SOIL TYPES AND THEIR RELATIONS WITH RADON CONCENTRATION LEVELS IN MIDDLE GOVERNORATE OF GAZA STRIP, PALESTINE

12Abstract. Determination of natural radioactivity has been carried out in surface and core agricultural soil samples collected from various sites in the Middle Governorate – Gaza Strip, Palestine. Mechanical and chemical analysis has been done to determine soil characteristics. Radon activity concentration measurements were carried out using solid state nuclear tracks detectors, Cr-39. The mechanical analysis results show that they belong to two classes, sandy loam and loamy sand. The sandy loam soil was observed in the eastern side of the study area, whereas the loamy sand was observed in western and middle parts. The radon concentration levels were higher in core samples and were proportionate to the soil depth. Also they were higher in sandy loam than loamy sand soil samples. The radon concentration levels had a positive correlation with fine grains (clayto silt-size) of soil sample which translocated from upper to lower horizons of soil during its development. Additionally, there was a positive correlation with pH and water content, whereas a negative correlation was observed with organic matter and potassium contents. The positive correlation referred to a large specific surface of fine grains which were located in lower horizons of soil and were able to adsorb more water and consequently led to high radon concentration levels.


INTRODUCTION
In addition to being the main source of continuous radiation exposure to human, soil acts as a medium of migration for transfer of radionuclides to the biological systems and hence, it is the basic indicator of radiological contamination in the environment. Moreover, the soil radioactivity is usually important for the purposes of establishing baseline data for future radiation impact assessment, radiation protection and exploration (Ramli et al. 2005). Various studies concerning radioactivity bound to soil were carried out by different researchers (e.g. Khatir et al. 1998, Tzortzis et al. 2003, Matiullah et al. 2004, Ramli et al. 2005, Veiga et al. 2006) and concerned various regions of the world. Most of these studies were concentrated on natural sources as the natural radiation is the largest contributor to the external dose provided to the world's population (UNSCEAR 2000). These dose rates vary from one place to another depending upon the concentration of natural radionuclides like 238 U, 226 Ra, 232 Th and 40 K present in soil. Data on natural radionuclides are still very scarce in Jordan and the previous studies are limited to specific regions, radionuclides and/or geological formation (e.g. El-Ghossain and Abu Saleh 2007, El-Ghossain and Abu Shammala 2012, Rasas et al. 2004, Ubeid andRamadan 2017).
The aims of this work were the determination of radon concentration levels in surface and core agricultural soil samples collected from various soil types in different sites in the Middle Governorate of Gaza Strip, Palestine. Additionally, identification of the relationships of these concentration levels with soil characteristics and their contents was examined.

STUDY AREA
The Gaza Strip is located in the southwestern Palestine, at the southeastern coast of the Mediterranean Sea, between 34°2'E and 31°45'N ( Fig. 1). It covers an area of approximately 365 km 2 , has a length of 45 km along the coastal zone, and its width ranges from 5 to 8 km in the central and northern regions to the maximum of 12 km in the south. It is located in the transitional zone between a temperate Mediterranean climate in the west and north, and the arid climate of the Sinai Peninsula in the east and south. The temperature gradually changed throughout the year, reached its maximum during the summer (August), and its minimum during the winter (January). The average monthly maximum temperature ranges between 17.6°C for January and 29.4°C for August. The average of the monthly minimum temperature for January is about 9.6°C and 22.7°C for August. The mean annual rainfall is 335 mm per year, and the average annual evaporation is 1,300 mm. Gaza topography is defined by three ridges (locally termed kurkar ridges) and depressions, dry streambeds and shifting sand dunes ( Fig. 1). The ridges and depressions generally extend in the NE-SW direction, parallel to the coastline. The surface elevation ranges from mean sea level to about 110 m above mean sea level. The depressions, which contain alluvial deposits, are about 20-40 m above mean sea level. The Gaza Strip is divided into five governorates, the Northern Governorate, Gaza Governorate, the Middle Governorate, Khan Younis Governorate, and Rafah Governorate. The study area is located in the Middle Governorate (Fig. 1).

GEOLOGICAL OVERVIEW
Stratigraphically, these ridges belong to the Kurkar Group (Bartov et al. 1981). This group is of a Pliocene-Pleistocene age, and within this group three formations can be distinguished: Ahuzam Fm, Pleshet Fm, and Gaza Fm (Bartov and Arkin 1980, Frechen et al. 2004, Galili et al. 2007, Ubeid 2010, 2011. The kurkar ridges belonging to the Gaza Fm consist of calcareous sandstones (locally termed kurkar) alternated with brown reddish fine-grained deposits (locally termed hamra) (Fig. 2) (Horowitz 1975, Abed and Al Weshahy 1999, Frechen et al. 2004, Ubeid 2010, 2011. In the southern part of the Gaza Strip these features tend to be covered by sand dunes. The soil in the Gaza Strip is mainly composed of three soil types: sandy soil, clayey soil, and loess soil (Ubeid 2011(Ubeid , 2013. The sandy soil was found along the coastline and middle parts of the Gaza Strip, in the form of sand dunes. The clayey soil is found along the northeastern part of the Gaza Strip. The loess soil is found around the Wadis.

Field sampling
Classical field work was carried in the Middle Governorate of Gaza Strip. Ten observed sites were selected to collect samples from agriculture soils. The coordinates for each site were recorded by the GPS device (Table 1). At each site, four fresh and represented samples were collected. One sample was taken from the surface, and the rest were core samples taken at a depth of 20, 40, and 60 cm. The samples were put in polyethylene bags and tightly closed. In addition to that, field observations for soil physical properties, time day sampling, temperature, and irrigation times were noted down. After field work the samples were directly transferred to the laboratory analysis. Primarily the coarse particles were removed from samples by the sieve (2,000 μm). Then, the samples were dried at 105°C for 24 hours in an oven. After drying, the sieving method was used for sandy soil samples by the sieve shaker to classify the particle sands of each sample, eight sieves were used (2,000, 1,180, 600, 425, 300, 212, 150, 63 μm). The initial weight of each sample was kept constant at 100 g, and the duration of shaking was about 15 minutes for each sample. The retained weight of sands in each sieve was determined separately. The results of grain-size analysis are generally expressed in terms of the percentage of the total weight of sample that passed through different sieves. For muddy soil samples, sedimentation analysis method (by using hydrometer and dispersive calgon) was done to separate the grain size. The data were processed using GRADISTAT software (Blott and Pye 2001) to obtain the grain-size distribution. The textural classes of the soil samples due to grain-size distribution results were defined by using the USDA soil texture triangle chart ( Table 2).

Chemical analysis
Chemical analysis for pH, water contents (WC), organic matter contents (OMC), and potassium contents of soil samples was done in laboratories of Geology Department in Al Azhar University -Gaza. For pH determination, around 20 gm of dry soil was transferred into a 100 ml beaker, and 40 ml distilled water was added and stirred well with a glass rod. This was allowed for half an hour with intermittent stirring. Then the pH meter 3310 electrode (Fenway UK) was used to determine pH value. The water contents were determined by drying soil samples in an oven up to 105°C for 24 hours. The water contents were measured by the weight difference between the wet and dry sample. Based on ASTM D2974 Standard Test Methods for Moisture, Ash, and Organic Matter of Peat and Other Organic Soils, the organic matter contents in the soil samples were determined. In this method the samples were heated in a muffle furnace up to 440°C. The organic matter was measured by the weight difference between the sample before and after drying.
For potassium determination, around 5 gm of dried soil was diluted in the solution of distilled water (100 ml) with sodium bicarbonate (0.5 gm). The solution was shaken for 30 min. After filtering the solution through 42 filter paper, the potassium contents were determined by the spectroscopy method, using a flame photometer instrument.

Radon concentration measurement
The sealed-cup technique and CR-39 detectors were used for measuring the radium content and radon exhalation rate from the soil samples (Fig. 3). 300 ml of dried soil samples are put in container (3L), CR-39 films are pasted on the lower side of cover, after tightening the cover, the containers were sealed with adhesive tape to minimize the leakage and left for 80 days so that the plastic tracks detectors (CR-39) can record enough alpha particles as a result of radon disintegrations. At the end of the exposure time, CR-39 films were etched, using a 6M solution of NaOH, at a temperature of 70°C, for about 5 hours. The detectors were washed with distilled water and left to dry. Each detector was counted visually using an optical microscope through the area within 3 mm 2 in 4 distinct regions, and the average number of tracks/mm 2 was determined from the measured average track densities on the CR-39, with detector efficiency of around 12.3 tracks cm -2 month -1 per Bq/m 3 . Radon calculations in this study were carried out using the following equations: Where: E is radon exhalation rate (Bq m -2 h -1 ) C Rn is the radon concentration (Bq/m 3 ) C Ra is the effective radium content (Bq/kg) ρ is the track density (tracks/cm 2 ) k is the detector sensitivity (Bq m -3 /tracks cm -2 h -1 ) λ is the decay constant (λ= 7.56 × 10 -3 h -1 ) V is the void volume of the container (cm 3

Soil characteristics
The results of grain-size analysis are presented in Table 2. It shows that the soil samples in the study area can be divided into two classes. The first class was sandy loam, which included samples 1, 2, 3, 4, and 10. This class was located in the eastern side of the study area. The second class was loamy sand, and it included samples 5, 6, 7, 8, and 9. This class was mostly located in the western and middle sides, where the sand dunes were dominant. Generally, this part of the study area characterized by dominant fine-to medium grain-size of sand (Ubeid and Albatta 2014). The results of grain-size distribution for the surface sample (1S to 4S and 10S) in the study area show that the fine-grained sediments (clay-to silt-size) in sandy loam class ranges from 26 to 34%, and the sand-grained ranges from 64 to 74%, whereas in the core samples (5C to 9C) of sandy loam class the clay-to siltgrained ranges from 26 to 50%, and the sand-grained from 54 to 74%. On the other side, the fine-grained (clay-to silt-size) in surface samples (1S to 4S and 10S) of loamy sand class ranges from 10 to 35%, and the sand-grained from 62 to 90%, whereas the clay-to silt-grained in the core samples (5C to 9C) of loamy sand class ranges from 16 to 38%, and the sand-grained from 62 to 84% (Fig. 4).
Overall, the results of grain-size analysis show that the fine-grained (clayto silt-size) sediments were found with high percentage in samples of sandy loam class which were located in eastern side of the study area. More precisely, the highest percentage was observed in the core samples in the same soil class. The lowest percentage of the fine-grained were observed in surface samples, especially in loamy sand classes which were located in western and middle parts of the study area. Table 3  However, the correlation between the grain-size of soil samples and their pH and water contents were characterized by an inverse relationship. This fact indicates that the pH and WC values increase with decreasing the grain-size, due to large specific area of fine-grained which were able to adsorb more water.

The radioactivity concentrations
The results of radioactivity concentrations of radon in soil samples of the study area are presented in Table 3, Table 4, and Fig. 9. It shows that the radioactivity levels of radon in the surface sandy loam soil class (1S to 4S and 10S) range from 3.35 to 17.57 Bq/kg, and from 1.94 to 20.86 Bq/kg in core samples (1C to 4C and 10C), whereas the levels of radioactivity concentrations in the surface loamy sand soil class (5S to 9S) range from 1.18 to 11.25 Bq/kg, and from 2.56 to 15.45 Bq/kg in core samples (5C to 9C). Fig. 10 shows that the linear correlation coefficient between radon concentrations and effective radium content was 0.98. This result shows the much stronger linear correlation between radon concentration and effective radium content. The linear correlation coefficient between the radon exhalation rate and effective radium concentration was 0.99 (Fig. 11). Table 4. Statistical descriptive of radon activity concentrations levels, pH, water contents (WC), organic matter contents (OMC), and potassium contents (K) in soil samples of the study area. E = Radon Exhalation Rate, C Rn = radon concentration, C Ra = effective radium Content, and AED = annual effective dose   Generally, the results of radioactivity concentration in the study area show that the concentration levels in core samples were higher comparing with surface samples. Moreover, the concentration levels were higher in both surface and core samples of sandy loam soil class which were located in eastern side of the study area comparing with surface and core samples of loamy sand soil class located in western and middle parts of the study area.
Overall, the radioactivity concentration levels had good correlation with fine-grained sediments. They were higher with the fine-grained sediments, which normally accumulated below the surface horizon called "B-Horizon". Consequently, the large specific surface of fine-grained sediments (clay-to silt-size) increased the adsorption of radon as well as the water contents and pH comparing with coarse-grained (sands) that accumulated in the soil surface. To sum up, the radioactivity concentration levels were directly proportional to these three factors as depicted in Figs On the other hand, the radioactivity concentrations were inversely proportional to organic matter and potassium contents in the soil sample, which were characterized by high levels in the surface and uppermost soil samples which were characterized by the accumulation of coarse-grained (sands) (Fig. 4). This suggests that fine-grained sediments were the main factor controlling the high levels of radioactivity, pH, and water contents. Thirty-five core samples were collected from ten sites in the Middle Governorate of Gaza Strip. These samples were analyzed for soil characteristics and radon exhalations. Mechanical and chemical analyses were carried out to determine soil characteristics, and the solid state nuclear tracks detector (Cr-39) was used for measuring radon activity concentrations. The soil samples in all observation sites were grouped into two classes, sandy loam and loamy sand. The sandy loam soil class which was dominated by fine grains (clay-to silt-size) was observed in the eastern side of the study area, while the loamy sand soil class was dominated by coarse grains (very fine-to fine sand-size), which was observed in western and middle parts of the study area. The grain-size distribution of soil samples shows that there was an inverse relationship between the grain-size and the depth of core samples.
The results of measuring samples show that the radon concentration levels were higher in core samples comparing with the surface sample of the study area. Also the levels were higher in the sandy loam class when compared with the loamy sand soil class. The average radon concentration levels in surface sandy loam samples range from 3.35 to 17.57 Bq/kg, and from 1.94 to 20.86 Bq/kg in core samples, while the average levels in surface loamy sand sample range from 1.18 to 11.25 Bq/kg, and from 2.56 to 15.45 Bq/kg in core samples. The radon concentration levels had a positive correlation with fine-grain (clay-to silt-size) sediments, pH, and water contents, and a negative correlation with organic matter and potassium contents. The positive correlation was referred to a large specific surface of fine grains which were able to adsorb more radon and water, whereas the negative correlation referred to the organic matter contents which had a good correlation with potassium naturally found in the surface of soil which lacks the finer fraction of the sediments, consequently, the ability of adsorption of radon going down.