location of the drinking water supply. For example, a ground water supply, far removed from any nuclear facility, is not likely to contain any artificial radionuclides. It should be analyzed initially for the naturally-occurring radionuclides 210Pb, 226Ra and total uranium. If these radionuclides are not present in sufficient abundance to explain the gross alpha or beta count, then analyses should be extended to additional members of the natural decay series, particularly 228Ra and dissolved 222Rn. The later is not considered to be a drinking water hazard, but its presence may explain a high gross alpha or beta count, and should prompt an analysis for indoor air levels.
On the other hand, if the sample is taken from a surface water supply located near a nuclear facility, then it should be analyzed for radionuclides that might be suspected from that facility. For a nuclear reactor, the analyses should begin with the four artificial radionuclides - tritium, 90Sr, 131I, and 137Cs - most likely to be found in emissions from a reactor. If their abundances are not sufficient to explain the gross particle count, then the presence of other fission products should be considered. In most cases this can be done quite simply, without the need for further analyses. If a gamma ray measurement was carried out with sufficient sensitivity to detect 131I and 137Cs at the beta screening level of 1 Bq/L, then any other gamma-emitting fission products present in the sample will also show up at this level.
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