Assessment of the danger of the release of deep hydrogen sulfide from the Black Sea to the surface

Degterev Andrey Kharitonovich Professor, Department of Radioecology and Environmental Compliance, Sevastopol State University 299033, Russia, Sevastopol', g. Sevastopol', ul. Kurchatova, 7 degseb@yandex.ru Other publications by this author

Kucherik Galina Valentinovna Doctor of Technical Science Head of the Department of Radioecology and Environmental Safety, Sevastopol State University 299033, Russia, Sevastopol region, Sevastopol, Kurchatov str., 7 GVKucherik@sevsu.ru

Introduction

The Black Sea is the largest body of water with a hydrogen sulfide zone. If we apply the terminology used in geological exploration, then the proven reserves of hydrogen sulfide in the water column of the Black Sea amount to five billion tons. In terms of volume, this corresponds to 3.5 trillion cubic meters. For comparison, this is more than the recoverable reserves of natural gas on the Sakhalin shelf ^[9].

A mixture of hydrogen sulfide with air is explosive starting with a concentration of hydrogen sulfide in the air of 50 g/m3. In addition, hydrogen sulfide is toxic, it is a gas of nerve action, deadly already at 1 g / m3.

Note that natural gas (and this is mainly methane) has an explosive concentration of almost the same, 55 g/m3, but at the same time no one is interested in questions like "when the Sakhalin shelf explodes", as is customary in relation to the Black Sea. Although Sakhalin has a higher seismicity than, for example, in the Crimea. Obviously, the proximity of the Black Sea to the European part of Russia with a high population density and recreational areas also plays an important role here.

It is of interest to assess the possibility of the release of hydrogen sulfide waters of the sea to the surface with their subsequent degassing, as well as the consequences of increasing the concentration of hydrogen sulfide in surface waters and in the air for coastal areas.

The current state of the hydrogen sulfide zone

The hydrogen sulfide zone has existed in the Black Sea for at least 7.5 thousand years, since the time when the straits between the Black and Mediterranean Seas opened once again after the ice Age. According to the study of bottom sediments ^[2], anaerobic conditions in the water column of the Black Sea have been preserved for all these millennia. This indicates the stability of the system of interaction between aerobic and anaerobic zones formed in the Black Sea.

The distribution of hydrogen sulfide in the Black Sea is well studied. Especially a lot of measurement data was obtained for the top layer with a thickness of 1000 m. Data on the concentration of hydrogen sulfide at depths from 1000 m to 2000 m is significantly less for technical reasons. At the same time, it is well known that the maximum concentration of hydrogen sulfide in the sea is reached at depths above 1500 m, where it is 10 mg/l. However, the largest amount of hydrogen sulfide per hundred meter layer of water is in the depth range from 500 m to 1500 m, which is due to the bowl-shaped basin of the Black Sea. It's just that the overlying layers have more area.

According to measurements, almost all deep-sea stations show an increase in the concentration of hydrogen sulfide with depth up to the bottom mixed layer ^[10]. A characteristic feature of hydrogen sulfide profiles is also a decrease in the concentration gradient with depth, starting from about 800 m (Fig. 1). This fact is usually associated with an increase in vertical mixing there due to the immersion of Mediterranean waters entering through the Bosphorus. In particular, the curve with a zigzag profile in Fig. 1 corresponds to the area of the water area near the Bosphorus.  A smooth averaged profile is usually given in the literature ^[2], which is well approximated by one-dimensional models.

When analyzing measurement data, it is necessary to take into account their accuracy, and at low concentrations, the sensitivity of the measurement method. As is known, at hydrogen sulfide concentrations of less than 1 mg/l, the traditional method of iodometry gives too large an error, and therefore the so-called methylene blue method is now used in these cases [ ]. Therefore, the results of measurements of hydrogen sulfide concentrations at the level of 30 mmol/l or less obtained earlier by the traditional method of iodometric titration should be considered unrepresentative. Accordingly, the conclusions about the upward or downward displacement of the boundary of the hydrogen sulfide zone, made on the basis of such measurements, cannot be considered fully justified.

Fig. 1. Hydrogen sulfide profiles in the Black Sea according to measurement data

It should also be borne in mind that, in fact, the concentration of hydrogen sulfide in seawater is usually understood as the total concentration of dissolved H2S gas and the hydrosulfide ion HS–. In this sense, hydrogen sulfide partially dissociates in water as a weak acid. Therefore, dissolved hydrogen sulfide itself, even in deep waters, accounts for only 2 mg/l out of 10 mg/l. This situation is similar to the carbonate system in seawater, where the dissolved CO2 gas itself accounts for a much smaller part of the dissolved inorganic carbon than the bicarbonate ion HCO3–^[3]. An important consequence of this feature of hydrogen sulfide is the delay in its release from water into the atmosphere when reaching the surface of deep waters, only 20% of hydrogen sulfide will immediately degass, the main part of it will be released later, after a shift in the chemical equilibrium in the H2S–HS--S2–system.

By itself, the concentration of hydrogen sulfide in the Black Sea waters of up to 10 mg/l is not unique. In the natural waters of the Caucasian mineral springs in the Sochi area, the concentration of hydrogen sulfide reaches 450 mg/l and there it is really necessary to dilute these natural waters at least twice for use in local sanatoriums. And doctors even recommend drinking water with a concentration of hydrogen sulfide up to 45 mg / l in glasses in certain cases.

Samples of the deep waters of the Black Sea are handled in chemical laboratories without special safety measures. Simply open a 0.9 l sample taken by a bathometer at a depth and take measurements. At the same time, the same 10 mg of hydrogen sulfide is gradually released into the laboratory air. Based on the volume of the room of the order of 50 m3, the concentration of hydrogen sulfide in the air will not exceed 0.2 mg / m3, even in the absence of ventilation.

Hydrogen sulfide and oxygen fluxes

The stability of the hydrogen sulfide zone for thousands of years has been due to the constant supply of hydrogen sulfide in the anaerobic zone and the natural dynamic equilibrium between this hydrogen sulfide flow and its compensating oxygen flow from the aerobic zone ^[6]. In the framework of the one-dimensional diffusion model, the oxygen flow is simply adjusted by changing the position of the upper boundary of the hydrogen sulfide zone. For example, in the simplest case of constancy of the coefficient of vertical turbulent diffusion Kz in depth, the oxygen flux density in the steady state is ^[1]:

                                           F = Kz Compare /(Hc – Nvks) .                                  (1)

Here, the saturating (equilibrium with the atmosphere) concentration of dissolved oxygen in surface waters is compared, Hc is the position of the boundary of the hydrogen sulfide zone (or a distributed source of hydrogen sulfide) and Nvcs is the thickness of the upper mixed layer. Since so far Hc >> Nvcs, then, in the first approximation, the oxygen flow is inversely proportional to Hc. This provides a negative feedback that prevents the rise of hydrogen sulfide to the surface of the sea.

Oxygen coming from the atmosphere in contact with dissolved hydrogen sulfide oxidizes it to sulfate ions, which are already included in the basic salt composition of seawater ^{[4, 11]}. At the same time, even if there is a constant source of hydrogen sulfide in the sea, an unchanged supply of hydrogen sulfide is maintained and the illusion is created that there are no flows at all, just between the aerobic and anaerobic zones there is a kind of impenetrable "lid" in the form of a well-defined pycnocline.

Of course, it is possible to force deep waters to the surface by, for example, an underwater explosion at a certain depth. However, it is not difficult to show that in this case the concentration of hydrogen sulfide in the air will be much less explosive. That is, that the "hydrogen sulfide bomb" will not work, although water masses with a concentration of hydrogen sulfide of about 5 mg/l will actually come to the surface.

In fact, the fact that the solubility of hydrogen sulfide in water ? = 5 g/l at its partial pressure of 1 atm, according to Henry's law, means that if the concentration of hydrogen sulfide in water C = 5 mg/l, then it is saturating for the partial pressure of hydrogen sulfide in air P = 10-3 atm:

                                                    C = P .                                                  ( 2 )

At the same time, according to the equation of state of an ideal gas, at a pressure of hydrogen sulfide P = 1 atm, its concentration in the air is equal to:

                                                    Svozd = ?/V? , ( 3 )

where the molar mass of hydrogen sulfide is ? = 34 g and the volume of 1 mole of an ideal gas is V? = 22.4 l. Hence, the yield is 1.5 g/m3.

The obtained estimate is an upper estimate, since the partial pressure of hydrogen sulfide in water is determined by the content of only H2S gas molecules in it, which in deep waters is 4 times less than 10 mg/l ^[3]. Taking into account this factor, even when undiluted deep waters come to the surface throughout the entire water area, the concentration of hydrogen sulfide in the air will be less than 1 g/m3. Compared to the explosive 50 g/m3, this is very small. And taking into account the strong mixing of the air above the sea, and this concentration in the air, dangerous in the sense of hydrogen sulfide poisoning, will be only directly above the surface of the water. Moreover, it is not deadly. Although the marine ecosystem associated with the aerobic layer will die in this case, since the mass of oxygen in the aerobic layer is much less than the mass of hydrogen sulfide in the anaerobic one.

An analysis of balance estimates of hydrogen sulfide and oxygen intake in the Black Sea shows that the annual production of hydrogen sulfide in the sea is 30-50 million tons, and approximately the same amount of hydrogen sulfide is oxidized in the sea per year ^[12]. At the same time, the net production of hydrogen sulfide (that is, the difference between these numbers) is estimated in a number of works as 7 million tons. It is obvious that such estimates related to predicting the growth of hydrogen sulfide contamination are statistically insignificant. This is the case when the resulting stream is the difference of two large numbers, and at the same time it is smaller than the errors in the estimates of these numbers.

A similar situation exists with monitoring of oxygen and carbon dioxide fluxes between the ocean and the atmosphere ^{[5, 8]}. When determining them, the gas exchange rate is used, the accuracy of which is estimated as 30%. As a result, unidirectional global gas flows into and out of the ocean in different waters and in different seasons are determined with this accuracy, but the resulting flows are not. Because their magnitude is an order of magnitude smaller, respectively, they are smaller and the measurement error. Thus, for example, it is not possible to estimate the flow of CO2 into the ocean according to gas exchange monitoring data. This is the so-called problem of estimating the difference between two large numbers.

Conclusion

The conducted research shows that regardless of the nature of various sources of hydrogen sulfide in the water column and at the bottom of the Black Sea, there is currently an approximate balance between the release of hydrogen sulfide and its oxidation to sulfates by dissolved oxygen. Most of the oxygen in this case comes from the atmosphere, therefore, through the position of the boundary of the hydrogen sulfide zone, the oxygen flow is adjusted. Quantification of the increase in hydrogen sulfide content in the sea by monitoring is statistically insignificant due to the smallness of the resulting flow compared to the production of hydrogen sulfide in the sea.

 The release of hydrogen sulfide to the surface in the foreseeable future is possible if the stratification of waters is disrupted by mechanical interference from the outside in the form of an underwater explosion or an asteroid falling into the sea ^[7]. However, the concentration of hydrogen sulfide in the air will not reach explosive values, although the aerobic zone may become completely hydrogen sulfide for a while.