Radon and radium Concentrations in drinkable water supplies of the Thu Duc region in Ho Chi Minh city, Vietnam

KHẢO SÁT NỒNG ĐỘ 222Rn VÀ 226Ra TRONG NƯỚC SINH HOẠT TẠI KHU VỰC THỦ ĐỨC, THÀNH PHỐ HỒ CHÍ MINH Huỳnh Nguyễn Phong Thu, Lê Công Hảo, Nguyễn Văn Thắng, Lê Qu c Bảo, Trương Thị Hồng Loan Khoa Vật lý Vật lý Kỹ thuật, Trường ĐH KHTN, ĐHQGHCM Tóm tắt Radon và radi là các nguồn bức xạ tự nhiên ảnh hưởng nhiều đến sức khỏe con người. Nồng độ 222Rn và 226Ra trong nước gồm 14 mẫu nước uống công cộng, 15 mẫu nước máy và 20 mẫu nước giếng thuộc khu vực Thủ Đức, thành phố Hồ Chí Minh được khảo sát trong nghiên cứu của chúng tôi. Các phép đo được thực hiện bằng detector RAD7. Phông được xác định bằng cách sử dụng các mẫu nước cất. Nồng độ 222Rn thay đổi phụ thuộc vào nguồn nước khảo sát. Tuy nhiên, các giá trị này hoàn toàn nằm dưới mức giới hạn của cơ quan bảo vệ môi trường Mỹ, 11,1 Bq.L1 . Nồng độ 222Rn cao nhất là (4,16 0,20) Bq.L1 . Vì vậy, xét về phương diện bức xạ ion hóa gây ra bởi 222Rn, các nguồn nước khảo sát an toàn sử dụng. Nồng độ 226Ra được xác định thông qua nồng độ 222Rn tích lũy sau 10 ngày nhốt mẫu. 10 mẫu nước giếng có nồng độ 226Ra vượt quá giới hạn cho phép, 0,185 Bq.L1 .

Author’s Accepted Manuscript

Radon and radium Concentrations in drinkable

water supplies of the Thu Duc region in Ho Chi

Minh city, Vietnam

Le Cong Hao, Huynh Nguyen Phong Thu, Nguyen

Van Thang, Le Quoc Bao

PII: S0969-8043(15)30174-3

DOI: http://dx.doi.org/10.1016/j.apradiso.2015.08.033

Reference: ARI7122

To appear in: Applied Radiation and Isotopes

Received date: 12 May 2015

Revised date: 13 August 2015

Accepted date: 24 August 2015

Cite this article as: Le Cong Hao, Huynh Nguyen Phong Thu, Nguyen Van Thang and Le Quoc Bao, Radon and radium Concentrations in drinkable water supplies of the Thu Duc region in Ho Chi Minh city, Vietnam, Applied Radiation and Isotopes, http://dx.doi.org/10.1016/j.apradiso.2015.08.033

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Radon and Radium Concentrations in Drinkable Water Supplies of the Thu Duc Region

in Ho Chi Minh City, Vietnam Names of the authors: Le Cong Hao, Huynh Nguyen Phong Thu, Nguyen Van Thang, and Le Quoc Bao

Title: Radon and Radium Concentrations in Drinkable Water Supplies of the Thu Duc Region in Ho Chi Minh City, Vietnam

Affiliation(s) and address(es) of the author(s): Nuclear Technique Laboratory, HCM, University of Science, Vietnam

VNU-E-mail address of the corresponding author: lchao@hcmus.edu.vn

Radon and Radium Concentrations in Drinkable Water Supplies of the Thu Duc Region in Ho Chi Minh City, Vietnam

Le Cong Hao, Huynh Nguyen Phong Thu, Nguyen Van Thang, and Le Quoc Bao Nuclear Technique Laboratory, VNU-HCM, University of Science, Vietnam

Abstract

The results of 222Rn and 226Ra activity measurements in drinkable water supplies of the Thu Duc region in Ho Chi Minh City, Vietnam, are presented in this paper The measurements were performed using a RAD 7 radon detector manufactured by Durridge Company, Inc Mean concentrations of 222Rn and 226Ra were found to be 0.11 ± 0.01 Bq.l-1 and 0.11 ± 0.02 Bq.l-1 in 14 drinking water samples They are 0.12 ± 0.01 Bq.l-1and 0.10 ± 0.02 Bq.l-1 in 15 tap water samples, respectively The mean 222Rn concentration of 1.40 ± 0.03 Bq.l-1 in the 20 groundwater samples of this study is also lower than the WHO advised level of 100 Bq.l-1 Fifty percent of groundwater samples analysed have 226Ra levels in excess of the USEPA recommended maximum contaminant level of 0.185 Bq.l-1 The occurrence of elevated concentrations of 226Ra in groundwater samples was explained by pH and alkaline conditions

have suggested that the source of the radionuclides dissolved in the water is the surrounding bedrock; in geological settings rich in 238U, like, for example, granite, also higher activity concentrations of the uranium series of radionuclides like 226Ra and 222Rn are expected (Wallner et al, 2007)

226

Ra and 222Rn are essentially soluble in water, thus enter groundwater by the dissolution of materials in water layers, removal of rock or soil surfaces and expulsion from minerals by radioactive decay (Sahin et al, 2013) 226Ra has a long half-life of 1600 years and behaves as calcium, tracing the calcium path in the body partially deposited in the bone tissue The alpha-particle emission of radium makes it a carcinogen, with the continual accumulation of 226Ra in the bone tissue being a known cause of bone cancer (Porntepkasemsan and Srisuksawad, 2008) 222Rn a decay product of 226Ra has a half-life

of 3.82 days and is a tasteless radioactive gas, inert, colourless, and odourless Therefore, human beings are exposed to 222Rn in two ways, either through inhalation or ingestion (Khattak et al, 2011) When radon decays after being inhaled or ingested, it releases energy that can damage parts of living tissue, which may lead to the unnatural reproduction of a cell and an increased risk of getting cancer

It is well known that the different radionuclides are not in radioactive equilibrium with each other due to differences in mobilisation from the rock and water chemistry So, for example, radon activities can be three to five orders of magnitude higher than U or Ra activities, probably due to absorption of the U and Ra into the host rock, while the gaseous Rn diffuses along microcrystalline imperfections into the interstitial waters (Wallner et al, 2007) High levels of radon in drinking water represents a potential health risk due to human exposure through both inhalation and ingestion of radon In water prior

to drinking, alpha particles emitted by radon and its decay product quickly lose their energy and are taken up by other compounds in water Therefore, the exposure of the ingested radon along with the intake of water is less than the inhaled radon from the radon exhalation from the same water (Somashekar and Ravikumar, 2010) In the case of radium, human exposure through the ingestion/consumption of radium in drinkable water should be taken into account

With regard to water contamination and public health, the purpose of this study was

The occurrence of elevated concentrations of 226Ra and the correlation between 222Rn and

226

Ra concentrations in groundwater samples were also presented

Experimental

Description of the site

Ho Chi Minh City (hereafter HCMC) is located in the south of Vietnam, and is the biggest city in Vietnam It is located from 10° 10’ to 10° 38’ North and 106° 2’ to 106° 54’ East It is 1,730 km from Hanoi and is at the crossroads of international maritime routes Figure 1 shows the location map of the site in Thu Duc in Ho Chi Minh City, Vietnam At present, four water resources are used for water supply in HCMC They are (a) the Dong Nai River, (b) the Sai Gon River, (c) groundwater and (d) rainwater The Dong Nai River originates from the Di Linh highland in Lam Dong province and connects to the East Sea through the Soai Rap estuary The section of the Dong Nai River

in HCMC spreads from District 9 and intersects the Nha Be River While a section of the Sai Gon River in HCMC originates from the Phu My commune to Thanh My Loi, District 2 and water from the Hoa An water intake station on the Dong Nai River is pumped to the Thu Duc water treatment plant with a capacity of 650,000 m3/day HCMC also has the following five aquifers, namely, (i) Holocene, (ii) Pleistocene, (iii) Upper Pliocene, (iv) Lower Pliocene and (v) Mesozoic Over 150,000 wells/boreholes were exploited in HCMC Three of the five aquifers play an important role in terms of water supply for HCMC: the Pleistocene aquifer (20–50 m), the upper Pliocene aquifer (50–100 m) and the lower Pliocene aquifer (100–140 m) (Institute for Global Environmental Strategies (IGES), 2007)

Water sampling

In this study, a total of 49 water samples were collected using the techniques proposed by the manufacturer (RAD7 RAD H2O) All water samples (without bubbles) were collected in dedicated 250-ml glass bottles 14 drinking water samples were collected from universities and dormitories (in the University Village), while 35 drinking water samples were collected from the source drilled wells (tube wells) and water taps During sampling a water, the water source flowed for 10 minutes before taking the

sample (RAD7 RAD H2O), in order to let out the water from a possibly stagnant pipe section and to obtain parameters characteristic of the fresh water pH values are an important indicator of water quality because water with a low pH can damage the piping

of the distribution system, leading to contamination (Sahin et al, 2013) The pH measurements for each sample/location were done by using a portable “OAKTON pH TESTER 30” device

222

Rn activity concentration measurements

A RAD 7 H2O detector (hereafter RAD-W) manufactured by Durridge Company was used to make direct readings of the radon concentrations in the water samples The RAD-

W setup consists of three components, namely: a) a water vial with an aerator; b) the desiccant tube and c) the alpha detector Figure 2 shows a diagrammatic illustration of the radon-monitor using a RAD-W for measuring radon concentrations in water samples For accurate readings, the RAD-W should be dried out thoroughly before making measurements High humidity levels reduce the efficiency of collection of the 218Po atoms, formed when radon decays inside the chamber As always, the relative humidity inside the instrument will stay below 10% for the entire 30-minute measurement period Then each measurement was carried out for three hours The RAD-W method employs a closed-loop aeration scheme whereby the air and water volumes are constant and independent of the flow rate The air is recirculated through the water and continuously extracts the radon until a state of equilibrium develops The RAD-W system reaches this state of equilibrium within about 5 minutes after which no more radon can be extracted from the water The extraction efficiency (percentage of radon removed from the water into the air loop) is around 94% for a 250 ml sample The exact value of the extraction efficiency depends somewhat on the ambient temperature, but it is almost always well above 90% The most significant background effects in the RAD-W are counts from radon daughters and traces of radon left from previous measurements

The RAD 7 has the unusual ability to distinguish between the ‘‘new’’ radon daughters and the ‘‘old’’ radon daughters left from previous tests Even so, a very high radon sample can cause daughter activity that can affect the next measurement (RAD7 RAD H2O) In order to determinate the background level, a distilled water sample is used

as a radon-free water sample The background sample was measured using the same

protocol The minimum detectable activity (MDA) was estimated to be 0.073 Bq.l-1

226

Ra Activity Concentration Measurements

After the latest results of radon activity in water, the bottles were tightly closed to

allow the concentration of radon from radium in the samples to increase The same

experimental method as was used for the radon measurements was followed to measure

the radium content of the samples The evaluation of the concentration of soluble radium

salts in water was performed after 10 days Considering that after that time the radon

concentration was reaching the secular equilibrium, the radioactivity of radium (226Ra)

soluble compounds and radon (222Rn) could be calculated from equation (1):

Rn C t Rn e C k Ra C

where CRn is the measured radioactivity of 222Rn after 10 days, Rn is the decay constant

of 222Rn, kC is the correction factor for both escape or leakage and counting efficiency

and CRa is the activity of radium compounds which are soluble in water that could be

considered constant during the time interval t of 10 days

The kC was determined by using a Standard Reference Material (SRM) capsule of

NIST The SRM capsule contained 226Ra with an activity of approximately 5 Bq The

SMS capsule was stored in 250 ml of distilled water for 10 days Equation (2) presents

the correction factor

Rn

NIST Rn

C

C ( )

C

where CRn(NIST) is the activity of radon calculated from NIST, and CRn is the measured

radioactivity of 222Rn from RAD-W The evaluation of the correction factor was found to

be 1.250.03

Dose assessment

The annual effective doses for ingestion and inhalation were estimated according to parameters introduced by a UNSCEAR report (UNSCEAR, 2006) The annual effective dose as a result of the intake of radon or radium, Ew (Sv.y-1) is calculated on the basis of the mean activity concentration using equation 3:

Ew V wC w (3) where,  (Sv.Bq-1) is the ingesting dose conversion factor of radon or radium (10-8Sv.Bq-1 for radon and 2.8x10-7 Sv.Bq-1 for radium), Vw is the water consumption rate (730 L.y-1 was assumedfor a ‘standard adult’ drinking the same water directly from the source point (Galán López et al, 2004, Somlai et al, 2007, Todorovic et al, 2012) and Cw

(Bq.l-1) is the radon or radium concentration in water

Results and discussion

Table 1 summarises the concentration results of radon and radium in the 14 drinking water samples collected from the University Village The highest radon concentration was measured in TDTT to be 0.36 ± 0.06 Bq.l-1 Mean concentrations of 222Rn in these water samples were calculated to be 0.11 ± 0.01 Bq.l-1 Following that, the highest radium concentration was found to be 0.18 ± 0.08 Bq.l-1 in the QT and the mean concentration of 226Ra was calculated to be 0.11 ± 0.02 Bq.l-1

The annual effective dose contributions from radon and from radium in the drinking water samples are also given in Table 1 It was found that the mean annual effective dose for ingestion (radon) ranged was calculated to be 0.78 ± 0.07 µSv.y-1 The mean dose contribution from radium in water to the annual dose resulting directly from the water intake ranges was found to be 22.57 ± 3.74 µSv.y-1

The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) has provided the mean dose from radon in water As a result of ingestion the mean radon dose is 2 µSv.y-1 (UNSCEAR, 2000) It is found that our average annual effective dose due to ingestion is well below the reference level values of UNSCEAR and

hence does not pose any health problems as a consequence of the radon dose received from drinking water in the study area

Similarly, the highest radon concentration in 15 tap water samples in Table 2 was found to be 0.20 ± 0.05 Bq.l-1 in M1 with an average value of 0.12 ± 0.01 Bq.l-1 and the average annual effective dose for ingestion was found to be 0.86 ± 0.07 µSv.y-1 The highest radium concentration was found to be 0.17 ± 0.08 Bq.l-1, then the mean annual dose resulting directly from water intake was 18.98 ± 3.482 µSv.y-1 It has been observed that these average values obtained are also under UNSCEAR safety limits

Table 3 summarises the concentration results of radon and radium in the groundwater samples collected from different drilled wells (tube wells) The recorded 222Rn activities

in 20 groundwater samples were found to vary from 0.44 ± 0.07 to 4.16 ± 0.20 Bq.l-1 with

an average value of 1.40 ± 0.03 Bq.l-1 consequently the average annual effective dose for ingestion was found to be 9.97 ± 0.20 µSv.y-1 The radium concentration was found to range from 0.08 ± 0.06 to 0.54 ± 0.12 Bq.l-1, with an average value of 0.18 ± 0.02 Bq.l-1then the mean annual dose resulting directly from water intake was 36.66 ± 3.74 µSv.y-1

In groundwater, findings suggested that the concentration of radium was consistently controlled by the geochemical properties of the aquifer systems, with the highest concentrations most likely to be present where, as a consequence of the geochemical environment, adsorption of the radium was slightly decreased (Szabo et al, 2012) The three water-chemistry groups defined by low pH, low dissolved oxygen (DO) concentrations, or by the combination of both factors were supported to explain the occurrence of elevated concentrations of radium in the groundwater samples This hypothesis was then confirmed by a study on the relationship between concentrations of

226

Ra and pH for twenty groundwater samples

Table 3 summarises the pH values and concentration results of radon and radium in the groundwater samples collected from different tube wells It was observed that the pH values range from 3.45 to 7.9, the lowest pH value was measured in G5 to be 3.45 and the highest pH value was measured in G16 to be 7.9, respectively The radium concentration was found to range from 0.08 to 0.54 Bq.l-1 with the lowest radium concentration measured also in the G8 (pH = 4.91) and the highest in the G1 (pH = 5.17) samples In this study low-pH conditions were most commonly found in the 20 groundwater samples

As a result, a pH of less than about 6.5 strongly enhances the mobility of radium into groundwater

Figure 3 shows the relationship between concentrations of 226Ra and pH for twenty groundwater samples collected from different tube wells It should be noted that concentrations of 226Ra in excess of 0.185 Bq.l-1 (US Environmental Protection Agency (USEPA), 2000) were observed in acidic water samples (pH was less than 6.3) at a frequency of 40% In particular, the maximum 226Ra concentration of 0.54 Bq.l-1 was associated with acidic water (pH = 5.17) in the G1 sample This result may be explained

by the fact that the Pleistocene aquifer is widely located under the whole area and is exposed in Thu Duc district and some others The high iron concentrations of groundwater in HCMC and low pH levels of most surveyed wells are the dominant reasons for high concentrations of 226Ra in these groundwater samples This means that under low pH values and (or) anoxic conditions, the iron compounds can dissolve and decrease the likelihood of the adsorption of radium onto aquifer materials enhancing the mobility of radium into groundwater Thus, low pH value is the most important water parameter linked to high radium concentration (Almeida et al, 2004)

With two exceptions in the cases of samples from G12 and G16 (pH was larger than 7.5), 226Ra concentrations were found to be in excess of 0.185 Bq.l-1 with a frequency of 10% These two values indicate that the occurrence of elevated concentrations of 226Ra in the groundwater samples was not only dependent on low pH levels but also on some other parameters (Szabo et al, 2012) In fact, 226Ra is chemically reactive and reacts similarly to other divalent alkaline earth cations such as Ca and Sr and is most similar to

Ba Thus, under high pH (pH>7.5) values, there is an increase of the mineral surface or increasing stability of inorganic complexes such as chlorides so that the increase in 226Ra mobility along with an increase in mineralisation is mostly attributed to competitive exchange with similar ions Therefore, the most significant reasons for elevated 226Ra concentrations throughout the G12 and G16 samples are the low DO concentrations and alkaline conditions

In order to investigate the origin of the radon from 20 groundwater samples, a relationship between radon and radium levels was investigated as in Figure 4 The

between 222Rn and 226Ra in these samples This suggests that the majority of 222Rn found

in water samples did not originate from 226Ra compounds soluble in water The biggest concentration of 222Rn may have originated from gas exhalation by the soil adjacent to the well due to a difference in physical-geographic characteristics of underground sources

of water The geographical characteristics of water regions with different concentrations

of 238U in rock, soil and water, or the temperature, salinity and turbidity of water may play a key role in accelerating the rate of increase of 222Rn concentrations in these 20 groundwater samples

Conclusions

From the results for the 14 drinking water samples, and 15 tap water samples, it can be concluded that these drinkable sources of water are low health risk from the standpoint of the concentrations of radon (100 Bq.l-1) and radium (0.185 Bq.l-1) in them

It was found that none of the radon concentrations of the twenty groundwater samples are higher than the advised limit set by the WHO of 100 Bq.l-1(World Health Organisation (WHO), 2008) The relationship between radon and radium levels was weak and 222Rn may have originated from differences in the physical-geographic characteristics

of underground sources of water

Concentrations of 226Ra were greater than 0.185 Bq.l-1 in 10 (50%) of the 20 samples analysed for this isotope Typically, for 40% of the 20 samples the pH of the water was lower than 6.3 and for 10% of these samples the pH of the water was greater than 7.5 The occurrence of elevated concentrations of radium in these waters was explained by pH and alkaline conditions

Acknowledgements

This work was supported by the grant C2014-18-26 for Vietnam National University -

Ho Chi Minh City (VNU-HCM) The authors would like to thank the reviewers, English proofreaders and editors for their thorough review and highly appreciated comments and suggestions, which significantly contributed to improving the quality of this manuscript