Decline in emissions also has negative implications

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Due to the burning of biomass and fossil fuels and, above all, due to agriculture, excessive quantities of reactive nitrogen are still being released into the atmosphere, soil and water with negative effects on biodiversity, the climate and human health. However, there was a differentiated analysis of nitrogen input pathways from the different sources reveals significant differences. While nitrogen inputs into soils that were primarily due to agriculture have elevated nitrate concentrations in the groundwater of many regions to values above the threshold of 50 mg per litre, atmospheric pollution is decreasing in large parts of Europe and North America due to emission-reducing measures. This means that less nitrogen is released into soils and water through atmospheric depositions. Moreover, Long-term measurements over the past 20 years clearly indicate that this is the case in Germany. For instance on average 35 mg less atmospheric nitrogen was released into the soils per square metre per year. According to studies conducted by UFZ scientists, this leads to 0.08 mg less nitrate per litre per year entering streams and drinking water reservoirs.

This positive development can also have negative implications became apparent when scientists started studying the causes of a brown coloration of water in reservoirs increasingly observable in Germany, northern Europe and North America. This brown coloration is especially problematic for drinking water treatment. In reviewing various hypotheses, they noted that the brown coloration of the water was strongly correlated with the decreasing concentrations of nitrate in the riparian soils surrounding the tributary streams of the reservoirs. This is due to the fact that the presence of nitrate in the riparian wetlands where most of the stream flow is generated ensures that carbon, phosphate and various metals remain bound to oxidized iron. Lower nitrate levels allow a chemical reduction of iron compounds and thus the mobilization of previously adsorbed substances. Thus, compounds previously bound to soil particles become mobile and are released into the streams with the rainwater. In the case of carbon this means that the concentration of dissolved organic carbon increases and is visible as the brownish colour of the water. In just under 40% of the 110 tributaries of drinking water reservoirs that were studied, the scientists found significantly increased DOC concentrations with an average of 0.12 mg more DOC per litre per year. The most significant increase was found in natural, forested, where nitrate concentrations in the water were less than 6 mg per litre.

In addition to DOC, phosphate concentrations are also increasing significantly in over 30 % of the tributaries. The calculated average 7 µg per litre per year tends to favour algae growth and is equally problematic for water quality in the long run. There is evidence that not only DOC and phosphate, but also adsorbed metals such as arsenic, vanadium, zinc and lead are increasingly becoming mobilized.

 

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