When you make silage, an effluent with a great potential for pollution is generated. Unless you put a concrete slab beneath the silage and drains at the sides to collect the effluent, pollution of groundwater and rivers and lakes can occur. Dungheaps in farmyards can cause similar problems to those described for silage and you need to take similar precautions to prevent pollution. In urban areas one of the greatest threats to groundwater quality is that posed by refuse dumps. Each one of us generates about a quarter of a tonne of refuse each year which has to be disposed of - usually by landfill.
Domestic refuse generates a noxious leachate when water is passed through the landfill.
This leachate can cause severe pollution of groundwater unless the landfill is located in a suitable area and designed and managed properly. To avoid pollution of groundwater, dumps need to be located where there is a thick unsaturated zone so that the leachate can be reduced in concentration before it reaches groundwater. The thickness of the unsaturated zone depends on the type of material beneath the dump - thicker in sands and gravels than in clays. Also, after each day's dumping the refuse should be covered with material which won't allow water into the refuse, because the more water you allow into the refuse the more leachate you generate. Even where there is a thick unsaturated zone and where you cover the refuse daily some contamination of the groundwater will occur.