Newsletter and Technical Publications
<Planning and Management of Lakes and
An Integrated Approach to Eutrophication>
CHAPTER 6. TECHNOLOGICAL AND MANAGERIAL ASPECTS OF EUTROPHICATION
6.4. Constructed Wetlands
The ecotones between lakes and terrestrial ecosystems are crucial for
protection of the lake ecosystem against anthropogenic impacts. The
transition area has the same function for a lake as the membrane has for a
cell: it prevents, to a certain extent, penetration of undesirable
components into the lake. Therefore, it is crucial to preserve the shore
ecotones around a lake and the wetlands in the watershed, independent of
the implemented management strategy. Any man-made construction should be
omitted in a zone 50 to 100 m from the lake shore line to keep the ecotone
Ecotones serve as a buffer zone, not only for pollutants, but also for
the species present in the adjacent ecosystems. Preservation of wetlands
at the lakeshore line may therefore be crucial for maintenance of the
biodiversity in the lake ecosystem - a function, which the manager must
not overlook in development of an appropriate lake management strategy.
Non-point or diffuse pollutants from the environment will inevitably
flow toward the lake, but the transition zone is able to transform and/or
adsorb the pollutants entirely or partially. It will thereby significantly
reduce the over-all irreversible effects on the lake ecosystems. The most
important processes occurring in the transition zone may be summarized as
- Nitrate is denitrified by the anaerobic conditions in the wetlands.
Organic matter accumulated in the wetlands converts nitrate to free
- Clay mineral is able to adsorb ammonium and metal ions.
- Organic matter is able to adsorb metal ions, pesticides, and
phosphorus compounds. Metal ions form complexes with humic acids and
other polymer organics, which significantly reduce the toxicity of these
- Biodegradable organic matter is decomposed aerobically or
anaerobically by the microorganisms in the transition zone.
- Pathogens are out-competed by the natural microorganisms in the
- Macrophytes are able to uptake heavy metals with high efficiency.
Other toxic substances may also be removed by macrophytes, but it is
hardly possible to indicate any general rule for the efficiency.
- Toxic organic compounds will, to a certain extent, be decomposed by
anaerobic processes in wetlands, depending on the biodegradability of
the compounds and the retention time in the wetlands.
- Soil with a high total metal content can remove phosphorus. Among the
four major metal ions (i.e., magnesium, calcium, iron, and aluminum)
calcium has the strongest correlation to phosphorus-sorption capacity. A
high pH also implies an increasing sorption capacity to soil with a high
metal content. This relationship between high calcium content and high
pH on the one side and high phosphorus-sorption capacity on the other
side may be utilized in construction of artificial wetlands. It is often
beneficial to transport a soil with a high phosphorus-sorption capacity
to the wetlands under construction. It may increase the
phosphorus-removal capacity even more than one magnitude. As much as 1
to 3 g phosphorus per kg of soil can be achieved by a 200 to 600 g of
major metals per kg of soil.
The denitrification potential of wetlands is often surprisingly high. As
much as 2,000 to 3,000 kg of nitrate-nitrogen can be denitrified per
hectare of wetlands per year, depending on the hydraulic conditions. This
is of great importance for the protection of lakes, because a significant
amount of nitrate is released by agricultural activities. As much as 100
kg nitrate-nitrogen per hectare may be found in the drainage water from
intensive agriculture. Since the denitrification is accompanied by a
stoichiometric oxidation of organic matter, this process also removes a
significant amount of organic matter. The phosphorus, bound in organic
matter or adsorbed to the organic matter, may, however, be released by
these processes. These processes should be examined carefully and
quantitatively in each case, including whether the released phosphorus
will flow towards the lake or towards the groundwater. These possibilities
should be included in the development of management strategies.
The adsorption capacity of the transition zone offers significant
protection against pollution by toxic substances, both heavy metals and
toxic organics i.e., primarily pesticides originating from agriculture.
The ratio concentration of heavy metals or pesticides in organic matter to
the concentration in water at equilibrium strongly depends on the
composition of the organic matter, but is usually between 50 and 5000,
which indicates that the transition zone has an enormous binding capacity
for these pollutants.
Table 6.3. shows an overview of the different types of wetlands that are
found adjacent to lakes: wet meadows, forested wetlands, marshes, bogs,
and shoreline wetlands. The characteristics of the seven types of wetlands
and their different ability to cope with the non-point pollution problems
are given in the table.
Table 6.3. Characteristic of wetlands
adjacent to lakes¹ (Patten et al., 1990).
|Type of wetland
||Ability to retain non-pollutants
||Grassland with waterlogged
soil. Standing water for a part of the year.
||Denitrification only in
standing water. Removal of nitrogen and phosphorus by harvest.
|Fresh water marshes
||Reed-grass dominated, often with peat
||High potential for denitrification, which
is limited by the hydraulic conductivity.
||Dominated by trees, shrubs. Standing water
not always for the entire year .
||High potential for denitrification and
accumulation of pollutants, provided that standing water is present.
|Salt water marshes
||Herbaceous vegetation usually with mineral
||Medium potential for denitrification.
||A peat-accumulating wetland with minor
||High potential for denitrification but
limited by small hydraulic conductivity.
|Shore line wetlands
||Littoral vegetation, often of great
importance for the lake.
||High potential for denitrification and
accumulation of pollutants, but area coverage.
¹ Classification of wetlands varies depending of
the author and region
The realization of the importance of wetlands, adjacent to
lake ecosystems, has resulted in the fact that drainage of wetlands has
ceased in many countries and that even previously drained wetlands are
restored. It is also considered to construct artificial wetlands to cope
with the diffuse pollution originating from agriculture, septic tanks, and
other sources. In accordance with the U.S.A. legislation, it is not
allowed to drain wetlands, unless another wetland of the same size is
installed somewhere else.
Construction of artificial wetlands is an attractive and
cost-moderate solution to pollution by diffuse sources and even
wastewater. First of all, wetlands are able to cope with the nitrogen and
heavy metal pollution from these sources. It is essential to properly plan
where to place the artificial wetlands, as their effects are dependent on
the hydrology (i.e., they should be covered by water most of the year and
have a sufficient retention time to allow them to solve the considered and
specific pollution problems), and on the landscape pattern (i.e., they
should protect the most vulnerable ecosystems, which are often lakes and
reservoirs). Furthermore, it is important to ensure that the wetlands are
not releasing other components, such as phosphorus, as mentioned above.
The following emergent macrophyte species are proposed to
be used in constructed wetlands: cattails, bulrush, reeds, rushes,
papyrus, and sedges. Submerged species can be applied in deep-water zones.
Species that have been used for this purpose include coon tail or horn
wart, redhead grass, widegeon grass, wild celery, and water milfoil.
It also should be kept in mind that a wetland, in most
cases, would reduce the water budget due to evapotranspiration. However,
wetlands reduce the wind speed at the water surface, and therefore may
also reduce the evaporation. It is important to consider these factors by
planning the construction of artificial wetlands. Finally, it should not
be forgotten that an artificial wetland is not fully developed over night.
In most cases, it will require two to four years for an artificial wetland
to obtain sufficient plant coverage and biodiversity to be fully
operational. It is, however, clear from the experience gained by the
relatively few constructed wetlands, that the application of models for
the wetland encompassing all the processes reviewed above, as well as for
the lake, is compulsory if positive results are to be expected.
Wetlands encompassing the so-called root zone plants may
also be utilized as wastewater treatment facilities. This application of "soft"
technology seems particularly advantageous for developing countries due to
its moderate cost.