Ponds, Pools and Lochans

5.3 Water sources for new ponds


Getting good water quality

The best design feature to give any new pond is a location where its water quality is going to be protected in the long term. With good water quality any pond (whatever its shape and structure) will eventually develop a high-value wildlife community. A pond which receives polluted water is, in contrast, unlikely to ever achieve its full wildlife potential (although design can offset some of the impacts of pollutants).

If a pond is badly polluted then it will almost certainly give long-term management problems: a characteristic of ponds is that they accumulate pollutants with age, so nondegradable pollutants that enter ponds (phosphates, heavy metals, persistent biocides) gradually build up as the pond ages. This leads to a progressive deterioration in water and sediment quality, fewer species and the beginnings of difficult problems like nuisance levels of algae, water fern or duckweed caused by gradual enrichment by nutrients. Pollution problems like these can really only be solved if all polluted sediments are removed and external pollutant inputs reduced.

It is critical, therefore, that new ponds which are created mainly for wildlife conservation are located so that the quality of the water they receive is as high as possible. For ponds which are specifically created to treat contaminated surface water runoff, wildlife potential will be enhanced where specific measures are taken to maintain good water quality in at least some areas associated with the waterbody (e.g. marginal pools, series of ponds etc.) (see Section 7).

The main water sources for ponds, and methods for ensuring these water sources are clean, are discussed below.

Apart from rainwater, there are three main sources of water for ponds: (i) groundwater, (ii) surface runoff and (iii) inflows. Individual ponds may be fed by one or more of these water types and the importance of each may vary during the year.

Click to enlarge image
Figure 31. Ponds can be fed by a variety of water sources including surface water, groundwater and ditch or stream inflows.
Surface water

Surface run-off can provide some of the best, and some of the worst, quality water for new ponds, depending largely on the naturalness of the ponds catchment. In agricultural and urban areas, surface water quality is often highly degraded, with high levels of soluble pollutants (nitrate, biocides etc.), together with phosphate, organics, hydrocarbons, oils and sediment-bound toxins which are washed-in with soil particles. Because surrounding landuse exerts such a strong influence on the quality of run-off, surface water ponds are best located in seminatural landscapes; where this is not feasible, the aim should be to control pollution sources in the drainage area around the pond (for example, by minimising pesticide usage) and to install buffer zones to intercept pollutants before they reach the pond.

The special case of SUDS ponds, which are designed to receive and treat contaminated surface water runoff from urban areas, is discussed in Section 7.

Groundwater

In intensively managed landscapes such as urban or arable areas, groundwater ponds are often less polluted than their surface water counterparts. Groundwater is, of course, not always pollutant-free, but it will at least have been pre-filtered subsurface before reaching a pond, removing sediment borne pollutants. In addition, since groundwaters move (albeit slowly), soluble pollutants entering a pond from surface runoff or inflows are continually diluted and carried away. For these reasons, groundwater ponds may sometimes support high quality wildlife communities even where their surroundings are degraded.

How to avoid pollution problems in new ponds

In designing ponds the best way to determine whether a new pond is likely to be polluted is to check the pond catchment for pollution sources. Pollution sources include:

  • streams, ditches or surface run-off draining arable farmland or intensive grassland (nutrients, biocides)
  • spray drift from adjacent farmland (biocides)
  • runoff from the farmyards of livestock enterprises
  • streams, ditches or surface run-off draining forestry plantations (may be acidified, can produce very large sediment loads from clear felling operations and trackway erosion which carries nutrients from fertilisers used to promote tree growth)
  • treated sewage effluent inputs
  • leakage from diesel and oil stores
  • drainage from road-runoff or urban areas (a wide range of pollutants)

Unpolluted waters supplies may be found in:

  • non-intensively managed grassland (i.e. less than 50 units of nitrogen applied annually)
  • groundwater (particularly in areas where it is not polluted by nutrients)
  • water draining from extensive semi-natural woodland, scrub, moorland, heathland or bogs.
How big an area is needed for an unpolluted surface water supply to a pond?

Comparatively tiny areas of non-intensively managed land can supply enough water for a good sized pond.

For example, 1 ha of land receiving 1500 mm of rainfall annually (the Scottish average) will receive about 15000 m3 of rainfall during the year. Assuming that half of this is lost through interception and evapotranspiration about 7500 m3 of water remains.

A pond with a surface area of 500 m2 and an average depth of 0.5 m holds 250 m3 of water - i.e. 1/30th of the volume available from 1 hectare of land.

Calculations for dew ponds - i.e. clay-lined ponds fed by runoff from the surrounds - suggest that a pond will hold water during the summer if it has a surface catchment approximately twice the surface area of the pond. So, a circular pond with a surface area of 100 m2 needs only a further 5 m all around it as collecting area to remain wet in most years.

Ponds_fig32.jpg (7813 bytes)

Figure 32. Jeremy Biggs, Joanna Drewitt, Katherine Bradshaw and Baroness Hilton inspect pond animals at Pinkhill Meadow Experimental Wetland site (Oxfordshire). British Dragonfly Society
Inflows and drains

Creating ponds that are linked to a temporary or permanent inflow always needs careful consideration because of its numerous disadvantages. In general, SEPA aims to discourage creation of on-stream ponds.

The quality of water in stream or ditch-fed ponds will reflect the quality of the inflow catchment and, in lowland Scotland (as elsewhere in Britain), most streams, rivers and ditches draining farmland and urban areas are polluted to a greater or lesser extent, especially by nutrients, and perhaps also by biocides. Overall, where a stream inflow is possibly polluted, it is best to avoid it. Note that a widespread myth in pond creation is the idea that ponds need an inflow to ensure that they do not become stagnant and unsuitable for wildlife. In fact, nothing could be further from the truth. Pond plants and animals are specifically adapted to live in still water. Ponds with inflows, will usually support a different biota but not a better community to ponds without.

In addition, on-stream ponds have a range of other problems associated with them. They often have significant construction costs and management requirements (e.g. flood spillways, fish passes). Ponds fed by streams or ditches draining all but the most pristine landscapes will quickly silt up - a pond with an inflow typically fills in 10 to 100 times as fast as one without. Consequently, these ponds need more frequent desilting than other pond types. This desilting can result in downstream pollution in any subsequent dredging operations.

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