Sadržaj hranjivih tvari u površinskim vodama rijeke Drave

The Drava is 725 km long river located in central Europe, with basin size of 40 150 km². It is a tributary of Danube, its source is in Italy and flows through Italy, Austria, Slovenia, Croatia and Hungary, and flows into Danube 2.5 km northwest from Aljmaš (Croatia). The Drava passes through several large settlements in Austria, Slovenia, Croatia and Hungary, and is a collector of wastewater from these settlements (areas with intensive industrial agricultural production). Since wastewaters are usually loaded with nutrients, the aim of this study was to collect and analyse concentration of nitrogen and phosphorus compounds and BOD5 and COD (whose concentration in water must comply with the permissible limit value in order to avoid nutrient overload of environment and eutrophication) in surface waters. Samples were taken twice a month over the period of ten years (year 2004 and 2013), at two locations – upstream from Osijek and in Donji Miholjac. Obtained results show that during analysed period concentration of total nitrogen was in the range of 0.325–4.59 mg/L (Osijek) and 0.74– 2.763 mg/L (Donji Miholjac), concentration of nitrites was 0.001–0.03 mg/L (Osijek) and 0.001– 0.078 mg/L (Donji Miholjac), and concentration of nitrates was 0.41¬–4.068 mg/L (Osijek) and 0.50¬–2.43 mg/L (Donji Miholjac). Concentration of orthophosphates was in the range of 0.005– 1.06 mg/L (Osijek) and 0.002–0.07 mg/L (Donji Miholjac), and total phosphorus was in the range of 0.030–2.15 mg/L (Osijek) and 0.018–0.406 mg/L (Donji Miholjac). Biological oxygen demand (BOD5) was in the range of 0.1–6.8 mgO2/L (Osijek) and 0.4–3.9 mgO2/L (Donji Miholjac). Chemical oxygen demand (COD-Mn) was in the range of 1.5–6.9 mgO2/L (Osijek) and 1.6–9.8 mgO2/L (Donji Miholjac). COD-Cr was in the range of 3.5–74.9 mgO2/L (Osijek) and 3–27.1 mgO2/L (Donji Miholjac). According to current regulations in Croatia, surface water quality of Drava River with regard to physico-chemical parameters can be characterized as good. Further continuous monitoring of water quality is necessary in order to improve reliability of water quality categorization, for which purpose specific polluting particulate matter should also be taken into consideration.