Development of a South African Guideline to Assess the Health Risk Associated with Naturally Occurring Radioactive Material in Water Resources

Transcription

1 Development of a South African Guideline to Assess the Health Risk Associated with Naturally Occurring Radioactive Material in Water Resources J Slabbert 1 1 PSI Risk Consultants CC psicc@iafrica.com Abstract. South Africa has a unique geology and an extensive mining industry. It is a mineral rich country and uranium mineralization, for example, is present in rocks which encompass almost the whole of the geological history of the country. This feature and mining activities can result in elevated levels of naturally occurring radioactivity in water resources in some densely populated areas. While radioactivity in water is relatively easily measured, given the appropriate equipment and radio-analytical expertise, the interpretation of the significance of the measured radioactivity to the domestic water user is beset with uncertainties and imponderables, especially when it comes to evaluating the actual risk to the consumer. This paper presents the development of a guideline to interpret radiological quality of water and facilitate communication with the general public. The guideline, together with associated computer software called WaterRad, makes it easier for all those concerned with evaluation of radioactivity in water to assess the fitness for use of a water resource. The radiological quality is evaluated in terms of a five-colour classification scheme already well accepted for chemical and microbiological water quality in South Africa. The classification is based on dose assessments using either screening or detailed radioanalytical results that include specific radionuclide equilibrium assumptions. The extent of radionuclide analysis of water is a function of the origin of the water, the potential impacts by mining and mineral processes and the scale of water use from a particular resource. The paper is a summary of a project performed for the Directorate Resource Quality Services 2 of the South African Department of Water Affairs and Forestry, which provided significant input towards the development of the guideline. 1 Introduction South Africa is a country receiving approximately half the world s average annual rainfall. Large parts of the country are arid. Protection of its water resources is therefore of prime importance. The National Radioactivity Monitoring Programme (NRMP). is being developed as one of several monitoring programmes of which the purpose is to determine the status and trends in the quality of South Africa s water resources on a national scale, and to provide an understanding of the natural and human factors that affect the quality of these resources. A guideline was required by the South African Department of Water and Forestry (DWAF) for interpreting the results of the monitoring programme and to classify the quality of a water resource. The guideline also had to allow clear communication of results to members of the public. This paper describes the methodology that was adopted in respect of naturally occurring radioactive material (NORM). NORM is of specific importance since South Africa s characteristic geology and extensive mining industry can result in elevated levels of radioactivity in water in some catchment areas. The water quality classification scheme is based on the life-time average committed effective radioactivity dose for a member of the public. It is a five-tier classification system. This approach is well accepted for chemical and microbiological water quality in South Africa. for classification of the water quality in the following classes: Blue (ideal), Green (good), Amber (marginal), Red (poor) and Purple (unacceptable). 1 PO Box 37759, Faerie Glen 0043 (Republic of South Africa) 2 Private Bag X313, Pretoria,0001 (Republic of South Africa) 1

2 2 Water analysis and calculation of the potential ionising radiation dose to a member of the public 2.1 Introduction The extent of radionuclide analysis of water should be a function of the following factors: The origin of the water The potential impacts by mining and mineral processes on water The scale of water use These factors serve as basis for three categories of water that are defined as follows: Category A water use: Untreated water from a natural resource (e.g. directly from a borehole, canal, river/stream or dam) with a low probability of being influenced by a mining and mineral processing activity. Category B water: Untreated water from any resource with a significant probability of being influenced by a mining and mineral processing activity e.g. a surface stream or borehole inside the potential impact radius of a mining and mineral processing facility. Category C water: Treated water from a formal water supplier (e.g. water service providers such as a municipal waterworks) providing drinking water to a large number of people. 2.2 The potential ionizing radiation dose for a member of the public The lifetime average annual dose associated with a water resource is calculated from the following expression [1, 2]: where: D= A i F (1) i i D is the lifetime average annual dose (msv/a) A i F i is the activity concentration of radionuclide i (Bq/L) is the proportionality constant for radionuclide i with units of (msv/a) per (Bq/L). The proportionality constant F i for nuclide i was determined from the following relationship: where: Fi = Cx ( DCF ) ixwx (2) x C x (DCF) I is the annual water consumption for an age group x (L/a). is the dose conversion factor (dose coefficient) for radionuclide i and age group x (msv/bq). Dose conversion factors for the various radionuclides and age groups were taken from IAEA Basic Safety Standards [3]. W x is the weighting factor for age group x. (3) 2

3 2.3 Radioanalysis of water Methods for assessing water quality must avoid overly conservative results that could create wrong perceptions especially in the public domain. These perceptions are usually difficult to change afterwards. However, it is still required to have the precautionary principle included in a water quality assessment method so that the probability is low for underestimating dose in situations of elevated NORM concentrations. Earlier studies in South Africa investigated the possibility of only using uranium concentration and gross alpha radioactivity concentration in a water resource to determine its classification. Two rivers in close proximity to extensive gold mining operations, the Klip River and Mooi River, showed that there is normally no generic statistical dose correlation for uranium chemical concentration and gross alpha concentration [1, 2]. It was deduced from these studies that each water resource has its own unique relationship between dose and uranium chemical concentration. This relationship can only be established after many samples over a period of time. It was also shown that gross alpha activity concentrations could not be accurately correlated with dose. Two different methods were designed, one for screening purposes and one for detailed risk analysis. These methods are used to determine the annual dose for the three categories of water. 2.4 Method 1: A screening method The screening method is designed to optimise costs without compromising the precautionary principle for situations where water consumption results in a dose that may give rise to concern. Measurement of two radionuclides, also referred to as a two-nuclide measurement vector, is required to perform an initial screening dose assessment. The two radionuclides are U-238 and Ra-226. Gross alpha activity and uranium chemical concentration do not allow a dose assessment of the required accuracy. However, these two parameters are also measured and are used to check the validity of analysing only for U-238 and Ra-226. Low-probability nuclide behaviour, for example a high relative abundance of Th isotopes, should be indicated by a low U concentration (µg/l) and a high gross alpha-activity concentration. This would then indicate a requirement to analyse for more radionuclides in the U-235, U-238 and Th-232 decay series. The dose is estimated using a six-nuclide calculation vector consisting of the following nuclides and assumptions: U-234 in equilibrium with U-238 Pb-210 and Po-210 in equilibrium with Ra-226 U-235 = U Note: U-235 was included since it has a fairly constant natural abundance in relation to U-238. The screening method has relatively poor accuracy at very low radioactivity concentrations (dose in the range 0.01 msv/a to 0.10 msv/a) when compared to a calculation using a radionuclide measurement vector consisting of all measurable nuclides in the U-238, U-235 and Th-232 decay series. The assumption of Pb-210 and Po-210 being in equilibrium with Ra-226 is used purely to introduce conservatism in the calculation and to flag a situation when the radioactivity concentration in the water may be high. 2.5 Method 2: A detailed method It was decided that water resources used by a large number of people would initially require a larger radionuclide measurement vector. As soon as acceptable correlation can be demonstrated between Method 2 and Method 1, say after a year s monitoring, Method 1 can be used on a routine basis. 3

4 Method 2 is similar to Method 1 except that larger radionuclide measurement and calculation vectors are used. A framework for decisions in respect of radioanalysis and dose calculation is presented in table 1. 3 Classification of the radiological quality of water The classification of water is based on potential ionising radiation dose where water from the same resource is consumed on a life-long basis. The classification scheme is presented in table 2. The guideline and classification scheme are supported by a computer program called WaterRad. It enables easy data processing and water classification as described in tables 1 and 2. 4 Conclusion The derivation of a guideline for assessing the radiological quality of water was described. A classification scheme for water resources was derived from this assessment methodology. An important requirement for the classification scheme was to correspond with existing non-radiological classification schemes of water quality used in South Africa. The ionizing radiation dose derived from life-long use of water from a particular resource, in most cases a hypothetical situation, serves as a basis for the classification scheme. Dose intervals were assigned specific colours, which enables easier communication with members of the public and technical staff outside the specialist field of radiation protection. 5 References 1. Report on the Radioactivity Monitoring Programme in the Klip River Catchment. Institute for Water Quality Studies: Department of Water Affairs and Forestry. (2000) 2. Report on the Radioactivity Monitoring Programme in the Mooi River (Wonderfonteinspruit) Catchment. Institute for Water Quality Studies: Department of Water Affairs and Forestry. (1999) 3. International Atomic Energy Agency, Vienna.. International Basic Safety Standards for Protection against Ionizing Radiation and for the Safety of Radiation Sources. Safety Series 115 (1996) 4. World Health Organization Guidelines for Drinking Water Quality, Geneva. (1993) 4

5 Table 1. Water Analysis and Decision Guide Category of water source A Untreated water from a natural source not influenced by mining and mineral processing B Untreated water from a natural source potentially influenced by mining and mineral processing Initial Analysis Method / Analyses Method 1 Screening method U-238 Ra-226 Gross alpha specific activity U chemical concentration Method 1 Screening method U-238 Ra-226 Gross alpha specific activity U chemical concentration Radionuclides in dose calculation and equilibrium assumptions U-238 (U-234) Ra-226 (Pb-210; Po-210) U-235 = U U-238 (U-234 Ra-226 ( Pb-210; Po-210) U-235 = U Annual Dose, D D < 1 msv/a D > 1 msv/a D < 0.3 msv/a 0.3 msv/a < D < 1 msv/a Further actions No further action required except to inform water users. Whenever gross alpha > (2 U Ra-226) then recommend a Method 2 analysis to determine the potential dose more accurately. Perform water evaluation using Method 2 If D 1 msv/a following Method 2 analysis, then no further action except to inform users and perform routine confirmatory monitoring. If D > 1 msv/a when using Method 2, intervention considerations are required. No further action if total exposure from all exposure pathways to the critical group, D 0.30 msv/a; if not, perform Method 2 analysis to determine the dose accurately. Re-assess dose by using Method 2. No further action if total exposure to the critical group 1 msv/a and an optimised constraint cannot be achieved at less than 1 msv/a Routine Monitoring / Comments Annual monitoring 3- monthly monitoring (initial period) 3- monthly monitoring Liaison between National Nuclear Regulator (NNR) and DWAF required. 5

6 Category of water source C Treated water delivered by a formal water distribution network (towns/cities) Initial Analysis Method / Analyses Method 2: U-238, U- 234, Ra-226, Th-230, Pb- 210, Po-210, Th-232, Th- 227, Ra-223, Gross alpha specific activity; U chemical concentration Radionuclides in dose calculation and equilibrium assumptions U-238 (Th-234, Pa- 234m) U-235 = U Pb-210 (Bi-210) Th-232 (Ra-228; Ac- 228) U-235 (Th-231; Pa- 231; Ac-227) Annual Dose, D D > 1.0 msv/a D 0.10 msv/a D > 0.10 msv/a Further actions Confirm dose by using Method 2. Intervention options to be evaluated (If mining or mineral processing is responsible for elevated radioactivity levels then definite remedial actions are required) No further action No further action if D 1.0 msv/a and cost effective lowering of the dose cannot be achieved; if D > 1.0 msv/a then intervention is required to lower dose to less than 1 msv/a. Routine Monitoring / Comments 3- monthly monitoring. Liaison between DWAF and NNR required. 3 - monthly monitoring during first year. For routine monitoring after first year use Method 1. There is a high probability that the water source will meet the recommended dose value of D = 0.10 when it is subjected to typical water treatment processes. WHO guideline: provisional level of D 0.10 msv/a. 6

7 Table 2. Radiological Classification of Water Class /Colour Class 0 (Blue - Ideal water quality) Class 1 (Green - Good water quality) Class 2 (Yellow - Marginal water quality) Dose range; msv/a > > 1 10 Typical Exposure Scenarios No health effects and water suitable for many generations. Most treated water falls in this water quality range. Additional dose above background (as a result of human activities) that falls inside this range, is difficult or impossible to determine and/or to distinguish with sufficient confidence from variations in background doses. Water suitable for lifetime use. It is the range of exposure from some natural and untreated water sources (e.g. ground water / wells) as well as water sources that could be influenced by mining and mineral processing activities. A dose between 0.2 to 0.8 msv/a [4] is the typical worldwide range of ingestion radiation dose resulting from water as well as food. A dose equal to 1 msv/a corresponds to the regulatory public dose limit for human activities involving radioactive material. The total natural background radiation from all exposure pathways, not only water, falls in this range Probably only a small number of natural water sources of this poor quality exist, resulting from exceptional geochemical conditions. Abnormal operating conditions at some nuclear authorised mineral and mining processes may result in a dose in this range when a person drinks the untreated water. Intervention will most likely be required to improve the quality of water that is released into the public domain. Intervention Decision Time Frames Intervention not applicable for this class of water. No intervention is required although the ALARA (As-Low-As- Reasonably- Achievable) principle applies. Intervention considerations within 2 years. Class 3 (Red - Poor water quality) > This range represents potential excessive exposure to ionising radiation and an unacceptable health risk. It is highly unlikely to find water of this poor quality in the natural environment. Intervention is required in less than 1 year. Class 4 (Purple - Unacceptable water quality) > 100 Severe acute health effects possible. It is extremely unlikely that water with this radiological quality will ever be encountered. Immediate intervention is required. 7