Study design
This was a descriptive study performed on two phases. In the first phase we covered all the wells and springs in Nablus city. In the second phase we selected a randomized sample that represents the residential tap water of the city.
The study area
Nablus city is located in the north of the West Bank and lies on two high mountains. Nablus citizens are provided by water from the municipality water net that drives the water from the main sources to the customers. Those citizens do not use other sources of water like a well in the house garden that is usually filled from rain or tanks as other cities and villages in the West Bank. Nablus water comes from artesian wells and natural springs. There are 6 wells, 4 main springs and 2 other small springs. Water from these wells and springs is pumped to collecting reservoirs and then to the houses and buildings' reservoirs and finally to the residential taps. There are 22 reservoirs, each reservoir received its water from different wells and springs so water arrived to one house does not come from a single main source. The municipality divides Nablus into 7 regions; each region is supplied mainly by a specific reservoir but this also may be varied according to the availability of water. This means, if there is no enough water in the reservoir; water will be pumped from other reservoir to that reservoir then pumped to the houses. There is an 8th region (old city of Nablus) which is supplied only by one spring called Qaryoon spring.
Sampling framework and data collection
In this study, samples were collected from the main sources (i.e. wells and springs), and from the residential tap water (selected randomly) in the eight regions of Nablus. The main wells are: Deir Sharaf and Sabstia wells in the northwest region, Badan and Faraa wells in the northeast region and Audala and Rujeeb wells in the southeast region. The springs are: Qaryoon, Ein Beit El Ma, Ein Dafna, Ras Al-Ein, Ein Al-Asal and New reservoir (commercial centre). While the regions or zones of Nablus are: Ein Dafna Zone, Northern Zone, Southern Zone, El Sumara zone, Ein Beit el Ma Zone, Ras Al Ein Zone, reservoir zone and the Old city.
Samples were collected in glass bottles (250 ml each). Three samples were taken from each main source. From each region we took 3 samples from the residential tap water while 10 samples were taken from the old city. Regarding the main sources, we were allowed to collect samples only from 9 sources as 3 of them were closed and were not pumping water at the time we conducted the study. The closed sources were Deir Sharaf well, Rujib well and new reservoir spring.
Experimental setup and measurement procedure
Research project was approved by the research committee at An-Najah National University faculty of medicine after the approval of the Institutional Review Board (IRB). Nablus municipality approval was also obtained. The researcher then received training in using the DURRIDGE RAD7 H2O device following the manual instructions [18]. Therefore, we used RAD water device, an accessory to the RAD7 device manufactured by DURRIDGE Company [18]. It was calibrated on 31 June 2010 and the next calibration time should be done in 31 June 2011.
Briefly, this device offers an accurate measurement, faster reading, it is portable and eliminates the need for noxious chemicals. The schematic diagram of this device is presented in Figure 1 below. Using RAD H2O technique employs closed loop concept, consisting of three components; (a) the RAD7 or radon monitor, on the left, (b) the water vial with aerator, in the case near the front, and (c) the tube of desiccant, supported by the retort stand above as marked in the figure.
The RAD-H20 method employs a closed loop aeration scheme whereby the air volume and water volume are constant and independent of the flow rate. The air re-circulates through the water and continuously extracts the radon until a state of equilibrium develops. The RAD-H20 system reaches this state of equilibrium within about 5 min, after which no more radon can be extracted from the water. The operation of this device is based on the following principle; (1) radon is expelled from a water sample by using a bubbling kit, (2) expelled radon enters a hemisphere chamber by air circulation, (3) polonium decayed from radon is collected onto a silicon solid-state detector by an electric field and (4) radon concentration is estimated from the count rate of polonium [18].
On the RAD7, one among the two available protocols (i.e., Wat-40 and Wat-250) will be selected depending on the size of vial (40 or 250 mL) that is being used for water sampling (here we used Wat-250 and sample size of 250 mL). This also decides the extraction efficiency or percentage of radon removed from the water to the air loop. For our used protocol of Wat-250, the extraction efficiency was usually very high, typically 95% for a 250 mL sample vial [18].
The 250 mL sample bottle was connected to the RAD-7 and the internal air pump of the radon-monitor was used for re-circulating a closed air-loop through the water sample, purging radon from the water into the air-loop. The air is re-circulated through the water continuously to extract the radon until RAD-H2O system reaches a state of equilibrium within about 5 min, after which no more radon can be extracted from the water. After reaching equilibrium between water, air, and radon progeny attached to the passivity implanted planar silicon detector, the radon activity concentration measured in the air loop was used for calculating the initial radon-in-water concentration of the respective sample. The RAD-7 allows determination of radon-in-air activity concentrations by detecting the alpha decaying radon progeny Po-218 and Po-214 using passivity implanted planar silicon detector. The radon monitor (RAD-7) uses a high electric field above a silicon semi-conductor detected at ground potential to attract the positively charged polonium daughters (Po-218 and Po-214) which are counted as a measure of radon-222 concentration in air.
The pump runs for 5 min, aerating the sample and delivering the radon to the RAD7. The system will wait a further 5 min and then it starts counting. During the 5 min of aeration, more than 95% of the available radon is removed from the water and the components automatically perform everything required to determine the radon concentration in the water. After 5 min, it prints out a short-form report.
The same thing is repeated again for 5 min later, and for two more 5-min periods after that. Thus, radon gas is collected through the energy specific windows which eliminate interference and maintain very low backgrounds and later counted for the radon concentration. Radon-222 activities are then expressed with uncertainty down to under ± 5%. At the end of the run (30 min after the start), the RAD7 prints out a summary, showing the average radon readings from the four cycles, counted a bar chart of the four readings, and a cumulative spectrum.
The RAD H20 enables the measurement of radon in water over a concentration range between 30 and 105 pCi/L (pico-Curie/liter). The lower limit of detection was less than 10 pCi/L [18]. The exact value of the extraction efficiency depends somewhat on ambient temperature, but it is almost always well above 90%. Furthermore, the temperature effect on accuracy is usually noticeable with the 250 mL vial at only very low or high temperatures. The RAD-H20 system has been calibrated for a sample analysis temperature of 20 C°. In our study, the mean ± standard deviation (M ± SD) temperature value for the wells' and springs' samples was 23.6 ± 0.74 and for the residential tap water samples was 20.0 ± 1.15. Therefore, a very limited or no effect of temperature was seen on the results.
The RAD7 calculates the sample water concentration by multiplying the air loop concentration by a fixed conversion coefficient that depends on the sample size. This conversion coefficient has been derived from the volume of the air loop, the volume of the sample, and the equilibrium radon distribution coefficient at room temperature. For the 250 mL sample volume, the conversion coefficient was around 4 [18]. In the analysis, we converted the picocurie (pCi/L) into Becquerel (Bq/L) unit using the formula that 1 pCi = 0.037 Bq.
Samples were taken in specific bottles designed for the RAD device and provided by the manufacturer. The collections of the samples and their analysis (for springs and wells) were done between 27th of November 2010 and 4th of December 2010. Tap water sampling and analysis from different regions of Nablus area were done between 7th of December 2010 and 20th of December 2010.
Sample analysis
To ensure the quality control and reliability of the sampling and measurement methods, each sample was analyzed in 4 cycles. The mean for these 4 cycles was then calculated in regard to the wells and springs [see Additional file 1: Wells and springs original data set in Bq]. Regarding the residential tap water, we took 3 samples from the houses receiving water from each region using the simple randomization. Each sample was analyzed in 4 cycles where we calculated the mean of these 4 readings and finally we calculated the mean for the 3 samples' means [see Additional file 2: Residential tap water original data set in Bq; see Additional file 3: Old city original data set in Bq].
Concentrations was measured by pCi/L unit as provided by the manufacturer then converted to Bq/L for easier comparison with the literatures. Analysis took place at the radon research laboratory at An Najah National University. All samples were analyzed within 3 days of collection because radon half life is 3.8 days [19]. The relative standard deviations of all the 4 cycles analyzed were within the 10% for their corresponding mean. The SPSS (statistical package for social sciences) software 16 was used for analysis [20].