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[email protected] 24 Pliska Stud. Math. (2015 ), 151–162 STUDIA MATHEMATICA ANALYSIS OF SURFACE WATER KEY POLLUTANTS OF THE TRIBUTARIES OF THE DANUBE RIVER IN BULGARIAN SECTION M. Filipova, I. Zheleva, A. Lecheva, P. Rusev Based on official data, a comparative analysis of the surface water along the rivers flowing into the Danube River in the transborder area Bulgaria– Romania is presented. The content of dissolved oxygen, nitrate nitrogen and Biological and Chemical Oxygen Demand (BOD5 and COD) for a five year period 2009–2013 is analyzed. The aim is the dynamics of these indicators and the reasons for the current exceedances to be traced and analyzed. Measures for improving the condition of the surface runoff are also proposed. 1. Introduction Pursuant to the operational European program for CBC Bulgaria - Romania for the period 2007–2013 [1] includes the border areas of both countries.