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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 intact.

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 follows:

1. Nitrate is denitrified by the anaerobic conditions in the wetlands. Organic matter accumulated in the wetlands converts nitrate to free nitrogen.
2. Clay mineral is able to adsorb ammonium and metal ions.
3. 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 ions.
4. Biodegradable organic matter is decomposed aerobically or anaerobically by the microorganisms in the transition zone.
5. Pathogens are out-competed by the natural microorganisms in the transition zone.
6. 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.
7. 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.
8. 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).

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.

(continued)

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