Risk Assessment of Exposure to Natural Radiation in Soil Using RESRAD-ONSITE and RESRAD-BIOTA in the Cobalt-Nickel Bearing Areas of Lomié in Eastern Cameroon

Round 1

Reviewer 1 Report

This manuscript is about the radiation risk assessment on a NORM site in Cameroon. 

The introduction is appropriate, the number and quality of the references are enough. 

Could you give example(s) from the world where cobalt/nickel/manganese deposit was monitored with similar tools?

The part of Material and Methods is very detailed and contains almost all necessary information. 

Could you mark the sampling points in Figure 1?

Results:

In Fig.2. the text can not be read (too small). Please correct it.

In the Introduction the Authors wrote: "This study is also a preliminary part of a major environmental radioactivity monitoring campaign..."

Could you give some details about this campaign in the Conclusion?

Author Response

Reviewer #1: Reviewer's feedback

This manuscript is about the radiation risk assessment on a NORM site in Cameroon.

The introduction is appropriate, the number and quality of the references are enough.

Could you give example(s) from the world where cobalt/nickel/manganese deposit was monitored with similar tools?

Reply: Thank you very much for your comments and suggestions that helped to improve the quality of the manuscript.

Regarding the monitoring of a cobalt/nickel/manganese deposit, this work is a first ever used combining RESRAD-Biota and RESRAD-ONSITE. Nevertheless, similar studies using one of the two codes have been performed in mining sites such as Khak-Sefid mining site, Ramsar, Iran conducted by Njinga and Tshivhase in 2018 and Tudor site by Ziajahromi et al. in 2015 using RESRAD-ONSITE and a study on "¹³⁷Cs distribution in the South Caspian region, transfer to biota and dose rate assessment" by Akbar Abbasi in 2019 using RESRAD-BIOTA. The above references are already considered in the manuscript.

The part of Material and Methods is very detailed and contains almost all necessary information.

Could you mark the sampling points in Figure 1?

Reply: Figure 1 has been replaced by another map showing the sampling points.

Results:

In Fig.2. the text can not be read (too small). Please correct it.

Reply: Figure 2 has been corrected.

In the Introduction the Authors wrote: "This study is also a preliminary part of a major environmental radioactivity monitoring campaign..."

Could you give some details about this campaign in the Conclusion?

Reply:

According to us, the findings of the work are already highlighted in the conclusion. However we added the following sentences:

“It should also be noted that the contribution of ²²⁶Ra to cancer risk is high compared to that of ²³²Th. Radium is therefore the major contributor to cancer risk.”

“Environmental assessment is planned during and after mining to evidence its impact for better radiological protection of members of the public and environment.”

Reviewer 2 Report

The manuscript was written well and the work is interesting to publish. The authors need to do some corrections: 

general corrections:

abstract needs additional improvement. 

Introduction: I would like the authors added other papers that are fitted to the manuscript topic.

Materials and methods: I don't understand why the authors measured the activity concentrations of Ra, Th and K in the samples by two various detectors? what is the aim of it?

results and discussion: they must improve the discussion and give scientific reasons. 

Minor corrections:

1- Table 2 present the site parameters of the present study, but where the references explain the ideal case.

2- Table, 3: Why the difference between activity concentrations of Ra, Th and K was observed in the samples? Does this mean due to the selection of samples?

3- lines 239-242, These not enough to illustrate the reasons for the presence of radionuclides in the sample, explain in detail, why the activity concentrations are high due to ............?

Author Response

Reviewer #2: Reviewer's feedback

The manuscript was written well and the work is interesting to publish. The authors need to do some corrections.

Reply: Thank you very much for your comments and suggestions that helped us improve the quality of the manuscript.

General corrections:

Abstract needs additional improvement.

Reply: Abstract has been improved as recommended.

Introduction: I would like the authors added other papers that are fitted to the manuscript topic.

Reply: Thank you very much for your comments. Two papers (Jang et al. (2021) and Tawfik et al. (2014)) that address similar topics have been added in the introduction section.

Materials and methods: I don't understand why the authors measured the activity concentrations of Ra, Th and K in the samples by two various detectors? what is the aim of it?

Reply: In this study, 30 soil samples were analyzed by the gamma spectrometry technique, one part using sodium iodide detector of the Centre for Nuclear Science and Technology in Cameroon and the other part using HPGe detector of the Institute of Radiation Emergency Medicine in Japan. The first idea was to measure all the samples using HPGe detector, more reliable than the sodium iodide detector. Unfortunately, due to some logistical issues only 15 samples were sent to Japan.

Results and discussion: they must improve the discussion and give scientific reasons.

Reply: the discussion has been improved with scientific reasons to support it.

Minor corrections:

• Table 2 present the site parameters of the present study, but where the references explain the ideal case.

Reply: Thank you very much for your comments. This Remark was taken into account and a reference was added.

The site specific parameters were estimated for the study area.

Table, 3: Why the difference between activity concentrations of Ra, Th and K was observed in the samples? Does this mean due to the selection of samples?

Reply: The variation in Ra, Th and K activity concentrations is due to the fact that the samples were taken at different locations; given the differences in the distribution of naturally occurring radionuclides in the rocks and soils that make up the geology of the area and the excavation of the soils during the exploration of the Nkamouna-Kongo cobalt-nickel deposit, it is obvious that the activity concentrations will be different. A statistical comparison of two sets of samples (cobalt-nickel deposit ant nearby town) was performed. The results of the analyses showed that the samples from the excavated area had higher activity concentrations than those collected in Lomié town. Similarly, the activity concentrations of the control sample were all lower than those of the other samples. Details of the comparison of two sets of samples have been added to the manuscript.

• lines 239-242, These not enough to illustrate the reasons for the presence of radionuclides in the sample, explain in detail, why the activity concentrations are high due to ............?

Reply: The activity concentrations of ²³²Th, ²²⁶Ra and ⁴⁰K are higher in the soil samples collected in the immediate vicinity of the nickel site than in those collected in the nearby town. The variation of activity concentrations in the study area could be attributed firstly to differences in the distribution of natural radionuclides in the rocks and soils that make up the geology of the area but also to the variable deposition of the clay content, which constitutes the formation of the geology of the investigated area and secondly to the soil excavation during the exploration of the cobalt-nickel deposit of Nkamouna-Kongo.  This paragraph has been added to the manuscript.

Reviewer 3 Report

Brief summary

The article presents assessment of doses and risks from naturally occurring radionuclides in the Nkamouna-Kongo mining area. The assessment is done using the RESRAD family of codes, based on gamma spectrometric analysis of thirty surface soil samples from the area.

Comments

The text is easy to follow and understand, and it is adequately structured. 

In the Introduction, authors focus on detrimental effects of mining on the environment and highlight health risks associated with the industry.

In Materials and Methods, first the study area is described. Then, in section 2.2, the authors describe sample collection and analysis. This part is first key weakness of the article, as it is stated (line 115) that the samples were collected at the cobalt exploration site and in the nearby town, random method was chosen for soil sampling. Thus, there’s almost no context – we don’t know if the samples come from waste dumps of the mine, represent natural background, or possibly some totally different material brought to the city from elsewhere to make roads. GPS locations and description of the collection points would be useful.

If the work aims to support the hypothesis of “mining results in higher radiation levels”, this should be proven by statistical comparison of two sets of samples – on sites clearly resulting from the mining activity (e.g. waste rock/soil dumps) and untouched sites representing natural radiation background. This is not done in this work. The [32] reference doesn’t solve the problem.

In sec. 2.3.2, the text is in some parts extremely similar to the the DOE-STD-1153-2019, yet the standard is only referred to at line 197. In the view of the reviewer, this is improper.

Given that it is not known whether the radiation situation results from the mining industry or is a naturally present situation, the justification of calculation of effect of cover is questionable. Maybe the radiation situation is a normal local background and adding 1 m of cover would only bring 1 m of the same soil?

In Results, gamma spectrometry of the samples shows that samples they collected have concentrations of the Ra-226, Th-232 and K-40 well within the range of normal background conditions (see Table 5 in UNSCEAR 2000 report, Annex B, p. 115).  Yet, the authors proceed with calculation of BCGs for the site and doses to biota and risks to humans living there.

Next, authors present results of calculations with the RESRAD codes, including BCG levels, but they do not use them.

Overall, key drawbacks of the study are

• Inappropriate sampling methodology and missing local background information.
• Analysis of site where the radiation levels are close to normal.

Author Response

Reviewer #3: Reviewer's feedback

Brief summary

The article presents assessment of doses and risks from naturally occurring radionuclides in the Nkamouna-Kongo mining area. The assessment is done using the RESRAD family of codes, based on gamma spectrometric analysis of thirty surface soil samples from the area.

Comments

The text is easy to follow and understand, and it is adequately structured.

In the Introduction, authors focus on detrimental effects of mining on the environment and highlight health risks associated with the industry.

In Materials and Methods, first the study area is described. Then, in section 2.2, the authors describe sample collection and analysis. This part is first key weakness of the article, as it is stated (line 115) that the samples were collected at the cobalt exploration site and in the nearby town, random method was chosen for soil sampling. Thus, there’s almost no context – we don’t know if the samples come from waste dumps of the mine, represent natural background, or possibly some totally different material brought to the city from elsewhere to make roads. GPS locations and description of the collection points would be useful.

If the work aims to support the hypothesis of “mining results in higher radiation levels”, this should be proven by statistical comparison of two sets of samples – on sites clearly resulting from the mining activity (e.g. waste rock/soil dumps) and untouched sites representing natural radiation background. This is not done in this work. The [32] reference doesn’t solve the problem.

Reply: Thank you very much for your comments and suggestions that helped us improve the quality of the manuscript.

The study area is located in a cobalt-nickel deposit and its surroundings. This deposit has been explored by mining companies. Mining exploration involves excavation of the soil which contaminates the environment. It is therefore necessary to evaluate the exposure of the surrounding population and the workers on the site. To do this, we took samples in the area where the soil was excavated and in a nearby town that constitutes a natural background of radiation. A control sample was taken from an area that had not been subjected to anthropogenic activity. A statistical comparison of two sets of samples was made. The results of the analyses showed that the samples from the excavation area had higher activity concentrations than those taken in the town of Lomié. Similarly, the activity concentrations of the control sample were all lower than those of the other samples. Details on the comparison of two sets of samples have been added to the manuscript.

The map of the study area has been replaced by another map showing the sampling points.

The reference [32] is put here for comparison with the data from the study conducted in Batouri and Betare-Oya.

In sec. 2.3.2, the text is in some parts extremely similar to the the DOE-STD-1153-2019, yet the standard is only referred to at line 197. In the view of the reviewer, this is improper.

Reply: this remark has been taken into account and the reference has been added in the text.

Given that it is not known whether the radiation situation results from the mining industry or is a naturally present situation, the justification of calculation of effect of cover is questionable. Maybe the radiation situation is a normal local background and adding 1 m of cover would only bring 1 m of the same soil?

Reply: The explored area having undergone soil excavation is already contaminated. To this effect, the addition of 1m of cover to the explored part of the site is justified which allows to estimate the level of cleanup of the contaminated area.

In Results, gamma spectrometry of the samples shows that samples they collected have concentrations of the Ra-226, Th-232 and K-40 well within the range of normal background conditions (see Table 5 in UNSCEAR 2000 report, Annex B, p. 115).  Yet, the authors proceed with calculation of BCGs for the site and doses to biota and risks to humans living there.

Next, authors present results of calculations with the RESRAD codes, including BCG levels, but they do not use them.

Overall, key drawbacks of the study are

• Inappropriate sampling methodology and missing local background information.
• Analysis of site where the radiation levels are close to normal.

Reply: Thank you for these important remarks. It is well known that during an analysis with RESRAD-BIOTA, there are 3 steps to follow. The first step is to enter the maximum concentrations of each radionuclide into the RESRAD-BIOTA software for a level 1. The second step is to compare the maximum radionuclide concentrations with the generic BCGs and then add up all the fractions for each radionuclide and medium. If the sum of the fractions is less than 1.0, then the assessment is complete and the reasoning and results can be documented; if not, the analysis phase can proceed. Although in this study the sum of the fractions is less than 1, we proceeded to the analysis phase in order to use the results obtained (internal and external doses) as a database that can be used as comparative data in the environmental assessment planned during and after the mining in order to highlight its impact for a better radiological protection of the public and the environment.

Round 2

Reviewer 3 Report

Thank you for taking the time to modify the text and consider the comments.

Splitting the dataset into background (west) and the mining (east) sites is a great change. If I understand it correctly from reply of the authors, then:

• The town (west) is considered to represent natural background.
• The mining area (east) is considered contaminated.
• One (unspecified) point in is nature, clear of any human influence as a control sample.

The map (which is the same as in [4]) is much clearer. Consider clearly identifying the “non-anthropogenic” point and using different markers for the background and mining site sampling sites.

In your reply, you state:

A statistical comparison of two sets of samples was made. The results of the analyses showed that the samples from the excavation area had higher activity concentrations than those taken in the town of Lomié. Similarly, the activity concentrations of the control sample were all lower than those of the other samples. Details on the comparison of two sets of samples have been added to the manuscript.

1) Statistical comparison of the two sets would be very welcome. The text on lines 257-269 provides ranges and mean values for the two sets, plus a single data point for the non-anthropogenic point. Differences between the sets in (Bq/kg) are:

K-40 158-140=18
Th-232 86-74=12
Ra-226 46-44=2

The authors declare that there is a significant difference. From the statistical point of view (as authors are doing a statistical comparison), this statement should be supported by results of a statistical test (possibly a t-test?).

2) Results for the two spectrometry methods yielded quite different results – could you comment on that?

3) The dataset is now split into two. Then, to evaluate the excess risk and excess effective dose received as a result of the mining activity, shouldn’t a difference against the normal background be considered? E.g. the dataset split should propagate into the RESRAD calculations, so it can be said what difference (if any) does it make if one lives on either of the sites.

4) I tried to reproduce your RESRAD-ONSITE results with the provided data, but I am getting a different value of t=0. Could be an error on my site or an implicitly expected, but not described value of some parameter in the text. Could you please recheck the RESRAD settings described in the article? (Or provide the RESRAD report file as a supplement to reviewers, so we can cross-check the results).

5) Where does the precipitation value for RESRAD come from? Why is it different than the annual rainfall?

6) On line 120, reference [4] is cited, as Gondji et al.. However, in the list of references, Gondji is used as first name (and thus abbreviated). Please correct.

Key drawback of the study is still not fully done comparison between a control group (Lomie points) and the mining site group (Kongo points).

Author Response

Reviewer: Reviewer's feedback

Thank you for taking the time to modify the text and consider the comments.

Splitting the dataset into background (west) and the mining (east) sites is a great change. If I understand it correctly from reply of the authors, then:

• The town (west) is considered to represent natural background.
• The mining area (east) is considered contaminated.
• One (unspecified) point in is nature, clear of any human influence as a control sample.

The map (which is the same as in [4]) is much clearer. Consider clearly identifying the “non-anthropogenic” point and using different markers for the background and mining site sampling sites.

Response: Thank you again for your comments and suggestions that helped us improve the quality of the manuscript.

Regarding the map of the study area, we improved it by using a particular identification marker for the control sample and different markers for the Lomié sampling sites representing the natural radiation background and the mining site (see Figure 1).

In your reply, you state:

A statistical comparison of two sets of samples was made. The results of the analyses showed that the samples from the excavation area had higher activity concentrations than those taken in the town of Lomié. Similarly, the activity concentrations of the control sample were all lower than those of the other samples. Details on the comparison of two sets of samples have been added to the manuscript.

1) Statistical comparison of the two sets would be very welcome. The text on lines 257-269 provides ranges and mean values for the two sets, plus a single data point for the non-anthropogenic point. Differences between the sets in (Bq/kg) are:

K-40 158-140=18

Th-232 86-74=12

Ra-226 46-44=2

The authors declare that there is a significant difference. From the statistical point of view (as authors are doing a statistical comparison), this statement should be supported by results of a statistical test (possibly a t-test?).

Response: Thank you for this important remark. Indeed, "The p-values were calculated for the comparison purpose. Firstly, the activity concentrations obtained for the immediate vicinity of the mining site (disturbed area by mineral exploration) with those obtained at Lomié (undisturbed area), Activity concentrations of ⁴⁰K, ²³²Th and ²²⁶Ra in the samples taken at Lomié vary respectively from 77 to 212 Bq kg^-1; 54 to 97 Bq kg^-1 and 34 to 50 Bq kg^-1 with an arithmetic mean of 140 Bq kg^-1, 74 Bq kg^-1 and 44 Bq kg^-1, respectively. At the disturbed area the activity concentrations range from 46 to 340 Bq kg^-1 ; 40 to 126 Bq kg^-1 and 36 to 54 Bq kg^-1, respectively for ⁴⁰K, ²³²Th and ²²⁶Ra with an arithmetic mean of 158 Bq kg^-1, 86 Bq kg^-1 and 46 Bq kg^-1 respectively. The t-test was performed and a p-value of 0.275 was obtained. This result indicates that, there is no statistically significant difference between the data obtained in the undisturbed and disturbed areas (p-value > 0.05).

Secondly, activity concentrations of ²²⁶Ra, ²³²Th, and ⁴⁰K in the control sampling from an area with no anthropogenic activity are 34 Bq kg^-1, 54 Bq kg^-1, and 77 Bq kg^-1, respectively. The p-values calculated to compare the activity concentrations of the area with high mining potential and of Lomié to those of the control sampling are all equal to zero (p-value = 0). Thus, the radionuclide concentrations in the immediate vicinity of the cobalt site and in Lomié are significantly higher than those in the control sampling". This paragraph has been added in the manuscript.

2) Results for the two spectrometry methods yielded quite different results – could you comment on that?

Response: In this study, 30 soil samples were analyzed by the gamma-ray spectrometry technique, one part using the sodium iodide detector of the Research Center for Nuclear Science and Technology in Cameroon and the other part using the HPGe detector of the Institute of Radiological Emergency Medicine in Japan. The first idea was to measure all samples using the HPGe detector, which is more reliable than the sodium iodide detector. Unfortunately, due to logistical problems, only 15 samples were sent to Japan. In fact, there is a difference in the results obtained because the samples were taken at different points.

3) The dataset is now split into two. Then, to evaluate the excess risk and excess effective dose received as a result of the mining activity, shouldn’t a difference against the normal background be considered? E.g. the dataset split should propagate into the RESRAD calculations, so it can be said what difference (if any) does it make if one lives on either of the sites.

Response: Thank you for the comment to improve and add to this article. Indeed, it should be noted that the maximum activity concentration values were used as input values in the RESRAD codes and that, due to secular equilibrium, the progeny concentrations are the same as those of the primordial radionuclides. Thus, the results presented in this manuscript are those of excess risk and excess effective dose received due to mining activity. In order to enrich the present study, we have also evaluated the excess risk and excess effective dose received due to normal background. The texts «Excess risk and excess effective dose due to natural background radiation (Lomié) were also evaluated. The maximum total dose due to natural background radiation from ²²⁶Ra, ²³²Th and ⁴⁰K and their progeny of 0.410 mSv yr^-1 was obtained at date t = 1 year and the total dose contributions calculated with RESRAD-ONSITE for each radionuclide at date t = 1 year for the ground pathway, are 0.0733 mSv yr^-1 for ²²⁶Ra, 0. 0169 mSv yr^-1 for ²³²Th and 0.0281 mSv yr^-1 for ⁴⁰K.

It is clear that the impact of the mining activity (excavation of the ground during exploration) is visible because the results show that the excess effective dose received due to the mining activity is greater than that received in the area representing the natural radiation background (Lomié)» has been added in the manuscript.

4) I tried to reproduce your RESRAD-ONSITE results with the provided data, but I am getting a different value of t=0. Could be an error on my site or an implicitly expected, but not described value of some parameter in the text. Could you please recheck the RESRAD settings described in the article? (Or provide the RESRAD report file as a supplement to reviewers, so we can cross-check the results).

Response: We checked again the RESRAD parameters described in the manuscript and ran the code again, the results are the same. Indeed, U-238 is the natural uranium, of which all the progeny would be in equilibrium from the start. From the point of view of radiological exposure, the one that really counts is Ra-226.  So we put the progeny in equilibrium from the beginning, that is, we added Ra-226 to the initial screen of radionuclides with the same concentration as U-238. Also, we had taken into account the use of distribution coefficients that considerably influence the results in RESRAD. The RESRAD report file has been attached.

5) Where does the precipitation value for RESRAD come from? Why is it different than the annual rainfall?

Response: Thank you for this important comment. The precipitation value for RESRAD is the same as the annual precipitation value. This is a typing error. We have corrected it in the manuscript.

6) On line 120, reference [4] is cited, as Gondji et al.. However, in the list of references, Gondji is used as first name (and thus abbreviated). Please correct.

Response: This reference has been corrected in the manuscript.

Key drawback of the study is still not fully done comparison between a control group (Lomie points) and the mining site group (Kongo points).

Response: Thank you, this comment has been addressed in question 1.

Author Response File: Author Response.pdf

Round 3

Reviewer 3 Report

Thank you for taking the time to modify the text and consider the comments and thank you for the thorough answers.

In the opinion of the reviewer the authors have now correctly compared the gamma spectrometry results from the disturbed and undisturbed area and found that there are no statistically significant differences between radionuclide concentrations measured at those sites.

That does make for an interesting study of the local conditions and provides a good base for extended study of the mining areas in the region.

However, authors still proceed with RESRAD evaluation of the site as contaminated area, even though they themselves prove that the radionuclide concentrations at the mining site are not elevated, compared to the Lomie city. The use of maximum concentration values does lead to a difference in doses at the two sites, however, then the results can hardly be presented as relevant for the whole contaminated area.

Quoting from User’s manual for RESRAD 7.2 (p. 30), available at https://resrad.evs.anl.gov/docs/RESRAD-ONSITE_7_2_UserGuide.pdf?v2

“Area of Contaminated Zone: The area of the contaminated zone is a compact area that contains the locations of all soil samples with radionuclide concentrations that are clearly (two standard deviations) above the background.“

Concluding:

The article has been vastly improved. However, the included statistical analysis confirms the expectation that the radiation concentrations on the disturbed site are normal in the region, at least when compared to the Lomie area. Therefore, working with the contaminated zone and excess risk concept seems to be unjustified.

The reviewer suggests that the RESRAD part is omitted, as the mean concentrations even at the disturbed site are statistically not different from the natural background, and the calculation of excess risk is therefore misleading.

Author Response

Reviewer: Reviewer's feedback

Thank you for taking the time to modify the text and consider the comments and thank you for the thorough answers.

In the opinion of the reviewer the authors have now correctly compared the gamma spectrometry results from the disturbed and undisturbed area and found that there are no statistically significant differences between radionuclide concentrations measured at those sites.

That does make for an interesting study of the local conditions and provides a good base for extended study of the mining areas in the region.

However, authors still proceed with RESRAD evaluation of the site as contaminated area, even though they themselves prove that the radionuclide concentrations at the mining site are not elevated, compared to the Lomie city. The use of maximum concentration values does lead to a difference in doses at the two sites, however, then the results can hardly be presented as relevant for the whole contaminated area.

Quoting from User’s manual for RESRAD 7.2 (p. 30), available at https://resrad.evs.anl.gov/docs/RESRAD-ONSITE_7_2_UserGuide.pdf?v2

“Area of Contaminated Zone: The area of the contaminated zone is a compact area that contains the locations of all soil samples with radionuclide concentrations that are clearly (two standard deviations) above the background.“

Concluding:

The article has been vastly improved. However, the included statistical analysis confirms the expectation that the radiation concentrations on the disturbed site are normal in the region, at least when compared to the Lomie area. Therefore, working with the contaminated zone and excess risk concept seems to be unjustified.

The reviewer suggests that the RESRAD part is omitted, as the mean concentrations even at the disturbed site are statistically not different from the natural background, and the calculation of excess risk is therefore misleading.

Response: Thank you once again for your feedback.

In fact, the main objective of this study is to calculate the doses and associated risks of natural radiation exposure at the cobalt-nickel zone of Kongo after the mineral ore exploration conducted at the site by the American mining company GEOVIC.

For this purpose, we selected the RESRAD as our main tool of calculation to obtain the doses and the associated risks. And the present data will be considered as a benchmark data which will be used as baseline for comparison with the data of the future studies during and after mining activities at the site.

The maximum concentrations of the primordial radionuclides were used as input values in RESRAD.

The RESRAD is the main tool for dose and risk assessment in our present study. The International Atomic Energy Agency is promoting the use of RESRAD codes by its Member States through the organization of training courses in Argonne National Laboratories. If the paper is accepted, this will contribute to enlarge the use of these codes in the African Region in particular where mining is growing.