by Pyotr ZAVYALOV, Dr. Sc. (Geogr.), Deputy Director of the Shirshov Institute of Oceanology (RAS), Pyotr MAKKAVEEV, Dr. Sc. (Geogr.), Head of the Biological Hydrochemistry Laboratory of the same institute

How does anthropogenic impact influence water quality characteristics of the Black Sea water areas adjoining estuaries of the rivers Mzymta, Sochi, Khosta, Kudepsta, Bitkha and others within the limits of the Sochi resort city? Data of ground and satellite observations provide an actual situation of pollution in this region. For a number of years staff members of the Shirshov Institute of Oceanology have been carrying on these observations.

AS EVIDENCED BY STATISTICS

In general for the World Ocean total many-year average volume of river runoff is estimated approximately at 40,000 km³ per year, which makes up about a quarter of water-balance input of the ocean (the remaining three quarters are provided by atmospheric precipitation to its surface). But for separate shelf regions and also internal and adjacent seas a relative value of continental runoff can be by an order of magnitude higher than for the whole ocean. For the Black Sea this index (about 340 km³ per year) exceeds atmospheric precipitation (240 km³ per year) and approaches evaporation value (390 km³).

River runoff is a main source of getting of dissolved and suspended substances of a continental origin including products of anthropogenic pollution into the sea. According to official data of the RF Ministry of Natural Resources and Ecology an annual runoff to the Russian seas includes 200,000 t of ammonium nitrogen, 60,000 t of phosphorus, 50,000 t of metals (iron, copper, zinc, etc.), 30,000 t of oil products and above 1,000 t of phenols. These and other substances of a continental origin have substantial and in many cases negative effect on ecosystems. Therefore, to forecast the routes of distribution of river runoff in sea shelfs is a major practical task.

The dynamics of such processes is rather complex and is not studied comprehensively. Continental waters, leaving the river estuary, spread over the surface and form in the sea specific structures called in modern

literature "river plumes". Their spatial scales vary from tens of meters to tens, and for large rivers also to hundreds, of kilometers. Besides, in most cases they maintain well-defined boundaries with the surrounding sea waters. It is just dynamics of plumes which can explain transfer processes of terrigenous (i.e. brought from dry land) impurities in a coastal area.

Being an inland and almost closed aquatic area the Black Sea* is especially subjected to the influence of a river runoff. The number of large and small rivers, flowing into it, approaches 1,000, including above 30 within the limits of Russia. All of them are comparatively small, and their total average many-year volume is about 7 km³ per year, which makes up only around 2 percent of the total freshwater runoff to the Black Sea, that's why specialists paid comparatively slight attention to them in scientific literature. However, though they are an insignificant component of the hydraulic balance for the sea as a whole, their runoff has an appreciable effect on the land-sea system locally, biological productivity of the Russian Black Sea shelf and also the quality of waters in this region.

The problem of their pollution is especially urgent in the aquatic areas close to big cities. First of all it refers to Sochi, the biggest health resort of the country. Industrial and domestic wastes getting to the sea with the runoff of the rivers Mzymta, Sochi, Kudepsta,

* See: M. Flint, "The Black Sea: Problems and Prospects", Science in Russia, No. 4, 2007.-Ed.

Khosta and others are one of the dangerous sources. The published data prove that concentration of oil products in the river water within the city exceeds the maximum allowable parameter 10-15 times and that of heavy metals 3-5 times. The intensive construction on the coastal line and especially construction of new port facilities in Adler near the estuary of the river Mzymta and also in the area of the central marine passenger terminal near the estuary of the river Sochi are another factor of seawater quality deterioration in Sochi in recent years. Plumes of turbid water with a high content of suspended and dissolved organic substances near the river estuaries include a considerable quantity of hard domestic wastes (plastic and glass bottles, polyethylene bags, paper, wood fragments, etc.). When the river plumes containing wastes get in contact with the coastal line of beaches and embankments, they essentially impair recreation conditions in a number of regions of the resort city.

NEW METHODS OF STUDIES

Monitoring of the seawater pollution and quality indicators is carried out in Sochi on a regular basis by the Rosgidromet services. The value of these observations is determined by long-term nature of the data. But their limited spatial and time resolution (to our knowledge, the question here is 4 measurements per year in 8 separate points) does not allow to describe details of pollutant distribution, localize their sources

Location of the testing grounds and example of a measurement organization scheme. Broken line--a ship's movement, sampling points are numbered, black circles--points of installed anchored current meters, black square--point of installed portable weather station.

and also trace their synoptic dynamics under the action of wind and coastal sea currents. These problems require measurements with much higher resolution and using special numerical models.

For this purpose the special Small Rivers of the Black Sea program was launched at the RAS Institute of Oceanology in 2006 within the framework of the annual all-embracing field works. In 2009-2013 studies were carried out within the Greater Sochi (rivers Mzymta, Kudepsta, Khosta, Bitkha and others). Practically all expeditions were carried out in the same period of the year, in spring (May), which usually agrees with flood runoff of rivers in this region.

The small-size ship measurements were taken in the form of 3-5 transverse coastal sections up to 4 km long, as a rule, from isobath 5 m to isobath 50 m with a distance between adjacent sections of 1-3 km. At each section several stops (stations) were made for measurements and water sampling. Continuous recording of parameters of the near-surface sea layer was conducted also when the ship moved between stops. The continuous-flow probing system and also an ultraviolet fluorescent lidar* specially worked out at the RAS Institute of Oceanology were used for this purpose. Flow-type sensors recorded temperature and salinity, oxygen concentration and separate chemical components in sea water during the ship's movement, while the lidar analyzed

* Lidar is an optical radar for remote sensing of air and water media.--Ed.

fluorescence spectra of dissolved and suspended substances in water and made express analyses of chlorophyll, suspended matter and dissolved organic substances. Thus, spatial resolution starting from units of meter unusually high for such measurements was provided.

Anchored stations equipped with current speed meters were installed on all testing grounds to study transfer of pollutants and terrigenous substances. Similar devices were installed also directly near river estuaries to trace changes of a river water discharge. Moreover, a portable weather station operated on the seashore, and 10-minute averaged data of wind speed and direction as well as basic meteorological elements were recorded during a whole period of measurements. Then all data were processed in a specially developed numerical model which will be discussed below.

WATER QUALITY INDICATORS

The areas of polluted water in the form of bright plumes almost always associated with river estuaries are clearly traced on the inland shelf for all testing grounds in the horizontal distribution of mineral and general suspended matter, dissolved organic substances and also of a majority of chemical indicators. The linear sizes of such tails differ for various regions of the city, the most extended of them were observed usually in Adler near the Mzymta river estuary. Their average sizes make up about 2 km (area of a respective region

reaches 4-5 km² on an average) but there were also observed "gigantic" tails (more than 50 km²). Our observations recorded the 16 km maximum extension of the polluted area from the Mzymta river estuary. It is noteworthy that this river brings to the sea by an order of magnitude more of terrigenous suspended matter than the river Sochi, though the average many-year volumes of their runoff differ only thrice. In general, the runoff structures in the estuary areas of the rivers Sochi, Kudepsta, Khosta and others have, as a rule, essentially lesser sizes than Mzymta, but at the same time they are not characterized by lesser concentrations of pollutants.

For example, a total concentration of dissolved organic substances in the seawater close to the Bitkha river estuary (Loo--Uch-Dere region) exceeds the background values more than 15 times, concentration of phosphates more than 3 times, silicon--18 times, nitrites and nitrates--6 times and ammonia nitrogen almost 40 times. This river is worthy of a separate discussion. The point is that the river runs through the largest area of domestic wastes in Sochi. The harm inflicted by the dump to regional ecology was a cause of protest meetings and a subject of a discussion in the local and central media. But a degree and a spatial scale of the seawater pollution associated with the dump area and the Bitkha river runoff were practically not studied. Our measurements showed for the first time that this pollution in the said region was really extremely high. True, luckily, the pollution has been localized in a relatively small zone and is mainly spreading in the direction of the Dagomys region along the coastal strip of 100-200 m wide and about 1 km long. The administration of the adjacent resorts has to inform their guests of the situation and think over protection measures. A high concentration of dissolved organic substances is typical also of plumes of other rivers in Sochi.

It should be noted that the situation in some indicators deteriorates every year before our eyes. For example, according to our measurements, the average content of terrigenous suspension in seawater near the Mzymta river estuary was increasing steadily and more than doubled for the last 6 years. The same is true, for example, for concentrations of silicon and nitrites. But the content of oxygen dissolved in seawater, on the contrary, decreased, and today the state of subsurface water is close to hypoxia which cannot but affect the ecosystem's functions and vital activity of biocommunities. But we should bear in mind that the content of a river runoff is very inconstant. The cited data relate to a limited number of point measurements made only in a spring period, therefore they surely do not expressly prove the existence of unidirectional yearly trends. Nevertheless, the noted tendencies seem quite instructive.

As a whole, the accumulated data leave no doubts that seawater in the resort city of Sochi is highly pol-

Maps of concentrations of dissolved organic substances (above) and mineral suspension (below) near the Mzymta river estuary (left) and Sochi (right). May of 2013. Dark regions indicate pollution.

luted, and runoff of the local rivers is the main source of this pollution. But the impurities are dissimilar in space and time, they are localized in the form of tails or plumes, whose sizes and location depend on the intensity of a river runoff and also conditions of wind and sea currents. We have already showed that the ecological situation is slowly changing every year, but the local effects of pollutants are apt to change faster. It is illustrated by satellite pictures of river plumes (shown in red and yellow colors). Obviously, their sizes and location change radically during several days and often even in one day.

The calculations show that the main cause of such changeability is wind effects. Indeed, river water is less concentrated than clean seawater, therefore it forms a stable and relatively thin, as if sliding on a sea surface, layer. A major part of wind energy is transferred just to this layer, that's why even a moderate wind can move quickly and efficiently polluted continental waters. Our ability to forecast, at least in a short-term perspective, the state of river plumes is important for practical purposes. We can get answers to many questions by means of numerical modeling. For example, what direction and force of winds is most risky for

Relative concentrations of mineral suspension in the seawater on the section taken along isobath 7 m. Arrows indicate location of estuaries of some rivers.

Average concentrations of some hydrochemical quality indicators of seawater near the estuaries of the rivers Mzymta, Kudepsta, Khosta, Sochi and Bitkha.

1 -content in water of a lower current of the river before exit to the sea;

2--content in sea surface waters opposite the river estuary, beginning of the "plume";

3--background content in sea surface waters at a distance of 2-3 km from the estuary.

Map of concentration of dissolved organic substances near the Bitkha river estuary.

Dependence of a plume pollution area of the Mzymta river on wind velocity given at different directions of wind.

carrying suspension and solid wastes from the Mzymta river to the city beaches of Adler?

NUMERICAL MODELING OF PLUMES

To calculate water movements in a coastal zone one uses usually numerical models based on the Euler method (the 18th century Swiss, German and Russian mathematician and mechanic, member of the St. Petersburg AS, Leonard Euler). This means that the sea together with the continental runoff is regarded in them as a continuous medium, and solve a complete system of equations of its movement (Navier-Stocks equation) on difference grids. But for river plumes such approach can meet with difficulties during reproduction of high-gradient zones near outer boundaries of the desalinated region and on its lower boundary. This can well be explained physically as due to low compactness the plume in many instances behaves as a "foreign" floating object on the sea surface, and it is difficult to describe it within the framework of the concept that it forms a sole continuous medium with the sea.

The Lagrange method (French mathematician and mechanic, foreign honorary member of Petersburg AS from 1776) is an alternative to the Euler method, and it considers balance of forces for certain volumes of water. Let us point out here that such method was not

Some hydrochemical indicators of sea waters near the Mzymta river estuary as measured in different years.

used before to solve the said problems. The numerical model based on this method was developed at the RAS Institute of Oceanology to reproduce transfer of pollutants in the Sochi water area. Plume is represented here as a set of an ample quantity of separate particles or elementary water "columns", which bend to friction force of wind, force of lateral and vertical friction with adjacent water layers, pressure gradient force and the Coriolis force*. Instead of solving transfer equations for a medium as a whole we follow the movement of separate particles tracing the plume, and each particle is treated as a material point. As water moves, it is mixed with its underlying layers in an elementary column, while its salinity changes from zero to values typical of seawater, which results in gradual dissipation of the plume. The new mathematical model turned out to be not only physically adequate but also very sparing as regards numerical resources and rapid action.

Alongside with the analysis of the field and satellite observations, the conducted experiments allowed to construct a physical classification of river plumes in the Sochi water area and establish a connection between distribution of terrigenous pollutions and characteristics of the wind effect.

* The Coriolis force (after the name of the 19th cent. French mechanic)-- one of the inertial forces introduced to register effects of rotation of a movable reading system on a relative movement of the body.--Ed.

Plumes of an isotropic or almost round shape are met most frequently (about 40 percent of all instances). As appeared they are mostly formed in the absence of wind or in case of a gentle wind. Turbid waters of such tail do not directly contact with the shore (except for the estuary zone), they are characterized by anticyclonic rotation and distribution in the south-western and western directions.

The south-eastern along the bank wind contributes to the formation of a tail spreading in the northern direction with the longest linear extension (in case of the Mzymta river plume--up to the Kudepsta region and further). But its area is not so large due to the limits of a coastal strip. Plumes of just such type greatly promote accumulation of pollutants in a coastal zone. They are observed approximately in 20 percent of cases. But the along the bank wind of north-western rhumbs causes appearance of pollution tails extended in the southern direction (also about 20 percent of cases) to the frontier with Abkhazia.

The second in prevalence (above 10 percent of cases) form of plumes is associated with the effect of southwestern winds (sea breeze). In such case the plume appears pressed to the shore area near the estuary, as a result of which it is accumulated in this zone and spreads but slightly to either side from the estuary. Pollution plumes of such type are characterized by the least values of the area and linear extent.

Satellite pictures illustrating quick changes of a distribution area, shape and direction of river plumes in the Sochi region.

Moderate and strong offshore north-eastern winds cause formation of the least prevailing form of plumes presenting a narrow and strongly extended strip in the south-western or western direction of a small area but relatively large horizontal extent. The offshore wind carries polluted waters to the open sea contributing to its rapid mixing and decontamination of the coastal zone.

Of interest are also model dependencies of an area of a river pollution zone in the sea on the wind speed. Very strong winds always cause pollution dissipation due to intensification of vertical mixing. But in the range of moderate winds these dependencies presuppose a maximum area of the plume at some intermediary values of their speed. The background sea currents and a number of other factors also exert influence on the formation of pollution plumes and transfer of terrigenous substances at the given conditions of a river runoff.

The general conclusion arising from our observations is as follows. The Sochi authorities face the problem of development of a system of measures on ecological control, while scientists should be concerned with the problem of pollution monitoring and forecast at the level of modern requirements for measuring technologies and numerical models.

The results cited in the paper belong to a big team. The authors are thankful, in particular, to the staff workers of the Shirshov Institute of Oceanology Boris Konovalov, Cand. Sc. (Biology), Alexander Osadchiev, Cand. Sc. (Phys. & Math.), Vadim Pelevin and Dmitry Solovyov, a research assistant at the Marine Hydrophysical Institute of the NAS of Ukraine, for the supplied material.