Ponds, Pools and Lochans

7.3 Two principles for working with contaminated surface water runoff

There are two main ways of maximising the value of constructed wetlands for aquatic wildlife:

  • design the waterbody or wetland to maximise water quality in some or all areas;
  • ensure that there are large areas of well vegetated shallows, to maximise the habitat for aquatic species which are most tolerant of contaminated water.
Maximising water quality in ponds

Although a variety of common and robust aquatic organisms tolerate mildly polluted water, many do not. This means that all design features which can minimise pollution in retention ponds will benefit wildlife. The employment of Best Management Practice features in SUDS schemes will do much to minimise pollutants reaching open waterbodies, and these measures should always be implemented to the full (see SEPA, 1998a, b, 1999 and 2000). Specifically:

  • Implement a full range of source control techniques (e.g. buffer strips, swales, porous pavements).
  • Design the SUDS treatment sequence to include structures which intercept and reduce the silt and pollutant loads in runoff water before it reaches pond or wetland habitats.
  • Establish a series of linked, vegetated pond basins, which progressively filter and clean the contaminated runoff, creating waterbodies which can sustain progressively higher quality wildlife communities.
  • Route cleaner sources of water (such as roof water) into separate isolated basins which provide a less contaminated habitat.

In addition to these standard methods, a fifth way of improving water quality in retention ponds is to use runoff from the slopes around the pond basin itself to provide a high quality water source for small surface runoff pools at the edge of the waterbody.

It is important that these small runoff pools are not directly connected to the main series of SUDS treatment waterbodies to ensure that they remain unpolluted, contributing a high water quality environment to the complex as a whole.

To do this, clean-water pools ideally need to be located above the water level of the main pond. The quality of the surface runoff will depend on the quality of the catchment area around the pond - so to be effective the pond banks need to be under semi-natural vegetation such as untreated grassland or scrub.

Maximising the availability of habitat for pollution tolerant species

Some common amphibians, fish, invertebrates and waterfowl appear to be quite tolerant of pollutants. For example, smooth newts and some water beetles may sometimes be seen in quite remarkably contaminated conditions. Similarly waterfowl are commonly present on quite degraded ponds and lakes, and even water voles can tolerate some levels of water pollution.

The plants and animals that suffer most from pollution tend to be species that live entirely below the water surface. Thus submerged plants often do badly in polluted ponds, whereas many tall emergents, herbs and grasses at the edges can thrive. Similarly, many of the species of mayflies, dragonflies and caddisflies which respire underwater, do not tolerate heavily polluted waters.

Other groups, particularly the air breathing invertebrate groups such as water beetles, bugs and water snails, appear to be far less affected by water quality - what they mainly need is good habitat structure (Williams et al., 1998b).

A key to designing wildlife-rich SUDS ponds and wetlands is, therefore, to incorporate extensive areas of marsh and very shallow water where diverse communities can survive under most water quality conditions.

Where this is done, urban run-off ponds can support comparatively uncommon animals. For example, in Wokingham (Berkshire) the water beetle Peltodytes ceasus, an indicator of high quality wetland habitats, was found in an urban drainage pond where well developed mixed marginal vegetation provided a habitat which compensated for the poor water quality in the main basin. Other groups such as mammals, birds and semi-terrestrial insects would also be likely to benefit from extensive marsh zones (e.g. water vole, scrub wetland birds such as reed buntings and reed warblers).

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