The large wetland area of Volga Delta presents a unique complex of aquatic landscapes. Processes of transformation and accumulation of chemical elements and compounds cause a diverse and dynamic geochemical pattern of this large area (about 20,000 km2). Natural geochemical anomalies are subjected to the growing technogenic pressure that results in redistribution of major and minor elements.
The biogeochemical research of aquatic systems has been undertaken in different parts of Volga delta both distinctly contaminated and non-contaminated (Astrakhanskiy biosphere reserve). Astrakhanskiy biosphere reserve presents a relatively undisturbed by men areas in the lowermost part of delta. For the research of contamination a few ship trips from Astrakhan town had been undertaken. Bottom sediments and main plant species were sampled mostly in sites with a low velocity of water, where we could expect the suspended solids to settle with the associated pollutants.
The interpretation of data obtained in the Astrakhanskiy biosphere reserve showed that the spatial variability of chemical elements in bottom sediments is determined mostly by the granulometrical composition; high concentrations of heavy metals (HM) are inherent to the clay fraction. Strong correlation with clay (r=0.8-0.9) had been revealed for major (Fe, Al, Mg, K, Ti) and minor (Ni, Pb, Rb, V, Zn) elements. Mobile forms of heavy metals ( Ni, Co, Cu, Pb, Cd) are strongly correlated with clay fraction as well. In general the amount of heavy metal pollution considered to be negligible: the area is ecologically clean.
The most important sources of the environmental pollution in the Volga delta are located in Astrakhan town. The disposal of contaminated industrial and domestic sewage into Volga river causes increased heavy metal levels in bottom sediments in the vicinity of Astrakhan. Concentrations of Pb, Cu, Ni are higher than background levels by a factor of 2-5. From Astrakhan downstream the contamination is followed up to 10-20 km. In lower parts some deeply ingrown inlets with a semi-stagnant or stagnant regime serve as traps for heavy metals. Contamination of bottom sediments in the Volga delta has been compared with Russian and Dutch standards. In general the level of contamination is relatively low, not higher than class 3, according to the Dutch standards (contaminated).
From the biogeochemical point of view two types of aquatic plants have been distinguished: non-barrier (Phragmites australis, Trapa natans, Salvinia natans, Ceratophyllum demersum) and barrier (Potamogeton). Concentrations of heavy metals in non-barrier plant species distinctly depend upon the environmental conditions and vary over a wide range. On the contrary barrier species are characterized by low variability of HM determined mainly by the biogeochemical specialization of plants.
Background levels of HM are the highest in Salvinia natans and Ceratophyllum demersum and the lowest in Phragmites australis. The mean concentrations of HM in Ceratophyllum demersum are more than 10 times higher in comparison with Phragmites australis. The natural variability of HM in aquatic plants is rather high. It depends on the regime of water streams and bodies. Concentrations of heavy metals in the same plant species are higher in closed creeks with a stagnant regime than in active channels with running water.
Biogeochemical research of contaminated sites revealed distinctly higher HM levels for non-barrier species. The factor of concentration is determined the highest (up to n*10) for Phragmites australis which accumulates pollutants both from water and bottom sediments. We consider Phragmites australis as one the best biogeochemical indicators of aquatic systems pollution. Contamination of aquatic plants is followed further from Astrakhan town downstream than pollution of bottom sediments.