The best wildlife ponds have clean water. Clean water with low levels of pollutants allows a wider range of plants to grow instead of a just a few dominating bullies that cause blooms. Many water supplies are now polluted so when you are making new ponds, or managing existing ones, you need to look carefully at where the water is coming from and assess how likely it is to be clean.
There are four main sources of water for ponds: rain water, surface water, groundwater and inflows (streams, ditches, rivers).
For further information please see the Pond Creation Toolkit.
Rain water
For garden pond owners: rain water should be the cleanest source of water for your pond.
You can collect rain water off your house, shed or greenhouse to fill up or top up your garden pond. Otherwise, be patient and let the cleanest possible water source - rain - fill up the pond naturally. Depending on the weather and the size of pond, this could take a couple of weeks, or a year but it will be well worth it.
Using tap water is bad for two reasons.
Firstly it is a waste of expensively treated and transported water, and often in short supply. Secondly, tap water contains high levels of nutrients, particularly phosphates. In a pond this leads to rampant growth of algae, turning your pond water green or brown, or of duckweed which blocks out light and oxygen, causing other plants to die.
Surface run-off
Many ponds are filled by surface water. This is water that runs into the pond from higher ground, running through the vegetation and the soil.
Surface water can be either the best or worst water supply depending on what the area around the pond is like. If the water runs off unfertilised grassland or natural awoodland then it is likely to be very clean, but if the water drains off an intensive arable field or a car park then it is very likely to be polluted.
Groundwater
This is water already present in the ground - especially in sand, gravel or peat. In urban and intensive agriculture areas it is usually the cleanest source of water because it has been filtered through the ground.
Stream, ditches and rivers
These might seem like an attractive water source but these are often polluted from agricultural run-off. Having a stream flowing into your pond will also bring in a large amount of silt which means it is likely to fill up faster.
