Madam, – Gavan Harte (May 18th) is mistaken in his assessment of the real cause of groundwater pollution. When he flushes his toilet in sustainable Blackrock, the contents are processed in a very expensive, nasally locatable, municipal wastewater treatment plant. Significant groundwater pollution is caused by overloaded municipal wastewater treatment in towns and villages throughout the country. He is correct that the political parties collectively owe an apology. However, this apology is not due to An Taisce, it is due to the public for not ensuring that dysfunctional plants were not replaced or upgraded in better times. Groundwater pollution caused by one-off housing is a red herring used by An Taisce to object to development. A correctly designed, installed and certified on-site wastewater treatment system is as sustainable as Mr Harte’s arrangement. – Yours, etc,