SECTION 2

Chronic Exposure Situations and Supporting Information for Assumptions made in the Proposed Guideline

4.    Chronic and persistent exposure situations

An example of chronic exposure from natural sources is that of cosmic radiation. People living at high altitudes receive more cosmic radiation than a person living at sea level.   An inhabitant of La Paz, Bolivia, receives approximately 6 times more cosmic radiation than a person living at sea level.  Cosmic radiation is an example of chronic exposure that is essentially uncontrollable or not amenable to control. It is generally excluded from the scope of regulations on radiological protection.

There is no water resource that does not contain NORM.  The potential health risk associated with drinking water will therefore mainly be the result of chronic exposure to elevated levels of dissolved NORM, because of the ubiquitous nature of NORM.  An example of such a situation is that of a community dependent on a water resource that has higher than typical concentration levels of natural occurring nuclides.  Some borehole water in the Namakwa Land region, for example, exhibits Ra-226 concentration levels in excess of 1 Bq/L.  The potential annual dose to an adult as a result of Ra-226 alone in such a situation is more than 0.2 mSv.  This is double the WHO recommended value of a 0.1 mSv/a [5].  (It is important to note that WHO also recommends that this value should be adapted to local conditions; increased or decreased depending on the site-specific conditions.)

When one assesses chronic exposure situations, the relevant quantity is the annual effective dose attributable to the exposure.  The dose as a result of chronic exposure from natural sources is termed the existing annual dose.  The example of water with a high natural concentration of radium is that of an existing annual dose for people drinking that water.  If a working activity, for example a mine, adds to the existing annual dose, the added component is termed the additional annual dose.   The additional annual dose is therefore the result of human activities and is additional to the background dose that would have existed prior to human activities.

Members of the public may claim different levels of radiological protection depending on the source of exposure.  The claim for protection is generally stronger when the source of exposure is a technological by-product rather than when it is considered to be of natural origin.  Mining and mineral processes that can potentially (or actually) increase radioactivity in water bodies are subject to regulatory control in order to limit the additional annual dose to the public.  Comparatively higher doses from purely natural radiation that is part of the existing dose (no contribution from practices or working activities) can be permitted.

5.    Intervention

Radioactive residues already existing in some habitats, for example historic mine tailings and that could influence the radioactivity in water resources, can be subject to protective actions through a process called intervention, which is intended to lower the overall exposure to people.

The justification of intervention in chronic exposure situations should be assessed by means of a decision-aiding process requiring a positive balance of all relevant long-term attributes related to radiological protection.  Attributes include:

·        avertable annual dose that can be achieved through intervention, for example, water treatment that removes the radium from borehole water,

·        expected reduction in anxiety caused by a situation of chronic exposure,

·        the reassurance provided by the intervention,

·        the social cost, harm and disruption that may be caused by the implementation of the intervention actions, and

·        other considerations by relevant stakeholders.

The ICRP [4] refers to generic reference dose levels for intervention expressed in terms of the existing annual dose.  These are useful in some situations such as exposures to high natural background radiation and radioactive residues that are a legacy from the distant past.  Generic reference levels, however, should not prevent protective actions at lower dose levels to reduce the contribution from dominant exposure components when it is justifiable.  According to the ICRP an existing annual dose approaching 10 mSv may be used as a generic reference level below which intervention is not likely to be justifiable for some chronic exposure situations.  Below this level, protective actions to reduce the dominant component to the existing annual dose are still optional and might be justifiable.  If, for example, cosmic radiation is the dominant component to a natural background radiation dose of 10 mSv/a, intervention may not be justifiable.  However, if water is the dominant component for a large population group for a specific situation where elevated NORM as a result of the peculiar geology of the area, impacts on the water resource, intervention may be justifiable.

An intervention exemption level equal to 1 mSv has been proposed in the ICRP report.  Natural sources of water with a high concentration should be regarded as falling in the same category of cases where an exemption level could apply.

Table 5.1 presents a summary of dose concepts in relation to intervention [4].

Table 5.1: Guideline for Decisions on Intervention For Different Levels of Annual Doses  

6.    Behaviour of naturally occurring nuclides in the environment

6.1.        Introduction

The magnitude of dose from ingesting water is to a large extent determined by those nuclides that have high dose coefficients and remain in solution.  The sections that follow are extracts from an article on the typical geo- and hydrochemical behaviour of important NORM nuclides [6].  This information forms part of the basis for a screening assessment in the proposed guideline methodology.  

6.2.        Uranium and thorium nuclides

Important primordial nuclides in nature are the long-lived nuclides thorium-232 (Th-232), uranium-235 (U-235) and uranium-238 (U-238) as well as potassium-40 (Ka-40). Thorium and uranium are concentrated in crustal rocks in an average Th:U ratio of about 3.5.   However, the various igneous, metamorphic and sedimentary rock types have widely different U and Th concentrations. Some metamorphic rocks, for example, have a high abundance of U and Th.  U is also found to be strongly enriched in certain organic sediments e.g. peat, lignite and asphalt. 

In a closed system the progeny of Th and U are present in concentrations determined by the concentration of parent U and Th isotopes and the time since the system became closed to nuclide migration.  In nature closed systems rarely exist and predictions regarding nuclide concentrations in water bodies invariably include large uncertainties. These nuclides and their decay products are found in ground and spring waters in element specific concentrations dependent on complex hydrogeologic processes and conditions (dissolution, transport and ion-exchange processes as well as redox potentials and pH-conditions of the aqueous system). These hydrogeologic processes result in non-equilibrium conditions between parent nuclides and their progeny.  However, characteristic behaviour in the natural environment can provide a basis for assumptions regarding probable behaviour of nuclides used in the radioactivity screening assessment methodology in the proposed guideline.  This characteristic behaviour is briefly discussed.

In the oxidised zone of the earth’s near-surface environment Th and U may both be mobilised, but in different ways.  Thorium has an extremely low solubility in natural waters. There is a close correlation of Th concentration and detrital content of water.  Th is almost entirely transported in particulate matter.  Th is bound in insoluble resistate minerals or is adsorbed on the surface of clay minerals. Even when Th (e.g.Th-230) is generated in solution by radioactive decay of U-234 it rapidly hydrolyses and adsorbs on to the nearest solid surface.  The soluble Th content of water was shown to be insignificant during the Klip and Mooi Rivers catchment radioactivity studies [1].  (However, sediments can transport NORM some distance from the point of origin.)

By contrast, U may either move in a detrital, resistate phase, similar to Th, or in solution as a complex ion.  Both elements appear in the 4⁺ oxidation state in primary igneous rocks and minerals, but U, unlike Th, can be oxidised to 5⁺ and 6⁺ states in the near-surface environment.  The 6⁺ oxidation state forms soluble uranyl complex ions which play the most important role in U transport during weathering.

Waters in the natural environment are variable in U content, depending mainly on factors such as contact time with U-bearing rock, U content of the contact rock, amount of evaporation and availability of complexing ions.  Groundwaters are somewhat enriched in respect of U when compared to surface waters especially in mineralised areas.

The ability of U to undergo inorganic dissolution and reprecipitation is probably the most important process in the natural environment to cause disequilibrium between the nuclides in the decay chains. Large variations of U can sometimes be observed in the same aquifer and are interpreted as due to Eh-pH changes which cause precipitation of U from solution along the flow direction.

6.3.        Other NORM nuclides

Disequilibrium between U-238 and U-234 in natural waters has been found to be the rule rather than the exception, for example, preferential leaching from radiation damaged sites in crystalline material, following decay down to U-234.  At relatively high concentrations of U, the Klip and Mooi Rivers studies indicate that disequilibrium is not pronounced and that equilibrium conditions can be assumed in a screening survey.

Radium-226 (Ra-226), the daughter of Th-230, is generally found in excess of its parent in most natural waters due to the greater solubility of Ra over Th.  In freshwaters, radium is found in highest concentrations in limestone regions where it is more soluble in HCO₃- waters.  Ra-228 is also found in excess of its parent Th-232 in natural waters.

Products of radioactive decay in the U and Th series include radon (Rn) gas of which three isotopes exist.  Rn-222 is the longest-lived and most abundant.  Loss of radon will cause disequilibrium between members of a decay chain.  Rn-222 has an appreciable solubility in water  (about 0.5 g/L at STP) and is often found in concentrations far in excess of the parent nuclide radium, Ra-226 (so-called unsupported Rn-222).  A Rn-222/Ra-226 activity ratio of 450 has been observed in groundwaters from central England [14].  Aeration of water and short half-lives make the contribution of radon negligible in ingestion dose calculations.  Normally radon is only considered for confined areas, e.g. showers using radon rich natural water and when inhaling the radon progeny are the exposure pathway.   This is considered a negligible exposure pathway since showers are usually supplied with treated water from which any dissolved radon would have been released during the treatment process.

Polonium-210 (Po-210) is largely insoluble.  In the hydrological cycle Po-210 generally follows its precursor Lead-210 (Pb-210).  Po-210 is generally more readily adsorbed than Pb-210 onto particulate matter.

The decay product Protactinium-231 (Pa-231 from U-235) is relatively insoluble when compared to uranium and radium, for example.

7.    Typical water quality criteria

7.1.        Criteria

Criteria that are generally used for drinking water quality assessments can be expressed in terms of any of the following quantities:

·        Dose

·        Activity concentration

·        Chemical concentration

·        Qualitative health risk e.g. stated as increased morbidity or cancer risk

·        Quantitative health risk e.g. fatality risk per year

A quantitative health risk for exposure to radioactivity can be estimated by using a nominal value of 5% per Sievert.  This value is for mortality from cancer after exposure to low doses for a population of all ages and as recommended by the ICRP [7].  Inclusion in the water quality guideline of a quantitative risk criterion expressed as a statistical mortality risk is, however, not recommended. The public generally finds it confusing when risk is expressed in statistical terms. Factors, for example, that result in increased public concern in health risk matters include the following [8]:

·        Unfamiliarity with a certain type of risk

·        Mechanisms or process of how a risk arises are not understood

·        Risk is scientifically uncertain and expressed in statistical terms

·        Delayed effects are part of the risk, for example when a health effect only manifest itself some time after exposure to a hazardous material

These factors are all elements of ionising radiation health risks. Communication with the general public on water quality in terms of radioactivity and radiation hazards, should preferably not include unfamiliar terms such as sieverts, becquerels and statistical fatality risk.   However, the method of communication must still allow a clear grasp of the radiation health hazard associated with a specific source of water.

The American Health Physics Society also recommends against quantitative estimation of radiation health risk below an individual dose of 50 mSv per year, additional to background radiation.  The reason given is that there exists no conclusive evidence of health risks for low dose rate annual doses up to 50 mSv  [9].   Statistically significant risks for solid tumors in the LSS cohort (Life Span Study cohort of the Japanese A-bomb survivors) are presently seen only above a dose of 100 mSv (short-term low-energy transfer doses).

7.2.        Guideline examples

It is essential that when preparing a guideline, other international guidelines be considered, especially for those countries with which important trade links exist, for example the United States and the European Community. 

The criteria used by the USA and the European Union are compared in the table that follows.

Table 7.1  US and EU Guidelines [10;11]

The German Commission for radiation protection, "Strahlenschutz-Kommission" (SSK), gave some recommendations on the basis of practicability to evaluate natural radioactivity in water [12].  The relative mobility of the ions of the primordial nuclides in water is stated to be in the order U⁶⁺ > U⁴⁺ >> Th⁴⁺ .   As a consequence of these different mobilities the Th-isotopes are ignored as constituents of aqueous systems. The same is mostly true for their radium progeny.  Ra-226 as progeny of U-238 is more relevant than Ra-228 in the Th-232 decay chain. 

The SSK has also recommended an additional annual dose of 0.500 mSv as limit from tap water where water resources are affected by former uranium mining activities.

The SSK further defines relevant natural nuclides in tap water to be U-238, U-234, Ra-226, Rn-222, Pb-210 and Po-210.  No Th-232 and U-235 progeny is included.

In South Africa domestic water refers to all uses to which water can be put in the domestic environment (drinking, food preparation, bathing, washing dishes, laundry and gardening).   The EU uses the concept of “water intended for human consumption” and is defined as follows [13]:

“- all water either in its original state or after treatment, intended for drinking, cooking, food preparation or other domestic purposes, regardless of its origin and whether it is supplied from a distribution network, from a tanker, or in bottles or containers;

- all water used in any food-production undertaking for the manufacture, processing, preservation or marketing of products or substances intended for human consumption unless the competent national authorities are satisfied that the quality of the water cannot affect the wholeness of the foodstuff in its finished form.”

The EU Directive also states that the directive does not apply, or a member state is exempt, when “water intended for human consumption from an individual supply providing less than 10 m³ a day as an average or serving fewer than 50 persons, unless the water is supplied as part of a commercial or public activity.” 

The EU Directive value of 0.100 mSv/a is described as an indicator parameter and the value has been selected to ensure that water intended for human consumption can be consumed safely on a lifelong basis.   The same value of 0.100 mSv/a is described by WHO as a screening value [5].

7.3.        UNSCEAR data on radioactivity in drinking water [14]

UNSCEAR provides information on nuclides in drinking water for different regions and countries in the world.  The information in Table 7.1 indicates that most countries report U-238 and/or Ra-226.  It is deduced that mostly these two nuclides are used for purposes of screening water for its radioactivity content.

The wide ranges that are reported for some countries stem from the fact that the data reflect all kinds of water sources, e.g. ground water, lakes and treated water.