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As the EU Water Framework Directive
(WFD) is translated into national legislation,
the nitrogen (N) concentration in many rivers will
have to be significantly reduced. In order to
implement suitable measures, the relevant pathways
and input quantities of nitrogen discharge
have to be identified. For the Middle Mulde
catchment in Saxony, Germany (2,700 km2), a
pathway-related model is developed to quantify
point and diffuse nitrogen discharges. Calculated
N-input via point sources is 2,400 Mg entering the
rivers in the study area every year. The main
point input representing over 50% of the total is
via municipal sewage treatment plants, followed
by the industrial direct dischargers and the combined
sewer overflows, amounting to approx. 20%
(462 Mg and 444 Mg) each. The diffuse nitrogen
discharges from the soil zone of the arable land
amount to a total of 8,050 Mg per year in the
1980s and 2,815 Mg in the 1990s. The inputs via
the groundwater flow make up the main proportion
for the 1980s with 5,303 Mg, followed by
1,480 Mg from tile drainage and 1,267 Mg from
direct runoff. In the 1990s the nitrogen discharge
fell sharply with the considerable fall in the
surpluses in the agricultural nitrogen balance
after German reunification with 1,934 Mg
(groundwater flow), 454 Mg (drainage runoff)
and 410 Mg (direct runoff). Sensitivity analyses
show that the parameter ‘‘total atmospheric
deposition’’ represents the highest uncertainties,
because of its uncertain quantification and the
high sensitivity of the results to this variable.
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