Journal of Environmental Radioactivity 44 (1999) 85—95

Occurrence of radon in the central region groundwater of Saudi Arabia Abdulrahman I. Alabdula’aly King Abdulaziz City for Science and Technology, P.O. Box 6086, Riyadh-11442, Saudi Arabia Received 1 February 1997; revised 15 April 1998; accepted 18 May 1998

Abstract Radon levels were measured in eight water supply municipalities of the Central Region of Saudi Arabia. Samples were collected from 77 wells and 6 treatment plants supplying drinking water to over 500 000 inhabitants. The well water radon level was in the range of 0.89— 35.44 Bq/l with an overall weighted geometric mean value of 8.80 Bq/l. Most of the raw water radon was removed by the plants’ treatment processes. Aeration and filtration resulted in 60.5% radon removal compared to a removal in the range of 78.7—96.5% in treatment plants containing reverse osmosis or electrodialysis processes along with aeration. The plants’ product water contains radon levels in the range of 0.15—5.71 Bq/l whereas two water systems with no treatment contain levels of 2.07 and 1.19 Bq/l.  1999 Elsevier Science Ltd. All rights reserved. Keywords: Groundwater; Radioactivity; Radon; Water treatment

1. Introduction The Central Region of Saudi Arabia which occupies about 23% of the Kingdom’s surface area of 2.25 Mkm relies on groundwater for domestic, agricultural and industrial uses. Only Riyadh depends on both treated groundwater and desalinated sea water. Groundwater treatment plants are located in the major cities and towns of the region. At present there are a total of 12 major plants in the region that utilize reverse osmosis (RO), electrodialysis (ED), and conventional water treatment processes. Six of those plants are located in the Riyadh area with a maximum total production capacity of 390 000 m/d. The remaining six are located in Buraydah, Unayzah, Al-Majmaah, Alzulfi, Al-Rass, and Al Quwaiyah (Fig. 1) (Alabdula’aly, 1995). Towns such as AlKharj and Shaqra have only water chlorination. The average daily water production of those public water supplies (excluding Riyadh) is about 240 000 m serving about 500 000 inhabitants. 0265-931X/99/$ — see front matter  1999 Elsevier Science Ltd. All rights reserved PII: S 02 6 5-9 3 1X (9 8) 0 0 06 3 - 0

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Fig. 1. Main cities of the Central Region of Saudi Arabia.

The main source of water for the Central Region of the country is the nonrenewable fossil groundwater stored in the sedimentary deep aquifers. The primary aquifers in the region are Saq, Tabuk, Minjur and Dhruma which consists mainly of sandstone, limestone and dolomite. The Saq and Tabuk sandstone formations are known to be the best aquifers in Saudi Arabia in terms of water quality and development potential. The average thickness of Saq aquifer ranges from 400 to 950 m

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whereas Tabuk aquifer has a thickness in the range of 300 —1100 m (MAW 1984). The average thickness of Minjur aquifer ranges from 185 to 400 m and that of Dhruma aquifer ranges from 185 to 375 m. The groundwater temperatures of the region aquifers depend on the depth of extraction with values ranging from 40 to 65°C. The water quality varies from one area to another but, in general, it is considered brackish with total dissolved solids of up to 6000 mg/l with an average value of about 2000 mg/l. The dominant ions are Ca, Cl, Na and SO . Wells are drilled to a depth of  up to 2000 m. The most commonly naturally occurring radionuclides in groundwater that pose health risks are Rn, Ra, and Ra. Uranium, if present in large amounts in drinking water is harmful due to its chemical toxicity. Radon-222 is an alpha-emitting noble gas that is found in various concentrations in groundwater. Exposure to radon has been associated with lung cancer in miners (Sevc et al., 1976; Archer et al., 1973). Since no limits for radon in drinking water supplies have been set yet, radon monitoring by regulatory authorities has not been established. This had led to a lack of knowledge about the geographic distribution of radon which is needed in water supply planning and development. The USEPA has proposed in 1991 a maximum contaminant level (MCL) of radon in public drinking water supplies serving more than 25 residences to be 11.11 Bq/l (USEPA, 1991). In the USA, an extensive survey studies have been undertaken to analyze for Rn-222 in groundwater supplies. The main factors that affect radon levels are the hydrology and hydrogeology of the aquifer in addition to the size of water resource and consumption rate. Extremely high concentration levels of radon are found in small public groundwater supplies, noncommunity, nontransient or private home groundwater supplies. Elevated levels were found primarily in Virginia, North and South Carolina, New England and at scattered locations throughout the Midwest and Western parts of USA. Only localized areas of the eastern parts showed relatively high levels of Rn-222 (370 Bq/l and higher) (Horton, 1985). Radon concentrations in 283 wells, mostly private, drilled into crystalline bedrock in the Piedmont region east of Atlanta, GA had a geometric mean value of 110 Bq/l. On the other hand, the corresponding values for a muscovite—biotite granite gneiss, and amphibolite inter-layered with muscovite—biotite schist were 230 and 100 Bq/l, respectively (Dillon et al., 1991). In Connecticut, analysis of water samples from 147 bedrock wells for radon showed that groundwater from Iapetos and Avalonian terrenes have significantly higher activities. The respective median values exceeded 133 and 237 Bq/l (Dupuy et al., 1992). The study of radon occurrence in groundwater supplies of the Central Region of Saudi Arabia was performed for the first time in 1993—1994. It was aimed at the evaluation of radon levels in the water supplies of the major cities and towns of the region in addition to the evaluation of its removal in the water treatment plants. Results for the Riyadh region indicate that radon levels are in the range of 0.27—10.73 Bq/l and 0.23—6.82 Bq/l for shallow and deep wells, respectively (Alabdula’aly, 1996). With water treatment plants cooling processes, radon was removed by a range of 74—96%, resulting in a maximum value of 0.56 Bq/l in the plants product water. The study results of eight major public water supplies in the central region of Saudi Arabia are presented in this paper.

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2. Materials and methods 2.1. Study area The study area under consideration covers the major public water supplies of the Central Region of Saudi Arabia with the exception of Riyadh city (Fig. 1). Samples were collected from eight of those supplies of which six have water treatment plants, and two have only chlorination. Table 1 lists the cities, towns, and villages that are supplied by those eight public water systems along with the treatment plant capacities, treatment processes, number of feeding wells, and number of collected samples. Buraydah plant supplies drinking water to the city of Buraydah, the second largest city in the region some 350 km northwest of Riyadh. The plant contains aeration, filtration and chlorination treatment processes and is supplied by 21 drilled wells in the vicinity of the city. Unayzah plant supplies drinking water to the city of Unayzah and contains softening, filtration, reverse osmosis, and post treatment (pH adjustment and chlorination) and is fed by 10 drilled wells near the plant. In addition to the water produced by its plant, Unayzah city is additionally supplied with water from 18 wells located within the city itself and their water is chlorinated and blended with that of the plant product. Al Rass water treatment plant was commissioned in February 1994 and supplies water to Al Rass and Al Badayi. Its treatment processes are similar to those of Unayzah plant with the addition of raw water aeration process. The plant is fed by 15 nearby wells. Alzulfi and Al Majmaah plants supply drinking water to the two cities and are fed by 5 and 3 wells, respectively. Both plants contain aeration, filtration, reverse osmosis and post treatment processes, in addition to that, Al Majmaah plant has a softening process as well. Al Quwaiyah plant is located about 150 km south west of Riyadh. The plant contains aeration, filtration, electrodialysis, and chlorination processes. It is fed by six wells and supplies water to the city of Al Quwaiyah. Al Kharj city which is located 40 km south of Riyadh is supplied with water from eight wells with only chlorination treatment. Shaqra public water supply consists of eight wells located about 65 km west of the city. Water is chlorinated and distributed to the city of Shaqra, some 200 km north west of Riyadh, and Ushaiqer, Marat, Athaithiah, Al Kraen, and Thurmeda. The average daily water supply was about 8200 m in 1992. 2.2. Sample collection Samples were collected from wells supplying water to eight public water supply systems at three different times, from influent to the existing six water treatment plants and at different stages of the treatment processes (2—10 times). Sampling was carried out according to the USEPA method (USEPA 1978), by connecting one end of a short hose to the sampling valve and the other end to a funnel. Water was allowed to slowly overflow for about 5 min and then 10 ml was drawn with a pipet. The sample was released into a scintillation vial that contains 10 ml of an organic scintillation cocktail. The vial was recapped, shaken, then stored in an ice box. Samples were collected in

6700 27000 8200

14 000 8700

96 000 51 000 51 000

Plant design production capacity (m/d)

1985 1980 1987 1985 — —

1985 1985 1994

Comissioning date

A, F, E, C C C

A, F, R, P A, S, F, R, P

A, F, C S, F, R, P A, S, F, R, P

Treatment processes

Saq. Minjur Saq.

Dhurma Minjur

Saq. Saq. Saq.

Aquifer

6 8 8

5 3

24
28 15

No. of supplying wells

4 7 6

4 3

22 23 8

No. of sampled wells

A: aeration; C: chlorination; E: electrodialysis; F: Filtration; P: post treatment (pH adjustment and chlorination); R: reverse osmosis; S: softening.
21 wells supplying water to plant and 3 wells directly to residence.  10 wells supplying water to plant and 18 wells directly to residence.  represent total wells production with only chlorination.

Al Quwaiyah AlKharj Shaqra

Al Quwaiyah AlKharj Shaqra, Marat, Athaithieh, Thurmeda

Buraydah Unayzah Ar Rass Al Badeyi Al Zulfi Al Majmaah

Buraydah Unayzah Al Rass

Al Zulfi Al Majmaah

Towns served

Water supply

Table 1 Scope of the survey of public water supplies of the Central Region A.I. Alabdula+aly /J. Environ. Radioactivity 44 (1999) 85—95 89

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duplicate, one immediately after the other. Counting was performed after a period of 8—16 h so that secular equilibrium of radon with its progeny is achieved. The vials used in the study are of teflon coated polyethylene from Zinsser, Germany. The used cocktail is a high efficiency mineral oil (NEF-957A) from New England Nuclear/Dupont, USA. 2.3. Radon determination All samples were analyzed for Rn using a commercial liquid scintillation counter (LSC) Wallac 1220 Quantulus. The technique was originally developed by Prichard et al. (1977). Detection limits of radon as low as 0.74 and 0.15 Bq/l for 40 and 1000 min, respectively, were reported using the LSC technique (Prichard et al., 1992). The counting time used throughout this study was 120 and 400 min for samples and their background, respectively. Standards for Rn counting were made from the dilution of a USEPA (National Bureau of Standard-Traceable) Ra standard solution. The original radon concentration which was present at the time of sampling was calculated by correction of radioactivity decay. Results are expressed in becquerel per liter (Bq/l) with an associated error of two-sigma confidence interval ($2p).

3. Results and discussion 3.1. Radon levels in well water The maximum, minimum and geometric mean values of radon levels in the 77 surveyed wells of the Central Region of Saudi Arabia are shown in Table 2. The maximum radon concentration of 35.44 Bq/l was found in a well feeding Al Majmaah water treatment plant whereas the minimum value of 0.89 Bq/l was found in a well feeding the city of AlKharj. Only 27 out of all sampled wells gave geometric mean radon values of more than 11.11 Bq/l. However at these levels, water is considered to be low in radon content and it would be expected that lower levels will be reached before consumption due to treatment and decay. Out of the three wells supplying water to Al Majmaah plant one was shallow and gave the highest radon levels (35.44 Bq/l). The same observation was noticed in Riyadh wells in which the shallow wells water contained on the average 3.44 Bq/l as compared to an average value of 2.29 Bq/l for the deep wells water (Alabdula’aly, 1996). High radon values in shallow wells could be the result of either underground faults or the pumping stress. The overall weighted geometric mean of radon in well water for the region is 8.80 Bq/l. By analogy, the observed values of radon concentration in drinking water of the southern region of Saxony and Thuringia of Germany ranges between 45 and 720 Bq/l, with a medium value 11 Bq/l (Freyer et al., 1997). In Finland, the mean radon concentrations found in the tap water and untreated groundwater were 24.8 and 59.2 Bq/l, respectively (Asikainen et al., 1980). In Spain, the data showed an arithmetic mean of 381 Bq/l and a maximum of 31 000 Bq/l in domestic drinking water (Sato et al., 1995). Cothern (1987) reported the averages for US water supplies at

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Table 2 Results of well water radon levels City

Radon concentration, Bq/l$2p No. of sampled wells

Buraydah Wells feeding the plant Wells feeding directly to distribution network ºnayzah Wells feeding the plant Wells feeding directly to distribution network Al Rass Al Zulfi Al Majmaah Al Quwaiyah Al Kharj Shaqra

Minimum

Maximum

Geometric mean

19

3.04$0.31

17.67$0.74

10.48$0.57

3

9.11$0.51

10.33$0.56

9.86$0.56

6

5.81$0.41

11.89$0.63

9.10$0.55

17 8 4 3 4 7 6

3.78$0.34 3.59$0.33 2.33$0.29 1.26$0.19 14.22$0.65 0.89$0.22 11.00$0.58

13.48$0.69 14.26$0.70 10.15$0.55 35.44$1.35 31.67$1.24 2.19$0.27 15.96$0.77

7.73$0.49 7.25$0.46 4.27$0.38 4.26$0.37 20.90$0.86 1.40$0.21 13.62$0.69

28.9 and 8.9 Bq/l for systems serving less than and greater than 1000 population, respectively. The respective values determined by the US National Inorganic and Radionuclides Survey (NIRS) as the population-weighted average are 22.3 and 7.2 Bq/l (Longtin, 1988). In the survey carried out by the American Water Works Service Co. (AWWSC), including 377 public groundwater supplies located in 15 states, the average and the median values were 25.4 and 11.8 Bq/l, respectively (Dixon et al., 1987a, b). The deep wells located in Dhurma and Minjur formations contained the lowest radon levels as compared to those located in Saq formation which gave values of 4 times that of the 2 aquifers. This is mainly due to the fact that Dhurma and Minjur formations are of predominantly limestone and sandstone which is expected to have low potential for the presence of uranium—radium series (Michel, 1987). On the other hand, Saq formation is predominantly sandstone with significant shale layers in the upper parts. In addition to that the sandstone layers of the Saq formation are thicker than those of the Minjur or Dhurma. The Saq formation is in more intimate contact with the basement complex rocks which may have lead to the accumulation of radioactive materials in that formation which subsequently became the source of radon. The distribution of radon in the 19 sampled wells supplying water to Buraydah plant and the 17 within Unayzah city is shown in Fig. 2. As shown, 50% of Buraydah wells have radon concentrations of less than 12.41 Bq/l compared to a level of 8.33 Bq/l for Unayzah wells. The values shown in Fig. 2 represent the average radon concentration of 4 measurements taken at different times.

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Fig. 2. Rn levels in Buraydah and Unayzah wells.

3.2. Radon removal by water treatment plants The average values of radon in samples collected from influent, after cooling or aeration and from the product water of the six major water treatment plants in the study area are shown in Table 3. The average percent removal of radon in the plants is shown in Fig. 3. It is evident that raw water (influent to plants) has radon levels that are slightly different than those from the well waters feeding into the plants (Table 2). This is mainly due to the differences in well water production in which the average values in Table 2 did not take this factor into consideration. The values in Table 3 are for the combined incoming water from all feeding wells. The removal of radon due to the use of diffused bubble aeration system has reached 56.9% as the case in Buraydah plant. No notable improvement in radon removal in that plant with other processes namely filtration, chlorination and water storage. Radon removal in Unayzah plant was found to be in the range of 26.7—56.8% with a geometric mean value of 34.9%. The low level of removal is attributed to the fact that no aeration or cooling is provided in the plant as well as the large amount of water used for blending which receives only filtration treatment. Three plants (Al Zulfi, Al Majmaah and Al Quwaiyah) have cascade cooling and have shown a radon removal range of 67.1—88.1%. Surface aeration in Al Rass treatment plant provided only 23% radon removal. However, the overall removal of radon in the latter four plants due to the entire treatment processes was in the range of 78.7—96.5% bringing the radon level to as low as 0.15 Bq/l. The aeration or cooling processes in the plants under study are not intended for radon removal but are for the purpose of water cooling, oxidation of iron and removal of gases present in the raw water. Aeration is considered by United States Environmental Protection Agency (USEPA) as one of the best available technologies in radon removal (Pontius, 1996). Diffused bubble aeration has been reported to reduce radon levels to as high as 95%

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Table 3 Radon concentrations at different locations within the drinking water treatment plants of the Central Region of Saudi Arabia Plant

Buraydah Unayzah Al Rass Al Zulfi Al Majmaah Al Quwaiyah

No. of samples

5 10 2 4 4 2

Geometric mean values of radon, Bq/l$2p Influent raw water

Cooled water

Product water

4.00$0.69 8.78$0.51 6.07$0.42 2.35$0.29 4.44$0.37 17.94$0.77

6.03$0.42 — 4.67$0.39 0.49$0.15 1.46$0.20 2.14$0.26

5.52$0.39 5.71$0.40 0.59$0.18 0.15$0.05 0.95$0.22 0.63$0.19

Fig. 3. Rn removal in six drinking water treatment plants of the Central Region of Saudi Arabia.

(Perkins & Hewett, 1988). Other methods of aeration such as the use of cascades can result in a radon reduction of more than 75% (Dixon & Lee, 1988). The radon level in all of the plants product water did not exceed the USEPA proposed MCL of 11.11 Bq/l. The maximum was 5.71 Bq/l which was found in Unayzah plant product water. Other plants with the exception of Unayzah plant have shown a geometric mean radon removal of 57.2% due to cooling only. Other processes have resulted in a removal of upto 96.5%. Since Unayzah city is supplied with water from both its treatment plant and wells located within the city (local wells), samples were collected from 28 districts and the results indicate that radon levels are

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in the range of 2.74—4.85, 2.30—4.67 and 1.04—7.44 Bq/l for districts supplied with water from the treatment plant, mixture of the plant product water and local wells, and directly from the local wells, respectively. For Shaqra the radon level in water supplied to the distribution network from the city water reservoir was found to be 2.07 Bq/l. Similarly, collected samples from Al Kharj water reservoir indicated an average radon level of 1.19 Bq/l. The reason for the low radon levels in the water being pumped to Shaqra and Al Kharj water distributions is the decay due to storage.

4. Summary and conclusions Water samples were collected from 77 wells and 6 treatment plants supplying drinking water to eight municipalities of the Central Region of Saudi Arabia. Results of radon content have indicated a maximum and minimum values of 35.44 and 0.89 Bq/l in wells supplying water to the region, respectively. The overall weighted geometric mean value was found to be 8.80 Bq/l. The high values of radon were found to occur in Saq aquifer and the low values were found in samples collected from wells located in Dhurma and Minjur aquifers. Most of the radon in raw water was removed undeliberately in treatment plants. When aeration and filtration processes were used, up to 60.5% removal was achieved. In treatment plants having reverse osmosis or electrodialysis processes along with aeration, the radon removal was found to be in the range of 78.7—96.5%. It can be concluded that the radon levels in the Central Region of Saudi Arabia drinking water are below the proposed MCL of 11.11 Bq/l. It is recommended, however, that periodical radon monitoring be carried out especially when new wells are drilled and added to the regions water supply.

Acknowledgements The work reported has been financially supported by King Abdulaziz City for Science and Technology (KACST). The author appreciates the assistance and cooperation of personnel from the different water authorities of the Central Region of Saudi Arabia.

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