ISSN 0097 8078, Water Resources, 2013, Vol. 40, No. 3, pp. 285–296. © Pleiades Publishing, Ltd., 2013. Original Russian Text © N.S. Safronova, E.S. Grishantseva, G.S. Korobeinik, 2013, published in Vodnye Resursy, 2013, Vol. 40, No. 3, pp. 274–286. WATER QUALITY AND PROTECTION: ENVIRONMENTAL ASPECTS
Hydrocarbon Gases (C1–C5) and Organic Matter in Bottom Sediments of the Ivankovo Reservoir on the Volga River N. S. Safronovaa, E. S. Grishantsevaa, and G. S. Korobeinikb a Moscow State University, Moscow, 119991 Russia E mail: [email protected] b Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, ul. Kosygina 19, GSP 1, Moscow, 119991 Russia Received June 21, 2011
Abstract—The results of studying the composition of hydrocarbon gases (C1–C5) and organic matter in bot tom sediments of the Ivankovo Reservoir in 1995, 2004, and 2005 are given. The methods used in the study include vapor phase gas chromatography, instrumental pyrolysis gas chromatography, and mass spectrom 13 etry for determining organic carbon δ Corg. The gas field of bottom sediments in different regions of the res ervoir varies widely in terms of gas saturation and the spectrum of hydrocarbon gases. This suggests the het erogeneous composition of organic matter in the sediments and different conditions of its input and transfor mation processes. The gases were found to contain saturated hydrocarbons from methane to pentane C1–C5, including isomers i C4 and i C5 and unsaturated compounds C2–C4. A correlation was found to exist between methane distribution and the distribution of its more high molecular homologues, which confirms their genetic relationship in bottom sediments. The obtained results show an increase in the rate of microbi ological processes and organic matter transformation for most regions in the Ivankovo Reservoir. The only exceptions are the zones of Moshkovichskii Bay and the sections at Gorodnya and Konakovo, where tech nogenic organic matter is being accumulated. The high information value of hydrocarbon gases as bio geochemical markers of the sources of organic matter and the rates of its transformation is demonstrated. The isotopic composition of organic matter carbon in the bottom sediments of the Ivankovo Reservoir δ 13C var ies from –26.1 to –30.86‰.
Keywords: bottom sediments, organic matter, gas hydrocarbons, vapor phase gas chromatography, pyrolysis gas chromatography, mass spectrometry DOI: 10.1134/S0097807813020085
INTRODUCTION the end hydrocarbon product of OM mineralization, and can serve as an indicator of those processes. How The role of bottom sediments (BS) in the life of a ever, the concentration of methane, especially in top water body is extremely high. The composition and sampling horizons, can demonstrate the effect of both properties of BS reflect the totality of biological, methanogenesis processes and the properties of meth chemical, and physical processes taking place in the ane as a very mobile gas with low sorption capacity and water body. The rates of biochemical processes of comparatively low water solubility. Because of this, the organic matter (OM) transformation are evaluated by geochemical data on methane distribution in BS a wide range of characteristics. The reduction of low should be studied along with other gas characteristics. molecular OM in BS of freshwater bodies is accompa nied by the production of the majority of methane and Studies show that the distribution of gaseous other low molecular hydrocarbons, whose concentra hydrocarbons (HC) in natural features is a function of tions can be used to evaluate the ecological state of the original OM and its transformation processes. The water body [4, 5, 8–10, 13, 14]. The accumulation of specific features of the distribution of gaseous HC can gases in BS depends on OM composition and concen be used as typical organic–geochemical characteris tration. Informative biogeochemical markers of OM tics [7, 12]. sources and its transformation processes are gaseous To collect information about the sources of matter hydrocarbons. Gasometry methods are in wide use in containing compounds of natural and anthropogenic geochemical studies [2, 3, 11, 18]. The total rate of OM, the following tasks were formulated: OM biodegradation processes is mostly assessed by the measured methane content, because methane forms to determine the concentrations of hydrocarbon and occurs in the biosphere almost ubiquitously; it is gases (С1–С5) and to consider the regularities in their
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N
Ivan’kovskii Pool Peretrusovskii Bay Soz R. 8 Omuntinskii Bay Tver 10 Volzskii Babninskii Bay Peremerki Cr. Pool 9 Moshkovicheskii Dubna Bay 12 7 ГРЭС 13 Vidogoshchi 6 Konakovo 1 2 3 Donkhovka R. Melkovo Ploski Redkino 4 14 11
5 Gorodishche Shoshinskii Pool Novozavidovskii
Fig. 1. BS sampling scheme in the Ivankovo Reservoir. Sections: (1) Gorodnya, (2) Melkovo, (3) Nizovka Volga, (4) Nizovka Shosha, (5) Gorodishche, (6) Ploski, (7) Konakovo, (8) Korcheva, (9) Klintsy, (10) Dubna. Bays: (11) Vesna, (12) Fedorovskii, (13) Korovinskii; (14) Redkinskii Canal. distribution in the top BS layers of the Ivankovo Res generalized characteristics of OM have recently ervoir; gained in popularity in geochemical studies [6, 16, 19]. to determine the concentration of total OM com Vapor phase gas chromatography and instrumental ponents; pyrolysis gas chromatography is used for comprehen to study the isotopic composition of organic carbon sive study of the composition of OM in BS. Mass spec 13 Corg for BS. trometry was used to study the distribution of isotopes 13 of organic carbon δ Corg. METHODS OF STUDIES Gas Chromatography The object of study was the BS of the Ivankovo Res ervoir, for which very few data are available in the lit Gases in BS were studied by vapor phase chroma erature about the concentration and distribution of tography. The analytical procedure included the low molecular HC [11, 15]. Field studies were carried extraction of gases from BS by equilibrium vapor out in 1995, 2004, and 2005. The sampling was carried method under static conditions and the subsequent out at 13 sections and in 7 bays from Tver City to determination of the composition and concentration Dubna Town (Fig. 1). of components on gas chromatography [18]. The multicomponent analysis of the composition To extract gases from BS, 1 g of sample material of gaseous and solid phases of BS was carried out for was placed in 15 ml glass flasks, distilled water was sediment samples taken in summer by GOIN tube added to 2/3 of flask volume, and the flasks were sealed from the upper horizon of BS (top 10–15 cm) in by self sealing rubber plugs. After shaking, the flasks points in areas subject to different level of technogenic were placed in a thermostat and exposed at 70°С for impact. 30 min. Next, 1 ml of gas was taken and placed in the OM composition is very diverse. Because of this, flow of carrier gas in “Tsvet 500" gas chromatograph the analytical methods for the separation and determi with a flame ionization detector (Russia). Analysis of nation of individual compounds and OM classes are methane and other light HC was carried out on "Tsvet very complicated and, sometimes, practically unreal 500" gas chromatograph with a flame ionization izable. The most widespread modern method for detector. HC were separated in a packed column with effective separation and analysis of complex multi modified aluminum oxide under isothermal regime of component organic mixture is chromatography [1, 2, column thermostat operation. The optimal conditions 17, 18]. Rapid instrumental methods for determining for the separation and quantitative determination of
WATER RESOURCES Vol. 40 No. 3 2013 HYDROCARBON GASES (C1–C5) AND ORGANIC MATTER 287 hydrocarbon components in gas phase were found The mass of the sample under study was 5–100 mg. [18]. Under optimal operation conditions of RE 2 analyzer Analysis procedure enable determining hydrocar in the specified pyrolysis cycle, the relative standard bons of the series С1–С5 with saturated, unsaturated, deviation for the characteristics to be evaluated varies and isomeric structure: СН4, С2Н6, С2Н4, С3Н8, from 1 to 8%. The lower concentration detection lim С3Н6, iС4Н10, С4Н10, С4Н8, ∑С5Н12 (the sum of its are 0.01–0.03 mg/g. pentanes and iso pentanes). The quantitative analysis was made by absolute calibration method. The lower boundaries of the concentrations determined by the Method for Determining the Isotopic Composition of Corg method were 0.001–0.01 μL/kg, depending on the δ13 compound being analyzed. The relative standard devi The isotopic analysis of Corg was carried out on ation varies from 2 to 6% depending on the concentra mass spectrometers Delta S and Delta Plus. The sedi tion and mass of HC. ment samples were first crushed in a ball mill Reatsch HM 200. Carbonates were removed by 10% HCl solu tion. The samples were washed and dried at 60°С. A Pyrolysis Method of OM Studying weighed portion of the sample was used to determine The method of pyrolysis gas chromatography is in the isotopic composition with the use of a CHN sys wide use for obtaining rapid and adequate information tem of mass spectrometer. The instrumental accuracy about the qualitative composition and the amount of was ±0.1%. The reproducibility of the entire cycle, OM in ecological–geochemical objects. including sample preparation, is not higher than ±0.3‰ PDB [2, 3]. The method is highly sensitive and rapid; it enables one to treat small samples by standard equipment. Of wide use now are pyrolysis methods developed for DISCUSSION OF RESULTS automatic analyzers of ROCK EVAL type and operat ing in fully automated regime—from loading the sam The major sources of anthropogenic impact onto ples to the output of data on the concentration of com the reservoir are the industrial plants of the towns of ponents and the evaluated characteristics in the form Tver and Konakovo and the settlements of Redkino of tables and plots [6, 16, 19]. and Melkovo; motor and railway transport; the dis Used in this study was automated ROCK EVAL charges of municipal and domestic wastewaters; agri 2/TOC analyzer (FIN and BEICIP FRANLAB, culture and recreation. France) [16], coupled with a personal computer with software packages ROCKDAT AND ROCKINT and The results of gas chromatography studies of the used to control the process of pyrolysis and to calcu gas phase of BS over the periods of expedition studies late the primary data and carry out their processing. (1995, 2004, 2005) are given in Table 1. The gas field of BS varies widely in different parts of the reservoir in BS taken in the reservoir were pre dried at the terms of both the level of gas saturation (a quantitative room temperature, blended, and powdered. Under characteristic) and the spectrum of gases (a qualitative programmed heating of the sample (25°C/min) from characteristic). This suggests the heterogeneity of OM 300 to 600°С in a flow reactor in an inert helium composition in the sediments and the difference atmosphere, RE 2 analyzer makes it possible to deter between its input conditions in different sampling mine the following OM characteristics: points. The heterogeneous character of OM deter S1—the amount of free HC (С1–С10) that are con mines the different resistance of its components to tained in the sample and that release at 300°С; decomposition and the different contributions of the S2—the amount of high molecular HC and OM forming gaseous HC into the total composition of BS cracking HC (С12–С36) that release within tempera gas phase. Saturated HC from methane to pentane are ture interval of 300–600°С; identified in the gases: С1–С5, including isomers i S3—the amount of СО2 that forms at the decom С4–iС5, and unsaturated compounds С2–С4.. position of OM or OM cracking within temperature interval 300–400°С, which serves as an indicator of Methane is a dominating component among the the presence of oxygen structures in OM molecules; saturated HC; it was recorded in all examined samples with a share in the total concentration of gases С1–С5 TOC—total Corg concentration in the samples (СН4/ΣС1 – С5 satur) varying from 75 to 99%. Studies evaluated by hydrocarbon parameters; [7, 12] have shown that methane homologs—HC T —pyrolysis furnace temperature at which HC max fractions С2–С3 can form because of biochemical output in the peak of S2 is maximal (peak crest); transformation of terrigenous OM of freshwater river HI—hydrogen index, HI = S2/TOC, which is used basins, such as the ecosystem of the Ivankovo Reser as a characteristic of the presence of hydrogen struc voir. The genesis of HC fraction С4–С5 can be due to tures in OM molecules; either terrigenous OM and freshwater plankton or OI—oxygen index (OI = S3/TOC) shows the pres technogenic pollution, since pentane essentially is the ence of oxygen structures in OM molecules in BS. first in the gasoline series of liquid petroleum HC.
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Methane concentration varies within wide limits 0.003 to 0.49 (up to 0.08 in the majority of points), (from 96 × 10–4 to 2429 × 10–4 mL/kg), depending on which suggests a very active processes taking place in the sampling site and time (Table 1). BS of the Ivankovo Reservoir, though at different The composition of HC gas mixture in BS sampled rates. In 1995, the maximal value of K (0.12) was in 1995 at the sections of Vidogoshch, Konakovo, obtained for BS of Ploski section, situated a short dis Korcheva, and the mouth part of Moshkovichskii Bay tance downstream of Gorodishche section. In 2004– show relatively low concentrations of methane and 2005, ethylene concentration in the samples increased saturated HC and the presence of homologs only of significantly. In two areas, K increased by an order of the series С2–С3. Such BS composition corresponds magnitude, implying that the rate of microbiological to the transformation of OM mostly of natural genesis processes decreases. DS sampled in the sections of in unpolluted parts of the water body. The composi Gorodnya (downstream of Tver City) and Gorodish tion of hydrocarbon gases in BS sampled in 2005 at che (at the site of mixing of OM rich water of the sections and in bays has changed. Relatively low con Shoshinskii Pool and the polluted water of the Volga centrations of methane and saturated HC of С2–С3 R. downstream of Tver C.) show the values of K of fractions were recorded at sections of Gorodnya, Gor 0.49 and 0.2, respectively. Technogenic OM, entering odishche, Ploski, Klintsy, the channel part of Dubna with municipal, domestic, and industrial wastewaters, section and the bays of Vesna, Korovinskii, and Per whose transformation in nature is slow, accumulate in etrusovskii outlet. The results of studying BS gas com Gorodnya section. The Shoshinskii Pool drains a wet position show the transformation of OM of mostly land area rich in organic matter. Further downstream, natural genesis in unpolluted areas within the reser in Gorodishche section, the transformation of tech voir. nogenic OM is more intense, which is likely due to Specific features of BS gas composition in Moshk water, rich in natural OM, which enters from the ovichskii Bay are the high methane concentration and Shoshinskii Pool. the presence of its homologs С2–С5. In 1995, higher The comparison of K values obtained for sediments concentration of saturated HC of the series С2–С4 was samples taken in identical sections in 1995 and 2005 recorded in this section; in 2005, HC of С5 series were has shown that the value of K dropped on the average recorded here. Moshkovichskii Bay receives the 2.5 times in the majority of the examined areas municipal and domestic wastewaters, effluents from a (Vidogoshchi, Ploski, Babninskii bay, Korcheva). The State District Power Plant (SDPP) and other indus value of K near the Moshkovichskii Bay has not trial plants in Konakovo Town. The gases from sam changed. This suggests that the environmental condi ples taken from Shoshinskii Pool near a bridge on the tions near Moshkovichskii Bay have not improved. Moscow–St. Petersburg highway, in addition to high The only exceptions are the sections of Gorodnya and methane concentration, the presence of its homologs Konakovo, where the value of K increased 8 and 1.5 up to С5 were recorded. The BS in Nizovka Shosha times, respectively. Thus, the concentration of tech section in 2004–2005 were also found to contain nogenic OM in Konakovo section is increasing insig hydrocarbons up to С5. This confirms that the techno nificantly, its accumulation in Gorodnya section is genic pollution from motor and railway transport still very rapid. This not only determines the level of OM has a negative effect on the ecological state of the res concentration, but also suggests the possibility of ervoir. changes in the occurrence form and the migration As can be seen from Table 1, most samples were capacity of heavy metals. also found to contain unsaturated HC. Unsaturated HC of the saturated series С4–С5 (butanes and HC С2–С4 (ethylene, propylene, and butylenes)— pentanes) were recorded in the study period in differ intermediate products of OM destruction—are very ent parts of the reservoir: reactive because of an unstable double bond. The pres in 1995, in areas of the Shoshinskii Pool and Ploski; ence of such compounds in gases in relatively high in 2004, in areas near Melkovo, Nizovka Shosha, concentrations suggest the permanent input into BS of Ploski, and Klintsy; new bioavailable OM, which experiences intense transformation as the result of biodegradation pro in 2005, in sections Nizovka Volga, Nizovka cesses, resulting in constant replenishment of unsatur Shosha, Moshkovichskii Bay, and Dubna. ated HC and even their accumulation. Among the It is worth mentioning that no increase in the con unsaturated HC in the examined samples, methane centration or evident accumulation of petroleum HC shows the highest concentrations, which, in a wide was recorded in those areas. It is likely that the con range, are 2–2500 times greater than that of the near centrations of petroleum HC are determined by the est saturated HC—ethane. The rates of the processes environmental conditions in the period under study. taking place in such cases are characterized by coeffi In the lower part of the reservoir, the dam near cient K, i.e., the ratio of saturated to unsaturated HC: Dubna T. serves as a mechanical barrier, reducing river K = (С2 – С4)sat/(С2 – С4)unsat. The lesser K, the more flow rate and causing the sedimentation of debris, intense is the process of OM transformation. The accompanied by OM accumulation. In the same area, value of K, which is much less than unit, varies from gases can accumulate, whose origin can be due to ter
WATER RESOURCES Vol. 40 No. 3 2013 HYDROCARBON GASES (C1–C5) AND ORGANIC MATTER 289 –0.04 –0.037 –0.067 0.03 0.029 0.052 0.025 0.019 0.023 0.037 is normal butane, 10 –0.06 kn –0.085 –0.047 –0.055 H 4 0.071 0.046 0.043 0.059 0.038 0.016 0.0098 0.048 0.03 0.055 0.044 0.049 0.047 0.035 0.033 0.0458 C n 0.0316 0.057 0.046 0.031 , 4 H 12 2 1 1 1 1 1 1 1 1 1 H 5 –72 /C