Finnish Forest Research Institute  Metla

Press release 02.05.2006

Riparian buffer zones effective in reducing leaching of soil from forest drainage areas into water systems

Riparian buffer zones appear to be a solution clearly superior to sedimentation basins in prevention of suspended solids loading in forest drainage areas. At best, riparian buffering may be used to prevent leaching of soil into water systems altogether. However, in order to ensure a good purification result, a sufficiently large area must be set aside for riparian buffering. The findings are a result of a joint study by the Finnish Forest Research Institute, the Forestry Development Centre Tapio and Forestry Centres.

Suspended solids loading is considered to be the most serious harm to water systems caused by forestry. Suspended solids carried into brooks and lakes impair survival conditions of e.g. fish and crayfish. Eutrophication of water systems may also increase in time, if the suspended solids start to release nutrients in oxygen-free conditions. Clouding of water along with increased suspended solids loading also impairs recreational use of water systems.

To date, attempts to reduce suspended solids loading have mainly involved use of sedimentation basins. However, sedimentation basins have proved to offer a limited solution as means of protecting water quality. For example, light and fine clay and peat particles are not retained by the basins. Neither do sedimentation basins work on nutrients carried by water in soluble form. Very rapid filling of basins during large suspended solids leachings and collapse of basin walls in areas prone to erosion may also create problems.

The Finnish Forest Research Institute, the Forestry Development Centre Tapio and regional Forestry Centres conducted a joint study in 1995-2001 on effectiveness of riparian buffer zones in prevention of suspended solids loading during improvement drainage works. The results show that riparian buffer zones are a solution to the erosion problem clearly superior to sedimentation basins. At best, riparian buffering may reduce suspended solids loading by up to a hundred percent, and mostly water running off a riparian buffer zone is at least 50% purer than the water running into the zone from the improvement drainage area. A good purification result (over 70 percent reduction in loading) requires a zone area of about one percent of the whole drainage area.

Unlike sedimentation basins, riparian buffer zones can also retain water-soluble nutrients. Preliminary results would indicate that also in terms of retaining nutrients, it is essential that riparian buffer zones cover at least about one percent of the drainage area.

The only known problem regarding water quality protection when using riparian buffering is the increased soluble phosphorus and soluble organic matter (humus) loading immediately after establishing the zone, found in some areas. Phosphorus and humus may be leached especially in cases where the riparian buffer zone is formed by blocking off ditches and restoring parts of an old drainage area.

Publication: Nieminen, M., Ahti, E., Nousiainen, H., Joensuu, S. & Vuollekoski, M. 2005. Capacity of riparian buffer zones to reduce sediment concentrations in discharge from peatlands drained for forestry. Silva Fennica 39(3).

A summary of the article in Finnish was published in Metsätieteen aikakauskirja 1/2006.

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