The surface water quality measurements in the year 2008, in accordance with the Agreement, 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 submerged 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 2008 or its Annexes. Figures in this Joint Report represent the data of selected parameters at selected monitoring sites. The data at jointly observed sites were not unified in 2008, so the data measured by the Slovak and Hungarian party were taken into account.

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

 

In Table 2-2 the limit values for surface water quality classification are given, which 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 were used in the surface water quality evaluation in 2008. Since 2007 the evaluation relates to a calendar year. The evaluation of hydro-biological elements was realized according to the 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 2008 from the water richness point of view belonged to average years. Extraordinary dry or extraordinary water rich months did not occurred. The average daily discharges in individual months often did not reached the long-term average values. There were not observed typical flood waves on the Danube, occurring due to snow melting, because the winter at the turn of the years 2007/2008 was mild. Sporadically only and shortly occurred flow rates exceeding 3000 m³.s^-1, and flow rates exceeding 4000 m³.s^-1 occurred only in March, July and August, when the yearly maximum of 4780 m³.s^-1 was recorded. The yearly minimum occurred in October and reached 900 m³.s^-1. Climatic conditions in 2008 were characterized by temperatures above the average from April to June, followed by temperature drop and rainy period in July and August. The warm autumn in September and October was substituted by cooler period in the second half of October. 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 1.1.2008 to 31.12.2008 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 from the previous year. At particular sampling sites the maximal values occurred in June, July and August, the minimal values occurred in January and February. The highest water temperature 24.5 ºC was measured in the Mosoni Danube at sampling site No. 1141 at Vének. The water temperature in the Danube fluctuated from 1.0 to 22.4 ºC. Similar development was recorded in the river branch system and in the Mosoni Danube at Čunovo/Rajka. The temperatures in the reservoir were lower in comparison with the previous year (maximally up to 20,8 ºC). The water in Mosoni Danube at Vének reached higher temperatures in comparison with the other sampling sites in the period from February till November. The most balanced temperature was characteristic for the water in the left side seepage canal (sampling site No. 317) where it fluctuated from 8.4 to 16.6 °C. The spread of temperatures in the right side seepage was higher and fluctuated from 4.5 to 18.0 °C. In comparison with the previous year higher temperatures (over 15 °C) on most of sampling sites were registered not until May (in 2007 they occurred already in April) and lasted till September, which was similar as in the year 2007. The course of the water temperature at selected sampling sites is documented on Fig. 2-3.

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 7.12 to 8.74. The highest values of 8.69 and 8.74 were recorded during sampling in May at sampling sites Medveďov (No. 112) and Komárno (No. 1205). Similar value (8.72) was recorded by the Slovak side as well at sampling site No. 3529 in the Mosoni Danube. In 2208 in the reservoir the pH values were not so variable as in the previous year, the pH varied from 7.87 to 8.47, with one value of 7.55 recorded in the navigation line (sampling site No. 307). Higher variability of pH values was in the river branch system and in the Danube old riverbed, where the pH ranged from 7.01 to 8.5. The pH in the left side seepage canal at Hamuliakovo sampling site (No. 317) was rather balanced and varied from 7.63 to 8.25. In the Mosoni Danube at Vének sampling site it fluctuated from 7.37 to 8.31. The Hungarian Party measured lot of low pH values in comparison to Slovak data. The difference is distinct particularly at jointly observed sampling sites (Fig. 2-4).

Specific electric conductivity

The specific electric conductivity of 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 June and July, depending on the sampling site. The lowest values in 2008 was recorded in the in the river branch system on Hungarian side (sampling sites No. 1112 and 1114) and in the Danube old riverbed at Dunakiliti (No. 0043), upstream of the submerged weir and reached 26.0 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 67.0 mS.m^-1. The course of specific conductivity at sampling sites in the Danube followed the conductivity values at the Bratislava sampling site (No. 109). The conductivity values fluctuated between 26.0 and 52.7 mS.m^-1. The conductivity varied in a similar range at sampling sites in the reservoir, in Mosoni Danube at Čunovo/Rajka and in the tailrace channel at Sap 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 39.9 to 46.7 mS.m^-1, the range in the right side seepage canal (No. 3531/0084) was wider and ranged from 33.0 to 45.6 mS.m^-1.

Suspended solids

The suspended solids content is closely related to the flow rate. It increases at flood waves. Higher content is characteristic mainly for the summer period. In comparison with the previous year higher values were recorded on most of sampling sites. The highest content of suspended solids 131 mg.l^-1 was recorded in July 2008 in the navigation line in reservoir (sampling site No. 307) and it was related to the discharge wave in July (flow rate above 4000 m³.s^-1). The discharge wave in August was caught by sampling at Bratislava, when the suspended solids content reached similar value – 126 mg.l^-1. Besides these values the suspended solids content in the Danube and in the reservoir ranged up to 94 mg.l^-1. The content in the Mosoni Danube at Čunovo/Rajka was lower and it fluctuated in a range from 1.2 to 50.0 mg.l^-1. In comparison to the previous years the suspended solid content significantly decreased in the Mosoni Danube at Vének and it ranged from 5 to 44 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 1.2 to 13 mg.l^-1. The suspended solids in the river branch system reached a maximal content of 88 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 surface water, therefore higher iron content occurs in samples taken during higher discharges. The iron concentrations in the Danube water in 2008 fluctuated in the range from 0.07 to 0.57 mg.l^-1, with one higher value of 1.19 mg.l^-1 measured on sampling site No. 109 at Bratislava during sampling, which caught the discharge wave in August. The iron contents in Mosoni Danube were similar; they varied from 0.13 to 0.58 mg.l^-1. The iron content at the confluence with the Danube (sampling site No. 1141 at Vének) usually reaches higher values during the whole year. In the river branch system the iron concentrations fluctuated from 0.04 to 0.74 mg.l^-1, while in the Ásványi river branch they reached only 0.39 mg.l^-1, similarly as in the river branches on the Slovak side. In the reservoir the iron content varied from 0.04 to 0.60 mg.l^-1. The lowest iron concentrations were characteristic for the seepage water. In the left side seepage canal at Hamuliakovo sampling site (No. 317) they varied from 0.08 to 0.14 mg.l^-1, in the right side seepage canal at Rajka they ranged from 0.05 to 0.30 mg.l^-1.

Manganese

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 fluctuated in the range from <0.02 to 0.14 mg.l^-1. The highest value was measured on sampling site No. 2306 at Medveďov. The manganese content in the reservoir, in the Mosoni Danube at Čunovo/Rajka and in the tailrace canal at Sap varied from <0.02 to 0.07 mg.l^-1, at Vének it was higher during the whole year, maximally up to 0.11 mg.l^-1. The highest values in the evaluated year were recorded in the left side seepage canal at Hamuliakovo (sampling site No. 317) and varied up to 0.19 mg.l^-1. The concentrations in the river branch system were similar to that in the Danube water. The only exception was the value of 0.20 mg.l^-1 recorded in the sample taken at sampling site No. 1112 at Helena in February.

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 year the values of individual parameters were similar to values in the previous year. The iron and manganese concentrations and the suspended solids content were influenced by the actual hydrological regime. 

2.3. Cations and Anions 

The quantitative ratio of the surface water ionic composition in the evaluated year 2008 showed high stability, just as in previous years. The seasonal fluctuation of individual ions content followed the changes of 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 individual 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 ammonium ion contents were similar as in the previous year. Low concentrations were recorded in the river branch system, in the tailrace channel at Sap and at sampling site in the Danube old riverbed as well. At these sites the ammonium ion fluctuated in a narrow range from <0.02 to 0.08 mg.l^-1, with one higher value of 0.12 mg.l^-1 measured on sampling site No. 4025 at Dobrohošť. In the main riverbed, in the Mosoni Danube and in the right side seepage canal higher values occurred in several months as well, maximally up to 0.21 mg.l^-1. The course of ammonium ion concentration in the Mosoni Danube at Vének was similar as in the Danube. Values fluctuated in a range from 0.02 to 0.14 mg.l^-1, with one high value in January reaching 0.30 mg.l^-1 (Fig. 2-5). The ammonium ion contents in the reservoir were similar as in the previous year and varied from 0.013 to 0.17 mg.l^-1, whereas higher values were less frequent.

Nitrates

Nitrates concentrations showed seasonal fluctuation, which was less noticeable in seepage canals. In winter months they reached higher values, afterwards they gradually decreased; minimal values they reached from June to September and to the end of the year slightly increased again. Seasonal fluctuation is related to the vegetation period and consumption of nutrients in the water. In the vegetation period the nutrient content mostly decreases to the half of the winter amount. Except the seepage canals the course of nitrates concentration at individual sampling sites was similar; higher values occurred mainly in the main riverbed and in the reservoir, slightly lower values were characteristic for sampling sites in the Danube old riverbed and river branch system. Concentrations in 2008 varied up to 12.4 mg.l^-1.One higher value occurred in June on sampling site No. 1141 in the Mosoni Danube at Vének, and reached 13.8 mg.l^-1. The other values and the course were similar as on the other sampling sites. Low concentrations of nitrates were characteristic for the seepage water, and varied from 2.1 to 7.8 mg.l^-1. The course of nitrates concentrations at selected sampling sites is shown on Fig. 2-6.

Nitrites

The nitrites 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 0.135 mg.l^-1. Higher values in the evaluated period occurred in the Mosoni Danube on sampling site No. 1141 at Vének, where they varied from 0.037 to 0.155 mg.l^-1, with one value of 0.277 mg.l^-1 recorded at the beginning of June. Different course on nitrites concentrations were registered in the seepage canals. The lowest nitrites concentrations in both, the left side and right side seepage canal were recorded in samples taken in February, highest values occurred in August and September. In the right side seepage canal the Slovak Party recorded one high value in June, reaching 0.330 mg.l^-1. The other nitrites concentrations in the seepage water varied between 0.015 and 0.080 mg.l^-1. In general it can be stated that the nitrites content in the evaluated year was slightly higher in comparison to the previous years.

Total nitrogen

The total nitrogen content is predominantly affected by fluctuation of nitrate-nitrogen and organic-nitrogen concentrations, which results from the quantitative ratio of nitrogen forms. The total nitrogen belongs to water quality parameters with significant seasonal fluctuation. Maximums are reached in colder months and minimums in warmer months (during the vegetation period). The tendency of fluctuation of the total nitrogen in the evaluated year in the Danube, in the Mosoni Danube, in the tailrace canal at Sap and in the reservoir was similar at all sampling sites. Total nitrogen concentrations varied from 1.1 to 4.0 mg.l^-1. The total nitrogen content in the Mosoni Danube on sampling site No. 1141 at Vének was higher in comparison with the Danube water and fluctuated from 1.58 to 5.14 mg.l^-1. Higher concentrations of total nitrogen were recorded in the right side river branch system as well. The concentrations varied from 1.38 to 4,63 mg.l^-1. The total nitrogen content in the left side river branch system was lower and the spread of values was smaller, it varied from 1.19 to 2,89 mg.l^-1. The seasonal fluctuation in the seepage canals is less remarkable and regarding the origin of the water the total nitrogen content is low. In the evaluated year it fluctuated from 0.81 to 1.93 mg.l^-1 in the left side seepage canal (No. 317) and it varied from 1.0 to 2.60 mg.l^-1 in the right side seepage canal at Čunovo/Rajka (No. 3531/0084). In general it can be stated that the total nitrogen contents at individual sampling sites in the evaluated year were higher than in the previous year.

Phosphates

Phosphates concentrations almost at every monitored sampling site developed similarly in the evaluated year. The exemptions were the sampling site on the Mosoni Danube at Vének (No. 1141) and the sampling site in the right side seepage canal at Čunovo/Rajka (No. 3531/0084). Higher values of orthophosphates concentrations are characteristic for colder months and during higher flow rates. The lowest values are typical for summer months during the vegetation period. The lowest concentrations in the evaluated year occurred already in April and May. 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 0.08 mg.l^-1. The concentrations in the left side seepage canal at Hamuliakovo fluctuated from 0.02 to 0.14 mg.l^-1. Higher concentrations were measured in the Danube water, where they mostly fluctuated from 0.03 to 0.25 mg.l^-1, except one higher value of 0.34 mg.l^-1 recorded at Bratislava sampling site (No. 109) at the beginning of July 2008. 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 - 0.24 mg.l^-1). In the Mosoni Danube at Vének sampling site significantly higher concentrations were recorded, in several occasions double (0.05 - 0.52 mg.l^-1) and they were higher than in the year 2007. In comparison with the previous year the phosphate concentrations were slightly higher at every monitored site – Fig. 2-7.

Total phosphorus

Concentration changes of the total phosphorus followed the seasonal changes of phosphates concentration only partially. Similarly to phosphates the total phosphorus reached higher values at higher discharges and during flood waves, because the increase of concentrations in the surface water is frequently caused by phosphorus bound to suspended solids. The total phosphorus content in the evaluated year was higher than in the year 2007. The concentrations at sampling sites on the Danube varied from 0.01 to 0.25 mg.l^-1. They were lower in the river branch system and varied from 0,02 to 0.20 mg.l^-1. The highest value from this range was recorded on the Helena sampling site (No. 1112). In the Mosoni Danube at Rajka (No. 0082) the maximal values reached 0.09 mg.l^-1. In the right side seepage canal at Rajka (No. 0084) only narrow range of concentrations was registered, which varied from <0.02 to 0.09 mg.l^-1. The content of total phosphorus in the reservoir and in the tailrace canal varied in the range from 0.03 to 0.14 mg.l^-1, with one higher value recorded at sampling site No. 307 in July. The highest concentrations of total phosphorus were characteristic for the sampling site on the Mosoni Danube at Vének. In the evaluated year they fluctuated here from 0.09 to 0.27 mg.l^-1. These values were higher 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. Contents of phosphates and total phosphorus can increase at higher flow rates. In general the long-term tendency of pollution decrease continues. The nutrients content at individual sampling sites recorded in the year 2008 was similar or slightly higher than in the previous year. The lowest and the most balanced nutrients 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 hydro-meteorological 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 year were recorded from June to September. The highest values occurred in the winter and spring months. On most of the sampling sites the maximal values were determined in the sample taken in January or in December. Exceptions were the sampling sites in the reservoir and in the tailrace channel at Sap, where the maximal values of dissolved oxygen content were recorded in May. These maximal values are obviously related to assimilation processes of phytoplankton. The highest value was recorded in the lower part of the reservoir (sampling site No. 309) and it reached 17.9 mg.l^-1. In general it can be stated that dissolved oxygen concentrations were similar to that recorded in the previous year. Except the reservoir the dissolved oxygen content varied from 4.5 to 14.9 mg.l^-1. The development of dissolved oxygen concentrations on most of sampling sites showed a seasonal character. Low concentrations below 7 mg.l-1 were registered at several sampling sites: in the Danube old riverbed below the submerged weir (No. 0042), in he Mosoni Danube at Rajka (No. 0082), in the right side river branch system at Helena (No. 1112) and particularly in the right side seepage canal at Čunovo/Rajka (No. 3531/0084), where unfavourable oxygen conditions were recorded four times in the period from July to October - Fig. 2-8.

COD_Mn and BOD₅

COD_Mn and BOD₅ parameters are used for expression of organic contamination of 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.

The decreasing tendency of COD_Mn values in the evaluated year 2008 continued. The highest values were registered in the Mosoni Danube at Vének (No. 1141), where they varied in a range from 2.9 to 6.0 mg.l^-1. In the right side river branch system the COD_Mn values fluctuated in the range from 1.8 to 4.2 mg.l^-1, in the left side river branch system in a narrower range from 1.4 to 2.7 mg.l^-1. At sampling sites in the Danube old riverbed the Hungarian Party recorded values from 1.7 to 4.1 mg.l^-1, while the Slovak Party recorded values in a range from 1.2 to 3.0 mg.l^-1 (Fig. 2-9). Similar values ranging from 1.1 to 4.0 mg.l^-1 were registered in the main riverbed. At sampling sites in reservoir were the values slightly lower, up to 3.2 mg.l^-1. The poorest water from the point of view of organic contamination was the seepage water in the left side seepage canal, where the COD_Mn values fluctuated from <0.8 to 1.4 mg.l^-1, in the right side seepage canal they varied from 0.8 to 2.5 mg.l^-1. Organic contamination expressed by the COD_Mn parameter was lower than in the previous year at every sampling site.

Contrary to COD_Mn in case of biological oxygen demand (BOD₅) increase of values was registered in 2008. The BOD₅ values in the Danube water varied from 0.7 to 6.5 mg.l^-1, with two higher values recorded in May on sampling sites at Medveďov (No. 2306) – 8.2 mg.l-1 and at Rajka (No. 0001) – 8.4 mg.l^-1, which represents the highest measured value in 2008. In 2007 the highest recorded value in the Danube water was 3.4 mg.l^-1. In the Mosoni Danube at Čunovo/Rajka sampling site the BOD₅ values were significantly lower than in the main riverbed and varied from 0.7 to 3.2 mg.l^-1, which means that they were similar as in the previous year. On the sampling site at Vének the BOD₅ values fluctuated from 2.0 to 7.6 mg.l^-1. Higher values were recorded in the river branch system as well and varied up to 6.7 mg.l^-1. Slightly lower values in comparison with the previous year were registered in the reservoir (up to 4.1 mg.l^-1), and in the seepage canals (up to 3.2 mg.l^-1). The lowest pollution is characteristic for water in the left side seepage canal at Hamuliakovo sampling site (No. 317), where the BOD₅ values varied in a narrow range only from <0.85 to 1.2 mg.l^-1. Overall it can be stated that the BOD5 values recorded by the Hungarian Party were significantly higher than values observed by the Slovak Party (Fig. 2-10).

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 in the Mosoni Danube at Vének no situation with oxygen shortage occurred. In comparison with the previous year slightly lower minimal values were recoded at several sampling sites. Based on the monitoring results it can be stated that the organic contamination expressed by COD_Mn was lower in comparison with the previous year. BOD₅ values at sampling sites observed by the Hungarian Party were significantly higher. At jointly observed sampling sites in some cases the difference between Slovak and Hungarian data was double. 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, but in the main riverbed. The water quality at Vének 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 (1992-2008) 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 and since 2008 the lead content as well. The courses of heavy metal concentrations in the evaluated period were similar as in previous year. No heavy metals were monitored in the left side river branch system on the sampling site at Dobrohošť (No. 3376).

Concentrations of mercury in the first half of the evaluated year mostly varied below the detection limit of the analytical method used. In June 2008 one of the sampling organisation (VÚVH) decreased the detection limit from 0.1 µg.l^-1 to 0.003 µg.l^-1, which resulted in obtaining of concrete values at sampling sites monitored by this organisation. The mercury concentrations varied in the range from <0.003 to 0.0182 µg.l^-1 and the highest value was recorded on sampling site at Bratislava (No. 109) in November. The maximal concentration recorded in the year 2008 was 0.04 µg.l^-1 registered by the Hungarian Party in February on sampling site at Medveďov (No. 2306).

Almost every cadmium concentration was below the detection limit of the applied analytical method. Only five values exceeded the detection limit: twice on sampling site at Medveďov (No. 2306) – 0.5 and 0.3 µg.l^-1, and once on sampling sites in Mosoni Danube at Rajka (No. 0082), in the river branch system at Helena (No. 1112) and in the Ásványi river branch (No. 1126) – 0.2 µg.l^-1.

The chromium content was low at all observed sampling sites. Major part of values was below the detection limit of the applied analytical method (<0.6 µg.l^-1). Concentration exceeding the limit value occurred once on seven sampling sites: in the tailrace channel at Sap (No. 3530), in the reservoir (No. 311), in the Szigeti and Ásványi river arms (No. 1114 and 1126), in the Mosoni Danube at Vének (No. 1141) and in the Danube old riverbed below the submerged weir at Dunakiliti (No. 0042) and at Dunaremete (No. 0006), where the highest value of 6.6 µg.l^-1 was recorded. Besides this value all the others varied up to 3.0 µg.l^-1.

Except the seepage canals, the arsenic was characteristic with low content at every sampling sites similarly to the previous period. Greater part of recorded concentrations remained below the detection limit of the applied analytical method. Sporadically occurred values exceeding the limit value, with a frequency from one to three times during the year, maximally reaching 2.1 µg.l^-1. The situation in the seepage canals was different. In the right side seepage canal the arsenic concentrations varied from <0.9 to 1.8 µg.l^-1, however the frequency of concentrations above the limit value was higher. The highest arsenic content was registered in the left side seepage canal at Hamuliakovo (sampling site No. 317) where it fluctuated in the range between 2.77 and 5.06 µg.l^-1.

Nickel concentrations in the evaluated year mostly varied in the range from <0.7 to 3.0 µg.l^-1. A lot of data were below the detection limit of the applied analytical method. As the most polluted seemed to be the Ásványi river arm, where the highest values were registered. The maximal value in 2008 was recorded in April and reached 6.3 µg.l^-1, in May another high value occurred, reaching 5.1 µg.l^-1. The all other recorded concentrations varied in the range from <0.7 to 2.6 µg.l^-1.

The highest concentrations from among the heavy metals were characteristic for copper and zinc. In 2008 the copper concentrations fluctuated up to 22.7 µg.l^-1. Values exceeding 10 µg.l^-1 sporadically occurred at sampling sites monitored by the Hungarian Party: in the Szigeti arm (11.2 and 15.9 µg.l^-1), in the Mosoni Danube at Rajka (10.9 and 11.2 µg.l^-1) and at Vének (10.6 and 12.0 µg.l^-1). The highest concentrations were determined in the Ásványi river arm – 20.3 and 22.7 µg.l^-1. The highest values recorded by the Slovak Party occurred in the reservoir and reached 2.8 µg.l^-1. Based on the results obtained at jointly observed sampling sites it can be stated that there were significant differences in copper concentrations determined by the Slovak and the Hungarian Party – Fig. 2-11.

In the Danube main riverbed and in the right side river branch system almost every measured zinc concentration was below 10.0 µg.l^-1, which is the detection limit of the applied analytical method. At the other monitored sampling sites the zinc content in 2008 varied up to 26.5 µg.l^-1. This maximal value was recorded in the reservoir at sampling site No. 307.

The contents of lead in 2008 varied from <0.7 to 4.8 µg.l^-1. The highest concentration was recorded in the main riverbed on sampling site No. 109 at Bratislava. Data obtained from the Hungarian side were significantly lower that the data measured by the Slovak Party and in the main riverbed they were exclusively below the detection limit of the applied analytical method.

In summary it can be stated that heavy metal concentrations, which were determined from the filtered sample, were low during the evaluated year, with occasional occurrence of higher values. Great part of the determined values was below the detection limit of applied analytical methods. The highest concentrations were characteristic for zinc and copper. 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. In 2008 the detection limit for mercury have changed. 

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 sites. In the year 2008 rapid increase of chlorophyll-a contents was registered at the beginning of May. Concentrations decreased afterwards and since July the chlorophyll-a contents remained low till the end of the evaluated year. The chlorophyll-a concentrations in the Danube water fluctuated from 1.0 to 75.8 mg.m^-3. Similar values and similar course was registered in the Mosoni Danube at Čunovo. Slight increase of chlorophyll-a concentrations was registered in the Danube old riverbed upstream and below of submerged weir (No. 0043 and 0042) in October (maximally up to 22.5 mg.m^-3). The chlorophyll-a contents in the reservoir were similar to that in the Danube water, however contrary to the other sampling sites a slight increase (up to 20 mg.m^-3) was documented in June as well. The highest chlorophyll-a concentrations in the evaluated year were recorded in the Mosoni Danube on sampling site No. 1141 at Vének, where the value in May reached 101.8 mg.m^-3. The chlorophyll-a contents in the river branch system were lower in comparison with the previous year and varied in the range from 2.0 to 53.9 mg.m^-3. Low values were registered in the seepage water. 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 <2.0 to 15.5 mg.m^-3. Apart from the seepage canals higher values were recorded in comparison with the previous year. The development of chlorophyll-a concentrations at selected sampling sites is showed on Fig. 2-12. 

Other biological parameters

The evaluation of biological elements of quality in 2008 at jointly observed sampling sites and at sampling sites monitored on the Hungarian side was carried out in harmony with the methodology agreed within the Trans-boundary Water Commission. At sites on the Slovak side, which are monitored by the Slovak Water Management Authority (SVP-BA) the evaluation used in previous period continued.

Biological parameters at jointly monitored sampling sites

The surface water quality observation in 2008 in the frame of Trans-boundary Water Commission was realized according to the “Surface water quality observation directive for Slovak-Hungarian boundary water bodies and for extended water quality monitoring on the Danube” accepted on the LXV. Session of the Slovak-Hungarian Trans-boundary Water Commission with regard to the national methods of observation of the ecological and chemical status of water bodies in accordance with the requirements of Water Framework Directive (WFD). The goal of WFD is to protect all kind of water bodies and water dependent ecosystems, improve their status and achieve a good water status, which results from the good ecological and good chemical status of water bodies.

Biological elements together with supporting hydro-morphological, physico-chemical and chemical quality elements determine the ecological surface water status. The evaluation of the ecological status of water bodies in 2008 for individual biological quality elements was focused on sampling sites not on water bodies. The basic principle of the evaluation is the type specificity and the comparison of changes in environment quality with reference values, which reflect the environment status without or with minimal anthropogenic influence. From among the biological quality elements the following belong to the ecological state evaluation: benthic invertebrates (macrozoobenthos), phytobenthos, macrophytes, phytoplankton and fishes. In 2008 no fishes were monitored and macrophytes were observed in Bratislava and Komárno only. The evaluation of observation results for individual biological elements was performed according to the classification schemes, which include limit values for classification into the respective quality classes in the range I.-V. quality class with corresponding ecological status: I. class – high, II. – good, III. – moderate, IV. – poor, V. – bad. In Table 2-3 ecological status evaluation of individual biological quality elements is given separately for each country. When the final ecological status was determined the rule of “the worst value” was used for classification (worst case approach). 

Table 2-3: Evaluation of ecological status for biological quality elements at jointly monitored sampling sites

 

Based on the biological quality elements evaluation results it can be stated that in 2008 sampling sites in the Danube, in the Mosoni Danube at Vének and in the seepage canal at Čunovo/Rajka were classified into moderate ecological status and the sampling site in the Mosoni Danube at Čunovo into bad ecological status (at this sampling site only the Slovak observation results were evaluated).

According to WFD the final evaluation have to be accompanied by a factor of reliability of proper ecological status evaluation (high, moderate or low factor of reliability). At determining of the reliability factor several criterions are taken into account, which are more or less in coherence with criterions agreed by ICPDR. Similarly to the ecological status the same rule of the lowest reliability factor is used for determining the final reliability factor of water bodies ecological status evaluation. 

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, June, July and August. 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 one or two occasions: in April, June and in September. An overview of biological quality elements evaluation 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 water quality elements it can be stated that according to phytoplankton at every sampling site I. quality class determined, which corresponds to the high ecological status. From the phytobenthos point of view except the only sampling site in the river branch system the high ecological status was determined as well. Only in the Ásványi river arm (sampling site No. 1126) good ecological status was classified in 2008, which corresponds to II. quality class. The results in comparison with the previous year were much better, particularly in case of phytoplankton (Tab. 2-4), which might be related to the process of creation and testing the methodology for monitoring and evaluation of biological quality elements.

Biological parameters at sampling sites monitored only by the Slovak Party

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

Phytoplankton

In the period between March and October 2008 twelve phytoplankton samples were taken at monitored sampling sites (Table 2-5). The abundance of phytoplankton at most of the sampling sites was lower in comparison to the previous year, which could be related to different climatic conditions. The highest abundance of phytoplankton was recorded in the reservoir at sampling sites No. 307 and 311. Only at these sampling sites abundance exceeding 10000 individuals/ml was registered in 2008. This value corresponds to the limit for mass growth of phytoplankton and it occurred in the first half of May. Similarly to the previous period the most abundant in the Danube old riverbed, in the reservoir, in the tailrace channel at Sap and in the river branch system were the centric diatoms (Bacillariophyceae - Centrales). In the left side seepage canal at Hamuliakovo sampling site the pennate diatoms (Bacillariophyceae - Pennales), the cryptomonades (Cryptophyceae) and the centric diatoms (Bacillariophyceae - Centrales) created a significant portion of phytoplankton.

The phytoplankton composition significantly determines the saprobe index of biosestone. The saprobe index in 2008 varied from 1.59 to 2.47, 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 environment offers suitable living conditions for a wide scale of organisms of high species diversity. From the comparison of saprobe indexes of biosestone in 2007 and 2008 results that in case of every sampling site slight increase of saprobe index was registered in 2008.

 

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 2008.

 

Macrozoobenthos

 

From the ecological point of view the macrozoobenthos observation in the flowing water bodies appears to be the most suitable method for bioindication. The samples could be relatively easily accessible and quickly processed. In the year 2008 the macrozoobenthos samples were taken at monitoring sites given in Table 2-6. In 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, Corophium curvispinum, Potamopyrgus antipodarum and Simulium sp. dominated at this sampling site in 2008. In sections with slowly flowing water stagnicolous and oligooxibiontic species appear, which are resistant to 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 year Dikerogammarus villosus, Lumbricidae g. sp. div., Lithoglyphus naticoides and Corophium curvispinum 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 2008 were Lumbricidae g. sp. div., Hypania invalida, Potamopyrgus antipodarum, Corophium curvispinum, Chironomus sk. reductus and Dikerogammarus villosus, Pisidium sp. and Caenis horaria, and at sampling site No. 308 Cricotopus sp. as well. In the river branch system species Dikerogammarus villosus, Cricotopus sp., Dreissena polymorpha, Corophium curvispinum and Valvata piscinalis dominated. Based on the determined species the saprobe indexes of macrozoobenthos were calculated, which fluctuated in the range from 1.99 to 2.81 (Table 2-6). Values in the Danube old riverbed varied from 2.02 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 2.06 to 2.81, 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. Evaluation of the overall ecological and chemical status of surface water bodies at jointly monitored sampling sites

(based on report “Evaluation of status of water bodies in Slovak-Hungarian boundary watercourses in 2008”, April 2009)

The working group for protection of water quality of Slovak-Hungarian boundary watercourses of the Trans-boundary Water Commission (TWC) elaborated the evaluation of the ecological and chemical status in 2008 paying regard to national methodologies prepared according to requirements of the Water Framework Directive (WFD). The evaluation was elaborated separately for observation results obtained by the Slovak Party and separately for Hungarian results. The evaluation was prepared according to methodologies, which were given in the abovementioned report.

Based on Slovak results the Danube between Bratislava and Komárno was according to the biological quality elements classified into moderate ecological status (III. class). The seepage canal at Čunovo was classified into the moderate ecological status as well, but the Mosoni Danube at Čunovo reached bad ecological status. According to the priority substances good chemical status was declared at every jointly monitored sampling sites.

Based on Hungarian results according to the biological quality elements, according to chemical and hydro-morphological elements the sampling site in seepage canal at Rajka was classified into the good ecological status (II. class), sampling sites in the Danube and Mosoni Danube at Vének were classified into moderate ecological status (III. class). At jointly monitored sampling sites good chemical status was declared as well, except the sampling site in the Mosoni Danube at Rajka. Here the measured copper concentrations did not meet the relevant National environmental quality standard, so the water did not reached good chemical status. (The Hungarian Party, except the priority substances, included into the surface water chemical status evaluation the group of relevant dangerous substances consisting of chromium, zinc, arsenic, copper and cyanides. In the Slovak evaluation these substances are included in the physico-chemical quality elements, which belong to the evaluation of surface water ecological status.) 

 

2.9. Quality of sediments 

In the evaluated year 2008, 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 at the beginning of October 2008 at six sampling sites. The Hungarian Party sampled the sediments in March and September 2008 at seven 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 as well.

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. The lead content in 2008 was lower than in the previous year at every sampling site. Contents of chromium, zinc and mercury exceeded the TEL limit value at two sampling sites, contents of copper and cadmium at three sites and content of arsenic at five sampling sites. However, the concentrations varied in the ranged >TEL and <PEL (Probable Effect Level), 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. Concerning the inorganic micro-pollution in 2008 the most polluted sediment was found in the reservoir at sampling site No. 311 at Šamorín, the best sediment quality was registered in the Danube old riverbed at sampling site No. 4016 at Dobrohošť.

In sediment samples taken on the Hungarian territory in case of mercury exceedings of Probable Effect Level (PEL) occurred, when the unfavourable effect on biological life occurs frequently, in more than 50 %. They occurred in samples taken at several sampling sites. During sampling in March concentrations exceeding the PEL limit value occurred at every sampling site, in September at three sampling sites: in the river branch system (in Szigeti and Ásványi river arms) and in the Danube old riverbed below the submerged weir at Dunakiliti (No. 0042), where the highest mercury concentration of 3.6 mg.kg^-1 was determined. Concentrations of other analysed heavy metals were low and major part of recorded values varied around the TEL limit values, when the unfavourable effect on biological life occurs rarely. The lead concentrations at every sampling site were below the TEL limit value. Concerning the results of inorganic micro-pollution it can be stated that the most polluted sediment in 2008 was found in the Mosoni Danube at Vének (No. 1141) and the best sediment quality was found in the right side seepage canal (No. 0084).

From among the organic micro-pollutants following components were analysed by the Slovak and Hungarian Parties in 2008: naphthalene, phenanthrene, anthracene, fluoranthene, chrysene and benzo(a)pyrene. Moreover the Slovak Party analysed lindan, heptachlorine, endrine, dieldrine, and sum of PCB’s, the Hungarian Party analysed acenaphtene, acenaphtylene, fluorine, pyrene, benzo(a)anthracene, dibenzo(a,h)anthracene, and 2-methynaphtalene. Concentrations of abovementioned organic micro-pollutants were very low. In case of lindan, heptachlorine, endrine, dieldrine, naphthalene, anthracene and sum of PCB’s the measured values on the Slovak territory did not reached TEL limit values. In case of phenanthrene, fluoranthene, chrysene and benzo(a)pyrene the TEL limit values were slightly exceeded at four sampling sites (in the Danube old riverbed at Sap – No. 3739 and in the reservoir – No. 307, 308 and 311). On the Hungarian territory concentrations slightly exceeding TEL limit values occurred only sporadically: in the Szigeti river arm (No. 1114) in case of acenaphtene and benzo(a)anthracene, and in the Danube old riverbed below the submerged weir at Dunakiliti (No. 0042) in case of pyrene, benzo(a)pyrene and benzo(a)anthracene. Moreover the Hungarian party analysed the indeno(1,2,3)pyrene, benzo(ghi)perylene, benzo(k)fluoranthene and benzo(b)fluoranthene, however these substances does not have limit values determined in the CSQG. In case of the organic micropollutants the sediments on the Slovak and Hungarian sides in the year 2008 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.

On the Hungarian side the total phosphorus and total nitrate content in sediments was analysed as well. The total phosphorus content in 2008 varied in the range from 28 mg.kg^-1 (in the seepage canal at Rajka) to 2332 mg.kg^-1 (in the Mosoni Danube at Vének). The lowest concentration of total nitrogen – 1460 mg.kg^-1 was measured in the right side seepage canal. The highest values were detected in the Ásványi river arm (4490 mg.kg^-1) and in the Mosoni Danube at Vének (4744 mg.kg^-1).  

2.10. Conclusions

Based on the long-term evaluation it can be stated, that the water quality at Bratislava sampling site (No. 109), which represents the water quality entering the region influenced by the hydropower structures, generally improved. 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 year 2008 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. In case of nutrients similar or slightly higher concentrations were registered in comparison with the previous year, however in long-term they did not violate the tendency of pollution decrease. The dissolved oxygen content remained preserved at the level of previous years, the organic pollution represented by COD_Mn was lower in comparison with the previous year 2007, however increase of BOD₅ values was registered in measurements carried out on the Hungarian side in the evaluated year. 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 year at jointly monitored sampling sites and at other Hungarian sampling sites was realised according to the actualised national methodologies for particular biological quality elements in harmony with the Water Framework Directive. Based on the evaluation results of the biological quality elements the sampling sites on the Danube, Mosoni Danube at Vének and in the seepage canal at Čunovo/Rajka were classified into III. quality class, which correspond to moderate ecological status and the sampling site on the Mosoni Danube at Čunovo/Rajka was classified into IV. quality class, which represents poor ecological status. The evaluation of sampling sites, which are observed only by the Hungarian Party was realised similarly, but only two biological elements were evaluated – the phytoplankton and phytobenthos. Sampling sites were classified into high ecological status (I. class), except the Ásványi river arm, where in case of phytobenthos II. class occurred, which represents good ecological status. The results in 2008 significantly differ from results in the previous year, when sampling sites were classified into II. and V. quality class, which corresponded to an ecological status from good to bad. Monitoring and evaluation of sampling sites observed only by the Slovak Party was carried out according to methodology used in previous years. Macrozoobenthos and phytoplankton were evaluated. According to the saprobe index of biosestone, which is significantly determined by the phytoplankton composition and according to saprobe index of macrozoobenthos the water at sampling sites was classified into II. and III. quality classes.

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 2008 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 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 around 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

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 two Parties have different detection limits, or 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.