CC BY
1EARTH SCIENCES
Domanov M.M.
doctor of geographical sciences, leading scientist, P.P. Shirshov Institute of oceanology, Russian Academy of Sciences.
Domanova E.G.
doctor of chemical sciences, senior research associate, Institute of Oil and Gas Problems, Russian Academy of Sciences
ESTIMATION OF THE RADIOECOLOGICAL SITUATION AND PECULIARITY OF CONNECTION BITUMEN HYDROCARBON STRUCTURES WITH CONCENTRATION OF NATURAL RADIONUCLIDES 226RA, 232TH, 210PB IN THE KARA SEA BOTTOM
SEDIMENTS
ABSTRACT
The assessment of a background radioecological situation in the bottom sediments of the Kara Sea is executed. Content of natural radionuclides 226Ra, 232Th and 210Pb in a surface layer of sediments of the Kara Sea is investigated. Common average content 226Ra, 232Th and 210Pb is 84±24 of Bq/kg. The gamma radiation dose from sediments varies within 0.030 - 0.084 jGr/h and average 0.056 jGr/h doesn't exceed admissible safe level (0.057 jGr/h). Results of the definition of group structure of the bitumoids and concentration 226Ra, 232Th and 210Pb in the surface layer of sediments are compared. Positive correlation link of concentration 226Ra and 232Th with the content of paraffin structures and negative communication 232Th and 210Pb with the content of naphthenic hydrocarbons is noted. In the sapropelic sediments positive correlation connection of contents of the long-chain polynuclear structures with depth is noted. It is shown that in sediments of sapropelic type the density of correlation of naphthenic hydrocarbons content with concentration 232Th and 210Pb increases in comparison with sediments of the mixed type. Well ex-pressed positive communication 226Ra, 232Th and 210Pb with polynuclear aromatic structures is shown.
Keywords: radioecology, bottom sediment, hydrocarbon, radionuclides, Kara Sea
Introduction
Under increasing external pressure from economic activities in Arctic seas natural radioactive background grows. The content of natural radionuclides in sediments grows especially significantly at oil and gas production on the sea shelf [13]. Change of natural radioactive background becomes the additional factor influencing a condition of an ecosystem. The available data indicate very complicated nature of links of radionuclides with organic substance of sediment which changes during early diagenesis [4-7]. It has been shown that there is correlation between of the distribution of bitu-minoid fractions and hydrocarbon structures of sediments with the concentration of natural radionuclides. A comparative analysis of bitumen hydrocarbon structures and natural radionuclides in sediments showed that Th, U and 226Ra correlate with a particular structural
group of hydrocarbons [8, 9]. In turn the structure of organic substance of bitumoid depends by nature origins of initial organic substance of a deposit. In this study, we investigated the structure of the hydrocarbon composition of bituminous substances and determined the amount of natural radionuclides 226Ra, 232Th and 210Pb in the surface layer of Kara Sea sediments in order to assess a relation between them in the sediments with different genetic type of initial organic matter.
Materials and methods
The material for the study was a 0-5 cm surface layer of bottom sediments of the Kara Sea collected by the 0cean-50 bottom sampler at the stations in cruise 128 of the research vessel Professor Shtokman and in cruise 22 Academic Boris Petrov
The positions of the stations and sediment characterization are given in Table 1.
Table 1.
Coordinates of stations in the area of investigation and characterization of sediments
Station Latitude N Longitude E Depth, m Sediment type
1 128-44 79.18 73.06 465 Pelitic oxidized ooze, dark brown
2 128-48 75.40. 63.41 172 Silt + pelite light brown
3 128-70 71.23 57.40 274 Silt + pelite light brown
4 22-118 70.30 58.11 203 Silt + pelite light brown
5 22-41 73.13 57.46 242 Pelitic oxidized ooze, dark brown
6 22-44 74.25 60.32 308 Silt + pelite, dark brown
7 22-51 75.30 65.39 335 Silt + pelite light brown
8 22-57 76.55 69.13 272 silt+pebbles
9 22-59 77.06 71.40 272 Silt, light brown
10 22-102 72.45 62.59 49 Silty ooze with sand,
Measurements of 226Ra, 232Th and 210Pb concentrations in the sediment were performed in the laboratory of dosimetry and environmental radioactivity at the Faculty of Chemistry of Moscow State University, using a gamma spectrometer with a GC - 3020 high purity germanium detector with a relative efficiency of 30% by the cobalt-60 line (1.332 MeV) and a resolution of 1.8 Kev according to this line. The GENIE-400 PC software was used. Radium-226 and thorium-232 were determined by the daughter products 214Bi (609 Kev) and 228Ac (583, 909 Kev), respectively. Prior to measurement, the sediment samples packed in sealed containers were held for a month for accumulating the decay products. The gamma irradiation dose in sediments has been calculated according UNSCEAR [10].
The qualitative and quantitative compositions of petroleum hydrocarbons were studied using standardized techniques of solvent extraction, chromatography, and IR spectroscopy. Bitumen chemistry study included cold extraction of sediment samples in an ultrasonic bath. The samples predried at 60°C were extracted three times with chloroform, and the extracts were combined and evaporated on a rotary evaporator. Chloroform extracted bitumen (CBA) was weighed and loaded onto a column of activated silica gel. By using different eluents (hexane, benzene, and a benzene-alcohol blend), the following CBA fractions were obtained: hydrocarbons (HC), benzene resins (bz. res.), alcohol-benzene resins (al-bz. res.) and asphaltenes (asph.).
To determine the quantitative composition of hydrocarbons in bitumens, absorbance values of the main IR absorption
bands at 1740, 1700, 1680, 1600, 1460, 1380, 1250, 1170, 1070, 1020, 970, 875, 860, 815, 770, 750, 725, 720, 700, and 675 cm-1 [11] were measured. The amounts of hydrocarbons and hetero compounds were calculated according to the procedure proposed in [12]. The instrument was calibrated with polystyrene of 0.025 mm thickness as a standard. The relative amount of aromatic structures was determined according to the optical density (D) ratio of the 1600 and 1460 cm-1 absorption bands (D1600 /D1460). The degree of branching was calculated using the D1380/D1460 ratio, which reflects the proportion of CH3 groups relative to total CH2 and CH3 groups of alkanes and cycloalkanes. The presence of (unsubstituted) benzene rings was determined using the absorption band at 675 cm-1; substituted monocyclic structures, the bands at 860, 770, 750, and 700 cm-1; longchain polycyclic structures, the band at 875 cm-1; and fused polycyclic aromatics, the band at 815 cm-1. The degree of oxidation of a sample was determined as the intensity ratio of the absorption bands of C=O groups (1740, 1700, and 1680 cm-1) to total CH2 and CH3 groups of alkanes and cycloalkanes (absorption band at 1460 cm-1).
Statistical analysis of data on the bitumen composition and concentration of radionuclides was performed by constructing correlation matrices with a statistical significance of p < 0.05 using the program STATISTICA 6 [13].
Results and discussion
The results of determining the concentration of 226Ra, 232Th and 210Pb in the surface layer of the Kara Sea sediments are presented in Table 2.
Table 2.
Concentration of 226Ra, 232Th and 210Pb in the surface layer of Kara Sea sediments (Bq/kg).
Station 226Ra 232Th 210Pb Total amount Dose ^Gr/h Depth, m
1 54±2.3 57.84±1.8 13.4±0.6 125±5 0.084 462
2 27.9±1.4 26.4±1.2 10.3±0.4 64±3 0.043 172
3 46.5±2.3 58.7±1.8 14.2±0.6 119±5 0.080 274
4 28.6±2.4 30.1±2.0 2.5±0.2 61±3 0.041 203
5 43.0±2.3 33.5±1.7 2.8±0.2 79±4 0.053 242
6 44.0±3.1 42.1±2.1 1.1±0.1 87±5 0.058 308
7 41.9±2.4 46.3±1.9 8.4±0.4 96±5 0.065 335
8 34.7±1.3 38.3±1.7 8.7±0.4 81±3 0.055 272
9 38.4±1.0 41.0±1.4 4.4±0.1 83±2 0.056 272
10 22.6±1.1 21.1±1.1 1.3±0.1 45±2 0.030 49
Average 38.1±9.6 39.5±12.4 6.7±4.9 84.3±24 0.056
The average 226Ra, 232Th h 210Pb contents in the sediments are 38.1±9.6; 39.5±12.4 and 6.7±4.9 Bq/kg, respectively. The maximum concentration of 226Ra and 232Th (54±2.3 and 57.84±1.8 Bq/kg, respectively) was observed at station 1.
Using data on concentration of radionuclides, the gamma irradiation dose for these sediments has been calculated according UNSCEAR [10]. The gamma irradiation dose in sediments on
the stations varies within 0.030 - 0.084 ^Gr/h (averages 0.056 ^Gr/h). This size doesn't exceed admissible safe level (0.057 ^Gr/h) recommended UNSCEAR. However, at station 1 dose of radiation exceeds admissible level a little (by 1,4 - 1,5 times).
Concentration 226Ra and 232Th increases in the sediment with increase in depth of the sea (Fig. 1)
60
50
m
11
m ^
Ph
<N m <N
Ö o
■b ö <u o Ö o O
40
30
A
A
20
60
50
M
11 m
cö
VO <N <N
40 ö
o
'S ■b ö <u o ö o ü
30
20
100
200 300
Depth (i )
400
500
Fig.1. Change in the concentrations of 226Ra (1, o) and 232Th (2, A) in the sediment with the depth of the sea
Bituminological Study with chloroform and their group composition are presented in
The content of bituminous substances obtained by extraction Table 3.
Table 3.
Composition of organic matter in Kara Sea bottom sediments, wt %
0
Station CBA HC Bz. res. Al-bz. res. Asphaltenes
1 0.011 22.9 11.4 54.3 11.4
2 0.011 25.0 7.6 50.0 17.4
3 0.013 16.8 6.3 61.05 15.18
4 0.016 21.8 7.3 58.0 12.9
5 0.013 28.3 10.9 53.2 7.6
6 0.012 25.8 11.2 55.1 7.9
7 0.014 29.0 8.0 58.0 5.0
8 0.015 33.9 8.7 51.3 6.1
9 0.012 28.4 6.3 60.0 5.3
10 0.01 29.4 8.8 54.4 7.4
Average 0.013±0.002 26.1±4.8 8.7±1.9 54.9±3.8 9.6±4.4
The group composition of CBA is characterized by a large amount of alcohol-benzene resins, having an average value of 54.87±3.75%, with the maximum value being for station 3. The average amount of the hydrocarbon (HC) fraction is also increased to 26.13±4.79% with a maximum value of 33.91% at
station 2. The maximum CBA content (0.016%) was found at station 4. An increased amount of benzene resins (11.43%) in the sediment from the deepest (465 m) station 1 also worth noting.
The distribution of hydrocarbon structures in the sediments from various stations is shown in Table 4.
Table 4.
Hydrocarbon composition of bitumens
Hydrocarbon structures, %
Stations paraffinic naphthenic aromatic Proportion of isoparaffinic
1 46.1 39.7 14.2 0.81
2 19.8 55.3 24.9 0.70
3 39.0 40.7 20.3 0.59
4 37.8 49.2 13.0 0.44
5 29.9 61.0 9.1 0.59
6 21.4 63.1 15.5 0.38
7 33.8 51.3 14.9 0.43
8 20.6 55.5 23.9 0.59
9 19.8 66.8 13.4 0.48
10 18.2 64.9 16.9 0.54
Average 28.6±10.0 54.7±9.1 16.6±5.0 0.56±0.13
The hydrocarbon composition of bitumens at all stations is made of paraffinic, naphthenic, and aromatic structures, wherein the naphthenic dominate (from 39.65 to 66.81%). Their average content is 54.74%. The maximum paraffininic (46.14%) and isoparaffinic content (0.81%) is found at station 1, and the maximum naphthenic content (66.81%) is found at station 9.
The maximum aromatics content (24.94%) is found to be at station 2. At all of the stations, there are both normal paraffins and isoparaffins. The investigation of the composition of aromatic entities showed that substituted monocyclic aromatic compounds prevail in the given areas (Table 5).
Table 5.
Proportions of aromatic structures in bitumens, %
Station Monocyclic Polycyclic
unsubstituted substituted fused long-chain
1 - 76.8 7.7 15.54
2 - 60.4 22.2 17.4
3 - 71.2 12.4 16.4
4 - 91.3 5.1 3.6
5 - 80.0 12.1 7.9
6 - 79.3 11.9 8.8
7 - 67.8 19.9 12.3
8 - 67.3 7.4 25.3
9 - 57.6 11.5 30.9
10 - 82.4 13.7 3.9
The genetic type of the original organic matter (sapropelic or coefficients obtained for the test samples of sediment bituminous humic) was determined according to the values of the spectral substances (Table 6).
Table 6.
Spectral coefficients characterizing the genetic type of original OM*
Station A1 A2 C1 C2
1 0.362 - 0.308 -
2 0.641 - 1.259 -
3 0.525 0.386 0.521 1.188
4 0.236 - 0.343 -
5 0.117 0.376 0.305 0.718
6 0.222 - 0.721 -
7 0.187 - 0.443 -
8 0.406 - 1.160 -
9 0.273 1.506 0.673 0.490
10 0.214 - 0.928 -
* A1 is the ratio of the amount of fused polycyclic to that of mono-Cyclic aromatics; A2 is the ratio of the amount of long-chain Polycyclic to that of monocyclic aromatics; C1 is the ratio between the amounts of aromatic and aliphatic structures; and C2 is the ratio of substituted arenes to normal alkanes.
For the sapropelic matter, the coefficients A1 and A2 are always less than unity. For the humic matter, these parameters are greater than unity. At stations 1, 4, 5, 6, 7, 10 A1 h A2 are less than unity (Table 6), a quantity that is inherent in sapropelic organic matter. The values of coefficients at stations 2, 3, 8, 9 are high and greater than 1, which is characteristic of humic matter. Therefore, taking all the parameters into consideration, we attribute the organic matter in the surface layer of Kara sea sediments to the mixed sapropelic-humic type.
Natural radionuclides 226Ra, 232Th and 210Pb and its relation to
group composition of bituminous substances in sediments
The data on the OM composition and the amount of the natural radionu-clides 226Ra, 232Th and 210Pb in the surface layer of bottom sediments were statistically processed using the software package STATISTICA 6 to establish possible correlations between these parameters. Table 7 presents the coefficients of correlation of the concentration of the natural radionuclides 226Ra, 232Th and 210Pb with the amount of CBA fractions and hydrocarbon structures in the sediments of different genetic type of initial organic substance from the Kara Sea.
Table 7.
Coefficients of correlation of radionuclides 226Ra, 232Th and 210Pb with the amounts of hydrocarbon structures in the Kara Sea
sediments. (data at a significance level of p < 0.05)
All stations, sapropelic and humic type sediments Stations 1, 4, 5, 6, 7, 10 sapropelic sediments
226Ra 232Th 210Pb 226Ra 232Th 210Pb
Paraffins 0,62 0.66 0.67 0.81
Naphthenes -0.66 -0.80 -0.70 -0.86
isoparaffinic 0.68 0.68
polycyclic 0.91 0.96 0.88
Depth 0.89 0.85 0.94 0.98 0.79
232Th 0.90 1.00 0.67 0.92 1.00 0.84
Positive correlation link of concentration 226Ra and 232Th with the content of paraffin structures and negative communication of the 232Th and 210Pb with the content of naphthenic hydrocarbons is noted. In the sapropelic sediments positive correlation connection of contents of the long-chain polynuclear structures with depth is noted. It is shown that in sediments of sapropelic type the density of correlation of naphthenic hydrocarbons content with concentration 232Th and 210Pb increases in comparison with sediments of the mixed type. In the sediments of sapropelic type positive correlation of the 226Ra, 232Th and 210Pb with polynuclear aromatic structures content well expressed
Conclusion
In the studied region of the Kara Sea the common average contents 226Ra, 232Th and 210Pb is 84±24 of Bq/kg. The gamma radiation dose from sediments varies within 0.030 - 0.084 ^Gr/h and average 0.056 ^Gr/h doesn't exceed admissible safe level (0.057 ^Gr/h). So nowadays in the studied region of the Kara
Sea of the significant influence of a technogenic press on the level of a natural radioactive background of bottom sediments is not noted.
Results of the definition of group structure of the bitumoids and concentration 226Ra, 232Th and 210Pb in the surface layer of sediments are compared. Positive correlation link of concentration 226Ra and 232Th with the content of paraffin structures and negative correlation link 232Th and 210Pb with the content of naphthenic hydrocarbons is noted. In the sapropelic sediments positive correlation connection of contents of the long-chain polynuclear structures with depth is noted. It is shown that in sediments of sapropelic type the density of correlation of content of naphthenic hydrocarbons with concentration 232Th and 210Pb increases in comparison with sediments of the mixed type. Well expressed positive communication 226Ra, 232Th and 210Pb with polynuclear aromatic structures is shown.
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