Wetlands and the "services" they provide
Wetlands perform a dazzling array of ecological "services" that we have only recently begun to appreciate. Water purification is one of them. Wetlands purify water by trapping sediments and retaining excess nutrients and other pollutants.
Pollutants enter wetlands primarily by way of runoff waters - rain and stormwater that travels over land surfaces to rivers and lakes. Runoff water may carry pesticides and fertilizers if these have been applied to the land. Other pollutants - notably heavy metals - are often attached to sediments and present the potential for further water contamination.
Wetlands protect surface waters from the problems of nutrient overload by removing the excess nutrients, some of which are taken up and used by wetland plants, and some of which are converted to less harmful chemical forms in the soil. Others simply settle to the bottom where wetland plants hold the accumulated sediments in place.
How we calculate the amount of water purified by wetlands
In 1999, the WWF Danube-Carpathian Programme and the WWF Auen Institut (Institute for Floodplains Ecology, WWF Germany), under the guidance of a UNDP/GEF team of experts conducted a study aiming to evaluate the wetlands of the Danube River Basin ("Evaluation of wetlands and floodplain areas in the Danube river basin").
They concluded that a scientifically defensible average nutrient reduction capacity cannot be obtained from available data. However, they said, it can be argued, based upon review of various studies and the general knowledge on all factors influencing the Danube basin, that a range of values for nutrient reduction can be proposed, the average being:
Nitrogen100-150 kg/ha/year
(average of 125 kg/ha/year )
Phosphorus
10-20 kg/ha/year
(average of 15 kg/ha/year )
The maximum amount of Nitrogen and Phosphorus pollution in the Danube River basin, based on the 2008 (latest available) figures from the TransNational Monitoring Network of the ICPDR which monitors physical, chemical and biological conditions in the Danube and its major tributaries, is:
Nitrogen - 3,4 mg/LPhosphorus - 0,412 mg/L
If we divide 125 kg/ha by 3,4 mg/L of Nitrogen we get 36 764m3 (36,764 million litres) per hectar per year. This makes 100,72 m3 per ha per day .
If we divide 15 kg/ha by 0,412 mg/L of Phosphorus we get 36 407m3 (36,407 million litres) per hectar per year. This makes 99,74 m3 per ha per day.
To calculate the total, we take the smaller figure of the two - 99,74.
If we convert this figure to "litres per second" we get 1,154 litres (1 litre and 154 ml) per second.
This means that a hectar of wetland purifies at least 1,154 litres per second.
According to WWF, the global average water footprint is about 3,400 litres per person per day. This includes water for drinking, cooking and washing, but even more for producing things such as food, paper, cotton clothes and others.
This means that a hectar of wetland purifies the water necessary for 29 people per day.
At this moment in time, this is the best available data for the calculation of nutrient reduction (water purification) by Danube wetlands.
How we calculate the amount of water that would be purified by restored wetlands along the Danube
In 2010 WWF published the study "Assessment of the restoration potential along the Danube and main tributaries". The authors of the study estimate the potential for floodplain restoration at about 800,000 ha along the Danube. This would mean that once restored, these wetlands would provide us with 923 200 litres of clean water per second, which is almost 1 million litres of clean water per second!