Biogeochemical Migration Patterns Of Heavy Metals
Biogeochemical migration patterns of HEAVY METALS
in THE ECOSISTEMS OF THE DNiester and Prut RIVERS
E.I. Zubcova
Institute of Zoology of Academy of Science, Chisinau, Moldova
The research on heavy metals migration is considered to be today one of the topical interest in modern hydrobiology, ecology and hydrochemistry and has a great theoretical and practical significance. The development of the theory on freshwater ecosystem functioning is impossible without a proper understanding of the processes of chemical element migration.
Heavy metals play an important biochemical role in functioning of freshwater ecosystems, their role being compared to that of catalyzes in chemical reactions. The environmental pollution in the last years has stimulated a lot of ecotoxicological research directed to establish the biological role of heavy metals for hydrobionts and assess the mankind impact of the dynamics of trace element content in freshwater ecosystems. Of a special importance is undertaken research on biological monitoring - a complex system of investigation, estimation and prediction of freshwater ecosystem status and on the evaluation of the hydrobionts role in migration and circulation of chemical elements.
The above mentioned directions of study are very important for Moldova, where there is a scarcity in good quality water and a continuos increase in mankind impact on the environment. Besides, the main aquatic arteries – Prut and Dniester rivers have a transboundary position, flowing from Ukraine and passing along Ukraine, Moldova and Romania.
On both Dniester and Prut rivers, dams were build and the main industrial centers of all three above-mentioned countries were concentrated here and thus water quality depends on human impact in these regions as well as on applying the environmental protective methods.
Among the main factors which determine the distribution and migration of heavy metals within the system " water-suspended matter-sediments" of the surface waters of Moldova are the dismembration of relief, the amount of rainfall, the physico-chemical characteristics of rocks, soil and water as well as
the hydrology and status of aquatic fauna and the chemical characteristics of heavy metals themselves. Significant effects have also the human activities such as hydrotechnical constructions, the chemization and irrigation of agricultural lands and wastewater discharge.
Zinc. Zinc content in water varies greatly between 5-6 to 4200 mg/l and in suspended matter - between 11 and 220 mg/l. The highest Zn concentration was estimated in Byc River and the lowest - in the reservoirs Dubasari and Costesti-Stinca. Downstream of the Dniester and Prut rivers, Zn content in water, suspended matter and sediments increases continuously. During high flooding period (spring) Zn content is higher than during low water period (summer-autumn). For Dnester river, dependencies of Zn concentration in water (Zna, mg/l) and suspensions (Zns, mg/l), on water debit (Q, m3/s) and content of suspended matter (S, mg/l) were revealed:
Zna=0.158Q + 0.224 S-14.4, R=0.89;
Zns= 0.030Q+ 0.208 S+20.2, R=0.92.
The dependence of Zn in water on the amount of Zn-containing fertilizers and pesticides used in agriculture (Zn, t), the volume of wastewater discharged in hydrographic network (R, mln m3/yr) and water debit (Q) were described by the following regression equations:
For Dniester river, Bender town: Zn= 0.0897 Z + 0.007 R + 0.370 Q, R=0.99;
For Raut river, Ustie village: Zn = 0,0112 Z + 34.154 R + 0.233 Q, R=0.99.
The multianual dynamics of Zn in Dniester river depends on the time factor (A) and water debit (Q): Zn = 4.258A + 0.043Q – 8418.9 , R=0.84.
The total amount of migration mobile forms of Zn accounts for 70-82 %, from which 30-56% are the adsorbed complexes and easily soluble carbonates, 6-35 % - are bound onto organic compounds and 15-28 % are associate with amorphous hydroxides of Fe and Mn. The aquatic migration coefficient of Zn in Inferior Dniester is 0,072 and in Prut river – 0,069, this reflecting an intense rate of migration of this element.
Copper. The range of Cu content varies within wide limits from 0,2 to 88,6 mg/l in water and from 2,0 to 270 mg/l in suspended matter. The highest values were found downstream of the rivers, in their tributaries and the lowest – in storage reservoirs.
The concentration of Cu in water and especially in suspended matter, during high flooding period was comparatively higher than during low water period. The multianual data allowed us to find a dependence of Cu concentration in Dniester river on time factor (A) and water debit (Q):
Cu=8.7606A+0.228Q-17248, R=0.87.
An analysis was made on the effect of a range of factors such as the amount of copper-based pesticide used in agriculture (V, t/yr.), the volume of discharged wastewater (W), and water debit (Q) on the dynamics of Cu content and was described by the equations:
For Dniester river, Bender town: Cu=0.00210 V+ 0.02 Q + 0.068 W–28.9, R=0.98;
For Prut river, Ustie vg: Cu=0.0028 V + 0.52 Q + 13.500 W – 43.23, R=0.98.
The quota for suspended forms of Cu in water account for 29-86%. The total content of mobile forms of Cu is 71-83%, from which 17-36 % are bound in adsorbed complexes and easily soluble carbonates, 17-36% - in organic forms and 15-37% are associated with amorphous hydroxides of Fe and Mn. The aquatic migration coefficient is equal to 0,0024-0,026, thus expressing a high migration capacity of Cu in Dniester and Prut rivers.
Nickel. The concentration of this metal in Moldova\’s waters varies within wide ranges. The highest Ni content (about 250 mg/l) were registered in river\’s tributaries, while the lowest (0,5 mg/l) in the storage reservoirs. The concentration of Ni in suspended matter depends on the water debit and the amount of suspended matter (S), this dependence being described by the equation: Ni = 0.598Q +0.006S –11.3; R= 0.85.
The seasonal dynamics of Ni was not very well expressed, but a difference of this metal during high flooding periods and lower water periods was observed. The dependence of Ni content on water debit (Q), the amount of raw wastewater (R) and the amount of suspended matter (S) were described by the equations:
For Dniester river Ni=1.6-0.001 Q + 0.080 R + 0.021 S, R=0.90;
For Raut river, Ni=1.4+ 0.3635 R + 0.010 S, R=0.85
In the last years, an awarning, even anomalous Ni level was identified (a few tens mg/l), which could be related to the increase in the amount of discharged industrial waste and sewage.
The concentration of Ni in suspended matter from Dniester and Prut rivers was 5-14 times higher than in the soils of these basins. The quota for mobile forms of Ni accounts for 70%, from which 18-39 % are in adsorbed complexes and easily soluble forms, 8-33% - are associated with organic complexes and 17-35 % - with amorphous hydroxides of Fe and Mn.
The aquatic migration coefficient of Ni down part of Dniester and Prut rivers was equal respectively to 0,089 and 0,093.
Manganese. The manganese content varies widely: in water these values being between 1,4 and 220 mg/l. In the most cases, Mn migrates in rivers in suspended forms. The content of this metal during low water period was higher than during high water period. This however, was not characteristic for suspended matter, where an inverse situation was observed and was probably related to an additional input of Mn from surface runoff from agricultural and urban sites (in such samples the concentration of this metal was quite high). Downstream of the Prut and Dniester rivers, the level of Mn increases, especially in suspended matter.
A data analysis of long standing of the total Mn content in Dniester river revealed the highest level during drought years and in a full water of Novodnestrovsk reservoir.
The quota for Mn in suspensions accounts for 54-87%, in reservoirs - 34-56%. Over 60% of Mn are bound in adsorbed complexes and easily soluble carbonates, organic matter and amorphous hydroxides. The migration capacity of Mn in Dniester and Prut rivers was relatively low, the value of aquatic migration coefficient being 0,002.
Lead. The concentration of Pb in surface waters of Moldova is relatively low and only in 15 % of case exceeds 5 mg/l. In suspended mater the Pb content fluctuates within wider limits: between 0,5 and 107 mg/l. An unusually high concentration of P (38-107mg/l) was found during 1994 and 1996 along the whole length of the Prut river, after the confluence with Jiga river. High concentrations of Pb were identified in Byc river (Chisinau location, where the concentration of this metal was 3-8 times higher than in other parts of the river).
Generally, the Pb concentration in Dniester and Prut rivers in suspended matter increases continuously, especially in suspensions. For Dniester river, a dependence of Pb content on water debit and the amount of suspended matter (S) was revealed: PbS=0.027 Q+0.058 S–1.3, R=0.95.
Only for Byc river dependence of the Pb content in water on the amount of discharged wastewater (R) and water debit (Q) was established:
Pb=0.5 + 1.067 R + 0.583 Q, R=0.98
The content of lead in water of Dniester river and Dubasari decreased (the highest concentrations were estimated after 1986, the increasing in Pb content being assumed to be due to the Chernobyl accident, when high Pb concentrations were found on the entire territory of Moldova).
Lead content in rivers reservoir suspensions was 10-33 times higher than in bed rocks and soils. Approximately 70 % of Pb is bound in adsorbed complexes and easily soluble carbonates, less than 5% is bound with organic matter and over 20% is associated with amorphous hydroxides of Fe and Mn. The value of aquatic migration coefficient of Pb in Dniester river is 0,017 and 0,015.
Aluminium. The lowest Al concentrations were found in storage reservoirs, while the highest - in the tributaries. In less than 10% of cases Al content exceeded 5mg/l. During the high flooding period, Al content was higher than during low water period. The dependence of Al level on water debit (Q) and the amount of suspended matter in the water (S) were described by the equations:
For Dniester river Al=0.012Q + 0.023S –0.8, R=0.84;
For Raut river Al=0.33Q + 0.012S + 1.2, R=0.82.
The major Al input in aquatic ecosystems originates from rocks and soils. The increase in Al concentration in the rivers occurs simultaneously with the increase in the amount of suspended matter.
Titanium. The dynamics of this trace element in surface waters of Moldova is highly influenced by natural factors. Over 95% of Ti migrate into the waters in suspended forms. In less than 5% of samples Ti exceeded 5mg/l. Titanium content in the water depends on water debit (Q) and the amount of suspended matter (S):
For Dniester Ti = 0.029Q + 0.018S –1.1, R=0.89;
For Raut Ti = 0.159Q + 0.011S – 0.7, R=0,86.
In suspended matter, the quota of the mobile forms of Ti accounted for 10% and the value of aquatic migration coefficient was 0,0004.
Cadmium. The content of this element in water varies between 0,25 and 8,15 mg/l and in suspended matter- between 0,25 and 5,85 mg/l. In Dniester river, the highest Cd concentrations were found near Bender, Tiraspol towns and Sucleia village and in the Prut river- near Leusheni village. Very high concentrations of Cd - 24,6 mg/l, were found in Byc river, near Chisinau, this level being 6-8 times higher than outside of the city.
Silver. The content of this metal in 80% of samples varied between 0,1 and 1,0 mg/l. High concentrations of Ag (5mg/l) were found only in 6 samples collected from Dniester river, 4 samples collected from Prut river and 8-12 samples collected from Raut and Byc rivers.
The highest Ag concentration was found during low water period. The quota of suspended forms of Ag exceeded those of dissolved forms (57-77%), while for Byc the situation was in verse.
Thus, it could be pointed out that the dynamics of heavy metals in water is a function of climatic and physico-geographycal patterns of the basin and the impact of mankind activities. Although the building of hydrotechnichal constructions led to significant changes in hydrological regimes of Prut and Dniester rivers, the mobility of heavy metals is affected very much by water turbulence and water debit (r=0,77-0,97)
The water suspensions of Moldovan aquatic ecosystems, are highly enhanced in Ni, Pb. Mo, Cu,Zn and Ag, while bedrocks are poor in Ti. The increased level of Cu, Zn, Ni, Pb and Ag concentrations in Moldova\’s rivers reflects a high level of pollution of these elements from human impact and the intensification of erosion processes in river basins.
The determination of the tolerable and optimal limits of heavy metals in the environment is very important. These limits are usually affected by the regional differences of ecosystems. Taking into account the above-mentioned statement, a number of experimental studies were carried out in rivers and reservoirs of Moldova. Firstly, the effect of metals on production and destruction of organic matter was studied, taking into account that these processes are the most important links for matter cycle and energy flux and are essential for ecosystem functioning.
The concentrations of heavy metals in Dniester and Prut rivers and Dubasari and Costesti-Stinca reservoirs in 85-94% of samples was considered as favorable, in 5-7% of samples a decrease in primary production was counted and in 2-6% of samples, a complete stopation in production and destruction processes was revealed, the last being characteristic especially for samples collected from Bender, Tiraspol, Ungeni, Cahul stations with a high level of wastewater discharge.
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