Introduction

River deltas presently draw significant attention due to increasing environmental problems in river basins. Deltas function in a strong relationship with fluvial and marine processes. Many of deltas present densely populated areas of high economic interests. A special attention is paid to the Volga delta. It is the unique natural object having huge economic and ecological value. Volga delta is one of the largest in the world. It has a length of 120 km and a coastline width of 200 km. Volga delivers about 85% of the total river run-off to the Caspian Sea. From the geochemical point of view the lowermost part of the Volga basin represents a unique complex of terrain and aquatic systems. Processes of transformation and accumulation of chemical elements and compounds cause a diverse and dynamic geochemical pattern of this large area. A large number of studies have been done in the Volga delta. Nevertheless its geochemical state remains insufficiently investigated. In the great extent it concerns a transport of polluting substances, their migration in suspended and dissolved forms, and accumulation in sediments. Our study aimed to receive quantitative characteristics of geochemical fluxes of heavy metals in the Volga delta.

MATERIALS AND METHODS

The research is based on results of fieldworks, which were carried out in 2004-2006. We have studied major systems of the deltaic branches (Bakhtemir, Buzan, Bolda, Staraya Volga), large fishery channels, and also near-shore zone of the Volga delta. The most detailed research has been done in territory of the Astrakhanskiy Biosphere Reserve. Fieldworks have captured the periods of a maximum high waters (May – June), recession high waters (July – August), autumn (September) and winter (December) low-water periods. We have sampled water, suspended matter (150 samples), and deltaic sediments (120 samples). The research has been carried out on the basis of methodology of the landscape geochemistry. We used methods of the cascade landscape-geochemical systems analysis, geochemical studying of aquatic systems, and the theory of geochemical barriers. Chemical analyses were done by AAS, XRFS, and ICP methods.

RESULTS AND DISCUSSION

Heavy metal content in suspended matter of the Volga delta (Table 1) in comparison with global averages is rather high, especially for Zn, Cu and Cd. Mean values of these elements during high-water period on the order exceed global estimations for the suspended matter and soils. Lead contents show low variability during the year, being 5-8 times higher than global average values. Less enrichment of suspended matter is characteristic for Mn and Ni. High values of heavy metals in the suspended matter can be explained by a number of reasons: very fine grainsize of the material, its enrichment with humus, sorption of HM by fine particles, water pollution, and deposition of contaminated aerosol particles. However, content of the suspended matter in the Volga delta is low (20-30 mg/dm³), so that content of heavy metals related to the water volume (mkg/dm³) is rather low as well.

Table 1. Mean heavy metal content (mg/kg) in suspended matter of the Volga delta, in comparison with global estimations

Subject		Mn	Zn	Cu	Ni	Pb	Cd
Suspended matter, Volga delta	May	1200	1690	1810	119	177	5,2
	July	5500	1320	509	107	204	2,8
	September	2900	860	750	82	203	3,9
	December	450	757	653	96	181	-
Suspended matter, global average values [Savenko, 2007]		1150	130	45	50	25	0,5
Soils of the world, average values [Bowen, 1979]		1000	90	30	50	35	0,35

Average estimations available in publications for the rivers of the world show, that as a whole the prevailing form of HM migration in river waters is suspended, making from 85 up to 99 % from the total HM content. Our study shows that in the Volga delta primary suspended transport of metals is kept for Cu, Mn, Pb and Ni (Figure 1), but share of these forms makes 60-80% which is less than average for the rivers of the world. Migration of Cd and Zn occurs mainly in dissolved forms. It is connected, first of all, to the general low sediment load in the Volga water.

Our study provides to reveal seasonal variability of the suspended matter in the Volga delta. For the major branches with high discharges and speeds of current, and, hence, transportation ability (e.g., Bakhtemir and Buzan) it is observed decreasing of the suspended matter in water from May by September, thus on high waters (May – June) there passes approximately half of the total sediment load for this period. For water subjects with less active hydrodynamic regime the high contents of the suspended matter is observed in July – August. It is supposed, that it is caused by high biological activity of water plants, i.e. prevalence of an organic component in the suspended material. For the near-shore zone of the Volga delta seasonal changes of amount of the suspended matter is not revealed. It is explained by small depths and intensive water mixing.

Figure 1. Mean percentage of suspended and dissolved forms of heavy metals in the Volga delta

Besides seasonal dynamics the spatial differentiation of suspended matter in water is observed. In May in the upper part of the delta amount of the suspended material is much higher, than in the lower part and near-shore zone. In July and September sediment load in the near-shore zone is higher, than in deltaic water streams. Most likely it speaks intensive growth of water vegetation during the warmest period of year.

We have studied also seasonal changes of HM dissolved forms content (Table 2). Most of HM shows the general decrease of contents from high water to low water period. Obviously, this is caused by their inflow with thawed snow from polluted areas of the river basin. Low water HM levels in the Volga delta (September) are mostly close to the global averages. Only Mn content is 2 times higher. This element also shows a different seasonal dynamics, being in a maximum value in July.

Table 2. Mean content (mg/dm³) of heavy metals, dissolved in water of the Volga delta

Subject		Mn	Zn	Cu	Ni	Pb	Cd
Volga delta	May	16	133	16	3,9	4,1	0,51
	July	31	25	9	2,0	5,6	0,48
	September	21	27	8	1,4	1,1	0,13
Average content in river waters [Dobrovolsky, 2003]		10	20	7	2,5	1,0	0,20

The obtained results have allowed carrying out a quantitative estimation of geochemical fluxes of heavy metals in the Volga delta (Table 3). For the estimation we have used available data on water budget [Polonski et al.,1998]. Annually the Volga delta receives about 18 thousand tons of Mn, 28 thousand tons of Zn, 13 thousand tons of Cu, 2 thousand tons of Pb, 1 thousand ton of Ni, and 100 tons of Cd. Transport of Mn, Cu, Pb, and Ni occurs mainly in the suspended forms, while Zn and Cd in the dissolved ones. The most part of heavy metals goes through the delta as a transit. Only about 15 % is accumulated in the Volga delta sediments.

Table 3. Transport of heavy metals in the Volga delta

Amount, thousand tons	Mn	Zn	Cu	Ni	Pb	Cd
Input of suspended forms	13,5	10,2	9,8	0,79	1,34	0,03
Input of dissolved forms	4,4	17,7	2,8	0,46	0,59	0,08
Total input	17,9	27,9	12,6	1,25	1,93	0,11
Accumulation in the delta	3,3	3,4	2,4	0,21	0,34	0,01

Geochemical study of bottom sediments in water objects of the Volga delta revealed the lythogeochemical uniformity of the delta water objects and Cr-Pb geochemical specialization of sediments. Generally low HM contents in sediments allowed concluding on prevalence of natural factors in the formation of a geochemical background in the Volga delta. Seasonal and long-term tendencies of HM content changes in bottom sediments of water objects in the lower part of the delta are revealed. On the basis of geoinformation study of the remotely sensed data zoning of the western part of the shallow near-shore area was carried out according to the water turbidity. Spatial analysis of HM distribution in bottom sediments allowed revealing a presence of two geochemical barrier zones within the near-shore area. The first zone is located adjacent to the channel mouths, leaving to the marine border of the delta, the second zone is put forward aside the sea on 20-30 km to the beginning of area of river and sea waters mixing.

CONCLUSIONS

The research has allowed revealing the basic geochemical features of heavy metal (HM) transport in aquatic systems of the Volga delta and its near-shore zone. The analysis of a ratio of the dissolved and suspended forms of metals has shown that in the Volga delta the relative contents of HM dissolved forms is essentially higher than the average world values. On the basis of repeated geochemical sampling in aquatic systems, which has captured all phases of the hydrological regime, the quantitative estimation of HM geochemical fluxes in the Volga delta has been carried out. Annual input of heavy metals to the Volga delta includes about 18 thousand tons of Mn, 28 thousand tons of Zn, 13 thousand tons of Cu, 2 thousand tons of Pb, 1 thousand ton of Ni, and 100 tons of Cd. Geochemical study of the HM behavior in sediments has led to the conclusion about prevalence of natural factors in the formation of a geochemical background of the Volga delta. The data analysis has allowed revealing a presence of two geochemical barrier zones within the near-shore area.

Acknowledgements

This study was supported by RFBR, projects 04-05-65073, and 06-05-08097.

REFERENCES

Bowen H.J.M., 1979. Environmental chemistry of the elements, Leningrad,. Acad. Press. 333 p.

Dobrovolsky V.V., 2003. Basics of biogeochemistry. Moscow, Academia, 397 p (in Russian).

Polonski V.F., Mikhaylov V.N., Kir’yanov S.V. (Eds.), 1998. The Volga River mouth area: hydrological-morphological processes, regime of contaminants and influence of Caspian Sea level changes, Moscow, GEOS, 280 pp. (in Russian).

Savenko V.S., 2007. Chemical composition of world river’s suspended matter, Moscow, GEOS, 175 p. (in Russian).