Radiological Analysis of Ground Water Jamal M. Sharaf

Main objectives Examination of concentration of natural radionuclides in ground water; Assessment of dose rate for each age group, related to water intake.

Radiation sources Naturally occurring radionuclides-uranium and thorium decay series. Manmade sources-fission products from manmade nuclear reactions Both sources emit the three basic types of radiation: Alpha particles - positively charged helium atoms, Beta particles - negatively charged electrons, Gamma rays - high-energy electromagnetic waves.

Radiation Dosimetry Measurements of ionization and energy absorption are the basis for radiation dosimetry. The absorbed dose (D):-the energy absorbed per unit mass , J kg–1, which is called the gray (Gy). The equivalent dose (H):-To allow for the different biological effectiveness of different kinds of radiation, H= QD. sievert (Sv). The effective dose (E):- different tissues of the body respond differently to radiation, E = ∑ wTHT sievert (Sv).

Cont. The committed effective dose [E(τ)]:- A measure of the total effective dose received over a lifetime (τ=70 years) following intake of a radionuclide (internal exposure): E(τ ) = ∑ wTHT(τ ). In the case of water, activity concentration is given in becquerels per litre (Bq/litre). This value can be related to an effective dose per year (mSv/year) using a dose coefficient (mSv/Bq) and the average annual consumption of water (litres/year).

WHO guidance levels for radionuclides in drinking-water The WHO guidance levels for radionuclides in drinking-water were calculated using the following equation: GL = guidance level of radionuclide in drinking-water (Bq/litre), IDC = individual dose criterion, equal to 0.1 mSv/year hing = dose coefficient for ingestion by adults (mSv/Bq), q = annual ingested volume of drinking-water, assumed to be 730 litres/year.

Cont. The recommended reference dose level (RDL) of the committed effective dose of 0.1 mSv/year is also equal to 10% of the dose limit for members of the population, recommended by both the ICRP (1991) and the International Basic Safety Standards (IAEA, 1996). The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR, 2000) has estimated that the global average annual human exposure from natural sources is 2.4mSv/year and therefore, no harmful radiological health effects are expected from consumption of drinking-water if the concentrations of radionuclides are below the guidance levels (equivalent to a committed effective dose below 0.1mSv/year).

Assessment of dissolved radionuclides in ground water The methodology described by the WHO (1993) for controlling radiological hazards in water involves the following steps: initial screening for gross alpha and/or beta activity to determine whether the activity concentrations are below levels at which no further action is required; and if these screening levels are exceeded, investigation of the concentrations of individual radionuclides should be determined and compared with the guidance levels. The outcome of this further evaluation may indicate that no action is required before a decision can be made on the need for measures to reduce the dose.

Cont. Screening procedure is a practical approach which maximizes both the reliability and the cost effectiveness of assessing the radionuclide content of water. The recommended screening levels for gross alpha and gross beta activity are 100 and 1,000 mBq/l, respectively. If either of the screening levels is exceeded, then the specific radionuclides should be identified and their individual activity concentrations measured. Where the sum exceeds unity for a single sample, the IDC of 0.1 mSv/year would be exceeded only if the exposure were to continue for a full year. Hence, such a result does not in itself imply that the water is unsuitable for consumption.

Measuring gross alpha and gross beta activity concentrations Methods for the analysis of gross alpha and gross beta activities in water Method Technique Detection limit Application International Organization for Standardization: Evaporation 0.02–0.1 Bq/l Groundwater with TDS less than 0.1 g/l American Public Health Association (APHA et al., 2005) Co-precipitation 0.02 Bq/l Surface water and groundwater (TDS is not a factor) United States Environmental Protection Agency (EPA), Method 900 0.1 Bq/l Monitoring drinking water supplies

Sample of analytical techniques used for radionuclide assessment Measurements Analytical Techniques Method Reference Minimum Detectable Activities Ra-226 Gamma spectroscopy from activities of short-lived radon progenies (Pb-214 and Bi-214) In-house method 50 mBq/l Po-210 Evaporation and electrodeposition onto silver disc followed by alpha spectrometry ISO standard 13161, <0.5 mBq/l Total Uranium U-234, 235 & 238 Inductively Coupled Plasma-Mass Spectrometry CP-MS ---

Radiological data used to calculate indicative doses for water Contribution of naturally occurring radionuclides to gross alpha activity Radiological data used to calculate indicative doses for water Radionuclide Ingested Dose Coefficient for Adults (Sv/Bq) Activity per radionuclide (mBq/l) equivalent to 0.1 mSv Po-210 1.2 x 10-6 117 Ra-226 2.8 x 10-7 500 Thorium-2 2.3 x 10-7 600 Uranium-234 4.9 x 10-8 3000 Uranium-235 4.7 x 10-8 Uranium-238 4.5 x 10-8

Cont. Uranium typically contributes significantly to gross alpha activity in water, despite its low ingestion dose coefficient. Calculated uranium activity concentrations could then be subtracted from the gross alpha activity concentration. Where the residual alpha activity was less than 100 mBq/l, the water source is deemed to be in compliance with the WHO guidelines (WHO, 1993). If the gross alpha activity minus the contribution from uranium is still approximately 100 mBq/l or greater, then the individual concentrations of Ra-226 and Po-210 should be determined and the dose arising from each component assessed.

Quality Assurance and Results A strong emphasis should be placed on quality assurance and reliability of data. Best laboratory practice should be ensured by accreditation of test procedures, through national accreditation to International standard, e.g. ISO. Analytical techniques should be validated both through exchange of samples with other laboratories and through analysis of certified reference materials for proficiency testing.

Health risk assessment Man-made radionuclides are often controllable at the point at which they enter the ground water . Naturally occurring radionuclides, in contrast, can potentially enter the water at any point, or at several points. For this reason, naturally occurring radionuclides in ground water are often less amenable to control. The global average annual dose per person from all sources of radiation in the environment is approximately 3.0 mSv/year: 80% (2.4 mSv) - due to naturally occurring sources of radiation, 19.6% (almost 0.6 mSv) - due to the use of radiation in medicine 0.4% (around 0.01 mSv) is due to other sources of man-made radiation (UNSCEAR, 2008).

Cont. Evidence of an increased cancer risk in humans is available at doses above 100 mSv. Below this dose, an increased risk has not been identified through many studies. The nominal risk coefficient for radiation-induced cancer incidence is 5.5 × 10−2/Sv. (ICRP, 2008). Multiplying this by an IDC of 0.1 mSv/year from drinking-water gives an estimated annual cancer risk of approximately 5.5 × 10−6. Therefore the individual dose criterion (IDC) of 0.1 mSv/year represents a very low level of risk that is not expected to give rise to any detectable adverse health effect.

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