Leachate collection pool at a rubbish dump. Leachate is polluted, toxic water formed by rainwater leaching toxic chemicals, decomposition products and heavy metals from waste items at a rubbish dump. If the leachate is not drained off, if can pollute the surrounding soil and enter the groundwater, contaminating drinking supplies. Drainage pipes (far left) bring the leachate into the pool, the base and sides of which are thick layers of plastic sheeting designed to contain the polluted water until it can be disposed of safely.