PART 2

 

Surface Water Quality

In accordance with the Agreement the surface water quality measurements in the year 2007 were carried out at 15 monitoring sites on Slovak territory and at 11 monitoring sites on Hungarian territory - Table 2-1, Fig. 2-1.

At all monitoring sites the influence of measures, described in the Agreement, on surface water quality was observed. Main factors, which could influence the water quality, are: the backwater effect upstream of the bottom weir, the increased discharges into the Danube downstream of Čunovo dam and into the Mosoni branch of the Danube, the water supply into the right side river arm system, and morphological changes in the riverbed.

Sampling and analysing were mainly performed according to the methods agreed by the Sub-commission for Water Quality Protection of the Slovak-Hungarian Trans-boundary Water Commission. The evaluation was realized according to the “Surface water quality observation directive for Slovak-Hungarian boundary waters and for extended water quality monitoring on the Danube” accepted by the Slovak-Hungarian Trans-boundary Water Commission.

Table 2-1: List of monitoring sites

* - jointly observed monitoring sites

Surface water quality and sediment quality data for the agreed monitoring sites, and time series diagrams of individual surface water quality parameters, are given in the Slovak and Hungarian National Reports on the Environment Monitoring in 2007 or its Annexes. Figures in this Joint Report represent the data of selected parameters at selected monitoring sites. In the water quality evaluation Slovak and Hungarian data were used at sites, which are observed by the Slovak or Hungarian Party only. At monitoring sites, which are included in the monitoring of Slovak-Hungarian boundary waters quality the data accepted by the Slovak-Hungarian Trans-boundary Water Commission were used (jointly observed sites are marked with * - Table 2-1). These monitoring sites on the Danube are Bratislava (109), Rajka (0001), Medveďov (112/2306) and Komárno (1205). On the Mosoni Danube it is the sampling site Čunovo/Rajka (3529/0082) Lock No. I and Vének (1141). Jointly observed is also the site on the right side seepage canal at Čunovo/Rajka (3531/0084) Lock No. II. 

Table 2-2: Agreed limits for surface water quality classification

Limit values for surface water quality classification used in the surface water quality evaluation were accepted by the Slovak-Hungarian Trans-boundary Water Commission (“Surface water quality observation directive for Slovak-Hungarian boundary waters and for extended water quality monitoring on the Danube”) and are given in Table 2-2. Evaluation of surface water quality up to the year 2007 was done for the respective hydrological year. The monitoring representatives on their negotiation held on April 25, 2007 agreed that since 2007 the evaluation would relate to a calendar. It was also agreed that the bacteria and zooplankton would be excluded from the list of determined parameters and the monitoring and evaluation of hydro-biological elements will be realized out according to the updated methodology agreed within Slovak-Hungarian Trans-boundary Water Commission in accordance with Water Framework Directive.

2.1. General evaluation of the actual year 

The year 2007 belonged to dry years. The average daily discharges often fluctuated below the long-term average discharges. There were no typical flood waves on the Danube, occurring due to snow melting, because the winter at the turn of the years 2006/2007 was mild. Higher discharges occurred n the second half of January, at the beginning of March and May, in July, November and December, but the average daily values did not exceeded 4000 m³s^-1. The only flood wave occurred in September when the discharges exceeded 7000 m³s^-1. The air temperature was rather high in the evaluated year. Since April the temperature gradually increased and the daily averages in May and June mostly exceeded 25 ºC. At the end of June extreme heat affected the Europe, which culminated in the second half of July. Above-mentioned hydrological and climatic conditions affected the surface water quality in the evaluated year.

2.2. Basic physical and chemical parameters 

Water temperature

The water temperature fluctuation in the evaluated period from 1.11.2006 to 31.12.2007 was similar on the monitored sampling sites. Its course showed a seasonal character. The maximal values during the summer did not exceed the maximal values in the previous year. Maximal values occurred in June, July and August, the minimal values occurred in January and February. The highest water temperatures were measured in the reservoir – the maximal value was 25.2 ºC at Kalinkovo (No. 308) at the end of July 2007. The water temperature in the Danube fluctuated from 4.4 to 21.5 ºC. Similar conditions were registered in the Mosoni Danube at Čunovo/Rajka and in the river branch system. The temperatures in the Mosoni Danube at Vének were higher and reached 23.8 °C. The most balanced temperature was characteristic for the water in the left side seepage canal (sampling site No. 317) where it fluctuated from 10.3 to 16.8 °C. The spread of temperatures in the right side seepage was higher and fluctuated from 4.5 to 18.6 °C. In comparison with the previous year higher temperatures (over 15 °C) were registered earlier. They occurred already in April in the reservoir, in the tailrace canal at Sap and in the left side river branch system at Dobrohošť, on the rest of observed sites they occurred in May and lasted till September and October.

pH

Higher values of pH occur in seasons corresponding to periods of increased assimilation activity of phytoplankton. The Danube water pH values in the evaluated period fluctuated in the range from 7.7 to 8.4. In the Danube old river they fluctuated in a wider range from 6.93 to 8.38. Similar situation was registered in the reservoir where pH values ranged from7.75 to 8.64. The highest value was recorded at sampling site No. 308 in the reservoir at Kalinkovo. Higher variability of pH values was in the river branch system and in the tailrace canal, which could be attributed to quicker temperature changes, to changes in phytoplankton abundance and chlorophyll content, and to changes in flow velocities. The pH values in the river branch system fluctuated from 6.73 to 8.63. In the left side seepage canal at Hamuliakovo (No. 317) was the pH balanced and varied from 7.71 to 7.94. The pH values in the Mosoni Danube were similar to that in the Danube, at Vének they varied from 7.2 and 8.2.

Specific electric conductivity

The specific electric conductivity of the surface water refers to the dissolved salts content of mineral origin. It has a seasonal character; values are higher in winter months, lower during the summer. Minimal values in the evaluated period were registered in May and July, depending on the sampling site. The lowest value was recorded in the Danube old riverbed at Dunakiliti (No. 0042), below the submerged weir and reached 29.3 mS.m^-1. Higher values during the whole year were typical for the sampling site on the Mosoni Danube at Vének (No. 1141), where the highest value for the evaluated year was recorded as well and reached 65.9 mS.m^-1. The course of specific conductivity in the Danube followed the conductivity values at the Bratislava sampling site (No. 109). The conductivity values fluctuated between 31.9 and 47.5 mS.m^-1. The conductivity in the reservoir was similar and varied from 29.3 to 48.3 mS.m^-1. In the Mosoni Danube at Čunovo/Rajka sampling site fluctuated from 29.6. to 56.2 mS.m^-1 and its course was similar to that in the Danube water. The dissolved solids content in seepage canals was stable. The electric conductivity values, similarly to the previous years, fluctuated in narrower range. In the left side seepage canal (No. 317) the electric conductivity fluctuated from 40.9 to 46.2 mS.m^-1, the range in the right side seepage canal (No. 3531/0084) was wider and ranged from 34.7 to 44.6 mS.m^-1. The specific electric conductivity at selected sampling sites is documented in Fig. 2-3.

Suspended solids

The suspended solids content is closely related to the flow rate. It increases at flood waves. In comparison with the previous year there were not recorded similarly high values of suspended solids. It is probably related to low and balanced discharges in the Danube throughout the year. The only significant flood wave occurred at the beginning of September, but it was not reflected in the water quality data, because samples were taken in the second half of the month. The highest content of suspended solids was recorded on September 18, 2007 in the Danube old riverbed, upstream of the submerged weir (No. 4016) and reached 205 mgl^-1. Besides this value the suspended solids content in the Danube fluctuated up to 105 mg.l^-1. In the reservoir it varied from <9.5 to 84 mg.l^-1; in the Mosoni Danube at Čunovo/Rajka it fluctuated from 2 to 46 mg.l-1 and at Vének it ranged from 6 to 60 mg.l^-1. The suspended solids content in seepage canals, regarding the origin of water, was low. In the left side seepage canal (No. 317) every measured concentration did not exceeded the detection limit value of the applied analytical method (<9.5 mg.l^-1). In the right side seepage canal the content fluctuated from 2 to 19 mg.l^-1. The suspended solids in the river branch system reached a maximal content of 67 mg.l^-1. When analysing the changes in suspended solids content at sampling sites in the Danube, it can be stated that the suspended solids content downstream of reservoir (at Medveďov sampling site) during flood waves is lower than in the Danube at Bratislava, which refer to settling effect of the reservoir.

Iron

The amount of suspended solids influences the iron content in the surface water. The suspended solids content follows the flow rates; therefore higher iron content occurred in samples taken during higher discharges. The iron concentrations were not monitored in 2007 at jointly observed sampling sites, therefore only two data were available for the evaluated period (November and December 2006). In case of other sampling sites the national data were used in the evaluation. The iron content in the Danube old riverbed mostly fluctuated in the range from 0.07 to 0.76 mg.l^-1. In relation to higher discharges higher values occurred occasionally (0.96 and 1.05 mg.l^-1 at Dunakiliti, upstream of submerged weir and at Dunaremete). The highest iron concentration was registered in September at Dobrohošť sampling site (No. 4016). The iron content in the Mosoni Danube varied from 0.14 to 1.24 mg.l^-1.In the river branch system the iron concentrations fluctuated from 0.01 to 0.79 mg.l^-1, with one higher values of 1.13 mg.l^-1 measured in the Ásványi river arm. In the reservoir the iron content varied from 0.06 to 0.71 mg.l^-1. The lowest iron concentrations were characteristic for the seepage water. In the evaluated period one higher value of 0.62 mg.l^-1 occurred in the left side seepage canal at Hamuliakovo sampling site (No. 317). The other iron concentrations in the seepage water fluctuated from 0.07 to 0.31 mg.l^-1.

Manganese

Similarly to iron either the manganese was not observed at sampling sites monitored by the Slovak-Hungarian Trans-boundary Water Commission. The manganese content increases with increasing discharges and during flood waves, corresponding to the higher amount of suspended solids. The manganese concentrations in the Danube old riverbed mostly fluctuated in the range from <0.02 to 0.18 mg.l^-1, with two higher values recorded in samplings from September (0.55 mg.l^-1 At Dunakiliti, upstream of the submerged weir and 0.62 mg.l^-1 at Dunaremete). The recorded values at sampling site No. 3739 at Sap and No. 4016 at Dobrohošť (upstream of the submerged weir) were mostly below the detection limit of the applied analytical method. The manganese content in the river branch system varied from <0.02 to 0.18 mg.l^-1. Low manganese content was registered in the reservoir too. The concentrations fluctuated in the range from <0.03 to 0.06 and except five values the remaining were below the detection limit of the applied analytical method In the tailrace canal at Sap (No. 3530) every registered value was below the detection limit, below 0.03 mg.l^-1.

Basic physical and chemical parameters - summary

The basic physical and chemical parameters in the Danube and in the river branch system connected to the Danube main riverbed show seasonal variations and some of them predominantly depend on the flow rate. The fluctuation of basic physical and chemical parameters in the Mosoni Danube and in the seepage canal reflects the different characteristics of these water bodies. The water quality in the Mosoni Danube is influenced by the Danube water and at Vének sampling site by its affluents and wastewater from Győr. The water in the seepage canal is influenced mainly by the leaking ground water. In the evaluated period (from 1.11.2006 to 31.12.2007) there were not recorded so high water temperatures as in the previous year. Differences were registered in suspended solids content as well. Extremely high values, which were recorded in previous years, were missing. This is related to low flow rates in the Danube. The iron and manganese concentrations were influenced by the actual hydrological regime.

2.3. Cations and Anions

The quantitative ratio of the surface water ionic composition in the evaluated period showed high stability, just as in previous years. The seasonal fluctuation of individual ions content followed the changes in conductivity. Changes of dissolved solids content are related to the flow rate fluctuation in the Danube. Compared to the long-term measurements the values of basic cations and anions have not changed. The development of cations and anions concentrations at sampling sites was similar. Higher content of salts is characteristic for the Vének sampling site on the Mosoni Danube due to its affluents and cleaned wastewater from Győr. The average values of sodium, potassium, chloride and sulphate ions at Vének exceeded the average values registered on the other sampling sites. The most stable ionic composition was characteristic for the seepage water.

2.4. Nutrients 

Ammonium ion

The decreasing tendency of ammonium ion concentrations in the Danube water continued in the evaluated period. The ammonium ion content in the Danube and tailrace canal fluctuated in the range from 0.02.to 0.128 mg.l^-1, in the Mosoni Danube at Čunovo/Rajka sampling site (No. 3529/0082) it varied in a narrow range from 0.02 to 0.09 mg.l^-1. Similar values were registered in the seepage channels as well. Low concentrations were recorded in the evaluated period in the river branch system, where they fluctuated in a narrow range from <0.02 to 0.04 mg.l^-1. Only in December 2007 values up to 0.09 mg.l^-1 occurred. At sampling site No. 3376 in the left side river branch system at Dobrohošť the ammonium ion content fluctuated from 0.013 to 0.103 mg.l^-1. Concentrations in the reservoir were slightly higher and varied from 0.013 to 0.154 mg.l^-1. The ammonium ion content in the Mosoni Danube at Vének (sampling site No. 1141) was multiple in several occasions in comparison with the other sampling sites (Fig. 2-4). However, contrary to the previous year the concentrations varied in a narrower range and fluctuated from 0.02 to 0.18 mg.l^-1, only in one occasion in January 2007 value of 0.35 mg.l^-1 occurred. From the long-term point of view it can be stated that the ammonium pollution at Vének significantly decreased.

Nitrates

Nitrate concentrations showed seasonal fluctuation, which was less noticeable in seepage canals. In the winter months they reach higher values, afterwards they gradually decrease; minimal values they reach from July to September and to the end of the year slightly increase again. Seasonal fluctuation is related to the vegetation period and consumption of nutrients from the water. In the vegetation period the nutrient content mostly decreased to the half of the winter/spring amount. The nitrates concentration in the Danube varied from 5.1 to 14.2 mg.l^-1, in the Mosoni Danube at Čunovo/Rajka and in the tailrace canal fluctuated in similar range from 2.5 to 12.6 mg.l^-1. Content of nitrates in the reservoir was higher than in the Danube water and it was similar like in the previous year. The highest value of 16.5 mg.l^-1 was recorded on sampling site No. 308 at Kalinkovo and in the sample from July the lowest concentration of the evaluated period was registered, which was less than 2 mg.l^-1. Values of nitrates content in the river branch system were similar to values registered in the Danube water (2.1 - 14.5 mg.l^-1). Low concentrations of nitrates were characteristic for the seepage water, and varied from 2.1 to 8.2 mg.l^-1. The highest contents were characteristic for the sampling site in the Mosoni Danube at Vének, where the nitrate concentration fluctuated from 3.1 to 16.9 mg.l^-1 (Fig. 2-5). However, the maximal values were lower than in the previous year.

Nitrites

The nitrite ion concentrations are considered as a temporary product of nitrification and denitrification processes. Their concentrations in the Danube, in the Mosoni Danube at Čunovo/Rajka, in the tailrace canal, in the reservoir and in the river branch system showed a seasonal variation and their course was similar. The concentrations varied in the range from 0.005 to 0.089 mg.l^-1. Higher values in the evaluated period occurred in the Mosoni Danube at Vének sampling site, where they varied from 0.030 to 0.135 mg.l^-1. Different course on nitrites concentrations were registered in the seepage canals. The lowest nitrite concentrations in both, the left side and right side seepage canal were recorded in samples taken in February, highest values occurred in from June to August. The concentrations of nitrites in the seepage water varied between 0.01 and 0.09 mg.l^-1. In general it can be stated that the nitrites content in the evaluated period varied in a narrower range in comparison to the previous years.

Total nitrogen

The total nitrogen content is predominantly affected by the fluctuation of the nitrate-nitrogen and organic-nitrogen concentrations, which results from the quantitative ratio of nitrogen forms. The total nitrogen belongs to the surface water quality parameters with significant seasonal fluctuation. Maximums are reached in colder months and minimums in warmer months (during vegetation period). The tendency of fluctuation of the total nitrogen in the evaluated period in the Danube, in the Mosoni Danube, in the tail race canal at Sap and in the right side river branch system was similar at all sampling sites. Total nitrogen concentrations varied from 1.1 to 4.0 mg.l^-1. Similar development of total nitrogen content was registered in the reservoir, where the seasonal fluctuation was the most remarkable. The total nitrogen content in the Mosoni Danube at Vének sampling site was higher in comparison with the Danube water and fluctuated from 1.05 to 4.99 mg.l^-1. Highest concentrations of total nitrogen were recorded in the right side river branch system. At Helena sampling site concentration of 5.53 mg.l^-1 was recorded in July 2007 and in the Szigeti river arm concentration of 5.28 mg.l^-1 was registered in May. Except these two values the other concentrations fluctuated from 1.32 to 3.42 mg.l^-1. The seasonal fluctuation in the seepage canal is less remarkable and regarding the origin of the water the total nitrogen content is low. In the evaluated period they fluctuated from 0.8 to 2.68 mg.l^-1. In general it can be stated that the total nitrogen contents at individual sampling sites in the evaluated period were lower than in the previous year.

Phosphates

Phosphates concentrations almost at every monitored sampling site developed similarly in the evaluated period. The exemptions were the sampling site on the Mosoni Danube at Vének (No. 1141) and the sampling site on the right side seepage canal at Čunovo/Rajka (No. 3531/0084). Higher values of orthophosphates concentrations are characteristic for colder months and during higher discharges. The lowest values are typical for summer months during the vegetation period. In the evaluated period they occurred already in April and May, in the left side seepage canal even in March, which could be related to the mild winter and suitable climatic conditions in the spring months. These low concentrations were often below the detection limit of the applied analytical method. The development of phosphates content in the right side seepage canal was more balanced and varied from 0.02 to 0.07 mg.l^-1. The concentrations in the left side seepage canal at Hamuliakovo fluctuated from 0.02 to 0.24 mg.l^-1. Slightly higher concentrations were reached in the Danube water, where they fluctuated from 0.03 to 0.24 mg.l^-1, with one extremely high value of 0.55 mg.l^-1 recorded at Rajka sampling site (No. 1203/0001) in November 2006. Concentrations similar to that in the Danube water were registered in the Mosoni Danube at Čunovo/Rajka sampling site, in the river branch system, in the reservoir and in the tailrace canal at Sap (0.02 - 0.23 mg.l^-1). In the Mosoni Danube at Vének sampling site significantly higher concentrations were recorded, in several occasions double (0.15 - 0.43 mg.l^-1), but they was lower the in the year 2006. In comparison with the previous year the phosphate concentrations were lower at every monitored site.

Total phosphorus

Concentration changes of the total phosphorus followed the seasonal changes of phosphates concentration and similarly reach higher values at higher discharges and during flood waves. The total phosphorus content in the evaluated period was lower than in the year 2006. The concentrations at sampling sites on the Danube varied from 0.03 to 0.17 mg.l^-1, with one higher value of 0.51 mg.l^-1 recorded in December 2007. The concentrations in the Danube old riverbed determined by the Hungarian Party were higher than concentrations determined by the Slovak Party (Fig. 2-7). The total phosphorus concentrations in the river branch system varied from 0.02 to 0.16 mg.l^-1. The highest values from this range were registered at the Helena sampling site (No. 1112). In the Mosoni Danube at Čunovo/Rajka sampling site, in the tailrace canal at Sap, in the left side seepage canal at Hamuliakovo and in the reservoir the total phosphorus content maximally reached 0.12 mg.l^-1.The lowest concentrations in the evaluated period were recorded in the right side seepage canal at Čunovo/Rajka sampling site (No. 3531/0084) and varied in a narrow range from <0.02 to 0.05 mg.l^-1. The highest concentrations of total phosphorus were characteristic for the sampling site on the Mosoni Danube at Vének. In the evaluated period they fluctuated from 0.09 to 0.24 mg.l^-1. However, these values were lower in comparison with the previous year.

Nutrients - summary

The individual nutrients show a seasonal fluctuation. Higher concentrations are characteristic for colder months; decrease of values is recorded in spring after getting warmer. The seasonal fluctuation relates with biochemical processes in the water, which are temperature dependent. The seasonality in the evaluated period was more remarkable in case of nitrates and total nitrogen content, less remarkable in case of ammonium ions. In general the tendency of pollution decrease continues. Further slight decreases were registered in nutrient contents at individual sampling sites. The decreasing tendency is evident at the Vének sampling site as well, which is related to improvement of wastewater treatment. The lowest and the most balanced nutrient values can be found in the seepage water, which results from its groundwater origin.

The nutrient content in the Danube water is potentially sufficient for development of eutrophic processes under other suitable conditions.

2.5. Oxygen regime parameters 

Dissolved oxygen

The dissolved oxygen content is besides the decay processes of organic pollution influenced by hydrometeorological conditions and by assimilation activity of phytoplankton. The dissolved oxygen content proportionally decreases with increasing water temperature in the Danube. Low oxygen concentrations in the evaluated period were recorded from June to August, in the river branch system up to September. The highest values occurred in the winter and spring months. On most of the sampling sites the maximal value was determined in the sample taken in April. In general it can be stated that the dissolved oxygen concentrations were lower compared to the year 2006. However the minimal values at individual sampling sites were similar or slightly higher than in the previous year. Except the reservoir the dissolved oxygen content varied from 5.7 to 14.7 mg.l^-1. The development of oxygen concentrations on most of the sampling sites showed seasonal character. The fluctuation at sampling sites in the reservoir had leaps in the vegetation period, which is related to quick temperature changes and assimilation activity during the vegetation period. Maximal concentration of 23.3 mg.l^-1 was recorded in July in the upper part of the reservoir at Kalinkovo, which strongly differs from the other measured values in the reservoir and is probably related to the specific situation in this part of the reservoir, where construction works were going on and the water was stagnant. The other concentrations in the reservoir varied in the range from 7.1 to 14.5 mg.l^-1. The dissolved oxygen contents in November and December 2007 were higher by 3-4 mg.l^-1 in comparison with the contents in November and December 2006. The dissolved oxygen fluctuation at selected jointly monitored sampling sites is documented on Fig. 2-8.

COD_Mn and BOD₅

COD_Mn and BOD₅ parameters are used for expressing the organic contamination of the water, they indicate the chemically and biologically degradable organic matter content. Higher values of COD_Mn and BOD₅ usually occur at periods with higher flow rates in the Danube, when the water contains higher amount of natural organic matter.

Only two COD_Mn values at jointly observed sampling sites were available in the evaluated period (November and December 2006) because the sub-commission for water quality of the Slovak-Hungarian Trans-boundary Water Commission did not monitored this parameter in 2007. On other sampling sites the COD_Mn values were lower than in hydrological year 2006. The highest values were registered in the right side river branch system and fluctuated in the range from 1.5 to 5.7 mg.l^-1. Slightly lower values were recorded at sampling sites in the Danube old riverbed, where varied from 1.3 to 5.4 mg.l^-1. Values at sampling sites monitored by the Hungarian Party were higher than at sampling sites observed by the Slovak Party (Fig. 2-9). In the left side river branch system the COD_Mn values fluctuated in a narrower range from 1.6 to 3.7 mg.l^-1. The COD_Mn values at sampling sites in the reservoir varied in the range from 0.8 to 3.7 mg.l^-1. The poorest water from the point of view of organic contamination was the seepage water in the left side seepage canal at Hamuliakovo, where the COD_Mn values fluctuated from <0.8 to 1.3 mg.l^-1.

The BOD₅ values at sampling sites in the Danube, in the Mosoni Danube at Čunovo/Rajka sampling site and in the tailrace canal varied in a similar range from 0.2 to 3.4 mg.l^-1. From April to June 2007 there were higher values recorded in the reservoir, maximally reaching 4.4 mg.l^-1. The greatest organic pollution is characteristic for the Mosoni Danube at Vének sampling site. In the evaluated period the BOD₅ values at this sampling site varied from 0.3 to 6.1 mg.l^-1. But higher values occurred in this period in the right side river branch system where the BOD₅ fluctuated in range from 0.2 to 6.24 mg.l^-1. The maximal values were recorded in April and December 2007 and in the Ásványi river branch in August as well, when the highest BOD₅ value of 6.24 mg.l^-1 was recorded. The lowest BOD₅ values were recorded in the left side seepage canal at Hamuliakovo sampling site, where they varied from <0.85 to 1.6 mg.l^-1. In the right side seepage canal they ranged up to 3.7 mg.l^-1.

Oxygen regime and organic carbon parameters - summary

The dissolved oxygen content remained preserved on the level of previous years, even on the mostly polluted sampling site on the Mosoni Danube at Vének no situations with oxygen shortage occurred. In comparison with the previous year the maximal values were slightly lower and the minimal values were slightly higher. Based on the monitoring results it can be stated that the organic contamination expressed by COD_Mn and BOD₅ values was lower than in the previous hydrologic year 2006. The water in the seepage canals remained the cleanest. The maximal values in the evaluated period were not recorded at the most polluted sampling site on the Mosoni Danube at Vének as it used to be in the previous period. The water quality got better thanks to investment into the wastewater treatment plant. The oxygen regime parameters in the Danube reflect the seasonal assimilation activity of the phytoplankton and organic carbon oxidation processes. The measured data (1992-2007) indicate decreasing tendency of organic load at the Bratislava section of the Danube, which is related to the substantial wastewater treatment improvement in the upstream Danubian countries.

2.6. Heavy metals

The joint monitoring from among heavy metals includes observation of zinc, mercury, arsenic, copper, chromium, cadmium and nickel contents. The courses of heavy metal concentrations in the evaluated period were similar as in previous year. Several higher values occurred, which were connected with higher discharges at the time of sampling. No heavy metals were observed in the left side river branch system at Dobrohošť sampling site (No. 3376).

Concentrations of mercury mostly varied below the detection limit of the analytical method used. Only eight values exceeded the detection limit. At sampling in August three values were registered in the Danube old riverbed (sampling sites No. 4016, 4025 and 3739), one value in the tailrace canal at Sap (No. 3530), one value in the reservoir at Kalinkovo (No. 308) and one value in the left side seepage canal (No. 317). One value exceeding the detection limit was recorded in March in the right side seepage canal at Čunovo/Rajka sampling site and one value occurred in the Mosoni Danube at Vének (No. 1141) in July. The highest concentrations 0.89 µg.l^-1 and 0.66 µg.l^-1 were recorded in the Danube old riverbed at Sap (3739) and at Dobrohošť (4025).

Almost every cadmium concentration was below the detection limit of the applied analytical method. Only three values exceeded the detection limit, from among two were at the level of the detection limit (0.05 µg.l^-1), and one value of 0.08 µg.l^-1 occurred in the Danube old riverbed at Rajka sampling site (No. 1203/0001) at December 2007.

The chromium content was low at all observed sampling sites. Major part of values was below the detection limit (0.6 µg.l^-1) of the applied analytical method. There were one or two values exceeding the detection limit at several sampling sites, mostly below 2.3 µg.l^-1. Higher concentration occurred only in the Mosoni Danube at Čunovo/Rajka sampling site (11 µg.l^-1) and extremely high values were recorded at sampling sites No. 309 and 307 in the reservoir in November 2007, when they reached 22,9 a 82,9 µg.l^-1 respectively. All other values in the reservoir were below the detection limit of the applied analytical method.

Arsenic concentrations in the Danube and right side river branch system varied from <0.9 to 1.4 µg.l^-1 (all concentrations determined by the Slovak Party in the Danube old river bed were below the detection limit of the applied analytical method, which was 1.7 µg.l^-1). The concentrations in the Mosoni Danube maximally reached 2.6 µg.l^-1, but most of the values were below the detection limit as well. The highest content of arsenic was registered in the left side seepage canal and varied from <1.7 to 13.4 µg.l^-1. In the right side seepage canal it varied in a narrower range from <0.9 to 1.3 µg.l^-1. The arsenic content in the reservoir and the tailrace canal at Sap was in all cases below the detection limit of the applied analytical method (<1.7 µg.l^-1).

Nickel content in the evaluated period mostly varied in the range from 0.7 to 1.8 µg.l^-1. A lot of data were below the detection limit of the applied analytical method at several sampling sites. In the left side seepage canal (No. 317) and at sampling sites in the Danube old riverbed observed by the Slovak Party (No. 4016, 4025 and 3739) all registered values of nickel were below the detection limit (<1.1 µg.l^-1). Sporadically higher values were registered on the other sites - 2.2 µg.l^-1 in the Mosoni Danube at Vének (No. 1141) in August, 2.6 µg.l^-1 in the Ásványi river branch (No. 1126) in February. The nickel concentrations in the reservoir were below the detection limit except three value registered in November 2006, when 2.5 µg.l^-1, 2.3 µg.l^-1 and 2.6 µg.l^-1 were recorded at sampling sites No. 307, 308 and 309 respectively.

The highest concentrations from among the heavy metals were characteristic for copper and zinc. The copper concentrations fluctuated up to 16.2 µg.l^-1. Values exceeding 10 µg.l^-1 sporadically occurred at sampling sites monitored by the Hungarian Party. The highest values recorded by the Slovak Party occurred in the reservoir and reached maximally 3.6 µg.l^-1. Based on the results obtained at jointly observed sampling sites it can be stated that there were differences in copper concentrations determined by the Slovak and the Hungarian Party.

The content of zinc in the Danube varied from <10 to 23 µg.l^-1, with one higher value of 39 µg.l^-1 measured in the Danube old riverbed upstream of the submerged weir (sampling site No. 0043). The zinc concentrations in the Mosoni Danube at Čunovo/Rajka sampling site fluctuated in the range from <10 to 20 µg.l^-1, at Vének up to 14 µg.l^-1. In the reservoir they fluctuated from 1.88 to 13.4 µg.l^-1, with one higher value of 26.4 µg.l^-1. The zinc concentrations in the seepage canals were lower. In the left side seepage canal they varied from 2.16 to 5.66 µg.l^-1 (detection limit 1 µg.l^-1), in the right side seepage canal were all values below the detection limit, which was 10 µg.l^-1 in this case. In the tailrace canal concentrations up to 9.58 µg.l^-1 occurred.

In summary it can be stated that heavy metal concentrations, which were determined from the filtered sample for the second time, were low during the evaluated period (1.11.2006 to 31.12.2007), with occasional higher values. Great part of determined values was below the detection limit of applied analytical methods. The highest concentrations were characteristic for zinc and copper. However, the highest value in the evaluated period was recorded in case of chromium in the reservoir in November 2007 (82.9 µg.l^-1). Except this value the other recorded values in the reservoir were low. The lowest concentrations were registered in case of cadmium and mercury. The detection limits of the individual heavy metals often correspond to the II. or III. class of surface water quality. The detection limits differ depending on the laboratory and moreover they could change during the evaluated period (in case of mercury, arsenic, chromium and cadmium).

2.7. Biological parameters 

Chlorophyll-a

The chlorophyll-a concentrations refer to the quantity of phytoplankton and provide information about the eutrophic state of water. The growth of algae on the respective section of the river is represented by relative increase of chlorophyll-a concentration between two compared profiles. In the evaluated period rapid increase of chlorophyll-a contents were registered already in April, which could be related with the mild winter and suitable climatic conditions in the spring months. Concentrations decreased afterwards and since July the chlorophyll-a contents remained low till the end of the evaluated period. The chlorophyll-a concentrations in the Danube fluctuated from 1.2 to 46.8 mg.m^-3. Similar values and similar course were registered in the tailrace canal at Sap. The situation in reservoir differs from the sampling sites on the Danube. The variability of the phytoplankton development and the culmination of chlorophyll-a content at individual sampling sites in the reservoir strongly depend on the local hydrological and climatic conditions. Rapid increase of chlorophyll-a content at sampling sites in the reservoir recorded at the beginning of April, occurred repeatedly at sampling sites No. 307 and 309 in May and at sampling site No. 309 in June too. Concentrations varied from <0.6 to 63.6 mg.m^-3. The highest chlorophyll-a concentrations in the evaluated period were recorded in the river branch system. Value of 85.3 mg.m^-3 was recorded at sampling site in the Ásványi river arm in April. Compared to the previous year the concentrations in the river branch system in 2007 were significantly higher. In the lower section of the Mosoni Danube two culminations were registered – in April and in August 2007. The chlorophyll-a amount was higher in comparison with the previous year, it varied up to 43.9 mg.m^-3, while in 2006 it maximally reached 19.5 mg.m^-3. Low values were registered in the seepage water only. In the left side seepage canal the chlorophyll-a content varied below the detection limit of applied analytical method, which was 6 mg.m^-3. In the right side seepage canal at Čunovo/Rajka sampling site it fluctuated from <2 to 12.3 mg.m^-3. The development of chlorophyll-a concentrations at selected sampling sites is showed on Fig. 2-11. 

Other biological parameters

In the evaluation of biological parameters in the frame of surface water quality significant changes were introduced. The monitoring representatives at negotiation held on April 25, 2005 in Győr agreed that in operation of environmental monitoring system Directives of European Union will be applied, including the Directive 2000/60/EC – the Water Framework Directive, and that the monitoring of hydro-biological elements will be carried out in harmony with the methodology agreed within the Trans-boundary Water Commission.

Biological parameters at jointly monitored sampling sites

In the frame of Trans-boundary Water Commission the ecological state evaluation at jointly monitored sampling sites the following biological elements of water quality were chosen: phytoplankton, phytobenthos (benthic diatoms), macrophytes and macrozoobenthos (benthic invertebrates). The evaluation was done according to the Water Framework Directive, on a basis of type-specific classification schemes elaborated separately for individual biological elements of water quality. Classification schemes include limit values for categorization into corresponding quality classes in a range from class I to class V, together with the respective ecological status: I – high, II – good, III – moderate, IV – poor, V – bad. On the basis of comparison of all indexes, which are entered into the multiparametrical index with boundary values for the given types, the water at observed sampling sites was categorized into the relevant quality classes.

The basis for determining the resulting ecological quality status at jointly monitored sampling sites were the partial quality classes determined by the Slovak and Hungarian Party. The resulting quality class and the partial classes are given in Table 2-3.

The ecological status of the Danube water in general can be evaluated as moderate, at sampling sites at Bratislava and Rajka as good. The sampling site at Komárno appeared to be problematic, because there was two classes difference in the macrozoobenthos evaluation (Tab. 2-3). The categorization into worse classes by the Slovak Party in case of macrozoobenthos was probably caused by including of several irrelevant biological indexes, or due to determination of too high boundary limits of the classification scheme for the individual quality classes.

The Mosoni Danube at Čunovo/Rajka sampling site and the right side seepage canal are categorized as artificial water bodies, for which no classification criteria were elaborated at present. The Hungarian Party did not evaluate these sampling sites, the Slovak evaluation is based on classification criteria for natural water bodies. This way was the ecological status of the Mosoni Danube at Čunovo/Rajka sampling site classified as good (II. class). Categorization into the III. class in case of macrozoobenthos is connected with probably too strict boundary limits derived for natural water bodies. The ecological status in the seepage canal was determined as high (I. class) according to the results for phytobenthos and phytoplankton.

The Mosoni Danube at sampling site at Vének was categorized by the Hungarian Party into IV. class, which correspond to poor ecological status. The Slovak Party evaluated the phytoplankton only – II. class. However, the Hungarian Party determined here the class V, probably due to low flow velocity and high number of taxons and high abundance of green algae, cyan bacteria and euglenas, which is caused by higher content of nutrients. 

Table 2-3: Quality classes for selected biological elements at jointly monitored sampling sites

The above evaluation is not based on definitive classification schemes so far. Their finalisation based on harmonisation of obtained results going on at present. The existing classification schemes for individual biological elements will be modified.

Biological parameters at sampling sites monitored only by the Hungarian Party

At sampling sites, which were monitored only by the Hungarian Party, the phytoplankton and phytobenthos were observed from among the biological elements of water quality. Phytoplankton samples were taken at four occasions: in April, May, August and September. The highest density of algae was registered at samples taken in the spring, when the diatoms of the Centrales order were the most abundant. The dominant phytoplankton species in the river branch system were mostly the same as in the Danube water, but the species diversity was higher. Phytobenthos samples were taken at two occasions: in May and in September. An overview of results is given in Table 2-4. 

Table 2-4: Quality classes for selected biological elements on the Hungarian side

Based on the obtained results from the monitoring of biological elements of water quality it can be stated that according to phytobenthos at every sampling site class II was determined, which corresponds to the good ecological status. From the phytoplankton point of view the good ecological status was determined in the Danube old riverbed at Dunaremete sampling site and in the river branch system at Helena sampling site and in the Ásványi river arm. The ecological status of the water in the Szigeti river arm was classified as moderate – III. class. The ecological status upstream and downstream of the submerged weir was classified as poor and bad respectively, which correspond to quality IV. and V. class.

Biological parameters at sampling sites monitored only by the Slovak Party

The monitoring and evaluation of biological elements of water quality was carried according to the methodology applied in the previous years.

Phytoplankton

In the year 2007 thirteen phytoplankton samples were taken at monitored sampling sites (Table 2-5). The abundance of phytoplankton at most of the sampling sites was higher in comparison to the previous year, which could be related to different climatic conditions. The highest values of the phytoplankton abundance were recorded in the reservoir. At every sampling site, except the site in the left side seepage canal, abundance higher than 10000 individuals/ml occurred. This corresponds to the limit for mass growth of phytoplankton and it occurred from the first half of April to the middle of May. Similarly to the previous period the most abundant in the Danube old riverbed and in the reservoir were the centric diatoms (Bacillariophyceae - Centrales). In the left side seepage canal at Hamuliakovo sampling site the pennate diatoms (Bacillariophyceae - Pennales) were dominant, and in comparison with the previous year significant increase was registered. Centric diatoms participated on the abundance only with small part, the cryptomonades (Cryptophyceae) occurred more significantly.

The phytoplankton composition significantly determines the saprobe index of biosestone. The saprobe index in 2007 varied from 1.56 to 2.36, which correspond to II.-III. water quality class (Table 2-5). The saprobe index of biosestone fluctuated in the range, which corresponds to beta-mezosaprobity. Such an environment offers suitable living conditions for a wide scale of organisms of high species diversity. From the comparison of saprobe indexes of biosestone in 2006 and 2007 results that in case of every sampling site slight decrease of saprobe index was registered in 2007. 

Table 2-5: Quality classes according to saprobe index of biosestone

Concerning the phytoplankton abundance, as a substantial determinant of the saprobe index, it can be stated that the hydropower system had no negative influence on saprobity either in 2007.

Macrozoobenthos

From the ecological point of view the macrozoobenthos observation in the flowing waters appeared to be the most suitable method for bioindication. The samples could be relatively easily accessible and quickly processed. In the year 2007 the macrozoobenthos samples were taken at monitoring sites given in Table 2-6. At section with quickly flowing water with gravely and stony bottom (sampling site No. 4025) rheophilous and oxibiontic macrozoobenthos species prevail indicating beta-mezosaprobity. Species Dikerogammarus villosus, Hydropsyche angustipennis, Cricotopus sp. dominated at this sampling site in 2007. In sections with slowly flowing water stagnicolous and oligooxibiontic species appears, which are resistant to a slight contamination. On these sections sandy and muddy bottom can be found – sampling sites in the Danube old riverbed No. 4016 at Dobrohošť and 3739 at Sap. In the evaluated period Dikerogammarus villosus, Corophium curvispinum, Lumbricidae g. sp. div., Lithoglyphus naticoides, Cricotopus sp. and Simulium sp. dominated in the macrozoobenthos.

There are places with different flow velocities in the reservoir. Depending on the flow velocity there exist different types of bottom substrates. Sandy and gravely substrate (sampling site No. 307) gradually changes into muddy substrate at places with slow flow velocity (sampling sites No. 308, 309 and 311). Dominant macrozoobenthos species in the reservoir in 2007 were Potamopyrgus antipodarum, Lumbricidae g. sp. div., Corophium curvispinum, Chironomus sk. reductus and Dikerogammarus villosus. Species Cricotopus algarum and Nais sp. occurred at sampling site No. 308 as well, and at sampling site No. 307 the species Chironomus sk. fluviatilis too. In the river branch system species Dikerogammarus villosus, Cricotopus sp., Dreissena polymorpha, Corophium curvispinum and Limnomysis benedeni dominated. On the basis of determined species the saprobe indexes of macrozoobenthos were calculated, which fluctuated in the range from 1.95 to 2.88 (Table 2-6). Values in the Danube varied from 1.95 to 2.27, which correspond to beta-mezosaprobity and were represented by II. and III. class of surface water quality. The saprobe index in the reservoir fluctuated from 2.08 to 2.88, which correspond to beta and alpha-beta mezosaprobity and were represented by quality classes from II. to IV. class depending on the sampling site location.

Table 2-6: Quality classes according to saprobe index of macrozoobenthos

The other aspects of development of macrozoobenthos communities are evaluated in Part 7 – Biological monitoring, where more detailed evaluation for Cladoceras (Cladocera), copepods (Copepoda), molluscs (Mollusca), dragonflies (Odonata), mayflies (Ephemeroptera) and caddisflies (Trichoptera) can be found.

2.8. Quality of sediments 

In the evaluated year 2007, similarly to the previous year, the Slovak and Hungarian Parties have realized unified evaluation of sediment quality according to the “Canadian Sediment Quality Guideline for Protection of Aquatic Life” (CSQG) published in 1999, revised in 2002.

The sediment sampling in the frame of the Joint Monitoring by the Slovak Party was carried out in September 2007 at six sampling sites. The Hungarian Party sampled the sediments in August and September 2007 at eight sampling sites. The situation of sampling sites is shown on Fig. 2-2. Besides the inorganic and organic micro components the total phosphorus and total nitrate content were analysed.

From among the inorganic micro-pollutants seven heavy metals were analysed: zinc, mercury, cadmium, chromium, lead, copper, arsenic and the Hungarian Party contrary to the Slovak Party analysed nickel as well, which has no limits determined in CSQG.

The heavy metals concentrations in sediments taken at sampling sites on the Slovak territory were low. All measured values fluctuated close to the Threshold Effect Level (TEL), when the unfavourable effect on biological life occurs rarely, in less than 25 %, and it corresponds to an uncontaminated natural environment. Contents of copper, arsenic, mercury and lead were lower than the TEL at every sampling site. The zinc content exceeded the TEL limit value at two sampling sites (No. 307 and 311 in the reservoir), the chromium content at four (No. 307, 308 and 311 in the reservoir and No. 4016 in the Danube old riverbed at Dobrohošť) and the cadmium at five sampling sites (No. 307, 308, 309 and 311 in the reservoir and No. 4016 in the Danube old riverbed at Dobrohošť). Their concentrations varied in the range >TEL<PEL, when the unfavourable effect on biological life occurs occasionally, in more than 25 % and less than 50 %. These concentrations represent potential possibility of ecotoxicological effect. All recorded values varied close to the TEL limit values. Concerning the inorganic micro-pollution in 2007 the most polluted sediment was found in the reservoir at sampling site No. 308 at Kalinkovo, the best sediment quality was registered in the Danube old riverbed at sampling site No. 3739 at Sap.

In sediment samples taken on the Hungarian territory two exceedings of Probable Effect Level (PEL) occurred, when the unfavourable effect on biological life occurs frequently, in more than 50 %. They occurred in case of mercury in samples taken in the Mosoni Danube at sampling site No. 0082 at Rajka and at sampling site No. 1141 at Vének. Concentrations of other analysed heavy metals were low and major part of recorded values varied below the TEL limit values, when the unfavourable effect on biological life occurs rarely. The arsenic concentrations at every sampling site were below the TEL limit value. Cadmium slightly exceeded the TEL limit value at Ásványi rive arm only (No. 1126). The chromium, copper and lead contents fluctuated in the range >TEL<PEL at two sampling sites, the zinc content at five sampling sites. However, no one value exceeded the half of this range. Concerning the results of inorganic micro-pollution it can be stated that the most polluted sediment was found in the Mosoni Danube at Vének (No. 1141).

From among the organic micro-pollutants three components were analysed by the Slovak and Hungarian Parties in 2007: naphthalene, phenanthrene and anthracene. Moreover the Slovak Party analysed lindan, heptachlorine, endrine, dieldrine, fluoranthene, chrysene, benzo(a)pyrene and sum of PCB’s, the Hungarian Party analysed acenaphtene, acenaphtylene, fluorine, dibenzo(a,h)anthracene and 2-methynaphtalene. Concentrations of abovementioned organic micro-pollutants were very low. In most cases did not reach the TEL limit values. Only two values at sampling sites in the river branch system on the Hungarian territory slightly exceeded the TEL limit values in case of dibenzo(a,h)anthracene. These sampling sites were situated in the Szigeti and Ásványi river arms. The Hungarian Party analysed the indeno(1,2.3)pyrene and benzo(ghi)perylene, but these components have no TEL and PEL values specified in the CSQG.

In case of the organic micropollutants the sediments on the Slovak and Hungarian sides in the year 2007 did not exceeded the PEL level in any case. The recorded concentrations in most cases varied below the TEL level, when no unfavourable influence on the biological life is expected. Concentrations slightly exceeding the TEL level were recorded in two cases.

On the Hungarian side the total phosphorus and total nitrate content in sediments was analysed as well. The total phosphorus content in 2007 varied in the range from 268 mg.kg^-1 (in the seepage canal at Rajka) to 1540 mg.kg^-1 (in the Mosoni Danube at Vének). The highest contents of total nitrogen were detected in the Ásványi river arm (3850 mg.kg^-1) and in the Mosoni Danube at Vének (3863 mg.kg^-1).

2.9. Differences in data measured by the Slovak and Hungarian Parties

This chapter is dealing with differences between Slovak and Hungarian data in surface water quality parameters at jointly monitored sampling sites and with results reached in elimination of these differences.

Several surface water quality parameters (iron, manganese COD_Mn and sulphates), where differences were detected in the previous period, were not determined in 2007 at jointly observed sampling points. In the evaluated period good concordance was reached in case of water temperature, nitrates and dissolved oxygen. Differences still persist at several parameters where the individual values measured by one of the Parties were different; the individual values of parameters measured by one of the Parties are higher or lower than values determined by the other Party (e.g. pH, dissolved solids, ammonium ion, nitrites, phosphates, the totals nitrogen, calcium, magnesium and BOD₅). In case of other parameters the course is similar, but the values are either higher or lower at one of the Parties. Higher values were determined by the Slovak party in case of conductivity; in case of total phosphorus, hydrogencarbonates and copper higher values recorded the Hungarian Party. Samples are shown on Figs. 2-10 and 2-12. 

2.10. Conclusions

Based on the long-term evaluation it can be stated, that the water quality in the Danube generally improved. Improvement of surface water quality was already observed at Bratislava profile (109), which represents the water quality entering the region influenced by the hydropower structures. The better surface water quality consequently influences the water quality in the territory influenced by temporary measures, realised according to the Agreement.

The surface water quality in the evaluated period (1.11.2006-31.12.2007) was similar as in previous years. Regarding the specific hydrological and climatic conditions in the actual period the observed surface water quality parameters did not reach extreme values. Increased values of some parameters in the Danube were related to higher flow rates in the Danube. The quantitative ratio of ionic composition of the surface water shows high stability in last years. At individual sampling sites further slight decrease was registered in case of nutrients. The dissolved oxygen content remained preserved at the level of previous years and the organic pollution represented by BOD₅ and COD_Mn was lower in comparison with the previous year 2006. When analysing the changes in suspended solids content at sampling sites in the Danube, it can be stated that the suspended solids content downstream of reservoir (at Medveďov sampling site) during flood waves is lower than in the Danube at Bratislava, which refer to settling effect of the reservoir. From among the heavy metals the highest concentrations were recorded in case of zinc and copper, the lowest contents were registered in case of mercury and cadmium. A large part of analysed concentrations was below the detection limits of the applied analytical methods.

The fluctuation of surface water quality parameters in the right side river branch system since introducing the water supply in 1995 follows their fluctuation in the Danube. The water quality in the Mosoni Danube differs in the upper and lower section of the river. The water quality at the sampling site at Čunovo/Rajka follow the water quality in the Danube, while the water quality on the lower section of Mosoni Danube (upstream of the confluence with the Danube) is formed by its affluents and local pollution from settlements. Compared to previous years the water quality significantly improved thanks to investments realized in the wastewater treatment plant. In general it can be stated that the content of all nutrients decreased at this sampling site, although their concentrations still reach the highest values in comparison with the other sampling sites. The cleanest water is characteristic for seepage canals, which results from its groundwater origin.

The monitoring of hydro-biological elements of the surface water quality in the evaluated period at jointly monitored sampling sites was realised according to the new methodology agreed in the frame of Slovak-Hungarian Trans-boundary Water Commission. Four biological elements of water quality were evaluated – macrozoobenthos, macrophytes, phytoplankton and phytobenthos. According to results the sampling sites on the Danube were classified into II. and III. quality class, which correspond to good and moderate ecological status and the sampling site on the Mosoni Danube at Vének was classified into IV. quality class, which represents poor ecological status. This evaluation is not based on final evaluation schemes, which have to be modified and finalised. The evaluation of the sampling sites, which are observed only by the Hungarian Party was realised similarly, but only two biological elements were evaluated – the phytoplankton and phytobenthos. According to phytobenthos the II. quality class was determined at every sampling site, which represents good ecological status. According to phytoplankton II. and III. quality classes were determined at several sampling sites and in the Danube old riverbed at Dunakiliti IV. and V. quality classes, corresponding to poor and bad ecological status. The monitoring and evaluation at sampling sites observed only by the Slovak Party was realised according to the methodology used in the previous years. Macrozoobenthos and phytoplankton were evaluated. According to the saprobe index of biosestone, which is significantly determined by the phytoplankton composition, was the water at sampling sites classified into II. and III. quality classes. According to saprobe index of macrozoobenthos the water in the Danube old riverbed and in the river branch system was classified into II. and III. quality classes, while in the reservoir the quality classes varied from II. to IV. class.

The sediment quality was evaluated according to the “Canadian Sediment Quality Guideline for Protection of Aquatic Life” (CSQG) published in 1999, revised in 2002. In case of inorganic contamination of sediments low concentrations of heavy metals were registered in 2007 at sampling sites on the Slovak territory. Every recorded value fluctuated below or close to the Threshold Effect Level (TEL), when the unfavourable effect on biological life occurs rarely. In sediments analysed on the Hungarian side two exceedings of Probable Effect Level (PEL), when the unfavourable effect on biological life occurs frequently were recorded in case of mercury. Concentrations of other analysed heavy metals were low and mostly varied below the TEL limit values. The registered concentrations in case of organic micro-pollutants mostly fluctuated below the TEL limit values. 

Table 2-7: Orientational classification of surface water quality parameters according to the agreed limits for surface water quality classification

*        all the data below the detection limit

**     most of the data below the detection limit

-         parameter was not observed at the sampling site 

In the Table 2-7 an orientational classification of selected sampling sites and selected surface water quality parameters was done. The orientational classification was realised using the limit values of five-classes system according to the trans-boundary water quality classification accepted by the Slovak-Hungarian Trans-boundary water Commission on its LXV. session and given in “Surface water quality observation directive for Slovak-Hungarian boundary waters and for extended water quality monitoring on the Danube”.

Some part of observed parameters shows seasonal fluctuation, which subsequently influences the classification into the quality classes. In case that a range is given (e.g. I-II) it means natural seasonal fluctuation of individual parameters or dependency on climatic conditions. The quality class in brackets means, that the measured value occurred only once or two times in the evaluated period (mostly during higher discharges or flood waves). The range with asterisks (e.g. I^*-II^*) represents a situation, when every recorded value was below the detection limit of applied analytical method, but the detection limit has changed in the evaluated period and simultaneously belonged to other quality class.

Based on comparison of water quality entering the influenced area (sampling site at Bratislava) and water quality leaving the influenced area (sampling site at Medveďov) it is evident, that the water quality leaving the system is very similar.