How can leachate cause pollution?
Pollution occurs when harmful substances usually produced by people, enter a river or soak into the ground, and
- cause damage to the animal and plant life living in a river, or
- remain in the ground to cause damage later.
It is easy to think of the ways in which pollution will harm a river, but if you think about it most of the water that soaks into the ground will move slowly through the ground and if it is polluted it will harm living things in the future when it flows out in streams, or is pumped up to become the water we drink, and water our crops with.
Leachate causes harm in many ways. The most important way it causes harm is by its potential to lower the dissolved oxygen (DO) content of any watercourse it may enter. Leachate normally contains high concentrations of dissolved nutrients and these act as a food source for aerobic aquatic micro-organisms. As a result of being fed, these organisms grow rapidly and in so doing they consume large amounts of oxygen from the water around them.
This reduces the dissolved oxygen in the water. Lowering the DO seriously effects the aquatic ecosystem of a river or stream etc. which needs this oxygen to survive. In a healthy river it is usually the fish which suffer first, but all aquatic life will be effected before long if the pollution continues.
There are other ways in which leachate can cause pollution, such as:-
- if it contains a lot of muds and silts, the natural life of the river may be simply buried or coated to such an extent that many perviously existing river creatures cannot survive;
- feeding fungal growth, or just depositing iron (rusty) staining which as in the previous example also physically coats all the normal river species so that they can no longer “breath” or receive the sunlight their leaves need to photosynthesise and grow;
- by introducing poisonous heavy metals into a river ecology (although this is not usually a problem where the waste disposal sites are regulated to ensure that little if any metals are disposed in the waste, as in the UK).
Summary:
Most people are familiar with the idea of sewage, and the very damaging effect that raw sewage pollution will have if it enters a stream or river. Although leachate volumes are normally much less, even a low strength leachate will usually be at least 10 times the already very damaging strength of sewage. Some leachates can be 1,000 the strength of sewage especially from young, rapidly filled and/or quite dry landfills.
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