5. DISCUSSION OF RESULTS
5.1 Annual Doses within the Mooi River Catchment for the Drinking Water
Exposure Route
The annual doses for the drinking water route of consumption, for
the Mooi River Catchment, are shown in Table 4 and Figure 3.
Table 4: Final annual total dose [a] in mSv/a for the drinking water
route, in the Mooi River catchment, arranged according to ascending dose. Also given is
the incremental dose [b] above the estimated background of 0,02 mSv/a.
| Site No |
Dose[a] Dose[b] |
|
Site No |
Dose[a] Dose[b] |
| 29 |
0,02 0,00 |
|
16 |
0,04 0,02 |
| 30 |
0,02 0,00 |
|
13 |
0,04 0,02 |
| 14 |
0,02 0,00 |
|
3 |
0,05 0,03 |
| 35 |
0,02 0,00 |
|
10 |
0,06 0,04 |
| 27 |
0,02 0,00 |
|
4 |
0,06 0,04 |
| 34 |
0,02 0,00 |
|
2 |
0,06 0,04 |
| 6 |
0,02 0,00 |
|
39 |
0,06 0,04 |
| 31 |
0,03 0,01 |
|
5 |
0,06 0,04 |
| 26 |
0,03 0,01 |
|
23 |
0,08 0,06 |
| 20 |
0,03 0,01 |
|
37 |
0,08 0,06 |
| 25 |
0,03 0,01 |
|
8 |
0,08 0,06 |
| 28 |
0,03 0,01 |
|
17 |
0,08 0,06 |
| 32 |
0,03 0,01 |
|
9 |
0,11 0,09 |
| 19 |
0,03 0,01 |
|
11 |
0,14 0,12 |
| 33 |
0,03 0,01 |
|
7 |
0,16 0,14 |
| 22 |
0,03 0,01 |
|
15 |
0,18 0,16 |
| 18 |
0,03 0,01 |
|
1 |
0,24 0,22 |
| 24 |
0,03 0,01 |
|
7a |
0,27 0,25 |
| 21 |
0,03 0,01 |
|
12 |
0,52 0,50 |
| 6a |
0,03 0,01 |
|
|
|
| 36 |
0,03 0,01 |
|
|
|
| 38 |
0,03 0,01 |
|
|
|
Applying the proposed interim water quality
guidelines to the mean annual doses calculated for the radionuclides, an annual dose map
for drinking water was produced. The dose map (Figure 3) shows that the radiological
quality of the water, at the majority of the sampling sites in the Mooi River Catchment,
is either in the ideal (blue, <= 0,1 mSv/year) or acceptable
for lifetime use (green; >0,1 to <= 0,25 mSv/year) class.
Two sites were in the yellow class (>0,25 to <=
1,0 mSv/year), implying suitability for interim use, including the need to establish the
origin, and consumption rate of the water at the site. No sites were in the red class
(>1,0 mSv/year), implying that there were no sites which were unsuitable for use, and
thus which needed active intervention.
In summary the following may be stated:
All sites had an associated annual radiation dose less than 1 mSv/year,
implying that at no site was the radiation dose at a level that would necessitate
consideration of immediate intervention, such as the necessity of immediately providing an
alternative water supply.
Two sites had a radiation dose level in the yellow class of >0,25 to
<= 1 mSv/year. These were:
Site 7a (West Driefontein mine process water before settling dams). This implies that
the water is radiologically suitable for drinking water use for an interim period, but
that a site specific investigation should be done, including the collection of information
on drinking water consumption.
Site 12 (Doornfontein gold mine service water). It was determined that
this site dried up, and that water was no longer being discharged. The radionuclide input
to the surface water from this site ceased for the further duration of the 1997 monitoring
survey.
Five of the sites were in the green class (acceptable for lifetime
use), with radiation dose levels between >0,1 and <= 0,25
mSv/year.
The large majority of the sites monitored (34 sites) had insignificant
radiation dose levels, and complied fully with the World Health Organization’s ideal
screening guideline for radioactivity in drinking water of <=
0,1 mSv/year. With respect to those sites at which there was no radiation problem from a
drinking water point of view, it was noteworthy that such sites included:
The two raw water intakes for drinking water treatment to the town
of Potchefstroom.
Most of the groundwater sites, including the Gerhardminnebron, and
the Turffontein eye.
The drinking water supply borehole of Welverdiend in the
municipality of Carletonville.
All but two of the mine water discharge points.
5.2 Discussion of Predominance of Uranium
The results of the monitoring in the Mooi River catchment have
shown that of the radionuclides measured, the parent radioactive element uranium, is
responsible for the major portion of the measured alpha activity.
A map representing the measured uranium-238 chemical toxicity values is
given in Figure 4, with the proposed colour classes. It can be seen immediately from this
map that at the lower end of the catchment the sites are all in the ideal (blue) class,
and that specifically the water of Potchefstroom is in the ideal class. The great majority
of the sampling sites in the catchment were acceptable as far as uranium is concerned,
with only 7 sites requiring further investigation from the viewpoint of uranium chemical
toxicity (6 in the yellow class and 1 in the red class). The six sites in the yellow class
for uranium chemical toxicity were:
Site 1: Luipardsvlei.
Site 7a: West Driefontein process water.
Site 7: West Drienfontein transfer water.
Site 11: Doornfontein Gold plant discharge in canal, upstream
Doornfontein
Site 15: Western Deep levels farm bridge down stream, no 7 Shaft Slimes dam.
Site 9: Mooirivierloop at Blaubank.
The single site in the red class for uranium chemical toxicity was site
12: Doornfontein Gold Mine no 3 shaft discharge.
It is noticeable from Figure 4, that the majority of sites of elevated
uranium concentration occur around the centre of the Mooi River catchment, with the
concentrations again decreasing as the river flows further west on course to Boskop Dam.
It is debatable what the reasons are for the decrease in uranium concentration after the
initial increase around the middle section of the Mooi River. It is noticeable that the
sites with elevated uranium concentrations almost all have contributions from mine water.
Important attenuating mechanisms downstream of the points of contamination are probably a
combination of sediment adsorption and dilution effects.
The majority of the sites not complying with the chemical drinking
water criterion for uranium are associated directly with discharges from gold mining
activities.
5.3 Annual Radiation Dose from Background Radiation Levels in Water
The radiation dose arising from the ingestion of the water at the
various sampling locations is made up of two components, the dose attributable to
background radioactivity in the water and the dose attributable to the additional
radioactivity originating from mining activities in the region. As explained in section
2.5, it is not possible to establish unequivocally the background radioactivity levels in
the water. However, for some sampling locations the radioactivity levels were very low,
and the dose corresponding to these levels was about 0,02 mSv/year. For one of those
sampling locations (C2H172Q01, site no. 34), there is no possibility of upstream mining
influence. It can be assumed, therefore that a value of 0,02 mSv/year represents an upper
bound value for the annual ingestion dose arising from background radioactivity in water.
World wide reference values for non-elevated levels of naturally
occurring radionuclides in water [3] correspond to an annual ingestion dose of between
0,01 and 0,02 mSv/year.
It was therefore assumed for the purposes of this investigation that
the annual radiation dose attributable to background radioactivity in water was
0,02 mSv/year. It will be seen from the results presented in Table 4 that this value
is so small that the uncertainty in its estimation is not critical to the outcome of the
investigation.
5.4 Relationship between Uranium Concentration and the Annual Dose
The IWQS (Appendix 8) and AEC (Appendix 9) methods of calculating
mean annual dose at each site, while they differed in the assumptions used to deal with
unmeasured nuclides, nevertheless gave very similar results, and essentially verified one
another.
As shown in Appendices 8 and 9, an
excellent linear correlation exists between the annual mean uranium concentration at a
site and the annual radiation dose for the drinking water route at that same site. This
correlation holds for the Mooi River catchment, but it should be noted that it may not
hold equally well for other catchments due to possible differences in radiochemical water
quality characteristics.
For all sites in the Mooi
River catchment, the following correlation between uranium in m
g/l and the total average annual lifetime dose in mSv/a was
found:
D = 0,0012895 Cu + 0,02128 (r2 = 0,98)
Where D is the annual radiation dose from
continuous drinking water use in mSv/year,
And Cu is the uranium concentration in the water in m g/l .
The implication of the existence of this correlation is that for
further monitoring purposes in the Mooi River catchment, only the uranium concentration
need be measured, from which the all nuclide dose can be accurately estimated. To
illustrate the high degree of accuracy with which the total annual radiation dose from
drinking water may be estimated from the uranium concentration alone, a comparison of the
total dose calculation from the full nuclide analyses (dose[a]) as compared to the all
nuclide dose as estimated from the uranium concentration alone (dose[c]) is shown in Table
5.
| Table 5: |
Annual Doses calculated for the
Mooi River catchment sites for drinking water. Annual doses [a] in (mSv/a) for drinking
water route, in the Mooi River catchment, for the 1997 sampling year arranged according to
ascending dose. Also given is the comparative dose obtained from the mean U-238
concentration (m g/l ) using the
linear regression; dose [c] = 0,0012895 x U + 0,02128 found for the study.
|
| Site no., |
Dose[a] Dose[c] |
|
Site no., |
Dose[a] Dose[c] |
| 29 |
0,02 0,02 |
|
16 |
0,04 0,05 |
| 30 |
0,02 0,02 |
|
13 |
0,04 0,04 |
| 14 |
0,02 0,02 |
|
3 |
0,05 0,07 |
| 35 |
0,02 0,02 |
|
10 |
0,06 0,05 |
| 27 |
0,02 0,02 |
|
4 |
0,06 0,07 |
| 34 |
0,02 0,02 |
|
2 |
0,06 0,05 |
| 6 |
0,02 0,03 |
|
39 |
0,06 0,05 |
| 31 |
0,03 0,02 |
|
5 |
0,06 0,06 |
| 26 |
0,03 0,02 |
|
23 |
0,08 0,05 |
| 20 |
0,03 0,02 |
|
37 |
0,08 0,10 |
| 25 |
0,03 0,02 |
|
8 |
0,08 0,10 |
| 28 |
0,03 0,02 |
|
17 |
0,08 0,10 |
| 32 |
0,03 0,02 |
|
9 |
0,11 0,12 |
| 19 |
0,03 0,02 |
|
11 |
0,14 0,13 |
| 33 |
0,03 0,02 |
|
7 |
0,16 0,18 |
| 22 |
0,03 0,03 |
|
15 |
0,18 0,17 |
| 18 |
0,03 0,03 |
|
1 |
0,24 0,22 |
| 24 |
0,03 0,02 |
|
7a |
0,27 0,30 |
| 21 |
0,03 0,03 |
|
12 |
0,52 0,50 |
| 6a |
0,03 0,02 |
|
|
|
| 36 |
0,03 0,03 |
|
|
|
| 38 |
0,03 0,04 |
|
|
|
5.5 Relationship between Gross Alpha Activity and the Annual Dose
Comparison of the
measured gross alpha activity results with the alpha activity calculated from individual
radionuclide measurements gave a reasonable linear correlation but with strongly scattered
individual data, indicating that individual gross alpha activity measurements should only
be used as a screening tool to identify whether the activity is high or low, and not as a
decision tool for determining the acceptability of radiological water quality.
The annual radiation dose (calculated from individual radionuclide
activities) was found to be linearly related to the annual average gross alpha activity in
the following way:
D = 0,02835 Aalpha + 0,021 (r2 = 0,856)
where D is the lifetime average annual radiation dose from continuous drinking water
use (mSv/a), and Aalpha is the gross alpha activity (Bq/l
).
5.6 Verification of Dose Calculations
To verify the doses calculated by the IWQS according to the methodology
described in section 3.5 above, the AEC performed an independent dose calculation using,
for the unmeasured radionuclides and unsampled sites, different assumptions from those
used by the IWQS. Details of the AEC’s assumptions and calculation methodology are
given in Appendix 8. A comparison of the results from the two calculation methods is shown
in Figure 6. It can be seen that, apart from one site (site 12), where the AEC calculation
gave a significantly lower dose than the IWQS calculation, the results were in good
agreement. The discrepancy with respect to site 12 can be explained by the fact that
limited data were obtained from this site, because the flow ceased during the course of
the study; in the AEC calculation, 6 months of data were represented by only one data
point.
| Figure 6: |
Comparison of doses calculated by the IWQS (method 1) and the AEC (method
2), using different assumptions with respect to the unmeasured radionuclides and sites not
sampled in the second phase |
The linear relationship
between uranium concentration and annual dose, derived using the AEC’s assumptions
with respect to unmeasured radionuclides and unsampled sites, was:
D = 0,00124 CU + 0,017 (r2 = 0,97)
This is very close to the relationship derived using the IWQS assumptions (see section
5.4), as can be seen from Figure 7.
| Figure 7: |
Comparison of IWQS and AEC relationships between uranium concentration and
dose, using different assumptions with respect to the unmeasured radionuclides and sites
not sampled in the second phase. |
5.7 Possible Uncertainties in Dose Calculations
Possible uncertainties in the estimation of the lifetime average annual dose are
examined in Appendix 10. The results can be summarized as follows:
Analytical uncertainties, based on a comparison between the radiochemical and ICP-MS
techniques, are estimated to be about 1,5%.
Uncertainties in projecting the present results to future years, assuming that the
variations in radionuclide concentrations observed during the year of study are purely
random, are estimated to be typically 20%. If there is a true seasonal component to the
variations, then the uncertainty will be less than this value.
Uncertainties in future dose estimations based only on uranium
measurements, arising from the derived linear relationship between uranium and dose, are
estimated to be less than 10%.
Uncertainties due to monthly rather than weekly sampling, based on
uranium data obtained in the first phase of the study, were estimated to range up to a
factor of 3, but such estimates arise as a result of the short (six month) sampling
period. The uncertainties would progressively decrease over longer sampling periods.
By far the greatest uncertainty is that arising from the assumption of
sole continuous use of the water for drinking purposes, which represents a ‘worst
case’ scenario. For an individual falling within any given age group, the dose
received will be directly proportional to the amount of water consumed while in that age
group.
5.8 Suspended Solids
It was not the intention of this study to measure radioactivity in the
solids suspended in the water. However, in order to obtain an indication of the possible
contribution of suspended solids to the radiation dose from ingestion, concentrations of
individual alpha-emitting radionuclides in the suspended solids were measured in samples
obtained from the 15 sites sampled in the final month of the monitoring programme
(December 1997).
In calculating the annual radiation doses from ingestion of the
suspended solids, it was assumed that the uptake factors were the same as those for the
dissolved constituents. The doses, expressed as percentages of the doses from filtered
water, were found to be very low:
| Average over 15 sites: | 2,3% ± 2,1% |
| Median: | 1,9% |
| Minimum: | 0,1% |
| Maximum: | 7,6% |
Contributions to the dose from the suspended solids were found to
originate mainly from the radionuclides thorium-230, polonium-210, actinium-227,
protactinium-231 and thorium-232. This contrasts with the situation for filtered water,
where the main contributors to dose were uranium-238, uranium-234 and radium-226.
5.9 Chemical Results: Sulphate
A summary of the sulphate concentrations found in the study is shown in
Figure 5. The reason for collecting chemical data in this study, was to enable
correlations with radiological data to be explored. An extensive search was made for
meaningful correlations between the radiological variables and the total dose, but no
statistically significant meaningful correlations were found.
The correlation between mean annual sulphate concentration and mean
annual uranium concentration was investigated, but found to be poor (r2 =
0,394). This implies that sulphate levels can, therefore, not be used as a surrogate for
indicating the possible presence of radioactivity. Sulphate in water is, however,
important from the viewpoint of drinking water in that it gives rise to traveller’s
diarrhoea in individuals not used to drinking high levels of sulphate. Sulphate also
accelerates corrosion in distribution systems and appliances.