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Figure 7.4: A comparison of water quality in the rivers in the Czech Republic, 1991–1992 (left image) and 2007–2008 (right image)

Note: Blue: Unpolluted or slightly polluted waters; Green: polluted waters; Yellow: heavily polluted waters; and Red: Very heavily polluted waters

Methodology for the map: Traditionally, surface water quality is classified into 5 categories (shown in legend). The basic classification for the maps below is the aggregate of the following indicators: BOD₅, COD_Cr, N-NH₄⁺, N-NO₃^-, Ptotal and the saprobic index of macroinvertebrate communities (the final class is the worst class of these indicators).

Source: http://issar.cenia.cz/issar/page.php?id=1775/The T.G. Masaryk Water Research Institute