The activity concentrations of the detected radionuclide ²²⁶Ra, ²³²Th and ⁴⁰K in the five offshore sand deposits are presented in Table 1.

In the Liaodong Bay, the concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K vary from 14.7 Bq・kg⁻¹ to 36.1 Bq・kg⁻¹, 25.3 Bq・kg⁻¹ to 56.8 Bq・kg⁻¹ and 405.8 Bq・kg⁻¹ to 919.4 Bq・kg⁻¹, with the mean values of 26.5 Bq・kg⁻¹, 41.8 Bq・kg⁻¹ and 691.9 Bq・kg⁻¹, respectively.

In the North Yellow Sea, the concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K vary from 7.2 Bq・kg⁻¹ to 66.3 Bq・kg⁻¹, 15.7 Bq・kg⁻¹ to 175.4 Bq・kg⁻¹ and 635 Bq・kg⁻¹ to 1044 Bq・kg⁻¹, with the mean values of 17.5 Bq・kg⁻¹, 39.4 Bq・kg⁻¹ and 857.1 Bq・kg⁻¹, respectively.

In the Zhoushan Area, the concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K vary from 15.0 Bq・kg⁻¹ to 24.0 Bq・kg⁻¹, 33.0 Bq・kg⁻¹ to 40.0 Bq・kg⁻¹ and 665.0 Bq・kg⁻¹ to 836.0 Bq・kg⁻¹, with the mean values of 19.5 Bq・kg⁻¹, 36.6 Bq・kg⁻¹ and 728.1 Bq・kg⁻¹, respectively.

In the Taiwan Shoal, the concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K vary from 5.6 Bq・kg⁻¹ to 39.9 Bq・kg⁻¹, 6.5 Bq・kg⁻¹ to 53.1 Bq・kg⁻¹ and 176 Bq・kg⁻¹ to 658.8 Bq・kg⁻¹, with the mean values of 20.7 Bq・kg⁻¹, 36.3 Bq・kg⁻¹ and 458.8 Bq・kg⁻¹, respectively.

In the Pearl River Mouth, the concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K ranges from 9.4 Bq・kg⁻¹ to 20.0 Bq・kg⁻¹, 9.0 Bq・kg⁻¹ to 22.0 Bq・kg⁻¹ and 152.0 Bq・kg⁻¹ to 319.0 Bq・kg⁻¹, with the mean values of 13.8 Bq・kg⁻¹, 13.5 Bq・kg⁻¹ and 245.7 Bq・kg⁻¹, respectively.

Obviously, the mean concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K radionuclides from these five sandy deposits are much comparable to those of the world average values of 35 Bq・kg⁻¹, 30 Bq・kg⁻¹ and 400 Bq・kg⁻¹ for ²²⁶Ra, ²³²Th and ⁴⁰K, respectively [17] and beach sands around the world (Table 2). The ²²⁶Ra concentrations from the five offshore sandy deposits in this study are all lower than that of world average value. However, the ⁴⁰K concentrations in Liaodong Bay, North Yellow Sea, Zhoushan Area and Taiwan Shoal are all much higher than that of the world mean value, and the ²³²Th concentrations from Taiwan Shoal and Pearl River Mouth are lower than that of the world average (Figure 2).

Since the distribution of natural radionuclides in the samples is not uniform, a common radiological index has been introduced to evaluate the actual activity level of ²²⁶Ra, ²³²Th and ⁴⁰K in the samples and the radiation hazards where

associated with these radionuclides, the radium equivalent activity (Raeq), which can be calculated from the relation [22] [31] :

Raeq = CRa + 1.43CTh + 0.077CK (1)

CRa, CTh, and CK are the specific activities of ²²⁶Ra, ²³²Th and ⁴⁰K, respectively, in units of Bq・kg⁻¹. In the definition of the radium equivalent, it is assumed that 10 Bq/kg of ²²⁶Ra, 7 Bq/kg of ²³²Th and 130 Bq/kg of ⁴⁰K each produce an equal gamma-ray dose rate [32] [33] .

The calculated values of Raeq for the five sand deposits in investigation are shown in Figure 3. The calculated values of Raeq range from 52.0 (Pearl River Mouth) to 139.8 (North Yellow Sea), with a trend that the Raeq is much higher in North Part of the Chinese Seas than that in the South. All values of Raeq in the studied samples are found to be lower than the criterion limit of 370 Bq/kg [34] , and therefore, do not pose any radiological hazard when used for construction of buildings.

In order to estimate the level of gamma radioactivity associated with different concentrations of certain specific radionuclides, known as the representative level index [5] [35] , the formula is given as:

RLI = CRa/150 + CTh/100 + CK/1500 (3)

where CRa, CTh and CK are the average activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K, respectively, in units of Bq・kg⁻¹. The mean RLI values varied from 0.39 (Pearl River Mouth) to 1.08 (North Yeloow Sea) (Figure 4). It is clear that these values do not exceed the upper limit for RLI, which is unity [5] .

The absorbed dose rates in indoor air (DR) and the corresponding annual effective doses (HR) attributed to gamma-ray emission from the radionuclides (²²⁶Ra, ²³²Th and ⁴⁰K) in building materials were evaluated using data and formula provided by UNSCEAR (2000) [17] and the EC (1999) [36] . In the UNSCEAR and EC reports, the dose conversion coefficients were calculated for the center of a standard room with the dimension of 4 m * 5 m * 2.8 m. The thickness of the walls, floors, ceiling and the density of the structure are 20 cm and 2350 kg/m³ (concrete), respectively. The resulting dose coefficients were found to be 0.92 nGy/h per Bq/kg for ²²⁶Ra, 1.1 nGy/h per Bq/kg for ²³²Th, and 0.080 nGy/h per Bq/kg for ⁴⁰K:

DR (nGy/h) = 0.92CRa + 1.1CTh + 0.080CK (4)

where CRa, CTh and CK are the activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K, respectively.

To estimate the annual effective dose rates, it is necessary to use the conversion coefficient from the absorbed dose in air to the effective dose (0.7 Sv/Gy) and the outdoor occupancy factor (0.2) proposed by UNSCEAR (2000). Therefore, the effective dose rate is determined as follows:

Outdoor (mSv/y) = DR (nGY/h) * 24 h * 365.25 d * 0.2 (outdoor occupancy factor) * 0.7Sv・Gy⁻¹ (Conversion factor) * 10⁻⁶

HR = DR * 8766 * 0.2 * 0.7 * 10⁻⁶ = DR * 0.00123 (5)

where DR is given by Equation (4).

The estimated results of DR and HR for all the studied marine sands range from 31.4 nGy・h⁻¹ (Taiwan Shoal) to 327.1 nGy・h⁻¹ (North Yellow Sea) and from 0.04 mSv・y⁻¹ (Taiwan Shoal) to 0.20 mSv・y⁻¹ (North Yellow Sea), respectively (Figure 5). And the estimated mean value of DR in all of studied samples is 107.9 nGy・h⁻¹, which is little bit higher than world average indoor absorbed gamma dose rate of 84 nGy・h⁻¹ [17] . Additionally, the estimated mean value of the annual effective dose rate of 0.13 mSv・y⁻¹ is also higher than the world average value (0.07 mSv・y⁻¹, [17] ).

The alpha index was developed as an assessment of the excess alpha radiation exposure caused by inhalation originating from building materials. The alpha index (Ia) is determined by the following formula [37] :

Ia = CRa/200 (Bq/kg) (6)

where CRa is the ²²⁶Ra activity concentration (Bq・kg⁻¹) in the building materials. The recommended exemption level and recommended upper level for the ²²⁶Ra activity concentration in building materials as are 100 Bq・kg⁻¹ and 200 Bq・kg⁻¹, respectively, as suggested by the Radiation Protection Authorities in Denmark, Finland, Iceland, Norway and Sweden and the upper level is in agreement with the action level given by the ICRP in Publication 65 (1994) and by the European Commission (1990) [39] . It was observed that the values of the alpha index in the studied marine sand samples are below the recommended unity (Figure 6(a)).

Another radiation hazard index, the gamma activity concentration index, Ig, has been defined by the European Commission (1990) [38] and Righi and Bruzzi (2006) [37] , which is given as:

Ig = CRa/300 + CTh/200 + CK/3000 (7)

The index Ig is corrected with the annual dose rate attributed to excess external gamma radiation caused by superficial material. Values of Ig ≤ 2 correspond to the dose rate criterion of 0.3 mSV・y⁻¹, whereas 2 < Ig ≤ 6 correspond to a criterion of 1 mSv・y⁻¹ ( [36] ). Therefore, the activity concentration index should be used only as a screening tool for identifying materials that might be of concern when used as construction materials; although material with Ig > 6 should be avoided, in that these values correspond to dose rates higher than 1 mSv・y⁻¹ [36] , which is the highest dose rate value recommended for the population [17] .

The gamma index Ig for the marine sands varies between 0.13 (Taiwan Shoal) and 0.62 (Taiwan Shoal) with an average of 0.45 (Figure 6(b)). Therefore, these marine sands can be exempted from all restrictions concerning radioactivity.

The external radiation hazard (Hex) and the internal radiation hazard, (Hin) was developed for the additional criteria to assess the radiological suitability of a building material [22] . And they are defined as follows:

Hex = CRa/370 + CTh/258 + CK/4810 (8)

and

Hin = CRa/185 + CTh/259 + CK/4810 (9)

where CRa, CTh and CK are the activities of ²²⁶Ra, ²³²Th and ⁴⁰K, respectively, in units of Bq・kg⁻¹.

The determined values of Hex vary from 0.09 (Taiwan Shoal) to 1.05 (North Yellow Sea) with an average of 0.32 (Figure 7). The Hin values range between 0.11 (Taiwan Shoal) and 1.23 (North Yellow Sea) with an average of 0.37 (Figure 7). However, the highest values of Hex and Hin indices are both found in the same sample from North Yellow Sea, with highest value of ²²⁶Ra activity in all samples, which make them larger than 1 (the criterion, [31] ).

The excess lifetime cancer risk (ELCR) was determined using the following equation [39] :

ELCR = HR × DL × RF 10)

where HR, DL and RF are the annual effective dose equivalent, duration of life (70 years) and risk factor (0.05 Sv⁻¹), respectively. The risk factor is defined as the fatal cancer risk per Sievert. For stochastic effects, the ICRP 60 uses a value of 0.05 for the public [39] .

The calculated range of ELCR is from 0.14 × 10⁻³ (Taiwan Shoal) to 1.41 × 10⁻³ (North Yellow Sea). The average ELCR values for the five marine sand

deposits are 0.54 × 10⁻³ (Liaodong Bay), 0.55 × 10⁻³ (North Yellow Sea), 0.50 × 10⁻³ (Zhoushan), 0.36 × 10⁻³ (Taiwan Shoal) and 0.20 × 10⁻³ (Pearl River Mouth), respectively (Figure 8). And it is very clear that most of the marine sands offshore China is slightly higher than the world average (0.29 × 10⁻³) [17] .

The activity of bone marrow and bone surface cells are considered to be origins of interest by UNSCER (1988). Therefore, the annual gonadal dose equivalent (AGDE) arising from the specific activities of ²²⁶Ra, ²³²Th and ⁴⁰K was calculated using the following formula [40] :

AGDE (μSv・y⁻¹) = 3.09 CRa + 4.18 CTh + 0.314 CK (10)

The mean AGDE values for each marine sand deposit are presented in Figure 9. The values of AGDE varied from 119.0 (Taiwan Shoal) to 1225.0 mSv・y⁻¹

(North Yellow Sea) and the average value was found to be 409.0 mSv・y⁻¹. The average values do not generally exceed the permissible recommended limits, indicating that the hazardous effects of the radiation are negligible. In the literature, the average AGDE value for the Eastern Desert of Egypt was found to be 2398 mSv・y⁻¹ [41] , for the Tamilnadu of 350.63 mSv・y⁻¹ [42] , for the Fırtına Valley (Turkey) of 550.5 mSv・y⁻¹ [43] .

Correlation analysis was carried out in terms of bivariate statistics to determine the mutual relations and strengths of association between pairs of variables through the calculation of the linear Pearson correlation coefficients. The results for Pearson correlation coefficients between all the studied radioactive variables of the marine sand deposits offshore China are shown in Table 3.

A high positive correlation coefficient is observed between ²³²Th and ²²⁶Ra (Figure 10(a)), because the radium and thorium decay series occur together in nature [42] . In contrast, a very weak negative correlation coefficient was observed between these two nuclides and ⁴⁰K (Figure 10(b) and Figure 10(c)), because ⁴⁰K is from different origin [42] . In addition, all radioactive variables have strong positive correlation coefficients with ²²⁶Ra and ²³²Th, while they are weakly negatively correlated with ⁴⁰K. All radioactive variables calculated are positively correlated with one another (Table 3).

Principal component analysis was performed on the whole data set (13 variables) to assess the relations between them. The rotated factor analysis was carried out via varimax rotation with Kaiser normalization. The rotated factor 1 and factor 2 values are shown in Table 4. Two principal components were yielded with eigenvalues > 1, explaining 98.26% of the total variance. From the

rotation space of component 1 and the component 2 (Figure 11), the first component accounts for 88.22% of the total variance and is mainly characterized by high positive loadings of concentration of ⁴⁰K, ²³²Th and most of the radioactive variables. While the second component accounts for 10.41% of the total variance and is mainly corresponds to positive loading of ²²⁶Ra and Ia. From the overall component analysis, it can be deduced that ⁴⁰K and ²³²Th dominantly increase the radioactivity in the entire marine sand deposits offshore China.