Treatment MeasuresWetlands : Water Quality and the Wetland

Urban stormwater runoff

Runoff, especially from urbanised catchments, is an important factor contributing to water quality degradation in receiving waters eg. waterways and bays. Urban stormwater runoff can be highly variable, with differing levels of pollutants such as nutrients (e.g. phosphorus, nitrogen), toxicants (e.g. metals such as zinc and nickel), or suspended solids (e.g. particles of soil or organic substances). Urban runoff can cause substantial degradation of receiving waters.

The benficial impact of wetlands on water quality

As stormwater passes through a wetland, the water quality will improve depending on the biological and chemical processes at the time. Variables that influence the treatment process include the, shape, storage volume or residence time (ie. time water takes to pass through the wetland) and soil type of the wetland, the species composition and number of the aquatic plants and management practises.

These factors have an influence on the length of time that water remains within the wetland - the greater the length of time, the greater the opportunity for the wetland processes to improve the quality of that water.

Nutrient removal in wetlands is not only due to uptake by flourishing plant growth, but is also due to physical processes such as the adsorption of nutrients to sediments, precipitation and sedimentation.

Periodic harvesting of plants may stimulate further plant growth and this may, in turn, enhance further nutrient removal.

The physical, chemical and biological processes in wetlands are influenced by the hydrological cycle. Seasonal wetlands are rich in flora and fauna, and the fluctuation in water level can affect the distribution and composition of emergent and fringing vegetation.

The leaf litter from the macrophytes can be a source of food for macro-invertebrates eg insect larvae. The abundance of higher level invertebrates usually corresponds to the quantity of micro-organisms present in the sediments and system ecology. Studies have also shown that seasonal inundation and exposure accelerates the processing of leaf litter and the amount of organic matter in the sediments. The distribution of organic rich soils in wetland basins may provide an indication of long term water levels and can therefore provide information to assist in the management of both artificially maintained or natural wetlands.

The water, sediments and plants in wetlands receiving urban runoff contain higher levels of heavy metals than wetlands not receiving urban runoff. Large aquatic plants e.g. Typha latifolia, Iris pseudacornis and Phragmitis australis are known to accumulate heavy metals in their tissues. As the macrophytes die and decay, there can be an increase in the concentration of heavy metals in the sediments.

The level of bacterial activity in a wetland is influenced by the fluctuation in water level, the season and the amount of orgainic matter in the sediments. Hydrogen sulfide, produced as a by-product of bacterial anaerobic processes, is also responsible for the sedimentation of dissolved heavy metals from water.

The fish populations in wetlands may influence not only the fluctuations in the fish-food organisms, but also the water chemistry. Fish and frogs can also be a biological agent in controlling invertebrates in the ecosystem.

Treatment/Design Targets

Newly developed Guidelines for developers as to the construction of wetlands requires the design to: